Simultaneous Reduction and Surface Functionalization of Graphene

Article pubs.acs.org/IECR

Simultaneous Reduction and Surface Functionalization of Graphene Oxide by Chitosan and Their Synergistic Reinforcing Effects in PVA Films Xiaming Feng,†,‡ Xin Wang,† Weiyi Xing,† Bin Yu,†,‡ Lei Song,† and Yuan Hu*,†,‡ †

State Key Laboratory of Fire Science, University of Science and Technology of China, 96 Jinzhai Road, Hefei, Anhui 230026, P. R. China ‡ USTC-CityU Joint Advanced Research Centre, Suzhou Key Laboratory of Urban Public Safety, Suzhou Institute for Advanced Study, University of Science and Technology of China, 166 Ren’ai Road, Suzhou, Jiangsu 215123, P. R. China ABSTRACT: Reduced graphene oxide (RGO) was functionalized by chitosan (CS), and then the CS-functionalized RGO was incorporated into poly(vinyl alcohol) (PVA) to obtain the PVA-based nanocomposite films by a solution casting method. The CS-functionalized RGO was characterized by TEM and FTIR, indicating that graphite oxide was reduced successfully by CS and CS-functionalized RGO dispersed well in the PVA solution due to the strong interfacial interaction. PVA/CS/RGO exhibited superior enhancements in tensile strength and glass transition temperature than PVA/CS and PVA/RGO, suggesting the synergistic reinforcing effect between CS and RGO. The significant reinforcement for PVA/CS/RGO nanocomposite is attributed to the strong hydrogen bonding among RGO, CS, and PVA molecules. Furthermore, the barrier effect of RGO and the synergism between CS and RGO effectively promote the formation of continuous and compact chars and protect the inner materials from burning out; as a result, the fire safety properties of the composites are significantly improved.

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INTRODUCTION Over the past few decades, biodegradable materials have received considerable attention as a result of the growing environmental concerns of nonbiodegradable materials.1−3 Among these biodegradable materials, poly(vinyl alcohol) (PVA) is a highly biocompatible and nontoxic synthetic polymer with high water solubility resulting from the hydroxyl groups in its side chain,4 and it has been applied in the field of food,5 membrane manufacturing,6 medicine,7 and so on. These applications have stimulated considerable interest in improving the properties of PVA. According to the reported literature, many efforts have been made to prepare high-performance PVA nanocomposites. For example, Huang et al. reported the preparation of PVA-based nanocomposites with pristine layered double hydroxides and apparently improved its mechanical properties.8 Moreover, PVA is highly flammable and drips easily once ignited.9 Therefore, it is also important to improve the flame retardancy of PVA as well as mechanical properties in many applications such as the textile industry, furnishings, adhesive, and packing materials. Since the demonstration of its existence, graphene has been an ideal filler for fabricating polymer-based nanocomposites. As is well-known, the dispersion and interface interactions of nanofillers in a polymer matrix are two key factors for property enhancements.10 However, due to the intrinsic van der Waals interactions, graphene or its derivatives usually easily reagglomerate, which makes dispersion and exfoliation difficult. So it is usually required to disperse graphene or its derivatives in a compatible solvent before it is incorporated into the polymer matrix.11−13 Thus, preventing the aggregation of graphene nanosheets is of significant importance to enhance the properties of polymer/graphene nanocomposites. Both covalent and noncovalent functional modifications of graphene © XXXX American Chemical Society

have been widely studied to improve the dispersion of graphene in polymers. Covalent modifications of graphene have allowed their effective dispersion in a solution.14,15 Layek et al. reported sulfonated graphene/poly(vinyl alcohol) composites prepared by solution casting method to improve the physical properties of nanocomposites.16 However, covalent modifications often cause defects in the graphene structures, which lead to the reduction in the mechanical and electronic properties of the graphene. In addition, organic solvent is used in the modification process, which would cause environmental problems. On the other hand, the noncovalent modification of graphene with surfactants and biopolymers has shown that the homogeneous dispersion of graphene can be acquired without damaging its pristine properties.17,18 However, most surfactants are toxic and difficult to remove from composites due to their strong adsorption on graphene surfaces, which limit the applications of the resulting composites in the food packaging area. In recent years, efforts have been made to simultaneously reduce and stabilize the graphene oxide dispersion by using natural product instead of toxic reducing agents. For example, vitamin C performs well in reduction of GO; however, in most cases, the product exhibited a highly agglomerated morphology without an external stabilizer.19 The protein bovine serum albumin has also been employed in reducing GO; although the resultant RGO formed a stable aqueous solution, an alkali is needed as a coreductant because of their weak reducing capability.20 Among the surfactants used Received: July 1, 2013 Revised: July 30, 2013 Accepted: August 9, 2013

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dx.doi.org/10.1021/ie402073x | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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Scheme 1. Schematic Procedure for the Synthesis of PVA/CS/RGO

The obtained viscous mud was diluted in 800 mL of water, and, then, hydrogen peroxide was added to reduce the unreacted oxidant until the slurry turned golden yellow. Wet GO was finally obtained after centrifugation and washing with dilute hydrochloric acid and hot deionized water. Graphene was obtained via green chemical reduction from GO. An appropriate portion of chitosan solution (2 wt % in acetic acid aqueous solution) was added into a GO aqueous solution with the mechanical stirring. The reduction was carried out at 95 °C for 24 h, and wet graphene was finally obtained after centrifugation and washing with the deionized water. A portion of the GO and graphene were dried at 60 °C for 24 h in a vacuum oven for further characterization. Preparation of PVA-Based Nanocomposite Films. Scheme 1 illustrates the preparation process of PVA/CS/ RGO nanocomposites, and Table 1 lists the formulations of the

to disperse the graphene, chitosan (CS) is very promising, which can simultaneously reduce and noncovalently modify the graphene oxide with environmentally friendly advantage and abundant resources.21 Besides its biodegradable properties, CS also displays unique properties, such as biocompatibility, antimicrobial activity, and excellent film-forming ability.22 Recently, CS and its derivatives have been reported for graphene modification to improve their dispersion state effectively.23 In addition, blends of CS and PVA have been reported to provide good mechanical properties and an approach for producing polymeric packaging films for specific purposes.24 Ma et al. reported the CS/PVA/GO system and investigated the improvement of properties of the CS nanocomposites.25 Lu et al. fabricated the CS/PVA nanofibers containing RGO by electrospinning, which were directly used in wound healing.26 As far as we know, fabricating the PVA/ CS/RGO system through the simultaneous reduction and functionalization of the graphene oxide has not been reported, and the synergistic enhancement between CS and RGO in the PVA nanocomposites has not been investigated. Therefore, CS is chosen to be a good candidate to prevent the aggregation of graphene in the PVA matrix and shows the synergistic effect with graphene to improve the properties of the nanocomposites. The current work investigated the preparation and characterization of simultaneously reduced and noncovalently modified graphene nanosheets using CS and their synergistic reinforcing effects in PVA-based nanocomposite films. The mechanical and thermal properties, fire behavior of the nanocomposite films were investigated in order to establish a structure−property correlation between the filler and the matrix.

Table 1. Formulations of All the Nanocomposite Films compositions (wt %) samples

PVA

CS

GE

RGO

GO

PVA PVA/CS PVA/CS/RGO-0.4 wt % PVA/CS/RGO-0.8 wt % PVA/CS/RGO-1.6 wt % PVA/RGO-0.8 wt % PVA/CS/GO-0.8 wt %

90.9 63.6 63.3 63.1 62.6 90.2 63.1

-27.3 27.2 27.1 26.8 -27.1

9.1 9.1 9.1 9.0 9.0 9.0 9.0

--0.4 0.8 1.6 0.8 --

------0.8

PVA and its composites. The nanocomposites were prepared by a solvent casting technique. To prepare PVA/CS/GO nanocomposites, PVA was dissolved in deionized water at 95 °C in a three-neck flask with mechanical stirring to form an aqueous solution (1.0 g mL−1). Then, the GO aqueous suspension (in 65 mL of deionized water) was dripped into the chitosan solution (45 mL 2 wt % in acetic acid aqueous solution) which was treated with ultrasonication for 30 min; together with 0.3 g of glycerol (GE) the obtained dispersion was poured into PVA matrix and stirred at 95 °C for 24 h. The glycerol is used as a plasticizer. Finally, the mixture was cast onto glass plates and dried at 45 °C in an oven for 2 days to form flat films which were peeled off and further heated at 80 °C for 1 day to remove residual water. To prepare the PVA/ RGO nanocomposites, hydrazine hydrate was added into the PVA/GO blending when it was treated with ultrasonication for 30 min, and the mixture was further stirred for 24 h. The PVA/ RGO films were prepared by the same method for PVA/CS/ GO films. The films were cut into certain species for measurements. Characterization. The morphology and structure of RGO were studied by transmission electron microscopy (TEM, JEM2100F, Japan Electron Optics Laboratory Co., LTD, Japan)

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EXPERIMENTAL SECTION Materials. Natural flake graphite (325 mesh; purity >99%) was supplied by Qingdao Tianhe Graphite Co., Ltd. (China). Chitosan (viscosity: 50−800 mPa·s, degree of deacetylation: 80−95%), PVA (polymerization degree: 1750 ± 50, hydrolysis: 94−95%, CP), glycerol, potassium permanganate (KMnO4, AP), sodium nitrate (NaNO3, AP), hydrazine hydrate (85% aq.), hydrochloric acid (HCl, 35% aq.), sulfuric acid (H2SO4, 98%), and hydrogen peroxide (H2O2, 30% aq.) were purchased from Sinopharm Chemical Reagent Co., Ltd. (China). Preparation of GO and RGO. GO was prepared by a modified Hummers’ method from flake graphite. First of all, H2SO4 (200 mL), flake graphite (6.0 g), and NaNO3 (6.0 g) were added into a four-neck flask and fully cooled to 3−4 °C in an ice bath. After a 15 min mechanical stirring, KMnO4 (25 g) was slowly added within 10 min. The mixture was further stirred for 1 h at 8−10 °C and then heated to 35 °C. The temperature was accurately controlled at 34−36 °C for 2 h. After that, 200 mL of H2O was dripped into the mixture, and the temperature of reactants was kept at 96−100 °C for 30 min. B



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Figure 1. FT-IR spectra of GO, RGO, and pure CS.

Institute). The char was placed on the copper plate and then coated with gold/palladium alloy ready for imaging. Laser Raman spectroscopy (LRS) measurements were carried out at room temperature with a SPEX-1403 laser Raman spectrometer (SPEX Co., USA) with excitation provided in backscattering geometry by a 514.5 nm argon laser line.

with an accelerating voltage of 200 kV. RGO was dissolved in deionized water with ultrasonication and then dripped onto copper grids before observation. X-ray diffraction (XRD) measurements were performed on a Japan Rigaku D Max-Ra rotating anode X-ray diffractometer equipped with a Cu−Kα tube and Ni filter (λ = 0.1542 nm). The scanning rate was 4° min−1, and the range was 5−40°. PVA nanocomposites were analyzed as films. Fourier transform infrared (FTIR) spectra of GO, RGO, CS, and PVA nanocomposites were obtained using a Nicolet 6700 spectrometer (Nicolet Instrument Company, USA). Dried samples were shaved to powder and mixed with KBr powders and pressed into tablets before characterization. Two parallel runs were done in the case of each sample. The tensile strength and elongation at break were measured according to the Chinese standard method (GB 13022-91) with a WD-20D electronic universal testing instrument (Changchun Intelligent Instrument Co., Ltd., China) at a crosshead speed of 50 mm min−1. The thermal behavior of the PVA nanocomposites was further investigated by a Q2000 differential scanning calorimetry (DSC) (TA Instruments Inc., USA). Samples (2−3 mg) were heated from 25 to 120 °C at a linear heating rate of 10 °C min−1; the temperature was kept at 120 °C for 10 min and then decreased at a linear rate of 10 °C min−1 from 120 to 25 °C. The temperature was kept at 25 °C for 10 min, and then the heating−cooling cycle was performed again. The data obtained from the second heating cycle were plotted. Microscale combustion colorimeter (MCC) was used to investigate the flammability characteristics of PVA nanocomposites according to ASTM D7309-07. Samples of about 5 mg were heated in nitrogen atmosphere at a constant heating rate of 1 °C s−1 from room temperature to 650 °C. The decomposition products were mixed with oxygen (20 mL min−1) and then combusted in the combustion furnace (900 °C). A scanning electron microscopy (SEM) image of the residue at 600 °C for 20 min in air was taken using a DXS-10 scanning electron microscope (Shanghai Electron Optical Technology

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RESULTS AND DISCUSSION Reduction of GO by Chitosan. CS is a biodegradable natural biomaterial and has also been utilized to stabilize RGO suspensions because it is hydrophilic and nontoxic and undergoes noncovalent interactions with RGO.27 GO has abundant hydroxyl and carboxylic groups, and CS has many amino groups along its macromolecular chains. Like other reducing agents such as benzylamine28 and hydrazine,29 the reduction of GO is presumably due to a direct redox reaction between GO and the −NH2 groups from CS. It is noteworthy that the resulting dispersion of RGO is very stable, indicating that CS is a good stabilizing agent for RGO. CS has −OH and −NH2 or OC−NH2 groups on its macromolecular chains, and these functional groups can form hydrogen bonding and electrostatic interactions with the residual oxygen-containing groups of RGO. These interactions lead to the amount of adsorption of CS macromolecules onto the RGO nanosheets, so the RGO nanosheets are stably dispersed in water. Figure 1 shows the FT-IR spectra of pure chitosan, GO and RGO. In the spectrum of CS, there are two characteristic bands centered at 1653 and 1599 cm−1, which correspond to the C O stretching vibration of −NHC(O)− and the N−H bending of −NH2, respectively. The GO has two absorption peaks at 1719 cm−1 (νCO) from carbonyl and carboxylic groups and at 1050 cm−1 (νC−O) peak from epoxy groups, which confirms the presence of oxygen-containing functional groups. The peak at 1719 cm−1 almost disappears in RGO due to the reduction of carbonyl groups, which proves that many oxygen groups have been removed during the transformation from GO to RGO. The new peaks at 2920 and 2852 cm−1 can be assigned to the stretching of −CH2− and −CH− groups, respectively, C



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Figure 2. TEM photos of RGO: (a) low magnification and (b) high magnification.

suggesting that some CS macromolecules have been adsorbed onto the RGO. The intensity of the peaks at 3417 (ν−OH) and 1084 (νC−O) cm−1 in RGO increases compared to that of GO, which results from the −OH groups of the adsorbed CS. Compared with pure CS and GO, both peaks at 1599 cm−1 related to the vibration of −NH2 and 1719 cm−1 belonging to CO stretching disappear in the spectra of RGO. Moreover, the band corresponding to the CO characteristic stretching band shifts to a lower wavenumber. These could be ascribed to the synergistic effect of hydrogen bonding between CS and the residual oxygen-containing groups of RGO and electrostatic interaction between polycationic CS and the negative charge on the surface of RGO. Figure 2(a) shows the TEM image of the RGO reduced by CS. There are some corrugations on the edge of the RGO. The black dots distributed on the surface of RGO layer and piled at the edges of the RGO are likely to be the CS that is adsorbed on the RGO.30 As shown in the high resolution of Figure 2(b), it can be clearly observed that there is an amount of black dots covered on the surface of RGO layer. Combined with the result of FT-IR, it can be concluded that the black dots are CS molecules attached on the RGO layer due to the hydrogen bonding and electrostatic interaction between CS and RGO. Characterization of PVA-Based Nanocomposite Films. Because the RGO can be simultaneously reduced and stabilized by chitosan solution, it is facile to fabricate the PVA/CS/RGO nanocomposite film by a solution casting procedure. The PVA/ CS/RGO nanocomposites with different RGO contents were prepared. For comparison, PVA/CS/GO and PVA/RGO were also fabricated with the GO (or RGO) contents of 0.8 wt %. The formulations of all the samples are listed in Table 1. The digital photographs of PVA, PVA/CS, PVA/RGO, and PVA/CS/RGO solutions are displayed in Figure 3. All the solutions were sonicated for 30 min and then placed for 15 days. It is clear that the PVA solution is colorless and transparent after completely dissolving in hot water, while the color of the PVA/CS solution is slightly yellow due to the existence of CS. As shown in Figure 3, the RGO appears as irreversible agglomerates in the PVA solution because of strong van der Waals interactions in the absence of stabilizers. However, no agglomerates are found in PVA/CS/RGO suspensions, indicating that the addition of CS can significantly improve the dispersion of RGO in PVA solution.

Figure 3. Photographs of solutions of PVA, PVA/CS, PVA/RGO-0.8 wt %, and PVA/CS/RGO-0.8 wt %.

For exploring the mechanism of the effect of CS on the dispersion state of RGO, FT-IR measurements were carried out, and the spectra are shown in Figure 4. As shown in the FTIR spectra of PVA/CS film, the absorption band at 2939 cm−1 is ascribed to the stretching of −CH2− or −CH− groups, the absorption band at 1627 cm−1 is ascribed to the amide I (CO stretching of −NHCOCH3), and the 1084 cm−1 is ascribed to the C−O stretching vibration absorption. The shift of

Figure 4. FT-IR spectra of PVA-based nanocomposite films (numbers on the left: ν−OH peak). D



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wavenumbers for ν−OH is related to the hydrogen bonding.31,32 The ν−OH wavenumber in the PVA/CS/RGO nanocomposite film shifts to higher wavenumbers compared with neat PVA, which could be ascribed to the presence of the chitosan molecules on the surface of RGO nanosheets. When the chitosan covered on the surface of RGO nanosheets, there is a large amount of hydrogen bonding between chitosan and the residual oxygen-containing groups of RGO. The incorporation of chitosan acts as a “bridge” and forms more hydrogen bonding with RGO and PVA, and, as a result, PVA/CS/RGO has higher ν−OH wavenumbers than PVA/RGO. XRD traces of PVA nanocomposite films are shown in Figure 5. It is well-known that the typical diffraction peak of GO is

Figure 6. Typical stress−strain curves of PVA nanocomposite films.

Table 2. Mechanical Properties of PVA Nanocomposite Films samples

tensile strength (MPa)

elongation at break (%)

PVA PVA/CS PVA/CS/RGO-0.4 wt % PVA/CS/RGO-0.8 wt % PVA/CS/RGO-1.6 wt % PVA/RGO-0.8 wt % PVA/CS/GO-0.8 wt %

35.7 46.2 53.8 64.5 56.2 39.9 48.6

451.9 102.3 103.2 76.5 73.2 229.1 126.6

MPa of pure PVA to 64.5 MPa of PVA/CS/RGO-0.8 wt %, and the rate of enhancement reaches up to 80.7%. As for the sole incorporation of RGO, the improvement of mechanical property of PVA/RGO is not very significant, which is attributed to the irreversible agglomerates of RGO nanosheets in the PVA matrix and the lacking of strong interaction between RGO and PVA molecules. From the results above, it can be found that the synergistic reinforcing effects exist in the PVA/CS/RGO nanocomposites. The presence of CS not only can prevent the aggregation of RGO in PVA matrix but also increase the interface interactions between RGO and PVA matrix by the hydrogen bonding. Thermal Behavior. DSC was performed to investigate the glass transition behavior of PVA nanocomposites. Figure 7 plots the DSC curves and the glass transition temperature (Tg) of PVA nanocomposites. Compared to the pure PVA, the Tg of PVA/CS decreases from 75.5 to 58.6 °C, which is attributed to miscibility of chitosan and PVA.36 When RGO is incorporated, the Tg is significantly enhanced and increases gradually as the RGO content increases. It can be attributed to the hydrogen bonding, which enhances the interfacial interactions between the RGO nanosheets and polymer chains and the great restriction of polymer’s chains motions. Compared to the PVA/ CS/RGO and PVA/CS/GO, we can find that the change trend of Tg is well corresponding with the result of FTIR, the larger amount of hydrogen bonding, the higher glass transition temperature. The Tg of PVA nanocomposite decreases after the sole addition of RGO, while the Tg of PVA nanocomposite increases conversely with the simultaneous incorporation of RGO and CS. Therefore, the incorporation of chitosan is the key factor, which not only compensate the reduction of hydrogen bonding caused by the ‘‘hydrogen bond barrier’’ due

Figure 5. XRD traces of (a) PVA, (b) PVA/CS, (c) PVA/CS/RGO0.8 wt %, (d) PVA/RGO-0.8 wt %, and (e) PVA/CS/GO-0.8 wt %.

observed at about 2θ = 10°, indicating an increase of interlayer spacing from 0.34 to 0.86 nm. The diffraction peak of PVA (Figure 5a) is located at 2θ = 19.5°. When the chitosan and the RGO (or GO) are incorporated, the XRD patterns of the PVA nanocomposites only show the diffraction peak from PVA and the characteristic peak of GO disappears, which implies exfoliation of GO in the PVA matrix. In addition, both the PVA/CS/RGO-0.8 wt % and PVA/CS/GO-0.8 wt % have a lower intensity than pure PVA, meaning that incorporating RGO (or GO) decreases the crystallinity of PVA. The decreased crystallinity is probably attributed to the hydrogen bonding between RGO (or GO) nanosheets, and chitosan can efficiently restrict and hinder the movement and arrangement of PVA chains. Mechanical Properties. Significantly enhanced mechanical properties are observed in polymer/graphene nanocomposites.33−35 In this work, tensile tests were carried out to evaluate the mechanical properties, as shown in Figure 6 and Table 2. The mechanical properties of the PVA nanocomposites are significantly increased compared to those of the neat PVA. The incorporation of the chitosan can obviously enhance the tensile strength of the PVA, which is originated from the strong hydrogen bonding between PVA and chitosan molecules. By the same reason, the value of elongation at break is decreased in terms of the restricted and hardly slipped molecules. Simultaneous addition of chitosan and RGO into PVA matrix leads to an obvious enhancement in tensile strength from 35.7 E



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PVA and its composites are composed of three steps, which indicate the decomposition process of the composites. From Figure 8, the pHRR and the total heat release (THR) of all the PVA composites are lower than pure PVA. Compared to the pure PVA, PVA/CS, and PVA/CS/RGO samples, it can be found that the incorporation of CS or RGO solely reduces the flammability of PVA, but the extent of the reduction is slight. When chitosan and RGO nanosheets are simultaneously incorporated into the PVA matrix, the HRR and THR values of PVA nanocomposites show a large reduction. The maximum reduction in HRR and THR value reach 37.3% and 36.8%, respectively. As is well-known, the chitosan is a kind of char forming agent used in the fabrication of the flame retardant materials, but the char degraded from chitosan is weak and easily collapses during the burning process. When the RGO nanosheets are incorporated into the system, the char degraded from chitosan in the PVA matrix can form the continuous and compact char layer, which can effectively prevent the flammable gas from escaping to combustion surface of the materials and further protect the inner materials from burning out. To explain the synergistic reinforcing effect on fire behavior of PVA nanocomposite films, the char residues of the composites after combustion were investigated, and the corresponding SEM images of the char residues are shown in Figure 9. The samples of PVA, PVA/CS, PVA/RGO-0.8 wt %, and PVA/CS/RGO-0.8 wt % are calcined in a muffle furnace in air at 600 °C for 15 min. It can be found that there are a large of holes left after thermal degradation of pure PVA, while the residue of PVA/RGO nanocomposite become more compact with the addition of RGO, meaning that incorporating RGO improves the formation of carbonaceous char during the thermal degradation. In addition, it is very interesting to observe many small particles on the surface of the char residue for the PVA/CS composites; it can be a sign of the char degraded from chitosan molecules during the combustion process. The reason for this phenomenon is that the chitosan is blended with PVA and there is not a strong interaction between the char particles degraded from chitosan and the char layer

Figure 7. DSC curves of PVA nanocomposite films marked with glass transition temperature.

to RGO (or GO) nanosheets but also bridge the RGO (or GO) nanosheets and PVA matrix as a compatilizer. DSC results provide an obvious evidence to demonstrate that the chitosan plays a significantly important role in the synergistic reinforcing effects in PVA nanocomposite films. Fire Behavior of the PVA Nanocomposites. Microscale combustion calorimetry (MCC) is a new technique for investigating the fire resistant properties of materials. MCC can measure the heat release process when very small samples (5−10 mg) are heated in a combustor. It directly measures the heat release of the combusted gases evolved during controlled heating of the samples. The peak of heat release rate (pHRR) is one of the most important parameters to evaluate fire safety, and a low value of HRR is an indication of low flammability and low full scale fire hazard.37 The MCC results for the PVA and its composites are shown in Figure 8. Compared to the neat PVA, the PVA nanocomposites acquire significant improvements in flame resistance. In Figure 8(a), the HRR curves of

Figure 8. (a) HRR and (b) THR curves of pure PVA and its composites. F



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Figure 9. SEM images of surface morphology of the residue of PVA nanocomposites: (a) PVA, (b) PVA/RGO-0.8 wt %, (c) PVA/CS, and (d) PVA/CS/RGO-0.8 wt %.

residual char of the PVA/CS/RGO is higher than the others, which results in the improvement of the thermal properties and fire resistance of the nanocomposites. Based on the SEM and Raman results aforementioned, a possible route of the char-forming mechanism is postulated in Scheme 2. As is well-known, there is hardly char residue left after the combustion of pure PVA, so the char formation of the PVA/CS/RGO mainly originates from chitosan and RGO. During the combustion of PVA/CS/RGO nanocomposites, the char degraded from chitosan covers on the surface of RGO and the RGO has extra-strong mechanical properties, and thereby the extra-strong char layer with sandwich structure is formed. These extra-strong sandwich char layers would become larger and larger, and finally a continuous and highly compact char layer would be formed on the surface of the burning materials. The continuous and compact char layer provides good barriers to protect the underlying polymers and inhibit the exchange of degradation products, combustible gases, and oxygen, which certainly lead to the superior flame retardancy of PVA/CS/ RGO nanocomposites.

degraded from PVA matrix. In the case of PVA/CS/RGO, as shown in Figure 9(d), it can be seen that the surface of the char residue is quite dense and smooth, and the char is a whole layer which has no holes left on the surface, indicating the synergistic char-forming effect between chitosan and RGO. LRS is further used to verify the existence of carbonaceous char in the residue and analyze the specific component of the char. The freshly made residues of PVA, PVA/CS, PVA/RGO0.8 wt %, and PVA/CS/RGO-0.8 wt % after the calcination at 600 °C for 15 min in the muffle furnace are investigated. From the Raman spectra of the residue (Figure 10), the spectra in all samples exhibit two broad and strong peaks with maximum intensity at about 1600 and 1380 cm−1. The former band (called the G band) corresponds to the stretching vibration mode with E2g symmetry in the aromatic layers of the graphite crystalline, whereas the latter (called the D band) represents the disordered or glassy carbons.37 As shown in Figure 10, each spectrum was subjected to peak fitting using the curve fitting software Origin 8.0/Peak Fitting Module to resolve the curve into 2 Gian bands. The graphitization degree of the char can be estimated by the ratio of the intensity of the D and G bands (ID/IG), where ID and IG are the integrated intensities of the D and G bands, respectively. Basically, the lower the ratio of ID/ IG, the better the structure of the char is. According to Figure 10, the ID/IG ratio follows the sequence PVA (3.56) > PVA/ RGO (3.38) > PVA/CS (2.88) > PVA/CS/RGO (2.66), indicating a higher graphitization degree and the most thermally stable char structure of the PVA/CS/RGO.38 The graphitized char formed during thermal degradation is very important in the control of heat spreading and the amount of volatiles because they are very stable at high temperature. The LRS results confirm that the graphitization degree of the

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CONCLUSIONS In this work, RGO nanosheets were simultaneously reduced and surface functionalized by the chitosan, and then incorporated into PVA. The mechanism for synergistic effect of chitosan and RGO on the property enhancement of PVA matrix was investigated. Through the reduction process of GO, there is an amount of chitosan molecules attached on the surface of RGO nanosheets, which result in a good dispersion state of nanofillers in the PVA matrix. Furthermore, chitosan can play an important “bridge” role in the loading transfer by forming the strong hydrogen bonding with PVA matrix and the G



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Figure 10. Raman curves of the residue of PVA nanocomposites after calcination at 600 °C for 15 min in air.

heat release. The improved flame retardancy is attributed to the synergistic char-forming effect between chitosan and RGO, which can effectively promote the formation of continuous and compact char and protect the inner of materials from further burning out.

Scheme 2. Illustration of the Char-Forming Mechanism of PVA/CS/RGO Nanocomposite Films

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AUTHOR INFORMATION

Corresponding Author

*Phone/Fax: +86-551-63601664. E-mail: [email protected]. Notes

The authors declare no competing financial interest. robust interface interactions with RGO nanosheets. PVA/CS/ RGO exhibit remarkable enhancements in mechanical, thermal properties, and the fire retardancy derived from the synergistic effects between chitosan and RGO. The hydrogen bonding among RGO, CS, and PVA molecules is believed to be the key factor to influence the glass transition temperatures; the surface functionalization by chitosan is a decisive factor for the stable dispersion of RGO in the PVA matrix; both the strong hydrogen bonding and the good dispersion result in significant enhancement in mechanical property. The fire safety properties of the composites are also improved, including the 37.3% reduction in peak heat release rate and 36.8% reduction in total

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ACKNOWLEDGMENTS

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REFERENCES

The work was financially supported by the National Basic Research Program of China (973 Program) (2012CB719701), the National Natural Science Foundation of China (No. 51036007), and the National Key Technology R&D Program (2013BAJ01B05).

(1) Zhu, J. F.; Zhang, G. H.; Lai, Z. C. Synthesis and characterization of maize starch acetates and its biodegradable film. Polym.-Plast. Technol. Eng. 2007, 46, 1135−1141.

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oxide and their synergistic reinforcing effects in chitosan films. ACS Appl. Mater. Interfaces 2011, 3, 4819−4830. (24) Srinivasa, P. C.; Ramesh, M. N.; Kumar, K. R.; Tharanathan, R. N. Properties and sorption studies of chitosan-polyvinyl alcohol blend films. Carbohydr. Polym. 2003, 53, 431−438. (25) Ma, J.; Liu, C. H.; Li, R.; Wang, J. Properties and structural characterization of chitosan/poly(vinyl alcohol)/graphene oxide nanocomposites. e-Polym. 2012, no. 033. (26) Lu, B. G.; Li, T.; Zhao, H. T.; Li, X. D.; Gao, C. T.; Zhang, S. X.; Xie, E. Q. Graphene-based composite materials beneficial to wound healing. Nanoscale 2012, 4, 2978−2982. (27) Fang, M.; Long, J.; Zhao, W. F.; Wang, L. W.; Chen, G. H. PHresponsive chitosan-mediated graphene dispersions. Langmuir 2010, 26, 16771−16774. (28) Liu, S.; Tian, J. Q.; Wang, L.; Sun, X. P. A method for the production of reduced graphene oxide using benzylamine as a reducing and stabilizing agent and its subsequent decoration with Ag nanoparticles for enzymeless hydrogen peroxide detection. Carbon 2011, 49, 3158−3164. (29) Stankovich, S.; Dikin, D. A.; Piner, R. D.; Kohlhaas, K. A.; Kleinhammes, A.; Jia, Y. Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide. Carbon 2007, 45, 1558−1565. (30) Guo, Y. Q.; Sun, X. Y.; Liu, Y.; Wang, W.; Qiu, H. X.; Gao, J. P. One pot preparation of reduced graphene oxide (RGO) or Au (Ag) nanoparticle-RGO hybrids using chitosan as a reducing and stabilizing agent and their use in methanol electrooxidation. Carbon 2012, 50, 2513−2523. (31) Salavagione, H. J.; Martinez, G.; Gomez, M. A. Synthesis of poly(vinyl alcohol)/reduced graphite oxide nanocomposites with improved thermal and electrical properties. J. Mater. Chem. 2009, 19, 5027−5032. (32) Jiang, L.; Shen, X. P.; Wu, J. L.; Shen, K. C. Preparation and characterization of graphene/poly(vinyl alcohol) nanocomposites. J. Appl. Polym. Sci. 2010, 118, 275−279. (33) Fang, M.; Wang, K. G.; Lu, H. B.; Yang, Y. L.; Nutt, S. Covalent polymer functionalization of graphene nanosheets and mechanical properties of composites. J. Mater. Chem. 2009, 19, 7098−7105. (34) Satti, A.; Larpent, P.; Gun’ko, Y. Improvement of mechanical properties of graphene oxide/poly(allylamine) composites by chemical crosslinking. Carbon 2010, 48, 3376−3381. (35) Vadukumpully, S.; Paul, J.; Mahanta, N.; Valiyaveettil, S. Flexible conductive graphene/poly(vinyl chloride) composite thin films with high mechanical strength and thermal stability. Carbon 2011, 49, 198− 205. (36) Grande, R.; Carvalho, A. J. F. Compatible ternary blends of chitosan/poly(vinyl alcohol)/poly(lactic acid) produced by oil-inwater emulsion processing. Biomacromolecules 2011, 12, 907−914. (37) Dong, Y. Y.; Gui, Z.; Jiang, S. H.; Hu, Y.; Zhou, K. Q. Carbonization of poly(methyl methacrylate) by incorporating hydroxyapatite nanorods during thermal degradation. Ind. Eng. Chem. Res. 2011, 50, 10903−10909. (38) Tai, Q. L.; Hu, Y.; Richard, K. K. Y.; Song, L.; Lu, H. D. Synthesis, structure-property relationships of polyphosphoramides with high char residues. J. Mater. Chem. 2011, 21, 6621−6627.

(2) Wu, H. X.; Liu, C. H.; Chen, J. G.; Yang, Y. J.; Chen, Y. Preparation and characterization of chitosan/alpha-zirconium phosphate nanocomposite films. Polym. Int. 2010, 59, 923−930. (3) Coelho, C.; Hennous, M.; Verney, V.; Leroux, F. Functionalisation of polybutylene succinate nanocomposites: from structure to reinforcement of UV-absorbing and mechanical properties. RSC Adv. 2012, 2, 5430−5438. (4) Suhrenbrock, L.; Radtke, G.; Knop, K.; Kleinebudde, P. Suspension pellet layering using PVA-PEG graft copolymer as a new binder. Int. J. Pharm. 2011, 412, 28−36. (5) Wu, J. G.; Wang, P. J.; Chen, S. C. Antioxidant and antimicrobial effectiveness of catechin - impregnated PVA-starch film on red meat. J. Food Qual. 2010, 33, 780−801. (6) Li, M. X.; Wang, H. T.; Wu, S. J.; Li, F. T.; Zhi, P. D. Adsorption of hazardous dyes indigo carmine and acid red on nanofiber membranes. RSC Adv. 2012, 2, 900−907. (7) Irani, M.; Keshtkar, A. R.; Mousavian, M. A. Removal of Cd(II) and Ni(II) from aqueous solution by PVA/TEOS/TMPTMS hybrid membrane. Chem. Eng. J. 2011, 175, 251−259. (8) Huang, S.; Cen, X.; Zhu, H.; Yang, Z.; Yang, Y.; Tjiu, W. W.; Liu, T. X. Facile preparation of poly(vinyl alcohol) nanocomposites with pristine layered double hydroxides. Mater. Chem. Phys. 2011, 130, 890. (9) Lin, J. S.; Liu, Y.; Wang, D. Y.; Qin, Q.; Wang, Y. Z. Poly(vinyl alcohol)/ammonium polyphosphate systems improved simultaneously both fire retardancy and mechanical properties by montmorillonite. Ind. Eng. Chem. Res. 2011, 50, 9998−10005. (10) Bao, C. L.; Guo, Y. Q.; Song, L.; Hu, Y. Poly(vinyl alcohol) nanocomposites based on graphene and graphite oxide: a comparative investigation of property and mechanism. J. Mater. Chem. 2011, 21, 13942−13950. (11) Kim, H.; Abdala, A. A.; Macosko, C. W. Graphene/polymer nanocomposites. Macromolecules 2010, 43, 6515−6530. (12) Choi, E. Y.; Han, T. H.; Hong, J. H.; Kim, J. E.; Lee, S. H.; Kim, H. W.; Kim, S. O. Noncovalent functionalization of graphene with end-functional polymers. J. Mater. Chem. 2010, 20, 1907−1912. (13) Khan, U.; May, P.; O’Neill, A.; Coleman, J. N. Development of stiff, strong, yet tough composites by the addition of solvent exfoliated graphene to polyurethane. Carbon 2010, 48, 4035−4041. (14) Wang, X.; Hu, Y.; Song, L.; Yang, H. Y.; Xing, W. Y.; Lu, H. D. In situ polymerization of graphene nanosheets and polyurethane with enhanced mechanical and thermal properties. J. Mater. Chem. 2011, 21, 4222−4227. (15) Xu, Y. F.; Liu, Z. B.; Zhang, X. L.; Wang, Y.; Tian, J. G.; Huang, Y.; Ma, Y. F.; Zhang, X. Y.; Chen, Y. S. A graphene hybrid material covalently functionalized with porphyrin: Synthesis and optical limiting property. Adv. Mater. 2009, 21, 1275−1279. (16) Layek, R. K.; Samanta, S.; Nandi, A. K. The physical properties of sulfonated graphene/poly(vinyl alcohol) composites. Carbon 2012, 50, 815−827. (17) Lotya, M.; Hernandez, Y.; King, P. J. Liquid phase production of graphene by exfoliation of graphite in surfactant/water solutions. J. Am. Chem. Soc. 2009, 131, 3611−3620. (18) Ge, Y.; Wang, J. L.; Shi, Z. X.; Yin, J. Gelatin-assisted fabrication of water-dispersible graphene and its inorganic Analogues. J. Mater. Chem. 2012, 22, 17619−17624. (19) Gao, J.; Liu, F.; Liu, Y.; Ma, N.; Wang, Z.; Zhang, X. Environment-friendly method to produce graphene that employs vitamin C and amino acid. Chem. Mater. 2010, 22, 2213−2218. (20) Liu, J.; Fu, S.; Yuan, B.; Li, Y.; Deng, Z. Toward a universal ″adhesive nanosheet″ for the assembly of multiple nanoparticles based on a protein-induced reduction/decoration of graphene oxide. J. Am. Chem. Soc. 2010, 132, 7279−7281. (21) Kumar, M. A review of chitin and chitosan applications. React. Funct. Polym. 2000, 46, 1−27. (22) Qiu, X. Y.; Yang, Y. H.; Wang, L. P.; Lu, S. L.; Shao, Z. Z.; Chen, X. Synergistic interactions during thermosensitive chitosan-betaglycerophosphate hydrogel formation. RSC Adv. 2011, 1, 282−289. (23) Pan, Y. Z.; Bao, H. Q.; Li, L. Noncovalently functionalized multiwalled carbon nanotubes by chitosan-grafted reduced graphene I


Simultaneous Reduction and Surface Functionalization of Graphene

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