Designing High Hard Block Content Thermoplastic ...

1

Designing High Hard

Block Content

Thermoplastic

Polyurethane (TPU)

Resins for Composite

Applications

A thesis submitted to The University of Manchester for the degree of

Doctor of Philosophy

in the Faculty of Engineering and Physical Sciences

2015

CHINEMELUM NEDOLISA

School of Materials

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TABLE OF CONTENTS

LIST OF TABLES .................................................................................................................. 9

LIST OF FIGURES .............................................................................................................. 10

ABSTRACT ........................................................................................................................... 13

DECLARATION................................................................................................................... 14

COPYRIGHT STATEMENT .............................................................................................. 15

ACKNOWLEDGEMENTS ................................................................................................. 16

CHAPTER 1 INTRODUCTION AND LITERATURE REVIEW........................... 18

1.1 INTRODUCTION ......................................................................................................... 18

1.1.1 THE RELEVANCE OF TPUs .................................................................................. 18

1.2 OUTLINE OF THE THESIS ......................................................................................... 19

1.3 AIMS OF THE RESEARCH WORK ........................................................................... 20

LITERATURE REVIEW ........................................................................ 21

1.4 POLYURETHANES ....................................................................................................... 21

1.4.1 CHEMISTRY OF PUs .............................................................................................. 22

1.5 THERMOPLASTIC POLYURETHANES (TPUs) ..................................................... 26

1.5.1 SYNTHESES OF TPUs ............................................................................................ 27

1.5.2 APPLICATIONS OF TPUs ...................................................................................... 29

1.6 FIBRE REINFORCED POLYMER COMPOSITES.................................................. 30

1.6.1 FIBRE ....................................................................................................................... 30

1.6.1.1 GLASS FIBRE ................................................................................................... 30

1.6.2 MATRIX ................................................................................................................... 32

1.6.2.1 THERMOPLASTICS ........................................................................................ 32

1.6.2.2 THERMOSETS ................................................................................................. 33

1.6.3 RULE OF MIXTURES IN DETERMINING THE PROPERTIES OF FIBRE

REINFORCED COMPOSITES ......................................................................................... 33

1.6.4 APPLICATIONS OF FIBRE REINFORCED COMPOSITES ................................ 36

1.7 STRUCTURE-PROPERTY RELATIONSHIPS OF TPUs ...................................... 36

1.7.1 EFFECT OF CHAIN EXTENDERS ........................................................................ 37

1.7.2 EFFECT OF HARD SEGMENT CONCENTRATION ........................................... 41

1.7.3 EFFECT OF ANNEALING ON THE THERMODYNAMIC, STRUCTURAL,

THERMO-MECHANICAL AND MECHANICAL PROPERTIES OF TPUs ................. 45

1.7.4 THERMODYNAMIC, STRUCTURAL, THERMO-MECHANICAL AND

MECHANICAL PROPERTIES OF TPU COMPOSITES ................................................ 48

CHAPTER 2 MATERIALS AND METHODS ......................................................... 53

2.1 MATERIALS .................................................................................................................. 53

2.1.1 CHEMICALS USED ................................................................................................ 53

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2.2 SYNTHETIC - CASTING - MOULDING PROCESSES ........................................... 54

2.2.1 SYNTHETIC ROUTE .............................................................................................. 54

2.2.2 TPU FORMULATION CALCULATION ................................................................ 56

2.2.3 DESCRIPTION OF TPU SYNTHESIS ................................................................... 62

2.2.4 SOLVENT CASTING .............................................................................................. 65

2.2.5 COMPRESSION MOULDING ................................................................................ 66

2.3 EXPERIMENTAL TECHNIQUES .............................................................................. 69

2.3.1 GEL PERMEATION CHROMATOGRAPHY (GPC) ............................................ 69

2.3.1.1 THEORETICAL ACCOUNT ............................................................................ 69

2.3.1.2 PARAMETERS USED ...................................................................................... 72

2.3.2 DIFFERENTIAL SCANNING CALORIMETRY (DSC) ........................................ 72


2.3.2.2 PARAMETERS USED ...................................................................................... 75

2.3.3 WIDE ANGLE X-RAY SCATTERING (WAXS) ................................................... 76


2.3.3.2 PARAMETERS USED ...................................................................................... 77

2.3.4 SMALL ANGLE X-RAY SCATTERING (SAXS) ................................................. 78


2.3.4.2 PARAMETERS USED ...................................................................................... 80

2.3.5 THERMO-GRAVIMETRIC ANALYSIS (TGA) .................................................... 81


2.3.5.2 PARAMETERS USED ...................................................................................... 82

2.3.6 TOMOGRAPHY ...................................................................................................... 83


2.3.6.2 PARAMETERS USED ...................................................................................... 85

2.3.7 RHEOMETRY .......................................................................................................... 85


2.3.7.2 PARAMETERS USED ...................................................................................... 86

2.3.8 DYNAMIC MECHANICAL THERMAL ANALYSIS (DMTA) ........................... 86


2.3.8.2 PARAMETERS USED ...................................................................................... 89

2.3.9 TENSILE TESTING ................................................................................................. 90

2.3.9.1 PARAMETERS USED ...................................................................................... 90

2.3.10 CREEP TESTING .................................................................................................. 90

2.3.10.1 THEORETICAL ACCOUNT .......................................................................... 90

2.3.10.2 PARAMETERS USED .................................................................................... 92

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2.3.11 OPTICAL MICROSCOPY ..................................................................................... 93

2.3.11.1 PARAMETERS USED .................................................................................... 93

CHAPTER 3 RESULTS AND DISCUSSION (I) TPUs .................................. 94

3.1 WEIGHT LOSS PROPERTIES .................................................................................... 94

3.2 THERMODYNAMIC - STRUCTURE PROPERTIES .......................................... 105

3.2.1 EFFECT OF CHAIN EXTENDERS AND HARD SEGMENT CONCENTRATION

......................................................................................................................................... 105

3.2.1.1 CAST TPUs ..................................................................................................... 105

3.2.1.2 MELT-QUENCHED TPUs ............................................................................. 114

3.2.1.3 SLOW-COOLED TPUs ................................................................................... 118

3.2.1.4 MOULDED TPUs............................................................................................ 121

3.2.1.5 ANNEALED MOULDED TPUs ..................................................................... 127

CHAPTER 4 RESULTS AND DISCUSSION (II) TPU COMPOSITES ..... 131

4.1 MELT VISCOSITY ...................................................................................................... 131

4.2 COMPOSITE ARCHITECTURE - TOMOGRAPHY ............................................. 134

4.3 OPTICAL MICROSCOPY ...................................................................................... 136

4.4 THERMO-MECHANICAL PROPERTIES .............................................................. 138

4.4.1 MASS AND VOLUME FRACTIONS OF TPU, POLYPROPYLENE AND

ISOPLAST COMPOSITES ............................................................................................. 138

4.4.2 DMTA OF UNANNEALED AND ANNEALED TPU COMPOSITES................ 139

4.4.2.1 2M13PD TPU COMPOSITES ......................................................................... 140

4.4.2.2 15PD TPU COMPOSITES .............................................................................. 143

4.4.2.3 16HD TPU COMPOSITES ............................................................................. 146

4.4.2.4 17HPD TPU COMPOSITES ........................................................................... 149

4.4.2.5 18OD TPU COMPOSITES ............................................................................. 151

4.4.2.6 14CHDM TPU COMPOSITES ....................................................................... 154

4.4.3 TPU COMPOSITES vs POLYPROPYLENE and ISOPLAST COMPOSITES . 157

4.4.3.1 70%HS TPU COMPOSITES vs POLYPROPYLENE vs ISOPLAST ............ 157

4.4.3.2 100%HS TPU COMPOSITES vs POLYPROPYLENE vs ISOPLAST .......... 161

CHAPTER 5 RESULTS AND DISCUSSION (III)................................................. 163

SPECIFIC TESTS ON 15PD TPUs .................................................... 163

5.1 ANNEALING STUDIES ON MELT-QUENCHED 15PD TPUs .......................... 163

5.1.1 MELT-QUENCHED 15PD TPUs (CONTROL SAMPLES) ............................... 163

5.1.2 ISOTHERMAL ANNEALING STUDIES ............................................................. 165

5.2 TENSILE PROPERTIES OF 15PD COMPOSITES vs POLYPROPYLENE vs

ISOPLAST COMPOSITES ............................................................................................... 172

5.2.1 UNANNEALED AND ANNEALED COMPOSITES ........................................... 172

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5.3 CREEP PROPERTIES OF 15PD COMPOSITES vs POLYPROPYLENE and

ISOPLAST COMPOSITES ............................................................................................... 175

5.3.1 UNANNEALED AND ANNEALED COMPOSITES ........................................... 175

CHAPTER 6 CONCLUSIONS AND FUTURE WORK ....................................... 180

6.1 CONCLUSIONS ........................................................................................................... 180

6.2 FUTURE WORK .......................................................................................................... 182

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LIST OF ABRREVIATIONS AND SYMBOLS

TPU Thermoplastic polyurethane

MDI Diphenylmethane diisocyanate

4,4’-MDI 4,4’-diphenylmethane diisocyanate

EO-PPO-EO Polypropylene oxide end-capped with ethylene oxide groups

NCO Isocyanate group

OH Hydroxyl group

CE Chain extender

12ED Ethylene glycol/1,2-ethanediol

14BD 1,4-butanediol

15PD 1,5-pentanediol

16HD 1,6-hexanediol

17HPD 1,7-heptanediol

18OD 1,8-octanediol

2M13PD 2-methyl-1,3-propanediol

14CHDM 1,4-cyclohexanedimethanol

DMAc N,N-dimethylacetamide

DABCO 1,4-diazabicyclo[2.2.2]octane

THF Tetrahydrofuran

C=O Carbonyl group

HS Hard segment

SS Soft segment

HP Hard phase

SP Soft phase

GPC Gel permeation chromatography

SEC Size exclusion chromatography

DSC Differential scanning calorimetry

SAXS Small angle x-ray scattering

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WAXD/S Wide angle x-ray diffraction/scattering

TGA Thermo-gravimetric analysis

DTG Derivative thermogravimetry

TMA Thermo-mechanical analysis

CT Computer tomography

FTIR Fourier transform infrared spectroscopy

DMTA Dynamic mechanical thermal analysis

SEM Scanning electron microscopy

w Weight-average molecular weight

n Number-average molecular weight

PDI Polydispersity index

Tg Glass transition temperature

Tm Melting temperature

TgHS Hard segment glass transition temperature

TgSS Soft segment glass transition temperature

TgHP Hard phase glass transition temperature

TgSP Soft phase glass transition temperature

TgMP Mixed phase glass transition temperature

TMMT Microphase mixing transition

TMST Microphase separation transition

TA Annealing endotherm

TM Melting transition (ordered hard segments after annealing)

ΔCp Heat capacity change

ΔCpHS Heat capacity change in hard segment

ΔCpSS Heat capacity change in soft segment

q* Scattering maximum (SAXS)

q Scattering vector (SAXS)

G’/ E’ Storage modulus

G’’/E’’ Loss modulus

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Tan δ Damping factor

E Young’s modulus

σ Stress

ε Strain

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LIST OF TABLES Table 1.1: Names and structures of chain extenders .............................................................. 25

Table 1.2: Mechanical properties of PU elastomers prepared by the four routes61

............... 29

Table 1.3: Krenchel factors for various fibre groupings ........................................................ 35

Table 2.1: Names, structures, acronyms and molar masses of the chemicals used for the

synthesis of TPUs .................................................................................................................... 53

Table 2.2: Molecular weights and polydispersity indices of synthesized TPUs ..................... 64

Table 3.1: Weight losses of 14bd TPUs .................................................................................. 97

Table 3.2: Weight losses of 14chdm TPUs ............................................................................. 99

Table 3.3: Weight losses of 2m13pd TPUs ........................................................................... 100

Table 3.4: Weight losses of 15pd, 16hd, 17hpd and 18od TPUs .......................................... 103

Table 3.5: Calculated d-spacings of the crystalline peaks of 70% HS- 15pd, 16hd, 17hpd

and 18od TPUs ..................................................................................................................... 112

Table 3.6: Calculated and observed d-spacings of Blackwell and Ross, and Born et al X-ray

studies of structure of polyurethane hard segments (MDI and 1,4-butanediol) ................... 112

Table 3.7: Calculated d-spacings of the crystalline peaks of annealed moulded 100% HS-

15pd, 16hd, 17hpd, 18od and 14chdm TPUs ........................................................................ 130

Table 4.1: Weight and volume fractions of TPU composites ................................................ 138

Table 4.2: Storage moduli data of unannealed and annealed 70% TPU vs Polypropylene vs

Isoplast composites ............................................................................................................... 158

Table 4.3: Storage moduli data of unannealed and annealed 100% TPU vs Polypropylene

and Isoplast composites ........................................................................................................ 162

Table 5.1: 1st heating DSC phase transitions of annealed melt-quenched 15pd TPU series171

Table 5.2: Data of the tensile properties of unannealed 15pd TPU composites vs

polypropylene and Isoplast composites ................................................................................ 172

Table 5.3: Data of the tensile properties of annealed 15pd TPU composites vs polypropylene

and Isoplast composites ........................................................................................................ 173

Table 5.4: Creep properties of the unannealed composites ................................................. 177

Table 5.5: Creep properties of the annealed composites ..................................................... 178

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LIST OF FIGURES Figure 1.1 Urethane linkage ................................................................................................... 22

Figure 1.2 Isomers of TDI ...................................................................................................... 23

Figure 1.3 Isomers of MDI .................................................................................................... 23

Figure 1.4 Structures of polyester and polyether polyols ....................................................... 24

Figure 1.5 Schematic illustration of hard and soft segments of a thermoplastic polyurethane

(TPU) ...................................................................................................................................... 26

Figure 1.6 Schematic representation of phase separation occurring in TPUs63

..................... 27

Figure 1.7 Values of efficiency factor or Krenchel factor for different fibre orientations82

.. 35

Figure 1.8 Hard-segment melting temperature versus number of methylene units of alkane

diol94

........................................................................................................................................ 38

Figure 1.9 Geometric isomerism of (a) even-numbered diol TPU and (b) odd-numbered diol

TPU94

...................................................................................................................................... 38

Figure 1.10 (a) Melting temperature and (b) melting/crystallization enthalpy versus number

of methylene units95

................................................................................................................ 39

Figure 1.11 Parallel and anti-parallel arrangements of odd and even-numbered diol-TPUs.

(a) Parallel and anti-parallel arrangements are possible for the odd-numbered diol-TPU

whereas only (b) anti-parallel arrangement is possible for even-numbered diol-TPU95

......... 40

Figure 1.12 Model showing the morphologies of TPUs in melt-quenched and microphase-

separated states60

..................................................................................................................... 43

Figure 1.13 Change in heat capacity of the soft segment glass transition as a function of

hard segment concentration108

................................................................................................. 43

Figure 1.14 Orientations of the fibres in the TPU matrix ...................................................... 51

Figure 2.1 Synthetic protocol used to synthesize the TPU .................................................... 55

Figure 2.2 The experimental set-up for TPU synthesis .......................................................... 62

Figure 2.3 (a) Schematic diagram of the mould and (b) photograph of the cast TPU-70

(15pd) sample.......................................................................................................................... 65

Figure 2.4 The schematic diagram of the compression moulding process ............................ 66

(a) The mould was first preheated in the compression moulder, (b) the polymer films and

fibre mats were layered together in the preheated mould, (c) the stacked-up polymer films

and fibre mats were compressed, (d) the compressed sample was left in the moulder to cool

to about room temperature and (e) the composite was then extracted from the mould .......... 66

Figure 2.5 Schematic diagram of GPC apparatus .................................................................. 69

Figure 2.6 Elution processes of large and small molecules ................................................... 69

Figure 2.7 Cell designs of power-compensation and heat-flux DSC instruments (where TS

and TR stand for the temperatures of the sample and reference respectively, ES and ER stand

for the heat energies of the sample and reference respectively. .............................................. 74

Figure 2.8 The thermal protocol of DSC runs ........................................................................ 75

Figure 2.9 X-ray diffraction process ...................................................................................... 76

Figure 2.10 Schematic diagram of small angle x-ray scattering technique............................ 78

Figure 2.11 Schematic diagram of thermo-gravimetric technique ......................................... 81

Figure 2.12 Schematic diagram of Computer Tomography instrumentation ......................... 83

Figure 2.13 Schematic representation of the squeeze-flow technique ................................... 85

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Figure 2.14 Phase lag in displacement of strain in comparison to the applied stress............. 87

Figure 2.15 Elastic deformation ............................................................................................. 87

Figure 2.16 Viscous deformation ........................................................................................... 88

Figure 2.17 Viscoelastic deformation .................................................................................... 88

Figure 2.18 Schematic diagram of dual cantilever DMTA method ....................................... 89

Figure 2.19 The relative response of stress – strain and their effect on creep modulus ......... 91

Figure 2.20 Creep diagram showing the three parts of strain responses of a material under

constant stress ......................................................................................................................... 92

Figure 3.1 Weight losses of TPU constituents ....................................................................... 95

Figure 3.2 12ed TPU series TGA curves obtained for the (a) weight loss (b) derivative

weight loss versus temperature ............................................................................................... 96

Figure 3.3 14bd TPU series TGA curves obtained for the (a) weight loss (b) derivative


Figure 3.4 14chdm TPU series TGA curves obtained for the (a) weight loss (b) derivative


Figure 3.5 2m13pd TPU series with hard segment concentrations (a) weight loss (b)

derivative weight loss as a function of temperature .............................................................. 100

Figure 3.6 (a) Weight losses (b) Derivative weight losses as a function of temperature of (I)

15pd TPUs, (II) 16hd TPUs, (III) 17hpd TPUs and (IV) 18od TPUs ................................... 103

Figure 3.7 1st DSC heating cycles of cast TPUs (a) 70%HS and (b) 100%HS ................... 105

Figure 3.7 (c) Microphase mixing temperature versus number of methylene units ............. 107

Figure 3.7 SAXS of cast TPUs (d) 70%HS and (e) 100%HS .............................................. 108

Figure 3.7 WAXS of cast TPUs (f) 70%HS and (g) 100%HS ............................................. 109

Figure 3.8 Projection of the conformation of Poly(MDI-Butanediol) proposed by Blackwell

and Nagarajan193

.................................................................................................................... 111

Figure 3.9 1st DSC heating cycles of melt-quenched TPUs (a) 70%HS and (b) 100%HS . 114

Figure 3.9 SAXS of melt-quenched TPUs (c) 70%HS and (d) 100%HS ............................ 116

Figure 3.9 WAXS of melt-quenched TPUs (e) 70%HS and (f) 100%HS ........................... 117

Figure 3.10 2nd DSC heating cycles of slow-cooled TPUs (c) 70%HS and (d) 100%HS .. 119

Figure 3.11 1st DSC heating cycles of moulded TPUs (a) 70%HS and (b) 100%HS ......... 121

Figure 3.11 (c) partially melted to 157°C, (d) cooled to -90°C, (e) reheated to 220°C and (f)

cooled to 25°C ...................................................................................................................... 123

Figure 3.11 SAXS of moulded TPUs (g) 70%HS and (h) 100%HS .................................... 125

Figure 3.11 WAXS of moulded TPUs (i) 70%HS and (j) 100%HS .................................... 126

Figure 3.12 1st DSC heating cycles of annealed moulded TPUs (a) 70%HS and (b) 100%HS127

Figure 3.12 WAXS of annealed moulded TPUs (e) 70%HS and (f) 100%HS .................... 129

Figure 4.1 Melt-viscosity properties of TPUs with different chain extenders ..................... 132

Figure 4.2 (a), (b) and (c) The internal architecture of the glass-TPU composite ............... 136

Figure 4.3 (a) and (b) Optical microscopy images of the TPU composite ........................... 137

Figure 4.4 (a) Storage modulus (b) tan delta of (I) unannealed 2m13pd TPU composites (II)

annealed 2m13pd TPU composites ....................................................................................... 140

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Figure 4.5 Storage moduli bar graph of (a) unannealed and (b) annealed 2m13pd TPU

composites............................................................................................................................. 141

Figure 4.6 (a) Storage modulus (b) tan delta of (I) unannealed 15pd TPU composites (II)

annealed 15pd TPU composites ............................................................................................ 143

Figure 4.7 Storage moduli bar graph of (a) unannealed and (b) annealed 15pd TPU

composites............................................................................................................................. 144

Figure 4.8 (a) Storage modulus (b) tan delta of (I) unannealed 16hd TPU composites (II)

annealed 16hd TPU composites ............................................................................................ 146

Figure 4.9 Storage moduli bar graph of (a) unannealed and (b) annealed 16hd TPU

composites............................................................................................................................. 147

Figure 4.10 (a) Storage modulus (b) tan delta of (I) unannealed 17hpd TPU composites (II)

annealed 17hpd TPU composites .......................................................................................... 149

Figure 4.11 Storage moduli bar graph of (a) unannealed and (b) annealed 17hpd TPU

composites............................................................................................................................. 150

Figure 4.12 (a) Storage modulus (b) tan delta of (I) unannealed 18od TPU composites (II)

annealed 18od TPU composites ............................................................................................ 151

Figure 4.13 Storage moduli bar graph of (a) unannealed and (b) annealed 18od TPU

composites............................................................................................................................. 152

Figure 4.14 (a) Storage modulus (b) tan delta of (I) unannealed 14chdm TPU composites

(II) annealed 14chdm TPU composites ................................................................................. 154

Figure 4.15 Storage moduli bar graph of (a) unannealed and (b) annealed 14chdm TPU

composites............................................................................................................................. 155

Figure 5.1 (a) 1st heating cycles (b) 2

nd heating cycles of melt-quenched 15pd TPUs ........ 163

Figure 5.2 1st heating DSC cycles of (a) 100%HS (b) 90%HS (c) 80%HS and (d) 70%HS

melt-quenched 15pd TPUs annealed at different annealing times (8, 24, 72 and 168 hours)166

Figure 5.3 TMMT/TM and TA vs HS concentration of 70%, 80%, 90% and 100%HS annealed

melt-quenched TPUs annealed for 8hrs ................................................................................ 169

Figure 5.4 ΔHtotal of TMMT/TM and ΔHtotal of TA vs hard segment concentration of 70%, 80%,

90% and 100%HS annealed melt-quenched TPUs annealed for 8hrs .................................. 170

Figure 5.5 Tensile properties of unannealed 15pd TPU composites vs polypropylene and


Figure 5.6 Tensile properties of annealed 15pd TPU composites vs polypropylene and


Figure 5.7 Creep properties of unannealed 15pd TPU composites vs PP and I composites 175

Figure 5.8 Creep properties of annealed 15pd TPU composites vs PP and I composites .... 177

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ABSTRACT

The effect of different chain extenders and hard segment (HS) concentrations on the

properties of thermoplastic polyurethanes (TPUs) and TPU composites were

investigated. The chain extenders used were 2-methyl-1,3-propanediol (2m13pd),

1,2-ethanediol (12ed), 1,4-butanediol (14bd), 1,5-pentanediol (15pd), 1,6-hexanediol

(16hd), 1,7-heptanediol (17hpd), 1,8-octanediol (18od) and 1,4-

cyclohexanedimethanol (14chdm). The hard segment concentrations of the TPUs

investigated were 70%, 80%, 90% and 100%. Only 70% and 100%HS were

investigated for the 17hpd and 18od TPUs. DSC results revealed that the cast

2m13pd and 14chdm TPUs had little or no melting transitions. Cast 14chdm TPUs

were amorphous which was attributed to the mixture of cis- and trans- geometric

isomers present in the 1,4-cyclohexanedimethanol chain extender. The reason for the

low crystallinities of 2m13pd TPUs was attributed to the branched structure of the

2m13pd extender as the methyl (–CH3–) pendant group hinders the crystallization of

the polymer chains which also imparts flexibility to the polymer chains. 12ed and

14bd TPUs displayed the highest melting transitions (about 220°C) and melting

enthalpies (ΔHTot) due to their high crystallinity levels. Similar trends in thermal

properties were observed for the 70%HS cast TPUs chain-extended with 15pd, 16hd,

17hpd and 18od. The microphase mixing transition (TMMT) values of 70%HS cast

TPUs chain-extended with 15pd, 16hd, 17hpd and 18od were 180±0.3, 190±0.5,

164±0.5 and 182±0.5°C. The TMMT values show that the even-numbered chain-

extended TPUs have higher melting transitions than the odd-numbered chain

extended TPUs. This observation could be linked to the odd-even effect of odd-

numbered and even-numbered diols (chain extenders). Multiple endothermic

transitions were observed for the melt-quenched and slow-cooled TPUs chain-

extended with 16hd, 17hpd and 18od. These multiple endothermic transitions were

attributed to the existence of polymorphic structures in the polymer chains. The

melt-quenching and compression moulding processes decreased the crystallinities of

the TPUs whereas the annealing process increased the degrees of crystallinity of all

TPU samples. Phase separation was observed for all the cast 70% and 100%HS

TPUs as revealed in the SAXS results. The SAXS peaks observed for the 70%HS

TPUs come from the HS-Soft Segment (SS) phase whereas the SAXS peaks

observed for the 100%HS TPUs come from the crystalline HS and the amorphous

HS. Crystalline peaks were seen on the amorphous halos of linear chain-extended

cast 70% and 100%HS TPUs as revealed by WAXS results. These peaks correspond

to the crystallinity of hard segments. It was observed that annealing the moulded

TPU samples at 80°C for 168 hours induced the formation of crystal structures

which have d-spacings of about 4.6Å (otherwise known as type-II crystals).

Compression-moulded TPU composites reinforced with woven glass-fibre mats

displayed storage moduli above 2 GPa at 25°C as revealed by the DMTA results.

Upon annealing, the storage moduli of the TPU composites increased above 4 GPa.

The storage moduli of the unannealed and annealed TPU composites compare well

with those of unannealed and annealed composites with polypropylene and

commercial TPU matrices. Tensile testing showed that the Young’s moduli of the

unannealed and annealed 15pd TPU composites were similar to those of

polypropylene and commercial TPU composites.

http://en.wikipedia.org/wiki/%C3%85

14

DECLARATION

No portion of the work referred to in the thesis has been submitted in support of an

application for another degree or qualification of this or any other university or other

institute of learning.

15

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16

ACKNOWLEDGEMENTS

I give all my thanks to the Almighty God for His goodness, mercies,

guidance, protection, unprecedented assistance and favour all through the period of

my research work and throughout my entire academic life till date.

I also express my heartfelt gratitude to my academic supervisor, Dr. Alberto

Saiani for the favour and assistance he showed to me in the pursuit of my PhD

programme. I really appreciate your care, support, guidance and tutelage shown

towards me throughout my research work. It was a great delight to work with you as

my supervisor.

I thank Prof. Aline Miller and my colleagues at the Polymers and Peptides

group for their contributions in the group meetings throughout my research work.

You all were helpful in many different ways.

I particularly thank Prof. Ian Kinloch, Dr. Arthur Wilkinson, Dr. Alan

Nesbitt, Mr. Andrew Zadoroshnyj, Mrs. Polly Crook, members of Materials Science

Centre workshop crew and members of Materials Science Centre IT team for their

academic and technical assistance throughout my research work.

I also thank my course mates for many years; Shicheng Li, Amir-Hossein

Milani and Basheer Al-Shammari for their lovely friendship and the fun times we

shared together. You guys are fun to be around.

I genuinely express my warm gratitude to my dearly beloved parents and

siblings for their massive encouragement, interest and support that kept me forging

ahead in my academic endeavour. Your words of encouragement and belief were

boosters to my undying academic pursuit even through thick and thin. I love and

thank you all so much. You all mean so much to me.

Finally, I deeply thank the Dr. Chris Lindsay, Huntsman Corporation

(Huntsman Polyurethane) and the School of Materials for their interest and funding

towards this research work. I am honoured to be a part and parcel of the Huntsman

organization in embarking on this research work.

My God shall bless you all and all the works of your hands. You all will be

remembered for the good you have done to me.

17

QUOTATION

On the road to success, failures are inevitable roadblocks that must be pushed aside

Chinemelum Nedolisa

18

CHAPTER 1

INTRODUCTION AND LITERATURE

REVIEW

1.1 INTRODUCTION

1.1.1 THE RELEVANCE OF TPUs

Thermosetting polymers have been widely used as composite matrices in various

applications especially in the automotive and aerospace sectors. Thermosetting

materials have excellent mechanical and thermo-mechanical properties largely due to

their cross-linked network-like architecture. Thermosetting polymers such as epoxy

and polyester resins have low viscosities which make them excellent materials to be

used as matrices in composite manufacture. Their low viscosities make them viable

materials to be used in different processing techniques such as compression

moulding, injection moulding, extrusion moulding, resin infusion etc. These

processing techniques are commonly used in the automotive and aerospace

industries. Moreso, their low viscosities make them compatible with different fibres

as there is ease of penetration within the fibre bundles and consequently, this aids the

formation of good fibre-matrix interfacial properties and therefore leading to

excellent mechanical and thermo-mechanical properties. Thermosetting composites

have good weight-to-strength ratio and are able to withstand external impacts1-14

.

Despite the excellent properties of thermosetting polymers which are advantageous

to them being used in structural applications, thermosetting polymers are non-

reprocessable and non-recyclable. The consequence is that many damaged

thermosetting materials become waste.

Thermoplastic polymers such as isotactic polypropylene (PP) have been widely used

in the manufacture of glass fibre reinforced composite materials15-29

. PP possesses

good mechanical properties which originate from its high level of crystallinity. The

disadvantage associated with the use of PP is mainly due to its high viscosity. The

high viscosity of PP often leads to the distortion of fibres during composite

processing.

Having established the significant disadvantages associated with thermosetting

materials as well as PP, the use of thermoplastic polymers therefore forms the main

19

crux of this research work. Thermoplastic polyurethane (TPU) is used as our choice

of thermoplastic polymer in this research work primarily due to its versatile physical

properties. TPU is a linear, segmented copolymer consisting of alternating hard and

soft segments. Being a copolymer, these versatile physical properties of TPU

originate from its microphase separation due to thermodynamic incompatibility

existing between the hard and soft segments. TPU can be employed in different

composite processing techniques due to its low viscosity. The recyclability and

reprocessability of TPU make it an excellent thermoplastic material to be used.

Furthermore, the properties of TPU can be attuned by varying the amounts of its

constituents as well as changing their chemical compositions and structures.

1.2 OUTLINE OF THE THESIS

This thesis consists of six distinct chapters.

Chapter 1 begins with the introduction of PU and TPU. The chemistry, syntheses

and applications of PU and TPU are also introduced. The constituents, processing

and applications of fibre reinforced composites are also highlighted. Furthermore,

different literatures about the structure-property relationships of TPUs and TPU

composites are reviewed. The aim of the research work is also revealed.

Chapter 2 gives comprehensive information on the starting raw materials, the

synthetic processes of the TPUs, the processing method of the TPUs/TPU

composites and the characterization techniques employed during the research work.

Chapter 3 provides information on the first part of the Results and Discussion. This

focuses on the weight loss, thermodynamic and structural properties of different

post-treated TPUs (i.e. casted, melt-quenched, slow-cooled, moulded and annealed

moulded TPUs).

Chapter 4 provides information on the second part of the Results and Discussion.

This focuses on the melt-viscosity, wettability, interfacial and thermo-mechanical

properties of TPU composites.

Chapter 5 provides information on the third part of the Results and Discussion. This

focuses on specific tests (tensile and creep) carried out on 15pd TPU composites.

The tensile and creep properties of 15pd TPU composites are evaluated.

20

Chapter 6 highlights on the conclusions derived from the research work and also

provide suggestions for possible research work that could be carried out for further

study.

1.3 AIMS OF THE RESEARCH WORK

The aim of this research work is to design high hard block content thermoplastic

polyurethane (TPU) resins which will be used for composite applications especially

in automotive and aerospace applications. In this research work, the thermodynamic,

structural, morphological, thermo-mechanical and mechanical properties of TPUs

and their composites are extensively investigated. This research work is divided into

two broad parts namely:

1. Understanding the thermodynamic, structural and morphological properties

of designed TPUs.

2. Elucidating the thermodynamic, structural, interfacial, architectural, thermo-

mechanical and mechanical properties of the designed TPUs as matrices in

glass fibre-reinforced composite making.

To broaden the scope of this research work, it was decided that the properties of the

TPUs and their composites can be attuned by changing certain parameters and

variables. These parameters and variables are as follows:

A. The effect of different chain extenders

B. The effect of increasing hard segment concentration

C. The effect of annealing

21

LITERATURE REVIEW

1.4 POLYURETHANES

Polyurethanes (PU) have found great industrial relevance due to their versatility in

properties. As a matter of fact, PU are seen all around us ranging from our footwear

to our cars. The versatile properties of PU have ushered the way for the advent of

new class of high-performance materials such as coatings, adhesives, elastomers,

fibres and foams. It is therefore important to review the history of this set of ‘unique

polymers’ called PU.

During the 1930s, it became distinct that the discoveries of nylon and polyamide

plastics by W. H. Carothers would be scientifically important and this therefore led

the German firm of Farbenfabriken Bayer to embark on a research to discover a

radically different synthetic route to structurally similar materials. In the light of the

above, Prof. Otto Bayer-led team (at I.G. Farbenindustrie at Leverkusen, Germany

presently Bayer AG) discovered in 1937 that linear polymers were formed from the

reaction between aliphatic di-isocyanates and aliphatic diols. However, their

synthesized polyurethanes were seen have poor thermal properties compared to

nylon but could be drawn into unyielding yarns (Perlon) or used as injection-

moulded thermoplastics (Durethan). Due to the unsatisfactory properties of the

newly synthesized polyurethanes, Prof. Otto Bayer and co-workers therefore

increased their scope of investigation by reacting aromatic polyisocyanates and

polyester diols. This research was successful which led to the discovery and large

scale industrial production of “I-rubber” in the 1940s. It was later discovered that the

reaction of linear alkyd resin with hydroxyl end groups and excess di-isocyanate

resulted in the formation of an ‘adduct’ with increased molecular weight but with

isocyanate end groups. These adducts reacted with water to form rubbers having

high tensile and tear properties. The use of short diols in chain elongation however

became a major development to polyurethane elastomers and this led to products

called Vulkollan® rubbers trade-named by Bayer.

22

1.4.1 CHEMISTRY OF PUs

The basic chemistry of PUs involves the reaction of the three major constituents,

namely: a diisocyanate, a polyol and a chain extender30

. The reaction of an

isocyanate and a hydroxyl group produces a urethane linkage as shown in Figure

1.1.

R1

N C O R2

O H R1

N

H

C

O

O R2

+

Isocyanate Hydroxyl Urethane

Figure 1.1 Urethane linkage

In the hydrogen bonding mechanism of PUs, the N-H group acts as proton donor

whereas the neighbouring oxygen from the carbonyl group or the oxygen from the

soft segments act as proton acceptors31

.

Diisocyanates are divided into aromatic and aliphatic. Aromatic isocyanates are

much more reactive than aliphatic isocyanates as the electron extracting property of

the benzene ring tends to uncover the isocyanate carbon for nucleophilic attack

whereas the electron-donating groups close to the isocyanate carbon tend to reduce

the reaction rate of the isocyanate group. Aliphatic isocyanates are preferred to

aromatic isocyanates when colour retention and clarity of materials in sunlight are

required.

Two commonly used aromatic isocyanates in PU production are toluene diisocyanate

(TDI) and diphenylmethane diisocyanate (MDI). TDI comprises of a mixture of the

2,4- and 2,6- isomers. The commonly used TDI product has 80% 2,4-isomer and

20% 2,6- isomer. MDI has three isomers: 4,4- MDI, 2,4- MDI and 2,2- MDI; these

can be polymerised to form oligomeric products with three or more functionalities.

Figures 1.2 and 1.3 show the isomeric structures of TDI and MDI.

23

N

N

CO

C O

2,4-TDI

NN C OCO

2,6-TDI

Figure 1.2 Isomers of TDI

CH2

NCOOCN

4,4-MDI

CH2

NCO

NCO

2,4-MDI

CH2

NCO

OCN

2,2-MDI

Figure 1.3 Isomers of MDI

The long elastic soft segment largely influences the low-temperature properties of

TPUs32

. Changing the chemistry and the molecular weights of the soft segment has

been reported to affect the physical, morphological and mechanical properties of

TPUs33-49

. Polyester and polyether-based polyols are the most commonly-used soft

segments in the manufacture of polyurethane elastomers. Structures of polyester and

polyether-based polyols are shown in Figure 1.4.

24

Figure 1.4 Structures of polyester and polyether polyols

The polyester diols are formed from the reaction of adipic acid and one or more

aliphatic diols in the series from ethylene glycol to 1,6-hexanediol whereas polyether

TPUs are commonly made from poly(oxytetramethylene) diols and

polytetrahydrofurans. Polyester based polyurethane elastomers have good physical

properties but tend to undergo breakage on the ester linkage when they come in

contact with water. Specialty polyester-based polyols of commercial interest include

polycaprolactones and aliphatic polycarbonates. Unlike polyesters, polyether based

TPUs are stable to hydrolytic cleavage. Therefore when properties such as resistance

to wet environments are desired, polyether based TPUs are favoured. Polyester based

thermoplastic polyurethanes are resistant to oil and hydrocarbon attacks. Specialty

polyether-based polyols of commercial interest include poly (oxypropylene) glycols

and poly (oxytetramethylene) glycols. Other specialty polyols include polysulfide,

polybutadiene and polydimethylsiloxane.

Chain extenders are low molecular weight short-chain diols. They are bifunctional

hydroxyl or amine compounds. There are two major classes of chain extenders,

namely: (a) Aromatic diols and diamines and (b) Aliphatic diols and diamines.

Aromatic chain extenders produce rigid materials for high performance applications

while aliphatic chain extenders are used in making softer materials. Chain extenders

play a significant role in influencing the structure and morphology of polyurethane

elastomers. Chain extenders react with the isocyanate increasing the length of the

25

hard segment chains. Chain extenders having a functionality of 3 or 4 are known as

cross-linkers. Chain extenders are added to isocyanate units to allow for hard

segment separation. This therefore results in an increment in modulus and the hard

segment glass transition temperature (Tg) of the TPU. Commonly used chain

extenders are seen in Table 1.1.

Table 1.1: Names and structures of chain extenders

Chain extender Structure

1,2-ethanediol (Ethylene

glycol) OHOH

1,3-propanediol

1,4-butanediol OH

OH

1,5-pentanediol OH OH

1,6-hexanediol OH

OH

2-methyl-1,3-propanediol OH OH

CH3

1,4-cyclohexanedimethanol OHOH

Hydroquinone bis(2-

hydroxyethyl) ether

Ethylene diamine

Hard segments with the highest packing densities and melting points result when the

carbon atoms in the chain extender are even-numbered. In this case, the hard

segments are aligned to give a straight three dimensional staggered C=O------H-N

hydrogen bonds. Hard segments having chain extender with odd numbered carbon

atoms are contracted in order to form hydrogen bonds and therefore attain a higher

energy conformation. The structure of the soft segment also influences the packing

of the hard segments. Polyether based polyols are less compatible with MDI which

26

therefore leads to stronger phase separation. Polyether based TPUs have more

complicated hard phase domains than polyester based TPUs. However, an increase

in the hydrocarbon chain length as well as molecular weights of polyether and

polyester polyols affect the phase separation of TPU50-55

.

1.5 THERMOPLASTIC POLYURETHANES (TPUs)

Thermoplastic polyurethanes (TPUs) are linear, block copolymers with alternating

hard and soft segments. The hard segment (HS) is usually the reaction product of a

diisocyanate and a chain extender whereas the soft segment (SS) is formed by

hydroxyl terminated compound known as a polyol.

Hard segment Soft segment

di-isocyanate

chain extender

polyol

Figure 1.5 Schematic illustration of hard and soft segments of a thermoplastic

polyurethane (TPU)

Figure 1.5 shows a schematic diagram of the different blocks or segments of a TPU.

TPUs exhibit versatile physical properties due to micro-phase separation arising

from the thermodynamic immiscibility existing between the HS and SS. The

morphology of thermoplastic polyurethane varies widely depending on the molecular

properties of its constituents52, 56-61

. The HS comprise of polymer chains formed

from diisocyanate and chain extender; the aromatic rings of the HS make them rigid.

The hard blocks may also be crystalline and thus impart majorly on the mechanical

properties of the polyurethane.

The flexible SS are responsible for the extensibility of a polyurethane. The

difference in the quantities of the HS and SS in a TPU affects the properties and

structure of the material. By changing the quantities of the HS and SS, we can

therefore obtain material ranging from a hard rubber to a soft engineering plastic62

.

27

At temperatures above the melting temperature of the HS, TPU becomes molten and

therefore can be processed using a range of processing techniques such as injection

moulding, extrusion, blow moulding or vacuum-forming. The phase separation of

HS and SS returns on cooling as chain mobility is reduced.

Figure 1.6 Schematic representation of phase separation occurring in TPUs63

Figure 1.6 shows the phase-segregated structure and morphology of TPU which

usually contains both hydrogen-bonded HS and non-bonded HS. The aggregation of

the hydrogen-bonded HS forms the crystalline phase of the TPU and consequently

imparts a phase-separated structure in the TPU architecture. The non-bonded HS are

randomly dispersed in the soft phase and therefore constitute the mixed phase of the

TPU architecture64-67

.

TPU combines the properties of a rubber (Elasticity) and those of a thermoplastic

(melt-processability). Elastic properties of TPUs are recovered on cooling. Phase

separation primarily depends on the rate of cooling and annealing parameters50, 52

.

1.5.1 SYNTHESES OF TPUs

TPUs are basically synthesized by two processes; the ‘one-step process’ and the

‘multistep process’. The one-step process involves a bulk mixture of difunctional

liquid diisocyanate and liquid diol which then cures to form an elastomer68-70

. To

achieve a TPU with better physical and mechanical properties, the multistep or two-

step process is preferred. The two-step process involves the preparation of a pre-

polymer which constitutes the first step. A pre-polymer is formed by the capping

reaction of a diol with a diiscyanate71

. The second step constitutes the addition of an

amine or a diol chain extender and other additives (such as solvent, catalyst) to the

28

pre-polymer already formed in the first step50, 54

. The one step process is mainly

employed in industry whereas the two-step process is mainly employed in research.

Po Hung Chen et al. reported on the preparation of polyurethane materials using bulk

polymerisation which involves the combination of different chain extenders (1,4–

butanediol and 1,6-hexanediol). The mixture of chain extenders decreased the

crystallinity of the hard segment thus improving the transparency of the TPU and

subsequently its mechanical properties56

.

Huibo Zhang et al. investigated four different synthetic methods. The 1st method

involved putting the polyol into a reaction flask with a mechanical stirrer and a

thermometer. Vacuum was applied at temperature of 100-120°C for 2 hours.

Temperature was then reduced to 50°C. Diisocyanate was then added dropwise into

the polyol as temperature was increased to 80°C and kept for 1 hour. A chain

extender and a catalyst were subsequently added to the reaction mixture. The 2nd

method consisted of the addition of polyol, diisocyanate and catalyst into the glass

reaction flask having a mechanical stirrer and a thermometer. Vacuum was applied

and the reaction was kept at temperature of 100-120°C for 2 hours. Temperature was

then reduced to 50°C. Chain extender was added and stirred for 10 minutes. The 3rd

method was quite similar to the 1st method having the same reaction conditions but

in this case polyol was added to diisocyanate in a drop-wise manner before the

addition of a chain extender and a catalyst. The 4th

method was entirely different

from other methods. In this method, polyol, chain extender and catalyst were first put

into the reaction flask. Vacuum was applied and the reaction was kept at temperature

of 100-120°C for 2 hours. Temperature was then reduced to 50°C. Diisocyanate was

finally added and stirred. Reaction was kept for 10 minutes. All four methods had

their mixtures cast into a preheated aluminum mould coated with Teflon, which were

then post cured in an oven at 120°C for 4 hours. All four polyurethane elastomers

from different synthetic methods gave different properties but polyurethane

elastomer from the 1st method was reported to have the best properties. From the

above synthetic methods, it can be observed that the dropwise addition of the

diisocyanate to the polyol employed in the 1st method proved to be the best synthetic

route in achieving TPUs with good mechanical properties. This is because there is

better bonding between the diisocyanate and polyol units and therefore microphase

29

separation is improved61

. Table 1.2 shows the mechanical properties of the PU

elastomers prepared by the four routes.

Table 1.2: Mechanical properties of PU elastomers prepared by the four routes61

Route 1 2 3 4

Tensile strength (MPa) 3.0 1.7 2.9 1.3

Elongation at break (%) 568 514 579 487

Hardness shore A 76 79 75 74

Modulus of elasticity (MPa) 5.5 2.3 4.9 1.7

1.5.2 APPLICATIONS OF TPUs

The unique and versatile properties of TPUs make them important materials for a

wide range of applications. These properties arise from the micro-phase structure and

morphology of TPU. Properties such as toughness, high resilience, low compression

set, resistance to abrasion, weather, tears, hydrocarbons and good low temperature

flexibility make TPUs good materials. TPUs also exhibit biocompatibility and this

property makes them tremendously useful in the medical world such as in

cardiovascular applications and making of some artificial organs (artificial heart).

Depending on the range of hardness, TPUs are used make certain automotive parts

such as fuel line connectors and seals amongst others. Abrasion resistant rollers of

some engineering machines are made of hard TPU. Other applications of TPU

include the making of wrist watches, football boots and other sports shoes such as

ski boots. The low temperature flexibility and impact resistance of TPU are

fundamental properties in the manufacture of ski boots52, 54

.

Films of TPUs have outstanding properties: abrasion resistance, puncture and tear

resistance, high elasticity, bondability and weldability. These films are therefore

often used for conveyor belts, welded hollow bodies, textile lamination, protective

coverings, sealing of foams and abrasion resistant coatings. Different types of hoses

are manufactured from TPUs. Due to properties such as good recovery after

deformation, cut resistance, good weathering properties and resistance to oil and

fuel, TPU can be injection moulded to form exterior automotive parts. Other

automotive applications include bearing bushings and gaskets for wheel components.

Polyether based TPUs display excellent compatibility with human blood and tissues;

they are therefore used in making catheters and tubes for blood50, 55

.

30

1.6 FIBRE REINFORCED POLYMER COMPOSITES

Fibre reinforced polymer composites are materials which consist of fibres (natural or

synthetic) incorporated within a matrix. The composite material possesses good

properties owning to the blend of individual properties of its constituents (fibre and

matrix). The physical and chemical properties of the fibre and matrix however stay

unaltered while in this bonded state. In general terms, the fibres are accountable for

the load-bearing ability of the composite whereas the matrix is responsible for

positioning and holding firm the ordering of the fibres. The matrix also functions as

load transfer path between the fibres and safeguards them from hazardous

environmental conditions such as humidity. There has been increasing demand in the

use of fibre reinforced materials over the last few years especially in engineering

applications. This increasing demand of fibre reinforced materials has led to the

decrease in the use of traditional materials, particularly metals. Composite materials

have shown to exhibit certain preferred features over metals. They find relevance in

applications requiring high mechanical properties as well as lightweight properties.

Properties such as physical strength, stiffness, impact resistance and dimensional

stability are generally improved when glass fibres are mixed with plastics. The

specific gravity of glass fibre reinforced composites is seen to be about one-fifth that

of steel and this plays a vital role in applications where light weight is required. Fibre

reinforced composites also display low thermal expansion compared to metals. They

also can be easily moulded or formed into various complicated parts and damaged

parts are easily reparable72-75

.

1.6.1 FIBRE

Commonly used fibres employed in polymer composites are glass and carbon fibres.

Glass fibres are most commonly used and are versatile in diverse applications. Other

fibres, such as boron, silicon carbide and aluminum oxide are used in small amounts.

Fibres are aligned in the matrix in either a continuous or discontinuous (chopped)

lengths74-76

.

1.6.1.1 GLASS FIBRE

Glass fibres are made of silica (SiO2), often combined with oxides of calcium, boron,

sodium, iron and aluminum. The vitreous state of silica is in the form of glass

whereas the crystalline state of silica is in the form of quartz. To achieve

31

crystallization, silica must be heated above 1200° C for a very long period of time.

The melting point of glass fibres remains unclear but they undergo softening at

2000° C and often begin to degrade at that point. Upon rapid cooling, the aligned

structure of glass fibre is distorted. In the polymeric form, glass fibre attains a

tetrahedral configuration consisting of SiO4 where the oxygen atoms are attached to

a central Si atom. Glass fibres are mostly non-crystalline but crystallisation can be

induced after long-term heating at elevated temperatures and thereby leading to

reduced strength. Certain materials are often added to the glass fibres to improve and

enhance certain properties of the glass fibre. This process is known as sizing. Size is

usually added at 0.5 to 20 % by weight. Size may include lubricants, binders and/or

coupling agents. Lubricants are applied in order to protect glass fibres from any form

of abrasion and breakage. Coupling agents are applied to glass fibres to enhance their

affinity to react with certain polymer matrices. In the case of thermoplastic

polyurethane (TPU) based glass fibre composite, the glass fibres will be much better

if they are functionalized with N-H groups. This aids in the reaction of N-H groups

with the isocyanate groups (N=C=O) of the thermoplastic polyurethane matrix and

will lead to the strong fibre-matrix cohesion of the composite material. Binders

and/or coupling agents also improve resin wet-out and reinforce the strength of the

fibre-matrix interface.

The most commonly used glass fibres for composite applications are E-glass (E

stands for electrical, known for great strength, stiffness, electrical and weathering

properties), C-glass (C stands for corrosion, exhibits better corrosion resistance than

E-glass but has low strength) and S-glass (S stands for strength, are costlier than E-

glass and has greater strength). E-glass accounts for most quantities of the world’s

glass production. Being in an amorphous form, the properties of glass fibres are

isotropic, that is to say, their properties are the same along the fibre as well as across

the fibre. The mechanical properties of fine glass fibres are outstanding as their

tensile strength is reported to be 35000 kgf cm-2

and this high tensile strength is due

to the fact that cracks are not present on the surface of the fibre. Damage caused by

the mishandling of the fibres would lead to the decrease in strength of the fibres.

Glass fibres show no degradation when exposed to sunlight and are very resistant to

chemical attacks. They can withstand temperatures as high as 500°C and will not

singe. They display a perfect Hookean elasticity when stretched without having a

32

yield point. The strength, high modulus and elasticity of glass fibres make them

excellent reinforcement for composite engineering applications. Applications of

glass fibres vary widely as seen in boats, automotive, aerospace, electrical and

sport75-76

.

1.6.2 MATRIX

Matrix is a group of materials used in making composites and they are usually used

in the form of resins, sheets or films. Commonly used polymer matrices for

engineering applications are thermoplastics and thermosets.

1.6.2.1 THERMOPLASTICS

Thermoplastics possess the simplest molecular architecture with chemically

unrestricted macromolecules. They are not crosslinked structures and therefore can

be easily melted and moulded into different shapes and forms. The long chain-like

molecules of thermoplastic material are held together by weak Van der Waals forces.

When heat is applied on a thermoplastic material, it becomes soft and pliable and

subsequently at high temperatures, it becomes a viscous melt. Upon cooling, the

material solidifies again. Several heating and cooling treatments can be repeatedly

carried out on thermoplastics without any noticeable degradation, giving room for

reprocessing and recycling. Stiffness and strength of thermoplastics originate from

the individual properties of the monomers that make up the long polymer chain.

Morphology also plays a role in the mechanical properties of the thermoplastics.

High molecular weight also plays a vital role in the properties of thermoplastics. An

important aspect of thermoplastics is related to whether they are crystalline (ordered)

or amorphous (random/disordered) in structure. In practical terms, it is not possible

for the structure of a moulded thermoplastic to be fully crystalline due to the

complicated physical nature of molecular chains. Some thermoplastics such as

polyethylene and nylon which are capable of achieving high level of crystallinity are

more correctly referred to as partially crystalline or semi-crystalline. The presence of

crystallinity inherent in thermoplastics that are susceptible to crystallization is related

to their thermal history as well as the processing parameters employed in making the

thermoplastics. Thermoplastics such as polystyrene and acrylics are always non-

crystalline. Generally, thermoplastics usually have higher density when they are in

their crystalline forms which are related to the close packing of the molecules.

33

Examples are polyethylene, polypropylene, polyvinyl chloride, polystyrene amongst

many others72, 74, 77-80

.

1.6.2.2 THERMOSETS

Thermosets possess a three-dimensional molecular network as a result of chemical

crosslinking of the polymeric chains and therefore leads to stiffening of the material.

They often require the combination of a resin and a binder. Thermosets are cured in

order to undergo complete polymerization. Curing in most cases takes place at room

temperature but can also be carried out at programmed heating processes in order to

achieve better characteristics. During curing, the long molecular chains of the

thermosetting material are interlinked by strong bonds so that the resultant material

sets and is no longer softened by the application of heat. If excess heat is applied to

the thermosetting material, they will only char and degrade rather than melt.

Mechanical properties of thermosets rely on the monomers involved in the build-up

of the polymer chain as well as the length and degree of crosslinking these

monomers undergo. Unlike thermoplastics, the final products are not reprocessable

and recyclable. Thermosets are rigid due to their high degree of crosslinking and

therefore exhibit brittleness on impact. Epoxy, unsaturated polyester and vinyl ester

are the three most commonly used thermosetting resins in composite applications72,

74, 77-78.

1.6.3 RULE OF MIXTURES IN DETERMINING THE

PROPERTIES OF FIBRE REINFORCED COMPOSITES

The rule of mixtures states that the modulus of a unidirectional fibre composite is

proportional to the volume fractions of the materials in the composite. This rule

explains that the modulus of the composite is simply the sum of the weighted

average between the moduli of the two composite constituents, depending only on

the volume fraction of fibres. The rule of mixtures can be used to determine the

density of a composite as well as other properties such as Poisson’s Ratio, strength,

thermal conductivity and electrical conductivity along the fibre axis74, 80-81

. The

properties of a composite material are derived from the individual properties of the

composite constituents (fibre and matrix). Thus if a stress is applied along the path of

the fibre alignment in a unidirectional composite, it is expected both constituents

should exhibit the same strain along the fibre axis, assuming that there is no

34

interfacial misalignment. The Young’s modulus of the composite can therefore be

written as74, 80

:

(Equation 1.1)

where Ec = Young’s modulus of composite

Ef = Young’s modulus of fibre

Em = Young’s modulus of matrix

Vf = Volume fraction of fibre

Vm = Volume fraction of matrix

The above equation is known as the isostrain rule of mixtures. For a composite

material in which the fibres are stiffer than the matrix, the fibre therefore is subjected

to higher stresses than the matrix and there is a re-allocation of the load.

In order to use the rule of mixtures to determine the properties of fibre reinforced

composites, the efficiency factor or Krenchel factor must be employed. The

Krenchel factor is used to predict the influence of fibre orientation on stiffness. This

is a term used to factor the rule of mixtures formula according to the fibre angle. The

Krenchel factor predicts that an estimate of an elastic response of a general multi-ply

laminate can be calculated by a summation analysis and of the contributions from

each bundle of fibres lying at a specific angle, θ, to the applied stress82

. An overall

composite efficiency factor or Krenchel factor, ηθ, can be defined as:

(Equation 1.2)

The approximate composite modulus can then be calculated using a modified rule of

mixtures as:

(Equation 1.3)

Values of the efficiency factor or Krenchel factor, ηθ, for a range of fibre

distributions can be seen in Figure 1.7 and Table 1.3.

35

Figure 1.7 Values of efficiency factor or Krenchel factor for different fibre

orientations82

Table 1.3: Krenchel factors for various fibre groupings

Composite type/Fibre orientation Krenchel factor, ηθ

Unidirectional composite loaded parallel to fibres 1.0

Biaxial composite loaded parallel to fibres 0.5

Random in-plane fibre orientation 0.375

Biaxial composite loaded ±45°to fibres 0.25

Random 3D fibre orientation 0.2

Unidirectional composite loaded perpendicular to fibres 0

36

1.6.4 APPLICATIONS OF FIBRE REINFORCED COMPOSITES

The benefits of using thermoplastic polyurethane (TPU) based fibre reinforced

composites in the manufacture of sporting materials are their light weight, vibration

reduction and flexibility properties. These properties play key role in the

manufacture of tennis rackets, helmets, surfboards, ski poles, hockey sticks amongst

others. Due to the different ranges of hardness of thermoplastic polyurethane

materials, certain thermoplastic polyurethane based composites are used in making

sport shoes, bags, gloves, jackets, mobile phone cases amongst others52

.

Epoxy based carbon fibre reinforced composites or those mixed with Kevlar fibres

are commonly used in making the wing and fuselage parts of aircrafts. These

composite materials can last for long stretch of years without any mechanical failures

as a result of their low coefficient of thermal expansion and high fatigue resistance

under cyclic loading at stresses. The structural integrity and durability created by

these composite materials have increased interests of their applications in making

other aerospace parts. Due to their low specific gravities, giving high strength-weight

ratios and modulus-weight ratios, epoxy based composite materials are used in

making the rotor blades for various helicopters. The flexibility of these blades makes

for easy swinging and twisting in the air and also adaptable to air resistance.

Fibre reinforced composites also play important role in automotive applications.

Epoxy resin based carbon fibre reinforced composites are used to make the body

parts of formula one (F1) cars. The fuel and driver lies in the survivor cell which is

made of advanced composites. The steering and brakes of the formula-one cars are

made of carbon composites due to their thermal and frictional properties. The helmet

worn by the driver is made of high performance carbon composites75

.

1.7 STRUCTURE-PROPERTY RELATIONSHIPS OF TPUs

The micro-phase separation arising from the thermodynamic immiscibility existing

between the HS and SS imparts the versatile physical properties of TPUs. However,

certain parameters contribute to the morphological, structural, thermodynamic,

thermo-mechanical and mechanical properties of TPUs. These parameters range

from the nature/composition of the TPU constituents to the processing/post-

treatment processing.

37

1.7.1 EFFECT OF CHAIN EXTENDERS

The use of chain extenders in the chain extension of TPUs proved to be prominent in

the manufacture of TPUs50

. The chemical structures and characteristics of chain

extenders (chain length, degree of branching etc) have been extensively reported to

influence the extent of interaction between the HS and SS30, 83-91

.

Wang and Kenney92

reported that 1,4-butanediol based TPU showed better thermal

properties than those of 1,5-pentanediol and 1,3-butanediol. The reason was that the

1,4-butanediol based TPU had strong crystal structures whereas 1,5-pentanediol and

1,3-butanediol TPUs had weak crystal structures due to their random intermolecular

arrangement. Phase segregation was lowest for 1,3-butanediol based TPU due to its

amorphous nature. Their FTIR results also showed that the hydrogen-bonded C=O

region of the 1,3-butanediol TPU was lower than the free C=O region. The reason

for the observation was due to the fact that only a small part of the hard segments

have been hydrogen-bonded.

In another publication, Sanchez-Adsuar and Martin-Martinez30

reported that ethylene

glycol based TPU showed better thermal properties than those of 1,4-butanediol and

1,6-hexanediol. The reason for their observation was that the short chain extenders

are easier to join with the pre-polymer during polymerization than the longer chain

extenders. This therefore results in high molecular weights of the TPU and

consequently leading to higher crystallinity and phase segregation.

Studies have shown that the glass transition temperatures (Tg) of the TPUs decreased

with increasing chain extender length. The decrease in Tg of the TPUs was attributed

to the polarity and the flexibility of the chain extenders. The polarity of the chain

extender decreases with increase in the number of methylene units in the chain

extender and therefore decreases in polarity results in low TgHS. Furthermore, as the

length of the chain extender increases, the flexibility of the TPU material increases

and consequently lowering its Tg93

. Camberlin et al94

reported that the Tm of the 1,2-

ethanediol, 1,3-propanediol and 1,4-butanediol HS (from 200°C to 208°C) were

higher than the other TPUs. Figure 1.8 shows that 1,4-butanediol TPU has the

highest melting temperature indicating its high crystallinity and stronger bonding.

However, the increase of the length of the chain extender decreases the Tm of the

TPU as well as the degree of cohesion. This was reported to be due to the structural

38

arrangement of the odd-numbered and even-numbered diols in the HS architecture.

Figure 1.9 shows that the even-numbered methylene units display hydrogen-bonded

carbonyl groups in a trans- arrangement whereas the odd-numbered methylene units

displayed hydrogen-bonded carbonyl groups in a cis- arrangement. The difference in

geometric arrangements could be traceable to the variations in the Tm.

2 4 6 8 10

160

170

180

190

200

210

Mel

ting

tem

pera

ture

(C

)

Number of methylene units, n

Figure 1.8 Hard-segment melting temperature versus number of methylene units of

alkane diol94

(a) trans (b) cis

Figure 1.9 Geometric isomerism of (a) even-numbered diol TPU and (b) odd-

numbered diol TPU94

In another publication, Fernandez et al95

investigated on the crystallization of linear

aliphatic TPUs with linear chain extenders with methylene units ranging from 5 (1,5-

pentanediol) to 12 (1,12-dodecanediol). It was reported that the Tm decreased with

increase in the methylene units for both odd- and even-numbered diols as a result of

the decrease in the hydrogen bonding interactions. It was also observed that the

melting and crystallization enthalpies for the odd- and even-numbered diols increase

39

with increase in the methylene units as shown in Figure 1.10. The flexibility

imparted by the increased methylene units promotes crystallization process.

4 5 6 7 8 9 10 11 12 13

110

120

130

140

150

160

170

Me

ltin

g te

mp

era

ture

(C

)


4 5 6 7 8 9 10 11 12 13

60

70

80

90

100

110

120

130

Mel

ting

enth

alpy

(J/

g)


Figure 1.10 (a) Melting temperature and (b) melting/crystallization enthalpy versus

number of methylene units95

The characteristic zig-zag plot shown above depicts an odd-even effect, as the even-

numbered diol-TPUs have higher melting transitions than their immediate odd-

numbered diol-TPUs. This occurs as a result of the arrangement of the polymeric

chains. The odd-numbered diol-TPUs were reported to have hydrogen bonds

arranged in either parallel or anti-parallel configuration whereas the even-numbered

diol-TPUs only take up anti-parallel configuration as shown in Figure 1.11. The

(a)

(b)

40

different arrangements of both the odd- and even-numbered TPU systems therefore

provide different crystal structure and packing which ultimately affects their melting

transitions95

.

Figure 1.11 Parallel and anti-parallel arrangements of odd and even-numbered diol-

TPUs. (a) Parallel and anti-parallel arrangements are possible for the odd-numbered

diol-TPU whereas only (b) anti-parallel arrangement is possible for even-numbered

diol-TPU95

a. odd-numbered diol-TPU

Parallel arrangement

b. odd-numbered diol-TPU

Anti-parallel arrangement

c. even-numbered diol-TPU

Anti-parallel arrangement

41

It can be observed that there is a difference in Tm in relation to the number of

methylene units, n as seen in Figure 1.8 and Figure 1.10 (a). The difference occurs

mainly due to the different synthetic routes employed by the two groups. Fernandez

et al used di-tert-butyl tricarbonate which is highly selective and reactive whereas

Camberlain et al used tetrahydrofuran solvent. It can therefore be deduced that the

use of di-tert-butyl tricarbonate yielded better phase-separated and uniform-

microstructured TPUs. The odd-even effect of n-aliphatic polyurethanes was clearly

shown in the results obtained by Fernandez et al, which corresponded to the odd-

even effect of TPUs reported by other authors96-102

. However, the -cis and -trans

configurations (Figure 1.9) observed by Camberlain et al corresponded to the anti-

parallel and parallel arrangements (Figure 1.11) observed for Fernandez et al.

1.7.2 EFFECT OF HARD SEGMENT CONCENTRATION

The multiple thermodynamic transitions in TPUs arise mainly from both the

chemical properties and the physical treatments carried out on the TPUs. Hard

segment concentration of TPUs also has a significant influence in the

thermodynamic, structural, mechanical and thermo-mechanical properties of

TPUs103-106

.

Frontini et al107

in their study discovered the existence of multiple Tgs for a 100%

hard segment (consisting of only MDI and 1,4-butanediol) PU cured at 110°C. An

exothermic transition and multiple endothermic transitions were present on the DSC

thermograph. Two Tgs for the 100% hard segment TPU occurred at 50°C (Ti,

intermediate transition) and 100°C (Th, hard segment glass transition). They showed

that for a 90% hard segment TPU (MDI, 1-4 butanediol and 10% polyol), there were

three transitions found and were assigned as Ts (soft segment glass transition), Ti and

Th. Ts and Th are usually associated with TPUs and they primarily constitute the

make-up of TPUs. The third Ti (intermediate transition) was reported by this group

to have originated from the rate of cooling administered to the TPUs from the melt

state. Due to the proximity of Ti and Th, it was reported that Ti was affiliated with the

HS107

.

Saiani et al58

in their thermodynamic investigation of freshly melt-quenched TPUs

using 2-methyl-1,3-propanediol as chain extender, reported that a broad Tg was

observed for a TPU with a hard segment content of 50%, indicating the presence of a

42

mixed phase system. For melt-quenched TPU samples with hard segment

concentration of 75% and above, Tg typical of HS (TgHS) was observed. The TgHS

values indicated the presence of an almost ‘pure’ hard segment. It was observed that

the TgHS values were seen to slightly increase with increasing hard segment

concentration, due primarily to the presence of SS residing within the HS.

Furthermore, the slight increase in TgHS may be due to the length of the hard segment

decreasing with increasing SS concentration. It was also observed that the heat

capacity change (ΔCpHS) resulting from TgHS decreases with decrease in hard

segment content. DSC showed that TPU with a hard segment concentration of 65%

did not exhibit TgHS despite the amount of hard segment present in the TPU system.

The reason for the observation was that a considerable amount of the hard segments

were dissolved with the soft segments in a mixed phase. It suggests that the 65% HS

TPU is in a homogeneous state. However, for TPUs with HS content higher than

65%, it was therefore suggested that the TPU system is no longer in a

homogeneous/mixed state but heterogeneous where 65% equivalent of the HS would

be involved in the mixed phase while the rest of the HS will account for the ‘pure’

TgHS. The high temperature transitions observed for the TPUs were attributed to the

mixing of the HS and SS and therefore referred to as ‘microphase mixing

temperature’ (TMMT). After annealing of the melt-quenched TPU samples two

endothermic transitions were observed. The first endotherm was reported to have

arisen from the melting of the ordered HS induced during the annealing process (TM)

whereas the second melting peak was due to the mixed phase of HS and SS

(TMMT)58

.

In another publication60

, SAXS results showed that phase separation of the hard and

soft phases decreased with increasing hard segment concentration. It was also

observed that the scattering maximum (q*) was the same for all the TPUs. The

reason for the observation was that the size of the electron density fluctuations that

influence the scattering maximum is the same for all the TPU samples. It was

therefore suggested that the microphase-separated morphology was the same for the

TPU samples having 2-methy-1,3-propanediol as chain extender60

. Figure 1.12

shows morphological models created for TPU samples with high hard block

concentration (> 65%) which suggests the morphologies of melt-quenched and

microphase-separated phase occurring in TPUs.

43

Figure 1.12 Model showing the morphologies of TPUs in melt-quenched and

microphase-separated states60

The effect of hard segment concentration on the phase transitions occurring in TPUs

was investigated by Koberstein et al108

. TgSS was observed for compression-moulded

TPU with low hard segment concentration (20%). The TgSS was seen to increase with

increase in hard segment concentration whereas change in specific heat capacity

(ΔCp) decreased with increase in hard segment concentration as shown in Figure

1.13.

20 30 40 50 60 70

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

Cp o

f sof

t seg

men

ts J

/(g.C

)

Hard segment concentration (%)

Figure 1.13 Change in heat capacity of the soft segment glass transition as a

function of hard segment concentration108

The reduction of ΔCp for SS (ΔCpSS) in higher HS content TPUs denotes that some

soft phase has been dissolved into the HS and interphase regions and thus was not

involved in the ‘pure TgSS’. The existence of multiple endothermic transitions was

44

traceable to the melting and melt re-crystallization processes occurring in MDI/1,4-

butanediol crystallites. It was also observed that a transition occurred in the range of

50- 90°C for TPU samples with HS concentration from 30% and above. This

transition was assigned to the hard segment glass transition temperature (TgHS)108

.

Koberstein and Stein proposed a model for microdomain structure based on a

concept of critical length of the hard segment units. They reported that hard segment

chains shorter than the critical length would be dissolved in the SS whereas HS

chains longer than the critical length would result in the formation of phase-

separated hard microdomains with a lamellar structure109-110

. This model also

proposed that the thickness of the hard segment is controlled by the critical hard

segment length. This therefore means that below the critical length, a mixed phase

structure with non-crystalline morphology is obtained.

Several authors have reported that the TgHS increased with increase in NCO content.

The increase in NCO content consequently decreases OH content and this therefore

decreases the soft TgSS. The intensities of the melting peaks (enthalpies) and Tm of

the TPUs were observed to increase with increase in hard segment aggregation111-113

.

WAXS results showed that amorphous halos were predominant for the TPUs with

hard segment content up to 50% with no trace of crystalline peaks whereas

crystalline peaks were observed on the amorphous halos of TPUs with hard segment

concentration of more than 50%. Tensile results showed that mechanical properties

such as Young’s Modulus and tensile strength increased with increase in hard

segment concentration (especially TPUs with hard segment content with more than

50%). The aforementioned mechanical properties are influenced by the degree of

hard segment concentration inherent in the TPU system. However, elongation and

yield strength decreased with increased hard segment content113

From the literatures reviewed, it is important to note that TgHS increases with increase

in hard segment concentration. It was also noted that TPUs with hard segment

concentration from 65% and below exhibited a mixed phase glass transition (TgMP).

This is primarily due to the mixture of HS and SS and is usually indicated by a broad

transition. Tensile properties such as Young’s modulus and tensile strength of TPUs

were increased relatively to the increase in their hard segment contents.

45

1.7.3 EFFECT OF ANNEALING ON THE THERMODYNAMIC,

STRUCTURAL, THERMO-MECHANICAL AND MECHANICAL

PROPERTIES OF TPUs

Goyert and Hespe114

reported that a distribution of crystallite size originated from the

cooling of molten TPU which was not only influenced by the length distribution of

the hard segmental chains but also by the kinetics of crystallization. DSC results

showed that moulded TPU (based on MDI, 1,4-butanediol and ethanediol-

butanediol-polyadipate) displayed multiple melting transitions of which the highest

melting peak was at 205°C. By annealing the TPU samples at different temperatures

(118, 135, 180 and 205°C) for 5 minutes, melting temperature increased to about

230°C. It was also observed that the melting range for the TPU sample annealed at

different temperatures contracted at the same time. The reason for the observation

was attributed to the incomplete melting of the HS as well as to recrystallization of

larger hard segment crystallites with better packing order. To better understand the

process of crystallization on annealing, the following illustration was made: When a

TPU is quickly cooled from melt; the hard segments form crystals that are

disarranged from one another. The crystal thickness of the HS is therefore relatively

low due to its less thermodynamically favorable arrangement. Upon annealing, the

crystal structures become ordered and therefore leading to a more

thermodynamically favourable organization. This organization subsequently leads to

the improvement of the melting transitions primarily due to the increase in crystal

thickness114

.

Several authors have reported that the crystallites formed as a result of annealing

were due to the presence of short-range and long-range dissociation of the HS in

their investigative studies on the morphological changes in TPUs via DSC

analysis115-116

.

Frontini et al. observed that the melting enthalpies and TgHS are dependent on

annealing temperature. It was reported that crystallization reaches a minimum with

respect to annealing temperature and then becomes negligible and this was due to the

fact that a non-crystalline materials can result for higher annealing temperatures. It

was reported the TgHS values increased with increase in annealing temperature but

subsequently reaches a constant value irrespective of the annealing temperature

46

employed. The above observation indicated that the degree of polymerization has

reached its maximum and therefore at this point, non-crystalline materials are

formed. Amorphous TPUs were formed by heating the TPU samples at a

temperature above the melting temperature (290°C) and then rapidly cooling. DSC

thermographs showed that the melt-quenched TPU samples had only TgSS and TgHS.

The reason for heating the TPU samples to such high temperature was to destroy

every form of crystallinity and thereby erasing the thermal histories inherent in the

TPU samples. It was observed that by employing different annealing treatments,

crystallinity was induced in the amorphous (melt-quenched) TPUs. WAXS results

also showed that the diffraction peaks increased with increase in annealing

temperatures and the change in crystalline peaks was traceable to some degree of

ordering of the hard segment chains. The crystalline peaks also suggested the

presence of more than one crystalline form (existence of polymorphs) in the

TPUs107

.

In another publication, this group investigated the effect of annealing on TPU and

Isoplast. The TPU sample which comprised 80% hard segment concentration (MDI

and the mixture of 4:1 molar ratio mixture of 1,4-butanediol and 1-phenyl-1,2-

ethanediol) and the polyol was a 1:1 ratio of polypropylene oxide diol and

polypropylene oxide endcapped with ethylene oxide. DSC results showed that the

TPU sample displayed Tgs namely: soft segment glass transition temperature (Ts) = -

50°C, intermediate transition (Ti) = 50°C and hard segment glass transition

temperature (Th) = 100°C. It was observed that the TPU sample was amorphous as

there was no melting transition. After annealing, the non-crystalline TPU sample

showed melting transitions indicating the presence of crystallites induced by

annealing. Melting endotherms and melting enthalpies were also seen to increase

with increasing annealing temperatures. However, Tgs observed for the untreated

TPU samples remained unchanged after the annealing process. Like the TPU sample,

Ti and Th were the same for the Isoplast (a commercial TPU) sample except that the

SS glass transition temperature (Ts) was at -20°C. Annealed Isoplast showed larger

melting endotherms and increased melting temperatures with consequent increase in

annealing temperature117

.

Saiani et al59

investigated the effect of annealing temperature on the thermodynamic

properties of TPUs. For isochronal annealing, the TPU samples were annealed

47

within the same duration of time (96 hours) at different annealing temperatures (60,

90, 120 and 160°C). It was observed that when TPU samples are annealed above the

TgHS, phase separation ensues. After annealing at 120 and 160°C, a Tg at -65°C was

observed for TPU with hard segment content of 65% and 75%. This low glass

transition was attributed to the soft phase glass transition temperature (TgSP). The

value of TgSP (-65°C) was observed to be higher than that of the pure SS glass

transition temperature (-73°C) and the reason for the observation was due to the

anchoring of the HS on the SS. As hard segment concentration increases, TgSP was

not observed for TPU with HS content of 85% and 95%, this was due to the

increased mobility restriction of the HS and therefore the small amounts of soft

segments anchoring on the HS were not able to contribute to TgSP. After annealing at

120°C, two endothermic transitions were observed for HS concentrations of 65%,

75% and 85% except 95%. The first endothermic transition was assigned to the

melting of the ordered HS initiated during annealing, (TM) whereas the second

endothermic transition was assigned to the microphase mixing of both the HS and

SS, which is otherwise called microphase mixing temperature (TMMT). It was also

observed that there were no endothermic transitions observed for 95% HS

concentration after annealing at 120°C. At 95% HS content, TMMT is not expected to

be observed however, since hard segments were ordered during the annealing

process but it was expected to observe TM. It was therefore suggested that after 96

hours of annealing, the hard phase of 95% HS content was still amorphous. After

annealing at 160°C, a clear, sharp melting transition was observed for TPU with

95% HS content. It was suggested that higher ordering of the HS occurred at higher

annealing temperature. This was confirmed by the presence of weak diffraction

peaks on the WAXS diffractogram. It was also observed that clear, sharp melting

peaks were observed by 65%, 75% and 85% HS contents. The reason for the

observation was due to the merging of TM and TMMT (i.e. TM + TMMT) at higher

annealing temperatures. The melting temperatures of these three samples (65%,

75% and 85%) were seen to be higher than that of 95% HS content and this therefore

confirms that only TM was observed for 95% HS content. After annealing at 60°C

and 90°C (annealing temperatures below TgHS), all the TPU samples except 95% HS

content had melting transitions at 180°C which corresponded to TMMT. TgSP at -65°C

were observed for TPUs with 65% and 75% HS content when annealed at 90°C.

48

This also confirmed that phase separation occurred even at low annealing

temperatures. However, no TgSP was observed for the TPU samples when annealed at

60°C. SAXS results showed the presence of scattering peaks for TPU with 65% HS

concentration annealed at 60, 90, 120 and 160°C for 72 hours. This therefore

suggests the presence of a phase-separated morphology for all the samples. As

annealing temperatures increased, it was also observed that there was no significant

decrease in the scattering peaks and therefore this suggested that the maximum level

of phase separation has been attained at higher annealing temperatures. Furthermore,

phase separation was seen to decrease with increasing annealing temperatures and

also scattering maximum (q*) shifted to smaller q values with increase in annealing

temperatures59

.

Annealing has shown to be a vital post-treatment process employed to effect better

thermodynamic, structural, thermo-mechanical and mechanical properties of TPUs.

This post-treatment process has led to extensive investigations elucidating its effect

on TPUs as reported in numerous publications33, 39-40, 46, 116, 118-127

.

From the literature review, it was imperative to note that although annealing

improves the properties of TPUs; however, increase in annealing temperature and

time also is very crucial in optimizing the properties of the TPUs. Crystallinity and

TgHS of the TPUs are improved due to annealing effect. For a 100% HS TPU, only

TM (melting of the HS) is observed whereas TM and TMMT (mixing of the HS and SS)

are observed for TPUs with lower HS concentrations (especially HS% of 70 to 90).

1.7.4 THERMODYNAMIC, STRUCTURAL, THERMO-

MECHANICAL AND MECHANICAL PROPERTIES OF TPU

COMPOSITES

Several studies have shown that the thermo-mechanical and mechanical properties of

TPU were improved when reinforcements were added. Impact studies of TPU

composites revealed that the glass fibre reinforcement contributed to the fracture

resistance of the TPU below its glass transition temperature. Fracture of the

composites often takes place between the interface of the fibre and matrix128-130

.

Jancar128

concluded that the presence of the short glass fibres helped in the increase

of the yield strength on the fibre-matrix interface of the composite. Therefore

deformation was drastically reduced in the fibre-matrix interface instead fracture

49

took place between the interface and the bulk TPU. Mateen et al129

found out that the

increase in the volume fraction of the glass fibre in a composite material led to a

steady increase in the impact resistance of the composite.

The stress-strain curve of the TPU composites showed that the stiffness of the TPU

was improved as glass fibres were incorporated into the TPU matrix. However, the

elongation properties of the TPU composites upon failure reduced drastically on

addition of glass fibres to the TPU matrix. The Young’s moduli of the TPU

composites were seen to increase with increasing glass fibre content (volume

fraction of glass fibres). The fracture toughness of the TPU composites were also

observed to increase linearly with increase in volume fraction of glass fibre131

.

Incorporation of fillers (calcium carbonate and mica) into pultruded glass fibre

reinforced TPU composites was seen to increase the mechanical (flexural strength

and flexural modulus) and thermal (heat distortion temperature; HDT) properties of

the composites as the mica and calcium carbonate content increased. Post curing in

the range of 100-150°C also facilitated better mechanical and thermal properties of

the composite materials132-133

.

Correa et al studied the properties of short fibre reinforced TPU elastomer

composites containing carbon and aramid fibres. It was reported that the addition of

the short fibres into the TPU elastomer gave significant improvements in hardness,

abrasion resistance and compression set. The aramid reinforced composites showed

better abrasion resistance when compared to carbon fibre. This was due to the fact

that the carbon fibres used were fragile and were easily broken. This caused the

reduction in the properties of the carbon fibre reinforced composite. According to the

tear resistance tests conducted for all the composite samples, the values of the

composites were higher compared to that of the pure TPU matrix. The aramid

reinforced composite had the highest tear resistance134

. In another publication, it was

reported the aramid fibre-based composites were seen to degrade above 400°C

which is higher than both carbon fibre-based composites and pure thermoplastic

polyurethane. The aramid fibre-based composites showed better properties than the

carbon fibre-based composites because of their better fibre-matrix interaction

(aramid-TPU interfacial bonding)135

.

50

Tensile and storage moduli of TPU composites made with short aramid fibre were

reported to increase with subsequent increase in fibre content embedded in the

composite materials. It was reported that the effect of increasing fibre content on the

modulus of the composite was pronounced at temperatures above the glass transition

temperature (Tg) of the thermoplastic polyurethane (-36.6˚ C). Damping properties of

the composites were also much improved with higher fibre quantity. Viscosity was

heightened and shear rate was reduced as the amount of fibre is enhanced. This

observation was as a result of the TPU elastomer-aramid fibre interaction136

.

Mothe et al. also investigated the thermal and mechanical properties of TPU

composites using a plant fibre known as ‘Curaua fibre’. In their study, it was

reported that increase in the weight of the curaua fibres into the TPU matrix led the

improvement in the storage modulus at a temperature range below the softening

region of the HS. This observation was reported to be influenced primarily by the

specific interaction of the curaua fibre with the HS domain of the TPU137

.

Kutty and Nando138

investigated on the mechanical and thermo-mechanical

properties of short Kevlar-Thermoplastic polyurethane composites. In their study, it

was observed that the tensile strength exhibited by the TPU composites was

attributed to the increase in fibre reinforcement of the TPU matrix. At low fibre

concentrations, the TPU matrix was observed not to have bonded properly onto the

fibres due to the fact that the matrix is not been restrained by enough fibres. This

lack of restraint resulted in poor interfacial bonding between the fibres and matrix.

As a result of the observation, there is a complete separation of the matrix away from

the debonded fibres at low strain as there is localized strain concentration within the

TPU matrix. At high fibre concentration, there is more restraint in the mobility of the

matrix and the stress is more uniform throughout the composite system. The

softening of the TPU matrix is lessened due to the improved reinforcement from the

fibres. DMTA results showed that the storage moduli of TPU composites increased

with increasing fibre content and the effect of the fibre reinforcement was more

noticeable especially after the glass transition temperature (Tg) of the TPU. The glass

transition temperature (Tg) of the TPU remained the same irrespective of the level of

the fibre concentration. Tan delta (tan δ) curves showed that the Tg values of the

TPU composites remained the same but the transition peaks broadened with increase

in fibre content. The broadening of the tan delta (tan δ) peaks due to fibres was

51

attributed to an interaction existing at the fibre-matrix interface. It was also reported

that a polymer entrapped in fibres exist in a different state compared to a polymer

without any reinforcement. The tan δmax peak was seen to decrease with increasing

fibre concentration and this can be attributed to the improvement in the damping

properties of the TPU composite. The observation is a characteristic feature of

Kevlar-TPU composites138

.

In other publications, Kutty and Nando reported that the Young’s modulus and

tensile strength of a pure, non-reinforced TPU decreased with increase in

temperature whereas the elongation at break was seen to increase. The decrease in

the mechanical properties of the TPU with increased temperature was attributed to

the softening of the TPU. At 20 phr (parts per hundred parts of rubber) of fibre

loading, the stiffness of the TPU increased with increasing temperatures. It was also

observed that the orientation of the fibres has an effect on the properties of the

composites. In this study, fibres were layered in longitudinal and transverse

directions as shown in Figure 1.14.

Figure 1.14 Orientations of the fibres in the TPU matrix

The difference in the orientation of the fibres led to the decrease of the appearance of

flaws in the TPU composite. In the transverse direction, the fibres were parallel to

the direction of fracture proliferation and therefore there is ease of fracture. This

form of fracture was seen to be synonymous with the fracture of the pure, non-

reinforced TPU matrix. In the longitudinal direction, the fibres were more resistant to

fracture proliferation139-142

.

52

Several authors have reported on the influence of the reinforcement on the

mechanical and thermo-mechanical properties of TPU. However, the mechanical and

thermo-mechanical properties of TPU composites were observed to be related to

fibre-matrix interaction. The fibre-matrix interfacial properties play a significant role

in the improvement of the overall properties of TPU composites1, 143-155

.

53

CHAPTER 2

MATERIALS AND METHODS

2.1 MATERIALS

2.1.1 CHEMICALS USED

The names, structures, acronyms and molar masses of the chemicals used in this

work are shown in Table 2.1. The suppliers of the chemicals are also listed.

Table 2.1: Names, structures, acronyms and molar masses of the chemicals used for

the synthesis of TPUs

Name Acronym Structure Molar

mass

(gmol-1

)

Supplier

4,4'-

diphenylmethane

diisocyanate

4,4-MDI

CH

2NCOOCN

250 Sigma

Aldrich

Voranol EP 2010 EO-PPO-

EO

C

H

H

C

H

H

* O C

H

H

C

H

C HH

H

O C C O

H H

H H

*o

n

m

n

2004 Dow

Chemicals

Ethylene glycol 12ED

OHOH

62.07 Sigma

Aldrich

1,4-butanediol 14BD

OHOH

90.12 Sigma

Aldrich

1,5-pentanediol 15PD

OH OH

104.15 Sigma

Aldrich

1,6-hexanediol 16HD

OH

OH

118.17 Sigma

Aldrich

1,7-heptanediol 17HPD

OH OH

132.20 Alfa Aesar

1,8-octanediol 18OD

OH

OH

146.23 Alfa Aesar

2-methyl-1,3-

propanediol

2M13PD

OH OH

CH3

90.12 Sigma

Aldrich

1,4-

cyclohexanedimeth

anol

14CHDM

OHOH

144.21 Sigma

Aldrich

N,N-

dimethylacetamide

DMAc

N

O

87.12 Sigma

Aldrich

1,4-

diazabicyclo[2.2.2]

octane

DABCO

N

N

112.17 Air

Products

Tetrahydrofuran

HPLC grade

THF O

72.11 Sigma

Aldrich

54

2.2 SYNTHETIC - CASTING - MOULDING PROCESSES

2.2.1 SYNTHETIC ROUTE

The TPUs used in this work were synthesized using a pre-polymer route. This

synthetic route involves two steps as schematically shown in Figure 2.1. Step 1 –

Synthesis of the pre-polymer by reacting the MDI with the polyol and Step 2 –

Synthesis of the TPU by reacting the pre-polymer with additional MDI and the chain

extender.

55

STEP 1 – PRE-POLYMER SYNTHESIS

C

H

H

OCN NCO C

H

H

C

H

H

O C

H

H

C

H

CH H

H

O C

H

H

C

H

H

On

m

n

* *

o

C

H

H

N C

H

O

O C

H

H

C

H

H

O C

H

H

C

H

CH H

H

O C

H

H

C

H

H

O C

O

N

H

C

H

H

n

m

n

OCN NCO

o

C

H

H

OCN NCO

n

STEP 2 – TPU SYNTHESIS

C

H

H

N C

H

O

O C

H

H

C

H

H

O C

H

H

C

H

CH H

H

O C

H

H

C

H

H

O C

O

N

H

C

H

H

n

m

n

OCN NCO

o

C

H

H

OCN NCO OH R OH

C

H

H

C

H

H

O C

H

H

C

H

CH H

H

O C

H

H

C

H

H

O C

O

N

H

C

H

H

n

m

n

* N C

H

O

O R O *o

p

Figure 2.1 Synthetic protocol used to synthesize the TPU

80°C for 2 hours

4,4-MDI Polyol (EO-PPO-EO)

Pre-polymer (polyol encapped with reactive MDI units)

excess

Chain extender

DMAc + DABCO 80°C for 2 hours

Thermoplastic Polyurethane (TPU)

56

2.2.2 TPU FORMULATION CALCULATION

The general formulation calculation for the synthesis of TPU is given with the

following sets of equations below. Prior to the synthesis, only three (3) quantities

were known and they include: (i) the total mass of TPU to be synthesized, (ii) the

mass of HS to be formulated and (iii) Isocyanate index (Isoindex).

A solution of 25% TPU (w/w) in DMAc was formed in the TPU synthesis and in

order to derive the amount of DMAc required for the TPU synthesis, the following

set of equations hold:

(Equation 2.1)

(Equation 2.2)

(Equation 2.3)

where mTPU = mass of TPU

mDMAc = mass of DMAc

Having known the mass of HS to be formulated, the following sets of equations

below reveal the constituents involved in the different steps of the synthetic process.

The mass of HS is the sum of the mass of MDI and chain extender.

(Equation 2.4)

where mHS = mass of hard segment,

mMDI = mass of MDI,

mCE = mass of chain extender,

Since

, (Equation 2.5)

where HS% = % hard segment

We can therefore deduce that

(Equation 2.6)

57

Addition of MDI was carried out in the two steps involved in the synthesis and the

equation is as follows:

(Equation 2.7)

where mMDI (STEP 1) = mass of MDI added in Step 1

mMDI (STEP 2) = mass of MDI added in Step 2

From Equation 2.4, we can therefore do the following expansion:

(Equation 2.8)

The major constituents of TPU are shown in the equation below:

(Equation 2.9)

where mSS = mass of soft segment,

The soft segment is also known as polyol and can be derived as follows:

(Equation 2.10)

where mpolyol = mass of polyol

We can also deduce that,

(Equation 2.11)

The polyol was only added in STEP 1 which involves the formation of the pre-

polymer and hence the following equation holds:

(Equation 2.12)

where mpre-polymer = mass of pre-polymer,

mpolyol = mass of polyol added in Step 1

Whereas STEP 2 involves the addition of the pre-polymer formed in STEP 1, more

MDI and chain extender to make up the TPU formulation

(Equation 2.13)

58

Having established the set of equations leading to the formulation of the TPU, it is

therefore important to know how the amounts of different constituents were derived.

Prior to commencing STEP 1, calculations were made in order to ascertain the

correct amounts of constituents involved in the synthetic process. The pre-polymer

formulation used in this research is calculated around having a molar ratio of the

number of moles of MDI and polyol as 6:1 respectively. With respect to the molar

ratio, the number of moles of MDI and polyol in the pre-polymer is 0.2568 and

0.0428 respectively. The calculated masses of the MDI and polyol needed for the

pre-polymer formulation are 64.2g and 85.8g respectively (See Chapter 2.2.3 for

details). The reasons for the above formulation were: (i) to ensure that the polyol is

completely reacted with the isocyanate units and there are still isocyanate reactive

sites left in the pre-polymer in order to build high HS concentration, (ii) to create

reactive sites which allow for the increase of more MDI units in the polymer

structure and as well as the addition of the chain extender (diol) units onto the MDI

units in STEP 2.

Having known the Isoindex (the amount of isocyanate needed to react with the

hydroxyl units calculated in terms of stoichiometric equivalents), we can therefore

calculate the amount of HS required in the STEP 2 of the synthesis.

(Equation 2.14)

where nNCO = number of moles of NCO

nOH = number of moles of OH

The number of moles of hydroxyl groups is therefore the sum of the number of

moles of hydroxyl groups in the polyol and the number of moles of hydroxyl groups

in the chain extender (diol). The following equation therefore holds:

(Equation 2.15)

where nOH-CE = number of moles of OH in chain extender

nOH-polyol = number of moles of OH in polyol

Equation 2.14 can be re-written as:

59

(Equation 2.16)

By incorporating Equation 2.15 into Equation 2.16, Equation 2.16 can be re-written

as follows:

(Equation 2.17)

The number of moles, n is calculated as follows:

(Equation 2.18)

where fn = functionality

m = mass

M = molar mass

The number of moles of OH per gram polyol, nOH-polyol is derived from its hydroxyl

value, OHv. The hydroxyl value of a polyol is the number of milligrams of potassium

hydroxide, KOH required to titrate the hydroxyl groups in the polyol.

The number of moles of OH per gram polyol is therefore given as:

(Equation 2.19)

where mKOH = mass of potassium hydroxide

MKOH = molar mass of potassium hydroxide in milligrams = 56100mg

Hydroxyl value, OHv has the units mgKOH/g polyol and therefore Equation 2.19 can

be re-written as:

(Equation 2.20)

Taking information from Equations 2.18 and 2.20, Equation 2.17 can be re-written

as follows:

(Equation 2.21)

where fnMDI and fnCE = functionalities of MDI and chain extender respectively,

MMDI and MCE = molar masses of MDI and chain extender respectively,

60

From Equation 2.21, the total amount of MDI, MMDI needed for the synthesis can be

derived by making it the subject of the equation.

When the total mass of MDI, mMDI needed for the synthesis is known, mass of chain

extender, mCE needed is gotten by subtracting the total mass of MDI, mMDI from the

mass of HS, mHS as shown in the equation below.

(Equation 2.22)

The amount of mass of MDI, mMDI (STEP 1) needed for the pre-polymer synthesis

(STEP 1) could therefore be derived from the following equation below

(Equation 2.23)

where mMDI (PP formulation) and mpolyol (PP formulation) are the masses calculated from the 6:1

molar ratio pre-polymer formulation of the MDI and polyol respectively. The masses

of the MDI and polyol needed for the pre-polymer formulation are 64.2g and 85.8g

respectively (as discussed earlier).

Having known mMDI (STEP 1) and mCE, the amount of MDI, mMDI (STEP 2) needed for the

TPU synthesis (STEP 2) can simply be gotten as follows

– (Equation 2.24)

Numerical example of the above TPU calculation formulation

Synthesis of 1,5-pentanediol TPU-70%HS

The calculations below were formulated for the synthesis of 100g of TPU for 70%

HS concentration.

61

From Equation 2.21, we can calculate the total amount of MDI required for the

synthesis.

From Equation 2.22, we can calculate the amount of chain extender, MCE from the

total mass of MDI, MMDI calculated in Equation 2.21.

From Equation 2.23, we can then calculate the amount of MDI, MMDI (STEP 1) needed

in STEP 1 – Pre-polymer Synthesis

Equation 2.24 then gives us the additional amount of MDI needed in STEP 2 – TPU

Synthesis

Total amount of TPU synthesized from the above calculations is given below as

shown in Equation 2.9

62

2.2.3 DESCRIPTION OF TPU SYNTHESIS

STEP 1 – PRE-POLYMER SYNTHESIS

All glassware and accessories used for the synthesis were dried before use in an oven

at 50°C overnight to remove any trace of water. The chain extenders and polyol

were dried too via rotary evaporation at 80°C for 3 hours using a rotational speed of

50rpm in order to remove any residual moisture. The presence of moisture can lead

to side reactions between the water and the MDI resulting in the formation of CO2

and amine. The dried chain extenders and polyol were then stored in sealed glass

jars. Molecular sieves were added to prevent moisture contamination of the dried

chemicals.

The pre-polymer synthesis was performed under dry nitrogen atmosphere at 80°C.

The experimental setup used is shown schematically in Figure 2.2. 64.2g of MDI

granules were inserted in the reaction vessel and stirred vigorously until fully melted.

85.8g of the polyol was then placed into a pressure equalizing dropping funnel,

which was subsequently mounted onto the reaction vessel. The polyol was added

drop-wise to the liquid MDI over 1.5 hours. After the drop-wise addition of the

polyol, the reaction mixture was left to be stirred for 2 additional hours. The pre-

polymer was then stored in a glass jar and placed in a pre-heated oven (40°C) until

needed for the second step.

Figure 2.2 The experimental set-up for TPU synthesis

63

STEP 2 – TPU SYNTHESIS

First, the required amount of chain extender, catalyst and solvent were introduced in

a dry reaction vessel. The vessel was then placed in the pre-heated oil bath (80°C)

and the mixture stirred mechanically. The vessel was then put under dry nitrogen

atmosphere.

In a separate dry glass jar, the required amount of the pre-polymer, MDI and DMAc

were added. The mixture was then stirred vigorously with a magnetic stirrer until the

MDI granules were dissolved. The above mixture was then poured into a pressure-

equalizing dropping funnel and then added drop-wise over 1.5 hours to the reaction

vessel. 180g of DMAc was added into the pressure-equalizing funnel and added

drop-wise. The reaction mixture was the left to be stirred for 2 hours. The

synthesized TPUs were stored in glass jars and kept in a dry, cool place.

Small amounts of samples were collected and diluted in THF for GPC analysis.

Table 2.2 below shows the number-average molecular weight n, weight-average

molecular weight w and molecular weight distribution MWD of the synthesized

TPUs. Molecular weight distribution is the same as polydispersity index.

64

Table 2.2: Molecular weights and polydispersity indices of synthesized TPUs

Synthesized TPUs w (g/mol) n (g/mol) Polydispersity Index

12ed-70

66440±4828

21020±2276

3.16±0.1

12ed-80

67250±7096

25280±2513

2.66±0.01

12ed-90

138700±14457

60690±6299

2.29

12ed-100

162400±18354

91960±9516

1.77±0.01

14bd-70

73850±7826

23920±2619

3.09±0.01

14bd-80

54760±5399

22150±2349

2.47±0.01

14bd-90

68320±6869

26010±2567

2.63

14bd-100

123500±13223

39070±3802

3.16±0.03

15pd-70

109130±11394

29936±3293

3.65±0.02

15pd-80

107400±11428

41000±4513

2.62±0.01

15pd-90

81515±8567

30650±3029

2.66±0.01

15pd-100

106710±10913

36090±3639

2.96

16hd-70

82450±8545

30860±2295

2.67±0.02

16hd-80

84910±9004

33450±3238

2.54±0.02

16hd-90

74660±7759

29490±2890

2.53±0.02

16hd-100

78920±8309

26470±2789

2.98

17hpd-70

87570±9726

39350±3989

2.23±0.01

17hpd-100

57090±6317

27460±2793

2.08±0.02

18od-70

59780±6366

22700±2436

2.63±0.01

18od-100

112400±12339

37380±3727

3.01±0.02

2m13pd-70

69730±7351

19160±1945

3.64±0.01

2m13pd-80

59400±6204

15550±1518

3.82±0.02

2m13pd-90

75770±7794

23930±2487

3.17±0.01

2m13pd-100

77870±8347

29390±2882

2.65±0.02

14chdm-70

48380±4926

14730±1609

3.29±0.03

14chdm-80

56840±5814

20560±2164

2.76

14chdm-90

79960±8618

36010±3736

2.22±0.01

14chdm-100

64550±6869

25700±2884

2.51±0.01

65

2.2.4 SOLVENT CASTING

Figure 2.3 (a) Schematic diagram of the mould and (b) photograph of the cast TPU-

70 (15pd) sample

The mould used comprises three layers: a polytetrafluoroethylene (Teflon) plate, a

hollow aluminum frame and a steel plate all with external dimensions of 125 x 125

mm. The thicknesses of the Teflon plate, hollow aluminum frame and steel plate

were 6, 10 and 10 mm respectively. The hollow aluminum frame had inner

dimensions of 100 x 100 mm. The Teflon plate was placed between the steel slab

(a)

(b)

66

and the aluminum frame. The mould was held together by allen key bolts (Figure

2.3a). The liquid TPU was poured into the mould and placed into a pre-heated oven

at 80°C for 3 days and left to air dry. On the third day, a vacuum (76cmHg) was

applied for 6 hours to ensure full drying of the polymer. The TPU films (Figure

2.3b) obtained were then placed into plastic bags and stored in a desiccator in order

to keep them dry until needed.

2.2.5 COMPRESSION MOULDING

(Preheating of mould) (a) (Layering of polymer and fibres) (b)

(Compression) (c)

(Composite) (e) (Cooling) (d)

Figure 2.4 The schematic diagram of the compression moulding process

(a) The mould was first preheated in the compression moulder, (b) the polymer films

and fibre mats were layered together in the preheated mould, (c) the stacked-up

polymer films and fibre mats were compressed, (d) the compressed sample was left

in the moulder to cool to about room temperature and (e) the composite was then

extracted from the mould

fibre mat

polymer film

67

Compression moulding was used for three distinct purposes: (1) preparation of thin

TPU films, (2) preparation of composites and (3) preparation of melt-quenched

samples.

The TPU samples were first flattened (compressed) in order to enable their proper

layering with the glass fibre mats for composite making. Flattening of the TPU

samples was done just below the melting temperature (160°C) of the TPU polymer.

The compression press was first preheated to 150°C - 160°C. The cast TPU sample

was put between two release films, placed in the preheated moulder and then

compressed. The thickness of the flattened film formed was about 0.45 mm. The

flattened film was then taken out of the press and left to cool.

Secondly, TPU samples and TPU composites were compression-moulded in a 150 x

100 mm flash type mould. The thickness of the mould was 2 mm. In this process, the

fibre mats were cut into sizes. The mass of the fibre mats were weighed and

recorded. The mould used was first preheated to 180ºC. The flattened TPU films

were then stacked up together with the fibre mats, put into the pre-heated mould and

then compressed. The compression pressure was 10 MPa. The compression in the

moulder lasted about 3 minutes and then the composite was cooled via water

cooling. Cooling process lasted about 12-14 minutes. The schematic diagram of the

compression moulding process is shown in Figure 2.4. Composite was then weighed

after the completion of the compression moulding process bearing in mind the pre-

weighed masses of the glass fibres mats. The mass of the polymer used was

obtained by the subtraction of the mass of glass fibre from the mass of the

composite.

(Equation 2.25)

where Mp = mass of the polymer,

Mc = mass of the composite, and

Mf = mass of the fibre

The composites used in this research all have 2 mm of thickness and contain 8 glass

fibre-mat layers. The percentage weight fractions of the polymer and fibre of the

composites were about 50:50%.

68

The composites of commercially available PP and Isoplast were also compression-

moulded and were used as benchmarks in comparison with our TPUs. PP granules

(100-CA50) were purchased from Ineos Polyolefins. Isoplast is a brand name for a

TPU resin manufactured by Lubrizol Advanced Materials Inc. Isoplast TPUs are

designed for rigid polymer requirements due to their high tensile strength and impact

resistance. These specialty TPUs combine the toughness and dimensional stability of

amorphous resins with chemical resistance of crystalline materials. Isoplast TPUs are

available in impact modified, clear and glassy grades. PP and Isoplast composites

were moulded at 200ºC and 240 ºC respectively. The PP and Isoplast polymers were

obtained in form of granules. The granules were first flattened in the compression

moulder at temperatures below the melting temperatures of the PP (180ºC) and

Isoplast (200ºC) polymers as the flattening procedure follows the same procedure as

earlier described. For PP and Isoplast composites, moulds were preheated to 200ºC

and 240ºC respectively. The thicknesses of the flattened PP and Isoplast films were

about 0.40mm.

The third purpose of using the compression moulding process is the making of melt-

quenched TPU samples. Melt-quenching of TPU samples was done in the press at

220˚C. The melt-quench procedure is outlined as follows: The mould was first

preheated to 220˚C in the press. The TPU sample was put into the preheated mould

between two release films and then compressed. The sample was left for 2-3 minutes

in the press and then cooled rapidly in liquid nitrogen.

The glass fibre mats used in making the composites was E-glass plain weave mat.

The glass fibre mats were sized with silane. The fibre mat had a thickness of 0.16

mm and density of 170g/sq.m. Glass fibre mats were composed of warp and weft

yarns. The ratio of the warp and weft was 50:50. The glass fibre fabric tape was

obtained from Glasplies Company.

Polytetrafluoroethylene (PTFE) glass coated release fabric/film was used in the

compression moulding and the melt-quenching process employed in this research

work. The PTFE glass coated release fabric was purchased from UMEC Process

Materials Limited.

69

2.3 EXPERIMENTAL TECHNIQUES

2.3.1 GEL PERMEATION CHROMATOGRAPHY (GPC)

2.3.1.1 THEORETICAL ACCOUNT

Pump Sample injection valve Series of columns Detector

solvent reservoir

Recorder

Figure 2.5 Schematic diagram of GPC apparatus

Series of GPC columns

Detector response

0 Elution volume

Time

Figure 2.6 Elution processes of large and small molecules

large small

molecules molecules

70

Gel permeation chromatography (GPC) which is also known as size exclusion

chromatography (SEC) is a type of liquid chromatographic technique in which

polymer molecules are separated relative to their molecular sizes. GPC is a powerful

analytical technique for determining molecular weights and polydispersity indexes of

polymers. Figure 2.5 shows a schematic diagram of the apparatus for gel permeation

chromatography.

In GPC, a dilute polymer solution is injected into a solvent stream, which flows

through a series of columns. The columns are packed with beads of permeable gel.

The commonly used GPC columns with organic solvents are rigid pore size beads of

either crosslinked polystyrene or surface-treated silica gel. The commonly used GPC

columns with aqueous solvents are pore size beads of water-expandable crosslinked

polymers (e.g. crosslinked polyacrylamide gels), glass and silica. The pore size of

the gel is usually within the range of 50 – 106 angstroms. Due to the size of the large

polymer molecules, they are unable to pass through the beads of the porous gel and

therefore they pass through the spaces between the beads. In this way, the large

polymer molecules are first eluted out of the columns. However, the small polymer

molecules are able to pass through the pores of the beads and also through the spaces

between the pores. These small polymer molecules therefore take longer time to flow

through the columns and therefore are eluted later than the large molecules. The

elution process is shown in Figure 2.6. The elution process in GPC therefore follows

in the order of decreasing molecular sizes of the polymer in solution. The

chromatogram obtained is a plot of polymer concentration against the elution volume

and this gives information on the molar mass distribution of the polymer53, 79, 156-158

.

The volume of solvent in the GPC instrument from the instance where the dilute

polymer solution is injected, via the columns until to the instance of the detection of

the overall polymer concentration can be taken as the total of the void volume Vo

(i.e. the volume of solvent in the system outside the pore size beads) and an internal

volume Vi (i.e. the volume of solvent within the pore size beads). The volume of

solvent needed to elute polymer molecules from the instance of injection to that of

polymer concentration detection is known as its elution volume Ve. Based on the

separation relative to polymer sizes, the equation therefore follows

Ve = Vo + KseVi (Equation 2.26)

71

where Kse is a fraction of the inner pore volume penetrated by some polymer

molecules. For the small polymer molecules that are able to pass through the inner

pore volume, Kse = 1 and therefore Ve = Vo + Vi, whereas for the very large polymer

molecules that are unable to pass through the inner pore volume Kse = 0 and thus Ve

= Vo. The commonly used flow rates in GPC systems is 1cm3 min

-1 and it has been

observed that Ve is unrelated to the flow rate. This therefore signifies that there is

adequate amount of time for the polymer molecules to pass in and out of the porous

beads until equilibrium concentrations, ci and co of the polymer molecules are

established within and outside the porous beads respectively. Since equilibrium

conditions are necessary for polymer-size separation, Kse can therefore be taken as

the equilibrium constant which is defined by Kse = ci / co (it is therefore worthy to

note that the rate of penetration of the polymer molecules into the porous beads is the

same with the solvent which therefore means ci = co and Kse = 1).

The following common parameters are measured in GPC experiments namely:

number-average molar mass ( n), weight-average molar mass ( w) and

polydispersity index (PDI).

The number-average molar mass ( n) is defined as the summation of the products

of the molar mass of each molecular specie multiplied by its mole fraction.

That is, n (Equation 2.27)

Where Xi is the mole fraction of molecules of the molar mass Mi and is defined as

the ratio of Ni to the total number of molecules. The above equation therefore can be

re-written as follows:

n (Equation 2.28)

The above equation depicts the arithmetic mean of the molar mass distribution.

The weight-average molar mass (Mw) is defined as the summation of the products of

the molar mass of each molecular specie multiplied by its weight fraction.

That is, w (Equation 2.29)

Weight fraction (wi) is defined as the mass of the molecules of molar mass Mi

divided by the overall mass of all the molecules available

72

That is, (Equation 2.30)

By integrating Equation 2.30 with Equation 2.29, Mw can be written as follows:

w (Equation 2.31)

Polydispersity index (PDI) is the ratio of weight-average molar mass ( w) to

number-average molar mass ( n).

w / n (Equation 2.32)

2.3.1.2 PARAMETERS USED

The parameters used in the GPC preparation of the synthesized liquid TPUs are as

follows. 80mg of liquid (DMAc) TPU was put into a small glass jar. 10ml of

tetrahydrofuran (THF) was then added to the glass jar containing the liquid TPU in

order to dissolve and dilute the polymer. A magnetic stirrer bar was put into the jar

to enable the stirring of the polymer solution. The solution was stirred and heated at

50° C for 2 days. On the 2nd

day, 4 drops of diphenyl ether (DPE) were added to the

solution. Diphenyl ether (DPE) is an organic compound which is used as an elution

standard. The solution was then filtered with a 5-micron filter to get rid of dust and

large contaminants which might create blockages in the GPC columns. The filtered

solution is then injected into the GPC instrument. Each GPC run takes about 35 to 40

minutes to complete. Diphenyl ether (DPE) peak marks the end of a GPC run.

2.3.2 DIFFERENTIAL SCANNING CALORIMETRY (DSC)


Differential scanning calorimetry is a thermo-analytical technique which measures

the difference in the input of energy to a sample and reference necessary to keep

them at the same temperature as both materials undergo heating or cooling at

controlled rate. The sample and reference are both retained at temperature preset by

the instrument programme during a thermal change (heating or cooling) occurring in

the sample. The quantity of energy needed to be supplied to or withdrawn from the

sample to sustain zero temperature difference between the sample and the reference

is the property measured in DSC. The sample and reference are kept in the same

surroundings, metal pans with separate platforms which contain a platinum

resistance thermometer and a heater. The mass of the sample and reference holders

73

are low which allows a quick response of the resistance thermometers. Comparison

of the temperatures of the thermometers is made and the electrical power supply to

the heaters is modified in such a way that the temperatures of the sample and the

reference are the same with the programmed temperature throughout the DSC run. In

DSC, the masses of the sample and the reference do not have to be similar since

power (energy) difference is measured and not temperature difference. The rate of

energy intake by a sample is correspondent to the specific heat of the sample since

the specific heat at any given temperature ascertains the measure of the thermic

energy required to alter the temperature of the sample by a certain degree. Any

transition followed by a difference in specific heat produces a discontinuity in the

power signal and the enthalpic changes of the exothermic or the endothermic

transitions give peaks whose areas are equivalent to the overall change in enthalpy.

In order to use DSC, the change in temperature between the sample and the reference

must be calibrated in this order:

(Equation 2.33)

Where ΔT = temperature difference between sample and reference, Δ (mCp) =

difference in total heat capacity between sample and reference, and R = heating rate

(°C/min).

It is a usual practice that the sample and reference pans used in DSC runs are similar;

with the reference pan being empty and therefore in view of the aforementioned, the

equation can be written as follows:

(Equation 2.34)

The above equation is therefore the common working equation. The term ‘mCp’ is

known as molar specific heat capacity and is defined as the amount of heat energy

required to raise the temperature of 1 mole of a substance by 1°C. All DSC

instruments must be calibrated (i.e. K is already known) either by the calculation of a

known specific heat-capacity sample (usually sapphire) or by using samples with

ascertained heats of fusion such as copper and aluminum.

As K is a slow function of temperature, calibration is done on the endothermic

transitions of indium and the following equation holds:

74

(Equation 2.35)

There are two main types of differential scanning calorimetry: (a) Power-

compensation DSC (b) Heat flux DSC. The cell designs of power-compensation and

heat-flux DSCs are shown in Figure 2.7.

Power-Compensation DSC Cell Design

Sample Reference

TS ES ER TR

Heat-Flux DSC Cell Design

Sample Reference

TS TR

Figure 2.7 Cell designs of power-compensation and heat-flux DSC instruments

(where TS and TR stand for the temperatures of the sample and reference

respectively, ES and ER stand for the heat energies of the sample and reference

respectively.

In the power-compensation DSC, there are different heating systems (furnace) for

the sample and the reference whereas in the heat-flux DSC, sample and reference are

heated in the same furnace. The temperature difference of the sample and reference

is always zero (0) in power compensation DSC whereas temperature differences are

measured in the heat-flux DSC. However, calibration becomes difficult in heat-flux

DSC when the temperature differences are to be converted to energy differences.

Single

heater for

sample and

reference

Separate

heaters for

sample and

reference

75

Power-compensation DSC gives a direct measurement of the energy difference

between both materials under heating or cooling conditions. In the power-

compensation DSC trace, the exothermic transitions (crystallisation, decomposition,

polymerisation and curing) appear as negative displacements whereas the

endothermic transitions (melting, vaporisation) appear as positive displacements.

Power-compensation DSC has heating rate up to 600° C. In the heat-flux DSC trace,

the exothermic transitions (crystallisation, decomposition, polymerisation and

curing) appear as positive displacements whereas the endothermic transitions

(melting, vaporisation) appear as negative displacements. Heat-flux DSC measures

temperature differences of the sample and reference79, 156-162

.


In this project, the thermodynamic properties of TPU, PP and Isoplast were

measured using a heat-flux DSC. 10mg of sample was cut and put into and

aluminum hermetic pan and sealed with hermetic lid. The pans were placed in the

DSC analyzer to begin the runs. All the samples were characterized from an initial

temperature of -90° C to a final temperature of 220° C at ramp rate of 20° C/min.

The samples underwent 4 cycles (heating and cooling cycles). The thermal protocol

for the DSC experiment is shown in Figure 2.8. The instrument used was DSC Q100

(TA INSTRUMENTS).

-175

-150

-125

-100

-75

-50

-25

0

25

50

75

100

125

150

175

200

225

250

TgHS

TgSS

Te

mp

era

ture

(C

)

Time

Melt-quenched sample

1st cooling

TMMT

2nd heating

Full DSC run for all samples

Annealed sample

1st heating Moulded sample

Figure 2.8 The thermal protocol of DSC runs

76

2.3.3 WIDE ANGLE X-RAY SCATTERING (WAXS)


Figure 2.9 X-ray diffraction process

Wide Angle X-Ray Scattering (WAXS) is a method that is used in the determination

of the crystallinities of materials. This technique measures scattering in electron

density observed at lengths lower than 100 Angstroms (Å) and therefore leading to

diffraction maxima of scattered intensity at larger angles say greater than 5°.

Information on d-spacing between 0.1 and 1nm are provided.

In this technique, incident x-rays are scattered by atoms in all directions and could

led to the occurrence of two sets of scattered beams. Firstly, the incident x-rays

scattered by the atoms in the planes (see Figure 2.9) are totally in phase and

therefore consolidate one another to form a diffracted beam of rays. The interaction

of diffracted beam of rays is known as constructive interference. Secondly, there is

also the occurrence of the scattered x-rays being out of phase in other angles of space

and therefore cancel out themselves. This phenomenon is known as destructive

interference and in this case, there is no scattering. From the above explanations, it is

therefore worthy to note that the crucial condition of scattering beam being

completely in phase must be achieved if diffraction is to occur and this gave rise to

the concept of Bragg’s law.

Bragg’s law gives an insight on the concept of x-ray diffraction by the following

equation; it is also known as the Bragg Equation


77

(Equation 2.36)

where n is called the order of diffraction and thus may assume any integer value that

is congruent with sinθ not greater than unity, is the wavelength of incident wave, d

is the spacing between the planes in the atomic lattice and θ is the angle between the

incident ray and the scattering planes.

Diffraction occurs only when the scattering taking place in an atomic lattice has

satisfied Bragg’s law. The consolidation of the diffracted x-rays makes them stronger

than the total of all the scattered rays in the same direction but is weak in relation to

the incident x-rays and this is because the atoms of any crystalline structure scatter

only a tiny amount of the energy emergent on them160, 163-165

.

The sharp peak observed from a typical WAXS curve is due to scattering from the

crystalline region whereas the broad underlying ‘halo’ is due to scattering from the

amorphous region. The shape of the amorphous halo can be obtained by melt-

quenching (rapid cooling) a molten sample. The mass fraction of the crystals xc can

be obtained by

(Equation 2.37)

where Aa is the area under the amorphous halo and Ac is the area remaining under

the crystalline peak.

On an x-ray diffractometer, the sample which is usually powder is positioned so that

its surface is parallel to the top of the sample holder. A slightly divergent x-ray beam

is then irradiated on the sample which is rotated slowly. Diffraction maxima occur at

positions where the planes lying parallel to the surface fulfil Bragg’s Law156

.


In this project, samples were measured in Philips X’Pert APD (PW 3710) with

generator settings of 40 mA, 50 kV. The anode material of the instrument is Copper

(Cu). Analysis was done from 5.0750 °2θ (start position) to 70.9450 °2θ (end

position). The scan type and scan step time used were continuous and 10s

respectively. The step size used was 0.0700 °2θ.

78

2.3.4 SMALL ANGLE X-RAY SCATTERING (SAXS)


Figure 2.10 Schematic diagram of small angle x-ray scattering technique

Small Angle X-Ray Scattering (SAXS) is a technique where the elastic scattering of

x-rays (with wavelength range of 0.1 to 0.2nm) by a sample which contains

structural irregularities in the nanometer region, is measured at very low angles

(normally 0.1 to 10°). Small Angle X-Ray Scattering (SAXS) is a similar technique

to WAXS. In SAXS technique, the distance between sample to the detector is longer,

that is to say, the electron density fluctuations are observed at lengths in the range of

100 Angstroms (Å) or higher and therefore scattered intensities are produced at

angles 2θ less than 5°. Variations in intensities mainly originate from the differences

in electron density and/or compositions of the polymer samples. Small angle x-ray

scattering is used to determine the particle shapes or their variations in sizes.

Structural irregularities can be seen in copolymeric materials and thus phase

separation of the copolymers can be measured by SAXS technique. Schematic

diagram of small-angle x-ray technique is shown in Figure 2.10.

SAXS can be used to produce peaks in intensity of long-range ordered structures.

The extent and angular spacing of the scattered intensity give information on the size

of minute particles or their surface area per unit volume, irrespective of the sample


79

or particles being either crystalline or amorphous. Long-ordered structures are

commonly seen in polymers and biological materials. The peaks of these samples

measured by SAXS indicate the uniformity or the difference in electron density in

varying directions156, 163

.

In a SAXS instrument, a monochromatic beam of x-rays is radiated to a sample from

which some x-rays scatter, while most of the x-rays go through the sample with no

contact with it. The scattered x-rays therefore create a pattern which is then detected

by a detector placed behind the sample. The detector is perpendicular to the path of

the main beam that hits the sample.

When a beam is incident on a material with a wave vector (k0), the atoms within the

material having been irradiated by the incident beam become origins of spherical

waves. The wave vector (k0) which can also be considered as the transmitted beam is

given as:

(Equation 2.38)

As in SAXS, elastic scatterings are marked by zero energy transfers in such a way

that the intensity of the scattered beam (k1) is equal to that of the transmitted beam

(k0) such that

(Equation 2.39)

SAXS patterns are expressed as scattered intensity as a result of the intensity of the

scattering vector (q). The scattering vector (q) can be seen as the criterion on which a

sample is detected and is an important factor to analyze the zero energy transfer

interactions such that

(Equation 2.40)

From Bragg’s law, distance probed is inversely relative to scattering vector (q) and is

given as:

(Equation 2.41)

80

(Equation 2.42)

where 2θ is the angle between the incident x-ray beam and the detector measuring

the scattered intensity and λ is the wavelength of the x-rays.

SAXS patterns are typically seen as scattered intensity as a function of the

magnitude of the scattering vector . One explanation about the

scattering vector is that it is a parameter with which a sample is detected. For

instance, in the case of two-phase sample (e.g. small particles in a liquid suspension),

the only difference that will result in scattering is the Δρ, which is difference in the

average electron density between the small particles and its immediate environment.

Variations in density due to atomic structures can only detected at higher angles in

the WAXS region. This therefore means that the total intensity of 3-dimensional

SAXS pattern is an invariant quantity corresponding to the square Δρ2. For an

isotropic pattern, this invariant quantity becomes where the integral

runs from q = 0 to wherever the SAXS pattern is assumed to end and the WAXS

pattern begins. There is also an assumption that the density does not vary in the

liquid or inside the small particles, that is to say, there is binary contrast.

At the high resolution end of SAXS pattern, scattering comes primarily from the

interface between two phases and therefore intensity should decrease with the fourth

power of q if the interface is smooth. In this regime, any other structural features of

the phases are random and have no effect on scattering parameters. This is known as

Porod’s law.

(Equation 2.43)

This therefore allows the surface area S of the particle to be measured by SAXS165

.


In this project, samples were measured in Hecus S3-MICRO X-ray scattering

facility. All tests were carried out using tungsten (W) filter and the scan time was

1000s. Silver Behemate (AgBeh) was used to calibrate the instrument before any

sample run. The thickness of the sample run was in the range of 1 – 2 mm. The

results were later analysed using fit2d software.

81

2.3.5 THERMO-GRAVIMETRIC ANALYSIS (TGA)


Figure 2.11 Schematic diagram of thermo-gravimetric technique

Thermogravimetric Analysis (TGA) is a thermo-analytical method. This technique is

based on the measurement of the weight of a material relative to temperature (or

time) in a regimented environment. This technique is appropriate for measuring the

chemical occurrences (only the ones that result in a change of mass) that take place

in a material as volatile products are given off during the heating process. Constant

measurements of sample temperature and sample weights are taken as the

temperature is ramped at a constant rate. The resultant graph of weight against

temperature is known as the thermogravimetric curve. The instrument used for

carrying out thermogravimetric studies is called a thermobalance; which is a

sensitive balance with a sample pan enclosed within the furnace.

The weight of a sample stays the same or is altered during heating unless the sample

integrates with its atmosphere. Weight loss of sample is encountered when volatile

gases entrapped within the sample are evolved. TGA is an essential method for

distinguishing between thermal occurrences arising from the physical transitions and

those arising from chemical transitions in a sample. Chemical transitions happen

spontaneously only on the heating of the sample and not on cooling and hence a

82

controlled cooling process is rarely employed in TGA. The thermogravimetric curve

shows sequences of weight-loss events as consecutive reactions resulting in the

evolution of gases take place at different temperatures (in particular, within different

temperature ranges) during the regimented heating process. Isothermal weight-loss

measurements resulting in weight versus time graphs are also employed specifically

for kinetic studies of decomposition reactions. TGA is also used to look essentially

at sample degradation. The schematic diagram of thermo-gravimetric analysis is

shown in Figure 2.11.

Apart from the common weight-loss curve (thermogravimetry), the thermobalance

can also be used for two other types of measurements namely: (i) Derivative

thermogravimetry (DTG) and (ii) Differential thermogravimetry.

In derivative thermogravimetry (DTG), the degree of the sample weight loss is taken

relatively as a result of the sample temperature. The derivative thermogravimetry

(DTG) curve shows better outcome of two or more processes taking place at the

same temperature than the common weight-loss TGA graph and the DTG graphs are

simpler to compare with the results obtained from other thermal analysis

techniques156-159, 166

.


All samples were placed on platinum pans and heated in an inert environment. The

sample weight is about 20mg. The temperature range for the controlled heating

process was 0°C - 1000°C. All experiments were carried out on a modulated TGA

2950 (TA Instruments).

83

2.3.6 TOMOGRAPHY


Figure 2.12 Schematic diagram of Computer Tomography instrumentation

Tomography is a technique that employs the use of x-rays to measure cross-

sectional, two-dimensional representations of a body or an object. This technique has

various applications such as airport screening, medical imaging etc. One of the

important parameters associated with radiation is its intensity I. Intensity I, is defined

as the quantity of photonic energy radiated through unit area per unit time.

Mathematically, it can be expressed as follows:

(Equation 2.44)

where h is Planck’s constant, v is the frequency of the photon of radiation emitted, S

is area and t is time.

When an X-ray beam of intensity I(0) is aimed at an object, some physical

transformations happen within the object. These transformations are accountable for

the attenuation of the radiation, its dissipation of energy and the attendant increase in

the temperature of the object. The mechanisms involved in these transformations

within the object include: (a) the photoelectric effect (absorption) and (b) coherent

and incoherent scattering.

84

For the photoelectric effect, the radiated photonic X-ray beams interact with the

electron shells of the atoms within the irradiated sample. Some of the incident beam

is used to overpower the binding energy of ejected electrons whereas some beams

interact with the photoelectrons in the form of kinetic energy.

(Equation 2.45)

where Eb is the electron binding energy, Ek is the kinetic energy transferred to the

photoelectron, Ei = hv is the energy of the incident photon.

The spaces in the lower electron shells enable other electrons from the higher shells

to migrate into them. The difference in energy between the electron that has migrated

to the lower shell and the electron that has moved from the higher shell is radiated as

a quantum of the secondary X-ray energy. In each element, only specific transitions

between electron shells are allowed and therefore the quanta of radiated beams have

well-defined intrinsic wavelengths.

Attenuation of radiated beams can also be influenced by both coherent (Rayleigh)

and incoherent (Compton) scattering. For the case of Rayleigh scattering, incident X-

ray photons on the sample diffract without loss of energy (elastic scattering) whereas

for Compton scattering, the radiated beams are diffracted and lose energy on

interaction with the electrons on the sample (inelastic scattering). The remaining

energy of the scattered quantum energy can be expressed by

(Equation 2.46)

where ζ is the ratio of the incident quantum energy to the remaining energy of the

target electron with which the quantum interacts, ξ is the angle of scattering, En =

hv’ is the energy of the scattered quantum, and v’ is the frequency of the scattered

quantum.

Having considered the factors that account for the attenuation of radiation, the total

value of the factor responsible for the attenuation can therefore be determined. This

is called the mass attenuation coefficient or the linear attenuation coefficient. This

coefficient is given as:

(Equation 2.47)

85

where μa is the true absorption coefficient caused by the photoelectric effect

(absorption), μc is the scattering coefficient for the coherent (Rayleigh) scattering, μn

is the scattering coefficient for the incoherent scattering167

. The schematic diagram

of computer tomography instrumentation is seen in Figure 2.12.


The processes involve capturing the images of the TPU composites and subsequently

reconstructing the captured images. Captured tomograms were analyzed with the

Avizo software. Several programs such as Orthoslice and Isosurface used to clearly

view the surface, cross-sectional and internal properties of the TPU composites.

Experiments were carried out on a X-Tek XT H 225 tomography machine.

2.3.7 RHEOMETRY


Figure 2.13 Schematic representation of the squeeze-flow technique

Rheology can simply be defined as the flow and deformation of materials. In

rheology, the flow aspect of materials is more emphasized than the deformation

aspect.

Rheology measurements (eg. viscosity) are used to evaluate the flow of a substance

especially liquids but also for soft solids or solids under conditions in which they

melt-flow. Rheological measurements are carried out on materials that have a

86

complex structure and morphology such as polymers. The flow of materials cannot

be ascertained by a single value of viscosity at a particular temperature. Viscosity of

a material can be influenced by a range of factors such as increase in temperature

affects the viscosity of polymeric substances. There are two groups of liquids

according to their viscosities, namely (a) Newtonian and (b) Non-Newtonian liquids.

A Newtonian liquid is one whose viscosity changes with temperature and pressure

but is not affected by the rate of deformation or time. A Newtonian liquid does not

exhibit any elastic properties or extensional irregularities. Conversely, a Non-

Newtonian liquid is one whose viscosity is affected by the rate of deformation and

time. It exhibits elastic properties and extensional irregularities168-169

.

A squeeze-flow technique developed by Huntsman Polyurethanes (Figure 2.13) was

used for rheological measurements. The essence of the rheology measurement was to

determine the melt-flow of the TPU especially when it is used to make composites.

The melt flow of the polymer (matrix) into the fibre mats influences the nature of the

fibre/matrix interface.


The TPU films were cut into 2mm-thick discs and placed on the platens of the

rheometer. The TPUs were heated according to their melting temperature range. The

melting temperature range used for the TPUs is from 160°C to 270°C. The normal

force and shear stress used were 5N and 0.2Pa respectively. The ramp rate, %strain

and frequency values used were 3°C/min, 0.5 and 1Hz respectively. All experiments

were carried out on an AR 2000 Rheometer (TA instruments).

2.3.8 DYNAMIC MECHANICAL THERMAL ANALYSIS (DMTA)


Dynamic mechanical thermal analysis (DMTA) measures molecular mobility in

polymers. DMTA determines the changes in mechanical properties (such as modulus

and damping) as a function of temperature and/or frequency. The technique depends

on applying a small sinusoidally changing stress on a material which consequently

varies the strain of the material. DMTA also gives information on the different

temperature transitions (glass transition temperatures) undergone by the materials

during the period of characterisation. The following are the properties measured by

87

DMTA instrument: (i) Storage modulus: E’ or G’, (ii) Loss modulus: E’’ or G’’ and

(iii) Damping factor: tan δ = (E’’/E’) or = (G’’/G’)

There are two methods of DMTA techniques namely: (a) Decay of free oscillations

and (b) Forced oscillations. Forced oscillation method is the most commonly used.

In this method, force is applied to the material and then allowed to vibrate after the

stress has been removed; hence the deformation of the sample is evaluated.

Deformation of the sample under stress makes it easy to measure the modulus

(stiffness) of the sample. By measuring the displacement in phase lag of the strain in

comparison to the applied force, the damping properties of the sample can be

ascertained. The damping is called tan delta and it is seen as the tangent of the phase

lag, which is an angle. The glass transition temperature (Tg) of the sample is seen on

the tan delta (tan δ) curve. This phase lag is shown in Figure 2.14.

Applied stress

Calculated Strain

δ

1.0s 1.5s 2.0s

Figure 2.14 Phase lag in displacement of strain in comparison to the applied stress

Depending on the nature of sample, there can be different responses to the sinusoidal

stress applied to the sample. For elastic samples, the stress and strain are in phase

and in this way, there is no energy loss. Tan δ is 0°. This is shown below in Figure

2.15.

Figure 2.15 Elastic deformation

Stress

Strain

88

Viscous materials have stress and strain totally out of phase and also energy is

completely lost. Tan δ is 90°. This is shown in Figure 2.16.

Stress Strain

Figure 2.16 Viscous deformation

Viscoelastic materials exhibit characteristics of both elastic and viscous materials

and as such some energy is reserved while some energy is lost. Tan δ is greater than

0° and less than 90° (0° < δ < 90°). This is shown in Figure 2.17.

Figure 2.17 Viscoelastic deformation

Dynamic mechanical parameters are described by the following equations;

Stress (σ) can be described mathematically as

(Equation 2.48)

where is the stress amplitude, is the cyclic frequency and is the time. The

mean stress for reversed loading in which the tensile and compressive amplitudes

are equal is zero. For a viscoelastic solid, there is a phase lag of strain behind the

stress. The resultant strain is

(Equation 2.49)

where is the strain amplitude and is the mean strain.

The dynamic mechanical features of a polymer are therefore explained in terms of a

complex dynamic modulus:

(Equation 2.50)

Stress Strain

89

where is known as the storage modulus and is a measure of the degree of

recoverable strain energy (elasticity of a material) and when the force applied is

small, it becomes approximately similar to Young’s modulus. is known as loss

modulus and it measures the degree of unrecoverable energy dissipation (viscous

property of a material). The phase angle ( ) is given by

(Equation 2.51)

The stiffness and the damping properties of a material can be explained by any two

of the quantities , and tan (the ‘loss’ tangent)156

.

DMTA is useful in determining the properties of viscoelastic materials such as

polymers. A material can be subjected to different mechanical tests using different

DMTA clamp designs. These various DMTA head designs are (a) Dual Cantilever

(b) Single Cantilever (c) Compression (d) Shear (e) Tension.

Although DSC is used for investigating the phase transitions of materials, DMTA is

a more sensitive technique and these phase transitions (Tgs) are easier to investigate.

Furthermore, DMTA is capable of evaluating weak transitions such as secondary

relaxations (sub-Tg transitions such as beta, gamma and delta relaxations). These

weak transitions are due to the rotation of branches of functional groups and the re-

ordering of crystalline regions53, 156-157, 170

.


In this research, the properties of the samples were measured using dual

cantilever clamp method. It is a flexural test which involves a fixed clamping of both

ends of a sample whereas the midpoint of the sample is oscillated under stress as the

temperature of sample is changed over time. Schematic diagram of dual cantilever

method is shown in Figure 2.18.

Composite material

Figure 2.18 Schematic diagram of dual cantilever DMTA method

90

The properties of different TPU, PP and Isoplast composites were measured. Firstly,

a rectangular shape of the sample was cut. The length, width and thickness of the

samples were 35mm, 12.8mm and 2mm repectively. Amplitude and frequency used

for the measurements were 10 microns and 1Hz respectively. The ramp rate was 3°

C/min. The start to final temperatures used was -100 to 170° C. The instrument used

was a DMA Q800 (TA Instruments).

2.3.9 TENSILE TESTING


The composite samples measured were cut in rectangular strips. The original

dimensions of the composite samples used for the tensile testing are 150 mm in

length and 12 mm in width. Composite tabs (with dimensions of 40 mm in length

and 12 mm in width) were glued to the composite ends. The final dimensions of the

composite sample used are as follows: the gauge length of the composite sample was

70 mm and the width; 12 mm. The test extension rate of the tensile testing was

2mm/min. The load cell was 25kN and an MTS 634.31F-24 extensometer set at

20mm gauge length was used. All experiments were carried out on an Instron 5569

at room temperature. BlueHill 2 software was used to run and analyze the composite

samples.

2.3.10 CREEP TESTING


Creep is a time-dependent mechanical property and is a measure of deformation

undergone by a polymer subjected to constant stress. In creep tests, polymers exhibit

large strains and their response is non-linear. If the stress is held over a period of

time, the strain increases and consequently creep sets in. The shortcoming of creep is

that the modulus of the material also decreases during the period of time the load is

applied, i.e. stress, σ is directly proportional strain, ε. This can be mathematically

written as follows:

(Equation 2.52)

where E is Young’s modulus. Since ε is not consistent with constant σ, E also varies

and E in this case is known as creep modulus. Figure 2.19 shows a diagram

91

illustrating the input and output results of the stress and strain respectively and how

that affects the creep modulus.

Figure 2.19 The relative response of stress – strain and their effect on creep modulus

The resulting strain e(t) after the application of a constant stress can be divided into

three parts:

(i) e1, an instantaneous response synonymous to that of an elastic material.

This occurs immediately at the instant of the application of stress;

(ii) e2(t), which tends towards a constant value as t moves to infinity;

(iii) e3(t), which becomes linear with time

92

The representation of the aforementioned parts of strain under constant stress can be

seen Figure 2.20.

Figure 2.20 Creep diagram showing the three parts of strain responses of a material

under constant stress

From Figure 2.20, if we assume that the creep process is linear; that is to say, each

part of the strain is proportional to the constant stress applied, a time-dependent

creep compliance J(t) can be defined as

(Equation 2.53)

The Boltzmann superposition principle explains that the strain observed at any

particular time in a viscoelastic system is dependent on the stress history of that

viscoelastic system to that time. This principle also assumes that every change in

stress makes an individual contribution to the strain at any particular time and these

individual contributions sum up the total strain observed on the sample53, 80-81, 160, 171-

172. The equation of the total strain can be seen as follows:

(Equation 2.54)


The composite samples measured had length and width specifications of 50mm and

12mm respectively. A 3-point bending clamp was used. Different duration of creep

measurements was also employed. All samples were carried out isothermally at

25°C. All experiments were carried out on a DMA Q800 (TA Instruments). The

93

stress applied for all the samples was 10MPa. The sample was loaded for 20 minutes

and the recovery time was 20 minutes.

2.3.11 OPTICAL MICROSCOPY

In this research work, the reason for the use of the optical microscopy technique was

to evaluate the interfacial properties of the TPU composite. The mechanical

properties of a composite material are dependent on its fibre/matrix interfacial

bonding. This technique therefore reveals the adhesion and wettability of the fibre by

the matrix. The optical microscope also enables us to see the defects and voids that

might be present in the composite material. These defects and voids could adversely

affect the properties of the composite material.


Prior to use of the optical microscope, the TPU composite was first embedded in a

Kleerset polyester resin and allowed to cure overnight. The composite sample was

then ground on silicon carbide sheets with specifications of P240, P400, P800 and

P1200. The composite sample was then later diamond-polished with grit

specifications of 6μm, 1μm, 1/4μm and the final polishing was on 0.04μm with

colloidal silica. Micrographs were captured on Olympus BH-2-reflected light

compound-microscope with a Zeiss MRm digital camera.

94

CHAPTER 3

RESULTS AND DISCUSSION (I)

TPUs

3.1 WEIGHT LOSS PROPERTIES

173-175Research has shown that the thermal decomposition of TPUs involves three

major steps:

The 1st step (I) is the production of primary amine, alkene and carbon dioxide from

MDI and chain extender — According to the representation shown below, the first

degradation step ‘1’ also known as ‘early degradation stage’ is believed to occur at

the breaking of hydrogen bonds within the HS region. Primary amine is produced by

the attachment of an alkyl group from the chain extender to the amine formed from

the isocyanate. Carbon dioxide is also produced from the isocyanate. The production

of alkene comes primarily from the chain extender since vicinal diols are produced

from the oxidation of alkenes. The chemical reaction for the first degradation step is

shown below.

The 2nd

step (II) is the dissociation of MDI and polyol (urethane linkage) to

isocyanate and alcohol — The second degradation step ‘2’ also known as ‘main

degradation stage’ (dissociation to isocyanate and alcohol) is believed to occur at

the breaking of the urethane linkage. The reason of the observation is that alcohol is

used in the formation of ether. The chemical reaction for the second degradation step

is shown below.

The 3rd

step (III) is the production of secondary amine and carbon dioxide from MDI

and other organic components — The third degradation step ‘3’ also known as ‘final

95

degradation stage’ (production of secondary amine and carbon dioxide) comes

primarily from the further combustion of isocyanate and other organic matters which

leads to the production of secondary amine and carbon dioxide. Char is also

produced after the evolution of volatile gases from the TPU in the final combustion

stages176-177

. The chemical reaction for the third degradation step is shown below

A representation illustrating the dissociation of TPU constituents is shown below.

MDI CE MDI PolyolCEMDIPolyol **

where MDI = Isocyanate, CE = Chain extender

The full elucidation of the degradation process of the polymers synthesized for this

work is beyond the scope of this thesis. TGA was used to assess the stability of the

samples and measure the onset of thermal degradation to inform the development of

processing protocols of these materials as will be addressed later. Figure 3.1 shows

the weight loss profiles of TPU constituents namely: MDI, polyol and chain

extenders (12ed, 14bd, 14chdm, 2m13pd, 15pd, 16hd, 17hpd and 18od).

0 200 400 600 800 1000

-20

0

20

40

60

80

100

Wei

ght (

%)

Temperature (ºC)

MDI

Polyol

12ed

14bd

14chdm

2m13pd

15pd

16hd

17hpd

18od

Figure 3.1 Weight losses of TPU constituents

2 1 3

96

12ed TPU series

Figure 3.2 shows the TGA curves of 12ed (Ethylene glycol) TPU series.

Figure 3.2 12ed TPU series TGA curves obtained for the (a) weight loss (b)

derivative weight loss versus temperature

As can be seen from Figures 3.2 (a) and (b), these set of samples have complex

degradation profiles with a significant number of weight loss steps being observed. It

was suggested that the peaks seen from 200°C to 300°C could be traceable to

unbonded HS (not bonded into TPU) which results from premature phase

separation178

. However, it can clearly be seen that the onset of thermal degradation

(onset of weight loss) decreases with increasing HS concentration going from 222°C

for the 12ed-70%HS sample to 177°C for 12ed-100%HS sample. The observation

could be traceable to the amount of polyol in the TPU system179

.

14bd TPU series

Figure 3.3 shows the TGA curves of 14bd (1,4-butanediol) TPU series. In Table

3.1, data on the weight losses in the different temperature ranges are presented:

160°C to onset of significant weight loss temperature, onset temperature to offset of

major degradation step and offset temperature to 1000°C.

0 100 200 300 400 500 600

0

20

40

60

80

100

Cast TPU-70 (12ed)

Cast TPU-80 (12ed)

Cast TPU-90 (12ed)

Cast TPU-100 (12ed)

We

igh

t (%

)

Temperature (ºC)

0 100 200 300 400 500 600

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Cast TPU-70 (12ed)

Cast TPU-80 (12ed)

Cast TPU-90 (12ed)

Cast TPU-100 (12ed)

De

ri. W

eig

ht (%

/ºC

)

Temperature (ºC)

(b) (a)

97

Figure 3.3 14bd TPU series TGA curves obtained for the (a) weight loss (b)


Table 3.1: Weight losses of 14bd TPUs

Casted

TPUs

Weight loss

T(onset)/˚C

Region I

(160˚C-288˚C)

%Weight loss

Region II

(288˚C-329˚C)

%Weight loss

Region III

(329˚C-1000˚C)

%Weight loss

14bd-70 288±0.74 1 63 36

14bd-80 253±0.78 4 54 40

14bd-90 146±0.05 6 44 50

14bd-100 174±0.28 12 46 42

Figure 3.3 shows that the 14bd TPUs are more thermally stable than the 12ed TPUs.

The 14bd-100%HS TPU was seen to have the highest weight loss (12%) within the

temperature range of 160°C and 288°C. The above observation is due to the early

degradation of the HS as earlier discussed. From Figure 3.1, it can be observed that

the main degradation of MDI and 14bd occurred at temperatures of 179°C and

171°C respectively. These temperatures are similar to the onset of thermal

degradation (T(onset)) of 14bd-100%HS TPU reported to be at 174°C as shown in

Table 3.1. Remarkably pronounced in Figure 3.3 (a) is the thermal stability of

0 100 200 300 400 500 600

0

2

4

6

8

10

12 Cast TPU-70 (14bd)

Cast TPU-80 (14bd)

Cast TPU-90 (14bd)

Cast TPU-100 (14bd)

De

ri. W

eig

ht (%

/ºC

)Temperature (ºC)

0 100 200 300 400 500 600

0

20

40

60

80

100

Cast TPU-70 (14bd)

Cast TPU-80 (14bd)

Cast TPU-90 (14bd)

Cast TPU-100 (14bd)

We

igh

t (%

)

Temperature (ºC)

(a)

(b)

98

14bd-70%HS TPU. The percentage weight loss measured from between 160°C and

288°C was 1%. The observation can be traceable to the amount of SS incorporated in

the TPU system. The elastic property of the soft segment (polyol) is believed to

improve the thermal stability of the TPU179

. Figure 3.1 shows that the main

degradation of the polyol occurred at 286°C and since 14bd-70%HS TPU has the

highest proportion of polyol (30%), there is therefore increased thermal stability in

the TPU system. Figure 3.3 (b) shows peaks marked by the main degradation stages

of the 14bd TPUs and from Table 3.1, 14bd-70%HS TPU had the highest weight

loss (63%) and this weight loss decreases with increase in HS concentration. The

main reason for the observations is that thermal degradation of TPU generally begins

in the hard block regions and considerable weight loss occurs in the ‘main

degradation stage’ which involves the dissociation of both the HS and SS regions180

.

This therefore corresponds with the thermal degradation profile of the polyol in

Figure 3.1. The observed good thermal stability properties of 14bd TPUs could be

attributed to the bond cohesion of the samples92, 115

.

From Figure 3.3 (a), the occurrence of the early degradation profiles observed for

both 14bd-90%HS and 14bd-100%HS TPUs could be attributed to the breakdown of

the short range order and long range order HS chains as shown by Frick and

Rochman115

.

14chdm TPU series

Figure 3.4 shows the TGA curves of 14chdm (1,4-cyclohexanedimethanol) TPU

series. In Table 3.2, data on the weight losses in the different temperature ranges are

presented: 160°C to onset of significant weight loss temperature, onset temperature

to offset of major degradation step and offset temperature to 1000°C.

99

0 200 400 600

0

20

40

60

80

100

We

igh

t (%

)

Temperature (ºC)

Cast TPU-70 (14chdm)




Figure 3.4 14chdm TPU series TGA curves obtained for the (a) weight loss (b)


Table 3.2: Weight losses of 14chdm TPUs

Casted

TPUs

Weight loss

T(onset)/˚C

Region I

(160˚C- 290˚C)

% Weight loss

Region II

(290˚C-345˚C)

% Weight loss

Region III

(345˚C-1000˚C)

% Weight loss

14chdm-70

198±0.36 7 59 34

14chdm-80

148±0.02 16 43 41

14chdm-90

149±0.11 18 35 47

14chdm-100 158±0.69 24 0 0

From Figure 3.4, the 14chdm TPUs generally showed very low thermal stability.

The percentage weight loss of 14chdm-100%HS between 160°C and 290°C was

measured to be 24%. 14chdm-70%HS TPU showed the lowest thermal degradation

(7% weight loss) between 160°C and 290°C. However, 14chdm-70%HS TPU had

the highest weight loss (59%) in the main degradation stage (290˚C - 345˚C) which

is mainly characterized by the degradation of the soft blocks as seen in Table 3.2.

(b) (a)

0 100 200 300 400 500 600

-10

0

10

20

30

40

50





De

ri. W

eig

ht (%

/ºC

)

Temperature (ºC)

100

One of the reasons for the observations could be traceable to the tacky nature of the

TPU samples arising from the molecular architecture of the chain extender.

2m13pd TPU series

0 200 400 600

-20

0

20

40

60

80

100

We

igh

t (%

)

Temperature (ºC)

Cast TPU-70 (2m13pd)




Figure 3.5 2m13pd TPU series with hard segment concentrations (a) weight loss (b)

derivative weight loss as a function of temperature

Figure 3.5 shows the TGA curves of 2m13pd (2-methyl-1,3-propanediol) TPU

series. In Table 3.3, data on the weight losses in the different temperature ranges are

presented: 160°C to onset of significant weight loss temperature, onset temperature

to offset of major degradation step and offset temperature to 1000°C.

Table 3.3: Weight losses of 2m13pd TPUs

Casted TPUs

Weight loss

T(onset)/˚C

Region I

(160˚C-270˚C)

% Weight loss

Region II

(270˚C-311˚C)

% Weight loss

Region III

(311˚C-1000˚C)

% Weight loss

2m13pd -70

216±0.96 4 41 55

2m13pd -80

153±0.35 5 39 56

2m13pd -90

128±0.73 9 32 59

2m13pd -100

145±0.92 14 0 0

0 100 200 300 400 500 600

0

10

20

30

40

50





De

ri. W

eig

ht (%

/ºC

)

Temperature (ºC)

(b)

(a)

101

In Figure 3.5, 2m13pd TPUs show similar degradation trend as 14chdm TPUs

discussed earlier except that the 2m13pd TPUs have better thermal stability

properties. Table 3.3 showed that from the temperature range of 160˚C to 270˚C, the

% weight losses of TPUs increased with increase in HS aggregation. Like the

14chdm TPUs, the 2m13pd TPUs are tacky as well due to the chemical structure of

the 2m13pd chain extender. The branched structure of the 2m13pd extender does not

allow easy packing of the polymer chains as the methyl (–CH3–) pendant group

distorts the crystallization of the polymer chains and also imparts flexibility on the

polymer chains181-182

.

It can also be suggested that from the above observations, the TPU samples (2m13pd

and 14chdm) are likely to still have residual solvent in them after the casting process

and therefore due to the tacky nature of the samples (especially the 80%, 90% and

100%HS samples), there is more likelihood for the samples to retain the solvent. The

boiling point of the solvent (DMAc) used in the synthesis of the TPUs is 165°C and

the entrapment of the solvent within the polymer will impart low thermal stability

properties of the polymer.

15pd, 16hd, 17hpd and 18od TPU series

Figure 3.6 shows the weight losses of 15pd (1,5-pentanediol), 16hd (1,6-

hexanediol), 17hpd (1,7-heptanediol) and 18od (1,8-octanediol) TPU series having

different HS concentrations (70%, 80%, 90% and 100%HS) whereas Table 3.4

shows a full data on the weight losses of 15pd, 16hd, 17hpd and 18od TPUs.

0 100 200 300 400 500 600

0

2

4

6

8

10

Cast TPU-70 (15pd)

Cast TPU-80 (15pd)

Cast TPU-90 (15pd)

Cast TPU-100 (15pd)

De

ri. W

eig

ht (%

/ºC

)

Temperature (ºC)

0 100 200 300 400 500 600

0

20

40

60

80

100

Cast TPU-70 (15pd)

Cast TPU-80 (15pd)

Cast TPU-90 (15pd)

Cast TPU-100 (15pd)

We

igh

t (%

)

Temperature (ºC)

(b)

(a) I.

102

0 100 200 300 400 500 600

0

20

40

60

80

100

Cast TPU-70 (16hd)

Cast TPU-80 (16hd)

Cast TPU-90 (16hd)

Cast TPU-100 (16hd)

We

igh

t (%

)

Temperature (ºC)

0 200 400 600

0

2

4

6

8

10

12

Cast TPU-70 (16hd)

Cast TPU-80 (16hd)

Cast TPU-90 (16hd)

Cast TPU-100 (16hd)

De

ri. W

eig

ht (%

/ºC

)

Temperature (ºC)

(b)

(a)

0 100 200 300 400 500 600

0

2

4

6

8

10

Cast TPU-70 (17hpd)

Cast TPU-100 (17hpd)

De

ri. W

eig

ht (%

/ºC

)

Temperature (ºC)

0 100 200 300 400 500 600

0

20

40

60

80

100

Cast TPU-70 (17hpd)


We

igh

t (%

)

Temperature (ºC)

(b)

(a)

II.

III.

103

Figure 3.6 (a) Weight losses (b) Derivative weight losses as a function of

temperature of (I) 15pd TPUs, (II) 16hd TPUs, (III) 17hpd TPUs and (IV) 18od

TPUs

Table 3.4: Weight losses of 15pd, 16hd, 17hpd and 18od TPUs

Casted TPUs

Weight loss

T(onset)/˚C

Region I

(160˚C - 300˚C)

% Weight loss

Region II

(300˚C - 346˚C)

% Weight loss

Region III

(346˚C-1000˚C)

% Weight loss 15pd-70

300±0.91 2 63 35

15pd-80

240±0.31 6 48 46

15pd-90

185±0.93 10 49 41

15pd-100

164±0.96 14 53 33

16hd-70

281±0.97 4 66 30

16hd-80

237±0.69 5 49 46

16hd-90

220±0.37 8 46 46

16hd-100

138±0.80 9 51 40

17hpd-70

292±0.36 2 71 27

17hpd-100

177±0.38 10 55 35

18od-70

300±0.91 2 66 32

18od-100

159±0.78 8 66 26

0 100 200 300 400 500 600

0

20

40

60

80

100

Cast TPU-70 (18od)

Cast TPU-100 (18od)

We

igh

t (%

)

Temperature (ºC)

0 100 200 300 400 500 600

-2

0

2

4

6

8

10

12

14 Cast TPU-70 (18od)

Cast TPU-100 (18od)

De

ri. W

eig

ht (%

/ºC

)

Temperature (ºC)

(b)

(a)

IV.

104

In Figure 3.6, the weight loss TGA curves of 15pd, 16hd, 17hpd and 18od TPUs are

alike and follow a similar trend to the 14bd TPUs discussed earlier. These set of

TPUs are generally stable within the temperature region of 160°C to 300°C. The

thermal stability of these samples could be attributed to the linearity of the chain

extenders and hence, there is better packing of the HS chains for crystallization. The

100% samples have the highest % weight losses in the early degradation stages

(160˚C - 300˚C) whereas the 70%HS TPUs have the highest % weight losses in the

main degradation stages (300˚C - 346˚C) of TPU. The reason for the high % weight

losses of the 70%HS TPUs is due to the degradation of the SS which occur primarily

at the main degradation stages of the TPU as earlier shown in Figure 3.1.

The reason for the high % weight losses of the 100%HS TPUs at early degradation

stages could be seen in the DSC melting endothermic transitions of the samples

which will be discussed later in this chapter. The 100%HS TPUs were seen to have

the lowest melting temperatures and this is ultimately due to the absence of the SS.

The melting transitions of TPUs involve the melting of both the HS and SS

(Microphase Mixing Temperature) as shown in studies carried out by Saiani et al 58-

60.

From the TGA results discussed, we can observe that the 70%HS samples of all the

different chain-extended TPUs showed the highest thermal stability whereas the

100% HS samples showed the least. The reason was due to the thermal stability of

the polyether based polyol as confirmed by the TGA result of the polyol (see Figure

3.1). However, the linear chain-extended TPUs (12ed, 14bd, 15pd, 16hd, 17hpd and

18od TPUs) showed better overall thermal stability properties than non linear chain-

extended TPUs (2m13pd TPUs) and cyclic chain-extended TPUs (14chdm TPUs).

The reason was due to the higher degree of crystallinity of the linear chain-extended

TPUs as well as the chemical structures of the linear chain extenders.

105

3.2 THERMODYNAMIC - STRUCTURE PROPERTIES

This chapter presents the phase behaviour of TPUs extended with non-

linear/branched (2m13pd), linear (12ed, 14bd, 15pd, 16hd, 17hpd and 18od) and

cyclic (14chdm) chain extenders. The phase-structure of these TPUs having

undergone different post thermal treatments will also be discussed. These thermal

treatments include casting, melt-quenching, slow-cooling, moulding and annealing.

The cast TPUs serve as a platform on which post-treated TPUs originate from. The

phase-separated morphology and the crystallinity of the cast TPUs will also be

discussed. Only 70%HS and 100%HS TPUs will be discussed in this section.

3.2.1 EFFECT OF CHAIN EXTENDERS AND HARD SEGMENT

CONCENTRATION

3.2.1.1 CAST TPUs

0 50 100 150 200 250

1st Heating Cycle

He

at F

low

- E

xo


Cast TPU-70 (12ed)

Cast TPU-70 (14bd)

Cast TPU-70 (15pd)

Cast TPU-70 (16hd)

Cast TPU-70 (17hpd)

Cast TPU-70 (18od)


Temperature (ºC)

0 50 100 150 200 250

1st Heating Cycle

He

at F

low

- E

xo


Cast TPU-100 (12ed)

Cast TPU-100 (14bd)

Cast TPU-100 (15pd)

Cast TPU-100 (16hd)


Cast TPU-100 (18od)


Temperature (ºC)

Figure 3.7 1st DSC heating cycles of cast TPUs (a) 70%HS and (b) 100%HS

Figure 3.7 (a) shows that the 70%HS TPUs chain-extended with 2m13pd and

14chdm displayed very low glass transition temperatures (Tg) with little or no

defined melting transitions. Cast 2m13pd-70%HS TPU showed a little melting

transition of 153±0.9˚C which shows that the sample is slightly semi-crystalline

whereas no melting transition was observed for the cast 14chdm-70%HS TPU which

(b) (a)

106

reveals that the sample is non-crystalline. The amorphous nature of the 14chdm TPU

samples could be attributed to the mixture of cis- and trans- geometric isomers

present in the 1,4-cyclohexanedimethanol chain extender. Non-crystallinity of the

TPUs could arise due to irregularities in the polymer chain arrangements183

. Also,

the presence of cyclohexane (bulky group) can restrict the mobility of the HS leading

to poor chain arrangement in the TPUs.

For the linear chain-extended TPUs, a similar thermodynamic trend was observed.

All samples showed melting transitions and these suggest that the samples are semi-

crystalline. No Tg was observed for these sets of TPUs. However, the melting

transitions observed for the 70%HS TPUs are referred to as microphase mixing

transition (TMMT) and is assigned to the mixing of the HS and SS. Pronounced is the

high melting transitions and melting enthalpies (ΔHTot) seen for the cast 12ed-

70%HS and 14bd-70%HS TPUs. These sets of samples have high crystallinity and

therefore melted at 220°C which is above the melting temperature of TPUs generally

observed in this research work. 14bd TPUs have been reported in several literatures

to have high crystallinity92, 112, 115, 184

. Melting temperatures of 14bd TPUs above

220°C were reported to coincide with the onset degradation temperatures of TPUs58

.

High crystallinity of the samples was confirmed by the high enthalpy values

associated with the melting transitions. It was also observed that the cast 12ed-

70%HS TPU displayed sharp, single melting peak. The observation is attributed to

the existence of a single crystalline structure in the polymer chains94, 125

.

Similar trend was observed for 70%HS cast TPUs chain-extended with 15pd, 16hd,

17hpd and 18od. The TMMT values of 70% cast TPUs chain-extended with 15pd,

16hd, 17hpd and 18od are 180±0.3, 190±0.5, 164±0.5 and 182±0.5°C. The TMMT

values show that the even-numbered chain-extended TPUs have higher melting

transitions than the odd-numbered chain extended TPUs. This observation could be

linked to the odd-even effect of odd-numbered and even-numbered diols (chain

extenders) used in the syntheses of TPUs as reported by several authors94-102

. The

odd-even effect was first reported for nylons which explain the formation of

hydrogen bonds based on the alignment of polymeric chains185-192

.

Fernandez et al95

reported on a characteristic zig-zag plot depicting odd-even effect

as the even-numbered diol-TPUs have higher melting transitions than their

107

immediate odd-numbered diol-TPUs. The reason for their observation is as a result

of the arrangement of the polymeric chains. The odd-numbered diol-TPUs were

reported to have hydrogen bonds arranged in either parallel or anti-parallel

configuration whereas the even-numbered diol-TPUs only take up anti-parallel

configuration (see Figures 1.10 and 1.11). The different arrangements of both the

odd- and even-numbered TPU systems therefore provide different crystal structure

and packing which ultimately imparts their melting transitions. Their characteristic

zig-zag plot of odd-even effect corresponds to our observation. The zig-zag plot of

the TMMT values of the 70%HS cast TPUs chain-extended with 15pd, 16hd, 17hpd

and 18od is shown in Figure 3.7 (c).

5 6 7 8

160

165

170

175

180

185

190

Mic

rop

ha

se M

ixin

g T

em

pe

ratu

re (C

)


Figure 3.7 (c) Microphase mixing temperature versus number of methylene units

In Figure 3.7 (b), the melting transitions of all the 100%HS TPUs are referred to as

melting of the HS (TM). Tgs and irregularities in melting transitions were observed

for the cast 2m13pd-100%HS and 14chdm-100%HS TPUs. The low Tgs observed for

these 100%HS samples means that the Tg has not arisen from to the soft segment

because the 100%HS TPUs contain no soft segment. The reason for this observation

is likely due to the presence of residual solvents in the samples. As a result of this,

these samples after the casting process were tacky. This observation can be

supported by the TGA results shown in Figures 3.4 and 3.5. TGA results of the cast

2m13pd and 14chdm TPUs showed early weight loss especially for the 100%HS

(c)

108

samples. The early weight loss was primarily due to solvent evaporation. The %

weight loss values of the cast 2m13pd-100%HS and 14chdm-100%HS TPUs were

14% and 23% respectively. The % weight loss (solvent evaporation) was seen to

decrease with decrease in HS concentration. It is therefore observed that more

solvent was retained in TPUs with higher HS concentration due to the increasing

amount of the HS. It can therefore be suggested that the irregularities in the melting

transitions could be attributed to the evaporation of the solvent in the TPU samples

(since the boiling point of DMAc is 165°C).

For the linear chain-extended TPUs (especially 15pd, 16hd, 17hpd and 18od), the

melting transitions of the 100%HS TPUs were observed to be lower than those of the

70%HS TPUs. The reason is that whereas TMMT for the 70%HS TPUs is assigned to

the mixing of the HS and SS whereas TM for the 100%HS TPUs is assigned to only

the HS. The low melting temperatures of the 100%HS TPUs is therefore due to the

absence of SS in the TPU structure. The odd-even effect in terms of melting

temperatures was also observed for the 100%HS TPUs.

1 2

0

380

760

1140

1520

1900


Cast TPU-70 (12ed)

Cast TPU-70 (14bd)

Cast TPU-70 (15pd)

Cast TPU-70 (16hd)

Cast TPU-70 (17hpd)

Cast TPU-70 (18od)


I (a

.u

)

q (nm-1)

1 2

160

320

480

640

800


Cast TPU-100 (12ed)

Cast TPU-100 (14bd)

Cast TPU-100 (15pd)

Cast TPU-100 (16hd)


Cast TPU-100 (18od)


I (a

.u

)

q (nm-1)

Figure 3.7 SAXS of cast TPUs (d) 70%HS and (e) 100%HS

Phase separation was observed for most of the 70%HS TPUs as seen in Figure 3.7

(d). Phase separation was observed as scattering peaks and the scattering maxima lie

(e) (d)

109

at different positions. This suggests that the length size of the electron density

fluctuations responsible for the scattering maxima is different for all samples. There

was no scattering peak observed for the cast 14chdm-70%HS indicating the presence

of a phase-mixed morphology in the TPU. The SAXS peaks observed for the

70%HS TPUs come from the HS-SS phase.

Figure 3.7 (e) shows that the scattering intensity peaks of the 100%HS TPUs are

lower than those of the 70%HS TPUs. The low scattering intensities observed for the

100%HS TPUs was partly because there are no soft segments in the TPU structure.

Since the 100%HS TPU has only one phase, it is somewhat contradictory that there

is a scattering peak present. It is therefore believed that the SAXS peaks observed for

the 100%HS TPUs come from the crystalline HS and the non-crystalline HS.

10 20 30 40 50 60 70

4300

8600

12900

17200

21500

In

te

nsity (a

.u

)


Cast TPU-70 (12ed)

Cast TPU-70 (14bd)

Cast TPU-70 (15pd)

Cast TPU-70 (16hd)

Cast TPU-70 (17hpd)

Cast TPU-70 (18od)


2 (º)

10 20 30 40 50 60 70

4900

9800

14700

19600

24500


Cast TPU-100 (12ed)

Cast TPU-100 (14bd)

Cast TPU-100 (15pd)

Cast TPU-100 (16hd)


Cast TPU-100 (18od)


In

te

nsity (a

.u

)

2 (º)

Figure 3.7 WAXS of cast TPUs (f) 70%HS and (g) 100%HS

WAXS results in Figures 3.7 (f) suggest the presence of low level crystallinities for

the 2m13pd- and 14chdm- 70%HS TPUs. Microcrystalline peaks were observed on

the amorphous halos of the 2m13pd-70%HS TPU whereas no crystalline peaks were

observed for the 14chdm-70%HS TPU. The low level crystallinity observed in

WAXS results correlate with the DSC results earlier discussed in Figure 3.7 (a).

Crystalline peaks were seen on the amorphous halos of linear chain-extended

70%HS TPUs (12ed, 14bd, 15pd, 16hd, 17hpd and 18od). However, sharp

(g) (f)

110

crystalline peaks were observed for the 70%HS TPUs chain-extended with 15pd,

16hd, 17hpd and 18od. The most prominent peaks observed for the 70%HS TPUs

chain-extended with 15pd, 16hd, 17hpd and 18od were seen at 19.01°, 19.34°,

18.90° and 19.19° respectively. These results correspond with the results reported by

Blackwell et al97-98, 125, 193-197

.

Blackwell et al97-98, 193-194

proposed a triclinic unit cell structure for Poly(MDI-

Butanediol) with unit cell dimensions: a = 5.2Å, b = 4.8Å, c = 35.0Å, α = 115°, β =

121°, and γ = 85°. The triclinic cell unit structure has a space group of P

(Pinacoidal) and contains two monomer units of a single chain. The above

propositions were made from single crystal x-ray analysis of the structure of a model

compound methanol-capped MDI (Me-M-Me). It was also proposed that the

Poly(MDI- Butanediol) has a monomer repeat of 18.95Å and the conformational

analysis showed that the polymer chains are arranged in a fully extended and

staggered structure (i.e. all trans). The conformational structure of Poly(MDI-

Butanediol) can be seen in Figure 3.8. However, it was also proposed that

Poly(MDI-Ethylene glycol) and Poly(MDI-Propanediol) adopt contracted and

unstaggered structures having monomer repeats of 15.0Å and 16.2Å respectively.

These monomer repeats are shorter than the predicted fully extended structure of

Poly(MDI-Butanediol) and are also seen to contain some gauche conformations.

These conformations have higher energy than the fully-extended all-trans structure.

It was observed that for butanediol and longer diol chain extenders, the structure

depends whether or not the diol chain extender is even or odd. The even diol

polymers adopt the lowest energy fully-extended conformation and this allows for

hydrogen bonding to take place in both direction perpendicular to the chain axis. In

contrast, such hydrogen bonding is not possible for the odd diol polymers in the

extended conformation and therefore they attain a contracted structure with higher

energy conformations.

It was also observed that both even and odd diol polymers adopted staggered chain

conformations with triclinic unit cells; however, the even diol polymers possess

higher crystalline order. In contrast to the above observation, the first two chain

extenders (ethylene glycol and propanediol) in the homologous series behave

differently. Ethylene glycol and propanediol- polymers adopt contracted unstaggered

structures. The reason was that the two diols are too short to allow the same order of







111

packing as the longer chain extenders. This therefore explains why polymers made

of butanediol and longer even diols have better overall properties. The hard segments

in these polymers are able to crystallize more in the lowest energy conformation and

consequently, there is more driving force for phase separation.

Figure 3.8 Projection of the conformation of Poly(MDI-Butanediol) proposed by

Blackwell and Nagarajan193

From Figure 3.7(f), the crystalline peaks observed for the 70%HS 12ed and 14bd

TPUs are diffuse and are difficult to resolve. For the 70%HS TPUs (15pd, 16hd,

17hpd and 18od), there were four prominent peaks observed on the WAXS

diffractograms. These four peaks were seen on the amorphous halos at ca. 19°, 20°,

22°and 24°. These peaks correspond to the crystallinities of HS as proposed by

Blackwell et al194

. These peaks were also observed by Pergal et al in their

research198-199

. The calculated d-spacings for the crystalline peaks can be seen in

Table 3.5. The calculated Bragg distances (Å) shown in Table 3.5 closely match

18.95Å



112

with the works carried out by Blackwell and Ross197

, and also Born et al200

shown in

Table 3.6.

Table 3.5: Calculated d-spacings of the crystalline peaks of 70% HS- 15pd, 16hd,

17hpd and 18od TPUs

70%HS

15PD

(2θ/°)

d (Å)

(calc)

70%HS

16HD

(2θ/°)

d (Å)

(calc)

70%HS

17HPD

(2θ/°)

d (Å)

(calc)

70%HS

18OD

(2θ/°)

d (Å)

(calc)

10.89 8.11 9.26 9.54 8.75 10.09 8.32 10.62

19.01 4.66 19.34 4.58 18.90 4.69 19.19 4.62

20.01 4.43 20.72 4.28 19.94 4.45 21.40 4.15

22.06 4.03 22.22 4.00 20.77 4.27 22.86 3.88

24.02 3.70 24.31 3.66 23.05 3.85 24.26 3.66

24.26 3.66

Table 3.6: Calculated and observed d-spacings of Blackwell and Ross, and Born et

al X-ray studies of structure of polyurethane hard segments (MDI and 1,4-

butanediol) Blackwell and Ross

197 Born et al

200

hkl d (Å) (calculated) hkl d (Å) (calculated)

01 4.77 011 4.27

101 4.21 02 4.90

02 4.96 012 3.89

102 3.90 1 3 3.57

103 3.59 013 3.55

004 7.66 014 3.25

0 4 4.61 04 4.60

104 3.31 004 7.66

06 4.49 06 4.04

07 4.18 008 3.83

0 8 3.75

9 3.43

From Table 3.5, the peaks seen at small angles were also observed in the work

carried out by Blackwell et al. on Poly(MDI-Hexanediol)194

. These peaks were







113

attributed to one of the ten reflections observed for the X-ray pattern of Poly(MDI-

Hexanediol). The d-spacing of the peak was observed to be 8.71Å. However, no

further explanation was given concerning the peak observed at small angle.

Blackwell et al. also observed that the x-ray patterns of Poly(MDI-Hexanediol) were

sharper than those of other diols. However, from Figure 3.7(f), the x-ray patterns of

15pd, 16hd, 17hpd and 18od TPUs were seen to be sharper than those of 12ed and

14bd TPUs.

The prominent peaks seen for the 70%HS 15pd, 16hd, 17hpd and 18od TPUs at

about 19° with d-spacing of about 4.6Å were also observed by many authors201-204

.

Briber and Thomas attributed the intense reflection at about 4.6Å to a type-II crystal

form. They commented that the type-II crystal structure is highly ordered and forms

classical negatively birefringent spherulites. The type-II crystal form is found in

unoriented films201

. This is therefore the case for our cast TPU samples.

The prominent peak intensities of the 70%HS 16hd, 17hpd and 18od TPUs were

seen to higher than that of the 70%HS 15pd TPU. The reason could be attributed to

higher monomer repeat. Blackwell et al observed that the monomer repeats of

Poly(MDI-Hexanediol), Poly(MDI-Heptanediol) and Poly(MDI-Octanediol) were

20.8Å, 20.7Å and 22.8Å whereas that of Poly(MDI-Pentanediol) was 18.6Å194, 205

.

In Figure 3.7(g), the crystalline peaks for the 100%HS TPUs chain-extended with

15pd, 16hd, 17hpd and 18od correspond with those of the 70%HS TPUs previously

discussed. However, their peak intensities were lower than those of 70%HS TPUs.

This observation could be attributed to the absence of SS in the polymeric chain.

This is because the soft segments contribute to the structural and morphological

integrity of the polymer. The 100%HS TPU chain-extended with 18od had the

highest prominent peak. The peaks at small angles were not too noticeable for the

100%HS TPUs.








114

3.2.1.2 MELT-QUENCHED TPUs

0 50 100 150 200

1st Heating CycleH

ea

t F

low

- E

xo

MQ TPU-70 (2m13pd)

MQ TPU-70 (15pd)

MQ TPU-70 (16hd)

MQ TPU-70 (17hpd)

MQ TPU-70 (18od)

Temperature (ºC)

0 50 100 150 200

1st Heating Cycle

He

at F

low

- E

xo

MQ TPU-100 (2m13pd)

MQ TPU-100 (15pd)

MQ TPU-100 (16hd)

MQ TPU-100 (17hpd)

MQ TPU-100 (18od)

Temperature (ºC)

Figure 3.9 1st DSC heating cycles of melt-quenched TPUs (a) 70%HS and (b)

100%HS

Due to the high melting temperatures associated with 12ed and 14bd TPUs, they

were not melt-processed for further thermal studies. Having established that 14chdm

TPUs are amorphous, it was therefore not necessary for them to undergo melt-

quenching processes.

It can be said that after the melting process, the TPU segments (HS and SS) are in a

homogeneous mixed state and therefore upon fast cooling, the polymeric molecules

do not have sufficient time to relax and attain an equilibrium state therefore they

quickly become immobile. Fast cooling of melted TPUs was done by dipping the

melted samples directly into a flask containing liquid nitrogen.

In Figure 3.9(a), broad Tgs were observed for the 70%HS TPUs chain-extended with

2m13pd, 15pd and 16hd. After the melt-quenching process, the TPU samples are in a

mixed state/phase and the broad transition observed is known as mixed phase glass

transition temperature (TgMP) as shown by Saiani et al58-60

. The mixed phase is a

dispersion of the SS with HS. TgMP values for the 70%HS TPUs chain-extended with

2m13pd, 15pd and 16hd were seen to be 40±0.6°C, 26±0.3°C and 22±0.1°C

(b) (a)

115

respectively. It was observed that as the samples are further heated, there appeared a

phase separation peak known as the microphase separation transition (TMST) marked

by an exotherm. Immediately after the microphase separation transition, an

endothermic transition known as the microphase mixing transition (TMMT) is

observed58-59

. After this transition, the HS and SS are once again in a homogenous

mixed state. No TMST was observed for 70%HS TPUs chain-extended with 17hpd

and 18od.

Pronounced melting-recrystallization-melting peaks were observed for the 70%HS

TPUs chain-extended with 16hd, 17hpd and 18od, that is to say, double melting

peaks and a recrystallization peak. DSC studies have revealed the occurrence of

multiple endothermic transitions of polyurethanes116, 118, 121, 206-209

. It is possible that

these multiple endothermic transition can be explained by the existence of

polymorphic structures in the polymer chain. The presence of polymorphic structures

for 14bd and 16hd TPUs has been reported125

.

Only a TgHS was observed for all the 100%HS TPUs as shown in Figure 3.9(b). This

is because the 100%HS TPU has no soft segment. The TgHS values of 100%HS TPUs

chain-extended with 2m13pd, 15pd, 16hd, 17hpd and 18od were 77±0.7°C,

69±0.4°C, 66±0.7°C, 43±0.3°C and 56±0.2°C. Having established that the

exothermic transitions observed for the melt-quenched 70%HS TPUs were as a

result of microphase separation (TMST); however, it is not enough to say that the

exothermic transition was only due to microphase separation because the same

exothermic transitions were observed for the 100%HS TPUs. Therefore, the

exothermic transition observed for the 100%HS TPUs is due to cold crystallization

(Tcc). The observed cold crystallization is as result of the frozen state of the HS

during the cooling process. During the cooling process, the hard segments do not

have sufficient time to crystallize before they vitrify.

116

1 2

0

16

32

48

64

80

MQ TPU-70 (2m13pd)

MQ TPU-70 (15pd)

MQ TPU-70 (16hd)

MQ TPU-70 (17hpd)

MQ TPU-70 (18od)

I (a

.u

)

q (nm-1)

1 2

0

5

10

15

MQ TPU-100 (2m13pd)

MQ TPU-100 (15pd)

MQ TPU-100 (16hd)

MQ TPU-100 (17hpd)

MQ TPU-100 (18od)

I (a

.u

)q (nm

-1)

Figure 3.9 SAXS of melt-quenched TPUs (c) 70%HS and (d) 100%HS

SAXS results in Figure 3.9(c) showed no phase separation peaks observed for the

melt-quenched 70%HS TPUs chain-extended with 2m13pd, 15pd and 16hd. This

therefore confirms the presence of mixed phase existing in the melt-quenched TPUs

due to rapid cooling effect. In addition, the absence of crystallization peaks on the

amorphous halos of the melt-quenched TPUs further supports that the presence of

only non-crystalline phases. Since crystallization arises from the HS for this set of

TPU samples, it is therefore deduced that the fast cooling (melt-quenching) of the

TPUs leaves no time for the crystallization of the HS thereby leading to a mixed,

amorphous phase. Also, since SAXS measures the electron density differences

between crystalline and non-crystalline moieties in a polymer or between soft phase

and hard phase. Low crystallinity of the polymer therefore means low electron

densities.

Small peaks were observed for the 70%HS TPUs chain-extended with 17hpd and

18od which suggests little phase separation occurring in the TPU due to the presence

of the SS. The scattering intensity associated with the phase separation peak is low

which also suggests that only a small amount of soft segments were involved in

(d)

(c)

117

phase separation. This therefore implies the phase separation observed for the

70%HS TPUs is negligible.

There was no phase separation peak observed for all the melt-quenched 100%HS

TPUs as shown in Figure 3.9(d). This indicates that there were no differences in

electron densities between the crystalline HS and the non-crystalline HS.

10 20 30 40 50 60 70

1800

3600

5400

7200

9000

MQ TPU-70 (2m13pd)

MQ TPU-70 (15pd)

MQ TPU-70 (16hd)

MQ TPU-70 (17hpd)

MQ TPU-70 (18od)

In

te

nsity (a

.u

)

2 (º)

10 20 30 40 50 60 70

2100

4200

6300

8400

10500In

te

nsity (a

.u

)

MQ TPU-100 (2m13pd)

MQ TPU-100 (15pd)

MQ TPU-100 (16hd)

MQ TPU-100 (17hpd)

MQ TPU-100 (18od)

2 (º)

Figure 3.9 WAXS of melt-quenched TPUs (e) 70%HS and (f) 100%HS

WAXS results in Figures 3.9 (e) and (f) showed that there are no crystalline peaks

observed for the melt-quenched samples. The absence of crystalline peaks on the

amorphous halos therefore confirms that the melt-quenched TPU samples are

amorphous. However, the sharp peak seen on the 100%HS 18od-TPU in Figure

3.9(f) can be attributed to impurities or artefacts arising from the WAXS instrument.

(e) (f)

118

3.2.1.3 SLOW-COOLED TPUs

0 50 100 150 200

1st Cooling Cycle

He

at F

low

- E

xo

SC TPU-70 (2m13pd)

SC TPU-70 (15pd)

SC TPU-70 (16hd)

SC TPU-70 (17hpd)

SC TPU-70 (18od)

SC TPU-70 (14chdm)

Temperature (ºC)

0 50 100 150 200

1st Cooling CycleH

ea

t F

low

- E

xo

SC TPU-100 (2m13pd)

SC TPU-100 (15pd)

SC TPU-100 (16hd)

SC TPU-100 (17hpd)

SC TPU-100 (18od)

SC TPU-100 (14chdm)

Temperature (ºC)

Figure 3.10 1st DSC cooling cycles of slow-cooled TPUs (a) 70%HS and (b)

100%HS

Figure 3.10(a) showed that there were no transitions (such as crystallization)

observed for 70%HS TPUs chain-extended with 2m13pd and 14chdm in the DSC

cooling process. It can therefore be suggested that the melted TPUs remained phase-

mixed upon cooling.

We can observe the presence of crystallization exotherms for the 70%HS TPUs

chain-extended with 15pd, 16hd, 17hpd and 18od. From the 1st heating DSC results

for cast TPUs earlier discussed, the samples are believed to be semi-crystalline

owning to their melting transitions. It is therefore suggested that crystallinity is

expected to increase with increasing HS concentration, that is to say, crystallization

peaks should be seen for higher hard block TPUs. However, this is not the case for

100%HS samples as seen in Figure 3.10(b). The crystallization peak noticed for

only the 70%HS TPUs chain-extended with 15pd, 16hd, 17hpd and 18od is related to

mobility of polymeric chains rather than the amount of crystallizable units present in

the polymer. The SS (polyol) used in this research work was reported to play a

plasticizing role in the TPU transitions observed in the DSC thermographs58

. This

plasticizing effect of the polyol can be said to play a vital role in the crystallization

(b) (a)

119

of the TPUs by enhancing the mobility of the polymeric chains. Furthermore, the

flexibility of the linear chain extenders (15pd, 16hd, 17hpd and 18od) inarguably

imparts on the mobility of the polymeric chain and is also responsible for the

crystallization of TPUs.

Further to the above discussion, due to the proportion of SS (30%) in the 70%HS

TPUs, the polymeric chains are more mobile during cooling (due to the lower Tg of

the SS) therefore the HS chains can arrange themselves into crystallites; whereas for

TPUs with higher HS concentrations the polymeric chains become restricted in

mobility, due to the higher proportion of the HS (of higher Tg), and therefore during

the cooling process the HS vitrify more rapidly thereby giving less time for

crystallization.

0 50 100 150 200

SC TPU-70 (2m13pd)

SC TPU-70 (15pd)

SC TPU-70 (16hd)

SC TPU-70 (17hpd)

SC TPU-70 (18od)

SC TPU-70 (14chdm)

He

at F

low

- E

xo

Temperature (ºC)

2nd Heating Cycle

0 50 100 150 200

2nd Heating Cycle

He

at F

low

- E

xo

SC TPU-100 (2m13pd)

SC TPU-100 (15pd)

SC TPU-100 (16hd)

SC TPU-100 (17hpd)

SC TPU-100 (18od)

SC TPU-100 (14chdm)

Temperature (ºC)

Figure 3.10 2nd DSC heating cycles of slow-cooled TPUs (c) 70%HS and (d)

100%HS

In Figures 3.10 (c) and (d), the TgMP values for the slow-cooled 70% and 100%HS

2m13pd TPUs were seen to be higher than that previously observed for the melt-

quenched 70% and 100%HS 2m13pd TPUs. The reason for this increase in Tg is that

during the slow cooling, the TPU segments have enough time to relax, and so doing

become partially phase separated. However, a broad glass transition was still

(d) (c)

120

observed for 70%HS 2m13pd TPU. This therefore means that the 70%HS TPU

segments are still phase-mixed after the slow cooling and are phase separated on

further heating. At HS concentrations higher than 70%HS, the melt-quenched TPUs

are no longer a homogeneous mixed phase thereby leading to the formation of a hard

phase and a mixed phase. It was reported by Saiani et al58

for 65% HS that part of

the hard segments are incorporated with the soft segments in a mixed phase with a

ratio of 1.8 : 1 and the rest comprises an almost pure hard segment phase 58-60

. As a

result of the restriction in the mobility of the mixed phase caused by the HS, TgMP is

not observed for samples with higher hard segment concentrations. It can therefore

be said TgHP was observed for HS higher than 70% (except 100%) since little amount

of soft segments still reside in them whereas only TgHS was observed for the

100%HS TPU. The Tg values of the 100%HS samples were also seen to increase due

to the restriction in mobility of the HS during the slow cooling process. There were

no melting transitions observed for 100%HS. The reason for the observation is due

to the increase of non-crystalline hard phases originating from the increased amount

of the 2m13pd chain extender in the TPU system. Due to the shortness of the chain

extender as well as its branched chemical structure, mobility of the HS chains tends

to be difficult and there are irregularities in the alignment of the TPU polymeric

chains. Furthermore, at high hard block concentrations (100%HS), we do not expect

to observe microphase mixing transition (TMMT).

As seen in Figure 3.10(c), only endothermic transitions were observed for the

70%HS TPUs chain-extended with 15pd, 16hd, 17hpd and 18od. This is due to the

crystallization that has taken place in the cooling process. After the cooling process,

the samples are still semi-crystalline; however, there was no Tg observed. No

endothermic transition was observed for the 70%HS TPU chain-extended with

14chdm which confirms that the sample is amorphous.

In Figure 3.10(d), TgHS was observed for all the 100%HS TPUs. The TgHS values of

the 100%HS TPUs chain-extended with 2m13pd, 15pd, 16hd, 17hpd, 18od and

14chdm were 93±0.6°C, 83±0.2°C, 80±0.7°C, 62±0.2°C, 80°C and 88±0.6°C. The

TgHS values observed for the slow-cooled 100%HS TPUs was seen to be higher than

that of the melt-quenched 100%HS TPUs. The reason was that during the slow-

cooling process, the HS chains had sufficient time to relax and attain equilibrium.

This means that the HS chains are better ordered, therefore TgHS is increased. It was

121

observed that the 100%HS 2m13pd and 14chdm TPUs had the highest TgHS values.

This could be attributed to the non-crystalline nature of the samples and hence there

was no melting transition observed for them. Prominent endothermic transition was

observed for the 100%HS 18od TPU and this suggests that it is more crystalline than

others.

3.2.1.4 MOULDED TPUs

0 50 100 150 200

He

at F

low

- E

xo

Moulded TPU-70 (2m13pd)

Moulded TPU-70 (15pd)

Moulded TPU-70 (16hd)

Moulded TPU-70 (17hpd)

Moulded TPU-70 (18od)

Moulded TPU-70 (14chdm)

Temperature (ºC)

1st Heating Cycle

0 50 100 150 200

1st Heating Cycle

He

at F

low

- E

xo







Temperature (ºC)

Figure 3.11 1st DSC heating cycles of moulded TPUs (a) 70%HS and (b) 100%HS

In Figure 3.11(a), it is observed that decreased melting enthalpies were seen for the

moulded 70%HS 2m13pd and 15pd TPUs compared to those of their corresponding

cast TPUs. The reason is because moulding decreases the crystallinity of the TPU

samples due to the heating and rapid-cooling. For the moulded 70%HS 2m13pd

TPU, the observation of low Tg (TgMP) was similar to that seen for the cast TPU

(which was previously discussed) although the moulded TPU has slightly higher Tg

value than the cast TPU. We can suggest that due to temperature and pressure, a new

mixed phase state was induced for the moulded TPU samples. The reason is because

the boiling point of the solvent (DMAc) is 165°C and therefore is expected to have

evaporated at the moulding temperature (which is 180°C). After TgMP, phase

(b) (a)

122

separation peak was observed. This suggests that phase separation occurred during

the moulding process or during heating in the DSC.

For the moulded 70%HS 14chdm TPU, low Tg was still observed. No melting

transition was observed for the moulded 70%HS 14chdm TPU implying that the

sample is amorphous.

Single endothermic transition was observed for the 70%HS 15pd TPU whereas for

the moulded 70%HS 16hd, 17hpd and 18od TPUs, melting-recrystallization-melting

peaks were observed. These endothermic transitions suggest the presence of

polymorphic structures in the TPUs. However, these endothermic transitions were

not present in the 1st heating DSC cycles of the cast 70%HS 16hd, 17hpd and 18od

TPUs. This therefore means that the polymorphic structures were formed during the

moulding of the cast TPUs (especially during the cooling process).

The above observations for the moulded 70%HS TPUs correlate with those of the

moulded 100%HS TPUs as shown in Figure 3.11(b). The striking difference is that

TgMP was observed for the moulded 70%HS TPUs whereas TgHS was observed for the

moulded 100%HS TPUs.

To explain the existence of polymorphism in the 16hd, 17hpd and 18od TPUs, the

moulded 70% and 100%HS 16hd TPU samples were partially melted in order to

understand the nature of the multiple endotherms in the TPUs. The DSC traces are

shown in Figures 3.11 (c) to (f).

123

Figure 3.11 (c) partially melted to 157°C, (d) cooled to -90°C, (e) reheated to 220°C

and (f) cooled to 25°C

0 50 100 150

He

at F

low

- E

xo

Moulded TPU-70 (partially melted)


Temperature (ºC)

1st Cooling Cycle CE: 16hd

0 50 100 150

He

at F

low

- E

xo



Temperature (ºC)

1st Heating Cycle CE: 16hd

(d) (c)

50 100 150 200

He

at F

low

- E

xo



Temperature (ºC)

2nd Cooling Cycle CE: 16hd

0 50 100 150 200

He

at F

low

- E

xo



Temperature (ºC)

2nd Heating Cycle CE: 16hd

(f) (e)

124

In Figure 3.11(c), the moulded TPUs were melted to about 160°C, that is between

the two melting temperatures. The 1st melting endotherms showed low enthalpies.

The TPUs were then cooled to -90°C (Figure 3.11(d)). The 1st cooling DSC

thermographs show that there is no crystallization peak observed on cooling. After

the cooling, only the second melting peaks were observed at higher temperatures.

The melting temperatures of the 70% and 100%HS TPUs were 191±0.1°C and

170±0.5°C respectively as shown in Figure 3.11(e). Figure 3.11(f) shows that

crystallization peak for 70%HS TPU was observed on cooling.

From the above observations, it is therefore clear to state that there are two distinct

crystal structures that melt at different temperatures. This therefore confirms the

existence of polymorphs in the 16hd, 17hpd and 18od TPUs. These results

correspond to those reported by Blackwell and Lee125

. Blackwell and Lee reported

on the existence/development of polymorphic/crystalline structures for TPU chain-

extended with 1,3-propanediol (PDO) 1,4-butanediol (BDO) and 1,6-hexanediol

(HDO); the HS concentrations of these TPUs were 51%, 52% and 54% respectively.

The development of polymorphic structures for these TPUs were apparent after

prolonged stretching and annealing as evidenced by their x-ray data. For the HDO

polymer, DSC data showed that a second/new peak of a contracted conformation

was observed at 190°C than that of an extended form already seen at a lower

temperature. A melt-quenching/reheating procedure as seen in Figures 3.11 (c) and

(e) was carried on the HDO polymer and it confirmed the existence of a second/new

peak of a contracted crystal form as only the second peak was observed after

quenching; this was also evidenced by their x-ray photographs. For the BDO and

PDO polymers, second/new peaks of extended conformations were observed at

lower temperatures than those of the contracted forms already seen at higher

temperatures.

125

1 2

0

200

400

600

800

1000







I (a

.u

)

q (nm-1)

1 2

0

60

120

180

240

300







I (a

.u

)q (nm

-1)

Figure 3.11 SAXS of moulded TPUs (g) 70%HS and (h) 100%HS

Decrease in the phase separation of the moulded TPU samples is confirmed by the

SAXS results in Figures 3.11(g) and (h). These show that there is a decrease in the

scattering intensity of the TPU samples compared to those of the cast TPUs in

Figures 3.7(d) and (e). The decrease in scattering intensities is due to the decrease in

electron densities differences of the TPU constituents resulting from the moulding

process. This reduction in the scattering intensity signifies that hard segments (which

are responsible for the crystallinity of the TPU) were partially hindered due to their

non-equilibrium state caused by the cooling process and therefore since SAXS is

sensitive to electron density heterogeneities of the polymeric chains, less crystallinity

of the polymer will lead to low scattering intensities.

(h) (g)

126

10 20 30 40 50 60 70

3700

7400

11100

14800

18500







In

te

nsity (a

.u

)

2 (º)

10 20 30 40 50 60 70

3200

6400

9600

12800

16000







In

te

nsity (a

.u

)2 (º)

Figure 3.11 WAXS of moulded TPUs (i) 70%HS and (j) 100%HS

In Figures 3.11 (i) and (j), there are very small crystalline peaks observed in the

amorphous halos of the moulded TPUs. The WAXS results therefore confirmed that

the crystallinity of the TPUs was reduced during the moulding process. Rapid

cooling involved in the moulding process decreases the rate of crystallization of the

HS. However, the small crystalline peaks still seen on the amorphous halos of the

moulded TPUs suggest that some crystallizable parts of the TPUs remained

crystalline after the moulding process.

(j) (i)

127

3.2.1.5 ANNEALED MOULDED TPUs

0 50 100 150 200

1st Heating Cycle

Ann. Moulded TPU-70 (2m13pd)

Ann. Moulded TPU-70 (15pd)

Ann. Moulded TPU-70 (16hd)

Ann. Moulded TPU-70 (17hpd)

Ann. Moulded TPU-70 (18od)

Ann. Moulded TPU-70 (14chdm)

He

at F

low

- E

xo

Temperature (ºC)

0 50 100 150 200

He

at F

low

- E

xo







Temperature (ºC)

1st Heating Cycle

Figure 3.12 1st DSC heating cycles of annealed moulded TPUs (a) 70%HS and (b)

100%HS

In Figures 3.12 (a) and (b), it was observed that after the annealing process, the TPU

samples showed melting transitions with sharp melting peaks. The melting

enthalpies of the annealed moulded TPUs were higher than those of the moulded

TPUs due to annealing. The sharp melting transitions were as a result of the ordering

of the HS during the annealing process118-119, 210

. Contrary to the 1st heating DSC

results obtained from the moulded 100%HS 2m13pd TPU where there was no

melting transition (Figure 3.11(b)), annealing above the TgHS induced the

development of ordered structure within the hard phase53

.

For the annealed moulded 14chdm TPUs, melting transitions were observed. These

melting transitions were formed as a result of increased crystallinity/ordering of the

HS due to prolonged annealing. Broad glass transition (TgMP) was observed for the

70%HS TPUs at -2±0.1°C and TgHS was observed for the 100%HS TPU at

71±0.1°C. A second melting peak was observed for the 70%HS TPU and this peak

(b) (a)

128

could be assigned to the melting of an ordered HS formed during annealing as

reported by Saiani et al58

.

For the annealed moulded 15pd and 17hpd TPUs, only single melting peaks were

observed whereas for the annealed moulded 16hd and 18od TPUs, recrystallization

peaks and double endotherms were observed. The melting enthalpies of these

annealed moulded samples were higher than their corresponding moulded samples as

during the annealing process, more HS crystallites are formed.

1 2

0

280

560

840

1120

1400

I (a

.u

)







q (nm-1)

1 2

0

70

140

210

280

350

I (a

.u

)







q (nm-1)

Figure 3.12 SAXS of annealed moulded TPUs (c) 70%HS and (d) 100%HS

Figures 3.12 (c) and (d) revealed that the scattering intensities of annealed moulded

TPUs were once again increased due to the annealing compared to the decrease in

scattering intensities observed in the SAXS results of the moulded samples (see

Figures 3.11 (g) and (h)). The increase in scattering intensities is due to increased

phase separation and crystallinity of the TPU segments.

(d) (c)

129

10 20 30 40 50 60 70

3000

6000

9000

12000

15000

In

te

nsity (a

.u

)







2 (º)

10 20 30 40 50 60 70

3300

6600

9900

13200

16500

In

te

nsity (a

.u

)







2 (º)

Figure 3.12 WAXS of annealed moulded TPUs (e) 70%HS and (f) 100%HS

Contrary to the WAXS results of the moulded TPUs (Figures 3.11 (i) and (j)),

crystalline peaks were seen on the WAXS diffractograms of the annealed moulded

TPUs confirming that the annealed moulded TPUs have increased crystallinities as

shown in Figures 3.12 (e) and (f).

In Figure 3.12 (f), two distinct crystalline peaks on the amorphous halos were

observed for the all the annealed moulded 100%HS TPUs. This therefore implies

that HS crystallites have been formed during the annealing process. The intensities

of the peaks seen at small angles for the annealed moulded TPUs were observed to

be higher than those of the moulded TPUs previously discussed (see Figures 3.11 (i)

and (j)). The calculated d-spacings for the crystalline peaks on the annealed moulded

TPUs can be seen in Table 3.6.

(e)

(f)

130

Table 3.7: Calculated d-spacings of the crystalline peaks of annealed moulded 100%

HS- 15pd, 16hd, 17hpd, 18od and 14chdm TPUs 100%HS

15PD

(2θ/°)

d (Å)

(calc)

100%HS

16HD

(2θ/°)

d (Å)

(calc)

100%HS

17HPD

(2θ/°)

d (Å)

(calc)

100%HS

18OD

(2θ/°)

d (Å)

(calc)

100%HS

14CHDM

(2θ/°)

d (Å)

(calc)

10.76 8.24 10.14 8.71 9.42 9.38 8.51 10.38 15.13 5.85

18.52 4.79 18.52 4.79 18.59 4.77 19.19 4.62 17.56 5.05

23.51 3.78 23.77 3.74 22.99 3.86 22.81 3.89 19.76 4.49

21.44 4.14

28.61 3.12

Some of the peaks observed in Table 3.7 correspond to those observed for the cast

TPUs seen in Table 3.5. It can therefore be deduced that annealing the moulded TPU

samples at 80°C for 168 hours induced the formation of type-II crystals and the

results are consistent with the findings of Briber and Thomas211

.

From the results discussed, it can be observed that TM (melting of the HS) was

observed for the 100%HS samples whereas TMMT (mixing of the HS and SS) was

observed for the 70%HS samples. The broad glass transition seen for the melt-

quenched, slow-cooled and moulded 70%HS 2m13pd TPUs suggested the presence

of a mixed phase system. Cast 14chdm TPUs were seen to be non-crystalline which

is due to the chemical structure of the chain extender. However, crystallinities of

2m13pd and 14chdm TPUs were increased on annealing. Multiple endothermic

transitions were observed for the 16hd, 17hpd and 18od TPUs. This was due to the

presence of polymorphic structures existing in the TPUs. Annealing improved the

thermodynamic and structural properties of the linear chain-extended TPUs.

Furthermore, the odd-even effect influenced the thermodynamic properties of the

linear chain-extended TPUs. The linear chain-extended TPUs showed better

thermodynamic and structural properties than the non linear and cyclic chain

extended TPUs. This is also due to the higher degree of crystallinity inherent in the

linear chain-extended TPUs. WAXS results showed that the reflections observed for

the cast and annealed moulded TPU samples were consistent with the findings of

Blackwell et al as well as Briber and Thomas.






131

CHAPTER 4

RESULTS AND DISCUSSION (II)

TPU COMPOSITES

4.1 MELT VISCOSITY

Prior to the use of the TPU samples as matrices for composite manufacture, it was

necessary to determine the melt-flow properties of the TPUs. Figure 4.1 shows the

melt-viscosity curves of the TPUs with different chain extenders.

200 210 220 230 240 250 260 270 280

1

10

100

1000

10000 Cast TPU-70 (14bd)

Cast TPU-80 (14bd)

Cast TPU-90 (14bd)

Cast TPU-100 (14bd)

Me

lt V

iscosity (

Pa.s

)

Temperature (ºC)

170 180 190 200 210

1

10

100

1000

10000

Cast TPU-70 (15pd)

Cast TPU-80 (15pd)

Cast TPU-90 (15pd)

Cast TPU-100 (15pd)

Me

lt V

isco

sity (

Pa

.s)

Temperature (ºC) (a) 14bd TPUs (b) 15pd TPUs

170 180 190 200 210

1

10

100

1000

10000

Cast TPU-70 (16hd)

Cast TPU-80 (16hd)

Cast TPU-90 (16hd)

Cast TPU-100 (16hd)

Me

lt V

iscosity (

Pa.s

)

Temperature (ºC)

170 180 190 200 210

1

10

100

1000

10000

Cast TPU-70 (17hpd)


Me

lt V

iscosity (

Pa.s

)

Temperature (ºC) (c) 16hd TPUs (d) 17hpd TPUs

132

170 180 190 200 210

1

10

100

1000

10000

Me

lt V

iscosity (

Pa

.s)

Temperature (ºC)

Cast TPU-70 (18od)

Cast TPU-100 (18od)

170 180 190 200 210

1

10

100

1000





Me

lt V

isco

sity (

Pa

.s)

Temperature (ºC) (e) 18od TPUs (f) 2m13pd TPUs

170 180 190 200 210

1

10

100





Me

lt V

iscosity (

Pa.s

)

Temperature (ºC) (g) 14chdm TPUs

Figure 4.1 Melt-viscosity properties of TPUs with different chain extenders

Figure 4.1 (a) shows melting curves for 14bd TPUs. 12ed and 14bd TPU samples

melted at high temperatures as earlier shown by their DSC thermographs in Figures

3.7 (a) and (b). The melting curves of 12ed TPUs were not presented and the reason

being that during the rheology test, the 12ed TPUs acted as a thermosetting polymers

showing random, zig-zag transitions. The melting range for these TPU samples was

seen to be between 200°C and 270°C. The observed melting range corresponded to

the degradation temperatures (from 220°C) of the TPUs. The reason for the high

melt-viscosity profiles observed for these TPU samples was primarily due to their

high degree of crystallinity and therefore they were resistant to melt easily92, 112, 115,

184. The high crystallinity primarily results from the hydrogen bonding existing

133

between the HS chains. The efficiency of the hydrogen bonding existing between the

HS depends on the chain extender. It was reported that the short-chain linear

extenders join effectively with the isocyanate units during polymerization30

. Due to

the high melting temperatures of 12ed and 14bd TPUs, they cannot be processed

without degradation and as a result, these TPU samples cannot be used as matrices

for the fabrication of composites in this research work.

Figure 4.1 (b) shows that the 15pd TPUs exhibited steady decrease in melt viscosity

with increasing temperature. The samples showed melt-viscosities which decreased

with increase in HS concentration. The observation corresponds to the 1st heating

DSC thermographs seen in Figures 3.7 (a) and (b) – the melting temperatures of

100%HS TPU was seen to be lower than that of 70%HS TPU.

Figure 4.1 (c) shows that the melt-viscosity curves of 16hd TPUs were higher than

those of 15pd TPUs. This observation also corresponded to the 1st heating DSC

thermographs seen in Figures 3.7 (a) and (b) – the melting temperatures of the 70%

and 100%HS 16hd TPUs were observed to be higher than those of the 70% and

100%HS 15pd TPUs. All samples were seen to begin to melt steadily from

approximately 190°C. However, during moulding these TPU samples melted easily

and this was due primarily to the impact of the applied pressure.

Figure 4.1 (d) shows that the 17hpd TPUs exhibited relatively little decrease in melt

viscosity with increasing temperature. It was however noticed that there was a sharp

decrease in the melt viscosities of the TPUs as samples began to melt from 170°C.

This observation also corresponded with the DSC traces of 17hpd TPUs in Figures

3.7 (a) and (b). The rheological traces of 17hpd TPUs were observed to be similar to

those of 15pd TPUs.

Figure 4.1 (e) shows that the rheological traces of 18od TPUs were similar to those

of 16hd TPUs. Melting began at about 185°C for 70% TPU and about 190°C for

100% TPU. Rheological curves showing an increase in melting temperatures of the

18od TPUs confirmed the DSC transitions shown in Figures 3.7 (a) and (b).

Figures 4.1 (f) and (g) show the melting profiles of 2m13pd and 14chdm TPUs

respectively. The melt viscosity profile of 2m13pd TPUs is the same as that of 15pd

TPUs. 2m13pd and 14chdm TPU samples exhibited relatively little decrease in

134

viscosity with increasing temperature. The DSC traces of these TPU samples

revealed that they displayed little or no melting transitions (see Figures 3.7 (a) and

(b)). 14chdm TPUs were observed to display the lowest viscosity properties

compared to all other TPU samples. The reason for the observation is due to the

amorphous nature of TPU samples owning primarily for the geometric isomerism

(mixture of cis- and trans- isomers) of the chain extender as discussed in the DSC

results in Chapter 3.

From the above discussion on the melt-viscosity traces of the TPUs, it is evident that

15pd, 16hd, 17hpd, 18od, 2m13pd and 14chdm TPUs were suitable matrices to be

used for composite making due to their easy melt-flow. Furthermore, it was observed

that there were similarities in the melting profiles of some of the linear chain-

extended TPUs. The melting profile of 15pd TPUs was similar to that of 17hpd

TPUs whereas the melting profile of 16hd TPUs was similar to that of 18od TPUs.

These similarities in melting transitions were also observed in the DSC results of the

TPUs as shown in Chapter 3 and were seen as the presence of the odd-even effect in

these TPUs94-102

.

4.2 COMPOSITE ARCHITECTURE - TOMOGRAPHY

After the compression moulding of the TPU composites, it was necessary to view the

internal features of the composite. The internal features of the composites were

viewed through x-ray tomography. These internal features include the arrangement

and alignment of the glass fibre mats in the composite material.

135

(a) Isosurface imaging - Central view of TPU composite

(b) Isosurface imaging - Lateral view of TPU composite

136

(c) Orthoslice imaging of TPU composite

Figure 4.2 (a), (b) and (c) The internal architecture of the glass-TPU composite

Figure 4.2 (a) and (b) show that the glass fibre mats were properly aligned in the

composite material. The isosurface imaging of the computer tomography was able to

reveal the arrangement of the fibre mats in the composite material. The lateral view

showed that the composite was made of 8 layered glass fibre mats. Figure 4.2 (c)

shows the make-up of the entire composite material. It was clear to see that 8 glass

fibre mats were embedded in a TPU matrix.

4.3 OPTICAL MICROSCOPY

It was also important to elucidate the interfacial properties of the TPU composite in

order to determine the mechanical properties of the composite.

137

(a) Polished composite image showing impregnation of TPU matrix in the fibre

mats

(b) Enlarged image of a fibre bundle showing the wetting of a fibre bundle by the

15pd TPU matrix

Figure 4.3 (a) and (b) Optical microscopy images of the TPU composite

The optical micrographs of the polished TPU composites in Figure 4.3 were taken at

Huntsman Polyurethanes, Belgium. Figure 4.3 (a) shows that there was a good

impregnation of the TPU matrix within the fibre bundles and also there was no

significant porosity observed within the fibre bundles. It therefore reveals that the

melt-viscosity of the TPU was low enough to penetrate into the fibre bundles. The

composite material was also transparent as the fibre mats in the composite were

visible to the eyes and this also confirmed good impregnation of TPU composite.

Figure 4.3 (b) shows that there is good wettability of the TPU matrix within a single

fibre bundle and this also confirmed the excellent melt-flow property of the TPU.

138

4.4 THERMO-MECHANICAL PROPERTIES

4.4.1 MASS AND VOLUME FRACTIONS OF TPU,

POLYPROPYLENE AND ISOPLAST COMPOSITES

8 fibre mats were stacked with TPU, PP and Isoplast films respectively and

compression-moulded into 2mm-thick composites. The weight fractions of the

composite constituents were calculated as shown in Table 4.1.

Table 4.1: Weight and volume fractions of TPU composites

Composite samples Mass

Fraction/%

(Fibre)

Mass

Fraction/%

(TPU)

Volume

Fraction/%

(Fibre)

Volume

Fraction/%

(TPU)

TPU comp-2m13pd-70 47

53

28 72

TPU comp-2m13pd -80

46

54

27 73

TPU comp-2m13pd -90

47

53

28 72

TPU comp-2m13pd -100

46

54

27 73

TPU comp-15pd-70

45

55

26 74

TPU comp-15pd-80

44

56

25 75

TPU comp-15pd-90

45

55

26 74

TPU comp-15pd-100

45

55

26 74

TPU comp-16hd-70

47

53

28 72

TPU comp-16hd-80

51

49

31 69

TPU comp-16hd-90

46

54

27 73

TPU comp-16hd-100

44

56

25 75

TPU comp-17hpd-70

48

52

29 71

TPU comp-17hpd-100

45

55

26 74

TPU comp-18od-70

46

54

27 73

TPU comp-18od-100

44

56

25 75

TPU comp-14chdm-70 47

53

28 72

TPU comp-14chdm-80

45

55

26 74

TPU comp-14chdm-90

45

55

26 74

TPU comp-14chdm-100

46

54

27 73

Polypropylene composite 54 46 32 68

Isoplast composite

48

52

29 71

139

The TPU composites processed in this research were manufactured by first

measuring the weights of the glass fibres. The weights of the composites were later

measured and therefore the weights of the polymers were deducted. Having

knowledge of the weights of the fibre and matrix, the weight fractions of the TPU

composite constituents were then determined. It is therefore important to also

determine the volume fractions of the TPU composite constituents so as to be able to

calculate the composite properties. Having established the weight fractions of TPU

composite constituents, the volume fractions of TPU composite constituents were

derived from the equation below:

(Equation 4.1)

where Vf = volume fraction of fibre

Wf = weight fraction of fibre

ρf = density of fibre = 2.6g/cm3

ρm = density of matrix (TPU & Isoplast = 1.130g/cm3; PP = 0.9g/cm

3)

4.4.2 DMTA OF UNANNEALED AND ANNEALED TPU

COMPOSITES

The TPU composites were cut with a band saw into sizes suitable for DMTA testing.

Annealing of the TPU composites was done at 80°C for 168 hours. The annealing

temperature chosen (80°C) is higher than the TgHS of the TPUs reported in this

research work. The reasons for the choice of annealing temperature is to avoid the

overlapping of TgHS with the annealing endotherm, TA (which occurs at 20 - 30°C

above the annealing temperature) and to optimize the ordering of the HS during

annealing58-60, 185

. Three samples of each TPU composite of different chain extender

series were tested. 70%, 80%, 90% and 100%HS TPU composites will be discussed

in this section.

140

4.4.2.1 2M13PD TPU COMPOSITES

-100 0 100 200

1

10

100

1000

10000

TPU comp(2m13pd-70)

TPU comp(2m13pd-80)

TPU comp(2m13pd-90)

TPU comp (2m13pd-100)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-100 0 100

0.0

0.2

0.4

0.6

0.8

1.0





Ta

n D

elta

Temperature (ºC)

I. Unannealed TPU composites

-100 -50 0 50 100 150 200

1

10

100

1000

10000

Ann. TPU comp (2m13pd-70)




Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-100 -50 0 50 100 150

0.0

0.2

0.4

0.6

0.8

1.0





Ta

n D

elta

Temperature (ºC)

II. Annealed TPU composites

Figure 4.4 (a) Storage modulus (b) tan delta of (I) unannealed 2m13pd TPU

composites (II) annealed 2m13pd TPU composites

(a) (b)

TgHP / TgHS

TgSP

(b)

(a)

141

70 80 90 100

0

5000

10000

15000

20000

25000

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at 25º C

E' at 100º C

TPU Composites

(2m13pd)

70 80 90 100

0

5000

10000

15000

20000

25000

Annealed TPU Composites

(2m13pd)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at 25º C

E' at 100º C

Figure 4.5 Storage moduli bar graph of (a) unannealed and (b) annealed 2m13pd

TPU composites

In Figure 4.4 – I (a), the storage moduli of all the TPU composites are seen to be

similar at low temperatures (-100°C). This could be attributed to the fact the 2m13pd

TPU samples attained a phase-mixed morphology during the moulding process

especially for the 70%, 80% and 90%HS concentrations58-60

. As the temperature

increased, the storage moduli of the composites were seen to decrease abruptly

between 0°C to 70°C. The characteristic drop in the storage moduli of the TPU

composites can be observed to have happened in the region after the occurrence of

TgMP. This observation correlates with the 1st DSC heating cycle observed (TgMP for

DMTA = 11±0.2; TgMP for DSC = 11±0.2) for the moulded 70%HS 2m13pd TPU as

shown previously in Figure 3.11 (a). Furthermore, pronounced peaks were observed

(especially for the 70% and 80%) at higher temperature and these peaks were

characteristic of phase separation which occurred after the TgMP. No phase separation

peak was observed for the 100%HS TPU composite. The occurrence of phase

separation also correlates with the 1st heating DSC exothermic peak transitions seen

for the moulded 70%HS 2m13pd TPU in Figure 3.11 (a). The storage moduli of the

composites were low at 100°C due to the low crystallinity of 2m13pd TPUs. Figure

4.4 – I (b) shows the tan delta curves of the composites. The main relaxation peaks

(b)

(a)

142

observed between 0°C to 100°C were attributed to the Tgs (TgMP for the 70%, 80%

and 90%HS and TgHS for the 100%HS) and the phase separation transitions of the

2m13pd TPU composites.

Figure 4.4 – II (a) shows that the storage moduli of the TPU composites were

increased after annealing. Pronounced improvement in the storage moduli of

annealed composites is seen at high temperature (100°C). During the annealing

process, the HS of the TPU become ordered thereby increasing the crystallinity,

which consequently led to the increase in mechanical properties118-119, 210

. Upon

annealing, the low temperature disordered segmental arrangements were erased due

to improved crystallization as the amorphous HS have been orderly arranged. The

improvement in the storage moduli as a result of annealing can be seen in Figure

4.5. Annealing also improved the phase-separated morphology of the 2m13pd TPU

composites as the TgSP and TgHP / TgHS were clearly seen on the tan delta curves of

the annealed 2m13pd TPU composites in Figure 4.4 – II (b). The peaks observed

for the unannealed TPU composites were erased as a result of annealing. The TgSP

was clearly seen at about -11±0.9°C for the 70%HS only whereas TgHS was clearly

seen at about 69±0.5°C for the 100% HS. Also, TgHP was also seen for the 70%, 80%

and 90%HS. Improvement in the crystallinity of the annealed TPU composites is

evidenced by the 1st DSC heating cycle of the annealed moulded 70% and 100%HS

2m13pd TPUs shown in Figures 3.12 (a) and (b). The Tgs of the TPU were more

noticeable in the DMTA technique than the DSC technique and this is because the

DMTA technique is more sensitive to the relaxation transitions occurring in the TPU

samples than the DSC technique.

143

4.4.2.2 15PD TPU COMPOSITES

-100 -50 0 50 100 150 200

1

10

100

1000

10000

TPU comp (15pd-70)

TPU comp (15pd-80)

TPU comp (15pd-90)

TPU comp (15pd-100)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-100 -50 0 50 100 150

0.0

0.2

0.4

0.6 TPU comp (15pd-70)

TPU comp (15pd-80)

TPU comp (15pd-90)

TPU comp (15pd-100)

Ta

n D

elta

Temperature (ºC)


-100 -50 0 50 100 150 200

1

10

100

1000

10000

Ann. TPU comp (15pd-70)




Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-100 -50 0 50 100 150

0.0

0.2

0.4

0.6 Ann. TPU comp (15pd-70)




Ta

n D

elta

Temperature (ºC)


Figure 4.6 (a) Storage modulus (b) tan delta of (I) unannealed 15pd TPU

composites (II) annealed 15pd TPU composites

TgSP

(a) (b)

TgSP

TgHP / TgHS

(b)

(a)

144

70 80 90 100

0

2000

4000

6000

8000

10000

12000

14000

TPU Composites

(15pd)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at 25º C

E' at 100º C

70 80 90 100

0

2000

4000

6000

8000

10000

12000

14000


(15pd)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at 25º C

E' at 100º C

Figure 4.7 Storage moduli bar graph of (a) unannealed and (b) annealed 15pd TPU

composites

In Figure 4.6 – I (a), it is observed that the storage moduli of the 70% and 80%HS

begin to decrease earlier than those of the 90% and 100%HS. The observation can be

attributed to the occurrence of the TgSP at -24±0.7°C. This also showed that the 15pd

TPU samples are phase separated and semi-crystalline because they exhibit both Tg

and melting transitions. The properties of TPU composites are influenced by the

morphological changes of the TPU212-214

. There were predominant transitions at

47°C observed for 90% and 100% and these transitions were attributed to the TgHP

and TgHS respectively. After the TgHP / TgHS, peaks were observed which were

attributed to cold-crystallization. Figure 4.6 – I (b) showed that TgSP was clearly

observed for 70%HS. 90% and 100%HS TPU composites were characterized by

prominent relaxation peaks attributed to the occurrence of TgHP / TgHS and cold

crystallization peaks. It was observed that the relaxation peak shifted to higher

temperatures with increase in HS concentration. It was also observed that the

magnitude of tan delta peaks also increased with increasing HS concentration. This

therefore relates to the composition of the TPU constituents (amount of NCO/OH) as

well as the HS transition temperatures103, 111

.

(b) (a)

145

The storage moduli of the TPU composites were increased after annealing especially

at 100°C and this is evidenced by the disappearance of cold crystallization as shown

in Figure 4.6 – II (a). The difference in the storage moduli of unannealed and

annealed TPU composites can be seen in Figure 4.7. Greater phase separation was

also achieved upon annealing as TgSP was prominently observed for only the 70%HS

in Figure 4.6 – II (b). The TgSP values of the 70%HS for both unannealed and

annealed 15pd TPU composites were -24±0.7°C and -20±0.2°C respectively. The

decrease in the TgSP value for the annealed 70%HS 15pd TPU composite was

attributed to the increased phase separation of the HS from the SS whereas the

increase in the TgSP value for unannealed 70%HS 15pd TPU composite can be

attributed either to the presence of HS in the SB phase or the mobility restriction

caused by the HS on which the SS were anchored215

. The TgHP was seen for the 70%,

80% and 90%HS TPU composites whereas TgHS was seen for the 100%HS.

146

4.4.2.3 16HD TPU COMPOSITES

-100 -50 0 50 100 150

100

1000

10000

TPU comp (16hd-70)

TPU comp (16hd-80)

TPU comp (16hd-90)

TPU comp (16hd-100)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-100 -50 0 50 100 150

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

TPU comp (16hd-70)

TPU comp (16hd-80)

TPU comp (16hd-90)

TPU comp (16hd-100)

Ta

n D

elta

Temperature (ºC)


-100 -50 0 50 100 150

100

1000

10000

Ann. TPU comp (16hd-70)




Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-100 -50 0 50 100 150

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35





Ta

n D

elta

Temperature (ºC)


Figure 4.8 (a) Storage modulus (b) tan delta of (I) unannealed 16hd TPU

composites (II) annealed 16hd TPU composites

(b) (a)

TgSP

TgHP / TgHS

(b)

(a)

TgSP

147

70 80 90 100

0

2000

4000

6000

8000

10000

12000

14000

TPU Composites

(16hd)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at 25º C

E' at 100º C

70 80 90 100

0

2000

4000

6000

8000

10000

12000

14000


(16hd)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at 25º C

E' at 100º C

Figure 4.9 Storage moduli bar graph of (a) unannealed and (b) annealed 16hd TPU

composites

In Figure 4.8 – I (a), the phase transitions of the unannealed 16hd TPU composites

were seen to be similar to those of the unannealed 15pd TPU composites discussed

in Figure 4.6 – I (a). The storage moduli of the 70% and 80%HS TPU composites

first began to decrease due to the occurrence of TgSP. These set of samples are phase-

separated and semi-crystalline. TgHP / TgHS and cold crystallization peaks were

observed for the 90% and 100% HS TPU composites. For the 90% and 100%HS

TPU composites pronounced melting-recrystallization-melting peaks are observed

(i.e. double melting peaks and a recrystallization peak) immediately after the cold

crystallization peaks. As discussed earlier, studies have revealed the occurrence of

multiple endothermic transitions in TPUs which are as a result of the presence of

polymorphic structures and this is the case with the 16hd TPUs116, 118, 121, 125, 206-209

.

The DMTA observations are similar to those discussed in the 1st heating DSC

transitions of the moulded 70% and 100%HS 16hd TPUs in Figures 3.11 (a) and

(b). Figure 4.8 – I (b) showed that TgSP was clearly observed for the 70% HS

whereas the 90% and 100% HS TPU composites were characterized by prominent

relaxation peaks attributed to the occurrence of TgHP / TgHS and cold crystallization

peaks.

(b)

(a)

148

In Figure 4.8 – II (a), annealing was seen to increase the storage moduli of the 16hd

TPU composites especially at 100°C. The cold-crystallization peaks were erased by

annealing. However, at temperatures above 150°C the melting-recrystallization-

melting peaks were still observed. These peaks were not erased during annealing

primarily because of the annealing temperature (80°C) used, which is too low to

erase the melting-recrystallization-melting transitions which occurred at higher

temperatures. Like the annealed 15pd TPU composites, TgSP and TgHP / TgHS were

clearly observed for annealed 16hd TPU composites as shown in Figure 4.8 – II (b).

The observation therefore confirmed that greater phase separation was achieved

during the annealing process. Figure 4.9 reveals the improvement in the thermo-

mechanical properties of the TPU composites due to annealing.

149

4.4.2.4 17HPD TPU COMPOSITES

-100 -50 0 50 100 150 200

10

100

1000

10000

TPU comp (17hpd-70)

TPU comp (17hpd-100)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-50 0 50 100 150

0.00

0.05

0.10

0.15

0.20

TPU comp (17hpd-70)


Ta

n D

elta

Temperature (ºC)


-100 -50 0 50 100 150 200

10

100

1000

10000

Ann. TPU comp (17hpd-70 )

Ann. TPU comp (17hpd-100)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-50 0 50 100 150

0.00

0.05

0.10

0.15

0.20



Ta

n D

elta

Temperature (ºC)


Figure 4.10 (a) Storage modulus (b) tan delta of (I) unannealed 17hpd TPU

composites (II) annealed 17hpd TPU composites

(a)

(b)

TgSP

TgHP / TgHS

(b)

(a)

TgSP

150

75 100

0

2000

4000

6000

8000

10000

12000

14000

TPU Composites

(17hpd)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at 25º C

E' at 100º C

75 100

0

2000

4000

6000

8000

10000

12000

14000


(17hpd)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at 25º C

E' at 100º C

Figure 4.11 Storage moduli bar graph of (a) unannealed and (b) annealed 17hpd

TPU composites

In Figure 4.10 – I (a), high temperature transitions (melting-recrystallization-

melting peaks) were observed for the composites. TgSP was prominently observed for

the unannealed 70%HS TPU composite as shown in Figure 4.10 – I (b). Upon

annealing, the phase transitional changes observed for the unannealed composites

were erased and the thermo-mechanical properties of the composites were enhanced

as shown in Figure 4.10 – II (a). The storage modulus of the annealed 70%HS TPU

composite decreased at relatively low temperatures due to the occurrence of TgSP.

However, the storage modulus of the annealed 100%HS TPU composite was seen to

be consistent over the temperature range showing no significant loss. The difference

in the storage moduli data of the unannealed and annealed TPU composites can be

seen in Figure 4.11. TgSP and TgHP were observed for the annealed 70%HS TPU

composite whereas only TgHS was observed for the annealed 100%HS TPU

composite as shown in Figure 4.10 – II (b).

(a)

(b)

151

4.4.2.5 18OD TPU COMPOSITES

-100 -50 0 50 100 150 200

100

1000

10000

TPU comp (18od-70)

TPU comp (18od-100)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-100 -50 0 50 100 150

0.00

0.02

0.04

0.06

0.08

0.10

0.12

TPU comp (18od-70)

TPU comp (18od-100)

Ta

n D

elta

Temperature (ºC)


-100 -50 0 50 100 150 200

100

1000

10000

Ann. TPU comp (18od-70 )

Ann. TPU comp (18od-100)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-100 -50 0 50 100 150

0.00

0.02

0.04

0.06

0.08

0.10

0.12



Ta

n D

elta

Temperature (ºC)


Figure 4.12 (a) Storage modulus (b) tan delta of (I) unannealed 18od TPU

composites (II) annealed 18od TPU composites

(b) (a)

(b)

TgSP

TgHP / TgHS

(a)

TgSP

152

75 100

0

2000

4000

6000

8000

10000

12000

14000

TPU Composites

(18od)S

to

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at 25º C

E' at 100º C

75 100

0

2000

4000

6000

8000

10000

12000

14000


(18od)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at -25º C

E' at 100º C

Figure 4.13 Storage moduli bar graph of (a) unannealed and (b) annealed 18od TPU

composites

In Figure 4.12 – I (a), high temperature transitions (melting-recrystallization-

melting peaks) were observed for the composites. The storage modulus of 70%HS

TPU composite decreased at about -50°C due to the existence of TgSP, clearly

observed in the tan delta graph in Figure 4.12 – I (b). The observation is consistent

with the 1st heating DSC transitions of the moulded 18od TPUs seen in Figures 3.11

(a) and (b).

The storage moduli of the TPU composites were greatly increased after annealing as

shown in Figure 4.12 – II (a). The high temperature transitions were still observed

for annealed TPU composites. The increase in the storage modulus of annealed

100%HS TPU composite at 25°C was the highest observed for all the annealed

100%HS TPU composites with linear chain extenders used in this research work.

This observation is in contradiction with Camberlin et al as it is expected that TPUs

with higher number of methylene units in the chain extender should exhibit lower

storage moduli. Camberlin et al. reported that 1,10-decanediol TPU showed lower

modulus than 1,4-butanediol TPU in the rubbery plateau region94

. The reason for the

decrease in the modulus was due to the flexibility of the TPU polymeric chain

imparted by the increasing length of the chain extender93, 95, 215

. This however is not

(b) (a)

153

the case in our work. The reason for this observation is that Camberlin et al used

about 30%HS for their studies whereas in this research work, higher %HS

concentrations (70% to 100%) were used. TgSP and TgHP were observed for the

annealed 70%HS TPU composite whereas only TgHS was observed for the annealed

100%HS TPU composite as shown in Figure 4.12 – II (b). The difference in the

storage moduli of the unannealed and annealed TPU composites can be seen in

Figure 4.13.

154

4.4.2.6 14CHDM TPU COMPOSITES

-100 -50 0 50 100 150 200

1

10

100

1000

10000

TPU comp (14chdm-70)




Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-100 -50 0 50 100 150

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6





Ta

n D

elta

Temperature (ºC)


-100 -50 0 50 100 150 200

1

10

100

1000

10000

Ann. TPU comp (14chdm-70)




Sto

ra

ge

M

od

ulu

s (M

Pa

)

Temperature (ºC)

-100 -50 0 50 100 150

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6





Ta

n D

elta

Temperature (ºC)


Figure 4.14 (a) Storage modulus (b) tan delta of (I) unannealed 14chdm TPU

composites (II) annealed 14chdm TPU composites

(a) (b)

TgHP / TgHS

TgSP

(b)

(a)

155

70 80 90 100

0

2000

4000

6000

8000

10000

12000

14000

TPU Composites

(14chdm)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at 25º C

E' at 100º C

70 80 90 100

0

2000

4000

6000

8000

10000

12000

14000


(14chdm)

Sto

ra

ge

M

od

ulu

s (M

Pa

)

Hard Segment (%)

E' at -100º C

E' at 25º C

E' at 100º C

Figure 4.15 Storage moduli bar graph of (a) unannealed and (b) annealed 14chdm

TPU composites

In Figure 4.14 – I (a), the storage moduli of all the TPU composites were seen to be

similar over the temperature range. Unlike the results of other chain extenders

discussed earlier, no TgSP was observed for the 70%HS TPU composite. The

observation confirmed the mixed phase state of moulded 14chdm TPUs as shown in

their 1st heating DSC transitions in Figures 3.11 (a) and (b). The 14chdm TPUs

were discussed to be amorphous primarily due to chemical structure of the 14chdm

chain extender (see Chapter 3). The amorphous nature of these TPU samples arose

from the mixture of the cis- and trans- geometric isomers of the 14chdm chain

extender183, 215-216

. The decrease in the storage moduli of TPU composites at about

20°C can be attributed to the softening of the HS. This HS softening was clearly

shown in the tan delta data in Figure 4.14 – I (b). The peaks observed at high

temperatures were due primarily to disruptions in the amorphous polymeric chains.

In Figure 4.14 – II (a), the thermo-mechanical properties of the TPU composites

were improved due to annealing. Induced crystallinity observed for the annealed

14chdm TPU composites corresponds to the 1st DSC heating transitions of the

annealed moulded 14chdm TPUs (see Figure 3.12 (a) and (b)). Furthermore, greater

phase separation as a result of annealing was also achieved which is evidenced by

(b)

(a)

156

the occurrence of TgSP at 11±0.6°C for the annealed 70%HS TPU composite. The

annealed 80% and 100%HS TPU composites showed higher storage moduli at high

temperatures (above 100°C). TgSP and TgHS were observed for the annealed 70% and

100%HS TPU composites, respectively, whereas TgHP was observed for the annealed

70%, 80% and 90%HS TPU composites, respectively, as shown in Figure 4.14 – II

(b). The storage moduli of the unannealed and annealed 14chdm TPU composites

can be seen in Figure 4.15.

157

4.4.3 TPU COMPOSITES vs POLYPROPYLENE and ISOPLAST

COMPOSITES

4.4.3.1 70%HS TPU COMPOSITES vs POLYPROPYLENE vs

ISOPLAST

-100 -50 0 50 100 150

1

10

100

1000

10000

Sto

rag

e M

od

ulu

s (M

Pa

)

Temperature (ºC)


TPU comp (15pd-70)

TPU comp (16hd-70)

TPU comp (17hpd-70)

TPU comp (18od-70)


PP composite

Isoplast composite

-100 -50 0 50 100 150

1

10

100

1000

10000

Sto

rag

e M

od

ulu

s (M

Pa

)

Temperature (ºC)







Ann. PP comp

Ann. Isoplast comp

Figure 4.16 Storage moduli of (a) unannealed composites (b) annealed composites

of 70% HS TPU composites having different chain extenders vs PP and Isoplast

composite

(a)

(b)

158

Table 4.2: Storage moduli data of unannealed and annealed 70% TPU vs

Polypropylene vs Isoplast composites

Unannealed / Annealed E’ at -100° C /

MPa

E’ at 25° C /

MPa

E’ at 100° C /

MPa

TPU comp-2m13pd-70 11333±704 /

10979±1690

4085±1076 /

6678±920

453±84 /

3865±470

TPU comp-15pd-70 12395±1892 /

12040±588

5190±732 /

5520±306

1666±185 /

3019±140

TPU comp-16hd-70 10615±666 /

10534±2021

3354±106 /

4962±789

1175±147 /

2861±426

TPU comp-17hpd-70 12353±1529 /

11445±1026

4255±630 /

6008±700

1601±248 /

3896±489

TPU comp-18od-70 11828±711 /

10891±586

3555±239 /

5459±618

1575±131 /

3688±612

TPU comp-14chdm-70 12122±1628 /

11582±1020

2103±459 /

7225±789

20±3 /

3217±339

PP composite 12264±484 /

12957±175

7899±342 /

8998±87

4929±312 /

5836±141

Isoplast composite 8249±315 /

6785±262

6857±203 /

5823±303

974±71 /

2955±206

Commercially available polypropylene (PP) and Isoplast (I) were used as

thermoplastic benchmarks for our designed TPU materials. The moulding

temperatures used in processing PP (200ºC) and I composites (240ºC) were different

from that of the TPU composites (180ºC). The granules were first flattened in the

compression press at temperatures (180ºC and 200ºC respectively) below their

melting temperatures. For PP and I composites, moulds were preheated to 200ºC and

240ºC respectively. Like the TPU composites, the PP and I composites were also

annealed at 80°C for 168 hours. The thermo-mechanical properties (storage moduli)

of 70% HS TPU composites with different chain extenders were compared with PP

and I composites.

In Figure 4.16 (a), the modulus of the unannealed PP composite at 25°C were seen

to be slightly higher than the unannealed TPU and I composites; however, at 100°C,

159

the modulus of the unannealed PP composite were seen to be much higher than the

unannealed TPU and I composites. The observation is due to the isotactic structure

of the homopolymeric PP used in this research work. It is therefore safe to conclude

that the PP is more crystalline than all the other composite samples. The crystallinity

of PP can be evidenced by WAXS result of moulded PP shown in Figure 4.17.

0 10 20 30 40 50 60 70

0

20000

40000

60000

80000

100000

120000

140000

Inte

nsity

(a.u

)

2(º)

Moulded Polypropylene

Figure 4.17 WAXS results of moulded PP

At -100°C, the unannealed I composite showed the lowest storage moduli. The

above observation is attributed to the glassy (amorphous) morphology of the Isoplast

matrix. The DSC trace of the I composite shows Tg to be 100°C as seen below in

Figure 4.18. From the DSC result, it can then be deduced that at room temperature

(25°C), the I sample is below its Tg.

160

0 50 100 150 200

-0.8

-0.6

-0.4

1st Heating Cycle

Hea

t Flo

w (W

/g)

Temperature (ºC)

Isoplast composite

Figure 4.18 1st heating DSC result of Isoplast composite

Figure 4.16 (b) shows the storage modulus of the annealed PP composite is slightly

higher than the storage moduli of the annealed TPU and I composites in all

temperatures. This is due to the higher crystallinity level of the PP sample whereas

for the annealed 70%HS TPU composites, only 70% can form a crystalline phase.

Table 4.2 shows the data of the storage moduli of the annealed and unannealed TPU,

PP and I composites. It can also be observed that the moduli of the TPU composites

can compete with those of the PP and I composites.

161

4.4.3.2 100%HS TPU COMPOSITES vs POLYPROPYLENE vs

ISOPLAST

-100 -50 0 50 100 150

1

10

100

1000

10000

Sto

rag

e M

od

ulu

s (M

Pa

)

Temperature (ºC)


TPU comp (15pd-100)

TPU comp (16hd-100)


TPU comp (18od-100)


PP composite

Isoplast composite

-100 -50 0 50 100 150

1

10

100

1000

10000

Sto

rag

e M

od

ulu

s (M

Pa

)

Temperature (ºC)







Ann. PP composite

Ann. Isoplast composite

Figure 4.19 Storage moduli of (a) unannealed composites (b) annealed composites

of 100% HS TPU composites having different chain extenders vs polypropylene and

Isoplast composites

(a)

(b)

162

Table 4.3: Storage moduli data of unannealed and annealed 100% TPU vs Polypropylene

and Isoplast composites

Unannealed / Annealed E’ at -100° C /

MPa

E’ at 25° C /

MPa

E’ at 100° C /

MPa

TPU comp-2m13pd-100

11778±6433 /

6427±1565

1676±456 /

6226±1469

13±68 /

5502±1279

TPU comp-15pd-100

8965±933 /

8175±314

6763±854 /

6801±120

1250±80 /

4017±242

TPU comp-16hd-100

9170±1827 /

8396±1477

7009±1537 /

7015±1157

1606±271 /

4202±521

TPU comp-17hpd-100

9774±966 /

5916±1040

4010±386 /

5764±987

1082±125 /

5557±929

TPU comp-18od-100

8794±1016 /

9757±1278

5040±736 /

8520±1152

1738±73 /

5852±770

TPU comp-14chdm-100

9172±2052 /

6067±282

4041±1226 /

5415±264

41±9 /

4967±548

Polypropylene composite

12264±484 /

12957±175

7899±342 /

8998±87

4929±312 /

5836±141

Isoplast composite

8249±315 /

6785±262

6857±203 /

5823±303

974±71 /

2955±206

In Figure 4.19 (a), the modulus of the unannealed PP composite at 25°C were seen

to be slightly higher than the unannealed TPU and I composites; however, at 100°C,

the modulus of the unannealed PP composite were seen to be much higher than the

unannealed TPU and I composites. In Figure 4.19 (b), the storage modulus of the

annealed 18od-100%HS TPU composite is almost similar with the annealed PP

composite. The reason for the observation is that for the annealed 100%HS TPU

composites, all 100% of the TPU polymer contributes to its overall crystallinity and

consequently much improved mechanical properties. From the Table 4.3, the storage

moduli of the annealed TPU composites compare well with those of annealed PP and

I composites.

163

CHAPTER 5

RESULTS AND DISCUSSION (III)

SPECIFIC TESTS ON 15PD TPUs

5.1 ANNEALING STUDIES ON MELT-QUENCHED 15PD TPUs

5.1.1 MELT-QUENCHED 15PD TPUs (CONTROL SAMPLES)

0 50 100 150 200

He

at F

low

- E

xo

MQ TPU-70

MQ TPU-80

MQ TPU-90

MQ TPU-100

Temperature (ºC)

CE: 15pd1st Heating Cycle

0 50 100 150 200

He

at F

low

- E

xo

MQ TPU-70

MQ TPU-80

MQ TPU-90

MQ TPU-100

Temperature (ºC)

2nd Heating Cycle CE: 15pd

Figure 5.1 (a) 1st heating cycles (b) 2

nd heating cycles of melt-quenched 15pd TPUs

Thermal analysis was employed to investigate the changes that occur as a result of

the segmental ordering during the annealing of TPU. Figure 5.1 shows the 1st and

2nd

DSC traces of melt-quenched 15pd TPUs with different HS concentrations (70,

80, 90 and 100%). These melt-quenched TPUs will serve as control samples with

respect to the annealed melt-quenched TPUs which will be discussed subsequently.

The DSC phase transitions of the melt-quenched 15pd TPUs have already been

discussed in Chapter 3.2.1.2 (see Figures 3.9 (a) and (b)). The melt-quenched 15pd

TPUs were annealed at 80°C (temperature above the HS glass transition, TgHS) to

elucidate annealing-induced ordering of these segmented TPUs. Various annealing

studies have been carried out on TPUs to investigate the morphological changes

(b) (a)

164

occurring within the segmental chains; and consequently the phase transitions of the

TPUs are altered as well33, 35-36, 39-40, 58-59, 63, 116, 118-121, 125-127, 208-209, 215, 217-220

.

DSC studies of annealed TPUs have reported on the existence of annealing

endotherms which are relative to the annealing temperatures employed. The

annealing endotherms are reported to result from the discontinuity of ordered

segments. Between the Tgs of the amorphous segments and the melting transitions of

the crystalline segments, there is a succession of ordered segmental morphologies.

Dissociation of these ordered segments requires a certain amount of energy and

therefore an endothermic transition is noticed. These endothermic transitions exist at

temperatures below the crystalline melting temperature, because the energies needed

for their dissociation are lower due to a lower degree of ordering than the crystalline

phases116, 118-119

.

When a sample is subjected to a particular temperature, order which disorders

beneath the annealing temperature will disappear and those segmental chains

affected will exist in a liquid-like state. If during annealing the liquid-like segments

are to return back to a solid state structure, there must be a reorganization of the

segments to an order which will be unalterable at the annealing temperature. More

liquid-like segments may be added to the more stable and ordered segments which

act as nuclei in a process similar to that of recrystallization. It is also suggested that

more stable segments may grow out of amorphous phases and thereby increasing the

number of the ordered chains. The annealing endothermic transition therefore

denotes the degree of order of the structures produced during the annealing process.

Higher annealing temperatures therefore lead to annealing endotherms with larger

melting peak areas as more structure is disorganized and reorganized with a higher

degree of crystal perfection116, 118-119

.

165

5.1.2 ISOTHERMAL ANNEALING STUDIES

0 50 100 150 200

He

at F

low

- E

xo

Ann. MQ TPU-100 (8hrs)




Temperature (ºC)

1st Heating Cycle

a) Annealed melt-quenched 15pd-100 TPUs at different annealing times

0 50 100 150 200

He

at F

low

- E

xo





Temperature (ºC)

1st Heating Cycle CE: 15pd

b) Annealed melt-quenched 15pd-90 TPUs at different annealing times

(a)

(b)

166

0 50 100 150 200

He

at F

low

- E

xo





Temperature (ºC)


c) Annealed melt-quenched 15pd-80 TPUs at different annealing times

0 50 100 150 200

He

at F

low

- E

xo





Temperature (ºC)


d) Annealed melt-quenched 15pd-70 TPUs at different annealing times

Figure 5.2 1st heating DSC cycles of (a) 100%HS (b) 90%HS (c) 80%HS and (d)

70%HS melt-quenched 15pd TPUs annealed at different annealing times (8, 24, 72

and 168 hours)

Figure 5.2 (a) shows the 1st heating DSC phase transitions of the melt-quenched

TPU-100%HS samples annealed at different annealing times. In the 1st heating DSC

(c)

(d)

167

cycle, annealing endotherms (TA) were observed for the melt-quenched TPU

100%HS samples. The temperatures of the annealing endotherms were seen to

increase with increase in annealing time. The annealing endotherms of the melt-

quenched TPU-100%HS at 8, 24, 72 and 168hrs were 137±0.05°C, 140±0.5°C,

151±0.9°C and 147±0.9°C respectively. It is somewhat interesting to see that the

annealing endotherm (TA) was observed for the 100%HS TPU. This therefore means

that the annealing-induced ordering is not dependent on the presence of the SS

phase. This also suggests that the ordering effected by annealing is an intraphase

occurrence rather than interphase between the HS and SS domains. It is therefore

safe to say that HS ordering can take place without the SS rubbery phase acting as a

plasticizer to enhance chain mobility118

. Since there are no SS in the 100%HS TPUs,

only melting temperatures of the HS (TM) are observed. The TM values of annealed

melt-quenched TPU-100%HS at 8, 24 and 168hrs were 164±0.2°C, 170±0.3°C and

174±0.02°C. No TM was recorded for the annealed melt-quenched TPU-100%HS at

72 hrs because the TA and TM coincide with each other. It is therefore difficult to

accurately resolve the TM for annealed melt-quenched TPU-100%HS at 72 hrs.

Figure 5.2 (b) shows the phase transitions of the melt-quenched TPU 90%HS

samples annealed at different annealing times. Like the 100%HS samples, the

temperatures of the annealing endotherms were seen to increase with increase in

annealing times. The annealing endotherms of the annealed melt-quenched TPU-

90%HS at 8, 24, 72 and 168hrs were 140±0.9°C, 142±0.5°C, 145±0.8°C and

148±0.01°C respectively. Since the 90%HS melt-quenched TPU sample contains

10% SS, it is therefore safe to deduce that the high-temperature endotherm is a

merger of TMMT and TM. On close observation, the TM appears as a shoulder on the

TMMT peak. However, due to the proximity of these two transitions, it is difficult to

resolve them apart. The temperatures of the microphase mixing transitions (TMMT)

were also observed to decrease with increasing HS concentration and this is due

primarily to the lesser proportion of SS with increasing HS content. For a 168-hour

annealing time, it can be observed that the temperatures of the annealing endotherms

increased primarily due to longer annealing time. This therefore implies that more

crystallisable HS were involved in the melting process. However, the TMMT

enthalpies were seen to decrease with increasing annealing times.

168

Figure 5.2 (c) shows the phase transitions of the melt-quenched TPU-80%HS

samples annealed at different annealing times. In the 1st heating DSC cycle,

annealing endotherms (TA) were observed for the annealed melt-quenched TPU-

80%HS samples. The annealing endotherms were seen to increase with increase in

annealing times. The annealing endotherms of the melt-quenched TPU-80%HS at 8,

24, 72 and 168hrs were 135±0.8°C, 139±0.3°C, 140±0.8°C and 146±0.7°C

respectively. The TA enthalpies of the annealed melt-quenched TPU-80%HS samples

are seen to be higher than those of the annealed melt-quenched TPU-70%HS

samples. The reason for this observation is the fact that more amorphous HS were

disrupted and reorganized in ordered structures as the HS concentration increases.

This therefore confirms that the annealed melt-quenched TPU-80%HS samples are

more crystalline than the annealed melt-quenched TPU-70%HS samples. The TMMT

values of the annealed melt-quenched TPU-80 at 8, 24, 72 and 168hrs were

172±0.02°C, 175±0.2°C, 174±0.4°C and 172±0.3°C.

Figure 5.2 (d) shows the DSC phase transitions of the melt-quenched TPU-70%HS

samples annealed at different annealing times. In the 1st heating DSC cycle,

annealing endotherms (TA) were observed for the annealed melt-quenched TPU-

70%HS samples. The annealing endotherms were seen to increase with increase in

annealing times. The annealing endotherms of the annealed melt-quenched TPU-

70%HS at 8, 24, 72 and 168hrs were 138±0.3°C, 140±0.5°C, 143±0.4°C and

146±0.5°C respectively. These annealing endotherms were in close proximity with

the microphase mixing transitions; TMMT (that is, melting of SS and crystalline HS

observed at higher temperatures). The annealing endotherms were also seen to be

observed at about 50-60°C above the annealing temperature (80°C). This observation

corresponds to those of Bogart et al118

. For the 8-hour annealing time, the annealing

endotherms were not prominently marked. As the annealing time increased, the size

of the annealing endotherms increased. The TA enthalpies of the melt-quenched

TPU-70%HS samples at 8, 24, 72 and 168hrs were 2.17J/g, 1.26J/g, 2.97J/g and

3.86J/g respectively. However, the TMMT values were seen to be consistent for the

annealed melt-quenched TPU-70%HS samples at different annealing times. The

TMMT values of the annealed melt-quenched TPU-70%HS at 8, 24, 72 and 168hrs are

175±0.6°C, 173±0.1°C, 174±0.4°C and 173±0.5°C. The TMMT enthalpies were seen

to decrease with increasing annealing times. Due to the increased crystallinity

169

induced by the annealing process, no Tg was observed for all the annealed melt-

quenched TPUs in the 1st heating DSC cycle. This is because the samples are

crystalline due to annealing and therefore amorphous regions that give rise to Tgs are

reduced.

It can be observed that annealing temperatures and times do not have any major

influence on the TM values as has been noticed from all the previous results. The

reason is that the crystallites of the HS have a well defined temperature at which they

melt. Therefore, while annealing is capable of changing the shape or the size of the

melting peak, it will not change its melting temperature116

. From the above results, it

is evident that annealing affects the thermodynamic and morphological properties of

TPUs. Although only one annealing temperature (80°C) was employed in this

research work, it has been observed that different annealing temperatures affect the

thermodynamic properties of TPUs as shown by several authors39, 59, 116, 118, 120

.

Table 5.1 shows the data for DSC phase transitions of the annealed melt-quenched

15pd TPU series at different annealing times.

Figure 5.3 shows a comparative graph of 70%, 80%, 90% and 100%HS annealed

melt-quenched TPUs after 8 hours of annealing with respect to TA and TMMT/TM.

70 80 90 100

135

140

145

150

155

160

165

170

175

DS

C tr

ansi

tions

afte

r 8h

r-an

neal

ing

at 8

0C

(C

)


TA

TMMT

/TM

Figure 5.3 TMMT/TM and TA vs HS concentration of 70%, 80%, 90% and 100%HS

annealed melt-quenched TPUs annealed for 8hrs

170

From Figure 5.3, we can observe that TMMT/TM decreases with increase in HS

concentration. The reason for the decrease is related directly to the decrease in the

amount of the SS in the TPUs (with 70%HS having the highest amount of SS

(30%)). This trend in the decrease of TMMT/TM is similar to that observed in the DSC

transitions for the cast, melt-quenched, slow-cooled, moulded and annealed moulded

15pd TPUs as discussed in Chapter 3.

Furthermore, TA values were observed to remain fairly constant with increase in HS

concentration. This therefore suggests that at the same annealing time, the annealing

effect on all the TPU samples (70%, 80%, 90% and 100%HS annealed melt-

quenched TPUs) is similar.

The melting enthalpy values (ΔH) of TMMT/TM and TA were seen to follow the same

trend with the above observations. ΔHtotal of TMMT/TM of the annealed melt-quenched

TPUs decreased with increase with HS concentration whereas the values of ΔHtotal of

TA remained fairly the same. This can be seen in Figure 5.4.

70 80 90 100

0

5

10

15

20

25

DS

C tr

ansi

tions

afte

r 8h

r-an

neal

ing

at 8

0C

(J/

g)


Htotal

of TA

Htotal

of TMMT

/TM

Figure 5.4 ΔHtotal of TMMT/TM and ΔHtotal of TA vs hard segment concentration of

70%, 80%, 90% and 100%HS annealed melt-quenched TPUs annealed for 8hrs

171

Table 5.1: 1st heating DSC phase transitions of annealed melt-quenched 15pd TPU

series

15pd series TA (˚C)

TMMT/TM (˚C)

ΔHtotal (J/g)

ANNEALED MELT-QUENCHED TPU-70 Annealing Temperature:80°C

Ann.MQ TPU-70 for 8hrs 138±0.3

175±0.6 2.17±0.07*

24.28±0.08


173±0.1 1.26±0.04*

20.41±0.02


174±0.4 2.97±0.04*

17.05±0.05

Ann.MQ TPU-70 for

168hrs

146±0.5 173±0.5 3.86±0.07*

14.65±0.06



172±0.02 3.39±0.06*

24.05±0.05


175±0.2 3.50±0.04*

19.07±0.07

Ann.MQ TPU-80 for 72hrs 140±0.8 174±0.4 4.20±0.07*

17.64±0.05

Ann.MQ TPU-80 for 168

hrs

146±0.7

172±0.3 3.02±0.03*

14.05±0.05



170±0.2 2.63±0.03*

17.22±0.02


171±0.6 2.99±0.09*

16.67±0.06


171±0.2 2.05±0.04*

10.88±0.09

Ann.MQ TPU-90 for

168hrs

148±0.01

168±0.5 6.15±0.04*

7.99±0.09



164±0.2 (TM) 4.26±0.07*

14.21±0.01

Ann.MQ TPU-100 for

24hrs

140±0.5

170±0.3 (TM) 4.77±0.07*

11.11±0.01

Ann.MQ TPU-100 for

72hrs

151±0.9 ------- 43.04±0.05*

Ann.MQ TPU-100 for

168hrs

147±0.9 174±0.02 (TM) 4.32±0.03*

10.86±0.06

Data marked with asterisk (*) connote transitions associated with annealing

endotherms (TA)

172

5.2 TENSILE PROPERTIES OF 15PD COMPOSITES vs

POLYPROPYLENE vs ISOPLAST COMPOSITES

5.2.1 UNANNEALED AND ANNEALED COMPOSITES

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0

50

100

150

200

250

300

Te

nsi

le S

tre

ss (

MP

a)

Strain (%)

TPU comp (15pd-70)

TPU comp (15pd-80)

TPU comp (15pd-90)

TPU comp (15pd-100)

PP composite

Isoplast composite

Figure 5.5 Tensile properties of unannealed 15pd TPU composites vs polypropylene


Table 5.2: Data of the tensile properties of unannealed 15pd TPU composites vs

polypropylene and Isoplast composites

Unannealed

composite

samples

15pd-70

15pd-80

15pd-90

15pd-

100

PP Isoplast

Young’s

Modulus (GPa)

8±0.1 10±0.9 9±0.4 11±0.5 10±0.2 9±0.6

Ultimate Tensile

Strength (MPa)

257±0.8 242±0.7 281±0.7 271±0.1 274±0.7 265±0.3

Elongation (%) 3±0.2 2±0.4 3±0.03 2±0.5 2±0.7 3±0.1

173

0.0 0.5 1.0 1.5 2.0 2.5

0

50

100

150

200

250

300

Te

nsi

le S

tre

ss (

MP

a)

Strain (%)





Ann. PP composite


Figure 5.6 Tensile properties of annealed 15pd TPU composites vs polypropylene


Table 5.3: Data of the tensile properties of annealed 15pd TPU composites vs

polypropylene and Isoplast composites

Annealed

composite

samples

15pd-70 15pd-80 15pd-90 15pd-

100

PP Isoplast

Young’s

Modulus (GPa)

11±0.6 11±0.6 13±0.9 11±0.2 11±0.9 9±0.3

Ultimate Tensile

Strength (MPa)

253±0.1 254±0.1 248±0.4 193±0.1 244±0.4 171±0.1

Elongation (%) 2±0.4 2±0.4 1±0.8 1±0.8 2±0.2 1±0.8

In Figure 5.5, the tensile properties of the unannealed 15pd TPU composites are

compared to those of commercially available PP and I composites. It was observed

that the Young’s moduli of the 15pd unannealed TPU composites were similar to

those of PP and I composites. The reason for the similarity in the Young’s moduli of

all composites is dominated by the modulus of the glass fibre rather than those of the

polymers. The fibre mats were aligned in 2-directional arrangement and therefore

follow the direction of the tension force applied to the composites. It is therefore

reasonable to say that the moduli observed for the all composites originate mainly

from the modulus of the glass fibre mats. All composite samples showed brittle

174

properties upon fracture, and therefore there is no plastic deformation which also

means that there is little or no yield. As a result of the brittle properties of the

composites, the ultimate tensile strengths (UTS) of all the composites are equal to

the fracture strengths (FS) of the composites as shown in Table 5.2. The 90% and

100%HS unannealed TPU composites were seen to have high UTS values of

281±0.7 and 271±0.1 MPa respectively. The aforementioned values compare

favourably with the UTS of the PP composite (274±0.7 MPa). The reason for the

high values observed for the 90% and 100%HS unannealed TPU composites could

be attributed mainly to the degree of HS concentration. It is therefore reasonable to

conclude that the increase in the UTS of the TPU composites is dependent on the

increase of the HS content. The increase in the amount of SS incorporated in the

TPU system therefore decreases the UTS of the TPU material and this is confirmed

by the UTS values of the 70% and 80%HS unannealed TPU composites which were

257±0.8 and 242±0.7 MPa respectively. The elongation (%) values of all the

unannealed TPU composites under tension were similar. The tensile strengths of the

glass fibre reinforced TPU composites were reported to increase with increasing

volume fractions of the glass fibre221-222

. In our case, the volume fractions of the

TPU composites are similar and that supports the reason there is similarity in the

Young’s moduli of the unannealed TPU composites as shown in Table 5.2. In

addition, the reason for the similarity in the elongation (%) values is likely to be due

to the load being applied mainly to the fibre mats rather than the polymer matrix.

In Figure 5.6, the tensile properties of the annealed TPU composites are slightly

improved due to annealing. The Young’s moduli of the annealed TPU composites

for 70%, 80% 90% and 100%HS are 11±0.6, 11±0.6, 13±0.9 and 11±0.2 GPa

respectively as shown in Table 5.3.

175

5.3 CREEP PROPERTIES OF 15PD COMPOSITES vs

POLYPROPYLENE and ISOPLAST COMPOSITES

5.3.1 UNANNEALED AND ANNEALED COMPOSITES

0 10 20 30 40 50

-0.4

-0.2

0.0

0.2

0.4

Str

ain

(%

)

Time (min)

TPU comp (15pd-70)

TPU comp (15pd-80)

TPU comp (15pd-90)

TPU comp (15pd-100)

PP composite

Isoplast composite

Figure 5.7 Creep properties of unannealed 15pd TPU composites vs PP and I

composites

All composite samples used in the creep experiments were subjected to a constant

stress of 10MPa with a creep time of 20 minutes followed by a recovery time of 20

minutes. Figure 5.7 shows the creep behaviour of the unannealed TPU, PP and I

composites. It can be observed that the unannealed 15pd-70%HS TPU composite

showed the highest creep strain (0.43%). The reason for this observation is due

mainly to the fact that the 15pd-70%HS TPU material contains the highest amount of

SS (30%) and therefore this imparts flexibility to the TPU material. The applied

stress (10MPa) therefore impacts more on the unannealed 15pd-70%HS TPU

composite due to the flexibility of the TPU composite. The unannealed 15pd-90%HS

TPU composite (0.35% strain) was also observed to have higher creep strain than the

unannealed 15pd-100%HS TPU composite (0.29% strain). It can therefore be

deduced that creep strain increases with decreasing HS concentration. This is

176

because the increase in HS concentration leads to increased stiffness of the TPU

materials which are more resistant to the applied stress. Contrary to the above

deduction is the decreased creep strain of the unannealed 15pd-80%HS TPU

composite. The reason of this observation could be traceable to the volume fraction

of the fibre mats present in the TPU composite sample. The unannealed 15pd-

80%HS TPU composite had similar creep strain as PP and I composites. The PP and

I composites showed that lowest creep strains, due to the PP and I composites having

higher fibre volume fractions than the TPU composites.

It can be observed that the unannealed 15pd-70%HS TPU composite has the highest

unrecovered % strain (0.09%) upon creep recovery whereas the unannealed 15pd-

100%HS TPU composite was seen to exhibit the lowest unrecovered % strain

(0.003%) upon creep recovery. The reason for the observation for the unannealed

15pd-70 TPU composite is due to the amount of SS present in the TPU material. The

SS used in this research work acts as a plasticizer and therefore viscoplasticity

(permanent deformation after creep) is likely to set in during the creep recovery

process. The SS is responsible for the extensibility properties of TPU; in other

words, a stretched TPU material can undergo plasticity due to the presence of the SS.

On the other hand, the HS exhibit elastic properties in the TPU material and that

explains the occurrence of the lowest unrecovered % strain observed for the

100%HS TPU composite. The creep modulus; E(t) and creep compliance; J(t) of the

unannealed composites can be ascertained for the creep results.

Creep modulus; E(t) is the ratio of applied stress to the time-dependent strain in

viscoelastic materials

(Equation 5.1)

where σ0 = applied stress

ε(t) = time-dependent strain

Creep compliance; J(t) is the ratio of time-dependent strain to applied stress in

viscoelastic materials. It is inverse/reciprocal of creep modulus. It can also be

defined as the susceptibility of a material to deform.

177

(Equation 5.2)

Following the formulae outlined, the E(t) and J(t) values of all the unannealed

composites can be calculated. The creep properties of the unannealed composites are

shown in Table 5.4.

Table 5.4: Creep properties of the unannealed composites

Unannealed

composites

15pd-

70

15pd-

80

15pd-

90

15pd-

100

PP I

Creep strain (%) 0.43±

0.01

0.24±

0.01

0.35±

0.02

0.30±

0.02

0.24±

0.02

0.23±

0.01

Unrecovered strain

(%)

0.09±

0.08

0.01±

0.01

0.05±

0.01

0.005±

0.001

0.05±

0.02

0.02±

0.01

Creep Modulus

(MPa)

2326±

53

4167±

167

2857±

154

3333±

208

4167±

321

4348±

181

Creep Compliance

(MPa-1

) x 106

430±

10

240±

10

350±

20

300±

20

240±

20

230±

10

The creep moduli and creep compliance data measured for all the unannealed

composites were seen to correspond with their respective creep strain (%) values as

discussed earlier.

0 10 20 30 40 50

-0.5

0.0

0.5

Str

ain

(%

)

Time (min)





Ann. PP composite


Figure 5.8 Creep properties of annealed 15pd TPU composites vs PP and I

composites

178

All composites were annealed in an oven at 80°C for 168 hours after which they

were tested to ascertain changes in their creep behaviour arising from annealing.

From Figure 5.8, it can be observed that creep strain (%) decreased steadily with

increasing HS concentration for the annealed TPU composites; with the annealed

15pd-70%HS TPU composite having the highest creep strain (0.43%). The reason

for the observation is mainly due to increased crystallinities of the TPUs as a result

of the annealing process. The HS are the only crystallisable part of the TPUs used in

this research work, it is therefore safe to say that after the annealing process, the

annealed 15pd-100%HS TPU composite is more crystalline than other TPUs with

varying HS concentrations and hence, it is the most creep resistant of all annealed

TPU composites. The creep strain (%) values of all the annealed composites were

seen to be reduced when compared to those of the unannealed composites earlier

discussed. On the other hand, the unrecovered strain (%) values of the annealed

composites were seen to be higher than those of the unannealed composites. The

reason is that annealing hardens the composite materials and therefore after an

applied stress is removed from the annealed composites, partial or no strain recovery

takes place. The creep moduli of the annealed TPU composites were also seen to

increase with increasing HS content. The observed creep moduli of the annealed

90% and 100%HS TPU composites were seen to be higher than the creep modulus

of the annealed PP composite. The annealed I composite was seen to exhibit the

highest creep modulus and the lowest unrecovered strain (%) than all the annealed

composites. The reason for this was due to the glassy nature of the Isoplast material,

therefore annealing of the material increases its resistance to deform/creep. The

creep properties of the annealed composites are shown in Table 5.5.

Table 5.5: Creep properties of the annealed composites

Annealed composites 15pd-

70

15pd-

80

15pd-

90

15pd-

100

PP I

Creep strain (%) 0.42±

0.02

0.28±

0.01

0.25±

0.01

0.19±

0.01

0.26±

0.02

0.17±

0.02

Unrecovered strain

(%)

0.10±

0.07

0.03±

0.03

0.03±

0.01

0.03±

0.02

0.05±

0.01

0.01±

0.01

Creep Modulus

(MPa)

2381±

108

3571±

123

4000±

154

5263±

263

3846±

275

5882±

619

Creep Compliance

(MPa-1

) x 106

420±

20

280±

10

250±

10

190±

10

260±

20

170±

20

179

From the tensile results discussed, all composites were brittle on breaking. The

tensile properties of the 8 fibre mat layered 15pd TPU composites compared

favourably with the tensile properties of the PP and I composites. However, the

tensile properties of all the composites were more influenced by the fibre

reinforcement. Annealing was seen to slightly improve the Young’s moduli of all the

composites. The elongation of the annealed composites was also seen to be lower

than those of the unannealed composites. Decrease in elongation was as a result of

increased crystallinity of the composites through annealing.

The creep strain values of the unannealed composites were seen to be slightly higher

than those of the annealed composites. This can be explained by stating that the

stress applied (10MPa) had more effect on the unannealed composites than the

annealed. Furthermore, annealing improved the creep moduli of the composites. The

creep properties of the 15pd TPU composites compared well with those of the PP

and I composites.

180

CHAPTER 6

CONCLUSIONS AND FUTURE WORK

6.1 CONCLUSIONS

The presence of mixed phase was found for the melt-quenched, slow-cooled and

moulded 70%HS 2m13pd TPUs as revealed in the DSC results. The mixed phase

morphology was marked by a broad glass transition (TgMP). The thermo-dynamic

observations and results found for 2m13pd TPUs correspond to the previous studies

carried out by Saiani et al58-60

. Cast 14chdm TPUs were amorphous which was

attributed to the mixture of cis- and trans- geometric isomers present in the 1,4-

cyclohexanedimethanol chain extender. 12ed and 14bd TPUs displayed the highest

melting transitions (about 220°C) and melting enthalpies (ΔHTot) due to their high

crystallinity levels. TgHP was seen for the 70%HS TPUs whereas TgHS was seen for

the 100%HS TPUs. TMMT was seen for the 70%HS TPUs whereas TM was seen for

the 100%HS TPUs. The TMMT values of the cast TPUs chain-extended with 15pd,

16hd, 17hpd and 18od show that the even-numbered chain-extended TPUs have

higher melting transitions than the odd-numbered chain extended TPUs. The

characteristic zig-zag plot of the TMMT values of 70% and 100%HS cast TPUs chain-

extended with 15pd, 16hd, 17hpd and 18od agree with the findings of Fernandez et

al95

. These findings were linked to the odd-even effect of odd-numbered and even-

numbered diols (chain extenders). The multiple endothermic transitions were found

for the melt-quenched and slow-cooled 16hd, 17hpd and 18od TPUs and this

confirmed the existence of polymorphic structures in the polymer chains. Annealing

improved the thermal properties of all the 70%HS and 100%HS TPU samples. TA

was seen for all the annealed melt-quenched 15pd TPUs. TA values of the different

HS concentrations were found to increase with increase in annealing times.

Furthermore, TA values were seen to remain constant with increase in HS

concentration and this therefore implies that at the same annealing time, the

annealing effect on all the annealed melt-quenched 15pd TPUs (70%, 80%, 90% and

100%HS) is similar.

Phase separation was seen for all the cast 70% and 100%HS TPUs as revealed in the

SAXS results. The SAXS peaks seen for the 70%HS TPUs come from the HS-SS

181

phase whereas the SAXS peaks seen for the 100%HS TPUs come from the

crystalline HS and the non-crystalline HS.

Crystalline peaks were seen on the amorphous halos of the linear chain-extended cast

70% and 100%HS TPUs as revealed by WAXS results. These peaks correspond to

the crystallinity of HS as proposed by Blackwell et al97-98, 125, 193-197

. It was shown

that annealing the moulded TPU samples at 80°C for 168 hours induced the

formation of type-II crystals and the results are consistent with the findings of Briber

and Thomas211

. The cast linear chain extender based TPUs showed better

crystallinities than the non-linear/branched and cyclic chain extender based TPUs.

This is due to the linearity of the chain extenders as polymer chains are easily packed

together and consequently, the polymer chains crystallize easily.

All TPU composites displayed storage moduli above 2 GPa at 25°C as revealed by

the DMTA results. Upon annealing, the storage moduli of the TPU composites were

further improved with values above 4 GPa. TgSP was seen for all the annealed

70%HS TPU composites, TgHP were seen for the annealed 70%, 80% and 90%HS

TPU composites whereas only TgHS was seen for all the annealed 100%HS TPU

composites. These aforementioned Tgs were seen more easily in the DMTA

technique than the DSC technique because of the higher sensitivity of DMTA

technique to the relaxation transitions occurring in the TPU samples. The storage

moduli of the unannealed and annealed TPU composites were found to compare well

with those of unannealed and annealed PP and I composites. Tensile results showed

that the Young’s moduli of the unannealed and annealed 15pd TPU composites were

similar to those of PP and I composites. The creep properties of the 15pd TPU

composites compared well with those of the PP and I composites.

182

6.2 FUTURE WORK

This research focused on only one type of soft segment (EO-PPO-EO) and one type

of di-isocyanate (4,4-MDI) in relation to different types of chain extenders. The

different types of chain extenders investigated in this research work can be further

studied with different polyol and isocyanate unit.

For the chemistry of the TPUs to be further investigated, polytetrahydrofuran

(PTHF) can be used as soft segment. Studies have shown that PTHF polyol promote

phase separation and therefore better mechanical properties of TPUs can be

achieved. Aromatic chain extenders such as hydroquinone bis(2-hydroxyethyl) ether

can be employed to enhance the mechanical properties of the TPUs.

For the TPU composites, the glass fibre mat used in this research work is silane-

sized. Further investigations on the TPU composites would be to use fibre mats that

are sized with NH- compounds and also glass fibres that are coated with an amino-

silane which are common. This could improve the mechanical properties of the TPU

composites as stronger interfacial bonding is likely to take place between the fibre

and the TPU matrix.

Particulate reinforcements were not investigated in this research work. These could

be a good alternative to the glass fibre mats. Commonly used nano-scaled particles

such as graphene, graphite, carbon nantubes, multi-walled carbon nanotubes

(MWCNT) and nanoclays can be used as reinforcements for the TPU composites.

Recent studies have shown that these particles exhibit good mechanical, conductive

and electrical properties.

Compression moulding has been the only processing technique employed in this

research work. It would be interesting to employ injection moulding as well as resin

infusion in the manufacture of the TPU composites.

Further mechanical tests can be carried out on the TPU composites such as izod

impact strength test and charpy impact test so as to elucidate the toughness of the

composites.

The annealing temperature used throughout this research was 80˚C. Annealing

temperature can be increased in order to further elucidate the effect of annealing on

183

the thermodynamic, structural, thermo-mechanical and mechanical properties of

TPUs and TPU composites. Studies have shown that increase in annealing

temperature leads to increase in phase separation occurring in the TPUs and

consequently the mechanical properties of the TPUs.

184

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