Thermal properties of polypropylene/ muscovite layered silicate composites: effects of organic...

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published online 17 April 2014Journal of Composite MaterialsMohd Firdaus Omar, Hazizan Md Akil, Mohd Fadli Ahmad Rasyid and Jamaliah Mohd Sharif

compatibilisersThermal properties of polypropylene/muscovite layered silicate composites: effects of organic modifications and

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JOURNAL OFC O M P O S I T EM AT E R I A L SArticle

Thermal properties of polypropylene/muscovite layered silicate composites:effects of organic modifications andcompatibilisers

Mohd Firdaus Omar1, Hazizan Md Akil2,3,Mohd Fadli Ahmad Rasyid2 and Jamaliah Mohd Sharif4

Abstract

Polymer-layered silicate nanocomposites containing polypropylene, compatibilisers, muscovite, and organomuscovite

were prepared through the hot compression technique. Muscovite particles were initially treated with lithium nitrate,

followed by cetyltrimethylammonium bromide, to obtain the organomuscovite through an ion-exchange treatment.

Maleic anhydride and polyhedral oligomeric silsesquioxane were used as compatibilisers to modify the polypropylene/

organomuscovite nanocomposite systems. The effectiveness of the ion-exchange treatment and the implementations of

both compatibilisers were successfully proven using the Fourier transforms infrared, X-ray diffraction, as well as the

transmission electron microscopy. The overall thermal properties of the polypropylene/muscovite layered silicate com-

posites were investigated by applying both differential scanning calorimetry and thermogravimetric analysis. It was found

that the polypropylene reinforced with organomuscovite recorded higher thermal properties in terms of crystallisation

temperature and thermal stability than its counterpart; whereas, the melting temperature and the degree of crystallinity

displayed an opposite trend. For the effect of the compatibiliser, it was found that compatibilised polypropylene/

organomuscovite nanocomposites recorded higher values of decomposition and crystallisation temperatures than

that of non-compatibilised polypropylene/organomuscovite nanocomposites. However, the melting temperature and

the degree of crystallinity of compatibilised polypropylene/organomuscovite nanocomposites were slightly lower, as

compared to non-compatibilised polypropylene/organomuscovite nanocomposites.

Keywords

Polypropylene/muscovite layered silicate composites, thermal properties, organic modifications, compatibilisers

Introduction

Polymer layered silicate nanocomposites (PLSNs) are anew class of composite materials composed of hybridorganic polymer; inorganic materials.1,2 For the pastfew decades, PLSNs have attracted remarkable atten-tion from both scientists and industries due to the excel-lent performance in terms of engineering properties,such as mechanical properties,3,4 environmentalstability,5 gas impermeability,6 flammability,7 etc. Thehighlighted properties are achievable at low particlecontent; typically less than 5%. The abilities of thelayered silicates to exfoliate, disperse individually, andpossess a high aspect ratio of silicate platelets within thepolymer matrix were several factors that attributed tothe immense popularity of PLSNs.

Theoretically, layered silicates are classified as clayminerals with a build of two structural units. The exam-ple of 1:1 structure is Kaolinite, where a silica tetrahe-dral sheet is fused to an aluminium octahedron, along

1School of Materials Engineering, Universiti Malaysia Perlis, Perlis,

Malaysia2School of Material and Mineral Resource Engineering, Universiti Sains

Malaysia, Pulau Pinang, Malaysia3Cluster of Polymer Composites (CPCs), Universiti Sains Malaysia, Pulau

Pinang, Malaysia4Malaysian Nuclear Agency (Nuclear Malaysia) Bangi, Selangor, Malaysia

Corresponding author:

Hazizan Md Akil, School of Material and Mineral Resource Engineering,

Universiti Sains Malaysia, 14300 Nibong Tebal, Pulau Pinang, Malaysia.

Email: [email protected]

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with the sharing of oxygen atoms. Montmorillonite(MMT)8 and saponite9 are categorised into 2:1 struc-tures. They are among the most commonly used layeredsilicates in fabricating the PLSNs. This was mainly dueto their outstanding ion exchange properties, as well astheir swelling behaviour. The development of PLSNshas evolved from a conventional-layered silicate to analternative-layered silicate (such asMuscovite) due to itswell-defined crystal structure, molecularly smooth sur-face, outstanding corona resistance, its high aspectratio, and its availability in large amounts with rela-tively low cost.10 In addition, the aspect ratio ofMuscovite is higher than that of MMT. The chemicalformula of muscovite is KSi3Al3O10 (OH)2, with a veryhigh layer charge density, close to 1.0 and equivalent perO10(OH)2. Muscovite belongs to a monoclinic structurewith the space group (C2/c); with cell parametera¼ 5.18A�, b¼ 8.99A�, c¼ 20.07A�, and �¼ 95.751.11

Several issues should be considered in order to fullyutilise the outstanding performance of Muscovite sili-cate composites, especially its compatibility and misci-bility issues between particles and matrix. Normally,muscovite particles face difficulty in dispersing withinpolymer matrix due to their preferred face-to-facestacking in agglomerated tactoids.12 Moreover, musco-vite tactoids are also difficult to disperse into discretemonolayers due to their intrinsic incompatibility ofhydrophilic-layered silicates with hydrophobic poly-mers.12 Based on this concern, specific modificationswere carried out on the muscovite particles in orderto overcome these dilemmas. Recently, organic cationicsurfactants (which are primary, secondary, tertiary, andquaternary alkylammonium) have been widely used inthe ion exchange treatment of muscovite-layered sili-cates. For example, Shimizu et al.13 and Kodamaet al.14 employed alkaline earth metal cations and tran-sition metal ions as their exchange cation. In addition,Bracke et al.15 and Meier et al.16 used Li as theirexchange cations for biotite muscovite and phlogopite.Unfortunately, the implemented exchange cations didnot significantly enlarge the basal spacing of muscoviteparticles through the ion exchange treatment. Based onthese findings, this study demonstrated a two-step ionexchange treatment in order to further enlarge the basalspacing of Muscovite particles. In the first step, thelayered silicates were treated using lithium nitrate,followed by quaternary alkylammonium cations.Although several previous researchers actively reportedon the characteristics of Muscovite-layered silicatenanocomposites,17,18 not all of them were focused onits thermal and degradation behaviours; especially onthe effects of organic modifications and compatibilisers.Due to this concern, an experimental report was pur-posefully designed to fulfil the lack of knowledge in thisspecific area.

To achieve these goals, a characterisation techniquebased on the differential scanning calorimetry (DSC)analysis was demonstrated to investigate the thermalcharacteristics. Furthermore, a thermogravimetric ana-lysis (TGA) was carried out to specifically obtain thethermal degradation behaviours of all polypropylene(PP)/muscovite-layered silicate composites. For theorganic modification, muscovite particles were treatedusing the two-step ion-exchange treatment. The effect-iveness of the ion-exchange treatment was characterisedusing Fourier transforms infrared (FTIR), X-ray dif-fraction (XRD) and transmission electron microscopy(TEM). For the effect of compatibilisers, two differenttypes were used: maleic anhydride (MAH) and polyhe-dral oligomeric silsesquioxane (POSS). The efficiency ofboth compatibilisers was examined using both XRDand TEM analysis. All PP/muscovite (PP/M)-layeredsilicate composites were prepared by a hot compressiontechnique. The composites’ response was then charac-terised in terms of their melting, crystallisation andthermal degradation behaviours; correlated with theeffects of organic modifications, as well as differentcompatibilisers.

Experimental details

Materials

PP Homopolymer (TITANPRO PM-255), with a meltflow index of 1.6 g/10min (at 230�C) and a polymerdensity of 0.91 g/cm3, was purchased from Titan PPPolymers (M). The Muscovite particles were obtainedfrom Bidor Mineral (M) Sdn. Bhd. with a cationexchange capacity (CEC) of 82meq/100 g. Organicallymodified muscovite (OM) was prepared by the cationexchange of lithium nitrate (LiNO3) and cetyldimethy-lethylammonium bromide (CTAB), supplied by SigmaAldrich (M) Sdn. Bhd.; and in accordance to themethod proposed by Yu et al.19 Two types of compa-tibilisers were used in this study: poly(propylene-g-maleic anhydride) copolymer (PP-g-MAH), with8–10wt% maleic anhydride (MA), and PP-methylpolyhedral oligomeric silsesquioxane (PP-POSS). Bothcompatibilisers were purchased from Sigma Aldrich(M) Sdn. Bhd.

Ion-exchange treatment of muscovite particles

In this study, muscovite particles were treated using atwo-step ion exchange treatment. The details of thetreatment are as follows.

The LiNO3 treatment of muscovite particles. The mixture of10 g of muscovite powder and 170 g of LiNO3 powderwere mechanically combined and then heated in a

2 Journal of Composite Materials 0(0)




furnace at 300�C, for the duration of 12 h. The resultantproduct was washed seven (7) times and filtered withde-ionised water. Subsequently, the product was driedat 110�C using a vacuum oven. The end product wasnamed Li-muscovite.

The intercalation of Li-muscovite with CTAB. The prepar-ation of Li-muscovite with cetyltrimethylammoniumbromide (Li-muscovite–CTABþ) was undertakenunder hydrothermal conditions. The ion-exchangetreatment was performed with various levels of modifi-cations; with a CEC CTAB/muscovite ratio of 1(82meq/100 g), 1.2 (92meq/100 g) and 1.6 (132meq/100 g). Firstly, the CTAB was dissolved in de-ionisedwater. The Li–muscovite was then mixed with theCTAB aqueous solution at room temperature.Secondly, the mixtures were put in a hydrothermalreactor (100mL) of Teflon-lined stainless steel auto-clave and heated at 200�C, for the duration of 12 h.After the reactions, all surfactant–muscovite productswere filtered and washed three (3) times with ethanol,dried in a vacuum oven at room temperature, ground inan agate mortar, and finally stored in a desiccator. Theobtained CTAþ-muscovite samples were named orga-nomuscovite (OM).

Sample prepartion

During the first stage of the sample preparation, mus-covite and OM particles were dried at 80�C in a vacuumoven, for 24 h, in order to remove excessive moisture.muscovite and PP were then mixed and compoundedusing a Haake internal mixer (Thermo Haake PolydriveR600) with a mixing temperature of 180�C and a rotorspeed of 50 r/min; for a period of approximately 8min.The processing condition involved melting the PP in thepreheated mixing chamber of the Haake internal mixerfor 4min. Well-dried mica nanoparticles were thenadded gradually within 30 seconds, and the mixing pro-cess was continued until 8min had passed. After 8min,the compounded specimen was discharged from themixing instrument and immediately sheeted through alaboratory mill at a 1mm nip setting. This was for the

purpose of specimen handling during the hot press pro-cess. The compounded composite sheets were then cutinto small pieces before being hot pressed. For musco-vite composites with compatibilisers (MAH and POSS),the compatibilisers were premixed with the PP matrix,prior to the preheating process. The sheeted specimenswere compression-moulded in an electrically heatedhydraulic press into 1mm and 3mm thick sheets; tobe used in both DSC and TGA. To achieve the mainobjective, the muscovite particles (OM particles and PPgranules) were prepared to produce composites withdiverse compositions, consisting of muscovite contentsbetween 1 and 5wt%; as shown in Table 1.Furthermore, 5wt% of OM was fixed to be used withthe different compatibilisers.

Material characterisation

FTIR. The FTIR analysis was carried out to examine theeffectiveness of the ion-exchange treatment on the mus-covite particles. In this study, FTIR analysis wasobtained using a precise Perkin-Elmer spectrum andone FTIR spectrometer; the attenuated total reflection(ATR) technique was adopted. Muscovite particleswere mixed with a dried powdery potassium bromide(KBr), and then grounded using an agate mortar andpestle in order to obtain fine particles. The mixture waspelletised by using a hydraulic press (Specac-1) at 8tonnes of pressure and then scanned 32 times. The scan-ning of the FTIR spectrometer was carried out in thewave length range of 4000–400 cm�1 with 4 cm�1

resolution.

XRD. The XRD is an important analytical tool toinvestigate the crystal structure of solids. In addition,this technique can also be manipulated to determine theintercalated structures of crystalline-ordered materials,especially clay. When organic species (such as surfac-tants or polymers) are introduced into the galleries, theadjacent clay platelets will move away from each otheralong the c axis. This shift will be recorded in the XRDanalysis. The 001 Bragg peak shifts to the left in the

Table 1. The recipes used and identification of polypropylene/muscovite-layered silicate composite.

Identifications PP (wt %)

Untreated

muscovite (wt %) Identifications

PP

(wt %)

Treated

muscovite (wt%)

PP/1% M 100 0 PP/1% OM 100 0

PP/3% M 97 3 PP/3% OM 97 3

PP/5% M 95 5 PP/5% OM 95 5

PP/10% M 90 10 PP/10% OM 90 10

M: muscovite; OM: organomuscovite; PP: polypropylene.

Omar et al. 3




XRD spectrum are in accordance with the Bragg for-mula in equation (1)

2d sin � ¼ n� ð1Þ

where n, �, d, and � are the order of interference, thewavelength of the X-ray radiation used in the diffrac-tion experiment, the spacing between diffractional lat-tice planes, and the incident angle of the x-ray to thelattice planes, respectively. The 001 peak of the XRDsilicate crystal orientation moved to lower 2� angles asintercalation increased. The shift in the scattering peakreflected the galleries of the silicate, indicating intercal-ation. Based on the disappearance or the decrease ofintensity of diffraction peaks, it was concluded that thesilicate was partially or completely exfoliated.20 Ananalysis was made on muscovite particles, before andafter the ion-exchange treatment. The XRD was con-ducted with a Phillips X-Pert Pro diffractometer.Powder samples were scanned in the interval of2�¼ 1��10� at 40 kV and 30 mA.

TEM. The TEM analysis is an important visual tech-nique that has been used to characterise layered silicatenanocomposites. It portrays clay morphology at thenanometer level. If TEM is used in combination withXRD, the accurate structure of the nanocomposite canbe elucidated. The morphology analysis was carried outwith a LIBRA� 120 TEM, ZEISS that applied an accel-eration voltage of 120 kV. Power Tome PC microtomewas used to prepare the specimens. An ultra-thin sec-tion of about 90 nm thickness was cut with a DiatomeUltra 450 diamond knife at room temperature.

Thermal analysis

DSC. DSC is a thermo-analytical technique used toinvestigate the crystallisation behaviour and meltingcharacteristics of the nanocomposites. Perkin ElmerDSC Pyris 1 was operated under nitrogen atmosphereat a flow rate of 50mL/min. Approximately 10mg ofthe specimens were placed in a standard 40 mL alumin-ium crucible pan. The DSC test was run using twoscans. The first scan was performed to eliminate theresidual stress from the tested specimens. In the firstscan, the sample was heated from 25�C to 200�C at aheating rate of 10�C min�1, kept isothermally at 200�Cfor 2min and cooled to 25�C at a cooling rate of 10�Cmin�1. The second scan was carried out to determinethe crystalline temperature (Tc) and melting tempera-ture (Tm), which ranged from 25�C to 200�C, at a simi-lar heating rate of 10�C min�1. Crystallisation entropy(�Hc) and melting entropy (�Hm) were calculatedfrom the exothermic and endothermic peak areas,respectively. The degrees of crystallinity (Xc) for each

of the tested composite specimens were calculated byapplying the following equation

Crystallinity ¼�Hm

�H0� 100% ð2Þ

where �Hm and �H0 are the melting heat of thesample and the melting heat of 100% crystalline,respectively. In this case, �H0 for PP was 209 J/g.The term �H0 is a reference value and representsthe heat of melting if the polymers were 100% crys-talline. This reference heat of melting was establishedfor each of the commonly used polymers and meltingheat of a 100% crystalline PP, whose value was esti-mated at 209 J/g.21

TGA. The thermal stability and the weight profile of thetested muscovite composites were examined using aPerkin Elmer TGA 7. Approximately 10mg of speci-men was heated from 30�C to 600�C, with a heatingrate of 20�C/min at nitrogen airflow of 50mL/min. Thechange in weight with temperature was manipulated todisplay a weight versus temperature plot.

Results and discussions

Effect of organic modifications on PP/M-layeredsilicate composites

FTIR analysis. It was expected that the ion exchangebetween Kþ and Liþ will occur during the first treat-ment process; whereas, CTAþ ions were expected toreplace Liþ ions during the second stage of the ionexchange treatment. In this work, the effectiveness ofthe ion-exchange treatment was experimentally provenby the FTIR analysis in Figure 1. It can be observed(i.e. refer to Figure 1) that several overlapping and newbands appeared before and after the ion exchange treat-ment. The bands, at around 830 cm�1, were attributedto the octahedral sheets occupied by a trivalent centralatom (O-H bending bands), which were attributed tothe silicate sheets in the muscovite structure.22 In allFTIR spectrums, there was a band at 3620 cm-1 corres-ponding to the OH group, between the tetrahedral andoctahedral sheets in the muscovite structure.Noticeably, it can be observed that the transmitterbands (i.e. which refer to the Muscovite) are similarfor all tested specimens, which confirmed that the struc-ture of aluminosilicate layers still remained the same;even after the ion-exchange treatment. After the LiNO3

treatment (i.e. refer to the Li-Muscovite FTIR spec-trum), a new band appeared at 600 cm�1, which corres-ponded to a band of Liþ19. This indicated that theLiNO3 treatment had successfully replaced the inter-layer cation of the muscovite.19





Furthermore, after the CTAB ion exchange treat-ment, new bands at 2918 and 2850 cm�1 appeared inthe FTIR spectrum of CTA-muscovite. These twobands were attributed to the CH2 asymmetric (vasCH2) and symmetric (vs CH2) stretching vibrationmodes of the alkyl chain of CTAB. It was widelyaccepted that the frequency and width of the CH2

stretching vibration mode were sensitive to the gauche/trans conformer ratio of methylene chains.23,24 Theband at 1487 cm�1 was assigned to the asymmetric bend-ing mode of the head [(CH3)3Nþ-] methyl group.25,26

This indicated that the head groups [(CH3)3Nþ-] ofCTAþ were anchored to the surface of aluminosilicate

layers of the muscovite. Based on the FTIR spectrumportrayed in Figure 1, the implementation of the ion-exchange treatment in this study was successful, andtherefore, ready for further investigation.

XRD analysis. The identification between the incrementin d-spacing of the silicate layer in the muscovite due tothe ion-exchange treatment between Li-muscovite andCTAB was expected. This basal spacing increment wascalculated using the Bragg formula in equation (1).Therefore, the X-ray diffractogram of neat muscovite(M), organomuscovite (OM), and PP-layered silicatenanocomposites are shown in Figure 2. The calculated

Figure 1. FTIR transmission spectrum of muscovite particles before and after ion-exchange treatment.

Figure 2. XRD diffractograms of (a) neat muscovite (M), (b) O (OM), (c) PP/muscovite (PP/M), (d) PP/organomuscovite (PP/OM).

Omar et al. 5




basal spacing values were collected together and aresummarised in Table 2. From Figure 2(a), it can beseen that the (001) plane diffraction peak of neat mus-covite (M) appeared approximately 2�¼ 8.940, whichcorresponded to the interlayer distance of d¼ 0.99mm.On the other hand, the (001) plane diffraction peak oftreated muscovite (OM) appeared around 2�¼ 7.25,with the interlayer distance of d¼ 1.22, as shown inFigure 2(b). Cumulatively, a 23% increment in basalspacing of the muscovite was recorded after the ion-exchange treatment with both LiNO3 and CTAB,which confirmed the existence of intercalation betweenmuscovite clay layers by surfactant molecules.

When reinforced with the PP matrix, it was foundthat nanocomposites with organomuscovite (PP/OM)recorded lower angles and higher values of basal spa-cing (i.e. d¼ 1.93 nm) than that of nanocomposites withuntreated muscovite (PP/M) (i.e. d¼ 1.30). This indi-cated that a more effective intercation and formation ofintercalated structures had occurred. This phenomenonwas further discussed and understood by the morphol-ogy analysis using the TEM observation in Figure 3.Theoretically, the shift of the silicate reflection indi-cated the intercalation of PP chains into galleries ofmuscovite particles. In addition, it was also believed

that the usage of longer surfactant alkyl chain lengthwould further increase the basal spacing of muscoviteparticles.

Morphology analysis. TEM permitted the direct visualisa-tion of intercalation and exfoliation of the nanocompo-site structure in a much smaller region than what wasinterrogated by XRD. The TEM micrographs of bothuntreated and treated Muscovite nanocomposites areshown in Figure 3. From Figure 3(a) and (b), it wasobserved that treated muscovite nanocomposites(PP/OM) displayed a good dispersion and orientationof organolayers; whereas untreated muscovite nano-composites (PP/M) contained larger tactoids. Withthe combination of XRD results, it was assumed thatOM layers were homogenously dispersed within the PPmatrix and had formed an intercalated structure. Qiaoet al.27 speculated that the modification of clay withLiNO3 and CTAB had lowered the electrostatic inter-actions between the clay layers by enlarging their intra-gallery spacing, thus forming an intercalated structureand an efficient dispersion of clay particles.

Thermal analysis

DSC analysis. The melting and crystallisation behav-iour of neat PP and PP-layered silicate nanocompositesare illustrated in both Figures 4 and 5. From the DSCthermograms in Figures 4 and 5, it was pre-concludedthat the use of muscovite particles (i.e. M and OM)recorded lower Tm than that of neat PP. This wasattributed to the confinement of chains and segmentsin the presence of muscovite particles.12 Consequently,it prevented the segmental rearrangement during crys-tallisation and restricted the formation of perfect crys-tals in the polymer matrix.12 In comparison, PPreinforced with OM recorded a greater reduction interms of Tm than PP reinforced with neat Muscovite;for each of the particle content. Well-dispersed OM

Figure 3. TEM micrographs of (a) 5 wt% PP/M nanocomposite and (b) 5 wt% PP/OM nanocomposites. M: muscovite;

OM: organomuscovite; PP: polypropylene.

Table 2. The basal spacing of neat muscovite and

polypropylene/muscovite-layered silicate

composite.

Sample

D (001)

peak (nm)

M 0.99

OM 1.22

PP/M 1.30

PP/OM 1.93






within the PP matrix had a severely disturbed segmen-tal arrangement during crystallisation, thus it formedfewer perfect crystals. This was the main reason why PPreinforced with OM recorded lower Tm values, in com-parison to PP reinforced with neat muscovite, for eachof the particle content. For the crystallisation tempera-ture (Tc), the values were also increased with increasingparticle content. Similar observations were previouslyreported by Parija et al. 2004.28 They believed that themuscovite particles acted as an effective nucleatingagent, and therefore increased the Tc. For PP rein-forced with OM, the nucleation phenomenon wasmore efficient since the dispersion of OM (i.e. nucleat-ing agent) was greater than that of neat muscovite par-ticles. It was believed that the well-distributednucleating agent (i.e. OM) exhibited homogenous ther-mal resistivity, and as a result, recorded higher Tc

values. For the degree of crystallinity (Xc), it clearly

exhibited a decrease in Xc values when there wasan increase in muscovite particles. This reduction pat-tern was attributed to the restriction of segmentalrearrangement during crystallisation; preventing theformation of perfect crystals, and therefore decreasingthe degree of crystallinity (Xc) (i.e. at higher Muscovitecontents). All recorded results from Figures 4 and 5were collected together and are summarised intoTable 3.

TGA. TGA was employed to investigate the thermal sta-bility of all tested PP/M layered silicate nanocompo-sites with various filler contents. The TGA curves ofneat PP and PP/M layered silicate nanocompositesare illustrated in Figure 6. From Figure 6(a) and (b),it can be seen that the thermal decomposition of alltested specimens had occurred in one stage betweenthe temperature of 350�C and 500�C. Statistically, the

Figure 4. DSC thermograms of (a) PP/muscovite and (b) PP/organomuscovite nanocomposites with various filler contents (heating).

Figure 5. DSC thermograms of (a) PP/muscovite and (b) PP/organomuscovite nanocomposites with various filler contents (cooling).

Omar et al. 7




decomposition temperature of neat PP was 402�C at the10% weight loss level. However, it was observed thatthe introduction of muscovite particles (i.e. 1, 3, 5,10wt%) had significantly increased the decompositiontemperatures (i.e. 423, 442, 442, and 442�C) of testednanocomposite specimens. This increment was attribu-ted to the dispersion of clay particles within the PPmatrix, where individual layers of intercalated clayplatelets acted as an insulator and restricted the thermalmotion of the molecular chain, thus enhancing the ther-mal stability of the nanocomposites.29 Among the twoMuscovite particles, it was found that PP reinforcedwith OM recorded a higher thermal decompositiontemperature (i.e. both T10% and Tmax) than its

counterpart, for each of the particle content. This wasmainly due to a better dispersion of layered silicatewithin the PP matrix for OM, as previously provenfrom the TEM observation in Figure 3. All recordedresults from TGA curves in Figure 6 were collectedtogether and are summarised in Table 4.

Figure 7 illustrated the derivative of the thermogra-vimetric (DTG) curves for both PP/M and PP/OM.From the DTG curves, the maximum decompositiontemperature (Tmax) was obtained and the values aresummarised in Table 4. From Table 4, it can be seenthat the addition of muscovite particles had signifi-cantly increased the Tmax of tested specimens. Forexample, PP/ 1% M, PP/ 3% M, PP/ 5% M, and PP/

Figure 6. TGA curves for (a) PP/muscovite and (b) PP/organomuscovite nanocomposites with various filler contents.





10% M nanocomposites had recorded an increment of11, 18, 14, and 20�C, respectively. Moreover, PP/ 1%OM, PP/ 3% OM, PP/ 5% OM, and PP/ 10% OM hadrecorded an increment of 15, 24, 18, 21�C, respectively.In comparison, PP reinforced with OM had shown ahigher Tmax than that of PP reinforced with neat mus-covites, for each of the particle content. This was againdue to a better dispersion of OM within the PP matrix

(i.e. as compared to the neat muscovite), where it hadsignificantly increased the Tmax.

29

Effects of compatibilisers on PP/M-layered silicatecomposites

XRD. The efficiency of compatibilisers was justifiedusing the XRD analysis, which was illustrated inFigure 8. From Figure 8, it can be seen that all func-tionalised PP/OM nanocomposites exhibited a higherbasal spacing than that of non-functionalised PP/OMnanocomposites as the d001 peak shifted to lowerangles. This indicated that the polymer-layered silicatenanocomposites were successfully formed using PP as amatrix.28 Cumulatively, the nanocomposites incorpo-rated with POSS compatibilisers showed a basal spa-cing of 1.39 nm, which corresponded to 6% of theincrement. Moreover, nanocomposites incorporatedwith MAH compatibilisers recorded 40% of basal spa-cing increment (i.e. 1.93 nm). Similar observations werereported by Martınez-Gomez30 and Mishra.31 Theybelieved that PP and compatibiliser intercalationsbetween muscovite clay platelets were the main factorsthat contributed to this significant increment. All cumu-lative XRD results are grouped together into Table 5.

TEM. In addition to XRD characterisation, TEM ana-lysis was also carried out to further clarify the degree ofdispersion of OM particles within the PP matrix, withthe employment of compatibilisers. Figure 9(a–c) rep-resented a series of TEM micrographs of PP/OM nano-composites with different compatibilisers. The blackspots represented the intersection of the silicate layer,while the white background corresponded to the PPmatrix. From the TEM micrographs, it can be observedthat the OM layers were intercalated and dispersedwithin the monolayer in the PP/OM/POSS and PP/OM/MAH specimens. PP/OM/POSS and PP/OM/MAH nanocomposites displayed a similar morphologywith a higher degree of disordered structures and inter-calated layers than that of non-compatibilised PP/OMnanocomposites. The nanocomposites prepared usingthe MAH compatibilisers displayed a slightly higherintercalation that was more apparent, as compared tothe POSS compatibilisers. Visually, it can be observedthat the structure of PP/OM/MAH nanocompositeswas intercalated with a stack of disordered layers andsmaller aggregates than that of PP/OM/POSS. Thisobservation was in mutual agreement with earlierXRD analysis, where PP/OM/MAH recorded ahigher basal spacing than that of PP/OM/POSS.Apart from that, non-compatibilised nanocompositesportrayed large tactoids of clay, which indicated thatthe clay was not well dispersed.

Table 4. Thermogravimetric analysis values of neat PP and PP/

muscovite-layered silicate nanocomposite with various filler

contents.

Specimens T10% (�C) Tmax (�C) Char residue (%)

PP 402 465 0

PP/1% M 423 476 0.2

PP/1% OM 435 480 0.4

PP/3% M 442 483 2.5

PP/3% OM 454 489 2.6

PP/5% M 442 484 3.0

PP/5% OM 457 485 3.0

PP/10% M 442 485 4.5

PP/10% OM 449 486 4.6


Table 3. DSC results of neat PP and PP/muscovite-layered

silicate nanocomposite with various filler contents.

Specimen Tc (�C) Tm (�C) �Hm (J/g) Xc (%)

PP 109 168 88.0 42.1

PP/1% M 116 166 85.0 40.7

PP/1% OM 113 164 80.0 38.3

PP/3% M 113 165 79.5 38.0

PP/3% OM 114 164 77.1 36.8

PP/5% M 111 164 76.5 36.6

PP/5% OM 114 163 76.2 36.5

PP/10% M 115 164 73.0 34.9

PP/10% OM 114 163 72.0 34.4


Table 5. The basal spacing of muscovite and polypropylene/

muscovite-layered silicate composites.

Sample d (001) peak (nm)

OM 1.22

PP/OM 1.31

PP/OM/POSS 1.39

PP/OM/MAH 1.93

M: muscovite; MAH: maleic anhydride; OM: organomuscovite; PP: poly-

propylene; POSS: polyhedral oligomeric silsesquioxane.

Omar et al. 9




Thermal analysis

DSC. Figures 10 and 11 demonstrated both themelting and crystallisation behaviour of PP/OM nano-composites as a function of different compatibilisers.From Figure 10, it can be observed that the use ofcompatibilisers had significantly influenced the meltingbehaviour of the PP/OM composites. Generally, bothcompatibilised nanocomposites (i.e. PP/OM/MAH andPP/OM/POSS) had recorded a lower Tm compared toPP/OM. Statistically, PP/OM/POSS recorded a 2.4%reduction of Tm than that of non-compatibilised PP/OM. In addition, PP/OM compatibilised with MAHcompatibilisers had recorded a lower percentage ofTm decrement, which was 1.8%. This decrement pat-tern was due to a better interaction between OM

particles and the PP matrix (i.e. with the appearanceof the compatibilisers), thus affecting the matrix chainmobility.32 Figure 11 showed the crystallisation behav-iour of PP/OM composites with different compatibili-sers. Interestingly, it was found that the presence ofcompatibilisers had further enhanced the nucleatingefficiency of OM, where PP/OM compatibilised withMAH recorded the highest increment of Tc value.This again confirmed that the incorporation of bothOM and compatibilisers within the PP matrix hadplayed a crucial role that acted as a remarkable nucle-ating agent, and as a result, increased the Tc of PP/OMnanocomposites. Regarding the perspective on thedegree of crystallinity (Xc), it displayed a contrarytrend with the incorporation of compatibilisers. PP/OM/MAH and PP/OM/POSS nanocomposites

Figure 7. DTG curves for (a) PP/muscovite and (b) PP/organomuscovite nanocomposites with various filler contents.





recorded 1.3% and 0.5% reduction of Xc values, ascompared to non-compatibilised PP/OM nanocompo-sites, respectively. This was mainly due to the presenceof OM particles and the production of a strong PP/OMnetwork, especially with the addition of compatibilisers

that can significantly restrict the mobility of PP poly-mer chains. Those beneficial restrictions did not allowthe polymer chains to move easily to form a crystalphase, and therefore, it indirectly contributed to thedecrease of the degree of crystallinity (Xc). All recorded

Figure 9. TEM micrograph of PP nanocomposite containing 5 wt% organomuscovite incorporated with different compatibilisers: (a)

PP/OM nanocomposite; (b) PP/OM/POSS nanocomposites; (c) PP/OM/MAH nanocomposites. M: muscovite; MAH: maleic anhydride;

OM: organomuscovite; PP: polypropylene; POSS: polyhedral oligomeric silsesquioxane.

Figure 8. XRD diffraction pattern of (a) OM; (b) PP/OM; (c) PP/OM/POSS; (d) OM/PP/MAH. M: muscovite; MAH: maleic anhydride;

OM: organomuscovite; PP: polypropylene; POSS: polyhedral oligomeric silsesquioxane.

Omar et al. 11




DSC results were collected together and are sum-marised on Table 6.

TGA. The thermal decomposition behaviour of theneat PP and the PP/OM nanocomposites with differentcompatibilisers were illustrated in Figures 12 and 13.Generally, it can be observed that the thermal decom-position of all tested specimens occurred in one stagebetween the temperatures of 350 and 500�C. Apart

Figure 11. DSC thermograms of neat PP and PP/organomuscovite nanocomposites with different compatibilisers (cooling).

Figure 10. DSC thermograms of neat PP and PP/organomuscovite nanocomposites with different compatibilisers (heating).

Table 6. DSC results of PP and PP/organomuscovite-layered

silicate nanocomposite with different compatibilisers.

Specimens Tc (�C) Tm (�C) �Hm (J/g) Xc (%)

PP 109 168 88.0 42.1

OM/PP 112 166 79.0 37.8

OM/PP/POSS 113 162 78.0 37.3

OM/PP/MAH 115 163 76.2 36.5





from that, it was interesting to note that bothcompatibilised PP/OM nanocomposites recorded ahigher decomposition temperature than that of non-compatibilised PP/OM nanocomposites; as well as theneat PP. Cumulatively, the decomposition temperaturefor the neat PP and non-compatibilised PP/OM were inthe range of 350–400�C and 380–420�C, respectively.Both compatibilised PP/OM nanocomposites (i.e. PP/OM/POSS and PP/OM/MAH) recorded a higherdecomposition temperature, at around 420–430�C. At10% of weight loss, the decomposition temperaturesfor PP/OM/POSS and PP/OM/MAH nanocomposites

were 449�C and 457�C, which corresponded to 3–5% ofthe increment, in comparison to non-compatibilisedPP/OM nanocomposites, respectively. It was proventhat the incorporation of compatibilisers can improvethe decomposition temperature of the PP matrix, aswell as the thermal stability of the PP/OM nanocom-posites. This remarkable improvement was contributedby the homogenous dispersion of the OM layer silicate,which was provided by the compatibilisers within thePP matrix. Furthermore, the compatibilisers had alsoimproved the interaction between the PP matrix andthe OM polar silicate surface by reducing the repulsive

Figure 13. DTG curves for neat PP and PP/organomuscovite nanocomposites with different compatibiliser.

Figure 12. TGA curves for neat PP and PP/organomuscovite nanocomposites with different compatibiliser.

Omar et al. 13




force between the filler/matrix interface.33,34

This resulted in a better dispersion of OM particleswithin the PP matrix, as well as a stronger interfacialadhesion between the PP/OM interface.34

Figure 13 presented the DTG curves for the neat PPand PP/OM nanocomposites with different compatibi-lisers. From Figure 13, it can be seen that the additionof compatibilisers significantly increased the Tmax ofnon-compatibilised PP/OM; which corresponded to0.8% and 1.0% of the increment for PP/OM/POSSand PP/OM/MAH, respectively. The increment wasinitially attributed to the restriction of thermalmotion due to the homogenous and well-dispersion ofOM particles within the PP matrix (i.e. with the add-ition of both compatibilisers). All recorded results fromTGA analysis were grouped together and are sum-marised in Table 7.

Conclusion

The effects of organic modifications and compatibiliserson the thermal properties of PP/M layered silicate com-posites were successfully carried out using both theDSC and TGA, respectively. From the results, the fol-lowing conclusions can be drawn:

. The organic modification of muscovite clay particleswas carried out using the two-step ion exchangetreatment. The effectiveness of the ion-exchangetreatment was successfully proven with the use ofFTIR, XRD, and TEM.

. DSC results of tested PP/OM nanocomposites dis-played a prominent improvement in terms of Tc, incomparison to untreated PP/M composites.However, for the Tm and the degree of crystallinity(Xc), it exhibited a contrary trend, whereby PP/OMrecorded a greater reduction compared to theuntreated PP/M specimens. For the TGA analysis,it was found that PP reinforced with OM recorded amore prominent thermal stability increment (T10%

and Tmax) than its counterpart (i.e. PP reinforcedwith neat muscovite).

. The efficiency of both compatibilisers (MAH andPOSS) was successfully determined using the XRDand TEM. Interestingly, it was found that the use ofcompatibilisers had significantly increased the com-patibility between the PP matrix and the muscoviteparticles. For DSC results, it was found that the Tm

and the degree of crystallinity (Xc) of compatibilisedPP/OM nanocomposites were slightly lower thanthat of non-compatibilised PP/OM nanocomposites.Contrarily, the Tc displayed an opposite trend,whereby compatibilised PP/OM nanocompositesrecorded higher Tc values as compared to non-compatibilised PP/OM nanocomposites.

. For the TGA analysis, it was interesting to note thatboth compatibilised PP/OM nanocomposites rec-orded higher decomposition temperatures (i.e. Tmax

and T10%) than that of non-compatibilised PP/OMnanocomposites; as well as the neat PP. Overall, PP/OM/MAH exhibited greater thermal performancethan that of PP/OM/POSS, due to higher polarity.

Funding

The authors would like to acknowledge the UniversitiMalaysia Perlis (Grant no: 9017-00014 & 9003-00390) andCluster of Polymer Composites (CPCs), Universiti SainsMalaysia (USM) (Grant no: 1001/PBAHAN/8043057 &

811070) for sponsoring and bestowing financial assistanceduring this research work.

Conflict of interest

None declared.

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