CHARACTERISATIONS AND PROPERTIES OF ...

School of Engineering

CHARACTERISATIONS AND PROPERTIES OF

NANOCOMPOSITES BASED UPON VINYL ESTER

MATRIX AND LAYERED SILICATE.

By

ABDULRAHMAN IBRAHIM S ALATEYAH

The thesis is submitted in partial fulfilment of the

requirements for the award of the degree of Doctor of

Philosophy of the University of Portsmouth

January 2014

Abstract

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Abstract

In this research, various concentrations of layered silicate based on vinyl ester

nanocomposites were prepared and the effect of the incorporation of layered silicate

into the polymer matrix on the different properties was investigated.

The characterisations of interlaminar structure of the nanocomposites by X-ray

Diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray Spectrometry

and Transmission Electron Microscopy are undertaken. This study revealed that the

incorporation of layered silicate into the polymer matrix formed uniformly distributed

nanocomposites structure at low clay content (i.e. 1, 2, and 3 wt.%). At 4 wt.% clay

loading, the partially intercalated / exfoliated nanocomposites system was observed as

proved by the different characterisations‟ results. However, the addition of more clay

such as 5 wt.% resulted in decreasing the overall intercalation level due to the existence

of aggregation layers.

The addition of layered silicate into the vinyl ester matrix increased the environmental,

mechanical and thermal properties, and the enhancements were correlated to the results

of the characterisations‟ outputs. The mechanical properties such as flexural, tensile,

nanoindentation, impact, and creep properties of neat samples were improved by the

incorporation of layered silicate. The presence of layered silicate into the polymer

matrix increased the tensile strength and modulus and flexural strength and modulus

up to 4 wt.% clay content. The level of intercalation of nanocomposites played an

important role in the improvements of the mechanical properties. So, the tensile and

flexural properties were correlated to the characterisations‟ results. At 5 wt.% clay

content, the modulus and strength of both tensile and flexural were reduced due to the

effect of aggregation layers where the interfacial interaction between the layered

silicate and the polymer is reduced.

The nanoindentation test showed that the addition of layered silicate increased the

reduced modulus and hardness of the nanocomposites compared to the neat vinyl ester.

The presence of only 1 wt.% clay loading increased the hardness and reduced modulus

at up to 13% and 11% respectively compared to the pristine polymer. The

improvement percentage of hardness and modulus at 2 wt.% were 31% and 19%

respectively. The ultimate improvements were observed at 4 wt.% clay loading, where

the enhancements in hardness and modulus were 56 and 50% respectively compared to

Abstract

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the neat vinyl ester. Further addition of clay resulted in marginal reductions in these

properties.

The impact properties of the neat vinyl ester and the nanocomposites were investigated

using a low velocity impact testing. The addition of layered silicate into the polymer

matrix showed that an optimum range of nanoclay reinforcement in the vinyl ester

matrix can produce enhanced load bearing and energy absorption capability compared

to the neat matrix.

Likewise, the influence of the clay addition into the neat polymer on the creep

relaxation behaviour at 25°C and 60°C was studied. In both cases, the presence of the

layered silicate remarkably improved the creep behaviour. The strain reduction is

related to the clay concentration level. The neat polymer illustrated higher strain

compared to the nanocomposites samples.

Moreover, the addition of layered silicate into the polymer matrix improved the

thermal properties. Thermal Gravimetric Analysis (TGA) showed that the

nanocomposites represent better stability compared to the neat polymer. The onset

temperature of the nanocomposites was higher than the neat polymer. At 1, 2, 3, and 4,

wt.% clay content, the improvements in onset temperature were 7 %, 4.2 %, 4 %, 2.5 %

respectively compared to the virgin polymer. In addition, the incorporation of layered

silicate into the polymer matrix increased the thermal conductivity. At 4 wt.% clay, the

thermal conductivity was increased by 12% compared to the neat polymer. Differential

Scanning Calorimetry (DSC) is also performed in order to study the effect of the

addition of layered silicate into the polymer on the glass transition temperature. The

level of intercalation is critical to the Tg values. The nanocomposites represented a

marginal reduction in Tg, however at 4 wt.% clay loading the Tg was as same as the

neat polymer which was traced to the well-dispersed structure.

Furthermore, the study of environmental measurements, which included the water

absorption behaviour and its effect on the nanoindentation test, was investigated. The

improvement of the water repellence behaviour was observed for the nanocomposites.

The enhancements in barrier properties were related to the clay content. At 5 wt.% clay

loading, the reduction of water uptake was about 1266% compared to the neat polymer.

The hardness and elastic moduli after water absorption was reduced due to the effect of

water molecules entering into the polymer chains. However, the higher amount of clay

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reinforcement led to less reduction in hardness due to the formation of the barrier

properties by the layered silicate. The hardness of neat polymer after immersing in

water was reduced by 30% whereas the hardness of 5 wt.% nanocomposites showed

only a reduction by 10.3% compared to the dry sample.

Keywords: Nanocomposites, vinyl ester, layered silicate, mechanical properties,

thermal behaviour, environmental measurement, XRD, SEM, EDS, and TEM.

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Table of Contents

ABSTRACT ................................................................................................................................ I

TABLE OF CONTENTS ........................................................................................................IV

LISTS OF FIGURES ............................................................................................................. VII

LISTS OF TABLES .................................................................................................................XI

DECLARATION.................................................................................................................. XIII

ABBREVIATIONS ............................................................................................................... XIV

ACKNOWLEDGMENTS ..................................................................................................... XV

DISSEMINATION ................................................................................................................ XVI

CHAPTER 1. INTRODUCTION .......................................................................................... 1

1.1 CHAPTER DESCRIPTION ....................................................................................................................... 1 1.2 INTRODUCTION ................................................................................................................................. 1 1.3 AIMS .............................................................................................................................................. 6 1.4 OBJECTIVES ..................................................................................................................................... 6 1.5 RESEARCH DESIGN AND STRATEGY ....................................................................................................... 7

1.5.1 Task One: Review of related literature.................................................................................. 7 1.5.2 Task Two: Design and production of mould .......................................................................... 7 1.5.3 Task Three: Sample fabrication............................................................................................. 7 1.5.4 Task Four: Carry out various testing ..................................................................................... 8 1.5.5 Task Five: Analysing of morphology structure ...................................................................... 8 1.5.6 Task Six: Analysing and discussion the results ...................................................................... 8 1.5.7 Task Seven: Dissemination .................................................................................................... 8 1.5.8 Task Eight: Writing of the thesis ........................................................................................... 8

1.6 THESIS OUTLINE ............................................................................................................................... 9

CHAPTER 2. LITERATURE REVIEW ............................................................................ 10

2.1 CHAPTER DESCRIPTION ..................................................................................................................... 10 2.2 POLYMER ...................................................................................................................................... 10 2.3 CLASSIFICATION OF POLYMER ............................................................................................................ 10

2.3.1 Classification based on the source of polymer. ................................................................... 10 2.3.2 Classification based on the polymer structure .................................................................... 11 2.3.3 Classification of polymers based on the polymerisation type ............................................. 12 2.3.4 Classification of the polymers based on molecular properties ........................................... 14

2.4 NANOCOMPOSITES .......................................................................................................................... 15 2.4.1 Background ......................................................................................................................... 15 2.4.2 Nanocomposites structures ................................................................................................ 16 2.4.3 Nanocomposite structural characterisation ....................................................................... 16

2.5 LAYERED SILICATE ........................................................................................................................... 19 2.5.1 Structure of layered silicate ................................................................................................ 19 2.5.2 Organic modification of layered silicates (OMLS) ............................................................... 21 2.5.3 Cation-exchange capacity (CEC) ......................................................................................... 24

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2.5.4 Layered silicate summary ................................................................................................... 25 2.6 NANOCOMPOSITE POLYMERS ............................................................................................................ 25

2.6.1 Vinyl polymers. .................................................................................................................... 25 2.6.2 Condensation (step) polymers ............................................................................................ 26 2.6.3 Polyolefins ........................................................................................................................... 26 2.6.4 Biodegradable polymers ..................................................................................................... 26

2.7 NANOCOMPOSITES PREPARATION ...................................................................................................... 27 2.7.1 In situ template synthesis ................................................................................................... 27 2.7.2 Intercalation of polymer from solution ............................................................................... 27 2.7.3 In situ intercalative polymerization .................................................................................... 32 2.7.4 Melt intercalation ............................................................................................................... 37 2.7.5 Direct mixing between the polymer and particles .............................................................. 43

2.8 NANOCOMPOSITES PROPERTIES ......................................................................................................... 43 2.8.1 Mechanical properties ........................................................................................................ 44 2.8.2 Barrier properties ................................................................................................................ 64 2.8.3 Thermal stability ................................................................................................................. 68 2.8.4 Flame retardance ................................................................................................................ 76 2.8.5 Optical property .................................................................................................................. 79 2.8.6 Rheological properties ........................................................................................................ 79

2.9 ADVANTAGES OF NANOCOMPOSITES BASED ON LAYERED SILICATE ............................................................. 80 2.10 NANOCOMPOSITES APPLICATIONS .................................................................................................... 81 2.11 CHAPTER TWO SUMMARY .............................................................................................................. 82

CHAPTER 3. EXPERIMENTAL PROCEDURES ........................................................... 84

3.1 CHAPTER DESCRIPTION ..................................................................................................................... 84 3.2 MATERIALS .................................................................................................................................... 84

3.2.1 Matrix ................................................................................................................................. 84 3.2.2 Reinforcement ..................................................................................................................... 84

3.3 MOULD GEOMETRY ......................................................................................................................... 85 3.4 CONCENTRATIONS OF LAYERED SILICATE FOR NANOCOMPOSITES PREPARATION ........................................... 86 3.5 SAMPLES PREPARATION PROCEDURES ................................................................................................. 86

3.5.1 Type 1 sample fabrication process ...................................................................................... 86 3.5.2 Type 2 sample fabrication process ...................................................................................... 88 3.5.3 Post curing .......................................................................................................................... 90 3.5.4 Cutting preparation for various testing .............................................................................. 90

3.6 THE CALCULATION OF NANOCOMPOSITES VOIDS VOLUME ........................................................................ 91 3.7 MORPHOLOGICAL CHARACTERISATIONS .............................................................................................. 92

3.7.1 Wide Angle X-ray Diffraction (WAXD) ................................................................................. 92 3.7.2 Scanning electron microscopy (SEM) .................................................................................. 92 3.7.3 Energy Dispersive X-ray Spectrometry (EDS) ....................................................................... 92 3.7.4 Transmission Electron Microscopy (TEM) ........................................................................... 92

3.8 MECHANICAL TESTING ..................................................................................................................... 93 3.8.1 Flexural testing ................................................................................................................... 93 3.8.2 Tensile testing ..................................................................................................................... 94 3.8.3 Nanoindentation ................................................................................................................. 95 3.8.4 Impact testing ..................................................................................................................... 96 3.8.5 Creep relaxation behaviour ................................................................................................. 96

3.9 THERMAL TESTING .......................................................................................................................... 97 3.9.1 Thermogravimetric Analysis (TGA) ..................................................................................... 97 3.9.2 Differential scanning calorimetric (DSC) ............................................................................. 97 3.9.3 Surface Thermal Conductivity ............................................................................................. 97

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3.10 ENVIRONMENTAL TESTING .............................................................................................................. 98 3.10.1 Water absorption behaviours ........................................................................................... 98 3.10.2 Contact angle measurements ........................................................................................... 99

CHAPTER 4. RESULTS AND DISCUSSION ................................................................ 100

4.1 CHAPTER DESCRIPTION ................................................................................................................... 100 4.2 RESULTS AND DISCUSSION OF THE TYPE 1 SAMPLES .............................................................................. 100

4.2.1 Nanocomposites voids content ......................................................................................... 100 4.2.2 Characterisations of the interlamellar structure and surface morphology ....................... 101 4.2.3 Mechanical properties ...................................................................................................... 106 4.2.4 Thermal properties............................................................................................................ 124 4.2.5 Environmental measurements .......................................................................................... 128

4.3 RESULTS AND DISCUSSION OF THE TYPE 2 SAMPLES .............................................................................. 134 4.3.1 Characterisations of the interlamellar structure and surface morphology ....................... 134 4.3.2 Mechanical properties ...................................................................................................... 136 4.3.3 Thermal properties............................................................................................................ 141

4.4 COMPARISON BETWEEN TYPE 1 AND 2 SAMPLES ................................................................................. 143 4.4.1 The processing parameters ............................................................................................... 143 4.4.2 Wide Angle X-ray Diffraction (WAXD) of type 1 and 2 sample ......................................... 143 4.4.3 The effect of processing parameters on the mechanical properties ................................. 145 4.4.4 The effect of processing parameters on the thermal properties....................................... 148

CHAPTER 5. CONCLUSIONS AND RECOMMENDATION FOR FUTURE

RESEARCH 149

5.1 CONCLUSIONS .............................................................................................................................. 149 5.1.1 Morphology of nanocomposites ....................................................................................... 149 5.1.2 Mechanical properties ...................................................................................................... 149 5.1.3 Thermal properties............................................................................................................ 151 5.1.4 Water absorption behaviour ............................................................................................. 151 5.1.5 Surface energy characteristics .......................................................................................... 152

5.2 RECOMMENDATIONS FOR FUTURE WORK ........................................................................................... 152

CHAPTER 6. REFERENCES ........................................................................................... 154

CHAPTER 7. APPENDIX ................................................................................................. 176

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Lists of Figures

FIGURE 1 THE RELATIONSHIP BETWEEN THE POLYMER PROPERTIES AND THE DEGREE OF

POLYMERISATION. ................................................................................................................ 1

FIGURE 2 DIFFERENT TYPES OF NANOCOMPOSITES. .................................................................... 4

FIGURE 3 LINEAR POLYMERS STRUCTURE. ................................................................................ 11

FIGURE 4 BRANCHED OR SIDE CHAINS POLYMERS STRUCTURE. ............................................... 11

FIGURE 5 NETWORK OR CROSS-LINKED POLYMERS STRUCTURE. .............................................. 12

FIGURE 6 HOMOPOLYMERS STRUCTURE. .................................................................................. 12

FIGURE 7 COPOLYMERS STRUCTURE. ........................................................................................ 12

FIGURE 8 ALTERNATING COPOLYMERS STRUCTURE.................................................................. 13

FIGURE 9 BLOCK COPOLYMERS ................................................................................................. 13

FIGURE 10 GRAFTED COPOLYMERS STRUCTURE........................................................................ 13

FIGURE 11 TERPOLYMERS STRUCTURE. ..................................................................................... 13

FIGURE 12 THERMOPLASTIC CHAINS MOLECULES STRUCTURE. ................................................ 14

FIGURE 13 THERMOSET PLASTIC STRUCTURE WHERE THE BLACK CIRCLES REPRESENT THE

CROSS LINK. ........................................................................................................................ 15

FIGURE 14 (2:1) LAYERED SILICATE STRUCTURE OR PHYLLOSILICATES. .................................. 20

FIGURE 15 THE INTERCALATION OF ALKALI IONS BETWEEN THE CLAY LAYERS. ...................... 22

FIGURE 16 ION EXCHANGE REACTION, WHICH ENHANCES THE SPACING BETWEEN THE LAYERS

AS WELL AS CHANGES THE PROPERTIES OF THE INDIVIDUAL SURFACE OF THE LAYER FROM

BEING HYDROPHILIC TO HYDROPHOBIC. ............................................................................ 23

FIGURE 17 ORGANISATION OF ALKYLAMMONIUM IONS IN MICA TYPE LAYERED SILICATES

WHERE THE VARIATION OF CHARGE IN THE LAYER IS EXHIBITED. DARK AREAS ARE

SILICATE LAYERS. (A) IS THE MONO LAYERS, (B) THE BILAYERS, WHEREAS (C) AND (D)

REPRESENTS THE MONO AND BIMOLECULAR ORGANISATION RESPECTIVELY. .................. 23

FIGURE 18 FLOWCHART DESCRIBES THE STEPS OF POLYMER INTERCALATION OF SOLUTION ... 28

FIGURE 19 FLOWCHART OF IN SITU POLYMERIZATION STEPS. ................................................... 32

FIGURE 20 INTERLAYER DISTANCE OF FLUORO-MODIFIED TALC (ME 100) IN FUNCTION OF AN

INCREASING AMOUNT OF AMINOLAURIC ACID USED AS THE ORGANIC MODIFIER (233).

REPRODUCED FROM REICHERT BY PERMISSION OF COPYRIGHT WILEY-VCH VERLAG

GMBH & CO. KGAA. ......................................................................................................... 34

FIGURE 21 BASIC STEPS OF MELT INTERCALATION METHOD. .................................................... 37

FIGURE 22 THE DISTRIBUTION OF NA+-MMT SLURRY INTO NYLON6 DURING COMPOUNDING:

(A) PUMPING THE CLAY TO THE SOFTENED MATRIX WITH STRONG STIRRING, (B-C) THE

SLURRY SHRINKAGE TO MINUTE DROPS THROUGHOUT THE MIXING, AND THE WATER OF

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THE CLAY BEING EVAPORATED WHEN IT TOUCHES THE PA6 MELT, (D) THE VACUUM

APPLIED IN ORDER TO ELIMINATE THE WATER WITH SILICATE LAYERS DISTRIBUTED INTO

THE PA6 WHICH SOFTEN AS ONE LAYER OR MULTI LAYERS (264). REPRODUCED FROM

HASEGAWA BY PERMISSION OF ELSEVIER SCIENCE LTD., UK. ......................................... 41

FIGURE 23 DIRECT MIXING PROCESS. ........................................................................................ 43

FIGURE 24 DIFFERENT NANOCOMPOSITES STRUCTURES EXHIBITED DIFFERENT CREEP VALUES

(316). REPRODUCED FROM SHOKUHFAR BY PERMISSION OF ELSEVIER SCIENCE LTD., UK.

........................................................................................................................................... 57

FIGURE 25 LOAD DISPLACEMENT CURVES FOR PA11 AND THE CORRESPONDING

NANOCOMPOSITES (320). REPRODUCED FROM HU BY PERMISSION OF ELSEVIER SCIENCE

LTD., UK. ........................................................................................................................... 59

FIGURE 26 HARDNESS AND MODULUS OF PA11 AND NANOCOMPOSITES SAMPLES (320).

REPRODUCED FROM YUCAI BY PERMISSION OF ELSEVIER SCIENCE LTD., UK. ................ 59

FIGURE 27 STORAGE MODULUS BEHAVIOUR OF NEAT POLYMER AND CORRESPONDING

NANOCOMPOSITES (281). REPRODUCED FROM UNNIKRISHNAN BY PERMISSION OF

ELSEVIER SCIENCE LTD., UK. ............................................................................................ 62

FIGURE 28 THE RELATIONSHIP BETWEEN THE STORAGE MODULUS AS WELL AS THE TAN IN A

FUNCTION OF TEMPERATURE (300). REPRODUCED FROM GANß BY PERMISSION OF


FIGURE 29 (A) IS THE MODEL OF TORTUOUS DIFFUSION PATH IN EXFOLIATED STRUCTURE OF

NANOCOMPOSITES AND (B) IS THE PATH OF CONVENTIONAL COMPOSITES. ...................... 64

FIGURE 30 THERMAL STABILITY FOR (A) NEAT EPOXY AND (B) EPOXY CLAY NANOCOMPOSITES

(351). REPRODUCED FROM LAKSHMI BY PERMISSION OF ELSEVIER SCIENCE LTD., UK. . 70

FIGURE 31 TGA CURVE (A) NEAT PU, (B) PU/KH550-SP (1%), (C) PU/KH550-SP (3%) AND (D)

PU/ KH550-SP (5%) NANOCOMPOSITES (282). REPRODUCED FROM CHEN BY PERMISSION

OF ELSEVIER SCIENCE LTD., UK. ....................................................................................... 71

FIGURE 32 TGA DTG OF NEAT POLYMER AND NANOCOMPOSITES SERIES UNDER (A) NITROGEN

AND (B) AIR (260). REPRODUCED FROM BARICK BY PERMISSION OF ELSEVIER SCIENCE

LTD., UK. ........................................................................................................................... 73

FIGURE 33 (A)TGA AND (B) DTGA CURVE OF A2 AND A2/MT-PCL NANOCOMPOSITES AT

HEATING RATE OF 10 K/MIN. (354). REPRODUCED FROM NAGUIB BY PERMISSION OF


FIGURE 34 HRR IN A FUNCTION OF TEMPERATURE OF EVA, EVA-NC0 (EVA + POLYMERIC

MODIFIER (20%)) AND EVA-NC5 (EVA+ POLYMERIC MODIFIER + 5.6 WT.% MMT) (291).

REPRODUCED FROM SHI BY PERMISSION OF ELSEVIER SCIENCE LTD., UK. ...................... 77

FIGURE 35 THE STRUCTURE OF VINYL ESTER (390). ................................................................. 84

FIGURE 36 MOULD GEOMETRY, (A) FLAT SHAPE; (B) DOG-BONE SHAPE. ................................. 86

FIGURE 37 FLOW CHART OF THE FIRST SAMPLE FABRICATION. ................................................. 87

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FIGURE 38 THE SECOND SAMPLE FABRICATION ......................................................................... 89

FIGURE 39 FAN ASSISTED OVEN FOR POST CURING; (A) BEFORE POST CURING, (B) AFTER POST

CURING. .............................................................................................................................. 90

FIGURE 40 (A) THE WATER JET CUTTING PROCESS, (B) THE EFFECT OF WATER JET METHOD ON

THE SAMPLES SURFACE AND (C) THE DIAMOND WHEEL CUTTER. ...................................... 91

FIGURE 41 THE PRINCIPLE OF THE THERMAL CONDUCTIVITY MEASUREMENT. ......................... 97

FIGURE 42 XRD RESULTS OF NEAT POLYMER AND THE CORRESPONDING NANOCOMPOSITES.

......................................................................................................................................... 102

FIGURE 43 SEM IMAGES AT 300 µM AND 50 µM OF (A) 2 WT.%, (B) 4 WT.% AND (C) 5 WT.%

NANOCOMPOSITES. ........................................................................................................... 103

FIGURE 44 EDS IMAGES AT DIFFERENT MAGNIFICATION (A) (55X) AND (B) (550X) OF 2 WT.%,

4 WT.% AND 5 WT.% NANOCOMPOSITES. ......................................................................... 105

FIGURE 45 TEM MICROGRAPHS AT 20 NM MAGNIFICATION OF (A) 2 WT.%, (B) 4 WT.% AND (C)

5 WT.% NANOCOMPOSITES. .............................................................................................. 106

FIGURE 46 THE EFFECT OF CLAY LOADING ON FLEXURAL STRENGTH. .................................... 108

FIGURE 47 THE EFFECT OF CLAY LOADING ON FLEXURAL MODULUS. ..................................... 108

FIGURE 48 THE RELATIONSHIP BETWEEN THE VOIDS CONTENT AND THE FLEXURAL MODULUS.

......................................................................................................................................... 109

FIGURE 49 THE RELATIONSHIP BETWEEN THE VOIDS CONTENT AND THE FLEXURAL STRENGTH.

......................................................................................................................................... 109

FIGURE 50 COMPARISON BETWEEN THEORETICAL AND EXPERIMENTAL DATA OF FLEXURAL

MODULUS AT DIFFERENT CLAY CONTENT. ....................................................................... 110

FIGURE 51 THE IMPROVEMENT AMOUNT OF TANGENT MODULUS FOR DIFFERENT CLAY

LOADINGS. ........................................................................................................................ 112

FIGURE 52 THE IMPROVEMENT OF TENSILE STRENGTH PROPERTIES OF NEAT POLYMER AND

NANOCOMPOSITES ............................................................................................................ 112

FIGURE 53 THE INFLUENCE OF THE VOIDS CONTENT ON THE TANGENT MODULUS .................. 113

FIGURE 54 THE INFLUENCE OF THE VOIDS CONTENT ON THE TENSILE STRENGTH ................... 113

FIGURE 55 THE RELATIONSHIP BETWEEN THE LOAD (MN) AND THE DEPTH (NM) OF THE NEAT

POLYMER AND CORRESPONDING NANOCOMPOSITES SAMPLES ........................................ 115

FIGURE 56 THE ELASTIC MODULUS OF CALCULATED VALUES AND EXPERIMENTAL RESULTS AT

VARIOUS CLAY CONTENT. ................................................................................................ 116

FIGURE 57 ZWICK/ROELL HIT230F DROP WEIGHT IMPACT TOWER AND SPECIMEN CLAMPING.

......................................................................................................................................... 118

FIGURE 58 THE RELATION BETWEEN THE WORK (J) AND THE CLAY LOADING WT.%. ............. 119

FIGURE 59 MAXIMUM FORCE VS. TIME TRACES OF THE IMPACT TEST. .................................... 120

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FIGURE 60 FRAGMENTATION CHARACTERISTICS OF (A) NEAT VINYL ESTER, (B)

NANOCOMPOSITES OF 2 WT.T% CLAY LOADING, (C) NANOCOMPOSITES OF 4 WT.T% CLAY

LOADING, AND (D) NANOCOMPOSITES OF 5 WT.% CLAY LOADING. ................................ 121

FIGURE 61 CREEP RELAXATION BEHAVIOUR OF NEAT POLYMER AND THE CORRESPONDING

NANOCOMPOSITES AT 25°C. ............................................................................................. 122

FIGURE 62 CREEP RELAXATION BEHAVIOUR OF PRISTINE MATRIX AND 2 AND 4 WT.%

NANOCOMPOSITES AT 60°C .............................................................................................. 123

FIGURE 63 TGA CURVE OF NEAT VE AND CORRESPONDING NANOCOMPOSITES. ................... 125

FIGURE 64 DTG CURVE OF BASE POLYMER AND DIFFERENT CONCENTRATIONS OF

NANOCOMPOSITES. ........................................................................................................... 125

FIGURE 65 DSC CURVE OF NEAT POLYMER AND THE CORRESPONDING NANOCOMPOSITES. .. 126

FIGURE 66 THE EFFECT OF LAYERED SILICATE INTO THE VINYL ESTER MATRIX ON THE

THERMAL CONDUCTIVITY. ............................................................................................... 127

FIGURE 67 WATER ABSORPTION BEHAVIOUR OF NEAT VINYL ESTER AND NANOCOMPOSITES.

......................................................................................................................................... 128

FIGURE 68 THE HARDNESS OF PRISTINE POLYMER AND NANOCOMPOSITES BEFORE AND AFTER

WATER ABSORPTION BEHAVIOUR. .................................................................................... 132

FIGURE 69 CONTACT ANGLE OF SOLVENTS (GLYCEROL AND WATER) DROPLET OF NEAT, 2

WT.% AND 4 WT.% NANOCOMPOSITES. ............................................................................ 133

FIGURE70 XRD CURVE OF CLOISITE 10A AND THE CORRESPONDING NANOCOMPOSITES. ..... 135

FIGURE 71 SEM IMAGES AT 50 µM OF (A) 1 WT.% AND (B) 2.5 WT.% NANOCOMPOSITES. ..... 135

FIGURE 72 TEM MICROGRAPHS AT 50 NM MAGNIFICATION OF (A) 1 WT.% AND (B) 2.5 WT.%

NANOCOMPOSITES ............................................................................................................ 136

FIGURE 73 THE EFFECT OF CLAY LOADING ON FLEXURAL STRENGTH. .................................... 138

FIGURE 74 THE EFFECT OF CLAY LOADING ON FLEXURAL MODULUS. ..................................... 138

FIGURE 75 THE TANGENT MODULUS IMPROVEMENTS AT DIFFERENT CLAY LOADING LEVELS.

......................................................................................................................................... 140

FIGURE 76 THE ULTIMATE TENSILE STRENGTH IMPROVEMENTS OF NEAT POLYMER AND

NANOCOMPOSITES. ........................................................................................................... 140

FIGURE 77 TGA CURVE OF NEAT VE AND CORRESPONDING NANOCOMPOSITES. ................... 141

FIGURE 78 DTG CURVE OF BASE POLYMER AND DIFFERENT CONCENTRATIONS OF

NANOCOMPOSITES. ........................................................................................................... 142

FIGURE 79 DSC CURVE OF THE NEAT VINYL ESTER AND THE CORRESPONDING

NANOCOMPOSITES. ........................................................................................................... 142

FIGURE 80 XRD CURVE OF CLOISITE 10A AND THE CORRESPONDING NANOCOMPOSITES OF

FIRST SAMPLE FABRICATION METHOD. ............................................................................ 144

FIGURE 81 XRD CURVE OF CLOISITE 10A AND THE CORRESPONDING NANOCOMPOSITES OF THE

SECOND SAMPLE FABRICATION METHOD. ........................................................................ 144

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Lists of Tables

TABLE 1 THE COMBINATIONS OF DIFFERENT MATERIALS (186). ............................................... 39

TABLE 2 YOUNG'S MODULUS OF HARD AND SOFT POLYURETHANE NANOCOMPOSITES BY MELT

AND SOLUTION INTERCALATION (223). .............................................................................. 46

TABLE 3 THE RELATIONSHIP BETWEEN THE NANOCOMPOSITES PROPERTIES AND THE

ULTRASOUND AMPLITUDE (292). ....................................................................................... 49

TABLE 4 THE INFLUENCE OF LAYERED SILICATE ON THE TENSILE MODULUS AND STRENGTH

(305). .................................................................................................................................. 51

TABLE 5 THE EFFECT OF THE ADDITION OF LAYERED SILICATE INTO THE POLYMER MATRIX ON

THE NANOMECHANICAL PROPERTIES (319). ....................................................................... 58

TABLE 6 VARIOUS CLAY TYPES THAT WERE USED IN THE EXPERIMENT. REPRINTED FROM

(323). .................................................................................................................................. 61

TABLE 7 E' VALUES AT DIFFERENT CLAY TYPES AND CONCENTRATIONS (323). ....................... 61

TABLE 8 VARIOUS POLYMERS AND CLAYS THAT WERE USED IN THIS STUDY (340). ................. 67

TABLE 9 DIFFERENT TYPES OF MATERIALS THAT HAVE BEEN USED IN THIS STUDY (351). ....... 69

TABLE 10 TGA IN NITROGEN AND AIR OF DIFFERENT CLAY TYPES AND CONTENT (323). ........ 70

TABLE 11 TGA RESULTS OF NANOCOMPOSITES AND NEAT POLYMER (282). ............................ 71

TABLE 12 GLASS TRANSITION TEMPERATURE FOR DIFFERENT POLYMERS BASED ON LAYERED

SILICATE (340).................................................................................................................... 72

TABLE 13 DSC VALUES FOR BASE POLYMER AND NANOCOMPOSITES SERIES (260). ................ 72

TABLE 14 CHEMICAL COMBINATIONS AND MOLECULAR WEIGHTS OF POLY(HB/PCL-PEG-

PCL) URETHANES (354). .................................................................................................... 74

TABLE 15 THE THERMAL DEGRADATION OF POLYURETHANE SAMPLES AND CORRESPONDING

NANOCOMPOSITES (354).................................................................................................... 75

TABLE 16 REINFORCEMENT PROPERTIES (392). ......................................................................... 85

TABLE 17 THE CONCENTRATIONS OF LAYERED SILICATE USED IN THE FABRICATION OF

NANOCOMPOSITES. ............................................................................................................. 86

TABLE 18 SAMPLE DIMENSIONS FOR DIFFERENT TESTS AND CHARACTERISATIONS. ................ 91

TABLE 19 VOIDS CONTENT OF DIFFERENT NANOCOMPOSITES SAMPLES. ................................ 100

TABLE 20 XRD RESULTS OBTAINED FROM DIFFERENT CLAY LOADING OF NANOCOMPOSITES.

......................................................................................................................................... 101

TABLE 21 FLEXURAL TEST RESULTS. ....................................................................................... 107

TABLE 22 COMPARISON BETWEEN CALCULATED AND EXPERIMENTAL RESULTS OF FLEXURAL

MODULUS. ........................................................................................................................ 110

TABLE 23 TENSILE TEST OUTCOMES FOR NEAT AND DIFFERENT CLAY LOADING SAMPLES. ... 111

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TABLE 24 THE NANOINDENTATION DATA EXTRACTED FROM THE LOAD-UNLOAD CURVE OF THE

EXPERIMENTAL NANOINDENTATION TEST. ...................................................................... 114

TABLE 25 THE THEORETICAL AND PRACTICAL ELASTIC MODULUS OF THE NEAT VINYL ESTER

AND THE CORRESPONDING NANOCOMPOSITES. ................................................................ 116

TABLE 26 THE MODULUS OF NEAT POLYMER AND THE CORRESPONDING NANOCOMPOSITES FOR

DIFFERENT MECHANICAL TESTS. ...................................................................................... 117

TABLE 27 IMPACT TEST RESULTS OF NEAT POLYMER AND THE CORRESPONDING OF

NANOCOMPOSITES. ........................................................................................................... 118

TABLE 28 THE STRAIN AMOUNT OF THE NEAT VINYL ESTER AND THE CORRESPONDING

NANOCOMPOSITES DURING THE CREEP TEST AT DIFFERENT INTERVALS TIMES AND

TEMPERATURES. ............................................................................................................... 121

TABLE 29 THE THERMAL CONDUCTIVITY OF PRISTINE VE AND THE CORRESPONDING

NANOCOMPOSITES. ........................................................................................................... 128

TABLE 30 THE MAXIMUM WATER UPTAKE AND DIFFUSION COEFFICIENT OF PRISTINE POLYMER

AND NANOCOMPOSITES. ................................................................................................... 130

TABLE 31 THE HARDNESS AND REDUCED MODULUS OF NEAT MATRIX AND CORRESPONDING

NANOCOMPOSITES BEFORE AND AFTER WATER ABSORPTION. ......................................... 131

TABLE 32 CONTACT ANGLE VALUES OF NEAT VINYL ESTER AND THE CORRESPONDING

NANOCOMPOSITES OF BOTH WATER AND GLYCEROL. ...................................................... 132

TABLE 33 XRD RESULTS OBTAINED FROM DIFFERENT CLAY LOADING OF NANOCOMPOSITES.

......................................................................................................................................... 134

TABLE 34 FLEXURAL TEST RESULTS. ....................................................................................... 137

TABLE 35 TENSILE TEST OUTCOMES FOR NEAT AND DIFFERENT CLAY LOADING SAMPLES. ... 139

TABLE 36 COMPARISON BETWEEN FIRST AND SECOND SAMPLE FABRICATION METHODS. ..... 143

TABLE 37 XRD RESULTS OBTAINED FROM DIFFERENT CLAY LOADING OF TYPE 1 AND 2

SAMPLES. .......................................................................................................................... 145

TABLE 38 COMPARISONS BETWEEN THE FIRST AND SECOND FABRICATION METHODS REGARD

THE FLEXURAL PROPERTIES. ............................................................................................ 146

TABLE 39 COMPARISONS BETWEEN THE FIRST AND SECOND FABRICATION METHODS IN TERM

OF THE TENSILE PROPERTIES. ........................................................................................... 147

TABLE 40 THE THERMAL PROPERTIES OF THE FIRST AND SECOND SAMPLE FABRICATIONS

PROCESS. .......................................................................................................................... 148

XIII | P a g e

Declaration

I hereby declare that this thesis is a presentation of my original research work and

effort and that it has not been submitted anywhere for any award. Where other sources

of information have been used, they have been acknowledged.

Signature: ……………………………………….

Date: ……………………………………………

XIV | P a g e

Abbreviations

AFM Atomic Force Microscopy PLA Poly(lactic acid)

CEC Cation-exchange capacity POE Polyolefin elastomer

13 C-NMR 13C solid-state nuclear magnetic resonance PP Polypropylene

DSC Differential Scanning Calorimetry PS Polystyrene

DTG Derivative Thermo Gravimetric Analysis PSF Polysulfone

EDS Energy Dispersive X-ray Spectrometry PU Polyurethane

EVA Ethylene vinyl acetate PUA Poly(urethane acrylate)

FTIR Fourier Transform Infrared Spectroscopy PVC Polyvinyl chloride

fwhm Full width at half maximum SEM Scanning electron microscopy

HCL Hydrochloric acid STM Scanning tunnelling microscopy

HRR The rate of heat release Td Decomposition temperature

MLR Mass loss rate TEM Transmission electron microscopy

MMT Montmorillonite Tg Glass transition temperature

OMLS Organo modification layered silicate TGA Thermo Gravimetric Analysis

o-MMT Organo montmorillonite THF Tetrahydrofuran

PA or PI Polyamide THR Complete heat released

PCL Polycaprolactone TTI Period to ignition

PE Polyethylene VE Vinyl ester resin

PEO Poly(ethylene oxide) WAXD Wide angle x-ray diffraction

PET Polyethylene terephthalate XRD X-ray diffraction

PHRR Heat release peak

XV | P a g e

Acknowledgments

It is a pleasure to thank the many people who made this thesis possible. This work

would not have been possible without the support from Dr. Hom Nath Dhakal under

whose guidance I chose this topic. I would like also to gratefully acknowledge him,

since he has been abundantly helpful and has assisted me in numerous ways. I specially

thank him for his infinite patience. The discussions I had with him were invaluable.

Also, I would thank him for helping me with the research material.

I would like to say a big thanks to Dr. Zhong Yi Zhang, who helped me with the many

test. Also, I would like to Thanks all the School of Engineering staff for all the help

and support provided.

I am very grateful to my parents Ibraihim and Monirah for their continuous prayers

for me and would also like to offer thanks to my brothers and sisters. No words can

express my gratitude to all my family.

I would also say a big thanks to my uncle Dr Ali Alateyah for his help and support.

I would also like to express my sincere thanks and gratitude to my wife Ibtihal

Alnugaythan and my daughter Ajawn for their support and help.

XVI | P a g e

Dissemination

Type Title Journal paper Status

Journal

Water absorption behaviour, mechanical and

thermal properties of vinyl ester matrix

nanocomposites based on layered silicate.

Polymer-Plastics Technology and Engineering 2014- 53(4)

Journal

Processing, properties and applications of

polymer Nanocomposites based on layer

silicates: A Review

Advances in Polymer Technology

2013 -32 (3)

Top 10 Accessed

Journal

Mechanical and thermal properties

characterisation of vinyl ester matrix


International Journal of Mechanical Science and

Engineering, 7(9), 0 - 8.

World Academy of Science, Engineering

and Technology, International Science

(WASET) Index 81

Most Accessed

Journal

Contact angle measurement of the vinyl ester

matrix nanocomposites based on layered silicate

International Journal of Materials Science and

Engineering, 7(12), 693 - 699.

(WASET) Index 84

Journal

The effect of processing parameters of the vinyl

ester matrix nanocomposites based on layered

silicate on the level of exfoliation.


Engineering, 7(12), 609 - 616.

(WASET) Index 84

Journal

Mechanical and thermal properties

characterisation of vinyl ester matrix

nanocomposites based on layered silicate: effect

of processing parameters.


Engineering, 7(12), 340 - 352

(WASET) Index 84

Journal

Characterisation and the influence of voids on

flexural and tensile modulus of layered

silicate/vinyl ester nanocomposites

American Journal of Materials Science

Vol.4, No.1, 2014

Journal

Characterisation and the influence of voids on

flexural and tensile strength of layered

silicate/vinyl ester nanocomposites

American Journal of Polymer Science

Vol.4, No.1, 2014

Conference

abstract

Mechanical properties of Vinyl ester based on

layered silicate. Saudi Scientific Conference

University of Edinburgh

01/02/2014

Journal

Nanoindentation and characterisations of vinyl

ester matrix nanocomposites based on layered

silicate: Effect of voids content

International Journal of Materials Engineering

(SAP) Accepted

Journal

Low velocity impact and creep properties of

vinyl ester matrix nanocomposites based on

layered silicate

International Journal of Plastics Technology

Under review

Supervision

Co-Supervised master student with a title (

Mechanical properties of vinyl ester matrix

nanocomposites)

University of Portsmouth Jun-Sep /2011

Supervision

Co-Supervised bachelor student with a title

(Polyester nanocomposites, syntheses and

characterisations).

University of Portsmouth Jun-Sep/2011

Reviewer

Mechanical Properties of Epoxy Resin; Based

Composites Loaded with Granite Stone Powder

from the Sergipe Fold Belt.

Journal of Material Research Nov-2013

http://www.manuscriptsystem.com/author/paperdetail.aspx?paperid=1007391



Chapter 1. Introduction

1 | P a g e


1.1 Chapter description

General understanding of the different material subjects which include materials

classification, the terms of polymers, nanocomposites, nanometre, and the polymer-

based nanocomposites are discussed in this chapter. Also, the aims, objectives and the

research design are included in chapter one.

1.2 Introduction

The materials classification can be based on the wide range of properties and their

structural application. In general, four huge families of materials can be obtained. For

example metals and alloys, ceramics and glasses, polymers and elastomers, and

composites (1). In this work, the polymer matrix nanocomposites are studied. The

polymer can be defined as the unique structural characteristic consisting of a very

enormous number of chain units which are connected together by covalent link (2).

The numbers of repeated units are called monomers, which are also expressed as the

degree of polymerisation. In the polymer, the large amount of the degree of

polymerisation, the better mechanical properties (tensile strength, ductility, and

hardness) as the chains are increased as shown in Figure 1(3).

Figure 1 The relationship between the polymer properties and the degree of polymerisation.

Polymers are widely used materials, owing to their advantageous properties such as

light weight and ease of manufacturing. However, polymers on its own, their certain

properties are inadequate unless they are modified through addition of fillers and

various reinforcements leading to the formation of composite or nanocomposite

Degree of polymerisation

Polymer

Properties


2 | P a g e

materials (4). For that reason and to overcome these drawbacks, suitable fillers

(additives) are applied in the neat polymers in order to enhance their properties.

Polymers with various particulate fillers have been successfully reinforced to improve

their stiffness and toughness, as well as enhancing their resistance to fire and ignition

and also their barrier properties. Addition of the particulate fillers often results in

unwanted properties such as brittleness and opacity as a result of inhomogeneous

dispersion on a nanometer scale between the polymer and the additives. However, if

homogenous dispersion on a nanometer scale could be achieved, the mechanical

properties could be further improved and/or new unexpected features might be

exhibited (5).

Composite materials have a fairly new class which are nanocomposites that are

particle-filled composites in which at least one dimension of the dispersion particles is

in the nanometer range. The matrix can be either single or multicomponent. The matrix

may be either metallic, ceramic or polymeric (6). In other words, nanocomposites are

multiphase solid materials in which one of the phases has at least 1, 2 or 3 dimensions

that are below 100 nm (7).

A nanometer equals one billionth of a meter, which means a nanometer equals

approximately 4 times an atom‟s diameter. In terms of the number of dimensions in the

nanometer range that are discerned, three different types of dimensions can be

observed. Firstly, identifying the isodimensional nanoparticles which comprise the

three dimensions are on the nanometer order. Secondly, when two dimensions of

nanometer scale where a large one is observed, this is called whiskers or nanotube.

However, the most extensive investigation is conducted into the clay (layered silicate),

which is a sheet of one or a few nanometers thick and 100-1000 nanometers in extent.

The reasons for the extent of this investigation of the clay is because of its ease of

availability and because the chemical interaction has been studied for a long time (8).

The most commonly used organophilic layered silicate is obtained from

montmorillonite (MMT). The structure of this layer is made of many stacked layers

and each one has a thickness of between 1.2-1.5 nm and the lateral dimension is from

100-200 nm. These layers order themselves to shape the stacks with a consistent gap

between them; these gaps are called the gallery or interlayer. The sum of galleries and

the thicknesses of single layer represent the repeat unit of the material of multilayer,


3 | P a g e

which is normally called d-spacing or basal spacing (d001). This spacing can be

calculated from the (001) harmonics obtained from x-ray diffraction (XRD)(9) .

The use of composites and nanocomposites made from inorganic substances of a

layered structure like clays has been a subject of elaborate research. However, the

subject is experiencing resurgence both in terms of research and industrial activity due

to the numerous properties that nanocomposites stand to provide. Several variables

associated with materials, that can be controlled, can have a profound influence both on

the properties and the structure of the nanocomposite such as the kind of the clay used,

the kind of pre-treatment, the polymer component chosen and the manner in which the

nanocomposite incorporates the polymer. Likewise, the relation between the

nanocomposite and the polymer material used is dictated by the processing method

chosen and the purpose for production (whether the user is a special processor or

integrated manufacturer) (10).

Polymer layered silicate nanocomposites have received much attention during the last

decade and have great interest both in the academic field and in industry (11), since

they often give more attractive improvement to material properties than both micro and

macro composite materials (12-16). The improvement can be mechanical (high

strength, modules, and flexural) or thermal (thermal gravity analysis). They also

exhibit different properties such as decreased gas permeability and flammability (16,

17), increased biodegradability and barrier properties (15). These materials reported to

be as 21st century materials as their unique of properties and design which are not

found on the traditional composites (18).

The particles which are dispersed into the matrix are nanoparticles such as Lamellar,

Fibular, Tubular, and Spherical. Each one has different functions since the

enhancement of mechanical properties as well as the barrier properties can be reached

by using Lamellar. However, for strength and rigidity, Fibular is used most and

preferred while Spherical can lead to an improvement in the optical and electrical

conductivity (6).

The reason for improvement of the material properties is the interfacial interaction

between the organically modified layered silicate (OMLS) and the matrix as opposed

to traditional composites. Layered silicate has a thickness of layer in the order of 1 nm


4 | P a g e

and very high aspect ratio. A little weight percent of OMLS is dispersed throughout the

matrix, thus giving a larger surface area for polymer filler interfacial interaction than in

conventional composites (15). However in polymers with low polarity such as

polyolefins, the improvement is not too significant since there is not compatibility

between the clay and polymer (19).

Relying on the strength of the interaction between the polymers and layered silicates

(which are treated or not), three varied phases of nanocomposites are

thermodynamically obtainable as shown in Figure 2.

Figure 2 Different types of nanocomposites.

First of all there are intercalated nanocomposite structures; this phase occurs when one

or more lengthened polymer chains is intercalated within the layered silicate. The

effect of that is a well-organized multilayer structure with reciprocally acting

polymeric and layered silicate layers. The dispersion between the platelets normally is

between 20-30 Å (8, 20-25). In this phase (intercalation), the introduction of the matrix

of polymer into the layered silicate happens in a “crystallographically regular fashion”

where the ratio of the layered silicate to the polymer is irrelevant. The properties of this

stage are similar to ceramic materials (16).

Secondly, flocculated structures nanocomposites, which are similar to the previous

stage in conception, but the difference is in the position of layered silicates since

normally they are flocculated. The reason for that is because of the introduction of

hydroxyls and the resulting interaction with the silicate layers (26). Finally, exfoliated

or delaminated structures nanocomposites. Usually, this phase is achieved when the

concentrations of the layered silicates are significantly lower than in the intercalation

Exfoliated Intercalated Flocculated


5 | P a g e

system. The exfoliation occurs when layered silicate layers are dispersed in a

continuous polymer matrix via median distance, which is dependent on the clay content

(16).

In other words, exfoliated or delaminated structures are produced when the layered

silicate layers are effectively separated from one another and separately incorporated in

the repeated polymer matrix (20-22, 24). The separation between the platelets in the

exfoliation stage is between 80-100 Å. Furthermore, the distance can sometimes be

more than 100Å (25). In the exfoliation structure, the increase in the distance between

the layered silicates is comparable to the radius gyration of the polymer. However, in

the intercalation structure, the expansion of the interlayer is comparable to an extended

chain. The exfoliation system is an attractive option since the properties of this

structure have been improved by the high area interaction between the clay and the

polymer which results in a homogeneous distribution (27). Even with the several

advantages of the exfoliation structure, it is difficult to reach a final exfoliation of the

clay. The reason for this difficulty is because of the large anisotropic structure of layers

of silicate where the outside dimensions are between 100-1000 nm. This makes it

difficult to locate them in a random way in the polymer sea even when they are

dispersed over huge distances such as when delaminated (28). Moreover, it was

revealed that the difficulty of reaching the exfoliation structure was assigned to the clay

surface nature as the clay is naturally hydrophilic and the polymer is hydrophobic,

which leads to difficulty in exfoliation in the polymer matrix (29). In addition, most of

the polymer chains in the mixture of clay and polymer are connected to the surface of

the layers of silicate. As a result of this, it can be thought that there are domains in

these hybrids where a number of long-range orders are retained and the layers of

silicate are positioned in a small number of favourable directions (16, 28).

The thermodynamic behaviour, which includes entropic and energetic factors of the

nanostructure, can determine the outcomes of nanocomposites structure and polymer

interaction. The low entropic penalty that is produced by the polymer chain

confinement when the incorporation of layered silicate took place could be

compensated by the increased conformational freedom of the surfactant chains as the

layers dispersed throughout the polymer sea. The difference in total change of entropy

between them can control the output nanocomposites structure whether immiscible,


6 | P a g e

intercalation or exfoliation. So, when the amount of total change in entropy is small,

less change in the system‟s internal energy will take place and the intercalation level

will be presented. Thus, for successful composite formation, it is important to have an

initial interlayer structure that leads in a maximum enhance in the conformational

entropy of the surfactant chains to compensate the penalty of polymer confinement. As

a result, more change in in the system‟s internal energy will be obtained and then the

exfoliation structure would be presented (30).

The mechanism of nanocomposites properties are correlated to the phase structure and

types. In immiscible nanocomposites system, the properties are similar to the

traditional composites. The intercalation nanocomposites structure provide with better

properties compared to the immiscible structure where the polymer chains reduce the

electrostatic force between layers and the uniform distribution of clay will take place.

The properties of exfoliation nanocomposites can be distinguished which attributed to

the high surface area provided by its structure. Thus, the angle between layered

silicates is high and the interfacial inetraction between the layered silicate and polymer

is significantly increased compared to intercalated structure.

1.3 Aims

The main aims of this study are to explore the structure and the properties of layered

silicate nanocomposites by employing various weight percent of layered silicate

reinforcements. The study further aims at investigating the effect of the presence of

layered silicate into the vinyl ester matrix on the overall properties. The study also aims

to pinpoint the gap and limitations and suggest the mechanisms that involve in

obtaining optimal properties enhancement in nanocomposite systems.

1.4 Objectives

This study is primarily devoted to the preparation and characterisations of neat vinyl

ester and the corresponding nanocomposites using novel preparation techniques, and to

perform a systematic experimental study on the properties of layered silicate reinforced

vinyl ester nanocomposites with various concentrations of clay contents, and

specifically the following objectives therein:

To review the relevant literature;


7 | P a g e

To gain insight into the differences between: conventional composites,

intercalated and exfoliated nanocomposites;

To fabricate vinyl ester nanocomposites reinforced with various concentrations

of layered silicate and study the effect of the various clay contents on the

different properties such as mechanical, thermal, environmental, and surface

energy;

To select and compare alternative methodologies for the preparation of vinyl

ester matrix nanocomposites based on layered silicate and the influence of

these processing parameters on the nanocomposites properties;

To investigate the morphology of nanocomposites by different characterisation

techniques and correlate the morphology structure to the different properties.

1.5 Research Design and Strategy

In order to achieve the overall aims and the objectives of the current thesis, the

following tasks were undertaken.

1.5.1 Task One: Review of related literature

The literature review is the basic infrastructure of the thesis. Reviewing the literature

can benefit the researcher and the reader to understand the whole subject and the

progress of the research. In the current research, extensive literature about the polymers

and the polymer nanocomposites are provided. Also, the effects of the addition of

layered silicate into the polymer matrix on the mechanical, thermal and environment

properties are presented.

1.5.2 Task Two: Design and production of mould

A special stainless steel mould was manufactured under the British Standard (BS)

geometry. For example, the tensile and flexural test need at least a specimen of 70 mm

x 20 mm x 3 mm, so the thickness of this mould was 3mm in order to meet the

requirements of BS.

1.5.3 Task Three: Sample fabrication

Series of the sample were produced which include the neat vinyl ester and the

corresponding nanocomposites with different clay contents. The clay concentrations

were as follows: 1, 2, 3, 4, 5 wt. %.


8 | P a g e

1.5.4 Task Four: Carry out various testing

Mechanical testing

Tensile, Flexural, nanoindentation, impact, and creep tests were carried out in

order to understand the effect of the incorporation of clay into the vinyl ester

matrix.

Thermal measurement

Thermogravimetric Analysis, Differential Scanning Calorimetric and Surface

thermal conductivity were used to evaluate the thermal properties.

Water absorption behaviour

The influence of the presence of layered silicate into the polymer matrix on the

water uptaking behaviour is carried out.

Other tests

The effect of the water absorption behaviour into the nanoindentation samples

was investigated. Concat angle measurment was also performed in order to

study the tension surface energy and the term of the wettability.

1.5.5 Task Five: Analysing of morphology structure

The morphology of the nanocomposites samples are investigated by utilising various

devices of characterisations which include Wide Angle X-ray Diffraction (WAXD),

Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectrometry (EDS)

and Transmission Electron Microscopy (TEM).

1.5.6 Task Six: Analysing and discussion the results

Intensive analytical discussions are presented. The comparison between the neat

polymer and the corresponding nanocomposites are performed.

1.5.7 Task Seven: Dissemination

Publish the findings in this research work in high impact factor scientific journal, and

enhancing the literature review section to be ready as a format of review paper.

1.5.8 Task Eight: Writing of the thesis

This task is the last one and combines all the above tasks to be together. Each chapter

will be connected to the others.


9 | P a g e

1.6 Thesis Outline

This thesis provides a useful understanding about the polymer-clay nanocomposites.

The contents of this research are divided into five chapters which are presented below:

Chapter one presents a general introduction about the different terminologies that are

used. Also, the aims, objectives and research strategy and design are included in this

chapter.

Chapter two contains a comprehensive review of current literature which includes the

types and structures of polymers, layered silicates and nanocomposites. In addition, the

polymer based on layered silicate nanocomposites properties, advantages and

applications are discussed.

Chapter three details the materials of polymer and layered silicate that were used in

the current study and the sample fabrication process which was followed. Experimental

procedures and characterisation techniques are also discussed in this chapter.

Chapter four discusses the mechanical, thermal, environmental and surface energy

properties of neat vinyl ester and the corresponding nanocomposites. Full explanations

about the effect of the incorporation of layered silicate into the polymer matrix on the

different properties are presented.

Chapter five summarises the current study.

References contain the references used in this work.

Appendices first page of each publication is presented.

Chapter 2. Literature Review

10 | P a g e



This chapter will present enormous background on the field of polymer

nanocomposites based on layered silicate. History, types, definitions, structures,

properties, preparation and applications of nanocomposites are described. The

nanocomposites usually reveal remarkable properties compared to conventional

composites. The concept of nanocomposite and its aims will be introduced.

2.2 Polymer

The word “Polymer” is obtained from Greek words which are “poly” and “mer”;

“poly” means many and “mer” means part or unit. It is a chain of small molecules

which are joining together in different fashions in order to form a single layer

molecule. The polymer represents the structure of repeating units on a large scale of the

joining macromolecules. The repeating parts are known as the monomers and are

linked together by covalent bonds. The formation of a polymer structure is derived

from the polymerisation process of the repeating monomer (31-33).

2.3 Classification of polymer

The polymer classification can vary and depends on different considerations. The

following classifications are the most widely studied and common in terms of polymer.

2.3.1 Classification based on the source of polymer.

2.3.1.1 Natural polymers

These kinds of polymers are found in plants or animals such as proteins, cellulose and

rubber (34, 35).

2.3.1.2 Semi-synthetic polymers

These can be defined as the chemical modification and treatment of the natural original

polymer. For instance cellulose acetate, cellulose methyl ethers and cellulose nitrate are

derived from the natural polymer which is the cellulose (34, 35).


11 | P a g e

2.3.1.3 Synthetic polymers

This is a polymer which can be prepared in the laboratory and is also usually called

man-made, polymers such as nylon 6,6 and polythene (34, 35).

2.3.2 Classification based on the polymer structure

2.3.2.1 Linear polymers

These polymers consist of long and straight chains (see Figure 3) such as high density

polythene, polyester and polyvinyl chloride. The linear structure usually exhibits high

mechanical properties (31, 35).

Figure 3 Linear polymers structure.

2.3.2.2 Branched or Side polymers

The monomers are joined together in order to form long chains with branched or side

chains, as depicted in Figure 4. For example, low density polythene and starch. It was

found that the properties of branched polymers such as tensile strength and melting

points are less than linear polymers (31, 35).

Figure 4 Branched or Side chains polymers structure.

2.3.2.3 The network or cross-linked polymers

The bi - or tri - functional monomers represent a very strong covalent bond between the

linear chains. The monomers are cross-linked together in order to form three-

dimensional chain polymers (see Figure 5). The properties of these kinds of polymer are

hard, rigid, and brittle. Bakelite and melamine are examples of the network structure

polymers (31, 35).


12 | P a g e

Figure 5 Network or cross-linked polymers structure.

2.3.3 Classification of polymers based on the polymerisation type

2.3.3.1 Addition polymers

These polymers consist of repeated additions of monomer molecules by double or

triple bonds. One example of the addition polymerisation is the addition of ethane and

polypropene which forms polythene. In addition, the PVC from vinyl chloride is a clear

example of addition polymerisation as seen in the equation below (31, 36, 37).

⏟

2.3.3.2 Homopolymers

The formation of the polymer occurs by the addition of single monomer (see Figure 6).

Figure 6 Homopolymers Structure.

2.3.3.3 Copolymers

The creation of the polymers by two different monomers by addition polymerisation is

termed copolymers, as depicted in Figure 7 .

Figure 7 Copolymers structure.

2.3.3.4 Alternating copolymers

The formation of these polymers is in an alternating fashion and the structure is

arranged as seen in Figure 8.

ABAAABB

AAAAAAA

AA


13 | P a g e

Figure 8 Alternating copolymers structure.

2.3.3.5 Block copolymers

Each repeating unit of one monomer is grouped together. Thus it can be defined as two

Homopolymers joined together (as illustrated in Figure 9).

Figure 9 Block copolymers

2.3.3.6 Graft copolymers

The polymer structure represents two different Homopolymers monomers which are

grafted together as seen in Figure 10.

Figure 10 Grafted copolymers structure.

2.3.3.7 Terpolymers

This type of structure contains more than two monomers in order to form the polymer

(31) (see Figure 11).

Figure 11 Terpolymers structure.

2.3.3.8 Condensation polymers

A reaction between two molecules or functional group to form one large molecule,

however these types of reaction eliminate the small molecules such as water as seen in

the following equation (31, 36, 37).

AACABCBA

AAAAAAA

BBBBBBB

B

AAAABBBB

ABABABA

Q


14 | P a g e

⏟

2.3.4 Classification of the polymers based on molecular properties

2.3.4.1 Elastomers

These polymers possess high elastic modulus and weak intermolecular forces. The

weak bonding allows the polymer to be stretched. A few crosslink bondings between

the chains permits the polymer to go back to its original shape (31, 32, 38).

2.3.4.2 Fibres

These can be defined as the thread forming solids and the structure of the fibres

contains very strong intermolecular forces. Thus, the fibres have high mechanical

properties (39).

2.3.4.3 Thermoplastic polymers

The structure of thermoplastic polymers usually consists of linear or branched chains

molecules which enable the polymer to be softened during heating and hardened during

cooling. The amount of the intermolecular forces is moderate between the elastomers

and the fibres. Figure 12 represents the structure of the thermoplastic polymers (31, 32,

40).

Figure 12 Thermoplastic chains molecules structure.

2.3.4.4 Thermoset polymers

The chain molecules are cross-linked or highly branched which explains the rigidity of

these polymers as seen in Figure 13. At high temperature, the polymer represents a high

value of cross-linking chains and become infusible. These polymers are unable to be

reused (31, 32, 38).


15 | P a g e

Figure 13 Thermoset plastic structure where the black circles represent the cross link.

2.4 Nanocomposites

2.4.1 Background

For the last fifty years, polymer matrix has been introduced with layered silicate. In

1949, Bower (41) carried out one of the earliest studies of the interaction between clay

mineral and a macromolecule when he showed the absorption of DNA by

montmorillonite. Although XRD proof was absent, the result included incorporation of

macromolecule in the lamellar structure of the silicate (8). The first study in this field

was by Carter et al. (42) who in 1950 combined polymer with layered silicate on the

nanometer size in order to form nanocomposites. In 1958, Hauser (43) filed a patent for

“Clay complexes with conjugated unsaturated aliphatic compounds of four to five

carbon atoms”. In the 1960s, in-situ polymerization of the vinyl monomer in interlayer

space Montmorillonite was studied by Uskov (44) and developed by Blumstein (45)

and Greenland (46). Later in that decade, the combination between organoclay and

thermoplastic polyolefin matrix was demonstrated by Nalsia et al.(47). They achieved

organoclay composites with tough solvent resistance and large tensile strength by

irradiation-induced cross linking. In 1975, Tanihara and Nakagawa (48) achieved

similar results to (47) by intercalating polyacrylamide and polyethylene oxide from a

hydrous solution. One year later, Fujiwara and Sakamoto (49) outlined initial hybrid

polyamide nanocomposites. Between 1976 and the mid 1980s, clay concentrations

were used in small amounts, rather than larger amounts: 1-10% instead of 50%. This

work was carried out by General Motors (GM) (50), Imperial Chemical Industries

(ICI)(51) and DuPont (52). GM took out an initial patent at around this time. The GM

patent utilised the use of clay minerals instead of antimony oxides while ICI developed

an idea about the use of “delaminated vermiculite” to impart self-extinguishing and


16 | P a g e

charring properties to large polystyrene beads. Likewise, DuPont established the flame

retardant properties of polymer-clay nanocomposites.

Despite these developments in nanocomposites, the usage of them was not very

extensive until researchers from Toyota (53-56) started specific tests of nylon layered

silicate nanocomposites, and then nanocomposites became widely extended in different

fields such as academic, government and manufacturing research (8). The reason for

the nanocomposites extension after Toyota‟s research was traced to the superior

enhancement in the overall properties of the resulting products that were observed.

2.4.2 Nanocomposites structures

Generally, dispersing the polymer in layered silicate does not essentiality form a

nanocomposite. It depends on the compatibility between the polymer and layered

silicate. In non-mixable situations, the weak physical interaction between the polymers

and layered silicates, weak mechanical properties will result. Likewise, it also leads to

agglomeration to take place which means to decrease the ability of strength as well as

providing poorer materials (21). As a result, when the organic component cannot be

able to intercalate with the inorganic component, the resulting mixture will have

properties that are similar to microcomposites structures which are not desirable (8, 20,

23). There are many correlative structural characteristics which function to differentiate

polymers based on layered silicate nanocomposites from a traditional system. One of

them is the percolation threshold which is much lower compared to conventional

composites. Also, the distance between particles is very small. Moreover, there is high

interfacial interaction between the polymer and layered silicate. The difference of

properties between them can be assigned to the high aspect ratio of layered silicate as

well as the nano dimension dispersion (57).

2.4.3 Nanocomposite structural characterisation

There are many techniques that are used to characterise nanocomposites structures. The

two correlative methods that are normally used are transmission electron microscopy

(TEM) and X-ray diffraction (XRD) (16, 20, 23, 58), or wide angle x-ray diffraction

(WAXD) (16, 21, 29, 59, 60). However, XRD and WAXD are the most widely used

techniques to evaluate the nanocomposites system. This is because of their availability


17 | P a g e

and ease of use (16). In addition, they are used to analyse the polymer melt

intercalation kinetics (16, 61). The XRD method enables an understanding in relation

to the distances between the silicate layers‟ structure by using Bragg‟s law = sin θ=

nλ/2d.

The structure of nanocomposites, whether intercalated or exfoliated, may be identified

by controlling the direction, intensity, and shape of the basal reflection that results from

the distribution of silicate layers (16). For instance, in the exfoliation structure of

nanocomposites, XRD or WAXD produces a loss of coherent x-ray diffraction that

results from the silicate layers. This is because this structure has a very large layer

distribution in the matrix of the polymer. However, the XRD or WAXD of the

intercalated structure, where the distribution of the layers is limited, produces a new

basal reflection that corresponds to the large interlayer height. Nevertheless, in the non-

mixable combination of polymer/organic modified layered silicate (OMLS), the basal

reflections characteristics of OMLS do not change because the silicates structure is not

affected (16, 20, 23, 62).

The effects of the intercalation structure of the polymer on the arrangement of the

layers of OMLS can be controlled by amendments to the full width at half maximum

(fwhm) of the basal reflections. If there is an increase in the coherent layers stacking

degree, the fwhm of the basal reflections will be decreased. Conversely, if the degree

of coherent layers are decreased, the intensity loss and peak enlarging will be observed

(62).

Despite the detrainment of the gallery spacing of the layers of silicate in the original

layered silicate where the nanocomposites have an intercalated structure (within1-4

nm) by XRD or WAXD, there is little agreement about the spatial distribution of the

silicate layers or any structures that are not homogenous in nanocomposites.

Furthermore, initially some of the layered silicates do not show a well-defined space

between the interlayer (which is the basal reflections). For this reason, the peak

enlargement and intensity loss are difficult to examine and analyse systematically. As a

result, the nanocomposites structure based on XRD or WAXD are uncertain. However,

TEM can provide information about the spatial distribution of the different phases, real

areas, structural problems, localised space and morphology. This makes possible a


18 | P a g e

qualitative understanding of the internal structure (12, 16, 63). Since the silicate layers

consist of heavy elements such as Al, Si, and O, they can be observed through TEM as

dark lines approximately 1 nm thick. However, the interlayer and the matrix consist of

light elements (i.e. C, H, and N) which appear as bright colours that are not as dark as

in layered silicates. In addition, the use of TEM needs special care which must be

applied to improve the result of the cross-section image of the specimen (58, 63). TEM

must be relied when the structure is midway between exfoliation and intercalation so

the broadening and enlargement of the diffraction peak is observed (20).

Also, 13C solid-state nuclear magnetic resonance (13 C-NMR) is used in order to

characterise the nanocomposites. This technique was first used by VanderHart et al.

(64) and resulted in a much improved understanding of nanocomposites morphology,

as well as the surface properties and, to a small degree, the exfoliation dynamics of the

polymer clay nanocomposites. NMR techniques help to measure the levels of the

structural exfoliation. This is a very important issue because the objective of NMR

measuring is to make a connection between the measured longitudinal relaxation T1

s

and 13C with the clay dispersion quality (16).

In addition, Fourier Transform Infrared Spectroscopy (FTIR) has been used by

different researchers to explain nanocomposites structures (65, 66). FTIR reveals the

differences in the bonding in the mixture and related nanocomposites. However, these

variations are extremely small and nowadays this method is not considered reliable for

the analysis of nanocomposites in the majority of cases (58).

Furthermore, Differential Scanning Calorimetry (DSC) can produce additional

information about intercalation. The high interaction between the chain intercalation of

the host species with the polymer can decrease their circular and transformational

motion. This situation is similar to that of reticulated polymer where limitations on its

mobility enlarge its glass transition temperature (Tg). Likewise, an increase is expected

to result in the nanocomposites because of the height of the energy threshold required

for the process. Obviously, this result is easily demonstrated by DSC (67).

Also, Scanning Electron Microscopy (SEM) is used to elucidate the nanocomposites

structure that works by focusing a beam of intense energy electrons on the surface of


19 | P a g e

the sample to produce a range of different signals. These signals provide a great deal of

information regarding the sample such as morphology structure, chemical combination

(dispersion), orientation of materials used to create the specimen, as well as the

crystalline structure (68, 69). In addition, Energy Dispersive X-ray Spectrometry

(EDS) can be used in the characterisation of nanocomposites. The principle of this

method is that when electrons are directed at the sample (in this case the electron beam

of the SEM) characteristic X-rays are emitted for all atoms with an atomic number

above that of Na. This enables an elemental distribution map to be created for any

element with Z > Na, in the case of the layered silicate the Al and Si can be detected.

Moreover, nanocomposites structures can be studied by Scanning Tunnelling

Microscopy (STM). This method enables an understanding of atomically resolved

imaging of the sample. In addition, this technique provides information about the

structural defects of nanocomposites (70).

Obviously, the main issue is that while the synthesizing of the layered silicate based on

polymer is taking place, properties of the product are demonstrated. Clearly, most of

the studies in this area concentrate on the structural characteristics of nanocomposites

and do not include the properties of the product (8).

2.5 Layered Silicate

2.5.1 Structure of layered silicate

Layered silicates composed of natural minerals and containing extremely thin layers

joined together with counter-ions, are used to synthesise nanocomposites. There are

three structures of layered silicate. Firstly, 1:1 layered silicate structure. They are the

basic building blocks and are composed of one sheet of tetrahedral where silicon is

surrounded by 4 oxygen atoms and one octahedral sheet where metal such as aluminum

is surrounded by 8 oxygen atoms. Sharing of oxygen atoms will be evident, whereby

the sheet of tetrahedral is connected to the octahedral sheet as in Kaolinite (71).


20 | P a g e

Figure 14 (2:1) layered silicate structure or phyllosilicates.

The second structure is 2:1 layered silicate or phyllosilicates (as shown in Figure 14)

which is the most widely used for the processing of nanocomposites. This structure

contains two dimensional layers which consist of one internal sheet of octahedral

connected at the tip to two external tetrahedral sheets. Thus the oxygen atoms of the

tetrahedral are associated with octahedral sheets. The thickness of layers is about 1nm.

However, the outside dimensions of thickness are different and can range from

0.000003 Centimeters to many microns or more. The variations in dimensions rely

upon the particulate of silicate, the place of exporter clay, and the method of

processing. For example, processing the clay by milling normally has lateral platelet

dimensions between (0.1-1.0 µm)(15, 20, 23).

Pyrophyllite structure is the basic structure of the class of 2:1 layered silicate

structures, where there is aluminum in the octahedral sheet and silicon in the

tetrahedral sheet, excluding any atomic substitution. As the layers do not extend and

expand in water, this structure has just an outside surface area and especially no inside

one (22).

Regarding the substitution of atoms in a 2:1 structure, there are two different kinds of

structure. Firstly, when the tetrahedral and octahedral sheets have a substitution, for

example between silicon and aluminum, it results in a mica structure. As a

consequence of this substitution, the surface of the mineral gains a negative charge,

which can be tackled by interlayer potassium cations that can balance the charge. The

potassium cations can be easily fitted tightly between the layers, since their size

corresponds with the hexagonal hole shaped by the silicon-aluminum tetrahedral layer.

As a result, both the interlayers collapse and the layers are cohered through the

attraction of the atom for electrons. This is an electrostatic attraction between the


21 | P a g e

potassium cations and the tetrahedral that has negative charge. As a consequence, the

structure of mica does not swell in water and, furthermore, does not have an internal

surface area similar to pyrophyllite (72). Secondly, montmorillonite structure can be

observed, where a trivalent aluminium cation in the original pyrophyllite structure in

the octahedral sheet is not wholly substituted by the divalent magnesium cation.

Montmorillonite is the most recognisable member of the collection of clay minerals

known as smectites. Generally, this structure has a negative charge that can be

stabilised by the ions of sodium and calcium which exhibit a hydrated inside layer

(73). As a result of this structure, there is a specific characteristic where these ions of

sodium and calcium do not fit in the tetrahedral sheet, as in the previous structure

(mica), and the layers are adhered together by a low force, so that the polar molecules

and water have a chance to access between the unit layers, which leads to expansion of

the lattice (22).

Besides Montmorillonite ((OH)4 Si8 Al4 O20·nH2O.), there are other layered silicates, such

as Hectorite (Na0.33(Mg 2.67 Li0.33)Si4O10(OH)2) and Saponite (Ca0.1 Na0.1 Mg2.25 Fe2+

0.75 Si3

AlO10 (OH)2•4(H2O)), which are the most widely used in nanocomposites materials.

The third structure is the 2:2 layered silicate system. Also, it is defined as tetramorphic

of four layer minerals; this is produced by the alternative of condensation of silica

tetrahedral sheet and alumina or magnesium octahedral sheet. An example of this kind

of structure is Chlorite (74).

2.5.2 Organic modification of layered silicates (OMLS)

Neat state layered silicates are only able to be combined with hydrophilic polymers

such as polyvinyl alcohol (46) and polyethylene oxide (75). In a non-mixable system,

there is less interaction between the layered silicate (organic) and the polymer

(inorganic) which leads to unsatisfactory properties such as mechanical or thermal.

However, the dispersion at nanometer scale can occur when a high interaction between

them is observed. As a result, nanocomposites provide exceptional properties that

cannot be compared with micro or conventional composites (16, 21, 29, 76). Thus, in

order to allow the layered silicates to combine with other polymers, the typical

hydrophilic silicate surface must be transferred to an organophilic surface. As such,

this can help to intercalate the layered silicates with several polymers. Normally, this


22 | P a g e

can be achieved by chemical reaction where ion-exchange of Na+,Ca

2+, or K

+ (77)

reacts with cationic organic surfactants. There are many ions that can be used such as

sulfonium and phosphonium. However, the most widely utilised ions in the exchanging

are Alkylammonium ions (16, 20, 67, 73). Due to the need to achieve an exchange of

the cations with the onium ions in the interlayers, the silicate needs to be expanded by

water. Thus Alkali cations are the most common and preferred in the formation of

interlayers. The reason for that is 2-valent and higher-valent cations stop the layers

from swelling by water. In fact, the hydrate structure of mono-valent intergallery

cations is the motivating influence for water swelling. There is also the possibility of

the natural clay to include divalent cations such as Calcium, which need exchange

processes with Sodium before the ion-exchange with onium salts (67). The Alkali

cations are called „exchangeable cations‟. Due to their composition, they are more

easily substituted by different positively-charged atoms or molecules (see Figure 15)

(78).

Figure 15 The intercalation of alkali ions between the clay layers.

The operation of exchanging ions, for example Alkylammonium in the organosilicates,

can lower the energy of the surface of the inorganic host and provide more hydrating

properties with a polymer matrix (21, 79). In addition, an increase in the interlayers‟

height can be observed when the long organic chains of surfactants (which have a

positive charge) are connected to the surface of the negative charge of the silicate

layers (24). Usually the modification results in enhancing the d-spacing over 2 nm (20).

As a result, it is possible for the polymers to be spread between the layers (as shown in

Figure 16) (8, 16, 78, 80).


23 | P a g e

Figure 16 Ion exchange reaction, which enhances the spacing between the layers as well as changes the

properties of the individual surface of the layer from being hydrophilic to hydrophobic.

Also, these cations offer a functional group which can react with the polymer matrix or

sometime acting as a catalyst for the polymerisation of monomer in order to develop

the strength of the interaction between the layered silicates and the polymer matrix (16,

81-84). Thus, relying upon the length and function of an onium chain, the layered

silicate could be engineered in order to enhance their affinity with chosen polymer (24,

78, 80, 85). For instance, it was thought that in mica types the chains shaping a mono

or bilayer are either parallel to the layered silicate or radiate away from the layers to

form a mono or bimolecular organisation called a “paraffinic” arrangement (86), as

depicted in Figure 17. Without doubt, the modification of the surface enlarges the d-

spacing of the clay and acts as a compatibiliser between both the hydrophobic polymer

and the hydrophilic clay, as shown in Figure 15 and Figure 16 (8, 80). The ion-exchange

reaction provides a new feature to the clay which illustrates the clay is organophilic

(79).

Figure 17 Organisation of alkylammonium ions in mica type layered silicates where the variation of

charge in the layer is exhibited. Dark areas are silicate layers. (a) is the mono layers, (b) the bilayers,

whereas (c) and (d) represents the mono and bimolecular organisation respectively.

A report published by Fisher and Koster (87) on the use of layered minerals

demonstrated that these platelets possess a certain degree of flexibility and have very

b a

c d


24 | P a g e

large surface areas in the range of several hundred m2/gram. Most types of clays have

also been found to possess ion exchange properties, providing them a vibrant

hydrophilic property and making them incompatible with many of the polymer

varieties. As such, one of the prerequisites for creating polymer-clay nanocomposites is

to alter the polarity of the clay to make it organophilic. This is achieved through an ion

exchange process using an alkyl-ammonium ion. In the case of montmorillonite, the

cations of sodium are exchanged by using ADA (12-aminododcanoic acid) (88):

Na+-CLAY + HO2C-R-NH3

+ Cl

- →HO2C-R-NH3

+ -CLAY + NaCl

In the general principle of ion exchange in the layered silicate,

X.clay + Y+↔ Y.clay + X

+ (89)

Kornmann (89), who conducted extensive studies on the synthesis and characteristics

of nanocomposites, has attributed the ion exchange process as having a profound

impact on the production of the nanocomposite. Although the procedures adopted so

far have largely increased the manufacturing cost, the clay material is relatively

cheaper without any restrictions on supply. Besides Montmorillonite, as well as other

varieties like hectorites, synthetic clays such as hydroalcite are also commonly used,

depending on the specific properties required.

In addition, the use of different size of alkylammonium ions with organoclays was

studied by Wang et al. (90). It was found that the higher the alkylammonium ions

length, the greater the enhancement in d-spacing of layered silicate. Likewise, the

modification of organoclays resulted in enhancing the properties of the nanocomposites

(91).

2.5.3 Cation-exchange capacity (CEC)

The cation-exchange capacity (CEC), which is a special property that can quantify the

overload negative charge of layered silicates and their ability to replace ions, is

normally exemplified in millequiv 100g-1

(20, 73, 92, 93). Depending on the source of

the layered silicates, the CEC has diverse capacities from one to another. For instance,

montmorillonites from various sources exhibit variations in CEC which can be from

90-120 millequiv 100 g-1

(67, 73, 79, 94). There are three types of clay that have a


25 | P a g e

large divergence in the CEC. Smectites, for example, contain a substantial amount of

CEC, approximately equal to 80-100 millequiv 100 g-1

. Illites have 15-40 millequiv

100 g-1

and kaolinites have the lowest amount of CEC at 3-15 millequiv 100 g-1

. Even

in the same kind of layered silicate, the CEC varies from one layer to another and is not

fixed (8, 16, 20). By and large, calculating the amount of clay CEC can be done by

establishing the difference in the loss on ignition between modified and unmodified

clays. A further calculation can be made of the variance between them in terms of the

molecular weight of Alkylammonium ions, where the amount of the organic substance

milliequivalents has been maintained by the clay. Once this has been computed, the

result of the CEC will be arrived at. In other words, the total of Alkylammonium ions

kept by organoclays is calculated. In addition, the CEC can be established via chemical

examinations of the clay (8).

2.5.4 Layered silicate summary

Taking everything into account, there are two specific characteristics of layered

silicates used in the field of polymers based on layered silicate nanocomposites. First

of all is the ability to disperse the silicate particles into single layers. Therefore, the full

distribution of single layers can lead to as high an aspect ratio as 1000. Conversely, the

weak distribution of particles can affect the ratio which may be as low as 10. Secondly,

there is the possibility of finding the perfect choice for the treatment of the surface of

layered silicate by the ion exchange reaction. The chemical composition of the material

is important as the clay sheets contain a charge along the surface as well as the edges,

with counter-ions present in the inter-layer spacing which balances this charge (10).

These two characteristics are related since the degree of distribution relies on the

gallery cation (15).

2.6 Nanocomposite polymers

There are many types of polymers that have been used with the layered silicate during

the nanocomposites preparation (16, 95). These polymers can be classified as follows.

2.6.1 Vinyl polymers.

These polymers have the addition of vinyl polymers which derive from popular

monomers, such as methyl methacrylate copolymers (96-98), acrylic acid (99, 100),


26 | P a g e

acrylonitrile (101, 102), styrene (91, 103-106), acrylamide (107, 108), 4-vinylpyridine

(109, 110), tetrafluoro ethylene and poly (N-isopropylacrylamide) (16). Likewise,

many polymers have been used, such as PVA (111, 112), poly(N-vinyl pyrrolidone)

(113, 114), poly(vinyl pyrrolidinone) (115, 116) , poly(ethylene glycol) (117),

poly(vinylidene fluoride) (118, 119), poly(p-phenylenevinylene) (120, 121),

polybenzoxazole (122, 123), poly(styrene-co-acrylonitrile) (124-126), ethyl vinyl

alcohol copolymer (127), and polystyrene–polyisoprene diblock copolymer (128-130).

2.6.2 Condensation (step) polymers

There are many polycondensates that have been used in the preparation of

nanocomposites with layered silicate: nylon 6 (20, 131, 132), poly(caprolactone) (PCL)

(133-137), poly(ethylene terephthalate) (138-143), poly(trimethylene terephthalate)

(144-146), poly(butylene terephthalate) (26, 147, 148), polycarbonate (PC) (149, 150),

polyethylene oxide (75, 151-153), ethylene oxide copolymers (87), poly(ethylene

imine) (154), poly(dimethyl siloxane) (155-159), polybutadiene (160), butadiene

copolymers (161-163), epoxidized natural rubber (164, 165), epoxy polymer resins

(166-168), phenolic resins (169, 170), polyurethanes (PU) (171, 172), polyurethane

urea (173, 174), polyimides (175-178), poly(amic acid) (179, 180), polysulfone (181,

182) , polyetherimide (85, 183), and fluoropoly( ether-imide) (184, 185).

2.6.3 Polyolefins

There are many examples of Polyolefins polymers which have been used in the

preparation of nanocomposites along with layered silicate. For instance, polypropylene

(186, 187), polyethylene (188-190), polyethylene oligomers (191), copolymers such as

poly(ethylene-covinyl acetate) (EVA) (192-194), ethylene propylene diene methylene

(EPDM) (195, 196) and poly1-butene (197, 198).

2.6.4 Biodegradable polymers

Polylactide (199-202), polyhydroxy butyrate (203-206), unsaturated polyester (207,

208), aliphatic polyester (209-211), and poly(butylene succinate) (PBS) (200, 212,

213) are the biodegradable polymers.


27 | P a g e

2.7 Nanocomposites preparation

At present, there are five different methods for the preparation of layered silicates

based on nanocomposites (18, 20, 23-25, 124, 214):

1- In situ template synthesis;

2- Intercalation of polymer or pre-polymer from solution;

3- In situ intercalative polymerization;

4- Melt intercalation;

5- Direct mixing between the polymer and particles.

2.7.1 In situ template synthesis

This method combines the mineral clay within the polymer by using an aqueous gel or

solution which interacts with both the silicate blocks and the polymer. The clay silica

sol, lithium fluoride and magnesium hydroxide sol are used as the precursor. In this

process, the polymer helps the nucleation and expansion of the inorganic host crystal

and becomes fixed within the layers as they rise. Despite the promoting ability of

dispersion of the layers of silicate in a single stage process, without the need to add an

onium ion, it seems that it has many drawbacks. For example, this synthesizing method

needs very high temperatures which affect the polymer and leads to a decomposition

stage. Moreover, there is the presence of aggregation that tends to result from the

expansion of the silicate layers. As such, although this is a commonly used method in

the production of double-layer hydroxide-based nanocomposites, it is far less well

developed for layered silicates. In addition, it was revealed that the length of the

layered silicate which synthesised by this method is limited. The combination of the

reaction conditions and the types of clay modification can control the morphology of

the nanocomposites (8, 20, 67, 215).

2.7.2 Intercalation of polymer from solution

2.7.2.1 Introduction

Figure 18 describes the steps of the solvent intercalation process. This process is based

on a solvent system where the layers of silicate are expandable and the polymer is able

to be dissolved. The silicate layers are introduced to the solvent first in order to be

expanded, and then combined with the polymer. The polymer chains are intercalated


28 | P a g e

between the silicate layers where the solvent in the interlayer is displaced. When the

solvent is removed, the structure of intercalated nanocomposites remains. The choice

of suitable solvent plays an important factor in the preparation of nanocomposites by

the intercalative solution. Thus, the combination between high solubility of the matrix

and perfect distribution of the layered silicate is the key factor of the selection of an

appropriate solvent to achieve sufficient high intercalation.

Figure 18 Flowchart describes the steps of polymer intercalation of solution

The solvent can be removed either by vaporisation, vacuum, or precipitation. The

solvents that can be used in this method include toluene, chloroform, or water. The

main advantage of this method is that the intercalated nanocomposite structure can be

synthesized based on high, low, or no polarity polymer. But the solvent is difficult

when used in a large quantity such as in industrial manufacturing (16, 20, 23, 216).

2.7.2.2 Reported works on the solvent method

Tan H et al. (217) carried out the production of epoxy-Na+-MMT nanocomposites by

sol – gel reaction of cationic triethoxysilanepropylamineformylethyl trimethyl

ammonium iodide (APS) and 3-glycidoxypropyl trimethoxysilane (GPS). The XRD

curves of the clay illustrated that the addition of more amounts of GPS led to

enhancing the d-spacing. Also, it was showed that the XRD results of the

nanocomposites for 1/2 ratio of APS/GPS revealed an exfoliation structure.

Chang et al. (218) carried out the production of PLA with various types of OMLS

nanocomposites by intercalative solution. Hexadecylamine-MMT (C16–MMT),

dodecyltrimethyl ammonium bromide–MMT (DTA-MMT), and Cloisite 25A were

introduced as organoclays in this report. A high mixing process for 4 hours was

followed in order to incorporate the 0.08 g of the layered silicate into 4.0 g of the

Elimination

of the solvent

Swelling

Solvent

Layered

silicate

Incorporation

Polymer

Nanocomposites

Vacuuming


29 | P a g e

matrix solution in the presence of the N,N,-Dimethylacetamide solution. The mixture

was casted and the elimination of the solution took place. This method resulted in an

intercalation nanocomposites structure of all the organoclays. The XRD presented an

increase in the basal distance of nanocomposites samples compared to the pristine clay.

The improvements in the interlayer were 6%, 60%, 33% of C16–MMT, DTA–MMT,

and Cloisite 25A respectively. Also, it was found that the organic modification of

layered silicate led to enhancing the basal distance which indicated more compatibility

with the polymer matrix.

Moreover, Poly(L-lactic acid)/layered silicate nanocomposites was successfully

prepared by cast solution in the presence of dichloromethane solution. Three different

clays were used which were Cloisite 15A, 25A, and 30B and their amounts were at

2.5,10, and15 wt.%. The XRD examined the level of the intercalation between the

layered silicate and the polymer. Pristine Cloisite 15A revealed 32.36 Å and the

intercalation of nanocomposites represented an increase in the basal distance which

was 38.08 Å. The authors reported that the nanocomposites structure was intercalated

in most areas of the sample. In the case of Cloisite 25A, the virgin clay reflected 20.04

Å and the incorporation of the polymer resulted in higher increment which was about

36.03 Å which was attributed to the partial dispersion of parallel layers of the clay

which exhibited an exfoliation system. Thus, a combination between intercalated and

exfoliated nanocomposites structure was obtained. The last clay, which was Cloisite

30B, exhibited 18.26 Å and the addition of polymer showed no reflection on the XRD

result which indicated an exfoliation structure was observed. It was revealed that the

key factor of dispersion is the enthalpic interaction between the polymer and clay. The

miscibility between the polymer and modifiers will help the layered silicate to be

exfoliated. Also, it was reported that the interaction of the C=O bonds located in the

PLA backbone in the Cloisite 30B modifier plays an important role in the level of

intercalation (219).

Choi et al. (220) produced PEO/MMT nanocomposites via a process of solvent casting,

where the chloroform acted as a cosolvent. The polymer was dissolved in the solvent

and then the layered silicate was incorporated into the solution at different loadings. A

nanocomposite intercalated structure had been obtained as confirmed by XRD. A


30 | P a g e

similar process and solvent were also used by Hyun, Y (221) and Lim, S (222) in order

to prepare the nanocomposites.

In addition, polyurethane /Cloisite 30B nanocomposites were prepared by solution

casting. Two types of polymer were used which include soft (SPU) and hard (HPU)

segments. The layered silicate was mixed with toluene and then added to the mixture of

polymer and dimethyl acetamide in order to stir them strongly. XRD showed the basal

distance of virgin organoclays which was 1.85 nm. At 3 and 7 wt.% clay loading, the

nanocomposites of soft and hard segments reflected an increase in the basal distance

which were at 3.95 nm and 3.85 nm respectively. The authors revealed that the

increment in SPU was traced to the existence of higher amounts of entropy which

helped to increase the interlayer. Also, it was observed that the resulting

nanocomposites structure was in between intercalated and exfoliated systems. TEM

confirmed the outcomes of XRD (223).

A new complex method for preparing polypropylene matrix nanocomposites based on

layered silicate by solvent solution was carried out by Kurokawa and Oya (224). The

layered silicate that was introduced is the hydrophobic hectorite. It was dried at 60ºC

and then was dissolved in toluene solution (10 wt %) in order to end up in translucent

sol. Methyl trimethoxysilane (MTMS) was added to the mixture at 1/3 of the mixture

weight. A stirring for six hours at 110ºC was applied on the sol. The reason for this

stirring is to allow the clay to be treated via MTMS. The next step was the elimination

of toluene by a small amount of pressure. A further heating was applied at 180ºC on

the silane surface (chemical compound SiH4) of the modified clay for 1 hour to get rid

of any unreacted chemical structure on the layered silicate surface. The modified clay

was dissolved in toluene for a second time to result in a sol, stirred at 120ºC for 30

minutes with 20 wt.% of polypropylene. The mixture was cooled at 60ºC and then

stirred with methanol; the addition of methanol was in order to wash the clay and

eliminate the unreacted MTMS. The final outcome of MTMS-Clay/polypropylene was

drying and shaping in a mould. XRD results showed an increase in diffraction in

polypropylene nanocomposites over those of conventional clay. The outcome

properties of the nanocomposites are controlled by the stiffness of the layered silicate.


31 | P a g e

The information can be confirmed via the enhancement in the modulus of three

bending tests of the final material (225).

The dispersion of the layered silicate into the cis-1,4-polybutadiene rubber by the

presence of the oil solution was undertaken by Gu Z et al. (226). The clay was mixed

with the oil solution and then the polymer was added. XRD showed an initial

indication about the nanocomposites morphology. The structure of the nanocomposite

revealed an intercalation structure at lower content; however the higher amount of clay

led to increase the 2 value and shifted toward a higher angle which indicated

agglomerated layers. XRD was not clear especially at low content which presented

poor peak resolution as well as overlapping of the diffraction pattern of the

nanocomposites structure. Thus the use of TEM was essential and to provide a clear

understanding of the structure. TEM of 3wt.% clay showed a mix of exfoliation and

intercalation structures. By the combination of two characterisations method, an

intercalation nanocomposites structure was obtained.

Herrera-Alonso, J. M et al. (227).carried out the production of polyurethane based on

Cloisite 10A, 20A, and 30B nanocomposites by the solution method. Different types

and amount of layered silicates were first dispersed in tetrahydrofuran (THF) solution.

The mixture was mixed for 8 hours and then the polymer was added to be mixed also

for 8 hours. Another process also was used utilising a sonicated device instead of

stirring. XRD was utilised to understand the nanocomposites morphology. Pristine C10

represented 4.7 ° which indicated 18.8 Å. The different amounts of nanocomposites

ranged from 2.4° - 2.6°. They were shifted toward lower angles indicating an

intercalation structure of nanocomposites. The result of the stirred and sonicated

mixture of C10A exhibited almost the same result of XRD. C20A presented the lowest

amount of improvement in d-spacing which was assigned to the presence of two tallow

groups within the layered silicate which shield the surface of layers and prevent the

penetration of polymer. C30B revealed almost the same finding of C10A. It was

suggested that the type of organo modification plays an important role in the dispersion

where C10A and 30B had only one tallow group present in the clay. TEM confirmed

the results from XRD and concluded that the sonicated process had a higher dispersion

than stirring.


32 | P a g e

Also, there were many successful nanocomposites preparations by intercalation

solution reported, for example, PEO/MMT nanocomposites (75), PSF/OMLS-

dimethylacetamide (181), PI/MMT-dodecylammonium- DMAC (228), and Poly (vinyl

alcohol)/Na+-MMT- water (229).

2.7.3 In situ intercalative polymerization


The basic steps of this method can be seen in Figure 19. This kind of technique involves

a process of combining the silicate layers with a monomer solution. The layers are

expanded within the monomer solution or liquid monomer where the monomer can

progress to the interlayer of the silicate layers. This allows the polymer to form

between the intercalated sheets. The initiator of polymerization can be either heat

(radiation via a suitable initiator of diffusion) or by an organic initiator or catalyst set

during ion-exchange cation inside the gallery. This can be introduced before the

swelling stage.

Figure 19 Flowchart of in situ polymerization steps.

This method produces a long polymer chain. Due to the balance rate of polymerization

inside and outside the interlayer, the silicate layers are exfoliated and the structure will

be in a disordered state (16, 20, 23, 230). The polymerisation technique includes

different types of method such as atom transfer radical polymerization (ATRP),

nitroxide-mediated polymerization (NMP), ring-opening polymerization (ROP), ring-

opening metathesis polymerization (ROMP), living cationic polymerization, living

anionic polymerization, and reversible addition–fragmentation chain transfer (RAFT)

polymerization (231).

Swelling

Monomer

Layered

silicate

Catalyst

Polymerisation

Nanocomposites


33 | P a g e

2.7.3.2 Reported works on in situ polymerisation method

Oral A et al. (232) carried out the preparation of the Poly (methyl methacrylate)/MMT

nanocomposites by photoinitiated free radical polymerization using intercalated

monomer. Methyl methacrylate monomer was first incorporated into the interlayer of

clay by “Click” reaction. XRD, TEM and AFM confirmed both the intercalation of the

monomer into the layered silicate as well as the exfoliation of the nanocomposites

structure with different amounts of clay (1 and 3 wt.%). A greater amount of clay, such

as 5 wt.%, resulted in partially exfoliated or intercalated morphology. It was suggested

that the clay content is an important parameter of the final structure.

Peng H et al. (233) carried out the production of the polyamide 6 based

nanocomposites by in situ polymerisation. At 2 wt.% layered double hydroxide, the

TEM showed an exfoliation nanocomposites system. It was observed that the PA6

based on layered silicate shows a different TEM picture in which the layers exhibit

face-to-face orientation which may be traced to the high aspect ratio of layered

silicates. Reichert et al. (234) produced nylon12 nanocomposites. This research

involved the utilisation of 12-aminolauric acid (ALA) as the layered silicate modifier

as well as the monomer. They stated that the expandable procedures for the clay rely

upon ALA loading in HCL. First, if the ALA loading level is low (less than 24

mmol/l), ion-exchange of the inorganic cations via enhancing the positive charge of

ALA will take place. Secondly, when the amount of ALA is greater than the HCL

(more than 24 mmol/l), the additional distribution of zwitterionic 12-aminolauric acid

into the gallery space will take place (as shown in Figure 20) which indicates better

intercalation between the layered silicates and the polymer. This study was undertaken

by XRD. Also, it was discovered that the expandability was found to be independent of

the flexible temperature, the loading of layered silicate and the type of acid utilised to

protonate the ALA. At high temperatures (280oC) and under high pressure (ca. 20 bar)

with the two kinds of expandable clay, the ALA will be polymerized. The use of TEM,

XRD, Energy Dispersive X-ray (EDX) and Atomic Force Microscopy (AFM) showed

that if they were not intercalated, the structures produced were partly exfoliated

nanocomposites.


34 | P a g e

Figure 20 Interlayer distance of fluoro-modified talc (ME 100) in function of an increasing amount of

aminolauric acid used as the organic modifier (233). Reproduced from Reichert by permission of

Copyright Wiley-VCH Verlag GmbH & Co. KGaA.

PMMA/clay nanocomposites were successfully prepared via ultrasound assisted

emulsifier-free emulsion polymerization method by Patra et al. (235). The XRD

represented an exfoliation and intercalation structure of nanocomposites which were

depending on the way of preparation. The use of the ultrasound helped to improve the

dispersion level. At 3% clay loading, the preparation of the nanocomposites with the

presence of the ultrasound represented no peak in XRD which indicated an exfoliated

structure whereas the disabling of ultrasound represented a peak at lower angle which

referred to the intercalated/exfoliated structure.

Greesh N et al. (236) undertook the study of the impact of Na-MMT on the

Poly(styrene-co-butyl acrylate) copolymers synthesised by Free-Radical

Polymerization in Emulsion. It was revealed that the output result provided an

intercalation structure. The reason for not achieving an exfoliation structure was

assigned to the presence of the organic modifier (sodium 1-allyloxy-2-hydroxypropyl

(Cops)) which acted as a hydrophilicity element. Also, the polymerisation group of

Cops had less reactivity on the monomer. As a result, these factors restricted the

entering of the monomers into the clay galleries.

Wu F and Yang G (237) revealed the study of poly(butylene terephthalate) based on

Na+-MMT nanocomposites prepared by in-situ bulk polymerization. Different layered

silicate concentrations were used (2-4-6 wt.%). At 4 and 6 wt.% layered silicate, the

XRD did not show any peak from 2° to 10° which indicated intercalated structures


35 | P a g e

were obtained. In order to confirm these observations, TEM was utilised. It was proved

that the resulting structure of 4 and 6 wt.% were intercalated-exfoliated structures

although it had some face-to-face agglomerated layered silicate. The aggregation part

was ascribed to the time before polymerisation as the layered silicates had the time to

aggregate before starting the reaction. Tasdelen A.M (238) carried out the preparation

of Poly(epsilon-caprolactone)/ Azide-MMT nanocomposites by copper (I) catalyzed

azide/alkyne cycloaddition (CuAAC) “click” reaction. The samples were examined by

XRD and TEM and both results showed an intercalation-exfoliation nanocomposites

structure.

In addition, a highly advanced new preparation system to produce large performance

polypropylene layered silicate nanocomposites (PPCN) via in-situ polymerization

under a mild situation was taken by Sun and Garces (239). This experiment improved

the processability, mechanical work and reacted efficiency compared with the

preceding approach. In this new study, there was no need for an external initiator for

the starting of the polymer polymerization such as methylaluminoxane or

perfluoroarylborates as well as the high temperature and pressure procedure conditions

which were not needed. The modified layered silicate was made by ion-exchange

reaction with amine combination as this can enable the activation of the polymerization

catalyst of metallocene olefin monomer. The organically treated layered silicate

powder is dipped in toluene which works as a solvent. The next step is to be mixed

with hydrolytic scavengers and a very large isotactic metallocene propylene

polymerization catalyst by mechanical stirring. The outcome composites are exposed to

a propylene gas stream at surrounding conditions at 25°C in order to produce PPCN. It

was observed that the clay loading is a major concern since the polymerization started

at a desirable loading. As a result, a stop-flow type of polymerization was utilised. The

ease of process, elimination of external reactors, high mechanical performance and the

effectiveness of the catalyst were to play an important role in making this approach

valuable for producing PPCN. Microscopic image and x-ray diffraction angle were

used to prove the changes in basal spacing of different concentrate layered silicate-

polymer nanocomposites within the polymerization procedures. XRD showed that the

use of the modified clay precursor had enhanced the d001-spacing compared to

untreated clay. The microscopic image was utilised to analyse the state of clay


36 | P a g e

dispersion whether there was an intercalation or exfoliation structure in the PPCN.

Also, Polypropylene/MMT nanocomposite was successfully prepared by in situ

method. Slurry polymerization procedure was utilised to polymerise propylene in

hexane. The XRD was used to study the morphology of nanocomposites. Base clay

showed 1.15 nm and by the addition of 3 wt.% layered silicate into the polymer, the

XRD reflected no peak which indicated an exfoliation structure. For further

investigation and confirmation about the level of dispersion, TEM was introduced.

TEM confirmed the previous results by XRD and illustrated a well uniform distribution

(240).

Qin X et al. (241) reported a novel approach of the preparation of PUA based on 2-

benzyl-2dimethylamino-1-(4-morpholinphenyl) butanone (BDMB)/MMT. The

BDMB/MMT was used as the clay and the initiator. The resulting clay mixture

represented an increase in the basal distance which was about 34% compared to

unmodified clay as calculated by XRD. XRD and TEM revealed an exfoliation

nanocomposites structure was obtained by dispersion treated MMT into PUA matrix.

Oral A et al. (242) studied the result of the Poly(cyclohexene oxide)/ Cloisite 30B

nanocomposites that were prepared by in situ photoinitiated activated monomer

cationic polymerisation. XRD, DSC, AFM and TEM were introduced to evaluate the

output nanocomposites structure. The initial XRD results revealed an increment in the

basal spacing from 1.84 nm to 4.86 nm which indicated an exfoliation structure. The

onset degradation temperature of the nanocomposites was reduced compared to the

neat matrix which was attributed to the catalytic activity of the clay on the matrix

degradation process. TEM and AFM confirmed the previous results.

Katoch S and Kundu P.P (243) carried out the production of unsaturated polyester

based on MMT nanocomposites by in situ polymerisation. Poly(ethylene terephthalate)

waste bottles were glycolyzed as precursors of polyester matrix. Benzoyl per-oxide

was utilised as initiator and styrene monomer to reduce the viscosity of the resin. The

XRD and TEM illustrated exfoliated nanocomposites and stack layers at the same time.

Agag T et al. (244) undertook the preparation of vinyl ester resin (VER) pre-polymer

based on different types of o-MMT by mixing them together followed by thermal

polymerisation. Depending on the kind of the modification used, intercalated and


37 | P a g e

exfoliated structures were obtained as confirmed by XRD.There were also many

studies reported by this method such as polyolefin/ fluoro-hectorite nanocomposites

(245), polyolfines/ hectorite nanocomposites (246) epoxy/ mica nanocomposites (247),

epoxy/MMT nanocomposites (248), Epoxy/various types of clay nanocomposites (249,

250).

At the end of this section it is important to mention that, although this method is

successful in processing different polymer-based layered silicates and it is a suitable

way to produce thermosetting polymers (79, 251-256), it has a number of downsides:

1- It is time-consuming as the process takes more than 24 hours.

2- The exfoliation phase tends to be less dynamically stable.

3- This method produces agglomeration with layered silicate.

4- The resin produced by this process is only of use for the manufacturers

themselves in order to have a specific production line (257).

2.7.4 Melt intercalation


This type of process can be defined as the annealing of a mixture that contains both the

polymer and the organic modified layers silicate where the temperature is above the

polymer softening point (molten state). The annealing can be achieved either statically

or under shear. Figure 21 presents the steps of this method. If the polymer is compatible

with the surface of silicate layers, it can move in the gallery of the layers and can form

either exfoliated or intercalated nanocomposite structures.

Figure 21 Basic steps of melt intercalation method.

Melt intercalation has further advantages over the previous methods. For instance, the

method is environmentally friendly because of the absence of an organic solvent.

Blending

Polymer

Layered

silicate

Annealing

Nanocomposites


38 | P a g e

Moreover, it is suitable for various manufacturing processes such as extrusion and

injection machining. Also, it can synthesise polymers that are not compatible with

other methods (8, 16, 20, 23, 230). The incorporation of the polymer via

polymerisation and solvent intercalation is limited because an appropriate monomer or

a special solvent that is compatible with the polymer and silicates is not always

obtainable. Also, the polymerisation and solvent methods are not always suited for all

polymer processes. At this point, it is worth mentioning that scientists dedicate their

time to tackling such downsides. This has ultimately resulted in the invention of the

melt intercalation method. This method is preferable in an environmental context to

other preparation techniques for nanocomposites based on layered silicates due to its

flexibility (21, 258). The melt intercalation method entails annealing the combination

of the polymer and clay and normally happens under shear. Also, this technique allows

the polymer to reach its softening point. Thus, throughout this process, the chains of

the polymer spread out between the silicate layers within the galleries. Straight

intercalation of the softened polymer into layered silicates is a novel process that does

not require any solvent. This approach can be achieved by heating the polymer over the

temperature of glass transition where the condition is static or running, and then

blending it with the layered silicate. The intercalation can be supported via the use of

treated layered silicate. The chain of polymer from the softened state is distributed in

between the interlayers of silicates to end up structuring intercalated or exfoliated

phases which depend on the level of diffusion. The main important aspect that plays a

major role in determining the outcome structure of nanocomposites is possibly

connected to the thermodynamics issues. In other words, the interplay of enthalpy and

entropy factors can help to know whether the resulting structure of the organically

treated layered silicate would be exfoliated, intercalated, or just dispersed within the

polymer matrix (62, 225, 259).

2.7.4.2 Reported works on the melt intercalation

Polypropylene/MMT nanocomposites were prepared by the melt intercalation method.

A twin-screw extruder was used to blend them and the mixture was moulded by an

injection moulding tool. The XRD was utilised to study the morphology of

nanocomposites. Base clay showed 1.15 nm and by the addition of 3 wt.% layered


39 | P a g e

silicate into the polymer, the XRD reflected a peak at 5.03° which represented 1.9 nm

which proved the intercalation of PP into the clay (240).

Table 1 The combinations of different materials (186).

Materials P PC PGC N NC PN PGN PNGC

Polypropylene 100 95 90 0 0 75 70 65

Polyamide 60 0 0 100 95 25 25 25

Organoclay 0 5 5 0 5 0 0 5

Fusabond 0 0 5 0 0 0 5 5

Motamedi P and Bagheri R (186) undertook the preparation of polypropylene (PP),

polyamide (PA6) and grafted polypropylene (PP-g-MA) based on Nanolin

nanocomposites. The combinations of samples are listed on Table 1. XRD represented a

basal distance of base clay about 2.2 nm. PC and PGC showed a reduction on the

interlayer space by about 0.2 nm and the authors revealed that the reduction on the clay

d-spacing when incorporated with PP and PP-g-MA has not been clearly understood.

Similar observations about the reduction of d-spacing of clay by the mixing with

polypropylene were reported. The authors tried to understand the negative impact of

the addition of layered silicate into PP polymer by summarising the process and the

limitation that were faced. The process of preparation followed two steps. The first one

was to apply shear stress in order to mix the combinations and transfer them from

micro-size to hundreds of nanometers. The second one was that the hydrophilic groups

in the polymer start to interact with cationic surfactants which allow them to

incorporate within the layered silicate. It was observed that the first process was

successfully applied. However, the second process needs very careful parameters to be

successfully achieved. First of all, the amount of the molecular weight of the PP and

PP-g-MA play an important role in the level of intercalation. Also, the screw rate and

clay d-spacing that were utilised in this experiment had a strong effect on the final level

of intercalation. On the other hand, NC showed an exfoliation structure which was

traced to the high polarity of PA6, low molecular weight, and the low viscosity. In

addition PNGC and PNC represented an intercalated nanocomposites structure. TEM

was used in order to give a clear picture of the resulting nanocomposites of PNGC and

PNC. It was observed that the layered silicate was intercalated with only PA6. The


40 | P a g e

authors traced this outcome to the hydrophobicity of the PP and the affinity between

the clay and the polar of end group in PA6.

Barick, A.K and Tripathy, D. K. (260) carried out the production of

polyurethane/Cloisite 30B nanocomposites by melt blending. 1, 3, 5, 7, 9 wt.% clay

loadings were introduced. XRD represented an exfoliation structure for all percentages.

TEM and SEM confirmed the clear separation of layered silicate. It was suggested that

the high amount of interaction between the polymer and platelets was traced to the

favourable attraction between hydrogen bonding bonyl groups in the polymer and

hydroxyl groups in the clay.

The study of the morphology of PLA/MMT nanocomposites prepared by the melt-

blending technique was carried out by Fukushima et al. (261). Cloisite 30B and PLA

were mixed in internal mixer at 190°C. A cooling process at room temperature for the

resulting batch was put in place. The clay basal distance was 1.9 nm as measured by

XRD. The addition of 3 wt.% layered silicate into the polymer matrix resulted in

increasing the basal distance up to 3.8 nm which indicated intercalated/exfoliated

structure as confirmed by XRD and TEM.

Dominkovics et al. (262) carried out the production of polypropylene/o-MMT

nanocomposites by the melt intercalation method. Different combinations were used

such as 0-6 wt.% clay and 0-50 wt.% of polymer. It was found that the addition of

layered silicate into a high amount of polymer resulted in a partially exfoliated

nanocomposites structure as proved by XRD. Poly(hexamethylene

terephthalate)/coliste 30B nanocomposite was successfully prepared by the melt

intercalation method. XRD revealed that the samples of nanocomposites represented an

intercalation level. It was shifted towards a lower angle compared to base clay where

the virgin clay reflected 1.85 nm and the nanocomposites showed 3.52 nm (263).

Another study was made by Hasegawa et al. (264). They developed a compounding

technique for producing PA6/MMT nanocomposites by utilising Na+-MMT water

slurry as an option for producing organically treated MMT, as shown in Figure 22. PA6

was mixed with Na+-MMT slurry via an extruder, and then the water was eliminated.

An exfoliated nanocomposites structure was obtained when XRD and TEM were used

http://www.sciencedirect.com/science/article/pii/S014139101000100X


41 | P a g e

to identify the internal structure. On the other hand, the PA6/ Na+-MMT did not show

any intercalation structure in the dry-compounding as proved by XRD.

Figure 22 The distribution of Na+-MMT slurry into nylon6 during compounding: (a) pumping the clay

to the softened matrix with strong stirring, (b-c) the slurry shrinkage to minute drops throughout the

mixing, and the water of the clay being evaporated when it touches the PA6 melt, (d) the vacuum

applied in order to eliminate the water with silicate layers distributed into the PA6 which soften as one

layer or multi layers (264). Reproduced from Hasegawa by permission of Elsevier Science Ltd., UK.

A further study was carried out by Zheng et al. (265) who utilised oligomerically

treated clay in order to produce PS/clay nanocomposites. This clay was prepared using

an ion-exchange with an oligomer. The oligomer consisted mainly of maleic anhydride

(MA), styrene (ST) and vinylbenzyltrimethyl ammonium chloride (VBTACl)

terpolymer. Thus it was termed MAST. The MAST was then dissolved in a solvent

which was acetone and was applied drop by drop to a distribution of clay in acetone

and distilled water. After this, the powder of MAST was shaped. The XRD represented

a peak at 1.8° of MAST clay which means 4.9 nm. On the other hand, the PS based

MAST nanocomposites reflected no peak which resulted in either an exfoliation

structure or immiscible nanocomposites were obtained. The used of TEM was to

permit the understanding of the right morphology of the nanocomposites. From both

XRD and TEM, it can be observed that the resulting structures were a mix between

immiscible, intercalated and exfoliated nanocomposites. The immiscible structure was


42 | P a g e

formed as a result from the lack of dispersed clay. Otherwise, the structure formed an

intercalated or delaminated system.

Kotek et al. (266) have produced the PP/ Cloisite 15A nanocomposites by melt mixing.

The polarity of polypropylene can be increased by inviting CI and SO2Cl groups via a

reaction with sulfuryl chloride which is below UV irradiation. High, medium, and low

amounts of CI and SO2Cl were used in order to know the effect of these groups into the

intercalation level and find an optimal amount. In addition, two preparation methods

were used in this experiment. The first one was to mix the PP, Chlorosulfonated PP,

and o-MMT in one step. The second method was to mix 75 wt.% Chlorosulfonated

polypropylene with 25 wt.% o-MMT in order to create a master batch. The master set

was subsequently mixed with polypropylene. The base layered silicate represented 3.15

nm as calculated by XRD. Both processes showed a good intercalation and exfoliation

nanocomposites structure at medium amount of SO2Cl group and at high amount of CI

which indicated that the chlorine was more effective in the o-MMT as had been

reported (267).

Polyethylene /clay nanocomposites by melt intercalation was studied by Hong S and

Rhim J (268). The layered silicate and the polymer were mixed together in the weight

ratio of 1:3 throughout high speed mixing in order to produce the master batch. The

resulting mixture was extruded by a twin extruder device. The d-spacing results

extracted from XRD showed an increase in the distance between layers indicating an

intercalation nanocomposites structure was obtained.

There were also many studies reported by this method such as poly(ethylene

terephthalate)/ H-S-A-MMT nanocomposites (269), polyamide 6 - styrene-butadiene-

acrylonitrile / Clay nanocomposites (270), Polystyrene (PS) /clay nanocomposites

(271), isotactic polypropylene/clay nanocomposites (272), and Polypropylene-MMT

nanocomposites (273).

2.7.4.3 Advantages of melt intercalation

1- Enlarges the commercial possibilities for nano-technology (62, 85);

2- Lower cost than in „in-situ‟ polymerisation;

3- Reduces the capital investment cost;


43 | P a g e

4- By using traditional devices such as extrusion, it is possible to produce

nanocomposites directly;

5- Allows greater freedom to the producers;

6- It is environmentally friendly (274, 275);

7- Increases the specialisation of polymer intercalation by reducing the

competition between polymer and main solvent intercalation (84);

8- Most thermoplastic polymers are prepared via this method (8).

Although this method was successfully used in preparing of different polymers based

on layered silicate, the melt intercalative method is not suitable for use with thermoset

polymers.

2.7.5 Direct mixing between the polymer and particles

This method is simple and without the need for any solvent, high temperature, or

catalyst. The polymer and layered silicate undergo a specific mechanical stirring at

specific speed (18) as seen in Figure 23.

Figure 23 Direct mixing process.

Velmurugan R and Mohan T. P (276) successfully prepared the epoxy based on MMT

nanocomposites by direct mixing. Organic-MMT and unmodified MMT were utilised

in this study. The XRD showed no reflection of the o-MMT where TEM was essential

to provide more understanding about the level of intercalation. The results of XRD and

TEM of o-MMT revealed an exfoliation structure of nanocomposites. However,

untreated clay represented a bit of aggregation layers.

2.8 Nanocomposites properties

Nanocomposites containing a polymer based on layered silicate usually show radical

improvement in materials properties compared with those of neat polymers. Properties

enhancement includes improved modulus, strength, heat resistance, biodegradability,

Polymer

Layered

silicate

Blending

Nanocomposites


44 | P a g e

and reduced gas flammability and permeability. The improvement is derived from the

high interfacial interaction between the polymer and the layered silicate.

2.8.1 Mechanical properties

2.8.1.1 The reinforcing mechanism of layered silicates

This primary mechanism, which has been stated to explain the reinforcing behaviour of

layered silicate, is the rigid fillers which are naturally opposed to elongating due to

their large moduli. Thus, when a quite softer polymer is reinforced with these kinds of

fillers and is located adjacent to the rigid fillers, the polymer will be restricted

mechanically. As a result, allowing a major amount of an applied force to be withstood

by the fillers will suppose that the bonding between the two phases is sufficient (277).

From these details, it can be seen that the higher the aspect ratio of the particles in

touch with the polymer, the bigger the reinforcing influence would be. This might

explain the reason that the layered silicate with high aspect ratio (800m2/g) leads to a

significant enhancement in modulus events with a small amount of particles in a

matrix. Therefore a low amount of particles in nanocomposites is preferable in order to

influence dramatic property enhancement (16).

The evidence of the influence of the level of exfoliation phase on the properties of

nanocomposites has been studied by Fornes and Paul (277). An analytical approach

was used in order to reveal how an insufficient exfoliation phase affected the stiffness

of nanocomposites. The modulus of a simple layered silicate stack in the parallel path

to its sheets was expressed by utilising the mixtures rule:

(1)

Where:

øMMT: Volume fraction of silicate layers in the stack;

EMMT: Modulus of MMT;

øgallery: Volume fraction of gallery space;


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Egallery: Modulus of the material in the gallery, which is expected to be much less than

EMMT.

øgallery can be expressed by X-ray basal spacing as follows:

(2)

Where:

d001: The repeat spacing between silicate particles;

tplatelet: The thickness of a silicate platelet.

Clearly, if the amount of platelets in a stack is equivalent to one, the structure will

confirm a separated exfoliation platelet.

2.8.1.2 Modulus and Strength

The reason for the layered silicate incorporation within the polymer matrix is to

enhance the modulus and strength of the materials. The enhancement of modulus and

strength of nanocomposites is attributed to the high modulus and strength of layered

silicate as well as the high aspect ratio which improves the interfacial interaction

between the polymer and layered silicate (74).

Poly(hexamethylene terephthalate) based on Cloisite 30B prepared by melt

intercalation and in-situ ring opening polymerization represented an increase in the

Young‟s modulus of about 20% and 25% respectively compared to virgin polymer

(278).

The preparation of the copolymer vinyl acetate based on nanocomposites was

undertaken by Peeterbroeck S et al. (279). Various types of layered silicate were used

which included Cloisite Na/ 20A/ 25A/30B, Nanofil 757/15, and Somasif ME100/

MAE. The organic modification process played an important role in the exfoliation

structure and properties. It was found that the addition of non-organo clays such as

Cloisite Na, Nanofil 757 and Somasif ME100 led to forming micro particles in the

resulting structure as confirmed by XRD. This result was expected because of the less

compatibility between the layered silicate and polymer which prevented the


46 | P a g e

nanocomposites structure being formed. Thus, the Young‟s modulus showed no

significant improvement. On the other hand, the treated clay exhibited a good

dispersion level and the modulus was duplicated compared to virgin polymer as in

Cloisite 20A.

Polyurethane nanocomposites based on layered silicate prepared by melt intercalation

and intercalative solution was studied by Finnigan et al. (223). Two types of polymer

were used which included soft and hard segments. Table 2 represents the values of

Young‟s modulus of both hard and soft polymer with different preparation methods. It

was revealed that the increasing of Young‟s modulus of hard host and especially by the

solution intercalation method was attributed to the high crystallinity of the resulting

polymer (280).

Table 2 Young's modulus of hard and soft polyurethane nanocomposites by melt and solution

intercalation (223).

Polymer Clay

content

Young‟s modulus (MPa)

Solvent intercalation Melt intercalation

Soft polymer 0 7.50 + 0.2 7.20 + 0.6

3 13.8 + 1.0 11.4 + 1.0

7 24.0 + 0.2 19.3 + 1.0

Hard polymer 0 50.0 + 5.0 61.0 + 5.0

3 86.0 + 2.0 81.0 + 3.0

7 134 + 10.0 119 + 13.0

Poly(methyl methacrylate) based on Cloisite10A, 93A and 30B by the melt blending

method was carried out by Unnikrishnan et al. (281). The tensile modulus was

increased up to 36% compared to base matrix by the addition of 3 wt.% clay. The

authors attributed the enhancement in the modulus to the high interfacial interaction

between the polar group of the polymer and the benzyl group of the layered silicate.

Hyper conjugation and the resonating structure generated by the benzyl group that was

located in the layered silicate were providing polarity in the benzene ring. Thus, the


47 | P a g e

interaction between the benzene ring and the polymer took place and resulted in

increasing the interface between the clay and polymer.

Tensile strength before and after thermal ageing at 120 °C for 72 hours of sepiolite/PU

nanocomposites was carried out by Chen et al. (282). The improvements of tensile

strength before ageing were about 45% and 65% at 2% and 5% clay loading compared

to the neat polymer. In addition, it was reported that the thermal ageing had a strong

effect on the tensile strength which was attributed to the oxidation of double bonds that

helped to speed up the scission of the matrix chains. Moreover, the incorporation of

layered silicate can increase the mechanical properties even under thermal ageing, as

proved by the result of tensile strength under the thermal condition.

Shelley et al. (283) revealed a 175% enhancement in yield stress associated with a

200% enlargement in tensile modulus for a nylon 6 nanocomposite consisting of only 5

wt.% layered silicate. The reason for improvements was attributed to the increase of

the amount of constrained chains of the polymer with the clay sheets.

Yao et al. (284) carried out the preparation of novel polyurethane based on layered

silicate. It was observed that the incorporation of clay led to enlarging the tensile

strength from 5.9 MPa to 8.3 MPa at 21.5 wt.% clay volume. It was revealed that the

increase in elasticity was traced to the plasticity effect of the gallery onium ion that

provided a dangling chain formation in the polymer matrix.

Moreover, the study of polyethylene based on layered silicate with different

intercalation agents (PE/JS and PE/DM) by melt intercalation was undertaken by Zhao

et al. (285). The JS and DM referred to (N- γ -trimethoxylsilanepropyl)

octadecyldimethylammonium chloride and dioctadecyldimethylammonium chloride

respectively. It was suggested that the level of intercalation depended on the type of

intercalated agent used. An improvement of tensile strength of nanocomposites with

different intercalation agents compared to neat polymer was observed. However, a

higher enhancement in tensile strength was observed by the addition of the JS agent.

This was traced to the good intercalation level between the polymer and layered silicate

by the presence of the JS agent as proved by XRD.


48 | P a g e

A recent study about the incorporation of layered silicate into the epoxy thermoset

matrix was carried out by Bashar et al. (286). A sodium montmorillonite layered

silicate was used in this study. The tensile modulus of neat epoxy was 2.88 GPa

whereas 3.15 and 3.33 GPa was seen for the epoxy based on MMT at 1 wt.% and 3

wt.% clay loading. Also, an improvement in tensile strength was observed. The

enhancement in tensile strength was 6.7% and 5.1% at 1% and 3% clay loading

respectively compared to virgin polymer. Likewise, enhancements in tensile strength

and modulus up to 25% and 34% respectively at 5% clay loading by the addition of

layered silicate into epoxy resin compared to the pristine polymer were obtained. The

authors traced these improvements to the interlocking and bridging of the layered

silicate with the matrix (287).

A modern report about the study of properties of polyamide 6 that was mixed with

layered silicate and melamine polyphosphate was carried out by Kiliaris et al. (288).

An enhancement in tensile strength and modulus on the nanocomposites compared to

the neat polymer was observed. The percentage of improvement in tensile modules was

12% and 17% at only 0.5% and 1.0% clay loading respectively. The tensile strength

was raised to 15% and 17% at 0.5% and 1% clay fraction volume.

In another new study, an exfoliation epoxy nanocomposites structure exhibited an

increase in tensile strength and modulus. The improvement of the strength and modulus

was 10% and 130% respectively at 5% clay loading compared to the pristine matrix.

The resulting enhancements in properties were attributed to the exfoliation

nanocomposites structure obtained from the new preparation method which was called

Nano-disassembling (289).

Rubbery and glassy epoxy based on different types of layered silicate including

Nanomer I.30E, I.28E, and Cloisite 10, 15, 20 A by in-situ polymerisation showed an

improvement in the tensile strength and elastic modulus of both rubbery and glassy

matrix at different clay types. It was observed that the glassy polymer exhibited a

marginal increase in mechanical properties which was attributed to the nature of the

material as the glassy polymer owned high strength and modulus which was difficult to

be improved (290).


49 | P a g e

The study of ethylene vinyl acetate copolymers based on layered silicate was

undertaken by Shi et al. (291). It was revealed that the addition of layered silicate into

the EVA matrix increased the yield strength as well as the modulus. The improvements

of strength and modulus were continuous and related to the clay content which was up

to 6 wt.%. It was found that the strength and modulus improvements were controlled

by the nanocomposites structure where the exfoliation system exhibited almost double

and triple the mechanical properties compared to the base polymer. It was revealed that

the improvements of strength and modulus were ascribed to the addition of polymeric

modifier and the nanoparticles. In the end it is worth mentioning that the copolymer

had a strong effect on the resulting properties.

Swain, S.K and Isayev, A.I (292) carried out the preparation of high-density

polyethylene (HDPE) /clay nanocomposites via a single screw compounding extruder

with different amounts of ultrasound amplitude. It was found that the ultrasound played

an important role in enhancing the extensibility of nanocomposites especially in the

tensile deformation. Table 3 represents the relationship between the nanocomposites

properties and various amount of amplitude. It can be seen that the presence of

ultrasound can increase the interfacial adhesion between the polymer and layered

silicate which can enhance the nanocomposites properties. Likewise, the elongation at

break and toughness were increased by utilising ultrasonic. The authors referred the

effect of the ultrasonic treatment on the elongation and toughness to the dependence of

these properties on the overall nanocomposites structure whether intercalated or

exfoliated (293-295).

Table 3 The relationship between the nanocomposites properties and the ultrasound amplitude (292).

Clay

(%)

Ultrasonic

amplitude (µm)

Elongation at

break (%)

Toughness

(MPa)

Yield

stress

(MPa)

5 0 487 + 28 62.1 + 5.3 18.1 + 1.0

5 5 887 + 58 112.4 + 7.3 19.0 + 1.1

5 7.5 973 + 62 132.3 + 6.8 19.5 + 0.8

Also, there were many studies which represented an improvement in Young‟s modulus

such as PCL/Clay nanocomposites (134, 296), PA6/ Clay nanocomposites (80, 274,


50 | P a g e

297), PLA/MMT nanocomposites (298), and PP/ Clay nanocomposites (299), and

tensile strength such as PA6/ clay nanocomposites (274, 297), star shaped styrene–

butadiene block copolymer based on Cloisite Na and Microfloc XFB (300), and

starch/poly (vinyl alcohol)/MMT nanocomposites (301).

2.8.1.2.1 Factors that influence the strength and modulus

There are many factors that may affect the nanocomposites structure and properties.

The first factor which plays an important role in the output properties of

nanocomposites can be the clay concentration. It is generally acceptable that the clay

loading has an optimal level which should be identified for the tensile strength

nanocomposites (302). It was suggested that the clay loading affects the resulting

structure, as in some polymers the clay content should be less than 10 wt.% in order to

obtain exfoliation nanocomposites. However, with greater than 10 wt.% clay,

intercalated or partially exfoliated nanocomposites structures can be formed (16, 20).

Polyaniline/MMT nanocomposites prepared by the in-situ polymerisation method was

carried out by Soundararajah Y et al. (303). Different concentrations of layered silicate

were used. It was reported that the addition of layered silicate increased the Young‟s

modulus. There is a constant increase in modulus by the increasing of layered silicate

content up to 23 wt.%, however, more clay content resulted in decreasing the overall

properties. This was attributed to the level of dispersion and the nanocomposites

structure obtained.

The study of mechanical performance of PA66 produced by the melt intercalative

method was undertaken by Liu and Wu (258). Epoxy intercalated layered silicate was

used in this study. The result of the tensile strength rose significantly from 78 MPa of

neat polymer to 98 MPa of 5 wt.% clay content. However, a reduction was observed

when the clay was above 5%. The tensile modulus showed a similar result to the tensile

strength but there was a small enhancement in modulus after 5 wt.% clay loading. The

reduction or small enhancement in properties above 5 wt.% clay loading was ascribed

to the layered silicate aggregation as reported.

Besides the clay content that influences the modulus and strength output, the

compatiblizer of the polymer plays an important role in the term of properties. PP/clay

nanocomposites with two different types of compatiblizer, including polypropylene


51 | P a g e

grafted maleic anhydride (PP-g-MA) and polyolefin elastomer grafted maleic

anhydride (POE-g-MA), was undertaken by Lai et al.(304). The tensile strength and

modulus of PP-g-MA represented an enhancement in tensile strength and modulus

compared to base polymer. However, the POE-g-MA decreased the properties which

were traced to less interaction with the layered silicate as proved by XRD.

Table 4 The influence of layered silicate on the tensile modulus and strength (305).

Preparation method

Clay

content

Wt.%

Modulus

(MPa)

Tensile

strength

(MPa)

Mixing of the curing agent

with MMT and then blended

with the polymer +

ultrasonic.

0 0.80 0.40 2 1.30 0.60

4 2.20 1.00

6 2.50 1.10 8 3.40 1.30

Mixing of the curing agent

with MMT and then blended

with the polymer.

0 0.80 0.40 2 1.25 0.59

4 1.75 0.75 6 2.20 0.82

8 1.60 0.85

The blending of the curing

agent with the polymer and

later on introducing the

layered silicate + ultrasonic.

0 0.80 0.40

2 1.25 0.59 4 1.77 0.80

6 2.10 0.80 8 2.50 1.00

The blending of the curing

agent with the polymer and

later on introducing the

layered silicate.

0 0.80 0.40

2 1.00 0.53 4 1.25 0.59

6 1.80 0.62 8 2.00 0.80

Apart from the compatiblizer of the polymer, the process parameter has an impact on

the nanocomposites properties. A recent report was revealed by M. Lipinska and J.M.

Hutchinson (305) about the study of epoxy (DER331) based on nanocomposites. The

octadecyl onium ion treated montmorillonite (MMT) was used in this study. Moreover,

polyetheramine (D-2000) was utilised as a curing agent. The authors investigated

different preparation methods and their output properties. The first and second

preparation method was the mixing of the curing agent with MMT and then blended

with the polymer with or without ultrasonic (US) respectively. The third and fourth

method was the blending of the curing agent with the polymer and later on introducing

the layered silicate to the mixture with ultrasonic or without. Table 4 represents the


52 | P a g e

effect of layered silicate content on the modulus and strength properties It was

observed that the blending of the curing agent with MMT directly helped to have better

properties with or without the ultrasonic. These improvements were assigned to

synergism between the layered silicate and curing agent. Thus, the preparation method

plays an important role in the resulting properties.

In addition, there are many factors that may influence the modulus and strength of the

nanocomposites. For instance, the nature of acid plays an important role in terms of

properties(20), the molecular weight (274), the organic-modification of layered silicate

and the catalyst used (306, 307), and the level of crystallinity (308).

2.8.1.2.2 Contradiction reports about the strength and modulus

Although there are many studies that reported drastic improvements in tensile strength

and modulus, different studies show a reduction in properties when the layered silicate

was introduced.

The study of the polyester matrix nanocomposites based on 5 and 10 wt.% layered

silicate was revealed by Chieruzzi et al. (309). Dellite®72T (D72T) and 43B (D43B)

types of clay were used. It was revealed that the introduction of the clay led to

decreasing the tensile strength because of the high viscosity of the mixture and the lack

of degassing. Thus, it was suggested that the addition of material helps to reduce the

viscosity. Fyrol® TEP based on triethyl phosphate was utilised as a viscosity reducer

(VR). However, after the addition of the VR, the properties were further reduced which

was attributed to the side effect of the chemical structure of the VR that reduced the Tg

and mechanical properties.

In addition, Polyethylene/MMT nanocomposites (310), Copolymer vinyl acetate

nanocomposites (279), and polyester/clay nanocomposites (311) represented a

reduction of mechanical properties by the addition of layered silicate.

In summary, clay loadings and sources, the nature of acid, the process parameters, the

molecular weight, layered silicate modification, catalyst, and the degree of crystallinity

can influence the level of exfoliation nanocomposites. Thus the amount of

improvement in either modulus or strength varies depending on the level of exfoliation.


53 | P a g e

2.8.1.3 Flexural modulus and strength

There are many reports that have been done on the bending modulus and strength of


The study of the properties of polyethylene nanocomposites was undertaken by Zhao et

al. (285). Various types of intercalation agents were used (PE/JS and PE/DM). With

regard to the kind of agents that were used, both nanocomposites samples showed an

improvement in flexural strength and modulus compared to virgin polymer.

A modern study of exfoliation epoxy nanocomposites based on layered silicate

structure exhibited an increase in flexural strength and modulus. The improvement of

the strength and modulus was 6% and 13% respectively at 5% clay loading compared

to the neat matrix. The resulting enhancements in properties were attributed to the

exfoliation nanocomposites structure obtained from the new preparation method which

was called Nano-disassembling (289). Similarly, Alamri, H and Low, I M (312) carried

out the study of mechanical properties of epoxy based on layered silicate. It was found

that at low contents of nano-fillers (1 wt.% clay loading), the flexural properties were

increased up to 58% and 77% for the strength and modulus respectively. The flexural

strength at 3 and 5 wt.% layered silicate showed no significant change compared to

base polymer. The strength of the materials is very sensitive to the level of

intercalation. So a lack of good dispersion will provide a negative side effect of the

strength outputs.

The investigation of the properties of unsaturated polyester based on clay

nanocomposites was carried out by Chieruzzi et al. (309). At 10 wt.% clay loading, the

flexural modulus was increased by 70% compared to virgin polymer.

2.8.1.4 Toughness

The debonding of layered silicate from the polymer matrix usually exhibits microvoids

in the nanocomposites structure which leads to provide the output material with

brittleness. This was examined by an accurate investigation for the cracking surface

and accompanied with transmission electron microscopy (TEM) (302, 313). For

example, Ethylene vinyl acetate based on clay nanocomposites showed a reduction in

toughness by the incorporation of different amounts of clay (291). Likewise, Poly


54 | P a g e

(methyl methacrylate) - layered silicate nanocomposites represented a marginal

reduction in toughness at low nanofillers content. However, at high amounts of clay,

the reduction was very high (281).

Although many reports revealed a dramatic reduction in toughness properties for

nanocomposites when the layered silicate was introduced, other studies reported a

small decrease in toughness or no change when the clay was incorporated.

A recent study revealed that the amount of fracture toughness for pristine epoxy and

nanocomposites were almost the same. However, the initial clay loading exhibited a

marginal improvement. This study utilised the epoxy as a polymer and Na+-MMT as a

layered silicate. The virgin polymer was 0.78 MPa.m0.5

whereas at 1% clay volume it

was 0.79 MPa.m0.5

and the addition of layered silicate as in 3% clay loading reduced

the toughness to 0.77 MPa.m0.5

. It was suggested that the intercalation structure played

an important role in the enhancement of the toughness (286). Likewise, exfoliation

epoxy nanocomposites based on a layered silicate structure, exhibited an increase in

impact strength. The improvement of the strength was 5% at 5 wt.% clay loading

compared to the neat matrix (289). In addition, the study of the effect of layered silicate

on the EVA matrix by using a microcellular injection mould was carried out by Hwang

et al. (314). This report revealed an increase in the impact strength by the addition of

layered silicate.

Also, there are many studies showing an improvement in toughness or that the

nanocomposites were almost as same as the base polymer, such as PP (315), nylon

(316), and PA66 (258) nanocomposites.

The previous reports were surprising to the principle of nanocomposites. It is generally

accepted that the size of particle plays a major role in the toughness property. In

addition, the size that acts to provide a tough material is more than 0.1 µm which is not

preferable to the nanometer size. Also, it is argued that the nanoparticles are very tiny

to offer toughening of their structure. Thus, the complete exfoliation phase does not

provide toughening of the material. As a result, the enhancement may be attributed to

the interaction between clay and polymer in the intercalation structure which may

alleviate the argument (80).


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2.8.1.5 Strain at break

Different reports in terms of elongation for the nanocomposites were obtained. A

recent study about the investigation of the properties of polyamide 6 exhibited a

reduction in nanocomposites elongation compared to pristine polymer. The decreased

amount was 7% and 23% when the level of clay was at 0.5% and 1%. The reduction of

elongation amount was attributed to two reasons. First, with the attendance of clay, the

ability to change orientation of polymer molecules was restricted. The authors also

traced the arising of reduction to the debonding of clay sheets from the polymer matrix

(288).

Ethylene vinyl acetate based on clay nanocomposites showed a reduction in elongation

by the incorporation of different amounts of clay. The reduction of the elongation was

to be less in the exfoliation structure compared to intercalation nanocomposites (291).

Recently, the epoxy-MMT revealed a marginal reduction in elongation at low

nanofillers content. The elongation of neat epoxy was 8.5%. However, at 1% and 3%

clay loading the figures were 7.21% and 3.77% respectively. The reduction of the

elongation was assigned to the high rigidity and stiffness of the particles which reduced

the ductility of the material (286). In addition, Poly (methyl methacrylate) - layered

silicate nanocomposites represented a reduction in the elongation. The reduction of the

elongation was proportional to the clay content. The decreasing was traced to the stiff

fillers that were incorporated. The stiff fillers restricted the mobility environment of the

polymer chains (281).

The preparation of the copolymer vinyl acetate based on nanocomposites was

undertaken by Peeterbroeck et al. (279). Various types of layered silicate were used as

mentioned in section 2.8.1.2. It was observed that the addition of clay to the polymer

matrix led to decreasing the elongation at break. The authors agreed that the

nanocomposites properties depend on the origin of the clay, whether modified or not,

and the level of dispersion. Polyurethane/clay nanocomposites by melt intercalation

and intercalative solution were studied by Finnigan et al. (223). Two types of polymer

were used which included soft and hard segments. With regard to the types of

preparation method or polymer types, the addition of clay reduced the elongation of

nanocomposites compared to neat polymer.


56 | P a g e

From the preceding reports, it can be seen that the addition of layered silicate provides

most of the polymers with brittleness. However, some results show an enhancement in

elongation at break. An enlargement in elongation at break is observed in the

incorporation of o-MMT into EVA12 polymer at low clay level, about 2%. The

increase was up to 2% clay loading and the addition of clay level leads to reduce the

strain as well as mechanical properties. This was assigned to the agglomeration of

layers (317).

The improvement in elongation for nanocomposites based on layered silicate was also

conducted by Yao et al. (284). The improvement in strain was up to 22.1% at 21.5%

clay content. The researchers assumed that the enhancement was ascribed in part to the

influence of plasticity of the intergallery onium ions which formed the dangling chain

in the polymer matrix.

A drastic enhancement in the tensile elongation with the addition of the clay into

poly(ε-caprolactone) was conducted by Chen and Evans (137). An unusual trend was

observed when the elongation exhibited a similar yield of neat polymer whereas the

ductility was much greater than the virgin one. However, the further addition of layered

silicate provides the material with brittleness and the yield area will be achieved.

Likewise, the improvement of the strain at break before and after thermal ageing at

120oC for 72 hours of sepiolite/PU nanocomposites was carried out by Chen et al.

(282). The improvements of elongation before ageing were about 35% and 50% at 2%

and 5% clay loading compared to the base polymer. In addition, it was found that the

thermal ageing had a strong effect on the elongation which was attributed to the

oxidation of double bonds that helped to speed up the scission of the matrix chains.

Moreover, the incorporation of layered silicate can increase the mechanical properties

even under thermal ageing, as proved by the result of strain at break under the thermal

condition. The rubbery epoxy based on layered silicate exhibited an improvement in

elongation at break. The improvement percentage of nanocomposites properties was

different which was attributed to the types and amounts of clay. However, the glassy

epoxy based on layered silicate showed a reduction in the strain at break (290). In

addition, the study of the poly(hexamethylene terephthalate)/ clay nanocomposites was

carried out by González-Vidal et al. (278). The incorporation of Cloisite 30B into the

matrix resulted in increasing the elongation at break from 2.58% to 3.46%.


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2.8.1.6 Creep properties

In the literature, the effect of layered silicate on the creep resistance was not studied in

a wide range. The study of creep behaviour of polymer layered silicate by inventing a

predictive modelling of calculating the creep was carried out by Shokuhfar et al. (318).

Regardless of the kind of matrix, the addition of layered silicate can enhance creep

resistance. Moreover, the level of intercalation was the key to reaching the optimal

creep resistance. For instance, the structure of intercalation, 33% exfoliation, 66%

exfoliation or ful exfoliation had different values of creep as depicted in Figure 24. The

more interfacial interaction between the matrix and layered silicate, the higher the

resistance of the creep. Also, the incorporation of clays into nylon 6 and polyamide 66

exhibited an enhancement in the creep properties. The authors attributed these

improvements of creep properties to the incorporation of nanolayers where these layers

can withstand the stress rather than the matrix.

Figure 24 Different nanocomposites structures exhibited different creep values (316). Reproduced from

Shokuhfar by permission of Elsevier Science Ltd., UK.


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2.8.1.7 Nanoindentation

The study of the influence of different loading levels of the layered silicate

reinforcement on the nanomechanical properties can be examined by the

nanoindentation test. The hardness and elastic modulus can be provided by this method

(319).

Aldousiri et al. (319) carried out the study of the nanoindentation of neat spent

polyamide-12 and the nanocomposites series. Table 5 represents the experimental data

of the base polymer and nanocomposites. It was found that the addition of layered

silicate led to increasing the hardness and modulus up to 116% and 73% improvements

respectively compared to the base polymer. The authors attributed the enhancement in

properties to the change in the properties matrix that was caused by the high aspect

ratio of layered silicate.

Table 5 The effect of the addition of layered silicate into the polymer matrix on the nanomechanical

properties (319).

Samples Max Depth (nm)(SD) Hardness (GPa)(SD) Reduced Modulus (GPa)(SD)

Spent PA-12 1160

(±28.55)

0.156

(±0.038)

2.212

(±0.341)

Spent PA-12/1 wt.% 1109

(±46.36)

0.161

(±0.062)

2.340

(±0.507)

Spent PA-12/3 wt.% 1061

(±33.22)

0.202

(±0.095)

2.900

(±0.872)

Spent PA-12/5 wt.% 908

(±59.48)

0.338

(±0.023)

3.835

(±0.690)

In addition, the investigation of the mechanical properties of PLA/layered silicate

nanocomposites was carried out by González et al. (298). The hardness and the

reduced modulus of the polymer based on MMT and sepiolite nanocomposites

calculated from the nanoindentation results were improved compared to the base

polymer. Good dispersion and good interaction are the keys of the improvement of the

hardness and reduced modulus of nanocomposites.


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Moreover, Yucai et al. (320) carried out a study of the effect of clay loading on the

nanoindentation properties of Nylon 11 nanocomposites based on layered silicate. It

was found that the addition of layered silicate increased the hardness up to 27% at 5%

clay loading. Figure 25 represents the loading-unloading curve of the nanoindentation

test. Also, the modulus of nanocomposites showed higher values compared to neat

polymer as seen in Figure 26.

Figure 25 Load displacement curves for PA11 and the corresponding nanocomposites (320). Reproduced

from Hu by permission of Elsevier Science Ltd., UK.

Figure 26 Hardness and Modulus of PA11 and nanocomposites samples (320). Reproduced from Yucai

by permission of Elsevier Science Ltd., UK.

Likewise, the hardness of nylon based on nanocomposites was increased by 15% at 5

wt.% clay loading compared to virgin polymer. The authors also revealed that the

strain rate had a strong effect on both the hardness and modulus of the materials. It was


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observed that with increasing the strain rate, the properties of nanoindentation are

increased too. The hardness of nanocomposites at 5% clay volume fraction with 2 s-1

(strain rate) was 0.119 GPa whereas with 20 s-1

it was 0.147 GPa. The improvement of

properties at high strain rate was attributed to the plasticity index. The materials behave

plastically at high strain rate (321). Moreover, Shen et al. (322) revealed an

improvement of hardness by the incorporation of layered silicate from 0.14 GPa of neat

nylon to 0.18 GPa at 10 wt.% clay level. Dhakal et al. (207) carried out the study of the

effect of layered silicate addition into the polyester matrix on the mechanical

properties. It was found that the hardness of nanocomposites extracted from the

nanoindentation loading–unloading curve was improved by 14% compared to the

virgin polymer with less than 5% clay loading.

2.8.1.8 Dynamic mechanical properties (DMA)

DMA is defined as the function of calculating the material‟s reaction to any types of

deformation, such as tensile, flexural and torsion, with the presence of temperature.

The outcomes of DMA are articulated by three important aspects: (A) the storage of

modulus which is termed as E' or G', (B) the loss of modulus which is known as E'' or

G'', and the last parameter (C) is the tan which is named as δ. The elastic phase to the

deformation represents the storage modulus whereas the loss of modulus shows the

plastic stage upon the deformation. In addition, the ratio between the loss and storage

modulus E''/E' represents the tan value. The above parameters are very helpful to find

out the action of the molecular movement such as the temperature of glass transition.

Dynamic Mechanical Thermal Analysis (DMTA) of poly(lactic acid) based on

different clays (Cloisite 30B, Fluoro-hectorite and Sepiolite, see

Table 6) at 5% and 7% clay content was examined by Fukushima et al. (323). It was

observed that the E' values below the Tg were about 50% improvement of the different

clay types compared to the base polymer. Above Tg, the enhancements were much

more pronounced as can be seen in the Table 7. This behaviour can be traced to the

restriction of motion of the chain above Tg as the incorporation of layered silicate takes

place. The variation of E' values of different clays can be assigned to the various aspect

ratios as well as the compatibility with the polymer.


61 | P a g e

Table 6 Various clay types that were used in the experiment. Reprinted from (323).

Type of clay Commercial

name Modifier structure Notation in text

Montmorillonitea CLOISITE 30B

CLO30B

Synthetic fluoro-

hectoritea SOMASIF MEE

(X + y)=2

SOM MEE

Sepiolite PANGEL S9 NA SEPS9

HT= Linear alkyl chains, R: C8-18 from hydrogenated tallow.

a Organic modifier content ca. 30 wt% according to the technical data sheet.

Table 7 E' values at different clay types and concentrations (323).

Samples E' at 30°C

(MPa)

E' at 68°C

(MPa)

E' at 80°C

(MPa)

Tan Delta

(°C)

PLA 2700 35 4 68

PLA + 5% CLO30B 3800 74 16 67

PLA + 7% CLO30B 4532 383 28 69

PLA + 5% SOM MEE 4330 76 25 64

PLA + 7% SOM MEE 5731 275 51 69

PLA + 5% SEPS9 4200 1100 21 72

PLA + 7% SEPS9 3530 900 13 72

Another study revealed an improvement of storage modulus of nanocomposites

compared to the neat polyurethane matrix. This enhancement was ascribed to the

restriction of movement of the matrix chains by the addition of layered silicate. Also, it

was found that the effect of hydrodynamic fillers was due to the incorporation of

nanosize particles in the matrix, which is well controlled by the filler content and shape

factor of layered silicate. The E' modulus showed a drastic reduction after 75°C which

O

H O

H X

Y

N HT

H3C

HO OH

N

H3C HT


62 | P a g e

can be attributed to the change in the material phase from the glassy to the rubbery

state (324). Moreover, the storage modulus of Poly(methyl methacrylate) based on

Cloisite10A, 93A and 30B by melt blending was increased compared to virgin polymer

(see Figure 27). The authors attributed the improvement in modulus to the high

interfacial interaction between the layered silicate and the polymer where, at the

organic and inorganic interface, the restriction of the polymer chains will take place

(281).

Figure 27 Storage modulus behaviour of neat polymer and corresponding nanocomposites (281).

Reproduced from Unnikrishnan by permission of Elsevier Science Ltd., UK.

In general, many factors influence the amount of the DMA of the nanocomposites

structure. One of the important factors is the polymer type. For instance, different

polymer matrix nanocomposites showed contradictory results in terms of the tan δ

(325, 326). In addition, the clay modification type and source plays an important role in

the change of DMA (307). For example, Krikorian and Pochan (219) revealed a study

of DMA on the virgin PLA and PLA based on the modification of organoclay

nanocomposites. The treated layered silicate was found to have less impact on the

matrix at high temperature which was traced to the lower thermomechanical stability of

the organoclay modifier at high temperature. Likewise, the study of the properties of

glassy and rubbery epoxy based on different types of layered silicate which includes


63 | P a g e

inorganic and organic modifier clay was carried out by Xidas P and Triantafyllidis K

(290). The glassy epoxy nanocomposites showed an improvement in storage modulus

at different kinds of clay. However, the rubbery epoxy nanocomposites exhibited the

same values of storage modulus of neat polymer or less. The reduction of the modulus

in rubbery epoxy was assigned to the large content of organic modifier in the polymer

matrix and the negative influence of the dangling chains of the modifier on the

interfacial adhesion of the matrix chains to the layered silicate surface. Thus the

properties of epoxy matrix in the glassy region will be affected. In addition, NBR/clay

nanocomposites based on O-MMT represented an improvement in storage modulus

compared to pristine polymer. The enhancement in thermo-mechanical properties was

attributed to the effect of the adhesion between the matrix and layers‟ surface. The

modifier molecules probably form a bridge bond between the clay and rubber which

limits chains motion near the inorganic-organic interface (327).

Apart from the preceding factors, the level of intercalation between the polymer and

layered silicate affect the value of DMA. González-Vidal N (278) carried out the

preparation of poly(hexamethylene terephthalate) / layered silicate nanocomposites by

melt blending and in-situ polymerisation. 3 wt.% clay loading of Cloisite 30B was

used. The storage modulus was increased by 15% and 40% for the nanocomposites

synthesis by melt intercalation and ring-opening polymerisation respectively. The

different enhancement in E' values was attributed to the level of dispersion.

An improvement of storage and loss modulus and the Tan values of star shaped

styrene–butadiene block copolymer based on the oligostyrene of modified MMT and

bentonite were reported. Figure 28 represents the enhancement of storage modulus by

the addition of layered silicate into the polymer matrix. The storage modulus of the

nanocomposites was sharply decreased which acted as plateau-like behaviour. The

authors revealed that the improvements in properties were traced to the chemical

affinity between the polymer and layered silicate. In other words, the higher the

interfacial interaction between the matrix and clay, the better miscibility between the

treated fillers and the matrix (300).


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Figure 28 The relationship between the storage modulus as well as the tan in a function of temperature

(300). Reproduced from Ganß by permission of Elsevier Science Ltd., UK.

2.8.2 Barrier properties

The nanocomposites usually exhibit a very high amount of barrier properties with even

a small amount of layered silicate (328). Tortuous paths help to understand the

principle of the barrier behaviour and its enhancement in the nanocomposites. This

action can be explained by the introduction of the impermeability of the layered silicate

into the matrix of polymer, so the intercalation molecules are placed in a wiggle shape

around the nanoparticles in a random way (16, 21, 29, 329), as seen in Figure 29.

Figure 29 (a) is the model of tortuous diffusion path in exfoliated structure of nanocomposites and (b) is

the path of conventional composites.

The equation of tortuous path can be expressed as follows:

Gas / Liquid Traditional composites

a b

L

W

δ


65 | P a g e

(3)

Where:

d’: Term as an actual distance that the penetrant has to go throughout the shortest

distance;

d: The spacing between diffractional lattice planes;

L: The length of the sheets;

W: The width of the sheets;

s: Clay loading.

Because of the ratio of large length to width of the sheets, this type of clay will

enhance the path length when compared to different nanoparticles geometry (16, 20,

330). From the model that was proposed by Nielsen L E (331), the influence of

tortuosity on the permeability can be simulated as

(4)

Where:

PPCN: The permeability of the nanocomposites;

PP: The permeability of the virgin polymer.

The preceding formula was proposed for the dispersal of small molecular size such as

micro-molecular size in traditional composites, however it has also been used for the

nano scale as in the diffusion and permeability in nanocomposites structure. A

contradiction in the results obtained from experimental outcomes and the practical

experiment was observed which was probably ascribed to the insufficiency of the

model or the orientation of the layered silicate sheet that was located in the

nanocomposites film plane (306, 330). This model was proposed to the excellent

arrangement of sheets within the structure where the direction of dispersal is normal to

the way of layers. Obviously, the arrangement of these sheets leads to improve the

tortuous path, and if these arrangements are failed, the barrier properties are failed also

(16, 330).


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Apart from the tortuous path equation, the Nielsen, the Cussler (332), the Barrel (333)

and the power low (334) equations are based on the supposition that the layered silicate

does not influence the dispersal of the polymer matrix whereas these nanoparticles are

affected by the molecular mobility in the matrix which is reduced by clay introduction

as proved by experimental observation. This reduction of the mobility was attributed to

the close connection of molecular mobility to the mass transport properties. Thus, the

diffusivity of tiny molecular is reduced which is not taken into account in the idea of

the tortuous path model (8).

A drastic enhancement in the barrier properties by the incorporation of layered silicate

into the PA6 matrix was reported, more than 20 times improvement in the O2

permeability for the nanocomposites compared to virgin polymer. The authors

attributed the increasing of barrier properties to the exfoliation structure of

nanocomposites (335).

Moreover, the addition of various concentrations of layered silicate into the Soy protein

via the solution intercalative method increased dramatically the O2 permeability which

can be used for food packing as the barrier properties are a major concern (336).

The incorporation of layered silicate within the polymer matrix (epoxy), can decrease

the helium permeability up to 70% by comparison with the base matrix (337). Also, a

reduction of O2 permeability for nanocomposites up to 30% at 3% clay fraction volume

was observed. It was observed that the modifications of layered silicate can decrease

the uptake of water (338). Moreover, the tortuous path of clay when introduced to the

epoxy matrix was found to increase the barrier properties to the moisture permeability.

The enlargement of barrier properties was proportionally increased to the clay content

(339).

In addition, Swain, S.K and Isayev, I.A (292) revealed that the incorporation of

layered silicate into the HDPE increased the oxygen permeability. The addition of 10%

clay loading into the polymer matrix increased the O2 permeability up to 5% compared

to neat polymer. Also, it was suggested that the use of ultrasound amplitude at low clay

loading can enhance the interfacial interaction between the layered silicate and the

polymer matrix which increases the barrier properties.


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The study of barrier properties of polyacrylate-layered silicate nanocomposites that

were prepared via emulsion polymerisation was carried out by Herrera-Alonso et al.

(340). Different polymers and clay concentrations were used in this report, as listed in

Table 8. It was found that the reduction of the permeability is proportional to the clay

volume. In addition, it was observed that the co-polymer showed better barrier

properties compared to the PBMA. The enhancement of co-polymer barrier membranes

was attributed to the addition of methyl methacrylate monomer (MMA) to the

preparation processes in which the addition of more MMA led to more enhancements

in the permeability. Moreover, the authors suggested that the preparation of

nanocomposites by solution polymerisation leads to greater improvement in the barrier

properties compared to preparation via emulsion polymerisation. This observation was

based on the comparison between the results by (341) and the current study. The

difference in the outcome properties of the different preparation methods was traced to

the optimal dispersion level that can be achieved. The emulsion polymerisation

produces the barrier membranes via the cast from the colloidal suspension, including

the matrix particles covered by the clay layers. Thus, the size of the polymer particles

limits the dispersion level of the clay layers to the particle‟s dimensions. However, the

solution polymerisation technique provides a better dispersion level between the

layered silicate and polymer as the last position of clay layers in the polymer is

diffusion-limited.

Table 8 Various polymers and clays that were used in this study (340).

Clay Polymer Clay polymer

Poly(n-butyl methacrylate) poly(n-butyl methacrylate-co-methyl methacrylate)

0 PBMA 0 CPBM/80-20

1 PBMA/1 1 CPBM/80-20/1

3 PBMA/3 3 CPBM/80-20/3

5 PBMA/5 5 CPBM/80-20/5

5 CPBM/90-10/5

5 CPBM/70-30/5

5 CPBM/60-40/5

(the numbers after CPBM represents the ratio between butyl methacrylate: methyl methacrylate)

Jawahar P et al. (342) revealed that the addition of layered silicate into the polyester

polymer enhanced the barrier properties. In consequence, the wear resistance was also

improved as the layered silicate created a protective layer on the matrix.


68 | P a g e

In addition there are many reports showing an improvement in the barrier properties of

nanocomposites by the addition of layered silicate, such as PA66 (258), epoxy (343-

345) PET (346) PLA (218) nanocomposites.

Taking everything into account, although the improvement of the diffusion coefficient

is widely reported in terms of polymer layered silicate nanocomposites, contrary

outcomes were also observed about the saturation absorption amounts of different

liquids or gasses. The concern about the enlargement of the saturation uptake is

normally traced to the clustering role. In addition, it is important to mention that the

complicated diffusion phenomena of the nanocomposites can be traced to the

synergism of phases with various permeabilities. The clay nature is the high absorption

behaviour and to specify the interfacial reaction with the solvents. In proper sequence,

the matrix can be counted as two phases, which are the crystalline and the amorphous

structures. The crystalline area is normally impermeable to penetrant molecules. The

attendance of the layered silicate is usually assumed to reduce the permeability, which

is assigned to the high number of tortuous path for the dispersal molecules that have to

avoid the impermeable sheets (347). Thus, the impact of the change in the chain

mobility, as well as the crystallinity system of the polymer that is traced to the presence

of the layered silicate, should be taken into account (8, 348).

2.8.3 Thermal stability

Nanocomposites normally result in improvement in thermal stability. This is because

the layered silicate tortuous path (349) acts as an insulator accompanied by the mass

transfer barrier to the product volatilisation produced during the decomposition (16,

330, 350). In addition, the layered silicate can help to leave a char block after complete

thermal decomposition, so the char acts as a protective barrier to insulate the flame

from going to the polymer. Moreover the char decreases the level of oxygen uptake as

well as the escape of the gasses from the polymer during the decomposition (351).

Thermo gravimetric analysis (TGA) is the most fundamental device used to

characterise the properties of thermal stability for pure polymer and polymer layered

silicate nanocomposite materials. TGA measures the reduction of weight that was

traced to the volatilisation of the material after distortion under high temperature as a

function of temperature besides time. The high temperature leads to two types of


69 | P a g e

degradation, which are the unoxidation degradation where the heating is applied under

an inert gas flow, and the oxidation degradation where the O2 is used besides the

heating (16, 330).

In most of the literature reports about the nanocomposites thermal stability behaviour,

the introduction of the layered silicate into the polymer matrix usually improves the

thermal stability since these layers act as an insulation and mass transfer barrier to the

volatilisation of the material during the degradation. Also, the layered silicate helps to

form the char after the thermal degradation (16, 330, 347, 350).

A modern study of the exfoliation epoxy nanocomposites based on layered silicate

structures exhibited an increase in thermal properties such as Tg and Td. The

improvement of the Tg and Td was 3.2% and 3.6% respectively at 5% clay loading

compared to the neat matrix (289). Likewise, the Tg temperature of epoxy

nanocomposites based on treated layered silicate was improved up to 15oC compared

to the pristine polymer. Also, the burning rate decreased by up to 58% at 10% clay

loading (352).

Table 9 Different types of materials that have been used in this study (351).

Materials Description

BDGE Epoxy resin (bisphenol A diglycidyl ether)

BPDG Epoxy resin (bisphenol A propoxylate diglycidyl

ether) BBDG Epoxy resin (bisphenolAbrominated diglycidyl ether)

TGDDM Epoxy resin (tetraglycidyl of

diaminodiphenylmethane) HDTMA Layered silicate (hexadecyltrimethylammonium)

HDTPP Layered silicate (hexadecyltriphenylphosphonium)

A study of different epoxy resins besides various types of layered silicate and their

thermal stability were undertaken by Lakshmi et al. (351). The descriptions of the

materials are presented in Table 9. Figure 30 represents the thermal stability for

untreated epoxy resin (a) and modified clay epoxy nanocomposites (b). It can be seen

that the initial decomposition temperature for (b) is higher than for (a). The authors

ascribed the improvements to the incorporation of layered silicate which acts as a

barrier. Also, these layers constrained the movement of the polymer chain. In addition,


70 | P a g e

the presence of inorganic substances such as Sio2, Al2o3 and MgO, high heat resistance

like phenyl units, and bromine atoms which were found in the epoxy and the

triphenylphosphine unit in the HDTPP-MMT layered silicate.

Figure 30 Thermal stability for (a) neat epoxy and (b) epoxy clay nanocomposites (351). Reproduced

from Lakshmi by permission of Elsevier Science Ltd., UK.

Table 10 TGA in nitrogen and air of different clay types and content (323).

Materials

In N2 In air

T5% (°C) Tmax (°C) T5% (°C) Tmax (°C)

PLA 308 366 300 352

PLA + 5% CLO30B 328 370 334 379

PLA + 7% CLO30B 324 371 330 381

PLA + 5% SOM MEE 316 371 322 379

PLA + 7% SOM MEE 321 370 331 375

PLA + 5% SEPS9 338 373 334 383

PLA + 7% SEPS9 335 373 331 383

TGA in air and nitrogen of poly(lactic acid) based on different clays (Cloisite 30B,

Fluoro-hectorite and Sepiolite) at 5% and 7% clay content were examined by

Fukushima et al. (323). The temperature of the experiment of TGA was at two stages


71 | P a g e

(T5% °C and Tmax

°C). A significant enhancement of TGA was obtained as can be seen

in Table 10. The enlargement of TGA in nitrogen can be attributed to the barrier

properties of layered silicate, and the char formation of layered silicate can explain the

improvement in TGA in air.

The effect of the addition of the modified Sepiolite on the thermal stability of PU was

studied by Chen et al. (282). The thermal stability was measured by TGA under N2.

The nanocomposites and neat polymer results are presented in Table 11.

Table 11 TGA results of nanocomposites and neat polymer (282).

KH550-Sp

weight/%

*Initial

degradation

temperature/°C

Residue weight

at 550°C/%

0 286 3.0

1 303 10.3

3 307 8.3

5 304 8.8

*The temperature for 5% weight loss

The residue weight at 550°C was improved by the incorporation of the layered silicate

up to 244% compared to the pristine polymer. The enhancement in the residue weight

was traced to the only escapes of the zeolite water and some of the coordinated H2O at

550°C (353), thus, the crystal structure is difficult to change. In consequence, the

Sepiolite will stay in the char leading to the enlargement of the residue weight.

Figure 31 TGA curve (a) neat PU, (b) PU/KH550-Sp (1%), (c) PU/KH550-Sp (3%) and (d) PU/ KH550-

Sp (5%) nanocomposites (282). Reproduced from Chen by permission of Elsevier Science Ltd., UK.


72 | P a g e

Figure 31 represents the TGA curve for the nanocomposites and neat matrix. It can be

seen that there are two weight loss stages in the curve. The first stage is between 280-

400°C which is attributed to the cleavage of the urethane set and simultaneous

secondary reaction (i.e. dimerization or trimerization). The second stage is between

400-490°C which is ascribed to the decomposition of the matrix polyol and the

stabilised urea (172). Moreover, the glass transition temperature of polyacrylate/clay

nanocomposites prepared by emulsion polymerisation was improved up to 32% and

66% for the polyacrylate and co-polymer respectively at 5% clay loading as listed in

Table 12 (340).

Table 12 Glass transition temperature for different polymers based on layered silicate (340).

Polymer Tg (°C)

Poly(n-butyl methacrylate)

PBMA 36.1

PBMA/1 45.9

PBMA/3 45.6

PBMA/5 47.5

poly(n-butyl methacrylate-co-methylmethacrylate)

CPBM/90-10/5 48.3

CPBM/80-20/5 55.5

CPBM/70-30/5 59.4

CPBM/60-40/5 60.1

Table 13 DSC values for base polymer and nanocomposites series (260).

Samples Tg(soft) Tm(soft) Tm(hard)

TBU -71 21.91 97.50

TBU1A -71 22.72 97.40

TBU3A -69 28.78 97.00

TBU5A -70 28.50 97.84

TBU7A -70 28.82 95.65

TBU9A -70 28.17 95.75


73 | P a g e

In addition, the thermal properties of the polyurethane matrix and the corresponding

nanocomposites were studied by Barick and Tripathy (260). As the matrix has two

segments, soft and hard, it has two melting points. It was found that the Tg and Tm of

both segments of nanocomposites were improved compared to the base polymer, as

presented in

Table 13.

Figure 32 TGA DTG of neat polymer and nanocomposites series under (a) nitrogen and (b) air (260).

Reproduced from Barick by permission of Elsevier Science Ltd., UK.

In the soft segment of nanocomposites, the enhancement in thermal properties can be

traced to the addition of layered silicate which helps to stiffen the soft segment as well

as limiting the movement of it. Figure 32 represents the TGA and DTG of pure polymer

and nanocomposites series under (a) nitrogen and (b) air. In nitrogen, it can be seen

that the onset temperature of the nanocomposite is higher than pristine matrix. It was

reported that the first degradation of the material under nitrogen can be traced to the

release of small molecular or unstable chain. The second one was ascribed to the


74 | P a g e

degradation of the matrix backbone. The residue was obtained at 500°C and enhanced

by the increasing of layered silicate.

TGA and DTG in air represented four degradation steps. The first was attributed to the

division of the urethane linkage to shape alcohols and isocyanates. The decomposition

of the polyol chain in O2 gas to yield volatile gaseous products (aldehydes, water,

carbon dioxide, etc) was the second stage of decomposition. This will help to hinder

the formation of metastable products which explains the decrease in their DTG peak.

The third stage was assigned to the prevention of completed volatilisation of the

resulting alcohol and isocyanates chain by dimerisation of isocyanates to carbodiimdes

which interact with the alcohol combinations to provide the stable urea that

decomposed. Higher amounts of residue in air were found at 400°C compared to the

nitrogen test. This was attributed to the formation of the metastable oxidation product

at that temperature. The last degradation was traced to the degradation of polymer

backbone.

Table 14 Chemical combinations and molecular weights of poly(HB/PCL-PEG-PCL) urethanes (354).

Sample code

Compositions a Mn

b/g

/

mol

Mw/Mnb

PHB/

wt.%

PCL/wt.

%

PEG/wt.

%

Connect

unit/wt.

% UPCL-PEG-PCL-HB30 (A1) 29.1 52.3 11.8 6.8 41.50

0

2.4

UPCL-PEG-PCL-HB50 (A2) 49.5 36.4 7.8 6.3 37.20

0

2.3

UPCL-PEG-PCL-HB70 (A3) 68.9 20.7 4.6 5.8 31.00

0

2.6 a Determined from 1 HNMR spectra, expected error 3%.

b Determined from GPC in CHCl3 at 30 °C.

A recent study on the thermal stability behaviour under nitrogen of poly(PHB/PCL-

PEG-PCL) urethanes based on organic MMT modification was carried out by Naguib

et al. (354). TGA and DTGA tests were used to evaluate the samples. Different

polymer compositions were utilised as can be seen in Table 14. Figure 33 represents the

TGA and DTGA curves of the A2 neat sample and the corresponding nanocomposites

samples of 5% and 10% modified MMT. It can be seen that the addition of layered

silicate to the polymer matrix can improve the thermal stability. The thermal

degradation of A1, A2 and A3 neat as well as nanocomposites samples are listed in

Table 15. The char yield was improved by the enlargement of clay loading. The

improvement in thermal stability was ascribed to the nature of layered silicate that


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acted as a barrier by hindering the permeability of volatile decomposition samples out

of the materials.

Figure 33 (a)TGA and (b) DTGA curve of A2 and A2/Mt-PCL nanocomposites at heating rate of 10

K/min. (354). Reproduced from Naguib by permission of Elsevier Science Ltd., UK.

Table 15 The thermal degradation of polyurethane samples and corresponding nanocomposites (354).

Sample T5%

°C

T1P

°C

T2P

°C

Tf

°C

Char yield

%

A1 228 260 299 386 2.9

A1/5 Mt-PCL 233 259 304 394 4.3

A1/10 Mt-PCL 238 272 325 395 7.1

A2 225 263 298 388 2.3

A2/5 Mt-PCL 234 270 303 410 4.7

A2/10 Mt-PCL 239 276 323 425 4.1

A3 203 256 - 391 2.7

A3/5 Mt-PCL 209 267 - 314 4.2

A3/10 Mt-PCL 220 267 - 403 7.6

Another study revealed that the total heat of reaction (ΔH dyn) of polyester

nanocomposites was decreased by the presence of clay with respect to neat polymer.


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The reduction was assigned to the prevention of the cross-linking by the intercalation

of the polymer and layered silicate which lowers the reactivity (309).

Moreover, many other polymers showed an enhancement in thermal stability by the

addition of layered silicate such as PS (355), PE/EVA/o-MMT (356), PLA (357, 358),

PCL (20, 359), epoxy (360) nanocomposites.

Apart from the previous polymers that showed little or significant improvement in

thermal stability, many studies represented a reduction or no improvement of the

thermal stability of nanocomposites compared to virgin polymer, for example, PA6

nanocomposites (361-363).

There was an argument about the reason for the reduction of thermal stability for the

nanocomposites material. For example, many researchers traced the reduction to the

aggregation of the clay layers as acting as a source of heating during the degradation

accompanied by a heating from outside (16). Furthermore, the treated clay includes the

alkylammonium cations which can be affected by the decomposition that follows the

Hoffmann elimination interaction. Thus the product can help the degradation of the

polymer. Also, the nature of the clay can distort the polymer as mentioned before.

Clearly, the treated layered silicate can follow two mechanisms. One of them is the

protection of the polymer by acting as a barrier which leads to enhancing the thermal

stability of nanocomposites. The second one is the effect of the clay on the degradation

of the polymer which reduces the thermal stability (285). In addition, There are many

parameters that can affect the thermal stability of the nanocomposites such as clay

loading (23, 317, 364, 365), and nanocomposites morphology (302).

2.8.4 Flame retardance

The applications of polymer based on layered silicate are getting expandable day after

day, and the flame retardance behaviour is very important for these applications. The

flame retardance property in conventional composites can be enhanced by either

utilising flame retardant polymers (PVC, fluoro-polymers) or by introducing flame

retardance fillers (aluminium-trihydrate, Mg(OH)2, organic brominated composites,

intumescent systems). The side effect of these fillers is that they provide the material

with many downsides. For instance, high density, lack of flexibility, low mechanical


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properties, environmental impact and the cost, can all be counted as major drawbacks

that such fillers cause (23, 58).

The method that is widely utilised to examine the flame retardance action of

nanocomposites samples and pure polymers is cone calorimetry which offers very

helpful information. The standard parameters for this method are ASTME 1354 and

ISO 5660 and the major principle of this technique is the O2 consumption. This method

expresses the relationship between the O2 mass that is consumed in air and the matrix

heat release within the consumption. The sample is exposed to a flux heat and then

many properties can be obtained. For example, the rate of heat release (HRR), heat

release peak (PHRR), period to ignition (TTI), complete heat released (THR), mass

loss rate (MLR), the average of CO yield and specific extinction area (23, 67).

Figure 34 HRR in a function of temperature of EVA, EVA-NC0 (EVA + polymeric modifier (20%)) and

EVA-NC5 (EVA+ polymeric modifier + 5.6 wt.% MMT) (291). Reproduced from Shi by permission of

Elsevier Science Ltd., UK.

The study of ethylene vinyl acetate/clay nanocomposites was carried out by Shi et al.

(291). Figure 34 represents the HRR plot of EVA and the corresponding

nanocomposites. The samples showed two different peaks which were at 351-396°C

and 485-499°C. The first peak was attributed to the combustion of vinyl acetate units.

The second peak was traced to the combustion of polyene backbone as well as the

ethylene units. It was observed that the addition of layered silicate into the EVA matrix

HRR

(°C)


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led to decrease the HRR value. In addition the THR which helps to maintain the

amount of fire hazard was decreased by the addition of layered silicate up to 16%.

Moreover, the authors revealed that the intercalation and exfoliation nanocomposites

structure exhibited almost the same amount of HRR and THR which indicated that the

flame retardance properties did not depend on the nanocomposites structure like other

properties.

Wang et al. (366) used a reactive phosphorous beside the layered silicate 5% based on

epoxy resin. The particles provide an enhancement in the limiting oxygen index, so

these fillers help to improve the flame retardance properties. The authors traced the

improvement of the flame retardance properties for nanocomposites compared to the

neat polymer to the synergism influence of phosphonium ions and the layered silicate.

Also, the exfoliation structure of nanocomposites that was obtained helps to improve

their properties. Moreover, an enhancement in the flame retardance behaviour of

nanocomposites can be obtained by the addition of epoxy carbon fibre composites

beside the layered silicate (367).

There are many studies that revealed an improvement of flame retardance for the

nanocomposites with respect to the neat matrix. For instance epoxy/clay (168, 368), PS

and PU based on a polymeric flame retardant and layered silicate (369), PLA/MMT

and Sepiolite (298), PP/ organoclay (370), PE / organoclay (285, 371), and EVA /

organoclay (371, 372) nanocomposites.

From the preceding information it can be easily understood that the reason for the

enhancement of the flame retardance properties for nanocomposites materials is

because the char formation acts as a barrier to protect the matrix. Also, to reduce the

heat and mass transfer from the flame to the polymer (23, 58) as well as slowing down

the matrix O2 uptake accompanied with the reduction of volatiles (23, 58, 328, 373-

375). In the polymer matrix nanocomposites based on layered silicate, the char

formation can be seen as a ceramic layer after the combustion of silicate layers (328,

356, 374). The char formation which was counted as a ceramic layer was obtained

from the intercalated nanocomposites structure as examined via XRD and TEM (373).


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In conclusion, although the satisfactory testing of the flame retardance on the

nanocomposites samples was in place, many reports need to be done in order to find

out the usage and limitation of these materials in a practical way (58).

2.8.5 Optical property

The traditional composites usually exhibit a dark colour to the final product which

decreases the optically property (376). However, the nanocomposites normally do not

affect the optical property of the materials. This is attributed to the size of the sheets of

the layered silicate which is 1 nm thick. Thus, the incorporation of layered silicate into

the polymer does not have a strong influence on the clarity of the samples (11, 328).

Also, it was observed that the nanoparticles influence the overall optical properties;

however, the effect is still reasonable with respect to conventional composites. In

addition, at small amounts of layered silicate (i.e. 1-3 wt.%), there is not much change

in colour and optical properties. The exfoliation structure of nanocomposites being

optically clear which is traced to the thickness of each clay layer that is smaller than

the wavelength of visible light (216, 377).

2.8.6 Rheological properties

In order to understand the processibility of the materials, the calculation of the

rheological properties, including dynamic oscillatory shear or steady shear, should take

place (16, 330). The study of rheological behaviour of nanocomposites based on

layered silicate is very important for two objectives. The first one is the

nanocomposites preparation sometimes includes melt processing, such as injection

moulding. The second objective is that the nanocomposites materials are sensitive to

the morphology, fillers‟ size, shape, and surface features of the incorporation phase.

Thus, the rheological properties provide better understanding of dispersion in

nanocomposites and can act as a complement to conventional methods of the materials‟

characterisation (230, 275, 330).

The study of the effect of layered silicate on the EVA and EVA-g-MA matrix by using

a microcellular injection mould was carried out by Hwang et al. (314). This report

revealed the relationship between the shear of viscosity and rate at 190°C. Also, the

shear thinning was investigated. It was found that the shear thinning of grafted EVA


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exhibited was influenced by the clay volume. At low shear rate, the nanocomposites

represented a clay dependent behaviour whereas nanocomposites showed clay

independent action at high shear rate. The addition of layered silicate did not change

the shear thinning but affected the viscosity extent. In the case of EVA

nanocomposites, the same shear thinning behaviour for the neat and nanocomposites

samples were observed, but the viscosity was decreased by the increasing of clay

content which was attributed to the weak bonding between the matrix and layered

silicate. The results of shear viscosity and thinning were obtained by the power low

model and Rabinowitsch correction equations respectively.

Work was done by Franchini et al. (378) about the study of the rheological properties

of epoxy based on different types of layered silicate. It was revealed that the viscosity

depended on the kind of clay or the modifiers. In addition, the processing parameters

affected the resulting mixture.

In addition, the shear thinning of PA6 nanocomposites at high shear rates was almost

the same as the virgin polymer. Moreover, the melt viscosity of the nanocomposites

was lower than the neat matrix. The authors attributed the reduction in the viscosity to

slip between the polymer matrix and the exfoliation clay sheets during the large

amount of shear rate or the reduction of the molecular weight of the nylon due to the

hydrolysis in the presence of the nanofillers (275). Likewise, the incorporation of

layered silicate into medium and low molecular weight of nylon polymer reduced the

viscosity of the material (274).

2.9 Advantages of nanocomposites based on layered silicate

From the extensive literature, the matrix nanocomposites based on layered silicate

usually show better properties compared to traditional composites. For example,

thermal stability, barrier properties, flame retardance, as well as mechanical properties

such as stiffness, strength and DMA (379). These improvements in different properties

in nanocomposites materials can be achieved by only a small layered silicate fraction

volume. Thus, these materials provide much lighter weight compared to micro or

macro composites, which make them useful in applications for some specific

requirements (16). In addition, the nanocomposites materials can be achieved at the

final stage of processing, such as injection or extrusion, by introducing the layered


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silicate which indicates that there is a preferable thermodynamic interaction between

the two phases (the polymer and clay). Therefore, a matrix based on nanocomposites is

commercially preferable because of their flexibility in manufacturing (328). Most

traditional composites have trade-off properties such as weight, mechanical properties

and cost. However, nanocomposites materials that contain layered silicate normally

have a lack of trade-off properties. Also, these materials can be created without

compromising other important properties (8, 379, 380). Apart from these advantages,

the weight of polymer based on nanocomposites has a strong effect on the recycling of

products and environmental concerns. It was agreed that the wide use of these

materials could reduce the consumption of gasoline by up to 1.5 billion litres, which in

turn decreases CO2 emissions. The barrier properties play an important role in the

advantages of nanocomposites materials. For example, the packing of food is one of

the applications for nanocomposites (381). In addition, the transparency of the

nanocomposites is the same as virgin polymer (382). Clays seems to behave as a

nucleating agent, which induces a larger level of polymer crystallinity beside the

crystallization rate (260, 323). In addition, the incorporation of layered silicate enlarges

the biodegradation of the resulting materials (209, 383). This action can be ascribed to

the hydrophilicity of layered silicate which attracts water molecules to the mixture and

ends up speeding up the hydrolytic degradation of the polymer. Another reason can be

the different pHs that occur in the mixture from the nanofillers after water absorption

which can catalyse the enzymatic activity of de-composition and provide more

biodegradable materials (384).

2.10 Nanocomposites applications

From the preceding advantages of nanocomposites materials, different possible

manufacturing applications were recommended. Due to the high enhancement of

different kinds of properties of nanocomposites such as mechanical, thermal and

barrier properties, these composites can be used for various applications in aerospace,

defence, the automobile industries, building sections, food wrapping, adhesives,

tooling, sealants, moulding, electronics, casting and construction (74, 292, 385, 386).

In addition, the nanocomposites based on layered silicate can be known as anti-

corrosive materials (338). Thus, it can be used in the coating of anticorrosive

applications on aircraft.


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Nanocomposites based on layered silicate can be used on the building of bulk-modified

sensors for the anions detection. The particles will provide the electronic conductivity

to the system. Due to the use of such a study being natural, the product will act in an

environmentally friendly way (387). In addition the nanocomposites are used in the

field of electrochemical biosensors (388).

The use of Nylon-6/MMT helps to improve the flame retardant properties. Thus, this

kind of nanocomposite was found to be used in the floor sheet of the vehicles of

Japanese railways (389).

A combination of thermoplastic, olefin-layered silicate nanocomposites was designed

to be used for an external automotive body part (step-assistance). The utilisation of

2.5% layered silicate level was put in place. The weight of this product was much

lighter than normal composites by up to 10 times. This study was announced by

General Motors and partners Basell, Southern Clay Products and Blackhawk

Automotive Plastics. The reduction in weight was up to 20% of conventional

composites which depend on the nature of parts and materials that were substituted by

that combination of nanocomposites (8, 380).

2.11 Chapter Two summary

An extensive review which reveals recent studies in polymer nanocomposites based on

layered silicate is presented. The background, morphology, preparations and properties

of these materials are discussed. The review covers and discusses various modern

methods and equipment in the preparation of nanocomposites, deeply focussing on the

morphology of nanocomposites and its effect on the overall properties. Although

nanocomposites based on layered silicate have been known for a long time, they have

received far greater attention nowadays. This is because of the interesting study by the

Toyota Research Group into enhancing the properties of PA6 nanocomposites. It is

likely that this examination by Giannelis and his co-workers (84) enabled the creation

of nanocomposites by means of a simple melt-stirring of the polymer and clay. In

various conditions, the layered silicate must be treated before mixing with the polymer.

This is because it needs to be mixable with most polymers. The outcomes of different

structures, whether they form a nanocomposite or not, rely upon many factors. The

polymer, clay source and modification, and the type of preparation, are all factors that


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have an influence. With regard to the improvement of properties, most of the

exfoliation structure of nanocomposites gains by enhanced strength and modulus. Also,

the barrier properties, storage of modulus, thermal stability and flame retardance have

been improved, according to many reports. By contrast, there are concerns regarding

some properties such as elongation and toughness. In addition, some properties were

improved more by the intercalation structure compared to exfoliation nanocomposites,

such as toughening and impact. The aggregation of layered silicate imparted side

effects of the resulting properties such as in flame retardance where the aggregation

layers acted as a source of heating. Even though the number of nanocomposites papers

expand day by day, some reduction of the properties have not yet been understood.

Furthermore, the effect of the layered silicate reinforcement in the water absorption

behaviour seems to be not fully investigated in the current literature. Also, the

relationship between morphologies structure and its effect on the properties is not fully

studied and discussed.

Chapter 3. Experimental procedures

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This chapter will detail the materials and the processes that were used in the production

of the neat vinyl ester and the corresponding nanocomposites samples. Tow sample

fabrication methods are presented. The characterisations and testing methods that are

employed on the specimens are also presented in this chapter.

3.2 Materials

3.2.1 Matrix

The matrix material used in this study was vinyl ester, which was purchased locally

from GRPMS and coded as AME 6000 T 35. Vinyl esters are chemically similar to

both unsaturated polyesters and epoxy resins. They were developed as a compromise

between the two materials, providing the simplicity and low cost of polyesters and the

thermal and mechanical properties of epoxies. Vinyl ester resin, a thermosetting

polymer, possesses high mechanical properties and superior resistance to moisture and

chemical attack than other polymer resins. Also, it offers excellent resistance to acids,

alkalis, hypochlorites, and many solvents. These vinyl esters have higher density cross

linking sites available, which leads to a more heat resistant polymer network. Vinyl

esters are unsaturated resins made from the reaction of unsaturated carboxylic acids

(principally methacrylic acid) with an epoxy such as a bisphenol A epoxy resin (390).

The typical structure of a vinyl ester resin is depicted in Figure 35.

Figure 35 The structure of Vinyl ester (390).

3.2.2 Reinforcement

The layered silicate that has been used is Cloisite® 10A which is classified as a natural

montmorillonite that is modified with a quaternary ammonium salt. It is a plastic

additive used to improve different physical properties such as barrier, flame retardance


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and reinforcement (391). The physical, mechanical, and thermal properties of

Cloisite® 10A are presented in Table 16.

Table 16 Reinforcement properties (392).

Physical, mechanical, thermal properties Parameters Comments

Specific Gravity 1.90g/cc -

Bulk Density 164 kg/m

3

Loose 265 kg/m

3

packed

Loss in Ignition 39.0 %

- Particle Size 2.00µm

10% Hardness, Shore D 83 5% Cloisite® reinforced Nylon 6

Tensile Strength, Ultimate 100.67 MPa 5% Cloisite® reinforced Nylon 6

Elongation at Break 8.00 % 5% Cloisite® reinforced Nylon 6

Modulus of Elasticity 4.67 GPa 5% Cloisite® reinforced Nylon 6

Flexural Modulus 3.78 GPa 5% Cloisite® reinforced Nylon 6

Izod Impact, Notched 0.686 N/m 5% Cloisite® reinforced Nylon 6

Deflection Temperature at 0.46 MPa (66 psi) 96 oC 5% Cloisite® reinforced Nylon 6

Moisture Content 2.00 % -

X-Ray Diffraction d-Spacing (001) 19.2 Angstroms -

3.3 Mould geometry

A 200 mm x 148 mm x 3 mm mould was used to fabricate nanocomposite samples as

shown in Figure 36 (a). This is a simple shape which allowed the material to fit in and

be taken out easily. Also, the manufacturing of this mould is easy and does not take a

long time. The thickness that has been used in this mould is under a British Standard

(BSI 2746:1998) for composite plastic materials, which is 3 ± 0,2 (393). Also, for the

tensile test, the minimum total length of the specimen in (BSI 2747:1998) is 150 mm,

which matches the value in this mould (394). In addition, another mould was used

which had dogbone-shaped geometry that was designed to get sample strips directly

suitable for tensile testing. However, these samples became stuck in the mould which

affected the surface and in some cases they were broken as can be seen in Figure 36

(b).


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Figure 36 Mould geometry, (a) Flat shape; (b) Dog-bone shape.

3.4 Concentrations of layered silicate for nanocomposites preparation

The information of different concentrations of layered silicate is presented in Table 17.

Table 17 The concentrations of layered silicate used in the fabrication of nanocomposites.

Concentrations Fabrication type 1 samples Fabrication type 2 samples

Neat Vinyl ester Preparation of Vinyl ester only. Preparation of Vinyl ester only.

VE + Nanoclay Mixing Vinyl ester with 1 wt. % nanoclay. Mixing Vinyl ester with 0.5 wt. % nanoclay.

VE + Nanoclay Mixing Vinyl ester with 2 wt. % nanoclay. Mixing Vinyl ester with 1 wt. % nanoclay.

VE + Nanoclay Mixing Vinyl ester with 3 wt. % nanoclay. Mixing Vinyl ester with 1.5 wt. % nanoclay.

VE + Nanoclay Mixing Vinyl ester with 4 wt. % nanoclay. Mixing Vinyl ester with 2 wt. % nanoclay.

VE + Nanoclay Mixing Vinyl ester with 5 wt. % nanoclay Mixing Vinyl ester with 2.5 wt. % nanoclay

3.5 Samples preparation procedures

Vinly ester /nanocomposites samples were prepared using the following two

techniques. The reasons for selecting two systems were to analyse the concentration of

nanoparticle sizes in dispersion, which is a critical factor in nanocomposite properties

as well as to find out the effect of the processing parameters on the overall peoperties.

3.5.1 Type 1 sample fabrication process

The flowchart of producing the neat vinyl ester and the corresponding nanocomposites

of first sample fabrication is presented in Figure 37.

a b


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Figure 37 Flow chart of the first sample fabrication.


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3.5.1.1 Fabricating of neat Vinyl ester samples

The quantity of Vinyl ester resin used for each composite sample was 150 g. The Vinyl

ester resin was evenly mixed for 3 minutes with 1.5% of the curing catalyst (Methyl

Ethyl Ketone Peroxide) MEKP for rapid curing. A frekote agent was applied on the

mould to smooth the surface of the sample and to help the sample to be taken out

easily. Then the resin was poured into the mould in a zigzag motion, to ensure

sufficient resin in all places. After the top mould was closed, the sample was inserted

into the compression moulding machine for 1.5 hours at 60 °C.

3.5.1.2 Fabrication of nanocomposites samples

Prior to this process, the layered silicate was dried for 3 hours at 120 °C in Heraeus fan

assisted oven in order to eliminate any exceeds moisture content which was around

3%. The vinyl ester resin was mixed with various concentrations of nanoclay at room

temperature using a mechanical mixer (2000 rpm) in an ultrasonic bath for 2 hours. A

degassing process was applied in the mixture for 3-4 hours then it was left overnight in

order to get rid of the remaining air bubbles naturally. A curing agent (MEKP) was

added to the mixture (1.5%). A frekote mould release was utilised in order to easily

take out the samples. The mould was closed and the composite panel was left to cure in

a hydraulic press at a temperature of 60 °C and at a compaction pressure of 1 MPa for

2 hours. The concentrations of the layered silicate were 0, 1, 2, 3, 4, 5 wt.%.

3.5.2 Type 2 sample fabrication process

Another processing parameters were used in order to investigate the effect of these

parameters on both the level of intercalation between the layered silicate and polymer,

and the overall properties. A flowchart of the fabrication process is presented in Figure

38.


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Figure 38 The second sample fabrication

3.5.2.1 Fabricating of neat Vinyl ester samples

In order to make neat vinyl ester panels, the vinyl ester was directly mixed with the

curing agent (MEKP) (mix ratio 1.5%) and then was poured in a steel mould. The

mould was closed and the composite panel was left to cure at ambient temperature

(20°C) for 24 hours.

Type 2 Samples

Neat vinyl ester

Mixing with MEKP (1.5 %)

Teflon (Mould release)

Pour into the mould

Compression molding machine

(1.5 hr at 60 °C)

Post curing (3 Hours at 80 °C)

Cutting the samples

Characterisations

Applying various tests

Vacuum degassing (3-4 Hours)

Nanocomposites

Mixing the layered silicate with the polymer (1 Hour and 1500 rpm)


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3.5.2.2 Fabricating of Nanocomposites samples

The vinyl ester resin was mixed with various concentrations of nanoclay at room

temperature using a mechanical mixer in an ultrasonic bath for 1 hour. A degassing

process was applied in the mixture for 3-4 hours. A curing agent (MEKP) was added to

the mixture (1.5%) with further gentle mixing before transfer of the mixture to the steel

mould. The mould was closed and the composite panel was left to cure at ambient

temperature (20°C) for 24 hrs. The clay loadings of this fabrication lot were 0, 0.5, 1,

1.5, 2, and 2.5 wt.%.

3.5.3 Post curing

After all the sample fabrication steps of type 1 and type 2 were completed, all of the

samples were subjected to post curing through a fan oven for three hours at 85 0C

(Figure 39 (a)). The reason for post curing is that curing at low temperatures usually

does not happen evenly. Post curing resettles the samples uniformly and ensures that

all specimens have a complete curing. It also helps to settle the molecules and volatiles

of the composites. After the post curing, the colour of the samples changed which

indicated a fully post curing treatment (Figure 39 (b)).

Figure 39 Fan assisted oven for post curing; (a) before post curing, (b) after post curing.

3.5.4 Cutting preparation for various testing

Each test requires a specific cutting shape and dimensions. Table 18 represents the

different dimensions required for each test and characterisation. In the first, the water

jet method was used for cutting the samples, as illustrated in Figure 40 (a). However, as

the samples were brittle, the surface finish of the edges of the cut samples were

affected by the force of the water which lead to small cracks appearing, as shown in

Figure 40 (b). Thus the premature failure could exist in many samples. As a result,

(a) (b)


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another cutting method was applied which is a diamond cutting wheel (see Figure 40

(c)). This method allows more flexibility and easy to control.

Table 18 Sample dimensions for different tests and characterisations.

Test and

characterisation Dimensions

Test and

characterisation

Dimensions

XRD 15 mm by 10 mm Nanoindentation 20 mm by 20 mm

EDS 15 mm by 10 mm Impact 60 mm by 60 mm by 6 mm

SEM 15 mm by 10 mm Creep 20 mm by 20 mm by 6 mm

TEM 15 mm by 10 mm TGA and DTG Powder (8 gm)

Contact angle 20 mm by 20 mm Surface thermal

conductivity

120 mm by 80 mm

Flexural 74 mm by 15 mm by 3mm Water absorption 20 mm by 20 mm

Tensile 148 mm by 20 mm by 3

mm

Figure 40 (a) The water jet cutting process, (b) the effect of water jet method on the samples surface and

(c) the diamond wheel cutter.

3.6 The calculation of nanocomposites voids volume

Nanocomposites voids were calculated by using the following equation;

(5)

Where Vv is the volume fraction of voids, the density of nanocomposites, wc the

weight percent of clay (%), the density of clay g/cm3, wm the weight percent of

matrix (%) and is the density of matrix g/cm3. The density of nanocomposites was

calculated as following;

(6)

c a b


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3.7 Morphological Characterisations

3.7.1 Wide Angle X-ray Diffraction (WAXD)

WAXD analysis on compression-moulded specimens was used to determine the clay

intercalation and interlayer spacing utilising a Philips APD 1700 X-ray diffraction

system with Cu Kα radiation (λ = 1.542A) generated at 40 mA and 40 kV. The basal-

spacings (the d-spacing, in Angstroms, between layers) were calculated using Bragg‟s

Law.

3.7.2 Scanning electron microscopy (SEM)

The morphology of vinyl ester /nanocomposite systems was investigated in a Hitachi

S4500 SEM working at an operating voltage of 8 kV. Block faces were prepared from

each material then ultrathin sections (63 nm) were collected using a diamond knife in a

Reichert Ultracut E ultramicrotome. Plasma etching was used to preferentially remove

the vinyl ester matrix and leave the clay particles sitting proud of the surface. After

adhering to SEM stubs, a thin layer of gold/palladium was applied to the specimens

prior to examination in a Quanta 250 FEG SEM.

3.7.3 Energy Dispersive X-ray Spectrometry (EDS)

The morphology of the VE/nanocomposite structure was further examined using a Jeol

JSM 6060LV microscope working at an operating voltage of 8 kV. The degree of

dispersion between the layered silicate and the vinyl ester matrix of the nanocomposite

samples was measured using Energy Dispersive Spectroscopy (EDS), a by-product of

the back-scattered electrons off the specimen from the electron beam. The principle of

this method is that when electrons are directed at the sample, characteristic X-rays are

emitted for all atoms with an atomic number above that of Na. This enables an

elemental distribution map to be created for any element greater than Na (atomic

number 11); in this case, the Al and Si found in the layered silicate and the Cl from the

vinyl ester. By scanning the beam in television–like raster and showing the intensity of

the selected sample, a map (image) of the distribution of elements can be produced.

3.7.4 Transmission Electron Microscopy (TEM)

TEM measurements on vinyl ester/nanocomposite systems were performed using a

high-resolution transmission electron microscope (Phillips CM12 with an associated


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Gatan digital camera system). The same block faces used to produce the sections for

SEM examination were also used for TEM.

3.8 Mechanical testing

3.8.1 Flexural testing

The flexural properties (strength and modulus) of the neat vinyl ester and the

corresponding nanocomposites samples were investigated by using the 3-points

bending test process under the specification of BS EN 2747:1998 (393). A span of 48

mm length was utilised in a 30 kN load cell. The load was applied midway between the

supports. The speed of crosshead was 2 mm/minute. The neat and nanocomposites

samples were loaded until any failure was observed and then their average values were

calculated. Equation (7) represents the calculation of flexural stress. Flexural modulus

was calculated using equation (8).

(7)

Where:

f: is the flexural stress, in megapascals;

F: is the force applied, in newtons;

L: is the span, in millimeters;

b: is the width of the specimen, in millimeters;

h: is the thickness of the specimen, in millimeters.

(8)

where:

Ef: is the modulus, in megapascals;

L: is the span, in millimeters;


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b: is the width of the specimen, in millimeters;

h: is the thickness of the specimen, in millimeters;

F: is a chosen difference in force, in newtons;

d: is the difference in deflection corresponding to the difference in force F,in

millimeters.

3.8.2 Tensile testing

The tensile testing was performed to determine the tensile strength, modulus and

elongation of neat and nanocomposite samples. The tensile tests on neat polymer and

nanocomposites samples were performed at room temperature at a crosshead speed of

10 mm/minute using Zwick (Z030) accordance with BS EN 2747:1998. The tensile

test specimens were prepared by utilizing a diamond wheel cutter into rectangle beams

from the nanocomposites slabs fabricated by a compression moulding method. The

average and standard deviation were calculated at least from five samples.

The ultimate tensile strength was calculated according to the following equation (9).

(9)

where:

is the ultimate tensile strength, in megapascals,

F is the maximum load, in newtons,

b is the initial average width of the specimen, in millimetres,

h is the initial average thickness of the specimen, in millimeters

The tangent modulus was determined by using equation (10).

(10)

Et :is the tangent modulus, in megapascals;


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F1: is the change in load, in newtons ;

Lo: is the gauge length, in millimeters;

b: is the initial average width of the specimen, in millimeters;

h: is the initial average thickness of the specimen, in millimeters;

Z1: is the increase in the distance between reference points corresponding to the

difference in load F1 expressed in millimeters.

3.8.3 Nanoindentation

Nanoindentation was used to gather elastic modulus and hardness in a nanometer scale.

All the nanoindentation tests in this work were achieved by using a NanoTest apparatus

(Micro Materials, UK). A Berkovich (three sided pyramidal) diamond indenter tip was

utilised throughout. Twelve symmetrical indentations (in the form of a matrix, 30 μm

apart) were applied on each sample. The test coupons were cut from the

nanocomposites prepared by compression moulding with approximate dimensions of

20 × 20 × 3.2 mm. The specimens were then fixed onto the nano-indentation fixture

using a suitable adhesive. The parameters used for all measurements were as follows:

Initial load: 0.1 mN; the maximum load for all indents: 5 mN; loading and unloading

rate (strain rate): 2.00 mN/S; dwell time or holding time at maximum load: 5 s.

3.8.3.1 Hardness and elastic modulus.

The measurement of the resistance to the deformation or damage can be known as the

hardness (395). The calculation of the hardness during the nanoindentation test can be

achieved by determining the peak load and the project area of contact as seen in

Equation 11. The elastic modulus can be calculated by knowing the un-loading portion

of the indentation depth (Equation 12) (321).

(11)


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√

√ (12)

Where Pmax, S, Ac and V are the driving force of the indenter, materials stiffness,

contact area at the time that the material is in contact with the indenter with the load

Pmax, and the Passion‟s ratio, respectively.

3.8.4 Impact testing

The impact strength of the neat vinyl ester and the corresponding nanocomposites

samples were determined by an instrumented falling weight impact tester (Zwick

Roell, HIT230F). The annular hole diameter on the specimen fixture was

approximately 4 cm. The specimen dimension utilised for the impact test was 60 mm x

60 mm x 6 mm. The total mass (kg), work capacity (J), and the height of release of the

load (mm) were 23.11kg, 25J, and 110 mm respectively which are under British

Standard (BS 6603). This process was carried out at room temperature. The energy

absorption of the samples was calculated from the curve of the maximum force-

deformation. The incident energies were obtained from adjusting the drop height of the

impactor and calculated using typical energy equation:

E (13)

where, is incident impact energy, is mass of the impactor, is gravity and is

height.

3.8.5 Creep relaxation behaviour

The creep test was carried out with the following parameters: 60 N loads, 25°C and

60°C, and 96 hours period times. Neat polymer, 2 wt.%, and 4 wt.% nanocomposite

samples were investigated. The specimens were prepared as 20 mm x 20 mm x 6 mm.

This size sample was used in order to adequately fit in between the grip where the

diameter of grip was 30 mm. The initial and final strains of each sample were

calculated from the strain-time curve.

iE m g h


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3.9 Thermal testing

3.9.1 Thermogravimetric Analysis (TGA)

TGA was performed using a TGA Q 50 V 6.1 from TA Instruments. The samples were

placed in a platinum crucible, and heated in a nitrogen filled environment at the heating

rate of 20 °C/min from room temperature to 600 °C. The initial weights of the samples

were approximately 8 mg. The data from the test is displayed as TG (weight loss as a

function of temperature) and as DTG (derivative thermal gravimetry, weight loss rate

as a function of temperature).

3.9.2 Differential scanning calorimetric (DSC)

The neat vinyl ester polymer and the corresponding nanocomposites were examined

using a TA instrument Q100 DSC. Both the empty reference and the encapsulated pan

were inserted inside the DSC device. The specimens were heated and cooled up to 200

°C at the rate 20 °C/min. The samples were used in this test were in the form of powder

extracted from a bulk sample by a knife. The DSC test was played in an inert nitrogen

atmosphere.

3.9.3 Surface Thermal Conductivity

Quick Thermal Conductivity Meter (QTM-500) was utilised in order to examine the

amount of the conductivity of neat vinyl ester and the corresponding nanocomposites.

The temperature in a function of time log (t) curve is plotted in a linear line. If the

angle of this line is increased, the thermal conductivity of the sample is decreased;

otherwise the thermal conductivity is increased. The following equation (14) and Figure

41 represent the relationship between the temperature and the angle.

Figure 41 The principle of the thermal conductivity measurement.

t1 t2

Tem

per

atu

re (

K)

Time log (t)

T1

T2


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(14)

Where

λ= thermal conductivity of the sample (W/mK).

q= generated heat/unit length of sample /time (W/m).

t1,t2= measured time length (sec). T1, T2= Temperature at t1, t2 (K).

3.10 Environmental testing

3.10.1 Water absorption behaviours

3.10.1.1 Weight gain study

A water absorption test of neat polymer and nanocomposites was carried out by

making specimens of 20 mm × 20 mm × 3.2 mm under the specification of BS EN ISO

62:1999. Four specimens at each loading were used in order to meet the requirements

of the British Standard. All the specimens were dried in an oven at 55 °C and then were

left to cool at room temperature in a desiccator before weighing them to the nearest

0.1 mg. This process was repeated until a constant weight was observed. The weighing

of dry samples was the first step and then the samples were soaked in tap water at room

temperature. By utilising an electronic balance, the samples were taken out from the

water path at regular intervals (24 hours) and the excess water was removed in order to

measure them. Immediately, the samples were returned to the water path. An electronic

balance (JF-2004, China) with an accuracy of 0.0001 g was used in this experiment. In

order to calculate the values of water uptake and the diffusion coefficient, the

following equations were used.

(15)

where MD is the dry mass and Mt the weight of soaked samples at selected time t.

(16)

Where M∞ is the maximum water uptake (equilibrium state) and h is the sample

thickness.


99 | P a g e

3.10.2 Contact angle measurements

Contact angle measurements were utilised in order to assess the affinity of the samples

used (neat and nanocomposites) with the plasticizers (water and gylcerol). In addition,

the study of the tension surface energy can be achieved by the contact angle

measurement. The angle of the droplet can be determined by the surface tension of the

liquid where the molecules in the bulk of the pure liquid are equally dragged in every

direction by the force of next liquid molecules, ending up creating a net force of zero.

However, if the affinity between the droplet molecules and the neighboring surface

molecules is not existed, the neighboring molecules will diffuse the droplet molecules

upward its surface. Thus, the contact angle measurement can understand the

thermodynamic issue related to the tension surface energy and wetting behaviour. 3 μl

volume drops of MilliQ grade water and glycerol were deposited on the surface of neat

and corresponding nanocomposites samples with a syringe. Pictures of the drops were

acquired through a digital camera positioned on a static contact angle analyzer. The θ

of the contact angle was measured automatically from the image setup. The image of

the solvent droplet can determine the diameter, D, and the height, h, and the following

equation was utilised in order to find out the contact angle of different surfaces.

(

)

(17)

Chapter 4. Results and discussion

100 | P a g e



In this chapter, the analysing and discussion of various properties of neat vinyl ester

and the corresponding nanocomposites obtained from experimental study are

presented. The effects of the incorporation of layered silicate into the vinyl ester

matrix on mechanical, thermal and the environmental properties of two types of

samples (type 1 and type 2) are evaluated.

4.2 Results and discussion of the type 1 samples

4.2.1 Nanocomposites voids content

The voids content in nanocomposites materials is a big concern for the industrial or

engineering designs. The pre-failure and the promoting of the local deformation of the

applications can be obtained by the existence of high content of microvoids (396).

Thus the study of the parameters which influence the content voids as well as the

percentage of these values can help to improve or maintain the quality of samples

fabricated.

The voids content in the nanocomposites sample were calculated using equations (5)

and (6) and the results are presented in Table 19.

Table 19 Voids content of different nanocomposites samples.

Samples Void content

(%)

Difference in

Percentage

Vinyl ester + 1 wt.% clay 1.06 0.00






101 | P a g e

4.2.2 Characterisations of the interlamellar structure and surface morphology

4.2.2.1 Wide Angle X-ray Diffraction (WAXD)

Wide Angle X-ray Diffraction (WAXD) is a method broadly utilised in the study of

intercalation or exfoliation which additionally characterizes nanocomposites and the

study of the interaction between layered silicate and the matrix. Nanocomposites show

dramatic enhancement in properties when neat polymers are compared with polymers

based on modified layered silicate. These improvements are attributed to the sufficient

dispersal of layered silicate within a polymer matrix. A micro-composite structure is

obtained when a weak interaction occurs between the clay and the matrix. X-ray

diffraction is utilised to show the intercalation or exfoliation structures by calculating

the intergallery spacing, in order to identify the structure of the nanocomposite (207).

In this section, the amount of d-spacing of the intergallery is presented and discussed

for different clay loading nanocomposites as well as for neat polymer. Table 20

represents the change of the basal spacing (doo1 spacing) of different samples of

nanocomposites that were calculated by Bragg‟s Law using the values extracted from

the XRD curves.

Table 20 XRD results obtained from different clay loading of nanocomposites.

Samples

2θ

values

at 20°

The

interlayer

distances

(nm)

d-spacing

improvement

%

Cloisite 10 A 20.00 0.443 00.00

VE + 1 wt.% clay 17.12 0.517 16.71

VE + 2 wt.% clay 16.86 0.525 18.51

VE + 3 wt.% clay 16.22 0.546 23.25

VE + 4 wt.% clay 13.84 0.639 44.24

VE + 5 wt.% clay 16.08 0.551 24.38

From Table 20 and Figure 42, it can be seen that the nanoparticles reinforced samples

show various x-ray diffraction patterns. The 2θ value for only Cloisite 10 A was 20°

which represents 0.443 nm basal distance. The first peak at 2θ value of 17.12° (1 wt.%


102 | P a g e

clay reinforced sample) illustrates the partial intercalated d-spacing of the clay at

approximately 0.517 nm with an improvement of the d-spacing about 17% compared to

base clay. At 2 wt.% clay loading, the angle was shifted toward a lower 2θ value which

was 16.86° and represented 0.525 nm of d-spacing. By the addition of 3 wt.% layered

silicate, the 2θ exhibited less amount than the previous clay loading which was 16.22°

and displayed 0.546 nm of the interlayer spacing. The peak for the 4 wt.% clay loading

sample at 2θ value has shifted towards a lower angle (13.84°) which indicated an

intercalated d-spacing of 0.640 nm. The improvement of the interlayer spacing at 4

wt.% was about 45% compared to the basal distance of base clay. This enhancement in

d-spacing value at the 4 wt.% reinforced samples indicated that the nanocomposites

structure was intercalated or partially exfoliated nanocomposites. In addition, the

enlargement of basal distance reflected a good dispersion of the layered silicate into the

polymer matrix. The d-spacing value of the 5 wt.% clay loading was 0.551 nm of

16.08° 2θ value. This reduction of 5 wt.% clay reinforced sample compared to 4 wt.%

clay loading was attributed to less interaction between the layered silicate and polymer

due to the insufficient mixing of high viscosity mixture at high amount of clay. Thus,

agglomeration layers were observed in the nanocomposites structure.

Figure 42 XRD results of neat polymer and the corresponding nanocomposites.

0

200

400

600

800

1000

1200

1400

1600

5 10 15 20 25 30 35 40 45 50 55

Inte

nsi

ty

2 Ө

Cloisite 10A

VE + 1 wt.% clay

VE + 2 wt.% clay

VE + 3 wt.% clay

VE + 4 wt.% clay

VE + 5 wt.% clay


103 | P a g e

A clear relationship between the gallery distance and the level of dispersion of the clay

in the matrix is proved by the 2θ values. In addition, the higher amount of interlayer

distance, the more intercalated and partially exfoliated structure. Thus, the

improvement in basal spacing led to enhancing the overall properties. In this study the

4 wt.% clay loading exhibited the highest value of d001 spacing. The decreasing in d-

spacing of 5 wt.% layered silicate indicated less interaction which resulted in less

improvement of overall properties, as will be discussed in the properties section.

Figure 43 SEM images at 300 µm and 50 µm of (a) 2 wt.%, (b) 4 wt.% and (c) 5 wt.% nanocomposites.

(300 µm) (50 µm)

(a)

(b)

(c)


104 | P a g e

4.2.2.2 Scanning Electron Microscopy (SEM)

The SEM examination in Figure 43 shows clearly the distribution of the clay through

the vinyl ester for each of the three levels of loading (i.e 2,4 and 5 wt.%). As the

selected images show below, the largest clay agglomerates are of a similar size for all

three samples, being around 30 to 35 microns in size. However, their frequency

increases with increase in loading, as does the degree of infilling between them with

smaller agglomerates. It can be seen that the 2 wt.% clay loading shows non

pronounced stacked layers and uniform distribution. At higher amounts of clay such as

4 wt.%, the intercalated / exfoliated structure is observed. The SEM image of 5 wt.%

clay loading exhibited a high number of stacked clay particles compared to 2-4 wt.%

clay. The results confirm the results of the XRD curves

4.2.2.3 Energy Dispersive X-ray Spectrometry (EDS)

Figure 44 represents the dispersion of different amounts of clay into the vinyl ester

matrix. It was observed that the incorporation of the layered silicate into the polymer

matrix was fairly homogeneous with a little bit of agglomerative layers at higher clay

loading level. In addition, it was found that the increasing of the clay concentrations

led to enlarging the clay agglomeration as the viscosity was increased. The layered

silicate in Figure 44 can be seen as white points which reflected the Si element. At 2

wt.% clay loading, the dispersion of clay into the polymer matrix was uniform and no

agglomeration layers were observed at different magnifications of EDS. By the

addition of more clay (i.e. 4 wt.%), the nanocomposites structure exhibited good

intercalation although the aggregation of few layers were obtained. As seen in Figure 44

(a), the aggregation of layered silicate appeared in one side of the sample, which was

attributed to the insufficient mixing process as the viscosity was increased. In addition,

the incorporation of high amounts of clay such as 5 wt.%, led to decreasing in the

homogeneity and enlarging the aggregation and the micro-voides in the

nanocomposites structure. The black circle on the EDS image represent the high

amount of agglomeration layers at high amount of clay loading (i.e. 5 wt.%). This

explains the reduction in the d-spacing value as was calculated by XRD and confirms

the findings by XRD and SEM.


105 | P a g e

Figure 44 EDS images at different magnification (a) (55X) and (b) (550X) of 2 wt.%, 4 wt.% and 5 wt.%

nanocomposites.

4.2.2.4 Transmission Electron Microscopy (TEM)

Figure 45 shows the TEM micrographs of 2, 4, and 5 wt.% nanocomposites samples at

higher magnification (50 nm), where the bright region represents the matrix sea and the

dark lines correspond to the stacked or individual silicate layers. Indications are from

the higher magnification images that greater levels of exfoliation of the clay particles

are achieved with lower nanoclay loading. At 2 wt.% clay loading, the TEM image

indicates good dispersion of layered silicate throughout the polymer matrix.

Intercalated / exfoliated structure is obtained at 4 wt.% clay loading as seen in Figure

45. The layered silicate shows uniform distribution with a few aggregation layers. At

high amount of clay (i.e. 5 wt.%), additional dark areas are observed indicating the

stack silicate layers and insufficient uniform dispersion. TEM images conclude that the


106 | P a g e

particles lumps are increased by the incorporation of more than 4 wt.% clay loading.

This was attributed to the high viscosity of the mixture where the ability of dispersing

the layered silicate and the polymer is restricted. It is acceptable that the higher amount

of clay loading mixed with the polymer, the less exfoliated and aggregated

nanocomposites structure (315, 397). These findings support the results by XRD, SEM,

and EDS.

Figure 45 TEM micrographs at 20 nm magnification of (a) 2 wt.%, (b) 4 wt.% and (c) 5 wt.%

nanocomposites.

4.2.2.5 Characterisation summary

From the characterisation results, it can be concluded that the higher the aspect ratio of

the particles in touch with the polymer, the bigger the reinforcing influence would be.

Therefore a low amount of particles in nanocomposites is preferable in order to

influence dramatic property enhancement. XRD, SEM, EDS and TEM showed that the

intercalation/ exfoliation nanocomposites structure was relation to the clay content up

to 4 wt.%. Addition of more layered silicate resulted in increasing the stack layers as

seen in the different characterisation techniques used which will reduce the exfoliation

and intercalation structure level as also proved by equation (1) page 44.


4.2.3.1 Flexural strength and modulus

The average results obtained from the four specimens tested are represented in Table

21.The results show a significant improvement in flexural strength and modulus for

nanocomposites compared to neat vinyl ester samples. The flexural strength and

modulus for the neat sample were 56.67 MPa and 2.87 GPa, whereas for 1 wt.% the

figures were 74.16 MPa and 4.67 GPa which was an improvement of about 30.86 %

(a) (b) (c)


107 | P a g e

and 62.71 %, respectively. Further improvements in flexural strength and modulus by

the addition of 2 wt.% clay loading where the strength and modulus were increased by

41.9 % and 70.38 % respectively compared to neat polymer. The flexural strength and

modulus were improved by 50% and 76% by the incorporation of only 3 wt.% clay

loading. The ultimate improvement in both strength and modulus was observed by the

addition of the 4 wt.% clay volume where the figures were 96.09 MPa and 7.05 GPa,

which was an improvement of about 69.56 % and 145.64 %, respectively.

Table 21 Flexural test results.

Samples

Average

maximum

Force (N)

(SD %)

Flexural

Strength

MPa

(SD %)

Improvement

in flexural

strength

%

Flexural

modulus

GPa

(SD %)

Difference in

percentage

increment

%

Neat VE 127.8

(+/- 9)

56.67

(+/- 10.5) 0.00

2.87

(+/- 9) 0.00

VE + 1 wt.% clay 155.28

(+/- 8)

74.16

(+/- 09) 30.86

4.67

(+/- 7) 62.71

VE + 2 wt.% clay 160.80

(+/- 7)

80.40

(+/- 8.5) 41.87

4.89

(+/- 5) 70.38

VE + 3 wt.% clay 165.75

(+/- 7.5)

85.37

(+/- 4) 50.64

5.07

(+/- 5) 76.65

VE + 4 wt.% clay 178.18

(+/- 8)

96.09

(+/- 5) 69.56

7.05

(+/- 4) 145.64

VE + 5 wt.% clay 131.60

(+/- 5)

69.60

(+/- 10.7) 22.81

4.75

(+/- 8) 65.50

The significant improvement in the 4wt.% clay loading is attributed to the properly

dispersed layered silicate within the host polymer as proved by XRD, SEM, EDS and

TEM. As seen in theTable 21, the layered silicate had a strong effect on the resulting

properties where continuous improvements in flexural strength and modulus properties

were observed and related to the clay content up to 4 wt.% clay loading. The

improvements of the flexural properties were traced to the high aspect ratio of layered

silicate which had high interfacial interaction within the polymer matrix. In addition,

the increment in the flexural properties can be further related to the load transferred

from the matrix to the reinforcement as a result of good matrix-reinforcement adhesion.

Figure 46 and Figure 47 elucidate the relationship between flexural strength and modulus

corresponding to the clay loadings of the neat polymer and the reinforced samples.


108 | P a g e

Figure 46 The effect of clay loading on flexural strength.

Figure 47 The effect of clay loading on flexural modulus.

The enhancement in flexural modulus can be related to the stiff fillers that are

incorporated into the matrix which help to enhance the material‟s modulus (312, 398).

In addition, the layered silicate restricts the mobility of the matrix chains under load

which leads to increase the modulus of the nanocomposites. Also, the modulus can be

increased if the clay and matrix chains‟ direction is with respect to the load orientation

(399). Usually the modulus of composites increases by the incorporation of layered

silicate due to the presence of the rigid fillers and high aspect ratio where the fillers

have higher modulus than polymer. The modulus depends on the ratio of particles

modulus to matrix material modulus (400). However, the mechanical strength relay

upon the interfacial interaction between the particles and matrix as opposed to the

modulus. This fact can explain the high improvements in modulus compared to

strength. Many studies of nanocomposites revealed an increment in modulus by the

presence of the nano fillers whereas the mechanical strength is decreased (401-403). In

other words, the adhesion level of the nanocomposites structure could be more

sensitive to strength whereas the modulus does not due to the influence of the stress

concentration factor.

0

20

40

60

80

100

120

0 1 2 3 4 5F

lexu

ral

stre

ss (

MP

a)

Clay loading wt.%

0

2

4

6

8

0 1 2 3 4 5

Fle

xu

ral

mo

du

lus

GP

a)

Clay loading wt.%


109 | P a g e

Figure 48 The relationship between the voids content and the flexural modulus.

Figure 49 The relationship between the voids content and the flexural strength.

As can be seen from Table 21, at higher clay reinforcement, i.e. 5 wt.%, both strength

and modulus have been reduced. The reduction of both strength and modulus of the

5wt.% clay loading may be related to the existence of aggregation layered silicate in

the nanocomposites structure which imparts a negative effect on the overall cross-

linking properties as seen in SEM, EDS and TEM. The reason for the agglomeration

could be because of the mixing parameter, i.e. the length of time for the mixing may

not have been adequate, as the volume of layered silicate was increased. Moreover, as

the clay loading increases, the viscosity of the nanocomposites get increased as a

result; at higher clay loading, insufficient degassing will be obtained. Thus, the

microvoids and porosity that are located in the structure will face difficulty in leaving

the matrix system during the shear mixing as seen in Figure 48 and Figure 49. As a

result, less interfacial interaction between the polymer and layered silicate will be

present (342, 404, 405). The improvement in flexural properties in the nanocomposites

samples in this study is in close agreement to the study that was undertaken by Kodgire

et al. (406).

0

2

4

6

8

0 2.45 6.4 9.03

Fle

xura

l mo

du

lus

GP

a

Voids content (%)

0

20

40

60

80

100

120

0 2.45 6.4 9.03

Fle

xura

l str

en

gth

GP

a

Voids content (%)


110 | P a g e

Moreover, the comparisons between the experimental data and the calculated values

based on the experimental outputs of flexural modulus of the neat vinyl ester and the

nanocomposites are presented in Table 22 and Figure 50. The theoretical results were

calculated using equation (8). It was found that the results are equally close together

which indicates good capability of the machine and parameters used. The marginal

difference in both values were may attributed it to the error of the software or errors of

machine set up and calibration.

Table 22 Comparison between calculated and experimental results of flexural modulus.

Samples

Experimental

(Software Output)

Calculated values

based on the

experimental outputs

Flexural modulus GPa

Neat vinyl ester 2.87 2.81






Figure 50 Comparison between theoretical and experimental data of flexural modulus at different clay

content.

0

1

2

3

4

5

6

7

8

0 1 2 3 4 5

fle

xura

l mo

du

lus

GP

a

Clay loading wt.%

Experimental

Calculated values based on the



111 | P a g e

4.2.3.2 Tensile strength and modulus

Table 23 Tensile test outcomes for neat and different clay loading samples.

Samples

Average

maximum

Force (N)

SD %

Tangent

modulus

GPa

SD %

Improvement

in Tangent

modulus

%

Ultimate

tensile

strength

MPa

SD %

Improvement

in UTM

strength

%

Elongation

%

SD %

VE 895.80

(+/- 10)

1.19

(+/- 11) 0

12.53

(+/- 6) 0.00

1.02

(+/- 11)

VE + 1 wt.% clay 1159.50

(+/- 8)

1.22

(+/- 14) 2.52

17.28

(+/- 7) 37.90

0.97

(+/- 9)

VE + 2 wt.% clay 1215.67

(+/- 9)

1.46

(+/- 10) 22.68

18.99

(+/- 9) 51.56

1.10

(+/- 10)

VE + 3 wt.% clay 1257.09

(+/- 6)

1.59

(+/- 9) 33.61

19.16

(+/- 8) 52.91

0.98

(+/- 4)

VE + 4 wt.% clay 1779.26

(+/- 7)

1.70

(+/- 4.5) 42.87

27.96

(+/- 5) 123.14

0.94

(+/- 6)

VE + 5 wt.% clay 1203.16

(+/- 3.5)

1.55

(+/- 7) 29.68

20.74

(+/- 4) 65.52

0.75

(+/- 5)

Four specimens in each clay loading were tested and the averages of those outcomes

were obtained and are presented in

Table 23.

Table 23 and Figure 51 represent the enlargement of the amount of tangent modulus for

nanocomposite samples compared to the pristine one. The clay loading had a strong

influence on the tangent modulus as the results were increased by the addition of

nanoparticles. For example, 4wt.% layered silicate represents the highest amount of

tangent modulus and was increased by up to 43 % compared to the property of the host

matrix.


112 | P a g e

Figure 51 The improvement amount of tangent modulus for different clay loadings.

Figure 52 shows the relationships between the loading levels of layered silicate and

ultimate tensile strength (UTM). The nanocomposite sample shows a significant

improvement with the comparison to neat polymer. By adding just 1% clay, the tensile

strength was increased up to 38%. 123% improvement in UTM was observed by

incorporation of only 4wt.% layered silicate loading. The reasons for the properties‟

improvements were discussed in section (4.2.3.1). The effect of the voids content on

the tensile modulus and strength are represented in Figure 53 and Figure 54.These results

obtained from this current study are similar to the study undertaken by Phang et al.

(302).

Figure 52 The improvement of tensile strength properties of neat polymer and nanocomposites

The effect of clay reinforcement on the elongation behaviour of polymer

nanocomposites has not been widely studied. The elongation is normally reduced with

the addition of nanoparticles such as MMT on polymers (8, 20, 407). The elongation is

a sign to measure the ductility of the material. There is a small improvement in the

elongation with 2wt.% clay loading which may be attributed to the conformational

effect at the layered silicate-polymer interface. Other nanocomposites samples had a

0

10

20

30

40

50

0 1 2 3 4 5

The

imp

rove

me

nt

of

Tan

gen

t m

od

ulu

s (%

)

Clay loading wt.%

0

20

40

60

80

100

120

140

0 1 2 3 4 5

The

imp

rove

me

nt

of

Ult

imat

e t

en

sile

str

en

gth

(%

)

Clay loading wt.%


113 | P a g e

marginal reduction of the elongation, up to 4wt.% clay loading. The result of this study

is in close agreement to the study by Wang et al. (408) where the improvement of

tensile strength was at low volume of layered silicate and further addition of clay (i.e

more than 5%) resulted in decreasing the properties.

Figure 53 the influence of the voids content on the tangent modulus

Figure 54 the influence of the voids content on the tensile strength

4.2.3.3 Nanoindentation behaviour

Table 24 shows the average data represented from the mechanism of loading-unloading

curves of the nanoindentation tests. The relationship between the load-depth curve of

neat polymer and the corresponding nanocomposites with different concentrations of

layered silicate is presented in Figure 55. As it can be seen, the addition of layered

silicate into the matrix led to gradual enhancement of the hardness of the

nanocomposites compared to the neat polymer. The neat polymer showed the lowest

amount of hardness (0.26 GPa) and the highest indentation depth (1134.85 nm). By the

addition of only 1% layered silicate, the hardness was increased to 0.293 GPa and the

indentation depth was decreased to 1009.8 nm. An improvement of 30.8 % of the

hardness was achieved by the addition of 2 wt.% layered silicate loading to the matrix

compared to the unfilled polymer. The maximum indentation depth of the polymer

0

0.5

1

1.5

2

0 2.45 6.4 9.03

Tan

gen

t m

od

ulu

s G

Pa

Voids content (%)

0

5

10

15

20

25

30

0 2.45 6.4 9.03

Ten

sile

str

en

gth

GP

a

Voids content (%)


114 | P a g e

when reinforced with 3% layered silicate was decreased to 897 nm and the hardness

was improved to 0.375 GPa. The highest improvement of hardness was observed in the

polymer based on 4% layered silicate nanocomposites which represented 0.406 GPa

with almost 57% improvement compared to the neat polymer. This indicates that the

layered silicate was well-dispersed throughout the matrix which resulted in intercalated

or intercalated partially exfoliated nanocomposites structures and confirms the results

by XRD, SEM, EDS and TEM. The addition of 5% layered silicate to the vinyl ester

polymer represents a marginal reduction of hardness (0.375 GPa) compared to the

reinforced polymer with 4 wt.% layered silicate. However, still 5% layered silicate

represents better hardness than neat polymer with an improvement about 45%.

Table 24 The nanoindentation data extracted from the load-unload curve of the experimental

nanoindentation test.

Samples

Max depth

(nm)

(SD)

Hardness

(GPa)

(SD)

Improvement of

hardness (%)

Reduced

modulus

(GPa)

(SD)

Improvement of

reduced

modulus (%)

VES 1134.85

(± 38.63)

0.26

(± 0.020) 0

3.7

(± 0.009) 0

VE + 1 wt.% clay 1009.8

(± 63.80)

0.293

(± 0.040) 12.70

4.11

(± 0.400) 11.10

VE + 2 wt.% clay 990

(± 76.44)

0.34

(± 0.063) 30.80

4.41

(± 0.022) 19.20


(± 75.53)

0.375

(± 0.071) 44.23

4.9

(± 0.688) 32.43


(± 69.83)

0.406

(± 0.084) 56.20

5.54

(± 0.581) 49.72


(± 68.69)

0.375

(± 0.077) 44.23

5.44

(± 0.695) 47.02

The reduction of the hardness and the increasing of maximum indentation depth at 5

wt.% clay loading compared to 4 wt.% clay volume fraction were assigned to the

existence of the aggregation layered silicate in the nanocomposites formation. Thus,

the interfacial interaction between the layered silicate and the matrix will be much less

than the non-agglomerated nanocomposites structure. In addition, the aggregation

layered silicate throughout the matrix structure lead to reduce the effect of the nanoclay


115 | P a g e

particles on the nanocomposites properties. In other words, the higher the layered

silicate amount, the insufficient mixing accuracy, as the high viscosity of the material

will be presented.

Figure 55 The relationship between the load (mN) and the depth (nm) of the neat polymer and

corresponding nanocomposites samples

The enhancements of the hardness amount signify the resistance of the materials to

plastic deformation. Thus, a clear view of the results showed that the layered silicate

has a strong effect on the change of the material‟s behaviour under plastic deformation.

The curves gradually shift leftwards with enlarging the layered silicate concentrations,

which indicated that the nanocomposites samples had more resistance to the

indentation. The reason for the enhancement of the hardness of the nanocomposites

samples compared to the neat polymer was assigned to the effect of the nano fillers that

owned a high aspect ratio of their particles. The results in Table 4 ensure the effect of

the nano particles on the material‟s behaviour compared to neat resin. It can be said

that the high interfacial interaction of the high aspect ratio particles and the polymer

matrix is the key of the enhancement of the hardness values as seen in Figure 55.

0

1

2

3

4

5

6

0 200 400 600 800 1000 1200

Load

(m

N)

Depth (nm)

Neat VE

VE + 1wt.% clay

VE + 2 wt.% clay

VE + 3wt.% clay

VE + 4wt.% clay

VE + 5wt.% clay


116 | P a g e

The reduced modulus of the neat vinyl ester represented 3.7 GPa. For 1%, 2%, 3%, 4%

and 5% clay loading, the reduced modulus equals 4.11, 4.41, 4.9, 5.54 and 5.44 GPa.

The reduced modulus was related to the clay content, however by the addition of more

clay loading, i.e. 5 wt.%, the reduced modulus tended to be marginally reduced. In

addition, the calculation of elastic modulus according to equation (7), the elastic

modulus values were close to the reduced modulus, which is illustrated in Table 25 and

Figure 56. The decreasing trend of the hardness and modulus at high amount of clay

loading were unpronounced and may be traced to the level of the clay distribution.

Table 25 The theoretical and practical elastic modulus of the neat vinyl ester and the corresponding

nanocomposites.

Samples

Experimental

(Software Output)

Calculated values

based on the


Elastic modulus (GPa)







Figure 56 The elastic modulus of calculated values and experimental results at various clay content.

0

1

2

3

4

5

6

0 1 2 3 4 5

Elas

tic

mo

du

lus

(GP

a)

Clay loading wt.%

Experimental

Calculated values


117 | P a g e

4.2.3.4 Modulus comparison

Table 26 illustrates different values of modulus of three various tests which include

tensile, flexural and nanoindentation of the neat vinyl ester and various

nanocomposites samples. As seen in the Table, the flexural and nanoindentation

modulus exhibited higher amounts of modulus compared to the tensile test which may

be attributed to the direction of these loads where the load was directed along to the

mixing flow direction. Also, the polymer chains expected to be aligned in different

directions because of the shear mixing. On the other hand, the tensile load was

perpendicular to the flow direction so the modulus was a bit reduced compared to the

other tests. In the end it is worth to point out that the preparation method, load

direction, frequency used in the test and surface finish can all affect the material

modulus.

Table 26 The modulus of neat polymer and the corresponding nanocomposites for different mechanical

tests.

Samples

Modulus (GPa)

Flexural Tensile Nanoindentation

VES 2.87 1.19 3.7

VE + 1 wt.% clay 4.67 1.22 4.11

VE + 2 wt.% clay 4.89 1.46 4.41

VE + 3 wt.% clay 5.07 1.59 4.9

VE + 4 wt.% clay 7.05 1.70 5.54

VE + 5 wt.% clay 4.75 1.55 5.44

4.2.3.5 Low-velocity impact response

The enhancement in nanocomposites mechanical properties can be achieved by the

addition of inorganic nano particles. However, the impact properties are a big concern

in the field of nanocomposites where many studies noted a reduction of impact

properties by the addition of layered silicate (274). In some applications in hard

working conditions, such as slide bearings, the impact properties are the main key to

meet the requirements of these applications (409). Thus, the study of the impact

properties is fundamentally important for many polymers.


118 | P a g e

Figure 57 Zwick/Roell HIT230F drop weight impact tower and specimen clamping.

In this study, the falling weight impact tests (see Figure 57) were carried out on the neat

vinyl ester and the corresponding nanocomposites. Different clay concentrations were

used which included 2, 4, and 5 wt.% clay loading. The impact test was used in order

to analyse and evaluate the effect of the incorporation of layered silicate into the

polymer matrix. Many parameters can be calculated from this test, such as the

maximum force (N) and energy absorption (J) in a function of time (ms) curve. Four

specimens from each group were tested and the average values were calculated as seen

in Table 27.

Table 27 Impact test results of neat polymer and the corresponding of nanocomposites.

Samples Fmax (N)

Improvement

of the peak load

(%)

Energy (J)

Improvement of

the energy

absorption (%)

Neat vinyl ester 1327 + 9.00 0 1.54 + 0.38 0

VE+ 2 wt.t % clay loading 1501 + 8.80 13.11 1.83 + 0.31 18.9



It can be seen from Table 27, the incorporation of the layered silicate into the vinyl ester

matrix resulted in enhancing the impact properties. The maximum force and energy

absorption were increased up to 42% and 59.74% respectively at 4 wt.% clay loading.

The improvements of the maximum load and energy were related to the amount of the

clay loading; however, at higher amounts of clay loading such as 5 wt.%, the force and

Impactor

mass

Specimen

clamping

system


119 | P a g e

energy absorption decreased compared to the 4 wt.% clay concentration as seen in

Figure 58 and Figure 59. These results indicated that the addition of nanofillers to the

polymer matrix not only increase the strength of the output product but also increase

the toughness.

One reason for the enhancement of the impact properties can be traced to the existence

of microvoids while mixing the nano layers and the polymer. When the impact load

was applied, the microvoids initiated the shear yielding of the combinations of vinyl

ester polymer and the layered silicate throughout the whole volume and at the start of

the crack propagation. Thus, the shear yielding distributed the mechanical stress and

enhanced the strength and toughness of the nanocomposites by absorbing the energy

(410). As can be seen in Table 19, the nanocomposites voids increased proportionally to

the clay content. Maximum voids volume was found at 5 wt.% clay loading which

represented 9.03 %. The voids percentage can conclude that the acceptable voids

content regard the impact properties in the existence parameters used is to be less than

6.5 % volume. Otherwise, less interfacial interaction between the layered silicate and

the polymer will take place.

Figure 58 The relation between the work (J) and the clay loading wt.%.

Also, it may be noted that the tortuous path of the clay layers when the interface

between them took place played an important role in the distribution of the mechanical

stress applied. As concluded by SEM, the distance between particles and clay volume

has an inverse relationship where the distance is decreased by the addition of more

clay. Thus, the tortuous path is increased and the crack propagation would take a long

path. This phenomenon explains the improvement of the impact properties at 4 wt.%

0

0.5

1

1.5

2

2.5

3

0 2 4 5

Wo

rk [

J]

Clay loading wt.%


120 | P a g e

compared to 2 wt.%. As a result, the intercalation system can provide better impact

properties than the exfoliation system (80).

Figure 59 Maximum force vs. time traces of the impact test.

At 5 wt.% clay loading, the impact properties were reduced which was ascribed to the

presence of the aggregation layers where the stress concentration factor was high.

When the stress concentration factor is high, the initiation of premature failure may

happen. In addition, the microvoids have contradictory functions regarding the impact

properties. Fewer amounts of microvoids will allow the yield shielding of the applied

load to be presented. However, the high amount of microvoids (i.e. 5 wt.%) will reduce

the interfacial interaction between the polymer and layered silicate, so premature

failure will dominate. Although the reduction of impact properties at high loadings of

clay was observed, still there was an improvement compared to neat vinyl ester. This

study was in close agreement to the report conducted by Lin et al. (411) where the

addition of layered silicate into the polymer matrix resulted in enhancing the impact

properties.

4.2.3.5.1 Fragmentation characteristics

The impact fracture shapes of different samples are shown in Figure 60. As it can be

seen, the neat polymer exhibited brittle fractures as the samples were fragmented when

undergoing the impact load. The diameter of the hole presented by the impact force for

neat vinyl ester was almost 40 mm. At 2 wt.% clay loading, the nanocomposite

samples showed better impact stability compared to neat polymer. The samples were

not wholly fragmented upon impact force. The hole diameter of 2 wt.%

nanocomposites was less than neat polymer which presented about 30mm. The best

impact resistance was found at 4 wt.% clay loading where the samples showed only

0

500

1000

1500

2000

25 30 35 40

Stan

dar

d f

orc

e [

N]

Test time [ms]

Neat VE

VE + 2 wt.t% clay

VE+ 4 wt.t% clay

VE + 5 wt.t% clay


121 | P a g e

about a 25 mm hole. At 5 wt.% nanocomposites, the sample showed less stability upon

the impact load compared to 4 wt.% clay loading and exhibited about 30-40 mm hole

diameter.

Figure 60 Fragmentation characteristics of (a) neat vinyl ester, (b) nanocomposites of 2 wt.t% clay

loading, (c) nanocomposites of 4 wt.t% clay loading, and (d) nanocomposites of 5 wt.% clay loading.

4.2.3.6 Creep relaxation behaviour

The creep relaxation measurement is a key for understanding the performance of the

product and the material processing. The test can help to evaluate the material‟s solid-

like behaviour and the effect of the incorporation of layered silicate into the polymer

matrix into the creep properties. In this test, the samples were subjected to a constant

load and the deformation levels were calculated in a function of time. The stress

relaxation processes provide an insight into the viscoelastic behaviour of the material.

Table 28 The strain amount of the neat vinyl ester and the corresponding nanocomposites during the

creep test at different intervals times and temperatures.

Sample

Initial Strain

(%)

Strain At 40

Hours

(%)

Strain At 60

Hours

(%)

Strain At 80

Hours

(%)

Strain At 95

Hours

(%)

25 °C 60 °C 25 °C 60 °C 25 °C 60 °C 25 °C 60 °C 25 °C 60 °C

Neat Vinyl ester 23 17 25 21 26 22 26 22 26.3 22.5

Vinyl ester + 2 wt.% clay 17 13 17.2 13.2 17.3 13.3 17.5 13.4 18 13.6

Vinyl ester + 4 wt.% clay 14 12 14.2 12 14.4 12.1 14.6 12.2 15 12.4

a b c d


122 | P a g e

Table 28 summarises the creep relaxation behaviour of neat vinyl ester and the

corresponding nanocomposites at 25 °C and 60 °C. At 25 °C, the elastic response of

the nanocomposite samples exhibited less disturbance in terms of shear flow as seen in

Figure 61. From the Figure, it can be seen that the pristine polymer presented higher

interval time and imposed stress. The initial part of the curve is termed „creep curve‟

and the remaining behaviour is called „relaxation level‟. According to the creep data,

the strain reduction is related to the clay concentration level. The neat polymer

illustrated higher strain compared to the nanocomposite samples where the strain

started at 23% wheras 2 wt.% and 4wt % nanocomposites showed at 17% and 14%

respectively. In addition, the strain of the neat polymer started to increase after 40

hours by 2%, which was attributed to the temperature. However, the nanocomposites

were almost stable during the mechanical stress applied and the temperature. The

enhancement of the creep properties depend on many reasons, such as the level of

intercalation between the clay and polymer, the clay source, and the clay shape. The

microstructural changes in the clay suspension can also help to improve the creep

behaviour. In addition, the presence of the layered silicate helps to improve the

microphase separation, so enhancement of the elasticity took place and the reduction of

the stress relaxation process was observed. Also, the layered silicate restricted the

motion of the polymer chains which help to withstand the mechanical stress.

Figure 61 Creep relaxation behaviour of neat polymer and the corresponding nanocomposites at 25°C.

A similar test of the previous creep behaviour was undertaken at 60°C in order to

evaluate the influence of the temperature on the neat and nanocomposite samples. Table

0

5

10

15

20

25

0 50 100

Stra

in [

%]

Test time [h]

Neat VE

VE + 2 wt.% clay

Ve + 4 wt.% clay


123 | P a g e

28 and Figure 62 show the creep relaxation behaviour of the selected samples. In the

same case as the 25°C, the nanocomposites exhibited good stability under the imposed

stress. The higher temperature (i.e. 60°C) represents better creep behaviour of the

nanocomposite samples compared to 25°C, which may be attributed to the

thermodynamic barrier where the enthalpic gain is translated to entropic gain. Thus,

enhancement of the conformational links between the layered silicate and polymer took

place (412). Also, the vinyl ester may be reorienting itself into a more ordered or

compact structure resulting in a stronger cross-linking plastic at higher temperature. It

can be seen that the neat vinyl ester started to deformed at 17 % of strain enlargement,

whereas 2 wt.% and 4 wt.% clay loading represented an initial deformation at 13 % and

12 % respectively. The neat polymer showed less stability where the temperature

affected the sample and led to increase the deformation by 20.5% at 40 hours. At 83

hours, the base vinyl ester started again to deform and the strain increased by 22%.

However, the nanocomposites were exhibited almost the same deformation level at the

initial and end time.

Figure 62 Creep relaxation behaviour of pristine matrix and 2 and 4 wt.% nanocomposites at

60°C

0

5

10

15

20

25

0 50 100

Stra

in [

%]

Test time [h]

Neat VE

VE + 2 wt.% clay

VE + 4 wt.% clay


124 | P a g e

4.2.4 Thermal properties

4.2.4.1 Thermogravemetric analysis (TGA) and Derivative Thermogravemetric

analysis (DTG)

Figure 63 and Figure 64 represent the TGA and DTG analysis respectively of neat vinyl

ester matrix and corresponding nanocomposites. The addition of layered silicate into

the polymer matrix resulted in enhancement of the thermal properties. As can be seen

in Figure 63, all nanocomposites show a slight decomposition at initial temperature

which could have been attributed to the degradation of organo modifiers and the

Hofmann elimination reaction. It was revealed that the enhancement of the

compatibility between the polymer matrix and clay can be achieved by the organo

modification of layered silicate such as alkyl-ammonium cation. However, a successful

preparation of nanocomposites will be followed by a degradation of alkyl-ammonium

cation at an earlier stage which will catalyse the degrading of the polymer (413). The

low clay content of nanocomposites represents higher amount of onset decomposition

temperature. The neat polymer shows 322°C, whereas at 1, 2, 3, 4, 5 wt.% clay

loading, the onset temperature were 342°C, 335°C, 334°C, 330°C, 321°C respectively.

At higher amounts of clay, the onset temperature was slightly reduced compared to

neat vinyl ester. The addition of layered silicate into the polymer matrix has

contradicting functions regarding the thermal properties. First, the creation of barrier

protection will provide enhancement of the thermal stability. The other one is the

catalysing effect of layered silicate which will decompose the polymer and the thermal

properties will be reduced. In addition, it was suggested that the lesser amount of clay

results in enhancing the thermal properties where the effect of barrier properties will

dominate; dispersion should also be achieved. On the other hand, the greater amount of

clay led to decrease the thermal stability where the catalysing effect will dominate. The

char yield of the nanocomposites was significantly increased which was proportional to

the clay content. The enhancement in onset temperature of nanocomposites compared

to the neat polymer were in close agreement to the studies by Zhao et al. (285), Gong

et al. (414) and Lee et al. (415)


125 | P a g e

Figure 63 TGA curve of neat VE and corresponding nanocomposites.

Figure 64 DTG curve of base polymer and different concentrations of nanocomposites.

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700

Wei

gh

t (%

)

Temperature °C

Neat VE

VE +1 wt.% clay

VE + 2 wt.% clay

VE + 3 wt.% clay

VE + 4 wt.% clay

VE + 5 wt.% clay

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0 50 100 150 200 250 300 350 400 450 500 550 600

Der

iv. W

eigh

t %

/°C

Temperature °C

Neat VE

VE + 1 wt.% clay

VE + 2 wt.% clay

VE + 3 wt.% clay

VE + 4 wt.% clay

VE + 5 wt.% clay


126 | P a g e

4.2.4.2 Differential scanning calorimetry (DSC)

The DSC characterisation method was utilised in order to investigate the change in

glass transition temperature of neat and corresponding nanocomposites as seen in

Figure 65. From the DSC results, it was found that the addition of layered silicate into

the polymer matrix resulted in a slight decrease in the Tg, however 4 wt.% clay

represented almost the same as base polymer. The neat matrix exhibited 143°C and for

nanocomposites of 1, 2, 3, 4, 5 wt.% showed 141°C, 141°C, 139°C, 143°C and 142°C,

respectively. As proved by XRD and EDS, the 4 wt.% clay represented a better

dispersion level compared to other nanocomposites samples. Thus the Tg of 4 wt.%

clay content did not change and it is understood that the level of interaction between

the polymer and fillers plays an important factor in the amount of Tg. One of the

reasons that may have reduced the Tg for nanocomposite specimens, for example, is the

lesser adhesion between the nano particles and polymer matrix where the clay often

does not act as barrier protection for the matrix from the heat or restrict the polymer

chain mobility.

Figure 65 DSC curve of neat polymer and the corresponding nanocomposites.

In addition, the formation of interphase, the matrix material close to the layered silicate

surface where the properties are different from the bulk material, between the clay

sheets can affect the Tg value. The formation of interphase is traced to the plasticisation


127 | P a g e

of the polymer surfactants (401). Also, it was revealed that the addition of clay

increases the free volume of the resin which will reduce its Tg and the

plasticising effect of silica layers that are located in the polymer domains can also

influence the Tg (416). Moreover, there are many factors that may affect the Tg results,

such as curing temperature and time (250-252), the modification of clay, the level of

dispersion and the space between sheets (401). The findings of this trend in Tg results

were found to be in close agreement to the work carried out by other researchers (250-

252, 401, 417-419).

4.2.4.3 Thermal conductivity

Table 29 and Figure 66 summarise the result of thermal conductivity of neat vinyl ester

and the corresponding nanocomposites. As can be seen, the incorporation of layered

silicate enhances the thermal conductivity compared to the base polymer. The

improvement of the conductivity was almost related to the clay loading; however, at a

higher amount of clay (i.e. 5 wt.%), the thermal conductivity did not show a significant

enhancement which could be attributed to the creation of microvoids in the structure.

As a result, the thermal conductivity will be scattered and the overall conductivity is

decreased. The optimal thermal conductivity was to be found at 4 wt.% clay loading

which was ascribed to the well-dispersed layered silicate as proved by XRD and EDS.

A similar trend in the enhancement of the thermal conductivity by the addition of the

layered silicate into the thermoset polymers was reported by Choudhury, et al. (420).

Figure 66 The effect of layered silicate into the vinyl ester matrix on the thermal conductivity.

0.185

0.19

0.195

0.2

0.205

0.21

0.215

0.22

0.225

The

rmal

co

nd

uct

ivit

y (W

/mK

)


128 | P a g e

Table 29 The thermal conductivity of pristine VE and the corresponding nanocomposites.

Sample

λ

(W/mK) at

30 °C

Improvement percent of the

thermal conductivity

Neat VE 0.1978 00.00

VE + 1 wt.% clay 0.2190 10.80

VE + 2 wt.% clay 0.2194 11.00

VE + 3 wt.% clay 0.2185 10.50

VE + 4 wt.% clay 0.2221 12.30

VE + 5 wt.% clay 0.2131 07.73

4.2.5 Environmental measurements

4.2.5.1 Water absorption behaviour

Generally, three water absorption curves can be achieved for composite and

nanocomposite materials; Fickian, non-Fickian and relaxation controlled. The nature of

the polymer (chemical structure), morphology and dimensions, and the type of particles

can distinguish the type of the resulting curve (421, 422). The water absorption

behaviour of neat vinyl ester and the corresponding nanocomposites samples is

depicted in Figure 67. Fickian diffusion behaviour was obtained from the result of the

water uptake curve. As shown in Figure 67, the neat polymer was rapidly absorbing the

water in the initial exposure to the water and then slows down until the equilibrium

state is reached. The nanocomposites samples showed different trends compared to the

neat polymer as the uptaking behaviour was a bit slow in the beginning.

Figure 67 Water absorption behaviour of neat vinyl ester and nanocomposites.

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

0.00 5.00 10.00 15.00 20.00 25.00 30.00 35.00 40.00

Mo

istu

re g

ain

(%

)

Time hrs (1/2)

Neat VE

VE+1%clay

VE+2%clay

VE+3%clay

VE+4%clay

VE+5%clay


129 | P a g e

The reduction of the absorption action of the water was attributed to the shape of the

layered silicate and its high aspect ratio. In other words, the direct path of the water

molecules to the polymer matrix was distributed by the tortuous path of layers which

caused the water uptaking action to be reduced (16, 423). This tortuous path model was

proposed by Nielsen (331) where the liquid or gas can enter throughout the sample in

an undirected path as the layered silicates shaped themselves in parallel array where the

orientation is perpendicular to the diffusion path. This model was applicable for the

nanocomposites structure as the result from the high content polymer of layered silicate

led to alter the water molecules path. Thus, the water absorption is much less for

nanocomposite samples than the unfilled polymer (424).

Another reason that may explain the reduction of water absorption at high clay levels

was assigned to the effect of layered silicate on the polymer crystallinity as a

nucleating agent (425). In summary, when the layered silicate was incorporated into

the polymer matrix, two mechanisms were observed. First, the hydrophilic surface of

clay platelets acted as an immobiliser of the moisture (426). The second mechanism is

the formation of a tortuous path by the clay platelets (311, 427). As a result, the barrier

properties of nanocomposites will be enhanced and the water molecules will be

hindered (421).

Table 30 represents the maximum water uptake (M∞) as well as the diffusion coefficient

(D). From the results, the higher amount of clay loading led to enhancing the barrier

properties of the samples and reduced the water absorption. An improvement of barrier

properties of about 63% by the addition of only 1wt.% layered silicate into the polymer

matrix compared to neat polymer was observed. At 2 wt.% clay, the improvement in

barrier properties is much pronounced, which was 109%. At 3 and 4 wt.% clay loading,

the maximum water uptake was improved by 341% and 594% respectively to neat

matrix. A dramatic enhancement in barrier properties was obtained by the addition of 5

wt.% layered silicate volume which was 1266%. However, there is no relation between

the diffusion coefficient and the nanocomposites loadings (347, 399, 423). This is

because the diffusion coefficient measures the water flow of one dimension along with

the thickness. However, the moisture weight gains curve measures the whole

directional sections of the samples. Thus, a clear understanding of the water absorption

behaviour can be obtained from the maximum water uptake curve (399). As the vinyl


130 | P a g e

ester is widely used in marine applications (428), this result can be used as a proof of

its valuable usage in these types of applications. The result of this study was in close

agreement to the studies by Alhuthali et al. (423), Ghasemi I and Kord B (421), Jong-

Whan Rhim (429) and Mohan TP and Kanny K (399).

Table 30 The maximum water uptake and diffusion coefficient of pristine polymer and nanocomposites.

Samples Maximum water

uptake (M∞)

Diffusion coefficient

(D)(mm2/s)

Neat VE 4.37 1.575 X 10 -3

VE + 1 wt.% clay 2.68 (63%) 1.352 X 10 -3

VE + 2 wt.% clay 2.09 (109%) 1.575 X 10 -3

VE + 3 wt.% clay 0.99 (341%) 1.507 X 10 -3

VE + 4 wt.% clay 0.63 (594%) 1.669 X 10 -3

VE + 5 wt.% clay 0.32 (1266%) 1.582 X 10 -3

4.2.5.1.1 The effect of water absorption on the nanoindentation properties

The influence of water absorption on the nanoindentation properties of vinyl ester

based nanocomposites was investigated. The influence of water absorption on

nanoindentation was chosen due the hardness and modulus of nanoindentation could

resemble other mechanical properties. The samples were placed in the water for 60

days at approximately 50°C. The nanoindentation test was applied on the specimens

that were used for the water absorption measurement in order to find out the effect of

the water molecules on the polymer properties. It is generally accepted that the water

molecules have a strong influence on the polymer behaviour (430-432). Table 31

represents the hardness and reduced modulus of neat polymer and corresponding

nanocomposites before and after placing the samples in water. As seen in Table 31, the

water molecules reduce both the hardness and modulus of all samples compared to dry

samples. It is obvious that the neat vinyl ester sample was the most affected by the

water action compared to the nanocomposites samples. The reduction of

nanomechanical properties of vinyl ester matrix was attributed to the softening of the

samples via the water absorbing which, in fact, decreased the interfacial interaction

between the layered silicate and the polymer. Also, the influence of the water


131 | P a g e

plasticisation on the vinyl ester matrix can be one of the reasons for a reduction in the

properties (312, 433).

Table 31 The hardness and reduced modulus of neat matrix and corresponding nanocomposites before

and after water absorption.

Samples

Before placing in water After placing in water

Hardness

(GPa)

(SD)

Reduced

modulus

(GPa)

(SD)

Hardness

(GPa)

(SD)

Reduced

modulus

(GPa)

(SD)

Neat VE 0.26

(± 0.020)

3.7

(± 0.009)

0.2

(± 0.032)

3.4

(± 0.412)

VE + 1 wt.% clay 0.293

(± 0.040)

4.11

(± 0.400)

0.24

(± 0.040)

4.0

(± 0.40)

VE + 2 wt.% clay 0.34

(± 0.063)

4.41

(± 0.022)

0.28

(± 0.044)

4.3

(± 0.49)

VE + 3 wt.% clay 0.375

(± 0.071)

4.9

(± 0.688)

0.32

(± 0.070)

4.8

(± 0.66)

VE + 4 wt.% clay 0.406

(± 0.084)

5.54

(± 0.581)

0.36

(± 0.047)

5.11

(± 0.34)

VE + 5 wt.% clay 0.375

(± 0.077)

5.44

(± 0.695)

0.34

(± 0.060)

5.00

(± 0.009)

The layered silicate surface was modified in order to have less deterioration with the

water molecules. This phenomenon explains the better hardness of nanocomposites

than the neat polymer after the water absorption behaviour (see Figure 68). In addition,

as has been discussed in the water absorption section (4.2.5.1), the tortuous path that

was established by the layered silicate reduced the entrance of the water molecules

inside the matrix (434-436). There have been many studies that revealed a reduction of

mechanical properties of nanocomposites due to the up-take of the water (312, 437,

438).


132 | P a g e

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

Neat VE VE+1%clay VE+2%clay VE+3%clay VE+4%clay VE+5%clay

Before water

After water

-10.3% -12.5%

-17%

-22%

-30%

-21%

Figure 68 The hardness of pristine polymer and nanocomposites before and after water

absorption behaviour.

4.2.5.2 Contact angle measurements

In order to study the subject of the wettability and surface chemistry of the

nanocomposites materials, the contact angle measurement is widely used and has an

empirical indication. This method determines the solid-liquid-vapour system by using

Young‟s equation (320, 439). The studying of the chemistry surface of nanocomposites

is very important as is the understanding of the wetting behaviour in relation with the

nanocomposites performance. Also, to achieve more advanced and desirable

nanocomposites applications (16, 320, 440, 441). The wettability can also describe the

progress of the intercalation between the polymer and layered silicate as the clay

galleries depend on the wetting at the melt/solid interaction (442).

Table 32 Contact angle values of neat vinyl ester and the corresponding nanocomposites of both water

and glycerol.

Sample Water droplet

(θ)

Glycerol droplet

(θ)


Vinyl ester + 2 wt.% clay loading 75.58 90.5

Vinyl ester + 4 wt% clay loading 61.67 70.1


133 | P a g e

The results of the contact angle measurement according to the experiment procedures

mentioned in the experimental part for the average of four samples are illustrated in

Table 32. The θ values of both liquids showed that the neat polymer represented more

hydrophobicity behaviour which means less wettability property. The affinity of the

solvent droplet to the sample surface was related to the clay content as seen in Figure

69. This was attributed to reduction of surface free energy of the nanocomposites and

the increment of the work adhesion. In addition, the incorporation of layered silicate

resulted in increasing the component polarity of surface energy which was attributed to

the hydroxylated silicate layers that make the system as a hydrophilic compound, as

well as the surface morphology. By the addition of layered silicate into the polymer

matrix, the angle of both liquids was decreased. At 2 wt.% clay loading, the θ values of

water and glycerol were decreased by 21% and 7% respectively compared to the neat

polymer. At 4 wt.% clay loading, the contact angle values decreased by up to 49% and

38% of glycerol and water droplets respectively compared to neat polymer. It was

found in the literature that the contact angle measurement can also provide the level of

intercalation between the layered silicate and the polymer (443). Thus, the decreasing

of the θ values were proportional to the clay content which indicated the enhancement

in the level of nanocomposites intercalation which can be correlated to the

characterisation techniques that were mentioned above.

XRD, SEM, EDS, and TEM show that the good intercalation level was related to the

clay content up to 4 wt.% clay loading. As a result, contact angle measurement has

proved the results by these characterisations and also proved the findings in the

literature regarding the relationship between the contact angle measurement and the

level of intercalation.

Figure 69 Contact angle of solvents (glycerol and water) droplet of neat, 2 wt.% and 4 wt.%

nanocomposites.


134 | P a g e

4.3 Results and discussion of the type 2 samples

4.3.1 Characterisations of the interlamellar structure and surface morphology

4.3.1.1 Wide Angle X-ray Diffraction (WAXD)

Table 33 and Figure70 represent the XRD values of neat clay and the corresponding

nanocomposites. It can be seen that the addition of layered silicate into the polymer

matrix increased the basal distance. The 2θ value for only Cloisite 10 A was 20° which

indicates 0.443 nm basal distance.

The first nanocomposites sample (i.e. 0.5 wt.%) exhibits 18.60° which illustrates the

partial intercalated d-spacing of the clay at approximately 0.477 nm with an

improvement of the d-spacing about 7.67% compared to base clay. At 1 wt.% clay

loading, the angle was shifted toward a lower 2θ value which was 16.95° and

represented 0.523 nm of d-spacing. By the addition of 1.5 wt.% layered silicate, the 2θ

was shifted toward upper angle and presented 19.20° with 0.464 basal distance. At 2

wt.% clay loading, the XRD represented a reduction of layered silicate distance

compared to lower clay percentage which was 0.459 at 19.30°. Likewise, the presence

of more clay (i.e. 2.5 wt.%) the 2θ was increased and represented 19.98° which was

almost as same as the neat clay.

Table 33 XRD results obtained from different clay loading of nanocomposites.

Sample No.

2θ

values at

20°

The

interlayer

distances

(nm)

d-spacing

improvement

%

Cloisite 10 A 20.00 0.443 00.00

Vinyl ester + 0.5 wt.% clay 18.60 0.477 07.67





The improvement of the interlayer spacing at 1 wt.% was about 18.05% compared to

the basal distance of base clay. This enhancement in d-spacing value at 1 wt.%

indicated that the nanocomposites structure was intercalated. In addition, the

enlargement of basal distance reflected a good dispersion of the layered silicate into the

polymer matrix. The reduction of the basal distance by the addition of more than 1


135 | P a g e

wt.% was traced to less interaction between the clay and polymer due to insufficient

mixing (time and speed) of the high viscosity mixture at high amounts of clay. Thus,

agglomeration layers were obtained in the nanocomposites structure.

Figure70 XRD curve of Cloisite 10A and the corresponding nanocomposites.

In summary, the 1 wt.% clay loading exhibited the highest value of d001 spacing. The

decreasing in d-spacing of above 1 wt.% layered silicate indicated less interaction

which resulted in less improvement of overall properties, as will be discussed in the

next section.

4.3.1.2 Scanning Electron Microscopy (SEM)

The SEM images can easily show the level of distribution of the clay through the

polymer as seen in Figure 71. It can be seen that the 1 wt.% clay loading shows non-

pronounced stacked layers and fairly uniform distribution. At higher amounts of clay

such as 2.5 wt.%, a high number of stacked particles compared to 1 wt.% clay was

observed. The results confirm the results of the XRD.

Figure 71 SEM images at 50 µm of (a) 1 wt.% and (b) 2.5 wt.% nanocomposites.

0

50

100

150

200

250

300

350

400

450

5 10 15 20 25 30 35

Inte

nsi

ty

2 θ

Cloisite 10A (a)

0.5 wt.% clay (b)

1.0 wt.% clay (c)

1.5 wt.% clay (d)

2.0 wt.% clay (e)

2.5 wt.% clay (f)

(f)

(e)

(d)

(c)

(b)

(a)


136 | P a g e

4.3.1.3 Transmission Electron Microscopy (TEM)

The level of dispersion of 1wt.% and 2.5 wt.% into the vinyl ester matrix are

illustrated in Figure 72. It was found that the addition of the layered silicate into the

polymer matrix was fairly homogeneous at lower clay loading (i.e. 1 wt.%), however

the addition of more than 1 wt.% clay imparted the structure with agglomerative layers.

In addition, it was observed that the enlarging of the clay concentrations led to

increasing the clay agglomeration as the viscosity was increased. The bright region

represents the matrix sea and the dark lines correspond to the stacked or individual

silicate layers. At 1 wt.% clay loading, the dispersion of clay into the polymer matrix

was fairly uniform and no agglomeration layers were observed. However, at 2.5 wt.%,

the nanocomposites structure exhibited less homogeneity and enlarged the aggregation

where additional dark areas are observed indicating the stack silicate layers and

insufficient uniform dispersion. TEM images summarised that the particles lumps are

increased by the incorporation of more than 1 wt.% clay loading in this study. The

results of TEM are correlated to the XRD and SEM findings.

Figure 72 TEM micrographs at 50 nm magnification of (a) 1 wt.% and (b) 2.5 wt.% nanocomposites


4.3.2.1 Flexural strength and modulus

The average results obtained from the five specimens tested are represented in Table 34

and Figure 73. These show a significant improvement in flexural strength and modulus

for low clay concentration of nanocomposites compared to neat polymer

samples.Figure 4 represents the relationship between the flexural strength and the clay

content of the neat polymer and the corresponding nanocomposites. The flexural

strength of the neat vinyl ester was 52.78 MPa whereas by the addition of only 0.5

wt.% clay loading the strength increased up to 58.14 MPa. A further enhancement in


137 | P a g e

flexural strength up to 37.33% compared to the pristine polymer by the presence of 1

wt.% clay volume which represented 72.49 MPa. At 1.5 wt.% clay loading, the

strength was increased by 32.34% compared to the neat matrix, however the strength

was decreased compared to 1 wt.%. A further strength reduction compared to the virgin

polymer was observed by the addition of more than 1.5 wt.% ; the strength was

reduced by 6.62 % and 16.96% by the addition of 2 wt.% and 2.5 wt.% clay loading

respectively.

Table 34 Flexural test results.

Samples

Average

maximum

Force (N)

SD %

Flexural

Strength

MPa

SD %

Improvement in

flexural strength

%

Flexural

modulus

GPa

SD %

Improvement in

flexural modulus

%

VE 110.29

+/- 15

52.78

+/- 18 00.00

2.85

+/- 7 00.00

VE + 0.5 wt.% clay 126.68

+/- 14

58.14

+/- 11 10.14

3.35

+/- 18 17.54

VE + 1.0 wt.% clay 124.29

+/- 9

72.49

+/- 13 37.33

4.55

+/- 19 59.64

VE + 1.5 wt.% clay 93.03

+/- 13

69.85

+/- 8 32.34

4.68

+/- 13 64.21

VE + 2.0 wt.% clay 86.99

+/- 20

49.29

+/- 9 -6.62

4.39

+/- 7.5 54.03

VE + 2.5 wt.% clay 61.79

+/- 12

43.83

+/- 3 -16.96

4.20

+/- 5 47.36


138 | P a g e

Figure 73 The effect of clay loading on flexural strength.

The flexural modulus followed almost the same pattern where after the addition of

more than 1.5 wt.% clay the modulus was reduced as seen in Figure 74. The significant

improvement in flexural strength and modulus of the 1wt.% clay loading under the

studied process parameters is attributed to the properly dispersed layered silicate within

the host polymer as proved by XRD, SEM and TEM. If the parameters such as mixing

time and mixer speed is increased, it can be envisage that the optimal clay loading from

1wt.% to higher level. As seen in Table 34, the clay had a strong influence on the

resulting properties where the enhancement in flexural strength and modulus properties

were related to the clay content up to 1 wt.% clay loading.

Figure 74 The effect of clay loading on flexural modulus.

The improvements and reduction of the flexural properties were discussed in section

(4.2.3.1).

0

10

20

30

40

50

60

70

80

0 0.5 1 1.5 2 2.5Fl

exu

ral s

tre

ngt

h M

Pa

Clay content (wt.%)

0

1

2

3

4

5

0 0.5 1 1.5 2 2.5

Fle

xura

l m

od

ulu

s G

Pa

Clay content (wt.%)


139 | P a g e

4.3.2.2 Tensile strength and modulus

Five specimens in each loading were tested and the averages of those values were

extracted and calculated from the tensile curve and are presented in Table 35. As seen in

Table 35, the enhancement of the amount of tangent modulus and ultimate tensile

strength for nanocomposite samples compared to the pristine polymer were observed.

The clay loading had a strong effect on them as the results were enhanced by the

presence of layered silicate.

The tangent modulus of the neat polymer represented 1.08 GPa. By the addition of

only 0.5 wt.% clay, the tangent modulus was increased by 6.48% compared to the neat

polymer and showed 1.15 GPa. Further enhancement in the tangent modulus was

observed by the incorporation of 1 wt.% clay loading where the modulus was increased

up to 1.30 GPa. At 1.5 wt.% clay, the modulus was almost as same as 1 wt.% and

represented 1.29 GPa. The addition of more clay led to decreasing the tangent modulus

as the examples of 2 wt.% and 2.5 wt.% showed as seen in Figure 75. Figure 76

represented the influence of the incorporation of layered silicate into the vinyl ester

matrix on the ultimate tensile strength. A maximum enhancement in the strength was

observed by the addition of 1 wt.% clay loading. Further presence of layered silicate

resulted in decreasing the nanocomposites strength.

Table 35 Tensile test outcomes for neat and different clay loading samples.

Samples

Tangent

modulus

GPa

(SD %)

Improvement in

Tangent

modulus

%

Ultimate

tensile

strength

MPa

(SD %)

Improvement in

UTM strength

%

Elongation

%

(SD %)

VE 1.08

(+/- 11) 0

13.05

(+/- 18) 0.00

1.30

(+/- 5)

VE + 0.5

wt.% clay

1.15

(+/- 14) 6.48

16.91

(+/- 14) 29.57

1.28

(+/- 13)

VE + 1.0

wt.% clay

1.30

(+/- 10) 20.37

19.01

(+/- 11) 45.67

1.29

(+/- 9)

VE + 1.5

wt.% clay

1.29

(+/- 9) 19.44

18.50

(+/- 7) 41.76

1.30

(+/- 8)

VE + 2.0

wt.% clay

1.29

(+/- 4.5) 19.44

18.11

(+/- 9) 38.77

1.01

(+/- 14)

VE + 2.5

wt.% clay

1.27

(+/- 7) 17.59

16.81

(+/- 13) 28.81

1.04

(+/- 6)


140 | P a g e

Figure 75 The tangent modulus improvements at different clay loading levels.

In addition, the elongation is normally decreased with the presence of nanoparticles in

polymers which is a sign to measure the ductility of the material (8, 20, 407). In this

study, the elongation was a bit reduced compared to the neat polymer.

Figure 76 The ultimate tensile strength improvements of neat polymer and nanocomposites.

0

5

10

15

20

25

0 0.5 1 1.5 2 2.5

Tan

gen

t m

od

ulu

s G

Pa

Clay content (wt.%)

05

101520253035404550

0 0.5 1 1.5 2 2.5Ult

imat

e t

en

sile

str

en

gth

MP

a

Clay content (wt.%)


141 | P a g e


4.3.3.1 Thermogravemetric Analysis (TGA) and Derivative Thermogravemetric

Analysis (DTG)

Figure 77 TGA curve of neat VE and corresponding nanocomposites.

TGA and DTG of base vinyl ester and the corresponding nanocomposites are presented

in Figure 77 and Figure 78 respectively. The incorporation of layered silicate into the

vinyl ester matrix increased the thermal stability compared to neat polymer. The TGA

curve shows that the nanocomposites exhibited a marginal decomposition temperature

at the initial stage which may be traced to the effect of the clay modifiers as well as the

Hofmann elimination reaction as discussed in section (4.2.4.1).The neat polymer

represents 320°C of onset temperature. By the addition of only 0.5 wt.% clay loading,

the onset temperature was increased by 4% which represented 331°C. Further additions

of layered silicate (i.e. 1 wt.%) enlarged the onset temperature by 6%; at 1.5 wt.% clay

loading, the onset temperature reduced to 321°C. In addition, the enlargement of char

yield was related to the clay content.

0

20

40

60

80

100

120

0 100 200 300 400 500 600 700

Lo

ss w

igh

t(%

)

Temperature (°c)

Neat VE

VE + 0.5 wt.%

clay

VE + 1.0 wt.%

clay

VE + 1.5 wt.%

clay


142 | P a g e

Figure 78 DTG curve of base polymer and different concentrations of nanocomposites.

4.3.3.2 Differential Scanning Calorimetry (DSC)

The study of glass transition temperature of neat vinyl ester and the corresponding

nanocomposites was carried out by DSC measurement. Figure 79 represents the effect

of incorporation of layered silicate into the polymer matrix. The neat polymer and the

nanocomposites samples almost showed the same Tg, however the addition of layered

silicate resulted in decreasing the Tg slightly. The neat polymer represented 142°C and

for nanocomposites of 0.5, 1.0, 1.5, 2.0, and 2.5 wt.% showed 140°C, 141°C, 139°C,

140°C and 140°C, respectively. It can be seen that the level of intercalation played an

important key for the Tg where 1 wt.% clay loading exhibited the highest amount of Tg

compared to the other nanocomposites and as proved by XRD, SEM and TEM that 1

wt.% had the best intercalation level.

Figure 79 DSC curve of the neat vinyl ester and the corresponding nanocomposites.

0

0.2

0.4

0.6

0.8

1

1.2

0 200 400 600 800

Deriv

. w

eig

ht

(%/º

c)

Temperature(ºc)

Neat VE

VE + 0.5 wt.% clay

VE + 1.0 wt.% clay

VE + 1.5 wt.% clay

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

0 50 100 150 200

Hea

t F

low

(w

/g)

Temperature (°C)

Neat VE

VE + 0.5 wt.% clay

VE + 1.0 wt.% clay

VE + 1.5 wt.% clay

VE + 2.0 wt.% clay

VE + 2.5 wt.% clay


143 | P a g e

4.4 Comparison between type 1 and 2 samples

4.4.1 The processing parameters

Different processing parameters were used in order to fabricate the neat vinyl ester and

the nanocomposites samples in order to study the effect of the preparation method on

the overall properties. The following parameters were utilised as tabulated in Table 36

Table 36 Comparison between first and second sample fabrication methods.

Parameters Type 1 samples Type 2 samples Details

Clay concentrations 1, 2, 3, 4, 5 wt.% 0.5, 1, 1.5, 2,

2.5 wt.%

Different clay contents were used in

order to study the effect of clay

content on the nanocomposites

properties

Mixing Time/speed 2 hours (2000

rpm)

1 hour (2000

rpm)

Various mixing time and speed were

utilised to find out the optimal time

and speed for mixing

Degassing time

3-4 hrs +

overnight

naturally

degassed

3-4 hour only

The process of degassing was a bet

improved for nanocomposites 1 in

order to decrease the flows of the

preparation

Clay drying 24 hours at 120

°C Nil

Study the effect of humidity on the

preparation of nanocomposites

Moisture storage of

clay About 3% Nil

Measuring the storage moisture helps

to study the influence of humidity on

the resulted nanocomposites properties

Mould release Frekote Teflon -

4.4.2 Wide Angle X-ray Diffraction (WAXD) of type 1 and 2 sample

A comparison between the two fabrications regarding the X-ray Diffraction (XRD) was

investigated. Figure 80 and Figure 81 represent the XRD curves of fabrication type 1 and

2 respectively. The 2θ values are presented in Table 37.


144 | P a g e

Figure 80 XRD curve of Cloisite 10A and the corresponding nanocomposites of first sample fabrication

method.

Figure 81 XRD curve of Cloisite 10A and the corresponding nanocomposites of the second sample

fabrication method.

As can be seen in Table 37, the processing parameters had a strong effect on the

intercalation level of the layered silicate and the polymer matrix. In the second

fabrication, the addition of more than 1 wt.% clay loading led to decrease the

improvement of the distance between individual sheets of layered silicate. This

indicates the existence of aggregation layers at even small amounts of clay. On the

other hand, the first sample fabrication exhibited better intercalation levels where the

0

200

400

600

800

1000

1200

1400

1600

5 10 15 20 25 30 35 40 45 50 55

Inte

nsi

ty

2 Ө

Cloisite 10A

VE + 1 wt.% clay

VE + 2 wt.% clay

VE + 3 wt.% clay

VE + 4 wt.% clay

VE + 5 wt.% clay

0

50

100

150

200

250

300

350

400

450

5 10 15 20 25 30 35

Inte

nsi

ty

2 θ

Cloisite 10A (a)

0.5 wt.% clay (b)

1.0 wt.% clay (c)

1.5 wt.% clay (d)

2.0 wt.% clay (e)

2.5 wt.% clay (f)

(f)

(e)

(d)

(c)

(b)

(a)


145 | P a g e

improvement of basal distance of layered silicate was up to 4 wt.% clay loading. In the

literature, it was revealed that the optimal clay loading was to be at 4 wt.% clay loading

and further addition of clay will end up having aggregation layers (444-446). Thus, the

first sample fabrication, which was followed, was in close agreement to the findings of

the literature.

Table 37 XRD results obtained from different clay loading of type 1 and 2 samples.

Type 1 samples Type 2 samples

Sample No.

2θ

values

at 20°

The

interlay

er

distanc

es (nm)

d-spacing

difference

increment

%

Sample No. 2θ values at

20°

The

interlayer

distances

(nm)

d-spacing

difference

increment

%

Cloisite 10 A 20.00 0.443 00.00 Cloisite 10 A 20.00 0.443 00.00

VE + 1 wt.%

clay 17.12 0.517 16.71

VE + 0.5 wt.%

clay 18.60 0.477 07.67

VE + 2 wt.%

clay 16.86 0.525 18.51

VE + 1.0 wt.%

clay 16.95 0.523 18.05

VE + 3 wt.%

clay 16.22 0.546 23.25

VE + 1.5 wt.%

clay 19.20 0.464 04.74

VE + 4 wt.%

clay 13.84 0.639 44.24

VE + 2.0 wt.%

clay 19.30 0.459 03.61

VE + 5 wt.%

clay 16.08 0.551 24.38

VE + 2.5 wt.%

clay 19.98 0.444 00.23

4.4.3 The effect of processing parameters on the mechanical properties

4.4.3.1 Flexural properties

Table 38 represents comparisons between the first and second sample fabrications

methods regard the flexural strength and modulus. The flexural strength and modulus

of the second fabrication were improved up to 37.33% and 64.21% respectively at 1

wt.% clay loading. Further addition of layered silicate (i.e 1.5 wt.%) reduced the

mechanical properties, which attributed to the presence of agglomerative layered

silicate as proved by XRD. However, The first fabrication method showed an

enhancement in flexural strength and modulus up to 69.18 % and 145.35 %

respectively at 4 wt.%.


146 | P a g e

Table 38 Comparisons between the first and second fabrication methods regard the flexural properties.


Samples

Flexural

Strength

MPa

(SD %)

Improvement

in flexural

strength

%

Flexural

modulus

GPa

(SD %)

Improvement

in flexural

modulus

%

Samples

Flexural

Strength

MPa

(SD %)

Improvement

in flexural

strength

%

Flexural

modulus

GPa

(SD %)

Improvement

in flexural

modulus

%

Neat

VE 56.67

(+/- 10.5)

0.00 2.87

(+/- 9)

0.00 Neat VE 52.78

(+/- 18)

00.00 2.85

(+/- 7)

00.00

VE + 1

wt.%

clay

74.16

(+/- 9)

30.58 4.67

(+/- 7)

62.34 VE + 0.5

wt.% clay 58.14

(+/- 11)

10.14 3.35

(+/- 18)

17.54

VE + 2

wt.%

clay

80.40

(+/- 8.5)

31.82 4.89

(+/- 5)

70.15 VE + 1.0

wt.% clay 72.49

(+/- 13)

37.33 4.55

(+/- 19)

59.64

VE + 3

wt.%

clay

85.37

(+/- 4)

42.35 5.07

(+/- 5)

76.29 VE + 1.5

wt.% clay 69.85

(+/- 8)

32.34 4.68

(+/- 13)

64.21

VE + 4

wt.%

clay

96.09

(+/- 5)

69.18 7.05

(+/- 4)

145.35 VE + 2.0

wt.% clay 49.29

(+/- 9)

-6.62 4.39

(+/- 7.5)

54.03

VE + 5

wt.%

clay

69.60

(+/- 10.7)

22.55 4.75

(+/- 8)

65.21 VE + 2.5

wt.% clay 43.83

(+/- 3)

-16.96 4.20

(+/- 5)

47.36


147 | P a g e

4.4.3.2 Tensile properties

Table 39 Comparisons between the first and second fabrication methods in term of the tensile properties.


Samples

Tangent

modulus

GPa

(SD %)

Improvement

in Tangent

modulus

%

Ultimate

tensile

strength

MPa

(SD %)

Improvement

in UTM

strength

%

Samples

Tangent

modulus

GPa

(SD %)

Improvement

in Tangent

modulus

%

Ultimate

tensile

strength

MPa

(SD %)

Improvement

in UTM

strength

%

VE 1.19

(+/- 11) 0

12.53

(+/- 6) 0.00 VE

1.08

(+/- 11) 0

13.05

(+/- 18) 0.00

VE + 1

wt.% clay 1.22

(+/- 14) 2.52

17.28

(+/- 7) 37.90

VE + 0.5

wt.% clay

1.15

(+/- 14) 6.48

16.91

(+/- 14) 29.57

VE + 2

wt.% clay 1.46

(+/- 10) 22.68

18.99 (+/- 9)

51.56 VE + 1.0 wt.% clay

1.30 (+/- 10)

20.37 19.01

(+/- 11) 45.67

VE + 3

wt.% clay 1.59

(+/- 9) 33.61

19.16

(+/- 8) 52.91

VE + 1.5

wt.% clay

1.29

(+/- 9) 19.44

18.50

(+/- 7) 41.76

VE + 4

wt.% clay 1.70

(+/- 4.5) 42.87

27.96

(+/- 5) 123.14

VE + 2.0

wt.% clay

1.29

(+/- 4.5) 19.44

18.11

(+/- 9) 38.77

VE + 5

wt.% clay 1.55

(+/- 7) 29.68

20.74 (+/- 4)

65.52 VE + 2.5 wt.% clay

1.27 (+/- 7)

17.59 16.81

(+/- 13) 28.81


148 | P a g e

4.4.4 The effect of processing parameters on the thermal properties

The comparisons between the two fabrications methods in term of TGA and DSC were

taken place. Table 40 represents the onset temperature and the glass transition

temperature of both preparation methods.

Table 40 The thermal properties of the first and second sample fabrications process.

Type 1 samples

Type 2 samples

Samples

The onset

temperature

(°C)

Tg

(°C) Samples

The onset

temperature

Tg

(°C)

VE 329 143 VE 320 142

VE + 1 wt.%

clay 339 141

VE + 0.5

wt.% clay 328 140

VE + 2 wt.%

clay 331 141

VE + 1.0

wt.% clay 330 141

VE + 3 wt.%

clay 332 139

VE + 1.5

wt.% clay 321 139

VE + 4 wt.%

clay 331 143

VE + 2.0

wt.% clay - 140

VE + 5 wt.%

clay 327 142

VE + 2.5

wt.% clay - 140

Table 40 concludes that the processing parameters had a strong effect on the neat

polymer nanocomposites properties. The achievement of good intercalation levels

between the layered silicate and the polymer matrix will help to improve the overall

properties.

Chapter 5. Conclusions and recommendation for future research

149 | P a g e


5.1 Conclusions

5.1.1 Morphology of nanocomposites

Nanocomposites morphology information is important for a full understanding of the

solid-state structure and its correlation to their physical properties. Thus, various

characterisation techniques, including XRD, SEM, EDS, and TEM, were utilised in

order to study the morphology of nanocomposites and the correlation between the

intercalation level and the overall properties. It was found that the addition of layered

silicate into the vinyl ester matrix was uniformly distributed up to 4 wt.% clay loading.

Further addition of clay resulted in increasing the percentage of aggregation layers and

the voids content. This can explain the relationship between the parameters and the

threshold level of layered silicate content. Thus, a low amount of nanofillers in

nanocomposites is preferable in order to influence dramatic property enhancement.


In this study, mechanical testing including tensile, flexural nanoindentation, impact,

and creep of a vinyl ester matrix based on layered silicate was investigated. The

layered silicate had a strong effect on the polymer properties. The higher the amount of

clay loadings, the better the matrix properties achieved, up to 4 wt.% clay. The reason

for the ultimate improvement found at 4 wt.% was traced to the high level of

intercalation and interfacial interaction between the layered silicate and the polymer

chains, as proved by the XRD, SEM, EDS and TEM.

The ultimate improvement of flexural and tensile strength of nanocomposite

samples was at 4 wt.% clay loading, which revealed increases of 69.56 % and

123.14 % respectively compared to the neat vinyl ester. The improvement of

the strength could be related to the large interface between the fillers and matrix

due the high aspect ratio of layered silicate. Also, the mechanical load can be

diverted from the soft matrix to the stiff fillers, which improves the mechanical

strength.

The flexural and tensile modulus were also affected and increased by the

incorporation of layered silicate. 145.64 % and 42.87 % were the improvements


150 | P a g e

for flexural and tensile modulus at 4 wt.% clay loading compared to the neat

vinyl ester. These enhancements can be due to the adhesion of polymer chains

to the stiff fillers by strong physisorption forces then the polymer chains will be

as one portion with the fillers and increase the modulus.

The nanoindentation test showed that the hardness and elastic modulus

improvements were related to the amount of clay loading up to 4 wt.%. The

enhancements of the hardness amount signify the resistance of the materials to

plastic deformation. Thus, a clear view of the results showed that the layered

silicate has a strong effect on the change in the material‟s behaviour under

plastic deformation.

The impact resistance of nanocomposites, which includes the maximum force

(N) and energy absorption (J), was dramatically improved by the addition of

layered silicate, by up to 42% and 60% respectively at 4 wt.% compared to neat

polymer. The enhancements of impact properties could be related to the yield

shielding formed by the existence of microvoids. In addition, the tortuous path

of the layered silicate played an important role in the distribution of the applied

mechanical stress, if the interfacial interaction between the layered silicate and

polymer is adequate. Also, theses paths helped to act as a crack deflection.

The creep processes provide an insight into the viscoelastic behaviour of the

material. The creep relaxation process was measured at 25°C and 60°C. Under

the creep and relaxation curve at both temperatures, the nanocomposite

exhibited less disturbance in terms of shear flow compared to the neat vinyl

ester. The strain reduction of the nanocomposite was proportional to the clay

concentration level. The addition of layered silicate into the polymer matrix

resulted in increasing the microphase separation where the elasticity of material

will be increased. As a result, the reduction of the stress relaxation process is

observed. At 60°C the creep properties were better than at 25°C, which may be

assigned to the thermodynamic changes in the morphology of nanocomposites

where the entropy gain is compensated by an enthalpy gain; in turn, the

increment of the linkage between the layered silicate and the polymer took

place. Also, the vinyl ester could be reorienting itself into a more ordered or

compact system resulting in a stronger cross-linking plastic at higher

temperature.


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Thermal properties such as TGA, DSC and surface conductivity represented an

improvement in the overall properties of the nanocomposite compared to the base

polymer, which was attributed to the presence of exfoliated clay particles.

TGA measurements showed that the onset temperature of the

nanocomposite was higher than the neat polymer. The low clay loading up

to 3 wt.% exhibited better onset temperatures than the high clay loading,

which was attributed to the creation of barrier sheets and a reasonable

amount of organic modifier. In higher amounts of clay, the amount of

organic modifier is predominant, so the catalysing effect of layered silicate

took place. The char yield was increased by the addition of layered silicate.

DSC revealed that the glass transition temperature of the nanocomposite

was decreased compared to the neat polymer; however, at 4 wt.% clay

loading, the Tg was the same as the neat polymer, which was traced to the

good dispersal of the nanocomposite morphology. The reduction of Tg

could be related to the free volume of resin, interphase, curing time, and

level of intercalation.

The improvement of the conductivity was almost related to the clay loading.

At a higher amount of clay (i.e. 5 wt.%), the thermal conductivity did not

show a dramatic enhancement, which could be traced to the creation of high

number microvoids in the structure.

5.1.4 Water absorption behaviour

This study presents the first efforts to evaluate the water absorption behaviour and its

effect on the hardness and elastic modulus of nanocomposites. The nanoindentation

test results show that the addition of layered silicate into the polymer matrix reduced

the entrance of water molecules within the surface. The concentration of layered

silicate shows an almost exponential decrease in equilibrium water uptake behaviour

for all nanocomposite samples compared to the neat vinyl ester sample. The reduction

of water uptake for the 5 wt.% sample was recorded at almost 13 times lower than that

of the neat sample. However, the diffusion coefficient (rate of water diffusion) was not

affected by the degree of clay loadings. Also the hardness and modulus of both virgin


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polymer and nanocomposite was reduced after the water absorption; however, the

higher amount of layered silicate led to a lesser reduction in hardness. The hardness of

5 wt.% nanocomposite was reduced by 10%, however a reduction of 30% in hardness

of the neat polymer was observed.

5.1.5 Surface energy characteristics

Studying the wettability and surface behaviour of the nanocomposite and the neat

polymer by contact angle measurement showed that the addition of layered silicate into

the polymer matrix reduced the angle of both water and glycerol droplet. The neat

polymer provided a hydrophobicity behaviour which means less wettability property as

the angle of water and glycerol droplet were high (θ 91.7 and θ 96.7 respectively).

The enlargement of the clay content increased the affinity between the droplet and the

surface of the sample which indicated higher wettability compared to the neat polymer.

This proved the finding in the literature regarding the relationship between the level of

intercalation and the wettability.

In summary, the reduction in properties by adding more than 4 wt.% clay loading can

be assigned to following reasons:

The amount of layered silicate at more than the threshold level can lead to

decrease the enhancements of the mechanical properties which are due to the

reduction in the intercalation level.

The flaws which occurred, such as trapped air bubbles and weak boundaries in

the preparation of the nanocomposite, can be two of the important reasons for

the reduction in properties. At high clay loading, the viscosity of the mixture

will increase due to the high interfacial between the layered silicate and the

polymer, so the number of these flaws will also increase. As a result, less

homogeneity of the structure will be observed.

5.2 Recommendations for future work

This PhD research work presents effort to determine different properties of layered

silicate based nanocomposites using various characterisation and testing

techniques. Analysis of experimental results reveal a promising prospects in

layered silicate based materials and technology.


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The processing parameters could be improved in order to study the effect of the

incorporation of high clay loading into the polymer matrix on the overall

properties.

The most advanced and available characterisations in the nowadays were used in

this project which include XRD, SEM, EDS, and TEM. However further

investigation by other characterisations may be utlised such as Atomic force

microscopy (AFM) in order to study more the morphology of the nanocompoistes.

The benefit of AFM that the sample does not need special preparation and can

provide with high resolution compared to TEM.

Compare properties obtained in this study against other clay types.

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