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Polymer Testing 31 (2012) 282–289

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Polymer Testing

journal homepage: www.elsevier .com/locate/polytest

Material properties

Functionalized-graphene/ethylene vinyl acetate co-polymer compositesfor improved mechanical and thermal properties

Tapas Kuila a, Partha Khanra a, Anata Kumar Mishra b, Nam Hoon Kim c, Joong Hee Lee a,b,c,*

aWCU Programme, Department of BIN Fusion Technology, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of KoreabBIN Fusion Research Team, Department of Polymer & Nano Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of KoreacDepartment of Hydrogen and Fuel Cell Engineering, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea

a r t i c l e i n f o

Article history:Received 27 October 2011Accepted 8 December 2011

Keywords:GrapheneEthylene vinyl acetateNanocompositesMechanical propertiesThermogravimetric analysis

* Corresponding author. Department of BINChonbuk National University, Jeonju, Jeonbuk 561-7Tel.: þ82 63 270 2342; fax: þ82 63 270 2341.

E-mail address: jhl@chonbuk.ac.kr (J.H. Lee).

0142-9418/$ – see front matter � 2011 Elsevier Ltddoi:10.1016/j.polymertesting.2011.12.003

a b s t r a c t

The surface functionalization of graphene and the preparation of functionalized graphene/ethylene vinyl acetate co-polymer (EVA) composites by solution mixing are described.Octadecyl amine (ODA) was selected as a surface modifier for the preparation of func-tionalized graphene (ODA-G) in an aqueous medium. The ODA-G was characterized byFourier transform infrared spectroscopy and X-ray photoelectron spectroscopy, whichconfirm the modification and reduction of graphite oxide to graphene. Atomic forcemicroscopy shows that the average thickness of ODA-G is ca. 1.9 nm. The ODA-G/EVAcomposites were characterized by X-ray diffraction and transmission electron micros-copy, which confirms the formation of ODA-G/EVA composites. Measurement of tensileproperties shows that the tensile strength of the composites (with 1 wt.% ODA-G loading)is w74% higher as compared to pure EVA. Dynamic mechanical analysis shows that thestorage modulus of the composites is much higher than that of pure EVA. The thermalstability of the composite with 8 wt.% of ODA-G is w42 �C higher than that of pure EVA.The electrical resistivity has also decreased in the composites with 8 wt.% of ODA-G.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Graphene has attracted considerable research interest inphysics, chemistry, bio-science and materials science [1–4].Its unique features, such as its excellent electrical conduc-tivity, mechanical flexibility, thermal conductivity andoptical transparency, give it practical applicability and makeit theoretically interesting [5–8]. It has been widely used asa nanofiller in the preparation of polymer composite mate-rials [9–14]. The percolation in mechanical, and electricalproperties of the polymer composites can be achieved byusing graphene at lower content than required for othercarbon-based nanofillers because of its large surface area

Fusion Technology,56, Republic of Korea.

. All rights reserved.

and good electrical conductivity [9–14]. However, thepreparation of homogeneous graphene-polymer compositesentails some difficulties [13]. First, polymer compositesrequire the volume production of nanofiller for industrialapplication. Second, pristine graphene does not dispersewell in polymers and has a tendency to form phase sepa-rated composites [10,14]. Strategies to overcome theseshortcomings are outlined below.

Graphene can be produced by several methods: micro-mechanical cleavage of natural graphite, chemical vapordeposition (CVD), plasma enhanced CVD, electric arcdischarge, epitaxial growth on electrically insulatingsurfaces such as SiC, unzipping of carbon nanotubes and thesolution-based reduction of graphite oxide [15–18]. Amongthese, the last method shows potential for the production ofgraphene sheets in the bulk quantities required for polymercomposites preparation. The compatibility of graphene withpolymers can be increased through appropriate surface

T. Kuila et al. / Polymer Testing 31 (2012) 282–289 283

modification. The surface modification of graphene occursthrough the formation of either chemical bonds (covalentmodification) or p–p interactions (non-covalent modifica-tion). Such types of surface modification of graphene facili-tates homogeneous dispersion of graphene in polymermatrices, leading to the formation of high-performancepolymer composites [10,13,14].

Ethylene vinyl acetate (EVA) copolymer is available inthe form of rubber, thermoplastic elastomer and plastic. Itis widely used in shoes, encapsulating electrical cables,water proofing, corrosion protection and componentpackaging [19,20]. However, bulk EVA has a few drawbacksin terms of its thermal stability and mechanical properties.In order to overcome this, different types of nanofiller havebeen successfully used to prepare various EVA composites[9,19,21,22]. It has been demonstrated that graphenepossesses some superior properties over other nanofillers.Therefore, to investigate the effects of graphene on themechanical and thermal stabilities of EVA, this workreports the preparation and characterization of compositesof EVA and functionalized graphene.

2. Experimental

2.1. Materials

EVA with 28 wt.% of vinyl acetate content was obtainedfrom Hanwha Chemical, Korea (EVA 1529; density at 23 �C,5944 kg/m3). Benzoyl peroxide (BPO) (Lancaster, England)wasusedas a crosslinkingagent toprepare sheets of neatEVAand its composites. Natural flake graphite (Sigma–Aldrich,Steinheim, Germany) was used as received. Sulphuric acid,hydrochloric acid, hydrogen peroxide and toluene werepurchased from Samchun Pure Chemical Co. Ltd. (Pyeong-taek-si, Korea). Potassium permanganate (Junsei ChemicalCo. Ltd., City, Japan) and hydrazine monohydrate (TCI, Tokyo,Japan)were used as received. Octadecyl amine (ODA) surfacemodifier was purchased from TCI (Tokyo, Japan).

2.2. Preparation of graphite oxide and surface-modifiedgraphene

Graphite oxide (GO) was prepared by a modifiedHummers method [23,24]. Surface modification of GO withODA was carried out according to the reported method[13,14]. In brief, 1 g of GO was dispersed in 100 ml water byultrasonication for 1 h with a Sonosmasher (ULH 700S) withapplied frequency of 20 kHz. The un-exfoliated GO wasremoved by centrifuging (Beckman Coulter, Allegra� X-22R)at 5000 rpmfor 15–20min. About3 gofODAwasdissolved in100 ml of hot ethanol; before adding to the GO dispersion at100 �C and stirred for 24 h. Hydrazinemonohydrate (w1ml)was then added and the mixture was refluxed for 12 h. Theresulting black powder was washed with a water-ethanolmixture and dried under vacuum at 75 �C. The finalproductwasdesignatedasODA-modifiedgraphene (ODA-G).

2.3. Preparation of ODA-G/EVA composites

Various ODA-G/EVA composites were prepared bya solution mixing technique as reported earlier [13,14]. In

brief, the desired amount of ODA-G (0.15, 0.45, 0.75, and1.20 g, respectively) was dispersed in 40 ml of toluene byultra-sonication at room temperature for 1 h. In a 1 L Wit’sapparatus, 15 g of EVAwas dissolved in 100 ml of toluene at100 �C under stirring. The ODA-G dispersionwas then addedto the EVA solution and refluxed at 100 �C for 12 h. Thecuring agent, BPO, was added and the composite solutionwas cast onto a glass Petri dish. The solvent was removedunder a hood at room temperature and then under vacuumat 75 �C. Fig. 1 shows the schematic depiction of the func-tionalization of graphene and the preparation of itscomposites with EVA. ODA-G/EVA-28 composites contain-ing 0, 1.0, 3.0, 5.0, and 8.0 wt.% of ODA-G have been desig-nated as E2G0, E2G1, E2G3, E2G5, and E2G8, respectively.

2.4. Characterization

X-ray diffraction (XRD) study of ODA-G, pure EVA andtheir composites was carried out at room temperature ona D/Max 2500V/PC (Rigaku Corporation, Tokyo, Japan) byusing Cu target (l ¼ 0.154 nm). The composite sampleswere hot pressed by compression molding (Carver, Model-4122, Wabash, Indiana, U.S.A) to prepare a thin film(w150 mm). These films were used for XRD measurementand other analysis.

Fourier transform infrared (FT-IR) spectra of pure GOand ODA-G were recorded over a wave number range of4000–400 cm�1 using a Nicolet 6700 spectrometer. Thepowdered samples of pure GO and ODA-GweremixedwithKBr and pressed into thin pellet for FT-IR study.

The chemical nature and elemental composition ofODA-G were characterized by X-ray photoelectron spec-troscopy (XPS) (AXIS-NOVA, Kratos Analytical Ltd, UK). TheXPS spectrum of the samples remaining after peak sepa-ration and background subtraction was compared to thatobtained after Shirley background subtraction followed bycurve fitting of the substrate contribution. The photoioni-zation cross sections for carbon and oxygen were taken as0.01367 and 0.04005.

The nano level dispersion of ODA-G in the EVA matrixwas recorded on a transmission electron microscope (TEM,H-7650, Hitachi, Japan) at an acceleration voltage of 100 kV.The ultra-thin sections were cut with a diamond knife fromthe compression molded sheets of the composite andplaced on a 300 mesh Cu-grid for TEM analysis.

Atomic force microscopy (AFM, XE-100, PARK System,South Korea) of ODA-G was performed in non-contactmode using a V-shaped ‘cantilever’ probe B. In order toprepare an AFM sample, the silicon substrate was cleanedby ultrasonication using carbon tetrachloride, 2-propanol,DI-water, piranha solution, DI water, methanol andmethanol-toluene (1:1 volume ratio) mixed solvent, insequence. About 2 mg of ODA-G was dispersed in 10 ml oftoluene by ultrasonication for 10–15 min. Then, thehomogeneous dispersion of ODA-G was spin coated ontothe cleaned silicon substrate for AFM imaging.

The tensile properties of neat EVA and its compositewere measured on a universal test machine (LR5K, LloydCo., England) at a strain rate of 50 mm/min at 25 � 2 �C.

Dynamic mechanical analysis (DMA) of neat EVA and itscomposites was carried out with a Q 800 DMA (TA

Fig. 1. Schematic depiction for the preparation of functionalized graphene and it’s composite with ethylene vinyl acetate co-polymer.

T. Kuila et al. / Polymer Testing 31 (2012) 282–289284

instruments, USA) in tension mode with an appliedfrequency of 1 Hz. The measurements were carried out inthe temperature range of �50 to þ100 �C with a heatingrate of 2 �C/min.

Thermal stability of the composites were carried out ona Q50 TGA (TA instruments, New Castle DE, USA) ata heating rate of 5 �C/minute from 60 to 600 �C in air. Ineach case w5 mg of samples were taken for this analysis.

Electrical conductivities were measured by a Keithley2000 apparatus (Keithley Instruments Inc., Cleveland, Ohio,USA). Thin films (w150 mm) of the composites were usedfor this purpose.

3. Results and discussion

3.1. FT-IR analysis

FT-IR spectra of pure GO and ODA-G are shown in Fig. 2.Pure GO shows characteristic peaks of hydroxyl (3437 and1398 cm�1), carboxyl (1715 cm�1) and epoxy (1117 and

1061 cm�1) groups. The peaks at 1398, 1715, 1117 and1061 cm�1 are absent in the spectrum of ODA-G, confirm-ing the removal of oxygen functionalities during reduction.The appearance of peaks at 2928 and 2852 cm�1 corre-sponds to the C–H stretching vibrations of CH3 and CH2groups in ODA-G, indicating the successful modification ofthe GO. The band at 1634 cm�1 is assigned to the carbonylstretching vibration, indicating the formation of amide-carbonyl (NHCO) bonds between the graphene sheets andODA molecules [25]. The peak at 1574 cm�1 furtherconfirms the formation of amide linkage.

3.2. XPS analysis

Fig. 3 (a & b) shows XPS of ODA-G in the C1s and N1sregions. The C1s spectrum of GO has been discussed indetail in our previous report [14]. It shows a considerabledegree of oxidation, with five components correspondingto carbon atoms in different functional groups: non-oxygenated ring C (284.6 eV); C–N (285.1 eV); C–O bonds

Fig. 2. FTIR spectra of pure graphite oxide and ODA-G.

Fig. 3. Deconvoluted XPS of ODA-G in (a) C1s region and (b) N1s region.

T. Kuila et al. / Polymer Testing 31 (2012) 282–289 285

(285.7 eV); carbonyl C (285.7 eV); and carboxylated carbon(288.6 eV) [14,26–28]. ODA-G shows similar oxygen func-tionalities, but with much reduced peak intensitycompared with GO. Elemental analysis of carbon, oxygen,and nitrogen determined by XPS analysis shows that GOcontains 68.5% carbon and 31.3% oxygen (atomic concen-tration). The oxygen content was found to decreasesignificantly in the ODA-G and is only 5.3%. The presence of4.4% nitrogen and 88.2% carbon are also observed in themodified graphene. Moreover, the peaks positions havebeen little changed compared with the spectrum of pureGO [14]. This is attributable to the strong chemical inter-action between ODA molecules and graphene. The changein chemical environment, i.e. from sp3 to sp2, also contrib-utes to the observed shift in peak positions and intensities.

Fig. 3b shows XPS of pure ODA-G in the N1s region. Thewell defined peak at 400.2 eV is attributable to nitrogenatoms on the surface of the modified graphene. Deconvo-lution of the main peak at 400.2 eV shows modes at 399.9and 401.1 eV, suggesting that two different types ofnitrogen atoms may be present in the modified graphene[29]. This suggests that the ODA molecules are attached tothe surface of graphene by strong chemical bondingthrough the epoxy and carboxyl groups.

3.3. AFM analysis

Fig. 4 shows an AFM image of ODA-G dispersion intoluene. The thickness of the ODA-G layer was found to beca. 1.8–2 nm. The observed higher thickness of functional-ized graphene is probably due to the attachment of ODAmolecules to its surface. The presence of sp3 character inthe modified graphene may also have been responsible forthe increase in thickness. The p–p stacking may occurduring reduction and results in the formation of multilay-ered graphene. Therefore, the observed higher thickness ofODA-G may be due to the formation of several layergraphene.

3.4. XRD and TEM analyses

Fig. 5 shows the XRD pattern of ODA-G, neat EVA-28 andtheir composites with different amount of ODA-G. Thebasal reflection peak of pure GO at 2q ¼ 11.3� is shifted to2q ¼ 3.8� in the ODA-G spectrum because of the interca-lation of long alkyl chains in the interlayer spacing of GO.This indicates the formation of several layer surface-modified graphene. The peak at 2q ¼ 26.0� is attributableto the formation of graphitic structures during the reduc-tion of the modified GO. The broad peak centered at2q ¼ 21.1� is attributable to the intercalation of long alkylchains between the stacked graphene layers. The spectrumof pure EVA shows two amorphous peak at 2q ¼ 20.8� and22.8�. The absence of characteristic peak of ODA-G in thecomposites indicates the delamination of graphene layersin the presence of EVA co-polymer. The XRD data also implythe homogeneous dispersion of ODA-G in the composites.However, XRD is not the best tool to determine crystal layerdelamination or the homogeneity of dispersion. Therefore,TEM was performed to assess the dispersion of ODA-G inthe composites.

Fig. 4. AFM image of ODA-G showing an average thickness of 1.9 nm.

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The morphologies of the exfoliated ODA-G/EVAcomposites were analyzed by TEM. Fig. 6 (a & b) showsthe TEM images of the composites with 3 and 8 wt.% ofODA-G. The gray-white areas represent the EVAmatrix; theblack areas are the graphene layers. Graphene layers areshown to be dispersed throughout the polymer matrix. The

Fig. 5. XRD patterns of pure EVA, ODA-G and their composites with 1, 3, 5,and 8 wt.% of ODA-G.

TEM image (Fig. 6b) of the composites with 8 wt.% of ODA-G shows that most of the ODA-G layers are not onlyhomogeneously distributed but also aggregated in the EVAmatrix.

3.5. Tensile properties

The effect of ODA-G on the tensile properties of ODA-G/EVA composites was studied and the results are shown inFig. 7. It is seen that tensile strength (TS) and tensilemodulus of the composites are higher than that of the pureEVA. The TS of the E2G1 shows maximum TS value of47.6 MPa. The increase in tensile strength is probably due tothe strong interfacial interaction between the amidogroups of ODA-G and the polar acetate groups of EVA.Moreover, the large surface of graphene can entirely fill thepolymermatrix at very lower ODA-G content [10]. The TS ofthe composites has been found to decrease gradually athigher ODA-G loadings (3, 5, and 8 wt.%). Such a drop inmechanical properties may be due to the increasingtendency of aggregation of ODA-G layers forming somedefects in the nanostructure of the composites. However,the TS values of the composites always remain higher thanthat of the pure EVA, showing the formation of mechan-ically durable composites. It can also be noted that theelongation at break (EB) of the composites decreasessteadily with the addition of ODA-G in the EVA co-polymer.This is attributed to the fact that the deformation of ODA-Gis generally much less than that of the polymermatrix, thus

Fig. 6. TEM images of ODA-G/EVA composites with (a) 3 and (b) 8 wt.% ofODA-G.

Fig. 7. Stress–strain plot of pure EVA and it’s composite with differentODA-G loadings.

T. Kuila et al. / Polymer Testing 31 (2012) 282–289 287

the filler forces the matrix to deform more than the overalldeformation of the composite [30]. The decrease in EB alsoindicates the improvement of stiffness in the composites[31]. The increase in stiffness has also been confirmed bythe measurement of Young’s modulus from the stress–strain plot of the composites, as shown in Table 1, whichshows that the Young’s modulus of the composites is muchhigher than that of the neat EVA.

3.6. Dynamic mechanical analysis

Variations of storage modulus (E0) and tan d of theODA-G/EVA composites between�50 and 90 �C are shownin Fig. 8 (a & b). Storage modulus of the compositesincreases with increasing ODA-G content. The homoge-neous dispersion of ODA-G in the EVA matrix restricts thechain mobility, resulting in the improvement of E0 values.Both pure EVA and the composites exhibit two tand peaks; one at ca. �10 �C and the other in the range of 23–46 �C. The appearance of two peaks in the tan d curvesindicates the presence of two different segments in theEVA copolymer. However, the second peak, possibly due tothe presence of polyethylene (hard segments), occurs ata much lower temperature than in neat polyethylene. Thisis due to the partial co-polymerization of ethylene andvinyl acetate. The height of both the tan d peaks decreasesignificantly in the composites as compared with pureEVA. This is due to the increased toughness or stiffness ofthe composites, which is directly related to their dampingability or their capacity to dissipate vibrational energy[32,33]. The position of the second tan d peak shifts tohigher temperatures with increasing ODA-G content; thefirst one has remained largely unmoved. The betterdispersion of ODA-G in the hard segments restrictssegmental motion of the polyethylene parts, increasingthe glass transition temperature (Tg).

3.7. TGA and electrical resistivity

Fig. 9 shows the TGA profiles of ODA-G, neat EVA andvarious composites. The onset of degradation temperatureof ODA-G is w135 �C and degradation is completed atw570 �C. The early degradation of ODA-Gmay be due to thedegradation of octadecyl amine chains from the surface ofgraphene. The weight loss at higher temperature isascribed to the loss of remaining oxygen functionalities ofODA-G. On the contrary, both the pure EVA and thecomposites reveal two steps of thermal degradation. Theinitial thermal degradation of neat EVA occurs between 240

Table 1Tensile and DMA data of pure EVA and ODA-G/EVA composites.

Sample TS(MPa)

EB (%) Young’smodulus

E0 (MPa)at �40 �C

E0 (MPa)at 25 �C

Tg tan dmax

at Tg

E2G0 27.2 1244.5 0.41 1489.6 105.3 24.0 0.180E2G1 47.6 982.3 0.86 1529.6 128.6 33.0 0.183E2G3 47.2 862.0 0.81 1558.2 162.0 33.7 0.167E2G5 40.2 692.0 0.88 1690.2 177.5 32.4 0.155E2G8 28.0 685.4 1.21 1862.1 304.7 46.0 0.143

Fig. 8. Variation of (a) Storage modulus and (b) tan d with temperaturechange for ODA-G/EVA composites.

Table 2TGA data of pure EVA and ODA-G/EVA composites.

Sample name T�15%(�C) T�25%(�C) T�50%(�C) T�75%(�C) T�95%(�C)

E2G0 354 403 434 449 472E2G1 359 406 437 454 479E2G3 396 422 446 457 485E2G5 407 434 457 468 497E2G8 423 440 460 470 514

T�x% indicates degradation temperature at x% weight loss.

T. Kuila et al. / Polymer Testing 31 (2012) 282–289288

and 357 �C as acetate groups decompose to CO2. Thecomposites reveal faster initial thermal degradation thanpure EVA due to the early degradation of ODAmolecules onthe surface of the modified graphene. The second

Fig. 9. TGA profiles of ODA-G, E2G0, E2G1, E2G3, E2G5, and E2G8.

degradation is due to scission of the main polyethylenechains; in pure EVA, it occurs between 370 and 515 �C[34,35]. This degradation in the composite with 1 wt.% ofODA-G is very similar to that of pure EVA. The compositeswith 3, 5, and 8 wt.% of ODA-G content exhibit significantlyenhanced thermal stability (Table 2). The thermal degra-dation temperatures at 50% weight loss of the compositesare 434, 437, 447, 458, and 460 �C for 0, 1, 3, 5, and 8 wt.%ODA-G, respectively. This is due to the homogeneouslydistributed graphene layers slowing the thermal degrada-tion of the polymer by inhibiting the emission of thermallydegraded small gas molecules and disrupting the supply ofoxygen from the surface to the specimen bulk.

Electrical resistivity measurements of the ODA-G/EVAcomposites show a sharp decrease to 2.3 � 107 ohm.cm at3 wt.% of ODA-G. At 5 and 8 wt.% ODA-G, the resistivity is4.5 � 106 and 1.3 � 103 ohm.cm, respectively. On thecontrary, pure EVA shows a much higher value(1013 ohm.cm) [36].

4. Conclusions

Covalent functionalization of graphene was performedusing ODA as a surface modifying agent. FTIR spectra showthat ODA molecules are doped on the surface of the gra-phene. XPS analysis of ODA-G provides evidence of theremoval of oxygen functionalities from the surface of GOand the formation of functionalized graphene. The surfacemorphology of ODA-G is further characterized by AFM,which confirms the formation of several layer graphene.ODA-G/EVA composites were prepared by the solutionmixing of EVA and ODA-G dispersed in toluene. Themorphology of the composites was confirmed by XRD andTEM analysis. Measurements of mechanical and dynamicmechanical properties show that the ODA-G/EVAcomposites are mechanically stable as compared to pureEVA. The thermal stabilities and electrical resistivity of theresulting composites are also much better than those ofneat EVA. Therefore, composites of ODA-G/EVA havepotential applicability as mechanically durable and ther-mally stable materials.

Acknowledgements

This study was supported by the National Space Lab(NSL) program (S1 08A01003210), the Human ResourceTraining Project for Regional Innovation, and the WorldClass University (WCU) program (R31-20029) funded bythe Ministry of Education, Science and Technology (MEST)and the National Research Foundation (NRF) of Korea.

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References

[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V.Dubonos, I.V. Grigorieva, A.A. Firsov, Electric field effect in atomi-cally thin carbon films, Science 306 (5696) (2004) 666–669.

[2] J.H. Jung, D.S. Cheon, F. Liu, K.B. Lee, T.S. Seo, A graphene oxide basedimmune-biosensor for pathogen detection, Angew. Chem. Int. Ed. 49(33) (2010) 5708–5711.

[3] K.P. Loh, Q. Bao, P.K. Ang, J. Yang, The chemistry of graphene, J.Mater. Chem. 20 (2010) 2277–2289.

[4] A.K. Geim, Graphene: status and prospects, Science 324 (5934)(2009) 1530–1534.

[5] Y.B. Zhang, Y.W. Tan, H.L. Stormer, P. Kim, Experimental observationof the quantum hall effect and berry’s phase in graphene, Nature438 (2005) 201–204.

[6] J. Wu, W. Pisula, K. Müllen, Graphenes as potential material forelectronics, Chem. Rev. 107 (3) (2007) 718–747.

[7] M.J. Allen, V.C. Tung, R.B. Kaner, Honeycomb Carbon: a review ofgraphene, Chem. Rev. 110 (1) (2010) 132–145.

[8] Y. Ouyang, J. Guo, A theoretical study on thermoelectric propertiesof graphene nanoribbons, Appl. Phys. Lett. 94 (2009) 263107.

[9] T. Kuila, S. Bhadra, D. Yao, N.H. Kim, S. Bose, J.H. Lee, Recentadvances in graphene based polymer composites, Prog. Polym. Sci.35 (11) (2010) 1350–1375.

[10] S. Stankovich, D.A. Dikin, G.H.B. Dommett, K.M. Kohlhaas, E.J. Zim-ney, E.A. Stach, R.D. Piner, S.T. Nguyen, R.S. Ruoff, Graphene - basedcomposite materials, Nature 442 (2006) 282–286.

[11] H.B. Zhang, W.Q. Zheng, Q. Yan, Y. Yang, J. Wang, Z.H. Lu, G.Y. Ji, Z.Z.Yu, Electrically conductive polyethylene terephthalate/graphenecomposites prepared by melt compounding, Polymer 51 (5) (2010)1191–1196.

[12] X. Zhao, Q. Zhang, D. Chen, Enhanced mechanical properties ofgraphene-based poly(vinyl alcohol) composites, Macromolecules 43(15) (2010) 2357–2363.

[13] T. Kuila, S. Bose, A.K. Mishra, P. Khanra, N.H. Kim, J.H. Lee, Effect offunctionalized graphene on the physical properties of linear lowdensity polyethylene nanocomposites, Polym. Test. 31 (1) (2012)31–38.

[14] T. Kuila, S. Bose, C.E. Hong, M.E. Uddin, P. Khanra, N.H. Kim, J.H. Lee,Preparation of functionalized graphene/linear low density poly-ethylene composites by a solution mixing method, Carbon 49 (3)(2011) 1033–1037.

[15] K.S. Kim, Y. Zhao, H. Jang, S.Y. Lee, J.M. Kim, K.S. Kim, J.H. Ahn, P.Kim, J.Y. Choi, B.H. Hong, Large-scale pattern growth of graphenefilms for stretchable transparent electrodes, Nature 457 (2009)706–710.

[16] B. Hsia, N. Ferralis, D.G. Senesky, A.P. Pisano, C. Carraro, R. Mabou-dian, Epitaxial graphene growth on 3C-SiC(111)/AlN(0001)/Si(100),Electrochem. Solid-State Lett. 14 (2) (2011) k13–15.

[17] Z. Sun, Z. Yan, J. Yao, E. Beitler, Y. Zhu, J.M. Tour, Growth of graphenefrom solid carbon sources, Nature 468 (2010) 549–552.

[18] S. Park, R.S. Ruoff, Chemical methods for the production of gra-phene, Nat. Nanotechnol. 4 (2009) 217–224.

[19] C. Nyambo, E. Nyambo, C.A. Nyambo, Thermal stability and flam-mability characteristics of ethylene vinyl acetate (EVA) compositesblended with a phenyl phosphonate-intercalated layered doublehydroxide (LDH), melamine polyphosphate and/or boric acid,Polym. Degrad. Stabil. 94 (4) (2009) 513–520.

[20] K.W. Park, S.R. Chowdhury, C.C. Park, G.H. Kim, Effect of dispersionstate of organoclay on cellular foam structure and mechanical

properties of ethylene vinyl acetate copolymer/ethylene-1-butenecopolymer/organoclay nanocomposite foams, J. Appl.Polym. Sci. 104 (6) (2007) 3879–3885.

[21] S. Duquesne, C. Jama, M.L. Bras, R. Delobel, P. Recourt, J.M. Gloaguen,Elaboration of EVA-nanoclay systems-characterization, thermalbehaviour and fire performance, Compos. Sci. Technol. 63 (8) (2003)1141–1148.

[22] S. Peeterbroeck, M. Alexandre, J.B. Nagy, C. Pirlot, A. Fonseca, N.Moreau, G. Philipin, J. Delhalle, Z. Mekhalif, R. Sporken, G. Beyer, Ph.Dubois, Polymer-layered silicate–carbon nanotube composites:unique nanofiller synergistic effect, Compos. Sci. Technol. 64 (15)(2004) 2317–2323.

[23] W.S. Hummers, R.E. Offeman, Preparation of graphitic oxide, J. Am.Chem. Soc. 80 (1958) 1339.

[24] S. Bose, T. Kuila, M.E. Uddin, N.H. Kim, A.K.T. Lau, J.H. Lee, In-situsynthesis and characterization of electrically conductive poly-pyrrole/graphene composites, Polymer 51 (25) (2010) 5921–5928.

[25] G. Wang, X. Shen, B. Wang, J. Yao, J. Park, Synthesis and character-ization of hydrophilic and organophilic graphene nanosheets,Carbon 47 (5) (2009) 1359–1364.

[26] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Klein-hammers, Y. Jia, Y. Wu, S.T. Nguyen, R.S. Ruoff, Synthesis ofgraphene-based nanosheets via chemical reduction of exfoliatedgraphite oxide, Carbon 45 (7) (2007) 1558–1565.

[27] S. Stankovich, R.D. Piner, X. Chen, N. Wu, S.T. Nguyen, R.S. Ruoff,Stable aqueous dispersions of graphitic nanoplatelets via thereduction of exfoliated graphite oxide in the presence of poly(-sodium 4-styrenesulfonate), J. Mater. Chem. 16 (2) (2007) 155–158.

[28] X. Fan, W. Peng, Y. Li, X. Wang, G. Zhang, F. Zhang, Deoxygenation ofexfoliated graphite oxide under alkaline conditions: a green route tographene preparation, Adv. Mater. 20 (23) (2008) 4490–4493.

[29] N. Liu, F. Luo, H. Wu, Y. Liu, C. Zhang, J. Chen, One step ionic-liquid-assisted electrochemical synthesis of ionic-liquid-functionalizedgraphene sheets directly from graphene, Adv. Func. Mater. 18 (10)(2008) 1518–1525.

[30] A. Romisuhani, H. Salmah, H. Akmal, Tensile properties of lowdensity polypropylene (LDPE)/palm kernel shell (PKS) bio-composites: the effect of acrylic acid (AA), Mater. Sci. Eng. 11 (2010)012001.

[31] K. Oksman, C. Clemons, Mechanical properties polypropylene-woodand morphology of impact modified flour composites, J. Appl.Polym. Sci. 67 (9) (1998) 1503–1513.

[32] W. Zhang, D. Chen, Q. Zhao, Y. Fang, Effects of different kinds of clayand different vinyl acetate content on the morphology and prop-erties of EVA/clay composites, Polymer 44 (26) (2003) 7953–7961.

[33] P. Fang, Z. Chen, S. Zhang, S. Wang, L. Wang, J. Feng, Microstructureand thermal properties of ethylene-(vinyl acetate) copolymer/rec-torite composites, Polym. Int. 55 (3) (2006) 312–318.

[34] M. Zanetti, G. Camino, R. Thomann, R. MuÈlhaupt, Synthesis andthermal behaviour of layered silicate-EVA composites, Polymer 42(10) (2001) 4501–4507.

[35] S. Peeterbroeck, F. Laoutid, J.M. Taulemesse, F. Monteverde, J.M.Lopez-Cuesta, J.B. Nagy, M. Alexandre, P. Dubois, Mechanical prop-erties and flame-retardant behavior of ethylene vinyl acetate/high-density polyethylene coated carbon nanotube composites, Adv.Funct. Mater. 17 (15) (2007) 2787–2791.

[36] N.C. Das, S. Yamazaki, M. Hikosaka, T.K. Chaki, D. Khastagir, A.Chakraborty, Electrical conductivity and electromagnetic interfer-ence shielding effectiveness of polyaniline-ethylene vinyl acetatecomposites, Polym. Int. 54 (2) (2005) 256–259.