effect of modified graphene and microwave irradiation on the mechanical and thermal properties of...

Research article

Received: 18 February 2014 Revised: 17 June 2014 Accepted: 25 June 2014 Published online in Wiley Online Library: 4 August 2014

(wileyonlinelibrary.com) DOI 10.1002/sia.5630

630

Effect of modified graphene and microwaveirradiation on the mechanical and thermalproperties of poly(styrene-co-methylmethacrylate)/graphene nanocompositesMukarram Zubair,a Jobin Jose,a Abdul-Hamid Emwasb

and Mamdouh A. Al-Harthia,c*

The effect of modified graphene (MG) and microwave irradiation on the interaction between graphene (G) and poly(styrene-co-methyl meth acrylate) [P(S-co-MMA)] polymer matrix has been studied in this article. Modification of graphene was performedusing nitric acid. P(S-co-MMA) polymer was blended via melt blending with pristine and MG. The resultant nanocomposites wereirradiated under microwave at three different time intervals (5, 10, and 20min). Compared to pristine graphene, MG showedimproved interaction with P(S-co-MMA) polymer (P) after melt mixing and microwave irradiation. The mechanism of improveddispersion and interaction ofmodified graphenewith P(S-co-MMA) polymermatrix duringmeltmixing andmicrowave irradiationis due to the presence of oxygen functionalities on the surface of MG as confirmed from Fourier transform infrared spectroscopy.The formation of defects onmodified graphene and free radicals on P(S-co-MMA) polymer chains after irradiation as explained byRaman spectroscopy and X-Ray diffraction studies. The nanocomposites with 0.1wt% G and MG have shown a 26% and 38%increase in storage modulus. After irradiation (10min), the storage modulus further improved to 11.9% and 27.6% of nanocom-posites. The glass transition temperature of nanocomposites also improved considerably after melt mixing and microwaveirradiation (but only for polymer MG nanocomposite). However, at higher irradiation time (20min), degradation of polymernanocomposites occurred. State of creation of crosslink network after 10min of irradiation and degradation after 20min ofirradiation of nanocomposites was confirmed from SEM studies. Copyright © 2014 John Wiley & Sons, Ltd.

Keywords: styrene; methyl methacrylate; copolymer; modified graphene; microwave irradiation

* Correspondence to: M. A. Al-Harthi, Department of Chemical Engineering, KingFahd University of Petroleum and Minerals, 31261 Dhahran, Saudi Arabia.E-mail: [email protected]

a Department of Chemical Engineering, King Fahd University of Petroleum andMinerals, 31261, Dhahran, Saudi Arabia

b NMR Core lab, King Abdullah University of Science and Technology, 23955,Thuwal, Saudi Arabia

c Center of Research Excellence in Nanotechnology, King Fahd University ofPetroleum and Minerals, 31261, Dhahran, Saudi Arabia

Introduction

Graphene, a single layer sp2-hybridized carbon atom arrangedin the two dimensional densely packed honeycomb crystallattice, has opened a new outstanding and cost-effective cor-ridor to formulate a broad variety of novel nano materials.[1]

The remarkable properties of graphene with low cost ofsource (graphite) have attracted interest in developing high-performance and low-cost polymer nanocomposites.[2–4]

Chemical modification or functionalization of graphene, suchas oxidation of graphene[5] by adding oxygen functionalitieslike hydroxyl, carboxylic acid, and other organic groups likephenyl isocynate,[6] prophyrin[7] and epoxy groups[8] has beenrecently investigated to succeed full exploitation of grapheneproperties in the polymer nanocomposites. The functionalizedgraphene (i) possess similar properties as graphene except apartly damaged carbon structure and (ii) functionalities presenton the surface may respond to the improvement of dispersionof graphene and interfacial interaction between graphene andpolymer matrix.Irradiation technique has been considerably used for the

modification of structural, electrical, mechanical, chemical, andother desired properties of the polymer nanocomposites.[9–12]

The radiation mechanism accounts for the generation of freeradicals on the polymer chains[13] and induced defects ongraphene.[14–16] This responses to the major reactions like cross-

Surf. Interface Anal. 2014, 46, 630–639

linking, chain scission (degradation) and grafting[17] in polymernanocomposites. This may result in the improvement of theinterfacial interaction between polymer matrix and graphene.

In this study, the copolymer of styrene and methyl methacrylate [P(S-co-MMA)] is used as a host material. This is widelyused in various fields, such as microelectronics, protectivecoatings, bio materials and solar technology etc.[18–20] Blends ofstyrene-methyl methyl acrylate copolymer with pristine andmodified graphene were irradiated using microwave radiationtechnique for different periods. The objective of this study is toexamine the changes in the mechanical and thermal propertiesof the P(S-co-MMA)/graphene nanocomposites by using twomodes: modification of graphene and microwave irradiation.Additionally, the effect of microwave irradiation on the chemical

Copyright © 2014 John Wiley & Sons, Ltd.

Microwave irradiated poly(S-co-MMA)/Graphene nanocomposites

structure, interaction of pristine and modified graphene onpolymer matrix, and surface morphology of the nanocompositesis discussed.

Experimental

Raw materials

Styrene (99%), methyl methacrylate (MMA, 99%), and benzoylperoxide were all purchased from Sigma-Aldrich and wereused as received. Tetra hydroforan, methanol, and nitric acid(97%) were obtained from Pure Chemika. Graphene (96–99%,50–100 nm) was purchased from Grafen Chemical IndustriesCo (Turkey).

Polymerization of poly(styrene-co-methyl methacrylate)

Poly(styrene-co-methyl methacrylate) is synthesized by freeradical polymerization. Benzoyl peroxide was used as initiator,0.1 wt% of total volume of monomers. Reaction took place inround bottom flask equipped with magnetic stirrer at 110 °Cfor 5 h under nitrogen environment. After reaction, tetrahydroforan (60ml per 10ml of monomer) was added in tothe round bottom flask and kept for 2–4 days to dissolve theproduct. The dissolved polymer solution is then precipitatedin excess amount of methanol and then dried in oven at40 °C for at least 24 h.

Scheme 1. Chemical oxidation of graphene using nitric acid.

Table 1. Composition of P(S-co-MMA) and its composites

Sample name CopolymercompositionP(S-co-MMA)*

P(S-cocont

P(S-co-MMA) 70.6/29.4

PG(0) 70.6/29.4

PMG(0) 70.6/29.4

PG(5) 70.6/29.4

PMG(5) 70.6/29.4

PG(10) 70.6/29.4

PMG(10) 70.6/29.4

PG(20) 70.6/29.4

PMG(20) 70.6/29.4

* Copolymer composition is calculated using Proton-NMR.

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Modification of graphene

Chemical modification of graphene was carried out throughthermal oxidation method.[21] First 300ml of concentrated nitricacid (69%, AnalaR grade) was added to 2 g of graphene (asreceived) in 1000ml round bottom flask. The mixture wasrefluxed at 120 °C for 48 h to produce maximum oxidation andthen cooled to room temperature. The reaction mixture wasdiluted with 500ml of deionized water and vacuum-filtered using3μm porosity filter paper. The washing operation using deion-ized water was repeated until the pH became similar to deionizedwater. The final product was then dried in a vacuum oven at100 °C. Chemical modification of graphene leads to the forma-tion of oxygen-based functionalities (carboxylic, carbonyl, andhydroxyl groups) on the defects sites and sides walls ofgraphene (Scheme 1).

Preparation of nanocomposites

P(S-co-MMA)/graphene (PG) and P(S-co-MMA)/modified graphene(PMG) nanocomposites were prepared using a MiniLab TorqueRheometer. 0.1 wt% of pristine and MG was added to 6 g ofP(S-co-MMA) copolymer and mixed for 10min at a tempera-ture of 180 °C at a speed of 60 rpm. Thin sheets of the com-posites with approximate thickness of 1mm were preparedby compressionmolding for 8min at a temperature of 140 °C under97MPa pressure and cooled to room temperature. Table 1 illus-trates the composition of different samples produced in this study.

-MMA)ent (g)

Graphene/modifiedgraphene content (mg)

Irradiationtime (min)

6 0/0 0

6 6/0 0

6 0/6 0

6 6/0 5

6 0/6 5

6 6/0 10

6 0/6 10

6 6/0 20

6 0/6 20

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Microwave irradiation method

Microwave irradiation of PG and PMG nanocomposites werecarried out at frequency of 2450MHZ at fixed power of1000watt with different treatment time. The irradiation wascarried out using domestic microwave oven with internalturnable table.

The detailed procedure for irradiation of sample is givenbelow

• Samples of dimension (4 × 10× 1mm) were treated at dif-ferent treatment time at constant power of 1000watt in thepresence of air.

• Irradiation was performed at cycle of 60 s in the presence ofair. After each cycle, the sample was then cooled to room tem-perature for 120 s, to avoid the effect of heat on the polymergraphene composite sample.

• Total irradiation treatment time was 5, 10, and 20min.

Characterization

XPS analysis

X-ray photoelectron spectroscopy studies were carried out in aKratos Axis Ultra DLD spectrometer equipped with a monochro-matic Al Kα X-ray source (hn= 1486.6 eV) operating at 150W, amultichannel plate and delay line detector under 1.0 × 10�9 Torrvacuum. The survey and high-resolution spectra were collectedat fixed analyzer pass energies of 160 and 20 eV, respectively.

Spectroscopic analysis

The Fourier transform infrared spectra (FTIR) are recorded byusing Nicolet 6700 spectrometer with resolution of 4 cm�1. In or-der to analyze the functional group like carbonyl and hydroxylgroup before and after irradiation of samples, the band range1500–1725 cm�1 was used. For Raman spectroscopy, RamanAramis (Horiba JobinYvon) instrument with Laser power of0.7mW and resolution of 473 nm was used. The composition ofstyrene and methyl meth acrylate in copolymer was calculatedby using Proton-NMR spectra estimated at room temperatureusing Bruker 500MHZ spectrometer.

Dynamic mechanical analysis

The dynamic mechanical properties of the samples before andafter irradiation is investigated in a temperature range of40–160 °C in the tension mode at a heating rate of 5 °C/minand a frequency of 1 Hz using Perkin Elmer dynamic mechan-ical analysis (DMA) Q-800. The dynamic mechanical propertiesare tested under nitrogen environment at a load of 5N withthe average sample size 4 × 10 × 1mm.

Differential scanning calorimetry

The glass transition temperature of the samples was determinedby using differential scanning calorimetry (DSC)-Q1000, TAinstrument. Samples are weighed with ±0.5mg accuracy, andexperiments were carried out under nitrogen environment. Thefirst stage of heating was carried out to remove the thermalhistory of the sample if any. The cooling step was performed ata rate of 5 °C/min, and the final heating at a rate of 10 °C/minwas carried out to determine the Tg of the sample.

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X-ray diffraction

X-ray diffraction (XRD) studies were carried out using D8 advancex-ray instrument with wavelength of λ = 1.542Aº and 2θ rangefrom 2° to 70°.

Scanning electron microscopy

Scanning electron micrographs were taken by using JSM-6460LV(Jeol) SEM. Prior to the experiment, the samples were cryo-fractured using liquid nitrogen, and the cross section was sputtercoated with gold for 2min to make the surface conductive.

Results and discussion

The presence of oxygen groups on the surface of MG not onlyimproved the interfacial interaction with polymer matrixduring melt blending but also developed greater influenceof microwave irradiation. Therefore, before and after microwaveexposure, the P(S-co-MMA)/modified graphene (PMG) comparedto P(S-co-MMA)/graphene (PG) nanocomposites, resulted inbetter improvement of the interfacial interaction betweenmodified graphene and polymer matrices as demonstrated inScheme 2. This assisted to develop cross-linked network andresults in enhanced mechanical and thermal properties of PMGnanocomposites.

XPS analysis

The modification of graphene with increased oxygen func-tionality on its surface was confirmed by XPS analysis. The XPSspectra of both modified and unmodified graphene revealedthe increased oxygen functionality after modification. The surveyspectra showed that the C¼O and O–C¼O components wereincreased by 0.6% and 0.5%, respectively (Fig. 1). The C/O ratioof graphene and modified graphene from the XPS analysis werecalculated as 98.4/1.6 and 97.4/2.6, respectively.

FTIR analysis

The structural changes in pristine graphene (G) after chemi-cal oxidation and for the nanocomposites before and after ir-radiation were examined using FTIR spectroscopy also. InFig. 2 for modified graphene (MG) spectra, the characteristicsvibrations include C–O stretching peak at 1016 and1102 cm�1, the C–O–C peak at 1260 cm�1, C¼C stretchingpeak at 1620 cm�1, and C–OH peak at 3443 cm�1.[22] The in-tensity of hydroxyl group in MG is lower than G (Fig. 2),which may be due to the reaction of hydroxyl group duringchemical oxidation.

In Fig. 3(a–b), the spectrum of nonirradiated and irradiated PGand PMG nanocomposites retained the similar trend except therewas some change in the intensity of the absorption band ofcarbonyl groups (C¼O) at peak 1725 and 1770 cm�1 andaromatic group of styrene (C¼C) at peak 1600 cm�1..[22]

For nonirradiated and 10min irradiated PG nanocomposites,the C¼O absorption band at peak 1725 cm�1 decreased to lowerintensity as compared to P(S-co-MMA). This attributes to thereaction of carbonyl group of P(S-co-MMA) with the graphenesurface after melt mixing and 10min irradiation.

In Fig. 3(b), the increasing behavior in the intensity of carbonylgroup at peak 1725 cm�1 of PMG nanocomposites up to 10minirradiation indicates the interaction between the oxygen

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Scheme 2. Improvement of interaction between graphene and polymer matrices through chemical oxidation and microwave irradiation.

Figure 1. XPS spectra of pristine graphene and modified graphene.

Figure 2. FTIR spectra of pristine and modified graphene.


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functionalities on the structure of MG and the methylacrylate(�COOCH3) functionality of P(S-co-MMA).[23] In addition, thepeak at 1600 cm�1 that corresponds to the aromatic vibration


of P(S-co-MMA) shifted to lower intensity level in nonirradiated and10min irradiated PG and PMG nanocomposites. This may be due tothe grafting of styrene chains on the graphene and MG surface.

At longer duration of microwave irradiation (20min), anincrease in the intensity of C¼O group at peak 1725 cm�1 of PGnanocomposites attributed to the photo degradation mechanismof PG nanocomposite (chain scission of carbonyl groups followedby the oxidation). This behavior is supported by the hypothesis ofchain scission followed by oxidation process as reportedearlier.[24,25] For 20min irradiated PMG nanocomposite, there isonly the reduction of intensity of C¼O group, which is associatedwith the chain scission and breakage of carbonyl bond ofmodified graphene with P(S-co- MMA). At 20min of irradiation,the degradation mechanism of PG nanocomposites reached tooxidation process while the carbonyl group chain scissionoccurred in case of PMG nanocomposites.

Raman analysis

Figures 4 and 5 show the assessment of Raman spectra of (a)pristine and modified graphene, (b) nonirradiated and irradiatednanocomposites. The main features of Raman spectra areD-band, G-band, and 2D-band at peaks 1357 cm�1, 1583 cm�1,and 2700 cm�1, respectively. The D-band (disordermode) associated


Figure 3. (a–b): FTIR spectra’s of control P(S-co-MMA) and nonirradiated and irradiated PG (b), nonirradiated and irradiated PMG (c).

Figure 4. Raman spectra of pristine and modified graphene.

Figure 5. Raman spectra of nonirradiated and irradiated PG and PMG.

Table 2. ID : IG ratio of pristine and modified graphene and non-irradiated and irradiated PG and PMG nanocomposites

Samples D-peak(�1357)intensity

G-peak(�1583)intensity

ID/IG

Graphene 92.1 863.1 0.11

Modified

graphene

939.1 1177.1 0.79

PG(0) 158.7 300.1 0.52

PMG(0) 673.2 802.3 0.83

PG(10) 1413.1 1491.6 0.94

PMG(10) 2091.7 2130.9 0.98

PG(20) 2639.8 2776.8 0.95

PMG(20) 3332.5 3421.6 0.97

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to the out-plane breathing mode of sp2 atoms. D-band is the reveal-ing of the existence of the disorder in graphene[26,27] and a best toolto evaluate the level of defects that appears in graphene. G-bandcorresponds to the E2g phonon at the center of the Brillouin zoneor due to the sp2 C¼C stretching vibrations.[28] The presence of de-fects on graphene acted as potential active sites to form covalentbonds with P(S-co-MMA) polymer chains during microwave irradia-tion. The 2D–band is used to inspect the quality of graphene.

In Raman spectra of modified graphene (Fig. 4), reduction in theintensity of G-peak and 2D-peak with respect to pristine graphenewas observed. This indicates the breakage of sp2 C¼C bond ofgraphene which results in the formation of oxygen-basedfunctionalities on the surface of graphene. Increase in the ratio ofintensity of D-band to the intensity of G-band (ID : IG) of modifiedgraphene compared to pristine graphene as shown in Table 2,clearly indicates the oxidation of graphene after modification.[29]

In Fig. 5, significant decrease in the intensity of G-peak and2D-peak in nonirradiated PG and PMG was observed com-pared to pristine and modified graphene. This may be dueto the breakage of pristine and modified graphene structureduring the melt blending and leads to the attachment of P(S-co-MMA) chains on pristine and MG surface. In addition, the ID :IG ratio (Table 2), which reveals the level of defects, is higher invalue of nonirradiated PMG compared to nonirradiated PG nano-composites. This is due to the better interaction and high grafting


Figure 7. X-ray diffraction of nonirradiated and irradiated PG and PMG.


of P(S-co-MMA) chain on the surface of modified graphenecompared to pristine graphene.[30]

In Raman spectra of 10min irradiated PG and PMG nano-composites (Fig. 5), the increase the intensity of D-band wasobserved. This refers to the formation of defects in pristine andmodified graphene induced by irradiation. The ID : IG ratio of PGand PMG nanocomposites (Table 2) increased from 0.52 to 0.96for PG and from 0.83 to 0.98 for PMG. This increase in the ID : IGratio is associated with formation of disorder in pristine and MGand was explained by Ferrari and Robertson theory[27] (that thecrystalline structure of graphene transform to nanocrystallinegraphene). This structural modification leads to the improvementin interaction and covalent bond formation between P(S-co-MMA) chains with pristine and modified graphene. Moreover,the ID : IG ratio, of nonirradiated and irradiated PMG is greaterthan the all PG nanocomposites (Table 2). This is attributed tothe better interaction of modified graphene with P(S-co-MMA)chains than pristine graphene after melt mixing and microwaveirradiation.

The Raman spectra of 20min irradiated PG and PMG nano-composites (Fig. 5) showed further increase in the intensity ofD-peak and G-peak. This refers to more defects formation onpristine and MG. The ID : IG ratio of 20min irradiated of PG andPMG showed the decreasing behavior compared to 10minirradiated PG and PMG nanocomposites. It means that at20min of irradiation, the pristine and modified graphene struc-ture start to transform from nanocrystalline structure to amor-phous phase enlightened by Ferrari and Robertson.[27] Theformation of amorphous structure of pristine and modifiedgraphene at 20min of irradiation may outcomes weakening theinterfacial interaction with P(S-co-MMA) chains and henceresulted in reduction in mechanical and thermal propertiesof the nanocomposites as discussed later in this article. Thisis also in conformity with the results obtained from DMAand DSC analysis.

XRD analysis

The changes appeared in the crystal lattice of graphene aftermodification and dispersion of the nano filler in the polymermatrix were evaluated using XRD patterns. Figure 6 displays thatthe diffraction peak of graphene observed at 26.9° and modified

Figure 6. X-ray diffraction of pristine and modified graphene.


graphene diffraction peak at 18.9°. The layer to layer spacing ofmodified graphene calculated using Bragg’s equation is0.47 nm, which is slightly higher than the pristine graphene(0.33 nm). This refers to the presence of oxygen functionalitiesand moisture content.[31] Reduction of diffraction peak intensityof G and MG is observed in the XRD patterns of nonirradiatedPG and PMG (Fig. 7), respectively. This is attributed to thebreakage of G and MG structure and leads to the exfoliation inthe P(S-co-MMA) polymer matrix after melt blending. Thepresence of oxygen functionalities (polar groups) on the surfaceof modified graphene, confirmed by FTIR, enhanced the interac-tion with microwave irradiation and caused better interaction ofMG in P(S-co-MMA) matrix. This caused further reduction ofdiffraction peak of MG in XRD patterns of 10minutes irradiated–PMG (Fig. 7).

DMA analysis

The storage modulus of control P(S-co-MMA), PG, and PMG nano-composites before and after irradiation were evaluated usingDMA (Table 3). After the addition of G or MG in P(S-co-MMA)

Table 3. Storage modulus of control polymer, PG and PMG nano-composites before and after microwave irradiation

Sample name Storage modulus (MPa) at

(40 °C) (100 °C)

Control P(S-co-MMA) (0) 1332 1184




PG(0) 1462 1165

PG(5) 1604 1198

PG(10) 1636 1169

PG(20) 1254 1025

PMG(0) 1603 1382

PMG(5) 1941 1631

PMG(10) 2048 1660

PMG(20) 1628 1305


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Figure 8. Storage modulus of control P(S-co-MMA), PG and PMG before and after irradiation.

Figure 9. Glass transition temperature of control P(S-co-MMA), nonirra-diated and irradiated PG.

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polymer matrix via melt blending, the mechanical properties ofboth PG and PMG nanocomposites enhanced compared tocontrol P(S-co-MMA). For example, incorporation of 0.1wt% Gand MG in PG and PMG resulted in an increase of storagemodulus (at 40 °C) of about 10% and 20%, respectively, com-pared to control P(S-co-MMA) (Fig. 8a–c).Upon exposure to microwave radiation, significant improve-

ment in storage modulus was achieved in control sample as wellas both the PG and PMG nanocomposites. At 10min irradiation ofthe nanocomposites, the storage modulus (at 40 °C) increasedfrom 1462MPa to 1636MPa for PG nanocomposite and from1603MPa to 2048MPa for PMG nanocomposite. This is aboutincrease of 11.9% and 27.76% of storage modulus after 10minof microwave irradiation (Fig. 8b–c) of PG and PMG nanocompos-ites, respectively. This enhancement in storage modulus mayrefers to the influence of three factors: (i) intrinsic mechanicalproperty of graphene and modified graphene, (ii) improvementin interaction of graphene and modified graphene in P(S-co-MMA) matrix due to structural changes by microwave irradiation,which is also proven by Raman results and (iii) formation ofcovalent bonds between G and MG with P(S-co-MMA) chains.The latter two factors (b and c) are observed stronger in case ofmodified graphene due to the presence of polar groups on itssurface, which improved the interaction of MG after microwaveradiation. Therefore, it results in more stronger and high storagemodulus composite compared to unmodified graphene polymernanocomposite.The storage modulus of control P(S-co-MMA) was reduced

significantly after 20min microwave irradiation. Similarly, thestorage modulus of PG and PMG were reduced (by 23% and20% with respect to 10 minutes irradiated samples) after aprolonged period of microwave irradiation (20min) (Fig. 8b–c).This is attributed to the impact of two factors: (i) chain scissionand photo degradation of the MMA in P(S-co-MMA), which leadsthe formation of oxygen-based functionalities and (b) transfor-mation of crystalline phase of G and MG into amorphous phaseas evident from Raman results. These two factors results in lowerinterfacial adhesion of G or MG with copolymer matrix and thusproduces a weak polymer graphene nanocomposites.

Figure 10. Glass transition temperature of control P(S-co-MMA), nonirra-diated and irradiated PMG.

DSC analysis

Glass transition temperature is a macroscopic property, which isthe measure of relaxation behavior of polymer and polymernanocomposites. The Tg of graphene based polymer composites,especially nonpolar polymer were not significantly tailoredcompared to the polar polymers. For example, there was an


approx 10 °C rise of Tg of PS-composite containing 1.5wt% ofnano gold. This represents a significant improvement in Tg ofPS-composite.[32] Figures 9 and 10 illustrated that there isincrease of 3.2 °C and 5.1 °C in Tg of the nanocompositescontaining 0.1wt% of G and MG, respectively. The higher value



of Tg of PMG compared to PG composite indicates that modifiedgraphene has better interaction with P(S-co-MMA) matrix due tothe presence of oxygen functional groups on the surface ofmodified graphene. In case of control P(S-co-MMA), the Tg washardly affected by microwave irradiation for different durations.There was marked improvement in the Tg of both PG and PMGthat was observed on exposure to microwave irradiation up to10min (i.e. increase from 93.21 °C to 97.77 °C (Fig. 9) and from95.14 °C to 100.04 °C (Fig. 10) of PG and PMG, respectively. Thisindicates a better interaction and covalent bonding betweenmodified or unmodified graphene with P(S-co-MMA) matrix.Degradation of PG and PMG nanocomposites produced becauseof the breakage of polymer chains that created weak interactionbetween graphene and P(S-co-MMA) matrix. The degradationof polymer nanocomposites outcomes in reduction of Tgvalue as observed at 20min of irradiation of PG and PGMnanocomposites (Fig. 10).

Figure 11. SEM images of the control P(S-co-MMA).

Figure 12. (a–f): SEM images of 0, 10 and 20min irradiated samples of PG (


SEM analysis

The surface morphology of the irradiated and nonirradiated PGand PMG nanocomposites were evaluated by SEM. The SEMimage of nonirradiated PG in Fig. 12(a) shows the fractured roughsurface with discrete patterns compared to control P(S-co-MMA)(Fig. 11). This reveals the reinforcement effect of graphene inthe polymer matrix.[33] In contrast, the good interfacial interac-tion between modified graphene and polymer matrix results amuch smoother and continuous surface morphology of PMGcomposite [Fig. 12(d)] with respect to control P(S-co-MMA) andnonirradiated PG nanocomposite. This is attributed to the highermechanical and thermal properties of PMG nanocompositecompared to PG nanocomposite. The rough surface of PG nano-composite [Fig. 12(a)] also revealed the low interfacial interactionbetween graphene and polymer matrix.

The interaction between graphene and polymer matrix wasimproved on exposure to microwave irradiation for 10min [Fig. 12(b)]. The rough and discrete surface morphology has changedinto smooth and continuous surface that resembles to the nonir-radiated PGM nanocomposite [Fig. 12(d)]. In Fig. 12(e), the fibrouslike cross-linked network structure has appeared on the PGMnanocomposite after 10min of irradiation. This fibrous structurestrengthened the PGM nanocomposite, which is in agreementwith the increased storage modulus and glass transition asreported earlier in Table 3.

In Fig. 12(c) and (f), breakage of polymer chains caused the for-mation of voids on the surface of PG and PGM nanocompositesafter 20min of irradiation. The degradation of polymer chains athigh microwave treatment made the nanocomposites weak andreduced the storage modulus as explained from the DMA results.In addition, Fig. 12(f) showed that some fibrous and cross-linkedstructure still remained in degraded PMG nanocomposite. Thisrestrained the strength and resulted in higher mechanical andthermal property compared to the control P(S-co-MMA).

Because FE-SEM is not enough to show the distributionof graphene in the polymer matrix, Transmission electron

a–c), and 0, 10 and 20min irradiated PMG (d–f).


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Figure 13. TEM images of PG and PMG before irradiation and after 10min irradiation.

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microscopy (TEM) of the nanocomposites were taken. TEM providea direct evidence of the formation of graphene nano sheets in thenanocomposites. There is considerable agglomeration of graphene,which exist as multilayered sheets in case of nanocomposites withmodified or unmodified graphene [Fig. 13(a) and (c)]. The distribu-tion has been different for the irradiated samples asmore individualgraphene nano sheets are visible [Fig. 13(b) and (d)]. Thesegraphene sheets were observed to be transparent and highly stableunder the electron beam. This confirms the improved compatibilitybetween graphene and P(S-co-MMA), and this is attributed to thecovalent bond interaction between the nano filler and the polymer.

Conclusion

This study proposed a new green novel method to enhance theinteraction between graphene and P(S-co-MMA) copolymermatrix. Modification of graphene was carried out using nitric acid.The nanocomposites of P(S-co-MMA) copolymer with pristine ormodified graphene were prepared via melt blending. Thepolymer nanocomposites were exposed to microwave radiationat different time duration to study its effect on the mechanicaland thermal properties. The mechanism of improved interactionand grafting of graphene or modified graphene on P(S-co-MMA)polymer chains during melt blending and microwave irradiationwas explained by Raman spectra. Modified graphene developeda better interfacial interaction with polymer matrix on micro-wave irradiation compared to pristine graphene. This resultedin higher mechanical properties and better thermal stability ofthe nanocomposites. Microwave irradiation up to 10min ofPMG nanocomposite resulted in 27.6% increase of storagemodulus, which is greater than that of 10min irradiated PGnanocomposite storage modulus (11.9% increase). The better


improvement of properties of PMG after melt blending andmicrowave irradiation (10min) is due to the presence of oxygen-based functionalities on the surface of modified graphene(confirmed by FTIR). Conversely at longer irradiation period(20min), the chain scission and photo degradation of the hostP(S-co-MMA) polymer chains lead to the reduction in mechanicaland thermal properties of the nanocomposites. The degradedpolymer nanocomposites are confirmed by the presence of cracksand holes with coarse rough surface using SEM study.

Acknowledgements

Thanks to Center of Research Excellence in Nanotechnology(CENT) for the support in this study.

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