European Polymer Journal 48 (2012) 252–259

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

Contents lists available at SciVerse ScienceDirect

European Polymer Journal

journal homepage: www.elsevier .com/locate /europol j

Macromolecular Nanotechnology

Self assembled graphene/carbon nanotube/polystyrene hybridnanocomposite by in situ microemulsion polymerization

Archana S. Patole a, Shashikant P. Patole b,c, So-Young Jung a, Ji-Beom Yoo b,c,Jeong-Ho An a,⇑, Tae-Ho Kim a

a Department of Polymer Science and Engineering, Sungkyunkwan University, Suwon 440–746, Republic of Koreab SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440–746, Republic of Koreac School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea

a r t i c l e i n f o a b s t r a c t

Article history:Received 1 July 2011Received in revised form 31 October 2011Accepted 9 November 2011Available online 18 November 2011

Keywords:NanocompositesGrapheneCarbon nanotubesPolystyrene

0014-3057/$ - see front matter � 2011 Elsevier Ltddoi:10.1016/j.eurpolymj.2011.11.005

⇑ Corresponding author. Fax: +82 31 292 8790.E-mail address: jhahn1us@skku.edu (J.-H. An).

Self-assembled graphene/carbon nanotube (CNT)/polystyrene hybrid nanocompositeswere prepared by water-based in situ microemulsion polymerization. The resulting nano-composites were used as filler in a host polystyrene matrix to form composite films. Anadmixture of the two types of carbon fillers provided better improvement in the thermaland mechanical properties compared to the neat polymer. The sheet resistance decreasedprogressively due to the formation of an extended conjugation network with the CNTbridging the gap between the graphene sheets coated with polymer nanoparticles. Thedetails of the analysis are presented.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction tivity. On the other hand, graphene has remarkably high

On the nanoscale, various carbon allotropes includingfullerene, single/multiwall carbon nanotubes (CNTs),graphite, diamond and graphene have attracted consider-able attention over the last two decades because of theirexcellent properties and wide applications [1]. Among thesematerials, carbon nanotubes and graphene have attractedtremendous interest since their discovery in 1991 and2004, respectively, and have emerged as a new class ofmaterials with potential application as actuators, solar cells,field emission device, field effect transistor, supercapacitorsand batteries. Moreover, CNT and graphene are representa-tive of one and two dimensional nanostructures, they aremutually complementary in both structure and propertiesand share many common points, such as ultrahigh mechan-ical strength and electrical conductivity [2]. However, theyhave their own drawbacks. CNT have superior mechanicalproperties but must be dispersed uniformly and form a net-work to achieve sufficient percolation for electrical conduc-

. All rights reserved.

electron mobility at room temperature with reported val-ues in excess of 15,000 cm2 V�1 s�1 but causes problem ofits restacking property [3]. Furthermore, for graphene onsilicon dioxide substrates, the scattering of electrons byoptical phonons of the substrate at room temperature hasa larger effect than scattering by graphene’s own phonons,which limits the mobility to 40,000 cm2 V�1 s�1 [4]. Oneof the future trends is to integrate them into a hybrid struc-ture, which would generate new potential for materialsresearch and applications. Such a hybrid structure showsexcellence flexibility and stretching ability and is expectedto have electrical conductivity and thermal dissipation inall directions [2]. One possible route to use this hybridstructure for applications would be to incorporate and dis-tribute it homogeneously into a polymer matrix or obtainCNT and graphene self assembled in in situ reactions [5].Graphene itself can be dispersed into a polymer matrixwithout CNT but such a system causes problems for energystorage applications owing to its restacking property.Therefore, it is important to preserve the high surface areagraphene. Graphene can form functional films using a rangeof solution processing methods including filtration [6],

Scheme 1. Multi step synthesis process for making the conductinggraphene/MWCNT/PS films.

A.S. Patole et al. / European Polymer Journal 48 (2012) 252–259 253

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

solution casting [7] and electrophoretic deposition [8].However, most of the above mention techniques suffer froma lack of architecture and property control leading to theloss the surface area for energy storage due to grapheneaggregation. Therefore, it is highly desirable to use onedimensional carbon nanotubes to physically separate thetwo dimensional graphene and preserve its high surfacearea [1].

Until now, many methods for incorporating CNT, graph-ene and hybrid systems into a polymer have been reported,such as solution blending, melt mixing, intercalation, micro/mini emulsion and bulk polymerization. Among the abovementioned approaches, microemulsion was employed inthis study, in which the product in aqueous form has aunique advantage because of the ease of manipulation, sim-ple, scalable, low cost and the absence of environmentalconcerns as well as its viability as a lightweight andeconomical material [9]. In these aqueous polymer emul-sions, the nanoparticles created exclude the volume andessentially push the CNT into the interstitial spacesbetween them. This dramatically reduces the space avail-able for the filler to form a conducting network resultingin an enhancement of electrical conductivity with asmall amount of filler. Many attempts have been made tofabricate the CNT/polymer and graphene/polymernanocomposites. Yu et al. examined the thermoelectricproperties-thermal conductivity, electrical conductivityand thermo power of a segregated-network CNT-polymernanocomposite as a function of the concentration of CNTat room temperature [10]. Shim et al. also reported the fab-rication of a conducting SWNT-modified cotton yarn that of-fers a uniquely simple yet remarkably functional solutionfor wearable electronics and smart textiles, with manyparameters exceeding the existing technological solutions,including those using carbon materials [11]. In the case ofthe graphene/polymer synthesis, Stankovich et al. reportedthat a polystyrene–graphene nanocomposite formed by thisroute exhibits a percolation threshold of �0.1 vol.% for theroom-temperature electrical conductivity, which is the low-est value reported for any carbon-based nanocomposite [5].Ramanathan et al. showed that single graphene sheets andtheir appropriate dispersion in a polymer would result ineconomically viable nanocomposites with excellentmechanical, thermal, electrical and barrier properties atextraordinarily low filler content [12]. There have been aconsiderable number of studies exploring the properties ofthe nanocomposites synthesized using CNT and grapheneas a fillers [13–15]. However, very few reports introducedthe graphene/CNT/polymer hybrid system. Recently Yanet al. attempted the synthesis of a GNS (graphenenanosheets)/CNT/PANI (Polyaniline) composite by in situpolymerization [16].

To the best of the authors’ knowledge, there are noreports describing the synthesis and properties of thegraphene/CNT/polymer hybrid system by in situ micro-emulsion polymerization. This study employed the sameapproach to synthesize such a hybrid system. The graph-ene sheets and CNT in the composites were self assembledinto a hybrid nanostructure. Their thermal and mechanicalproperties are reported. This novel hybrid nanostructurehas the potential to combine the major advantages of its

individual components to produce significant improve-ment in their current nanoelectronic applications.

2. Experimental

2.1. Materials

Styrene, divinyl benzene (DVB), sodium dodecyl sulfate(SDS), Azobisisobutyronitrile (AIBN), and 1-pentanol werepurchased from Sigma Aldrich and used as received.

2.2. Synthesis of MWCNT and graphene

A previous article discussed the synthesis of MWCNT bychemical vapor deposition and the synthesis of graphenefrom expandable graphite [17,18].

2.3. Nanocomposite formation

Before polymerization, the MWCNT were purified bythermal treatment at 300 �C for 1 h in air to remove theamorphous carbon. The purified multiwall carbon nano-tube MWCNT (0.250 g) and graphene (0.250 g) were dis-persed in 2.2 g of SDS and 10 g of 1-pentanol in 200 mLof deionized water and sonicated for 4 h. SDS and 1-pent-anol were used as a surfactant and hydrophobe, respec-tively. The mixture was charged with a MWCNT andgraphene aqueous dispersion into a 1000-mL four neckglass reactor equipped with a condenser, dropping funnel,stirrer and nitrogen inlet. The reactor was then placed in anice bath. Styrene (10 g), DVB (1.66 g) and AIBN (0.1 g) asthe monomer, crosslinking agent and initiator, respec-tively, were mixed through a funnel prior to polymeriza-tion. The mixture was then sonicated in a nitrogenatmosphere at 0 �C for 4 h. After homogeneous mixing,the ice bath was replaced with an oil bath for heating. Poly-merization was then carried out by increasing the oil bathtemperature to 85 �C for 4 h. The resulting product wasthen isolated in a mixture of methanol and water, anddried at 60 �C under vacuum for 24 h. The dried nanocom-posite product was then used for further analysis.

2.4. Film formation and characterization

Graphene/MWCNT/PS was used to prepare the PS films.In typical experiments, a concentration of the graphene/

254 A.S. Patole et al. / European Polymer Journal 48 (2012) 252–259

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

MWCNT/PS nanocomposites sample varying from 2 to20 wt.% was selected as filler to produce the compositefilms. The host matrix was prepared by dissolving 5 mgof PS beads in 20 ml of Tetrahydrofuran (THF). The filler(2–20 wt.%) was then added to the host matrix. The solu-tion was then cast on a glass plate and dried at room tem-perature to form free standing, 600 lm thick and uniformfilms. Detail recipe of synthesis and film formation wasshown in Scheme 1.

3. Experimental methods

The morphology of graphene and graphene/MWCNT/PSnanoparticles was examined by scanning electron micros-copy (FESEM; JSM6700F, JEOL) and transmission electronmicroscopy (HRTEM; JEOL 300 kV) to explore the struc-tural properties. The spectroscopic analyses were carriedout using a Fourier transform infrared spectrometer(FTIR-Nicolet IR 200), UV–visible spectrometer (S-4100,SCINCO) and a Micro-Raman Spectrometer (Invia Basic,Renishaw Co. England) using an Ar–ion laser at 640 nm

Fig. 1. (a) SEM image of exfoliated graphene from expandable graphite, (b–d) sethe graphene/MWCNT/PS nanocomposite.

as the excitation light source. Thermogravimetric/differen-tial thermal analysis (Tg/DTA) of PS nanoparticles andgraphene/MWCNT/PS nanocomposite were carried outusing a thermobalance (TGA 2050) from room temperatureto 600 �C, at a rate of 10 �C/min in a continuous nitrogenflow. The dynamic differential scanning calorimetry (DSC)experiments were carried out with a thermal analyzer(DSC 2920) at heating rates of 5 �C/min in a continuousnitrogen flow. The sheet resistance (Rs) of the compositefilms was measured using a 4-point probe resist meter(AIT CMT-SR2000N). GPC analysis was carried using THFas a solvent on the instrument (Agilent 1100 S).

4. Results and discussion

Fig. 1a shows a SEM image of the as-exfoliated graph-ene after thermal shock on expandable graphite usingchemical vapor deposition. The exfoliated graphene sheetswere several micrometers in size. The delicate graphenecould be folded over repeatedly without breaking. Suchcloth like graphene layers were observed in abundance

lf assembled graphene/MWCNT/PS nanocomposite and (e) TEM image of

Fig. 2. (a) Comparative Raman spectrum of graphene, MWCNT and graphene/MWCNT/PS nanocomposite. (b) Comparative FT-IR spectrum of PS andgraphene/MWCNT/PS nanocomposite.

A.S. Patole et al. / European Polymer Journal 48 (2012) 252–259 255

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

on the experimental sample. A previous report showedthat a single layer to thirty layers could be prepared in asample [18]. Fig. 1b shows the nanocomposite in whichthe MWCNT coated with PS nanoparticles was sandwichedbetween the several micrometer thick graphene sheets andwell separated CNT wrapped on graphene. A cloth likebending pattern, crumbled and folding of graphene onthe borderline was observed in the nanocomposite. Thestrength of graphene could tolerate angstrom-radius bend-ing without breaking. Fig. 1a demonstrated perhaps thegraphene have sharpest bending point formed of all knownmaterials and its characteristics play an important role inthe low strain regime of the composites [19]. Fig. 1c andd displays the self assembled structure of the graphene/MWCNT/polystyrene. The MWCNT coated with the poly-mer appears to be a bridge between two graphene sheets.TEM (Fig. 1f) also shows that the PS nanoparticles form abridge between the MWCNT and graphene, ultimatelyresulting in a self assembled structure. In Fig. 1d shows rel-atively dense and uniform network of carbon nanostruc-tures formed with nanotubes bridging between graphenesheets. Fig. 1e gives a magnified view of crumbled graph-ene in the composites. It also shows the PS grafted agglom-erated MWCNT at the folding edges of graphene as well aswell separated PS-anchored MWCNT on the graphenesurface.

Fig. 2a shows the Raman spectra of functionalizedgraphene/MWCNT. All spectra were excited with visible(640 nm) laser light. The main features in the Raman spec-tra of MWCNT are the so-called D (1351) and G (1572)bands, which are known as the disorder induced and in-plane E2 g zone center modes, respectively, in addition tothe D0-band at 1604 cm�1, which could be affected directlyby the disorder in the CNT, was also observed in the pris-tine MWCNT and nanocomposite. This band in the nano-composite was attributed to an increase in the number ofdefects along the tube body [20]. In graphene the mostnotable features of the spectrum are the G peak at�1580 cm�1 and the second order Raman band (2D) ataround 2703 cm�1 is used widely as an indicator of highquality of graphene. However, in the nanocomposite, theintensity of the D (ID) and G (IG) bands were lower than

those of graphene and MWCNT. Moreover, the broadeningof the 2D band along with a decrease in intensity revealedthe creation of defects on the graphene [18]. Significantlydecreased peak intensity of the 2D band and all of thesecharacteristic absorption peaks revealed a high amount ofpolymer chain tethered on the surface of graphene andMWCNT in the nanocomposite. Furthermore, the ID:IG ra-tio indicate the quality of the sample. Therefore, whilecomparing the ratio of ID/IG for graphene, MWCNT andnanocomposites show that the nanocomposites have thelowest ratio, and consequently a higher defect level dueto grafting of the polymer chain on the high surface areaof graphene as well as to the passivation of dangling bondsin the MWCNT.

Fig. 2b shows the FT-IR spectrum of the PS and graph-ene/MWCNT/PS nanocomposite. The overall spectral pat-tern and number of bands for the PS and nanocompositewere almost similar, except for their intensity. The largeabsorption peak at 3200–3800 cm�1 was assigned to theabsorption of water inside the PS nanoparticles [21]. Thepeaks at 2925 and 2850 cm�1 were assigned to the asym-metrical and symmetrical stretching vibrations of ACH2,respectively. The peak at 1450 cm�1 was assigned to theflexural vibrations of ACH2. The prominent changes inthe spectrum were observed in the region 1587–1678 cm�1. The peak at 1641 cm�1 and 1596 cm�1 for PSmerged to form a new peak at 1632 cm�1, which mightbe due to the formation of a CAC bond between MWCNT,graphene and PS during polymerization initiated by radi-cals on both the MWNT and graphene. This has also beenobserved in the case of PMMA [22]. The vibrationalspectrum of the polystyrene resin beads in the re-gion >1500 cm�1 consisted of several absorption bandswith few structural characteristics, which indicates that itis extremely difficult to obtain an accurate vibrationalassignment and spectroscopic barcode in this region.

Fig. 3 presents the UV–visible spectra of the PS nanopar-ticles and nanocomposite at wavelengths from 200 to800 nm. THF was used as the solvent, which extends thePS chain easily and allows the chromopher to absorb lightwithout significant mutual interference. PS showed ashoulder at approximately 200–400 nm due to the

Fig. 3. UV–visible spectrum of PS and graphene/MWCNT/PS nanocom-posite in a DMF solution.

256 A.S. Patole et al. / European Polymer Journal 48 (2012) 252–259

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

associative interactions between the neighboring phenylgroups. This is because the phenyl group away from thethree carbon atoms and the adequate flexibility of the PSchain in PS allowed free rotation of two phenyl groupsabout the carbon–carbon bond. In the 200–400 nm region,peaks appeared at different wavelengths from lower tohigher. The peaks at lower and higher wavelengths reflectthe isolated phenyl ring and interacted phenyl groups,respectively [23]. The same peaks appeared in the nano-composite but with a decrease in intensity. This may bedue to the addition of graphene and MWCNT, where thePS chains become tangled and cause a screen effect thatdoes not allow the absorption of light by the chromopherinside. The valance to conduction (Vn–Cn) transition ofmetallic MWCNT from 400 to 600 nm, which is called thefirst van Hove transition of metallic nanotubes (M11),was examined [24]. Graphene generally shows two peaks;a broad peak at 273 nm characteristic of a p–p⁄ electrontransition in the polyaromatic system of graphene layers,and a sharp peak at 222 nm, which is probably related toa p–p⁄ electron transition in the polyene type structurefrom the defects of the graphene sheets [25]. In the nano-composite, the absorption peak of graphene and PS mightoverlap, resulting in a broad peak at 273 nm. However,the SEM images revealed few MWCNT anchored by thepolymer dispersed on graphene and many sandwiched be-tween two graphene sheet remained in the form of bun-dles. These bundled MWCNT showed a broader peak inthe first van Hove transitions.

Gel permeation chromatography (GPC) analysis wascarried out on the samples to determine the effect ofgraphene and MWCNT on the molecular weight and poly-dispersity index (PDI) of PS and nanocomposite. In pres-ence of MWCNT and graphene, the molecular weight(Mw) and polydispersity index increased. The PS nanopar-ticles and nanocomposite showed a molecular weight of46,065 and 65,829 and polydispersity index of 168,890and 228,510, respectively. This provides definitive infor-mation of the role of the MWCNT and graphene in thein situ polymerization process. MWCNT affected the poly-merization process by opening p-bonds on the MWCNTsurface making them available to an initiator for radical

polymerization, such as AIBN [26]. The increase in molecu-lar weight shows that the MWCNT and graphene partici-pated in the reaction and consumed the AIBN. The Mw ofthe polymer chain goes increased with decreasing AIBNconcentration due to the lack of accessibility of the initia-tor to the monomer to initiate and form a new polymerchain. Moreover, it was reported that fillers like graphene,MWCNT and Carbon black tend to generate active free rad-icals on the surface due to a mechanochemical effect [27].Therefore, the generation of such radicals due to the con-sumption of AIBN and the mechanochemical effect initi-ates polymerization on graphene and MWCNT, andcontinues to grow the polymer chain on it. However, evenwith the use of the same amount of monomer in the nano-composites, the PDI of the nanocomposite was higher thanthe Pristine PS, which is possibly due to the presence offree PS particles in the aqueous phase. Moreover, most ofthe PS particles were anchored to the MWCNT, resultingin a decrease in the concentration of polymer anchoredto the graphene sheets compared to MWCNT, as observedin SEM, causes to the higher polydispersity index. A poly-dispersity of this magnitude is consistent with diversegrowth rates of the individual polymer chains due to lo-cal-site heterogeneity [16,28].

A thermal transition of a segmental motion of the PSchain in the pristine PS and PS in nanocomposite due tothe effect of an interaction with MWCNT and graphenewas examined by DSC, as shown in Fig. 4a. This effectwas improved by the presence of such a carbonaceous typeof filler. The temperature required to alter the behavior ofthe PS chains in nanocomposite from a glassy to rubberystate was increased by 70 �C compared to that of pure PS.This might be because the heat provided by the systemwas absorbed by the PS chain as well as by graphene andMWCNT in the nanocomposites, which tended to shiftthe Tg to a higher temperature. However, graphene andMWCNT tend to store heat energy. Therefore, in the pres-ence of these types of carbonaceous fillers, the heat energyprovided to the PS chain in the nanocomposite at lowertemperatures is not sufficient to alter the behavior of aPS chain from a glassy to rubbery state. Moreover, a broadshoulder peak was observed between the 150 and 170 �C.This may be due to the PS chain at the edge of graphene,which was not shielded by the carbonaceous fillers, begin-ning to become flexible from 150 �C. The MWCNT sur-rounded by the PS chain, which is sandwiched betweentwo graphene flakes, causes a shielding effect due to thepresence of graphene and MWCNT. This causes slower heatabsorption by PS chain at the MWCNT interface and abroadened peak as well. In the microemulsion at highertemperatures, the PS nanoparticles were anchored to theMWCNT due to Brownian motion in an aqueous mediumby etching the adsorb SDS [29]. On the other hand, manytypes of filler, such as carbon black, graphite silica anquartz, etc., under grinding and ultrasonication, generatelarge active radicals on the surface of the fillers that inducepolymerization due to a mechanochemical effect [27].Therefore, during the collision process between PS nano-particles and fillers, SDS on graphene and MWCNT are re-moved by Brownian motion, and these active centersbecome responsible for initiating polymerization of the

Fig. 4. The comparative thermal properties of PS and graphene/MWCNT/PS nanocomposite (a) DSC, (b) TGA and (Inset b) DTA.

A.S. Patole et al. / European Polymer Journal 48 (2012) 252–259 257

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

monomer to form polymer chains due to the strong con-finement effect of the PS chains on the graphene sheetsand MWCNT, shifting Tg to higher temperatures.

Fig. 4b shows the effect of the MWCNT and grapheneconcentration on the decomposition behavior and thermalstability of the PS and nanocomposites as well as shownthe decomposition behavior of MWCNT and graphene.Both PS and nanocomposite decomposed in a one step pro-cess, and the TGA curve profile of the nanocompositesshifted towards a higher temperature compared to thatof pure polystyrene. The inset of Fig. 4b shows the DTAof TGA, which gives the peak temperature at which themaximum weight loss occurred. The DTA of pure PSshowed a single large peak from 380 �C. The degradationat this temperature was initiated by scissoring of theCAC bond by the thermal energy and was accompaniedby transferred hydrogen to the site of scission. The nano-composite also showed a broad single peak but at 25 �Chigher than that of the pure PS. A previous study of Poly-propylene(PP)/MWCNT and PP/PP grafted with maleicanhydride(PP-g-MN)/clay reported that the peak tempera-ture was increased by 17 and 12 �C, respectively, relative tothat of the respective pure polymer. This effect was attrib-uted due to the barrier labyrinth effect of the clay plate anddispersed nanotubes, which caused a decrease in the diffu-sion of the degraded product of the polymer to the gasphase [30]. In simultaneous research, a similar barrier ef-fect was indicated by the numerous carbon tubes and sev-eral micrometer large graphene sheets in the sampleimpeding the transport of degradation products of polymerinto the gas phase, which was responsible for the enhancedpeak temperature of the nanocomposites. However, thepeak intensity of the nanocomposites was lower than thatof pure PS, which was shifted to a higher peak temperature,indicating the nanocomposite to have higher thermalstability.

Fig. 5 shows a SEM image of the PS composite films pre-pared on glass slides using a solution casting technique.The samples were coated with Platinum (Pt) by sputteringto prevent charge build up. Fig 5a shows a plan view of thenanocomposite film containing 20 wt.% nanocompositebased on the PS matrix. The graphene flakes and MWCNT

were well dispersed in the PS matrix. Moreover, only par-tial sections of the graphene sheets were visible; i.e., partsof the graphene were not observed because they were in-side the matrix. The end of such cloth like graphene ap-pears to be folded and crumbled. The graphene coatedwith the MWCNT network on its surface were also ob-served in the film. However, it can be clearly visible inthe nanocomposite due to the several micrometer size ofthe graphene sheets. Furthermore, the observed wavinessshowed that the nanotube must be curved in the PS as a re-sult of its long length. Fig. 5b and c shows magnified viewof the morphology of graphene and MWCNT in the com-posites. The graphene sheets were pulled out from the ma-trix PS. In addition, MWCNT showed greater dispersabilityin PS, where less bundled nanotubes can be seen in Fig. 5c.Fig. 5e shows that the graphene paper was bended in thepolymer matrix. A higher magnification image Fig. 5f con-firmed that the edges of graphene were folded and its thinseparated layer was exposed from the polymer matrix.However, the agglomeration of MWCNT was also observed,as shown in Fig. 5d.

Fig. 6a shows the storage modulus (E0) obtained fromDynamic mechanical analysis (DMA) of the PS and nano-composites films containing mixing 20 wt.% nanocompos-ite based on the matrix PS. E0 is the storage modulus,which is representative of the elastic behavior. The changein modulus shows the change in rigidity, and ultimatelythe strength of the sample. The films were characterizedin the constant frequency temperature scan (100 HZ,2 �C min�1) to determine the effect of the elastic anddamping behavior of the films. It shows the effect of theMWCNT and graphene concentration on the storage mod-ulus. As expected, the storage modulus of the nanocom-posite film was higher than PS film. Therefore, increasesin the storage modulus suggest that the presence ofMWCNT and graphene may transfer more stiffness to thepolymer matrix or these carbonaceous fillers act as rein-forcements in the polymer matrix by transferring the loadfrom the polymer to these fillers.

Fig. 6b shows that the presence of MWCNT and graph-ene in the nanocomposites leads to a progressive increasein peak height and a move to a higher temperature in loss

Fig. 5. SEM images of (a) Top view (b, c, and e) cross view (d) close view of the dispersion of MWCNT at the edges of the cross view and (f) magnify view ofthe graphene sheet.

Fig. 6. Comparative study of the (a) Storage modulus and (b) Loss modulus of the PS film and nanocomposite film as a function of the nanocompositereinforced into the PS matrix.

Fig. 7. Sheet resistance of the films as a function of the nanocompositecontent in the PS matrix.

258 A.S. Patole et al. / European Polymer Journal 48 (2012) 252–259

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

modulus. MWCNT and graphene restrict the segmentalmotion of the polymer chain in the nanocomposites,resulting a higher Tg. Furthermore, MWCNT and grapheneact as a resistance to the viscous flow of the polymer chainin the glass transition region. Interestingly these carbona-ceous fillers show improved elastic properties of the com-posites in the Tg region, suggesting good compatibilitybetween the matrix polymer and nanocomposite in thefilm at elevated temperatures.

Fig. 7 shows the relationship between the concentrationof the nanocomposite present in the PS matrix and sheetresistance. The observed relationship is consistent withthe expectation that the film resistivity decreases withincreasing nanocomposite filler content in the polymermatrix. The film with a 2 and 20 wt.% nanocomposite inthe PS matrix showed a film resistivity of 2.7 � 107 X/hand 1.0 � 105 X/h, respectively, indicating that electrical

A.S. Patole et al. / European Polymer Journal 48 (2012) 252–259 259

conduction within the composite film occurs via CNT andgraphene percolation. The improvement in sheet resis-tance in the nanocomposite was attributed to the forma-tion of an extended conjugation network within the CNTbridging the gap between the graphene sheets. The largegraphene sheet provides most of the total surface area,whereas the CNT act as wires connecting the large pad to-gether in the composite matrix film [31]. Furthermore, theresistivity in both faces of the film were in the same orderof magnitude, which is indicative of a uniform nanocom-posite distribution within the film and across the filmsurfaces.

MA

CRO

MO

LECU

LAR

NA

NO

TECH

NO

LOG

Y

5. Conclusions

This paper reports a novel strategy for functionalizinggraphene and MWCNT together with polystyrene nanopar-ticles using an in situ microemulsion technique. In thisinterconnected nanocomposite network, the large graph-ene sheet provides most of the total surface area, whereasthe CNT act as wires connecting the large pad together inthe composite matrix film. Such a structure provides a con-ducting path that decreases the sheet resistance and im-proves the thermal and mechanical properties ofpolystyrene films. Overall, this novel hybrid nanostructurehas the potential to combine the major advantages of itsindividual components to produce significant improve-ment in their current applications.

Acknowledgment

This study was supported by Basic Science ResearchProgram through the National Research Foundation of Kor-ea (NRF) funded by the Ministry of Education, Science andTechnology (2011-0006268) and a grant (No. 10037449)from the Fundamental R&D Program for Core Technologyof Materials funded by the Ministry of Knowledge Econ-omy, Republic of Korea. This work was partly supportedby the GRRC program of Gyeonggi province [(GRRC Sung-kyunkwan 2010-B10), Development of Carbon nano com-posite material for light-weight vehicle]. S.P.P. is gratefulto the Korean government for the BK-21 fellowship.

References

[1] Yu D, Dai L. Self-assembled graphene/carbon nanotube hybrid filmsfor supercapacitors. J Phys Chem Lett 2010;1:467–70.

[2] Li C, Li Z, Zhu H, Wang K, Wei J, Li X, et al. Graphene nano-‘‘patches’’on a carbon nanotube network for highly transparent/conductive thinfilm applications. J Phys Chem C 2010;114:14008–12.

[3] Geim AK, Novolselov KS. The rise of graphene. Nat Mater2007;6:183–91.

[4] Chen JH, Jang C, Xiao S, Ishigami M, Fuhreri MS. Intrinsic and extrinsicperformance limits of graphene devices on SiO2. Nat Nanotechnol2008;3:206–9.

[5] Stankovich S, Dikin DA, Dommett GHB, Kohlhaas KM, Zimney EJ, StachEA, et al. Graphene-based composite materials. Nature2006;442:282–6.

[6] Xu Y, Bai H, Lu G, Li C, Shi G. Flexible graphene films via the filtrationof water-soluble noncovalent functionalized graphene sheets. J AmChem Soc 2008;130:5856–7.

[7] Becerril HA, Mao J, Liu Z, Stoltenberg RM, Bao Z, Chen Y. Evaluation ofsolution-processed reduced graphene oxide films as transparentconductors. ACS Nano 2009;2:463–70.

[8] Wu ZS, Pei S, Ren W, Tang D, Gao L, Liu B, et al. Field emission ofsingle-layer graphene films prepared by electrophoretic deposition.Adv Mater 2009;21:1756–60.

[9] Yu C, Kim YS, Kim D, Grunlan JC. Thermoelectric behavior ofsegregated-network polymer nanocomposites. Nano Lett2008;8:4428–32.

[10] Sun Q, Deng Y, Wang ZL. Synthesis and characterization ofpolystyrene-encapsulated laponite composites via miniemulsionpolymerization. Macromol Mater Eng 2004;289:288–95.

[11] Shim BS, Chen W, Doty C, Xu C, Kotov NA. Smart electronic yarns andwearable fabrics for human biomonitoring made by carbonnanotube coating with polyelectrolytes. Nano Lett 2008;8:4151–7.

[12] Ramanathan T, Abdala AA, Stankovich S, Dikini DA, Herrera-AlonsoM, Pineri RD, et al. Functionalized graphene sheets for polymernanocomposites. Nat Nanotechnol 2008;3:327–31.

[13] Koerner H, Price G, Peatce NA, Alexander M, Vaia RA. Remotelyactuated polymer nanocomposites—stress-recovery of carbon-nanotube-filled thermoplastic elastomers. Nat Mater2004;3:115–20.

[14] Razal JM, Coleman JN, Munoz E, Lund B, Gogotsi Y, Ye H, et al.Arbitrarily shaped fiber assemblies from spun carbon Nanotube gelfibers. Adv Funct Mater 2007;17:2918–24.

[15] Putz KW, Compton OC, Palmeri MJ, Nguyen ST, Brinson LC. High-nanofiller-content graphene oxide–polymer nanocomposites viavacuum-assisted self-assembly. Adv Funct Mater 2010;20:3322–9.

[16] Yana J, Weia T, Fana Z, Qianb W, Zhanga M, Shena X, et al.Preparation of graphene nanosheet/carbon nanotube/polyanilinecomposite as electrode material for supercapacitors. J Pow Sources2010;195:3041–5.

[17] Patole SP, Alegaonkar PS, Lee HC, Yoo JB. Optimization of waterassisted chemical vapor deposition parameters for super growth ofcarbon nanotubes. Carbon 2008;46:1987–93.

[18] Patole AS, Patole SP, Kang H, Yoo JB, Kim TH, Ahn JH. A facileapproach to the fabrication of graphene/polystyrene nanocompositeby in situ microemulsion polymerization. J Coll Interf Sci2010;350:530–7.

[19] Schniepp HC, Kudin KN, Li JL, Prud’homme RK, Car R, Saville DA, et al.Bending properties of single functionalized graphene sheets probedby atomic force microscopy. ACS Nano 2008;2:2577–84.

[20] Jorio A, Pimenta MA, Souza Filho AG, Saito R, Dresselhaus G,Dresselhaus MS. Characterizing carbon nanotube samples withresonance Raman scattering. New J Phys 2003;5:139.1–17.

[21] Huanga ZL, Liua LX, Leib H, Zhao YD. Vibrational spectroscopicencoding of polystyrene resin bead: a combined FT-IR andcomputational study. J Mol Struct 2005;738:155–9.

[22] Park SJ, Cho MS, Lim ST, Choi HJ, Jhon MS. Synthesis and dispersioncharacteristics of multi-walled carbon nanotube composites withpoly(methyl methacrylate) prepared by in-situ bulk polymerization.Macromol Rapid Commun 2003;24:1070–3.

[23] Li T, Zhou C, Jiang M. UV absorption spectra of polystyrene. PolymBull 1991;25:211–6.

[24] Goyanes S, Rubiolo GR, Salazar A, Jimeno A, Corcuera MA,Mondragon I. Carboxylation treatment of multiwalled carbonnanotubes monitored by infrared and ultraviolet spectroscopiesand scanning probe microscopy. Diam Rel Mater 2007;16:412–7.

[25] Zhang W, Cui J, Tao C, Wu Y, Li Z, Ma L, et al. A strategy for producingpure single-layer graphene sheets based on a confined self-assemblyapproach. Angew Chem Int Ed 2009;48:5864–8.

[26] Jia Z, Wang Z, Xu C, Liang J, Wei B, Wu D, et al. Study on poly(methylmethacrylate): carbon nanotube composites. Mater Sci Eng A1999;271:395–400.

[27] Lu L, Zhou Z, Zhang Y, Wang S, Zhang Y. Reinforcement of styrene–butadiene–styrene tri-block copolymer by multi-walled carbonnanotubes via melt mixing. Carbon 2007;45:2621–7.

[28] Li L, Lukehart CM. Synthesis of hydrophobic and hydrophilicgraphitic carbon nanofiber polymer brushes. Chem Mater2006;18:94–9.

[29] Patole AS, Patole SP, Yoo JB, Ahn JH, Kim TH. Effective in situsynthesis and characteristics of polystyrene nanoparticle-coveredmultiwall carbon nanotube composite. J Polym Sci Part B: PolymPhys 2009;47:1523–9.

[30] Kashiwagi T, Grulke E, Hilding J, Harris R, Awad W, Douglas J.Thermal degradation and flammability properties ofpoly(propylene)/carbon nanotube composites. Macromol RapidCommun 2002;23:761–5.

[31] Tung VC, Chen LM, Allen MJ, Wassei JK, Nelson K, Kaner RB, et al.Low-temperature solution processing of graphene–carbon nanotubehybrid materials for high-performance transparent conductors.Nano Lett 2009;9:1949–55.