Sodium deoxycholate functionalized graphene and its composites with polyvinyl alcohol

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IOP PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 44 (2011) 445302 (9pp) doi:10.1088/0022-3727/44/44/445302

Sodium deoxycholate functionalizedgraphene and its composites withpolyvinyl alcoholLanwei Wang1, Ruijuan Liao1, Zhenghai Tang1, Yanda Lei1 andBaochun Guo1,2,3

1 Department of Polymer Materials and Engineering, South China University of Technology,Guangzhou 510640, People’s Republic of China2 State Key Laboratory of Pulp and Paper Engineering, South China University of Technology,Guangzhou 510640, People’s Republic of China

E-mail: psbcguo@scut.edu.cn

Received 20 July 2011, in final form 1 September 2011Published 18 October 2011Online at stacks.iop.org/JPhysD/44/445302

AbstractSodium deoxycholate (SDC), a kind of bile derivative, is used to noncovalently functionalizegraphene. Stable and high concentration (up to 20 mg ml−1) of graphene colloid is obtained.The stabilization mechanism is revealed to be hydrophobic interaction, electrostatic repulsionand hydrogen bonding. Single-layer and few-layer graphene are obtained in the colloid.Subsequently, the obtained graphene sheets are incorporated into a polyvinyl alcohol (PVA)matrix by solution casting to fabricate PVA/graphene composites. Morphological observationssubstantiate the homogeneous dispersion of graphene in the PVA matrix and strong interfacialadhesion between them. Significant improvements in tensile strength and modulus of thecomposite films are observed.

(Some figures may appear in colour only in the online journal)

1. Introduction

Graphene, as a very promising carbonaceous material, is anatom thick, two-dimensional planar sheet composed of sp2

hybridized carbon atoms arranged in a honeycomb lattice [1].Graphene has attracted considerable attention due to its highthermal conductivity [2, 3], superior mechanical properties[3–5] and excellent electronic transport properties [3, 6].However, its practical application has been hampered as itssolubility in water or organic solvents, which is critical to itsprocessing, is far from satisfied. Graphene oxide (GO), whichcan be synthesized in large quantities, has been considered tobe a promising precursor for bulk production of graphene byreduction (rGO) through a reduction process.

Polymer/graphene composites with significantly improvedmechanical properties [7–10] or electrical conductivity[7, 11, 12] have been prepared using GO as the precursorof graphene. However, graphene sheets obtained from rGOtend to form irreversible re-stacked graphite-like agglomerates

3 Author to whom any correspondence should be addressed.

through strong π–π and van der Waals interactions [3].The prevention of re-stacking is particularly important forgraphene sheets because most of their unique properties areonly associated with individual layers or few layered sheets.Some methods containing covalent [13–15] and noncovalent[16–19] functionalization of graphene have been used forobtaining dispersed graphene sheets. Noncovalent modifi-cation of graphene is particularly competitive in fabricatingpolymer-based composites as the noncovalent modificationpermits larger retention of the intrinsic properties of graphene.

Sodium deoxycholate (SDC), a kind of bile derivative, hasbeen employed to modify carbon nanotube (CNT) surfaces[20–23]. Due to the same surface chemistry, SDC mayalso adsorb onto graphene layer via similar interactions.Actually, SDC has been used to disperse graphite directly[24–27]. Due to the larger area of β side (1.9–2.3 nm2)

[24] per surfactant molecule than linear chain surfactants(e.g. sodium dodecylbenzene sulfonate, SDBS) [28, 29], itcan be readily adsorbed on hydrophobic graphene surfaces[25]. In comparison with SDC, linear chain surfactants

0022-3727/11/445302+09$33.00 1 © 2011 IOP Publishing Ltd Printed in the UK & the USA

J. Phys. D: Appl. Phys. 44 (2011) 445302 L Wang et al

Figure 1. Chemical structure of the SDC molecule.

adsorb onto graphene surface only through their alkyl chains.However, the β side of SDC can be anchored onto the graphenelayer by the strong hydrophobic interaction [26, 27, 30]. Inthe meantime, the terminal carboxylate can supply enoughnegative charge, and the electrostatic repulsion can makethe graphene dispersion stable. The direct dispersion ofgraphite into graphene layers with the aid of SDC canonly yield a graphene colloid with a very low concentration(around 0.026 mg ml−1) [25]. Such a low concentration is notpractical for compounding with polymers for the fabricationof polymer/graphene composites.

In this work, to make a stable graphene colloid, SDCwas investigated as an effective stabilizer for graphene,which is reduced from GO. Much higher concentrationof graphene (up to 20 mg ml−1) was obtained. To checkthe effectiveness of graphene as reinforcement for polymer,polyvinyl alcohol (PVA), a typical water-soluble syntheticpolymer, was used to blend with the graphene colloid and castto form PVA/graphene composites. The interaction betweenPVA and graphene, and the effect of graphene content onthe properties of PVA/graphene composites were investigated.The prepared PVA/graphene composites exhibited a significantenhancement of mechanical properties at low graphenecontent, and this may be attributed to the uniform dispersionof thin graphene layers and the strong interaction between thetwo components.

2. Experimental

2.1. Materials

Natural graphite powder (purity of 99.9%) was purchasedfrom Qingdao Xinghua Graphite Co., Ltd, China. PVA withan average polymerization degree of 2400 and hydrolysisdegree of 88% was purchased from Yunnan Yunwei Co., Ltd,China. SDC was purchased from Shanghai Yuanju Biologicaltechnology Co., Ltd, China. The structure of SDC is depictedin figure 1. Other reagents were analytically pure and used asreceived.

2.2. Preparation of graphite oxide

Graphite oxide was synthesized from natural graphite powerusing a modified Hummers method [31]. Typically, 5 gof graphite powder and 2.5 g of sodium nitrate were addedinto 115 ml of concentrated sulfuric acid in an ice bath.

Under vigorous stirring, 15 g of potassium permanganate wasgradually added and the temperature of the mixture wascontrolled below 5 ◦C. The ice bath was removed, and themixture was stirred at 35 ◦C for 0.5 h and then diluted with230 ml of deionized water. After stirring for 15 min at 95 ◦C,700 ml of deionized water and 12.5 ml of hydrogen peroxideaqueous solution (30%) were added to the above solution. Theobtained graphite oxide slurry was re-dispersed in deionizedwater. The mixture was then filtered and washed withdiluted hydrogen chloride aqueous solution to remove metalions. Finally, the product was washed with deionized waterrepeatedly until neutral. The graphite oxide was collected bycentrifugation at 8000 r/min and dried in air.

2.3. Preparation of SDC-stabilized graphene colloid andgraphene paper

SDC-functionalized graphene nanosheets were fabricated byreducing GO. Specifically, 50 mg of GO and 200 mg of SDCwere dispersed in 100 ml deionized water under sonicationfor 1 h. The obtained yellow GO colloid was reduced with40 µl of hydrazine monohydrate (80 wt%) for 24 h at 80 ◦C.After the reduction, a stable colloid was obtained. The SDC-functionalized rGO (SDC-G) paper was prepared by filteringthe colloid through a nylon membrane (0.22 µm) and wasrepeatedly washed with deionized water to remove excessSDC. The SDC-G paper was dried and peeled off.

2.4. Fabrication of PVA/graphene composites

The typical synthesis procedure for PVA/graphene compositeswas as follows. PVA (5 g) was dissolved in 95 ml of deionizedwater at 90 ◦C, and the solution was subsequently cooled toroom temperature. The obtained graphene colloid was thengradually added to the PVA aqueous solution and sonicated for0.5 h at room temperature. The homogeneous PVA/graphenesolution was slowly casted on a polystyrene substrate. Aftermost of the water was evaporated at room temperature, thefilms were vacuum dried at 40 ◦C until weight equilibrium wasattained. The composite films (about 70 µm) containing 0, 0.3,0.6, 1.0, 1.5, 2.0 and 3.0 wt% graphene relative to PVA contentwere obtained.

2.5. Characterization

Thermogravimetric analysis (TGA) was performed undernitrogen atmosphere with TA TGA Q5000 at a heatingrate of 10 ◦C min−1. Fourier transform infrared (FTIR)spectroscopy was performed using a Bruker Vector 33spectrometer (Buchen, Germany). Raman spectra of thesamples were measured by a LabRAM Aramis Ramanspectrometer (HORIBA Jobin Yvon, France). UV–vis spectrawere collected on a Scinco S-3150 spectrometer (Korea).X-ray diffraction (XRD) patterns were recorded using a BrukerD8 ADVANCE x-ray diffractometer with Cu Kα radiation(λ = 1.54 Å). X-ray photoelectron spectroscopy (XPS) wascarried out on a Kratos Axis Ultra DLD (UK). Scanningelectron microscopy (SEM) images were obtained on ZEISSEVO 18 operating at 10 kV. Atomic force microscopy (AFM)

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Figure 2. Comparison of appearance and weight loss of GO, U-Gand SDC-G during heating, Tyndall effect of SDC-G colloid.

images were acquired in tapping mode on a Veeco MultimodeV scanning probe microscope. The electrical conductivitywas measured by the four-probe method on a Keithley 2365Ainstrument. Differential scanning calorimetry (DSC) wascarried out using a Perkin–Elmer Pyris 1 under nitrogenatmosphere. The samples were heated from room temperatureto 230 ◦C, maintained at this temperature for 1 min, then cooledto room temperature, and heated again to 230 ◦C. The heatingand cooling rates were 10 ◦C min−1 in all cases. The tensilestrength of the composites was determined on a Zwick/RoellZ010 machine. The extension rate was 5 mm min−1 and theinitial gauge length was 20 mm. The species were cut intostrips of 50 mm ×6 mm× ∼ 70 µm. No less than five parallelmeasurements were carried out for each sample.

3. Results and discussion

3.1. Functionalization and stabilization of rGO by SDC

Preparation of graphene via chemical reduction of GO isone of the most promising methods. However, the currentchallenge is the easy agglomeration of graphene layers, whichlargely causes their dispersion to deteriorate and preventsthem from application in polymer composites. So, exploringan effective stabilizing system for preparing well-dispersedgraphene with high quality is essential. In this study, theaddition of SDC into the GO solution does not change thestability of the solution. The interactions between GO andSDC may be due to the hydrogen bonding between the twocomponents. After the reduction, the obtained SDC-G colloidis still stable (inset of figure 2). The nature of the SDC-G colloid is evidenced by the Tyndall effect and salt effect.As demonstrated in figure 2, a diluted SDC-G dispersiongives rise to distinct Tyndall effect. In addition, the SDC-G colloid coagulates when the NaCl solution is added. Thesephenomena demonstrated that SDC-G is a typical lyophobiccolloid stabilized by electrostatic repulsion. In comparisonwith SDC-G, the unmodified graphene (U-G) obtained by thedirect reduction of GO irreversibly agglomerates. Different

Figure 3. UV spectra (a) and FTIR spectra (b) of GO, SDC, U-Gand SDC-G.

SDC/GO ratios were attempted to examine the minimum valuenecessary for stabilizing the resulting rGO. It is found that aSDC/GO ratio of 2 is sufficient for the stabilization of rGO.By washing the SDC-functionalized rGO with plenty of water,the ‘free’ SDC on SDC-G could be removed. The TGA curvesof SDC-G and U-G are compared in figure 2. It is foundthat the weight losses of U-G are much less than that of GO,suggesting that most of the oxygenic groups of GO, whichmay be released during heating, are converted into conjugatedsp2 structure [32]. It can also be calculated that there is about16 wt% SDC on the SDC-G. The maximum concentrationof the SDC-G is determined by weighing. Our results showthat when SDC/GO is 4, the maximum concentration of SDC-G is as high as 20 mg ml−1. Compared with the reportedvalue, our result is impressive. For example, when sodiumlignosulfonate was used as a stabilizer, stabilizer/GO is 10/1to stabilize graphene at a concentration of about 2 mg ml−1

[33]. As reported by Zhang et al when a triblock copolymer(PEG-OPE) is used as a stabilizer, a high stabilizer/GO ratio(57/1) is necessary to suspend graphene with a concentrationof 5.2 mg ml−1 [34]. Therefore, the SDC is a more efficientstabilizer for graphene.

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Figure 4. AFM images and section analysis of GO and SDC-stabilized graphene.

To reveal the stabilizing mechanism, UV and FTIR spectrawere used to characterize the possible interaction betweenSDC and graphene. The UV spectra of GO, SDC-G, SDCand U-G are shown in figure 3(a). The spectrum of GOdispersion exhibits a maximum at 231 nm (π–π* transitions ofaromatic C=C bonds) and a shoulder at about 300 nm (n–π∗

transitions of C=O bonds) [33]. For U-G, the absorptionpeak of the aromatic C=C bonds is red-shifted to 267 nm,suggesting that the electronic conjugation within graphenesheets is restored [3]. In the spectrum of SDC aqueoussolution, there is nearly no characteristic absorption peak forits nonconjugated structure. But for the aqueous solutionof SDC-G, the maximum absorption is further red-shifted to274 nm, indicating that SDC could interact with graphenevia the hydrophobic interaction, which is proposed to bethe dominant mechanism for adsorbing SDC onto graphenesheets [27].

The FTIR spectra of GO, SDC and SDC-G are shownin figure 3(b). In the FTIR spectrum of GO, the presenceof C–O (νc−o at 1043 cm−1), C–O–C (νc−o−c at 1225 cm −1),C–OH (νC−OH at 1400 cm−1), C=O (νC=O at 1728 cm−1) incarboxyl and O–H (νO−H at 3400 cm−1) indicates that thesurface of GO is decorated by oxygenic groups, which isessential for its stable dispersion in water. As for SDC-G,the peak at 1640 cm−1 may be assigned to skeletal vibrationsof unoxidized graphitic domains [35]. In addition to thisobservation, the C=O peak intensity at 1720 cm−1 for SDC-Gis largely depressed. The result evidences the successfulreduction of GO. Compared with pure SDC, its –COO− peakat 1566 cm−1 is shifted to 1548 cm−1 in SDC-G, which isindicative of the formation of hydrogen bonding between SDCand graphene. Consequently, in addition to the hydrophobic

interaction, the hydrogen bonding between SDC and grapheneis also responsible for the adsorption of SDC onto graphene.Meanwhile, the electrostatic repulsion provided by the SDC-functionalized graphene sheets makes them stably dispersedin water.

The dispersion of GO and SDC-G suspension wascharacterized by AFM, as shown in figure 4. GO shows aheight of about 0.9 nm, suggesting a single-layer graphenesheet [36]. Compared with GO, the average thickness of SDC-G is increased to about 2.6 nm, but taking into considerationthat SDC molecules or micelles are adsorbed on both sides ofthe graphene layer [37], it is reasonable to believe that most ofthe SDC-G exists in the form of single layer.

3.2. Properties and microstructure of SDC-G

Generally, electrical conductivity of rGO reflects the extent ofreduction and restoration of the conjugated structure [38, 39].The SDC-G paper was prepared via vacuum filtration. Theaverage electrical conductivity of the paper is about 308 S m−1

(with an average thickness of 16 µm). Compared with thatof GO (about 0.014 S m−1), the conductivity of the SDC-Gpaper is increased by four orders of magnitude, indicating thesuccessful restoration of graphite structure.

Raman spectroscopy is convenient for characterizingcarbonaceous materials. Figure 5(a) illustrates the Ramanspectra of GO, U-G and SDC-G. The peaks at 1350 cm−1

and 1604 cm−1 of GO are ascribed to D band and G band,respectively. The G band corresponds to the first-orderscattering of the E2g mode and the D band originates fromthe breathing modes of sp2 rings and requires a defect forits activation by double resonance [40]. From the Raman

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Binding Energy (ev)

Figure 5. Raman spectra (a) and XPS C1s spectra (b) of GO, U-Gand SDC-G.

spectra in figure 5(a), the chemically reduced graphene viahydrazine hydrate (U-G and SDC-G) also have fundamentalvibrations at 1598 cm−1 (G band) and 1350 cm−1 (D band).The D/G intensity ratios of SDC-G and U-G are 1.47 and 1.06,respectively, which are larger than that for GO (0.96). Thischange suggests that more sp2 domains are formed during thereduction of GO, which agrees very well with the results byRuoff et al [32]. Meanwhile, compared with U-G, the G bandof SDC-G is stiffened from 1580 to 1598 cm−1, which couldbe ascribed to the hydrophobic interaction between SDC andgraphene [27]. The intensities of the second-order Ramanpeaks (2D) are sensitive to the ordering structures on thebasal plane of the graphene [41]. In this work, as shownin figure 5(a), the intensities of the overtone 2D band withrespect to the D and G peaks are small, suggesting the presenceof disordered graphene sheets in these samples. In addition,both the shift of 2D peak and the reduction in its magnitudewith respect to G peak are indicative of reduction of GO [42].Compared with GO, the 2D peaks for U-G and SDC-G areshifted to higher wavenumbers. Also, the 2D/G intensityratios for U-G and SDC-G (0.12 and 0.14, respectively) are

Figure 6. (a) SEM image of the SDC-G paper and (b) XRDpatterns of GO and SDC-G.

lower than that for GO (0.16). These observations indicate thesuccessful reduction in U-G and SDC-G.

The reduction of GO and the structure of SDC-G were alsofurther studied by XPS, which was already used to characterizethe surface composition of rGO [43, 44]. Figure 5(b) shows theC1s XPS spectra of GO and SDC-G. From the spectrum of GO,there are clearly four types of carbon with different chemicalvalences. These peaks at 284.5 eV, 286.6 eV, 287.8 eV and289.0 eV are assigned to C=C, C–O, C=O and O–C=O,respectively [35]. After the reduction, the C1s XPS spectrumof SDC-G reveals that the content of the oxygenic groups isobviously decreased from 43.9% for GO to 2.8% for SDC-G.It is believed that the reduction in the presence of SDC as astabilizer is still effective in restoring the conjugate structureof graphene. Moreover, there exists an additional componentat 285.8 eV for SDC-G, corresponding to the C in the C=Nbonds of hydrazone [44].

Appearance and SEM side-view images of the SDC-Gpaper are shown in figure 6(a). It can be seen that laminargraphene sheets are tightly stacked. To characterize the

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microstructure of SDC-G, XRD experiments were performedand the corresponding results are shown in figure 6(b). Thecharacteristic peak of GO sheets appears at 2θ = 9.7◦ and itsinterlayer distance is 0.9 nm according to the Bragg equation.In SDC-G, the diffraction peak of (0 0 2) at 2θ = 26.4◦

is ascribed to the d-spacing of 0.345 nm, which is relatedto the interlayer distance between SDC and graphene. Ourresults are very consistent with those of previous reports[45, 46]. However, two peaks at 8.5◦ and 17.5◦, correspondingto interlayer distances of 1.05 nm and 0.51 nm, are found.Considering that the β side of the SDC molecule is particularlysuitable for stacking into layers, they readily form micelles[23]. And the two peaks may be attributed to the stackedstructures of the SDC micelles [37].

3.3. Morphology and microstructure of PVA/graphenecomposites

In view of the excellent stability of the resultant SDC-Gand the presence of various hydrophilic groups such ascarboxyl and hydroxyl, one may expect the homogeneousdispersion of SDC-G in a water-soluble polymer matrix.PVA is a synthetic polymer that has attracted considerableinterest. However, low mechanical properties of purePVA restrict its applications [47]. Consequently, thePVA/graphene composites were fabricated as a model toexamine the effectiveness as reinforcement. The compositesare prepared by solution mixing. Since the filler dispersionand the polymer–filler interfacial interaction are the two mostimportant factors governing the comprehensive performance ofpolymer composites, the morphology and the microstructureof the PVA/graphene composites were characterized.

The SEM image of the fractured cross-section of thePVA/graphene composite with a loading of 1.5 wt% is typicallyshown in figure 7(a). Most of the graphene nanosheets arefully exfoliated and clearly well dispersed in the PVA matrix.The image also reveals that the crumpled graphene nanosheetsrandomly disperse in the PVA matrix. The graphene sheets donot just align parallel to the surface of the sample film, whichmay be attributed to the PVA/graphene strong interaction,which will be disclosed later. This phenomenon is inagreement with that of other reports [8, 48, 49].

The XRD patterns of pure PVA and PVA/graphenecomposites are shown in figure 7(b). The (1 0 1) peak ofpure PVA appears at 2θ = 19.3◦. However, once SDC-Gis incorporated, the XRD patterns of the PVA/graphenecomposites show only the PVA diffraction peak. Thecharacteristic diffraction peak of the SDC-G paper, as shownin figure 6(b), is not observed in the composites. This impliesthat graphene sheets are randomly dispersed and completelyexfoliated [8, 49]. This result is in good agreement with thatof the SEM analysis above.

Interfacial interaction between graphene and PVA ischaracterized by FTIR spectroscopy. The FTIR spectra of thePVA/graphene composites with different loading of grapheneare shown in figure 8(a). As the loading of graphene increases,the characteristic bands for –OH (around 3340 cm−1) and C–Ostretching (around 1050 cm−1) are continuously red-shifted,

Figure 7. (a) SEM image of the fracture surfaces of thePVA/graphene composite with a graphene loading of 1.5 wt% and(b) XRD patterns of pure PVA and PVA/graphene composites.

indicating the hydrogen bonding between the –OH of PVAand the remaining oxygenic groups in SDC-G.

The glass transition and melting behaviour of the PVAmatrix in the composites were also investigated and the resultsare summarized in figure 8(b) and table 1. As shown infigure 8(b), it can be seen that with the addition of graphene, theglass transition temperature (Tg) of PVA increases graduallyfrom 70.8 ◦C for pure PVA to 74.2 ◦C for the PVA/graphenecomposite with 3.0 wt% graphene loading. The increase inTg is attributed to the restricted chain mobility of the matrixby graphene, which is induced by the interfacial interactionbetween the PVA chain and SDC-G. Such a kind of restrainingeffect of graphene on the chain mobility has also beensuggested in other systems [50, 51].

Table 1 shows the changes in the melting enthalpy (�Hm)

of PVA in the PVA/graphene composites. And the degree ofcrystallinity (χc) of the PVA matrix is also calculated fromthe ratio of enthalpies (�Hm/�H0), which are the ratio of

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Figure 8. FTIR spectra (a) and glass transition (b) of PVA andPVA/graphene composites.

the measured and 100% crystalline melting enthalpy. The�Hm of 100% crystalline PVA, �H0, is taken as 138.6 J g−1

[52]. Compared with PVA, it is observed that �Hm and theconsequent χc of the composites are slightly increased withincreasing graphene content when the graphene content isbelow 1.5 wt%. This observation may be explained accordingto the nucleating effect induced by the incorporated graphene.When the graphene content is increased to 3.0 wt%, theχc of PVA decreases to some extent, suggesting that thecrystallization of PVA is suppressed due to the sufficiently highinterfacial interactions at that graphene content. The reportedPVA/GO composites also exhibit a similar crystallizationbehaviour [51].

3.4. Mechanical performance of PVA/graphene composites

Graphene, as a prospective material, has recently beenhighlighted to reinforce polymer composites due to its largeinterfacial area and high aspect ratio of the nanosheet [53, 54].The mechanical performance of the composites was expectedto be significantly enhanced by the good dispersion of

Table 1. Crystallinity and melting enthalpy of pure PVA andPVA/graphene nanocomposites.

Graphene content (wt%) �Hm (J g−1) χc (%)

Pure PVA 25.4 18.30.3 29.6 21.40.6 32.8 23.71.0 25.9 18.71.5 26.7 19.33.0 21.1 15.2

Figure 9. Typical stress–strain curves of the PVA/graphenecomposites.

graphene sheets in the PVA matrix and the strong interfacialinteraction between the two components. Figure 9 shows therelationship between graphene loading and tensile strengthof the composites and inset is the enlarged view of strainbetween 2% and 9%. The mechanical performances of thePVA/graphene composite film are summarized in table 2.The modulus is calculated from Hooke’s law (2–6% strain).The tensile strength and modulus of the composites aresignificantly increased with increasing graphene loading. Forexample, at a graphene content of 1 wt%, the tensile strengthof the composite is almost doubled and the modulus of thecomposite is over four-fold compared with those of purePVA. The prepared SDC-G contains many hydrophilic groupsas a consequence of adsorption of SDC molecules onto thegraphene surface via hydrophobic interaction between them.Consequently, SDC-G can strongly interact with the PVAchain via hydrogen bonding. As a result, a homogeneousdispersion of graphene and strong interfacial adhesion areachieved, which lead to the significant improvement of tensilestrength and modulus of the composites. Simultaneously, dueto the restricted chain mobility caused by the strong interactionbetween PVA and graphene, the elongation at break of thecomposite film is decreased with increasing graphene loading.

4. Conclusions

In conclusion, a kind of bile acid derived surfactant, SDC,is used to noncovalently functionalize graphene and fabricate

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Table 2. Mechanical properties of PVA/graphene composites.

Grahene content (wt%) Pure PVA 0.3 0.6 1.0 1.5 3.0

Break strength(MPa) 28.6 ± 2.7 35.1 ± 2.8 48.4 ± 2.6 55.2 ± 4.9 60.7 ± 6.4 80.8 ± 5.8Tensile modulus (GPa) 0.17 ± 0.01 0.24 ± 0.02 0.72 ± 0.03 0.80 ± 0.03 1.08 ± 0.03 1.48 ± 0.03Break strain (%) 197.8 ± 14.8 191.4 ± 16.4 185.8 ± 14.4 147.5 ± 9.6 86.3 ± 6.6 15.2 ± 4.5

a stable colloid with high concentration (up to 20 mg ml−1).The stabilization mechanism is revealed to be hydrophobicinteraction, electrostatic repulsion and hydrogen bonding.The obtained functionalized graphene shows significantlyreinforced effect on PVA. At a graphene loading of 1 wt%, thetensile strength and modulus of the PVA/graphene compositeare doubled and over four-fold, respectively. This workprovides a new way for the preparation of functionalizedgraphene colloids with high concentration and graphene-basedpolymer composites with high mechanical properties.

Acknowledgments

The authors acknowledge financial support from the NationalNatural Science Foundation of China (50873035 and50933001), the Guangdong Natural Science Foundation(151008901000137), Fundamental Research for the CentralUniversities (2009ZZ0007) and the Program for New CenturyExcellent Talents in University (NCET-10-0393).

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