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Thermochimica Acta 569 (2013) 127– 133

Contents lists available at ScienceDirect

Thermochimica Acta

jo ur nal ho me page: www.elsev ier .com/ locate / tca

hermal stability and kinetic study of poly(ethyl methacrylate-o-acrylonitrile) nanocomposites prepared by in situ polymerizationn presence of an Algerian bentonite

ouad Kadia, Saïd Djadouna,∗, Nicolas Sbirrazzuolib

Université des Sciences et de la Technologie Houari Boumediene, Laboratoire des Matériaux Polymères, Faculté de Chimie, BP 32 El-Alia, Bab-Ezzouar,6111 Algiers, AlgeriaUniv. Nice Sophia Antipolis, CNRS, LPMC, UMR 7336, 06100 Nice, France

r t i c l e i n f o

rticle history:eceived 20 April 2013eceived in revised form 19 June 2013ccepted 22 June 2013vailable online 1 July 2013

eywords:

a b s t r a c t

Poly(ethyl methacrylate-co-acrylonitrile) (PEAMN20) nanocomposites were successfully prepared viafree radical in situ polymerization using a bentonite from Algeria modified by Hexadecyltrimethy-lammonium chloride (HDTMA). X-ray diffraction (XRD) and transmission electronic microscopy (TEM)investigations revealed that depending on the OMMT loading, intercalated or partially exfoliatednanocomposites were obtained. These nanocomposites exhibited an overall improved thermal stabil-ity and an increase in their glass transition temperature compared to the pure copolymer (PEMAN20) asevidenced by Thermogravimetric (TGA) and Differential Scanning Calorimetry (DSC) analysis.

aghnia bentoniteoly(ethyl methacrylate-co-acrylonitrile)ntercalated-partially exfoliatedanocompositehermal propertiesinetic of thermal decomposition

Apparent activation energies (E�) of thermal decomposition of the elaborated nanocomposites, deter-mined by Tang method, were higher than those of the virgin copolymer.

The significant increase of (E�), observed with PEMAN20/OMMT (1 wt%) confirmed the change in thedegradation mechanism with OMMT loading and the ratio of intercalated/exfoliated structures.

© 2013 Elsevier B.V. All rights reserved.

ransmission electron microscopy.

. Introduction

Interest in polymer/clay nanocomposites has increased signif-cantly recently and many papers are reported in the literature1–4]. Because of their natural abundance, low cost, high cationxchange capacity (CEC), swelling behavior, adsorption propertiesnd large surface area [5–7], smectite clays such as montmorillonitehich is the major constituent of bentonite, are widely used inany industrial applications. In general, intercalated or exfoliated

olymer-clay nanocomposites with significantly improved prop-rties and extended utilities compared to conventional ones cane prepared, mainly by in situ polymerization, solution intercala-ion and melt mixing [8–10]. The recommended way to achievehe best dispersion of clay particles in a polymer matrix is to usen situ polymerization or intercalation from solution methods, sincet is relatively easy to achieve exfoliation of the aggregates of lay-red silicates as compared to melt blending. The drawbacks of this

ethod is that it is not often used for most industrial polymers and

t is rather environmentally unfriendly [11,12].

∗ Corresponding author. Tel.: +213 771 755 043; fax: +213 21 247 311.E-mail addresses: matpolylab@yahoo.fr, djadoun.said@gmail.com (S. Djadoun).

040-6031/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.tca.2013.06.028

Moreover, the properties of nanocomposites prepared by thesemethods are somewhat superior to those prepared by melt inter-calation. It is however important to note that these methods arelimited due to requirements of compatibility between the claysurface and the polymer matrix, in order to produce exfoliatednanocomposites.

Several studies dealt with clay nanocomposites based oncopolymers of acrylonitrile and other monomers such styrene,methyl methacrylate or butadiene but to our knowledge, thepreparation of nanocomposites based on poly(ethyl methacrylate-co-acrylonitrile) in presence of an Algerian organically modifiedbentonite (OMMT), has not been reported.

In the present contribution, we will report on an attemptto prepare for the first time, nanocomposites based on ethylmethacrylate and acrylonitrile via in situ free radical polymeriza-tion in tetrahydrofuran (THF), using azobis-isobutyronitrile (AIBN)as initiator in presence of different loadings of a bentonite fromAlgeria, purified and organically modified by hexadecyltrimethy-lammonium chloride (HDTMA).

The structure, morphology and thermal stability of the elabo-

rated materials will be investigated in a second step by FourierTransform Infrared (FTIR) spectroscopy, X-ray diffraction (XRD),Transmission electronic microscopy (TEM), Differential ScanningCalorimetry (DSC) and Thermogravimetry (TGA).

1 ica Acta 569 (2013) 127– 133

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In the last part of this study, apparent activation energies (E�) ofegradation of pure PEMAN20 and its nanocomposites with differ-nt amounts of OMMT will be estimated using Tang et al. method13].

. Experimental

.1. Materials

Bentonite from Maghnia (Algeria) was kindly supplied by Ben-onite Company of Algeria and analyzed by the central laboratoryf the ENOF. This clay containing SiO2 (55–65%), Al2O3 (12–18%),e2O3 (1–3%), Na2O (1–3%), CaO (1–5%), K2O (0.76–1.75%) and MgO2–3%), was purified [14] and organically modified by a cationxchange method between ions of Hexadecyltrimethylammoniumhloride (HDTMA) and sodium cations of pure Maghnia bentoniteMMT) as previously described [15,16].

.2. Synthesis

A Copolymer of ethyl methacrylate with acrylonitrile was syn-hesized by free radical polymerization in THF at 60 ◦C, using annitial mixture containing 35 mol% of acrylonitrile and 0.1% by

eight of azobis-isobutyronitrile (AIBN) as initiator. The copoly-er, obtained by precipitation in a large excess of methanol, was

urified by repeated dissolution/precipitation, dried in a vacuumven at 60 ◦C for several days and characterized by FTIR, Protonuclear Magnetic Resonance spectroscopy (1H NMR), DSC and TGAnalysis.

Appropriate amount of OMMT (1, 2 and 4% by weight) was dis-ersed in THF under magnetic stirring for 24 h at room temperature.imilar amounts of ethyl methacrylate and acrylonitrile used torepare the virgin copolymer were added to the clay suspensionnd stirred vigorously for another 24 h. 0.1% by weight of AIBNas then added to the solution that was maintained at 60 ◦C for

h under nitrogen atmosphere. The synthesized materials werebtained by precipitation from an excess of methanol, purified andried under the same conditions than the virgin copolymer [17].

.3. Characterizations

FTIR spectra of thin films of PEMAN20 and PEMAN20/OMMTybrids, cast from THF solutions onto potassium bromide (KBr)isks, were recorded at room temperature on a Perkin-Elmer spec-rometer One. 60 scans were signal averaged at a spectral resolutionf 2 cm−1.

The content of acrylonitrile in PEMAN determined using 1H-MR performed on a Brucker Advanced spectrometer operatingt 200 MHz in CDCl3 at 25 ◦C, using tetramethylsilane (TMS) as thenternal reference standard, was found to be 20 mol%. The intrin-ic viscosity of PEANAN20, determined in THF at 30 ◦C in the usualay, was of 1.20 dl g−1.

X-ray patterns of pure (MMT) and modified Maghnia bentoniteOMMT), virgin PEMAN20 and PEMAN20/OMMT prepared by in situolymerization in presence of (1, 2 and 4% by weight of OMMT)ere recorded on a Philips PW3710 diffractometer in the range of

Theta (2–50). The monochromatic radiation applied was CuK�� = 1.5406 A).

As a complementary technique to XRD, TEM was used to investi-ate the morphology of the elaborated materials using a JEOL 1400quipped with a MORADA SIS numerical camera at an accelerationoltage of 120 kV.

Differential Scanning Calorimetry (DSC) runs were carried outn a Mettler-Toledo DSC 821 equipped with a FRS5 sensor andTAR© software for data analysis. Temperature, enthalpy and tauag calibrations were steadily done by using indium and zinc

Fig. 1. FTIR spectrum of (a) MMT, OMMT and HDTMA, (b) PEMAN20 andPEMAN20/OMMT materials.

standards. Samples of about 5–10 mg were placed in 40 �L alu-minum crucibles and sealed hermetically. The experiments wereperformed under a N2 atmosphere (80 mL min−1) at 10 K min−1.

The glass transition temperatures (Tg) of the synthesizedPEMAN20 copolymer and its PEMAN20/OMMT nanocompos-ites were determined by differential scanning calorimetry usingMettler-Toledo 821 DSC analyser at 10 K min−1. Several runs werecarried out and average values were considered.

Experiments of a thermogravimetric analysis were carried outon a TA instruments analyser TGA-Q500 under a nitrogen atmo-sphere from 30 ◦C to 650 ◦C, using aluminum pan and platiniumnacelle. Non-isothermal TG of the samples (5 to 10 mg) was per-formed under nitrogen at constant heating rates of 5, 10, 15 and20 K min−1.

3. Results and discussion

3.1. FTIR and 1H NMR

FTIR spectra of pure bentonite (MMT), organically modified ben-tonite (OMMT), (PEMAN20) copolymer and its (PEMAN20/OMMT)materials (Fig. 1) show the characteristic bands of pure ben-tonite at 462 cm−1 (Al–O), 524 cm−1 (Mg–O) and 1038 cm−1 (Si–O),additional bands at 1473 cm−1, 2850 cm−1 and 2922 cm−1 (C H),indicating that intercalation of the alkyl-ammonium ions withinthe interlayer spacing of the bentonite and peaks at 2240 cm−1

(C N) and 1728 cm−1 (C O) confirming the presence of acryloni-trile and ethyl methacrylate moieties of PEMAN20 respectively.

PEMAN20 copolymer was also characterized qualitatively by 1HNMR. Fig. 2 displays the 1H NMR spectrum of PEMAN20 and itsdifferent spectral assignments. The 1H NMR spectrum was thenused to determine the copolymer composition of the synthesizedcopolymer considering the relative intensity of (–OCH2) protons inthe ethyl methacrylate and the rest of the protons using the Eq.(1). The result showed that the synthesized copolymer PEMAN20contains 20 mol% of acrylonitrile, in good agreement with the onecalculated using literature reactivity ratios [18].

FAN% = I( CH)AN

I( CH) + I( O CH ) /2× 100% (1)

AN 2 EM

FAN% indicates the mole fraction of acrylonitrile comonomer (AN)in the copolymer, I ( CH)AN and I ( O CH2)EM represent theintensity of protons peaks of ( CH ) group of acrylonitrile and

S. Kadi et al. / Thermochimica Acta 569 (2013) 127– 133 129

Table 1Copolymer composition data for acrylonitrile (AN) and ethyl methacrylate (EM).

Copolymer Initial fractions ofcomonomers in the feed (f)

I ( CH)AN I ( O CH2)EM Copolymer fraction (F)

fAN% fEM%

PEMAN20 35.0 65.0 1.64

(t

3

fiaaa1m

ao

F4

Fig. 2. 1H NMR spectrum of PEMAN20.

OCH2) group of ethyl methacrylate respectively. Table 1 showshe comonomer mole fractions in the feed and in the copolymer.

.2. X-ray diffraction

X-ray diffraction patterns of pure (MMT), organically modi-ed bentonite (OMMT) and of (PEMAN20/OMMT) nanocompositesre illustrated in Fig. 3. The characteristic peak of MMT observedt 2� = 6.40◦, corresponding to a d-spacing of 1.38 nm, shifted to

lower angle of 5.80◦ with OMMT increasing the d-spacing to.52 nm, indicating the intercalation of the Hexadecyltrimethylam-onium ion into the layer of this clay.

The formation of an intercalated nanocomposite is usually char-

cterized by the observation of a XRD peak at 2� lower than thene of the organically modified clay, while the absence of XRD

ig. 3. X-ray diffraction patterns of MMT, OMMT and PEMAN20/OMMT (1, 2 and wt%) nanocomposites.

FAN% FEM%

11.38 22.4 77.6

peak is a necessary condition but not sufficient for the identifica-tion of an exfoliated nanocomposite. Indeed, the relatively broadpeak and a small shoulder depicted respectively at d-spacing of2.20 nm and 3.20 nm with PEMAN20/OMMT prepared in presenceof 2% by weight of OMMT indicate the formation of an interca-lated nanocomposite. Intercalated nanocomposite with a lowerd-spacing of 2.10 nm was observed as OMMT loading was increasedto 4 wt%.

As it is shown in Fig. 3, this main peak, tends to disappear asOMMT loading decreased to 1 wt% by weight and a diffraction peakof low intensity is observed at 2� = 2.40◦, corresponding to a d-spacing of 3.67 nm. This indicates a better intercalation with a lowerOMMT loading and that incomplete exfoliation of the stacked lay-ers may have occurred within the copolymer matrix. The presenceof some tactoids is also confirmed by the weak peak appearing at ad-spacing of 1.52 nm with PEMAN20/OMMT (2 wt%).

3.3. TEM

The dispersion of OMMT in the different PEMAN20/OMMT (1,2 and 4% by weight) hybrids was examined by TEM as displayedin Figs. 4 (a, b) and 5 which show some representative TEMmicrographs. In agreement with XRD the formation of mixed inter-calated/exfoliated nanocomposite structures were evidenced athigh magnification by the presence of stacked layers randomlydispersed in the PEMAN20/OMMT (2 wt%), of a thickness rangingfrom 8 to 25 nm and a length varying between 60 to 180 nm. Someindividual exfoliated silicate sheets are also shown. Although onlya diffraction peak of very small intensity and a shoulder wereobserved at around 2� = 2.4◦ and 3.30◦ with PEMAN20/OMMT con-taining 1 wt% of OMMT, a higher number of stacks of a smallerthickness varying between 4 and 14 nm and an average d-spacinglarger than 3.5 nm were confirmed by TEM as shown in Fig. 4a withPEMAN20/OMMT containing 1 wt% of OMMT, indicating the pres-ence of more exfoliated structures. TEM images also confirmed thepresence of some weakly intercalated stacks of much larger thick-ness. Fig. 5 shows selected TEM images of PEMAN20/OMMT (4 wt%)of intercalated structures confirming the OMMT loading effect ingood agreement with XRD analysis.

3.4. DSC analysis

Table 2 gathers the glass transition temperatures of virginPEMAN20 and its corresponding PEMAN20/OMMT nanocompos-ites prepared in presence of different OMMT loadings. A single Tg

is observed with this copolymer and its nanocomposites. The high-est Tg increase of 12 ◦C, compared to the virgin copolymer, wasobserved with PEMAN20/OMMT (1 wt%) nanocomposite of par-tially exfoliated structure. However, the Tg of the nanocompositesdecreases with an increase of OMMT loading but remains higherthan that of virgin PEMAN20.

This increase in Tg is probably due to the retarding effect ofclay platelets on the segmental motions of the polymer chains atthe organic–inorganic interface. Furthermore, the confinement of

PEMAN20 chains between the silicate layers as well as silicate-surface polymer interaction in the nanostructured hybrids resultsin an enhancement in the Tg compared to that of the virgin matrix[19].

130 S. Kadi et al. / Thermochimica Acta 569 (2013) 127– 133

Table 2Thermogravimetric data, Tg and E� (Kissinger method) values of PEMAN20 and its nanocomposites of different OMMT loadings.

PEMAN20 PEMAN20/OMMT (1 wt%) PEMAN20/OMMT (2 wt%) PEMAN20/OMMT (4 wt%)

T10% (◦C) 341 250 352 354T50% (◦C) 387 373 397 400Tmax (◦C) 396 407 405 402Char at 550 ◦C (%) 2 12 16 28Tg (◦C) 50 62 58 55

Fm

3

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ig. 4. TEM image of (a) PEMAN20/OMMT (1 wt%) and (b) PEMAN20/OMMT (2 wt%)aterials.

.5. TG analysis

Poly(ethyl methacrylate-co-acrylonitrile) was prepared by freeadical polymerization and contains 80 mol% of ethyl methacry-ate. Its degradation under nitrogen showed a small peak at09 ◦C, a shoulder at 348 ◦C and a major peak at 396 ◦C, attributed

Fig. 5. TEM image of PEMAN20/OMMT (4 wt%) nanocomposite.

Fig. 6. TG (a) and DTG (b) thermograms of OMMT, HDTMA, PEMAN20 andPEMAN20/OMMT (1, 2 and 4 wt%).

respectively to the scission at the vinylidene chain-end units andto random scission of polymer main chain (Fig. 6).

The mechanism of improvement of thermal stability in poly-mer nanocomposites is not fully understood. As it has been alreadyreported, that the improved thermal stability for polymer/claynanocomposites is mainly due to the formation of char whichhinders the out-diffusion of the volatile decomposition products[20–23].

The observed shift of the degradation curves ofPEMAN20/OMMT nanocomposites to higher temperature isan evidence of improved thermal stability of these latter hybridscompared to pure PEMAN20. Indeed, the small peak at 309 ◦C dis-appeared gradually with an increase of OMMT loading. Moreover,as shown in Table 2, the temperature at which the degradationstarts to become significant, taken at a temperature corresponding

to 10% of mass loss T10% and the temperature at 50% mass loss,T50% of the nanocomposites (2 and 4% by mass) increased ascompared to the virgin PEMAN20. All prepared nanocompositesexhibited also higher Tmax than virgin PEMAN20. The char formed

S. Kadi et al. / Thermochimica Acta 569 (2013) 127– 133 131

Table 3E� values (Tang et al. method) of PEMAN20 and its nanocomposites of different OMMT loadings.

PEMAN20 PEMAN20 OMMT1% PEMAN20 OMMT2% PEMAN20 OMMT4%� (%) E� (kJ mol−1) Error R2 E� (kJ mol−1) R2 E� (kJ mol−1) R2 E� (kJ mol−1) R2

10 132 6,6 0,97086 128 6,4 0,98907 E� 8,95 R2 182 9,1 0,8142520 178 8,9 0,99734 206 10,3 0,99484 179 8,95 0,99706 213 10,65 0,9794230 179 8,95 0,99692 243 12,15 0,99319 179 9,4 0,99751 223 11,15 0,9277940 179 8,95 0,99713 306 15,3 0,99783 188 9 0,96904 212 10,6 0,9783950 157 7,85 0,98497 278 13,9 0,99848 180 9,35 0,93882 179 8,95 0,9970960 178 8,9 0,99708 269 13,45 0,99313 187 8,95 0,96749 179 8,95 0,93791

5 179 8,95 0,99776 / / / 179 9,55 0,93805 / / /

191 / 0,96476 / / /

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70 177 8,85 0,99713 248 12,4 0,999880 157 7,85 0,99 191 9,55 0,914190 152 7,6 0,98113 / / /

ith PEMAN20/OMMT nanocomposites at 550 ◦C was significantven with low OMMT loading and increased up to 28% withMMT loading of 4 wt%. This improvement in thermal stability ofEMAN20 nanocomposites may be mainly due to the intercalationf polymer matrix into the clay galleries, which act as a barrier forhermal degradation.

The degradation behavior of PEMAN20 prepared in presence of wt% of OMMT was different from that of PEMAN20 or nanocom-osites of higher OMMT loadings. In addition to a peak relative tohe degradation of the surfactant that appeared at around 250 ◦Cnd another one at 363 ◦C, absent with PEMAN20/OMMT (2 and

wt%), PEMAN20/OMMT (1 wt%) hybrid showed the small peakt 309 ◦C observed with PEMAN20 and a major degradation peakt a higher temperature of 407 ◦C. Although its T10% and T50%ere much lower than those of PEMAN20 or nanocomposites ofigher clay content, this PEMAN20/OMMT (1 wt%) nanocompositef partially exfoliated structure exhibited the highest Tmax whichecreased slightly with an increase of OMMT amount as withEMAN20/OMMT (4 wt%) of intercalated structure but remainedigher than that of pure PEMAN20.

This result is in agreement with those of C. Marazzato et al.24] that reported a decrease in thermal stability of nanocompos-tes based on functionalized polyethylene in nitrogen atmosphereor nanocomposites having higher exfoliation level. These authorsxplained these observations by the better dispersion of the residuen an exfoliated nanocomposite, remaining after the thermal degra-ation of the clay which leads to an increase of the catalytic sites.

At low filler loading, exfoliation structure dominates, but themount of exfoliated silicate layers is not sufficient to improve thehermal stability [20].

.6. Thermal decomposition kinetics

The activation energies of the non-isothermal degradation ofirgin PEMAN20 and its PEMAN20/OMMT nanocomposites of dif-erent OMMT loadings were estimated using the approximationroposed by W. Tang et al. [13].

Non-isothermal TG of the different materials was performednder nitrogen at constant heating rates of 5, 10, 15 and 20 K min−1.

The TG and DTG curves for the decomposition of PEMAN20opolymer and of typical PEMAN20/OMMT nanocomposites atifferent heating rates are shown in Figs. 7(a, b) and 8 (a, b) respec-ively.

Application of the approximation proposed by W. Tang et al. [13]o the basic rate equation, gives the following equation:

n

(ˇi

T1.894661˛,i

)= ln

(A˛E˛

R

)− ln g(˛) + 3.63504095 − 1.894661

ln E˛ − 1.00145033

(E˛

RT˛,i

)(2)

Fig. 7. Thermograms TG (a) and DTG (b) of PEMAN20 at different degradation rates(5, 10, 15 and 20 K min−1).

where is the constant heating rate (dT/dt), E� the apparent activa-tion energy at a given extent of conversion (weight of materialsvolatilized/initial weight of materials), A� the pre-exponential fac-tor at a given extent of conversion, R the gas constant, g(�) amathematical function whose form is related to the reaction mech-anism, T˛,i the temperature corresponding to a given extent ofconversion for the i heating rates used. In this study, a value of E�

was computed for each value of lying in between 0.01 to 0.90 witha step of 0.01. Activation energies E� versus conversion obtainedfrom the slope of the variation of ln (ˇ/T1,894661) versus 1/T˛,i lines

are illustrated in Fig. 9 and Table 3.

The E� of degradation were calculated from 10 to 90% conversionfor PEMAN20. Because of the observed char, the E� of degrada-tion for PEMAN20/OMMT (1, 2 and 4 wt%) nanocomposites were

132 S. Kadi et al. / Thermochimica A

Fd

cc

pp

hi

Fm

ig. 8. Thermograms TG (a) and DTG (b) of PEMAN20/OMMT 2% at different degra-ation rates (5, 10, 15 and 20 K min−1).

alculated from 10 to 70–80% conversion only and plotted with theonversion in Fig. 9.

The E˛ values for PEMAN20/OMMT (2 wt%) nanocomposite wereractically similar to those of virgin PEMAN20 and indicate that theresence of 2% by weight of OMMT affected

slightly the degradation behavior of PEMAN20.Increasing the content of OMMT from 2 wt% to 4 wt% led to

igher E values. The apparent activation energy first increasedn the conversion range of 10–30%, went through a maximum at

ig. 9. Variation of the apparent activation energy versus conversion using Tang’sethod for PEMAN20 and its PEMAN20/OMMT nanocomposites.

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cta 569 (2013) 127– 133

30% conversion then decreased progressively and finally leveledoff above 50% conversion.

As shown in Fig. 9, a significant increase of E� was how-ever observed with PEMAN20/OMMT (1 wt%) compared to virginPEMAN20 or PEMAN20/OMMT (2 or 4 wt%) nanocomposites, con-firming the change in the degradation mechanism with the clayloading and the ratio of intercalated/exfoliated structures. The E�

increases in the conversion range (10–40%) from 128 kJ mol−1 to306 kJ mol−1, then decreased to 191 kJ mol−1 for = 80%.

4. Conclusions

This study showed, for the first time, that intercalated and par-tially exfoliated nanocomposites based on ethyl methacrylate andacrylonitrile copolymer were successfully prepared by free radicalin situ polymerization, using a bentonite from Maghnia (Algeria),modified by hexadecyltrimethylammonium chloride and THF assolvent.

The intercalated and partially exfoliated structures of the elab-orated nanocomposites exhibited an overall improved thermalstability investigated by TGA and an increase in Tg compared tothe virgin copolymer.

Nanocomposites prepared with 1% by weight of OMMT were ofpartially exfoliated structure and led to the highest Tg and to thehighest degradation temperature at maximum.

The E� values for PEMAN20 were practically similar to those ofPEMAN20/OMMT (2 wt%) nanocomposite, increased with the con-tent of OMMT and suggest an improved thermal stability of thenanocomposites compared to the copolymer matrix.

The significant increase of apparent activation energies,observed with PEMAN20/OMMT (1 wt%) compared to virginPEMAN20 or PEMAN20/OMMT (2 or 4 wt%) nanocomposites, con-firmed the change in the degradation mechanism with the clayloading and the ratio of intercalated/exfoliated structures.

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