Gas permeation and mechanical properties of polypropylene nanocomposites with thermally-stable...

EUROPEAN

European Polymer Journal 43 (2007) 3727–3736

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Macromolecular Nanotechnology

Gas permeation and mechanical properties ofpolypropylene nanocomposites with

thermally-stable imidazolium modified clay

V. Mittal *

Department of Chemistry and Applied Biosciences, Institute of Chemical and Bioengineering, ETH Zurich, 8093 Zurich, Switzerland

Received 17 March 2007; received in revised form 28 May 2007; accepted 10 June 2007Available online 24 June 2007

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Abstract

Dialkyl imidazolium salt with better thermal stability than the commonly used dimethyldioctadecyl ammonium salt wassynthesized and ion exchanged on the montmorillonite surface. Polypropylene nanocomposites with different volume frac-tions of the obtained organo-montmorillonite (OMMT) were prepared and the effect of the modified clay on the gas barrierand mechanical properties was studied. Wide angle X-ray diffraction (WAXRD) and transmission electron microscopy(TEM) were used to investigate the microstructure obtained. Thermal behavior of the composites analyzed by thermo-gravimetric analysis was observed to enhance significantly with the filler volume fraction. The gas permeation throughthe nanocomposite films markedly decreased with augmenting the filler volume fraction. The decrease in the gas perme-ation was even more significant than through the composites with ammonium treated montmorillonite. Better thermalbehavior of the organic modification owing to the delayed onset of degradation hindered the interface degradation alongwith detrimental side reactions with polymer itself. Transmission electron microscopic studies indicated the presence ofmixed morphology i.e., single layers and the tactoids of varying thicknesses in the composites. The crystallization behaviorof polypropylene remained unaffected with OMMT addition. A linear increase in the tensile modulus was observed withfiller volume fraction owing to partial exfoliation of the clay.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Polypropylene; Layered silicates; Thermal stability; Gas permeation

1. Introduction

A relatively new approach to address the ever-increasing requirement of high-end polymeric prod-ucts is represented by organically modified plate likelayered silicates which can be dispersed into nano-

0014-3057/$ - see front matter � 2007 Elsevier Ltd. All rights reserved

doi:10.1016/j.eurpolymj.2007.06.015

* Tel.: +41 44 632 5688; fax: +41 44 632 1082.E-mail address: [email protected]

meter thin platelets in the polymer matrix [1–3].Such polymer nanocomposites have attractedtremendous attention owing to the substantial prop-erty improvement at minimal filler volume fractions.The properties of the composites are synergisticallyderived from the organic and inorganic componentsof the composites, therefore, a uniform dispersionand distribution of the filler is of utmost impor-tance. Aluminosilicates, especially montmorillonite,has received significant attention as the inorganic

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3728 V. Mittal / European Polymer Journal 43 (2007) 3727–3736

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filler to prepare such composites owing to their easyswelling in water leading to 1 nm thick platelets[4,5]. The exchange of alkali metal cations presenton the surface of the platelets with long chain alkylammonium cations is thus achieved to reduce thesurface energy and to make the treated montmoril-lonite more compatible in the polymer matrix lead-ing to its nanoscale dispersion [6,7]. The variousproperties which could be enhanced significantlyinclude the mechanical and barrier performance,heat deflection temperature, ablation and flamma-bility resistance etc [8–12].

Melt intercalation has been by far the most con-venient way to synthesize polymer nanocomposites.However, significantly higher temperatures arerequired for melt intercalation process and the sub-sequent processing protocols. The organic ammo-nium modifications have been observed to haveonset of degradation as low as 180 �C by TGA stud-ies [13]. The thermal degradation was observed tofollow the Hoffmann degradation path involvingthe early breakage of weaker C–N bond in theammonium modification. Rupture of even a smallnumber of such bonds can significantly change thethermodynamics of the system and interfacial inter-actions because of the changes in the structure ofthe surfactant and production of low molecularweight species consequently affecting the physicaland mechanical properties like viscosity, molecularweight, glass transition temperature and flammabil-ity etc. as reported in the literature [14–18].

As polypropylene is a material of choice for anumber of applications, inorganic fillers are com-monly compounded with polypropylene to furtherenhance its properties [19–21]. However, the highmelt temperatures required along with the highmechanical shear used in the melt intercalationand processing operations poses a concern for thethermal stability of the ammonium modification.Temperatures more than 200 �C are generallyemployed for such processes which are almost equalor higher than the initial onset of degradation of thealkyl ammonium groups. Furthermore, the funda-mental theoretical investigations on the thermody-namics and kinetics of polymer melt intercalationand hybrid formation have assumed the perfect sta-bility of the organic modification even at highertemperatures [22,23]. Therefore, no theoretical stud-ies are also available which systematically quantifythe thermal stability of OMMT at high melt interca-lation temperatures and its possible effect on thephysical and mechanical performance of nanocom-

posites. However, many approaches have been triedto circumvent this limitation of thermal stability ofthe ammonium cations attached to the clay surface[24–27], out of which the use of other thermally sta-ble cations such as phosphonium, pyridinium andimidazolium can offer a good alternative to theammonium salts [28,29]. Intensive thermal studiescarried out especially on imidazolium salts haveproved their much better thermal response as com-pared to the alkyl ammonium cations both in thepresence and absence of oxygen as degrading atmo-sphere [28]. Although the thermal stability of suchsalts was observed to be inversely proportional tothe length of the alkyl chain attached to the imi-dazolium group, but still the thermal stability valuesfor the long alkyl chains required for better interca-lation and hence exfoliation of the clay plateletswere high enough to sustain the higher compound-ing temperatures [30,31]. Increasing the substitutionof the imidazolium ions also was reported toincrease the thermal stability due to the removalof the ring hydrogens. However, only a few studiesusing these imidazolium salts for the synthesis ofpolymer nanocomposites with mostly polystyreneas matrix have been reported [32,33]. He et al.reported the synthesis of polypropylene nanocom-posites with monoalkylimidazolium montmorillon-ite using in situ polymerization approach [34].

The goal of the present investigation was to studythe effect of thermally stable imidazolium treatedmontmorillonite on the gas permeation andmechanical behavior of polypropylene nanocom-posites. A dialkyl chain imidazolium salt was syn-thesized and the composites containing differentvolume fractions of the OMMT were prepared bymelt intercalation. Correlations of the final compos-ite properties were established with the inorganic fil-ler volume fraction.

2. Experimental

2.1. Materials

Sodium montmorillonite with a trade name ofCloisite Na was purchased from Southern ClayInc. (Gonzales, Texas, USA). The cation exchangecapacity (CEC) of the sodium montmorillonite wasdetermined to be 880 leq g�1 by exchanging itssodium ions with Cu(trien)2+ [35]. 1-Decyl-2-methy-limidazole and octadecylbromide were purchasedfrom Aldrich (Buchs, Switzerland). Polypropyleneused was the homopolymer grade H733-07 from

V. Mittal / European Polymer Journal 43 (2007) 3727–3736 3729

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Dow (Dow Plastics, Horgen, Switzerland). It has amelt flow index of 7.5 g/10 min (230 �C at 2.16 kgload) and a density of 0.9 g cm�3.

2.2. Synthesis of Imidazolium salt and filler surface

modification

To synthesize the required imidazolium salt(1-decyl-2-methyl-3-octadecylimidazolium bromide),2.22 mL of 1-decyl-2-methylimidazole and 3.15 g(1.05 molar excess) of octadecylbromide wereplaced in 50 mL round bottom flask along with5 mL of ethyl acetate as reported earlier [32]. Thecontents were stirred at room temperature for 2 hunder nitrogen. The reaction mixture was thenplaced in an oil bath heated at 55 �C and the reac-tion was allowed to continue overnight. The reac-tion mixture was cooled to obtain a whiteprecipitate of imidazolium salt which was filteredand washed extensively with ethyl acetate. The saltwas then dried at room temperature under reducedpressure. The synthesis of the salt was confirmed byproton NMR spectrum collected on a Bruker 300spectrometer with observing frequencies of300 MHz. Fig. 1 shows the structure of the imidazo-lium salt synthesized. For the exchange of the imi-dazolium cation on the clay surface, it was alwaysensured that the OMMT is free from any excessmodifier molecules after the exchange, which if pres-ent can adversely affect the properties of compositesowing to their low temperature stability [13,36,37].The exchange of the imidazolium salt on the mont-morillonite surface was achieved by following theprocess reported earlier [38]. Both degree ofexchange and the purity of the modified clay weremonitored by Hi-Res TGA. If the TGA thermo-gram indicated the presence of unattached modifiermolecules, the washing step was repeated until thepurity of the product was satisfactory. Finally themodified clay was suspended in 400 mL of dioxane,sonicated and freeze dried. The freeze dried clay wassieved (60 lm mesh) to obtain the fine OMMTpowder.

N

N

Br+

Fig. 1. Structure of the 1-decyl-2-methyl-3-octadecylimidazoliumbromide.

2.3. Composite preparation and characterization

The required amounts of OMMT and polymerwere calculated on the basis of desired inorganicvolume fraction as reported earlier [39]. Twin bladecompounder (Plasticorder W 50 EH, Brabender,Duisburg, Germany) was used to prepare polypro-pylene-OMMT composites and a compoundingtemperature of 180 �C was used. All the detailsregarding the composite preparation and the pro-cessing parameters can be found elsewhere [38].

Thin films (�100 lm) of the composites as well aspure polypropylene were pressed by compressionmolding [38]. OX-TRAN 2/20 ML (Mocon, Minne-apolis, MN) modules at 23 �C and 0% RH wereused to measure oxygen permeation through thesefilms. The gas permeation rates were normalizedwith respect to thickness of the films. An averageof four measurements for each sample has beenreported. Density and thickness of the films werecalculated as reported earlier [39].

Dumbbell shaped samples (type 5B) werestamped out of the compression molded nanocom-posite and pure polymer plates using a stampingpress (H. W. Wallace, Croydon, Surrey, England)with a die conforming to the standard ISO 527-2.Tensile tests on the stamped samples were carriedout (ISO 527-1) at room temperature using a ZwickZ020 universal testing machine with testXpert 9.01software (Zwick, Ulm, Germany) coupled with aVideo-Extensometer V4.19.02 (Messphysik, Fur-stenfeld, Austria) for accurate measurement. Thedrawing speed of 0.1 mm min�1 was used for themeasurement of elastic modulus and modulus wasdetermined in the range of 0.05–0.25% strain. Aspeed of 6 mm min�1 was used for the measurementof other tensile properties and an average of fivemeasurements was taken.

DSC 7 (Perkin Elmer, Norwark, CT) was used tocarry out differential scanning calorimetric measure-ments at a heating rate of 10 �C min�1, adopting theprocedure for measuring heat capacities [40]. A tem-perature range (100–180 �C) was used to calculatethe melting enthalpies of the composite samplesfollowed by necessary corrections to represent thethermal behavior of pure polypropylene in thecomposites.

High-resolution (Hi-Res) thermogravimetricanalysis (TGA) of OMMT and composites, inwhich the heating rate is coupled to mass loss, thatis, the sample temperature is not raised until themass loss at a particular temperature is completed,

a

b

Fig. 2. (a) TGA thermograms depicting the thermal degradationbehavior and (b) dynamic thermogravimetric analysis; I: imi-dazolium treated montmorillonite and II: ammonium modifiedmontmorillonite.


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was performed on a Q500 thermogravimetric ana-lyzer (TA Instruments, New Castle, DE). All mea-surements were carried out in air in thetemperature range 50–900 �C at a heating rate of20 �C min�1. Both degree of exchange and the pur-ity of the OMMT were monitored by Hi-Res TGA.Dynamic TGA measurement of OMMT was carriedout by first conditioning the sample to 140 �C fol-lowed by heating to 180 �C and keeping the sampleat this temperature for 90 min. TGA of the compos-ites was also performed at the same specifications inorder to study the thermal behavior of the compos-ites as a function of the increasing amount ofOMMT. X-ray diffraction patterns were collectedon a wide angle Scintag XDS 2000 diffractometer(Scintag Inc., Cupertino, CA) using Cu Ka radia-tion (k = 0.15406 nm) in reflection mode andbright-field TEM (Zeiss EM 912 Omega) was usedto study the microstructure of the nanocompositesas reported earlier [39]. The SEM investigationwas carried out on Hitachi S-900 field emissionscanning electron microscope at an acceleratingvoltage of 15 kV.

3. Results and discussion

Dialkyl imidazolium treated montmorillonitewas employed in the present study to investigatethe effect of more thermally stable organic modifica-tion on the properties of OMMT-polypropylenenanocomposites. The compounding temperatureswere kept low in order not to degrade the modifica-tion and the polymer, but high enough to ensurehomogenous mixing. The long octadecyl chains, incommonly used dimethyldioctadecyl ammoniumsalts, help in significantly separating the clay plate-lets thus reducing the attractive forces among themand their similar chemical nature as polypropyleneis also expected to help in better intermixing at theinterface. The imidazolium salt synthesized in thepresent study also contained two long alkyl chainsthus expecting to achieve similar compatibilizingforces. Alkyl chains of two different lengths wereused in order to avoid its possible phase separationat the interface with the polymer which sometimeshave been reported for dimethyldioctadecyl ammo-nium modification owing to its more ordered struc-ture [41].

A comparison of the thermal behavior of themontmorillonite treated with dimethyldioctadecylammonium [38] and 1-decyl-2-methyl-3-octadecylimidazolium salts have been shown in Fig. 2. As is

clearly visible in Fig. 2a, the degradation of ammo-nium treatment starts earlier than the imidazoliumcounterpart by around 50 �C. A sharp degradationwas also observed in the ammonium treatment atthis point, which indicates the presence of someunattached modifier molecules which are difficultto wash especially in dimethyldioctadecyl ammo-nium treated clay. This early degradation can seri-ously marginalize the improvement in the compositeproperties [13,36,37]. Apart from the delayed onsetof degradation in the imidazolium treated clay, thepeak degradation temperature also is higher byalmost 35 �C as compared to the ammonium treatedclay. A dynamic thermal analysis of the two mont-morillonites in air was also carried out and theresults are shown in Fig. 2b. The OMMT pow-ders were stabilized at 140 �C and then heatedto 180 �C and kept at 180 �C under isothermal

Fig. 3. Relative oxygen permeability of imidazolium OMMT-PPcomposites as a function of filler volume fraction compared with2C18 ammonium OMMT-PP composites [39]; d: 2C18 ammo-nium OMMT composites and m: imidazolium OMMT compos-ites. The dotted lines only serve as guides.


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conditions for 90 min. As is clearly visible, theammonium treated clay shows faster thermal degra-dation than the imidazolium counterpart. Althoughthe difference in the amount of degradation of thetwo organo-montmorillonites is not significant,but owing to the fact that the main mechanism ofthe degradation being the breakage of the weakerC–N bond, therefore, breaking of even small num-ber of such bonds can seriously change the structureof the surfactant and its interaction with the poly-mer. Thus, the thermal analysis clearly confirmedthe better thermal behavior of the imidazolium trea-ted montmorillonite.

Table 1 shows the oxygen permeation coefficientsof the composites containing 0–4 vol% of theOMMT. The oxygen permeation through the com-posite films was observed to decrease substantiallyas the filler volume fraction was increased. The oxy-gen permeation decreased from 89 cm3 lm/m2 day mmHg for the pure polypropylene films to48 cm3 lm/m2 day mmHg for the composite filmscontaining 4 vol% filler fraction, thus, bringingabout a decrease of almost 50% in the oxygenpermeation. For comparison, relative oxygen per-meation values of the composites containing imi-dazolium treated clay have been plotted alongwith the composites containing dimethyldioctadecylammonium modified clay with same volume frac-tions in Fig. 3. Dimethyldioctadecyl ammoniumtreated clay was observed to have the best perme-ation resistance among a number of other OMMTwith different ammonium salts as reported earlier[38]. A decrease of around 35% in oxygen perme-ation was observed for this clay at 4% filler volumefraction. A much better oxygen permeation behav-ior for the imidazolium treated montmorillonitecan definitely be attributed to its better thermal sta-bility. As the permeation properties are sensitive tothe interface and interfacial interactions, therefore,the improved permeation behavior can be more

Table 1d-Spacing and oxygen permeability of OMMT-polypropylenenanocomposites as a function of filler volume fraction

Filler volumefraction

d-Spacing Permeability coefficienta

cm3 lm/m2 day mmHg

0.00 – 890.01 2.52 710.02 2.28 610.03 2.29 530.04 2.24 48

a In the range of 5% error.

safely attributed to the interplay of the thermal sta-bility and resulting interfacial interactions [12,38]. Apermeation value of 80 cm3 lm/m2 day mmHgthrough the composite film containing 3 vol% ofuntreated montmorillonite also confirms the impor-tance of interfacial compatibility between the phasesof composite. An important thing to note is the rel-ative lower d-spacing of the clay as compared to the2C18 ammonium treated clay (2.24 as compared to2.51). The composites with imidazolium modifiedmontmorillonite therefore also have lower basalplane spacing as compared to the 2C18 counterparts(Table 1). As has been observed earlier that thehigher d-spacing may lead to enhanced exfoliationof the clay platelets owing to the decrease of attrac-tive forces among them, therefore, it may be possi-ble that the imidazolium composites may havesuffered in this regard. Therefore, the improved per-meation resistance is a combined result of betterthermal stability and possible lower extent of exfoli-ation owing to lower basal plane spacing and henceless stronger interfacial interactions. Apart fromthat, a decrease in the basal plane spacing was alsoobserved with respect to the increase in filler volumefraction as detailed in Fig. 4 as well as Table 1, but itwas too insignificant and it can be safely said thatno or minimum intercalation in the compositescould be observed. However, owing to better inter-facial interactions and the effect of shearing forces,a decrease in the tactiod thickness and henceincrease in the aspect ratio can still be expected.

Fig. 4. X-ray diffractograms of the pure OMMT and the OMMTin the polypropylene composites; I: pure OMMT, II: 1 vol%OMMT-polypropylene composite, III: 2 vol% OMMT-polypro-pylene composite, IV: 3 vol% OMMT-polypropylene compositeand V: 4 vol% OMMT-polypropylene composite.

Fig. 5. (a)–(c) TEM micrographs of 3 vol% OMMT-polypropylenealuminosilicate platelets, (d) SEM micrograph of the same composites.


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As the X-ray peaks have a qualitative nature owingto the sensitivity of the peak intensity and peakwidth to the platelet orientation, processing condi-tions and the sample preparation, estimation ofextent of intercalation and exfoliation as well asthe tactoid thickness is therefore very difficult. Sub-sequently, to supplement the X-ray diffractionresults, microscopic analysis of the composites wascarried out with transmission electron microscopy.Fig. 5a–c shows the TEM micrographs of compositecontaining 3 vol% of the filler fraction. A mixedmorphology with single clay layers and tactoids ofdifferent thickness were observed. The clay layerswere also observed to be bent, folded and com-pletely misaligned. Similar morphology was alsoobserved in the polypropylene composites withdimethyldioctadecyl ammonium treated montmoril-lonite [38]. The micrographs were also taken at

nanocomposites. The dark lines represent the cross-section of

Table 2Tensile properties of OMMT-polypropylene nanocomposites as afunction of filler volume fraction

Fillervolumefraction

Tensilemodulusa

(MPa)

Yieldstressb

(MPa)

Yielsdstrainc

(%)

Stress atbreakd

(MPa)

0.00 1510 36 9.4 290.01 1668 31 8.5 230.02 1810 28 6.0 220.03 1918 26 4.4 230.04 2007 24 3.9 21

a Relative probable error 5%.b Relative probable error 2%.c Relative probable error 5%.d Relative probable error 15%.

a

b

Fig. 6. Relative (a) tensile modulus and (b) yield stress ofpolypropylene nanocomposites plotted as a function of inorganicvolume fraction of filler. The dotted lines serve as guides.


GY

different magnifications and no global orientation ofthe platelets could be observed at any magnification.However, one important thing to notice here is thata partial exfoliation could still be achieved evenwithout the use of a compatibilizer, generally usedin synthesizing polypropylene nanocomposites.Fig. 5d also shows the SEM micrograph of the3 vol% OMMT-polypropylene composites. The thinplaty tactoids could be observed which though aremisoriented, but are also uniformly distributed.Thus, it confirms that the morphology suggestedby the TEM microgaphs was present throughoutthe microstructure of the composites.

Tensile properties of the OMMT – polypropylenenanocomposites containing different filler volumefractions have been detailed in Table 2. As can be

b

a

Fig. 7. Relative (a) stress at break and (b) yield strain ofpolypropylene nanocomposites plotted as a function of fillervolume fraction. The dotted lines serve as guides.

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seen, the modulus increased monotonously withaugmenting the filler volume fraction (Fig. 6a) indi-cating enhanced stress transfer to the inorganicplatelets. An increase of nearly 35% was observedfor the composite containing 4 vol% of the OMMT.Relative yield strength (Fig. 6b) of the compositeswas observed to decay with filler volume fractionindicating lack of attractive interactions at the inter-face and the presence of tactoids. The other tensileproperties of the composites have been plotted withrespect to the filler volume fraction in Fig. 7. Thestress at break also had slight decrease with the fillervolume fraction possibly due to the strain hardeningof the confined polymer and also the presence oftactoids (Fig. 7a). Decrease of yield strain(Fig. 7b) was observed owing to similar stain hard-ening because of the confined polymer chains indi-cating the brittleness with increase in filler volumefraction. Relative oxygen permeation has also beenplotted against the relative tensile modulus in Fig. 8.A good correlation was observed to exist betweenthe two, indicating the synergistic improvement ofthe properties. It also indicates that if the interfaceof the clay surface modification is optimallydesigned, better composite properties and micro-structure can still be achieved even without the useof external compatibilizers.

It has been earlier reported that incorporation ofOMMT did not have any significant effect on thecrystallization of polypropylene under the process-ing conditions used [39]. Calorimetric behavior ofthe imidazolium composites was also carried out

Fig. 8. Decrease of relative oxygen permeability of the polypro-pylene nanocomposites plotted against the increase in relativetensile modulus as a function of filler volume fraction.

in order to verify this observation. The calorimetricresults of the composites with respect to filler vol-ume fraction have been shown in Table 3. Therewere no observable variations in the onset and peakmelting and crystallization temperatures. Also thedegree of crystallinity calculated from the enthalpyof melting of polymer in the composite comparedwith the enthalpy of purely crystalline polymerwas found be fairly constant. This leads to the con-clusion that the improvements in the permeationand mechanical properties of the composites couldbe solely attributed to the exfoliation and interfacialinteractions.

The thermal behavior of the composites carryingdifferent filler fractions was analyzed by thermo-gravimetric analysis in order to study the effect ofOMMT on the stability of the composites. Fig. 9ashows the TGA thermograms of pure polypropyl-ene and the OMMT – polypropylene nanocompos-ites. Addition of only 1 vol% of the inorganic fillerfraction improved the thermal behavior tremen-dously. Further additions also significantlyimproved the thermal degradation behavior of thecomposites with the composite containing 4 vol%of the filler showing the best thermal response.Thus, the thermal behavior of the OMMT – poly-propylene composites was tremendously improvedon the incorporation of the clay. The synergismbetween polypropylene and the OMMT is clearlyevident from Fig. 9b, where the TGA thermogramsof the imidazolium clay, pure polypropylene and thecomposite containing 4 vol% of the filler have beencompared. The thermal response of the compositewas better than any of its constituents indicatingthe better dispersion of the clay.

Table 3Calorimetric behavior of OMMT-polypropylene nanocompositesas a function of filler volume fraction

Fillervolumefraction

Tm, onseta

(�C)Tm

b

(�C)DHm

(J/g)Crystallinityc

Xc

Tc,onsetd

(�C)Tc

e

(�C)

0.00 152 162 96 0.58 114 1120.01 152 163 94 0.57 113 1100.02 155 166 96 0.58 114 1110.03 152 163 97 0.59 115 1110.04 153 161 95 0.57 115 111

a Onset melting temperature.b Melt peak temperature.c Degree of crystallinity calculated using DH of 100% crystal-

line PP = 165 J/g [42,43].d Onset crystallization temperature.e Peak crystallization temperature.

a

b

Fig. 9. (a) TGA thermograms of pure polypropylene andpolypropylene nanocomposites containing different filler inor-ganic volume fraction and (b) comparison of the TGA thermo-grams of pure PP (II), imidazolium clay (I) and thenanocomposite containing 4 vol% of OMMT (III).


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4. Conclusions

In this work, polypropylene nanocompositeswith imidazolium treated montmorillonite were pre-pared and subsequently characterized for their per-meation and mechanical properties. The isothermaland non-isothermal thermogravimetric analysisconfirmed the delayed onset and peak degradationfor the imidazolium surface modification ionicallybound to the montmorillonite surface as comparedto the dimethyldioctadecylammonium counterpartcommonly used. The oxygen permeation propertiesimproved markedly with a decrease of nearly 50% at4 vol% of filler fraction significantly better than2C18 ammonium composites. A mixed morphologyconsisting of clay tactoids of varying thicknessesrepresented the microstructure of the composites.A partial delamination could still be achieved even

without the use of commonly added compatibilizers.Tensile modulus linearly increased with filler vol-ume fraction. Decreases in other tensile propertiesindicated lack of stronger interfacial interactions,strain amplification and the presence of tactoids.The improvement in the overall composite proper-ties seem to be a combined result of better thermalstability and interfacial interactions. The additionof OMMT did not affect the crystallization behaviorof polypropylene. The thermal stability of the com-posites was observed to improve significantly withincreasing filler volume fraction.

Acknowledgements

Financial support from the Swiss National Sci-ence Foundation (SNF) and TOP NANO 21 ofthe ETH board is greatly acknowledged. The authoris grateful to Prof. M. Morbidelli and Prof. U. W.Suter for allowing the use of their research facilitiesat ETH Zurich. Expert help of Dr. N. Matsko forTEM investigations is highly appreciated.

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