In Situ Thermal Reduction of Graphene Nanosheets Based Poly

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In-situ Thermal Reduction of Graphene Nanosheets Based Polymethyl Methacrylate Nanocomposites with Effective Reinforcements Xinya Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie5035978 • Publication Date (Web): 22 Dec 2014 Downloaded from http://pubs.acs.org on December 23, 2014

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In-situ Thermal Reduction of Graphene Nanosheets Based Polymethyl Methacrylate Nanocomposites with Effective Reinforcements

Journal: Manuscript ID: Manuscript Type: Date Submitted by the Author: Complete List of Authors:

Industrial & Engineering Chemistry Research ie-2014-035978.R2 Article 18-Dec-2014 Sheng, Xinxin; South China University of Technology, Xie, Delong; South China University of Technology, Cai, Wenxi; South China University of Technology, Zhang, Xinya; South China University of Technology, Zhong, Li; South China University of Technology, Zhang, Huiping; South China University of Technology,

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In-situ Thermal Reduction of Graphene Nanosheets Based Polymethyl Methacrylate Nanocomposites with Effective Reinforcements Xinxin Sheng, Delong Xie, Wenxi Cai, Xinya Zhang*, Li Zhong, and Huiping Zhang School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou, 510640, China

ABSTRACT: The incorporation of graphene nanosheets (GNSs) into a polymer matrix can effectively enhance its thermal and mechanical properties. We report a facile and eco-friendly method for preparing polymer nanocomposites with homogeneously dispersed GNSs in the polymethyl methacrylate (PMMA) matrix via firstly grafting functionalized graphene oxide (GO) using PMMA miniemulsion, then melt blending the grafted GO with PMMA matrix, and simultaneous in-situ thermal reduction of GO in the matrix. The results show that the GNSs exhibit exfoliated morphology and good distribution in the obtained nanocomposites. A 37.9% enhancement in tensile strength and a 61.4% increase of Young’s modulus with respect to the polymer matrix are achieved by incorporating only 1.5 wt.% GNSs loading. The experimental derived Young’s modulus agrees well with the theoretical simulation. Moreover, the storage modulus of the nanocomposites increases by 45%, while the glass transition temperature (Tg) increases by 7.5 °C at 1.5 wt.% GNSs loading. KEYWORDS: graphene nanocomposites; miniemulsion; in-situ thermal reduction; melt blending; reinforcement

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1. INTRODUCTION Graphene nanosheets (GNSs) represent the single-atom-thick form of two dimensional sp2 bonded carbon atoms arranged in a hexagonal lattice.1 GNSs have attracted remarkable research interest since their discovery,2 because of their wide applications in the areas of chemistry, physics, and composites.1, 3-5 One of the most promising applications of GNSs is in fabrication of multifunctional polymer nanocomposites because of their excellent properties, such as high Young’s modulus,3 outstanding thermal conductivity,6 high aspect ratio,7 and superior electrical conductivity.2 Graphene oxide (GO) nanosheets, a derivative of GNSs, contain a variety of oxygen functional groups (e. g., –OH, –COOH, and epoxide). Thus, the GO nanosheets are hydrophilic,8 resulting in their incompatibility with hydrophobic monomers or polymers such as PMMA and polystyrene (PS). Therefore, GO is usually functionalized to enhance the compatibility with hydrophobic monomer or polymers. Various techniques including esterification,9 silanization,10,

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and polymer grafting12-14

with surface functional groups of GO, such as –OH and –COOH, by forming covalent bonding have already been explored to functionalize GO nanosheets surface. The reduction of GO is accompanied by the elimination of oxygen-containing groups, which is necessary to disperse GNSs uniformly into the hydrophobic polymer matrixes. In general, two effective reduction strategies, namely, chemical reduction14,

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and thermal reduction16-19 for the

fabrication of polymer/GNSs nanocomposites from GO. Chemical reduction is highly effective, but it uses large amounts of organic solvents or hazardous reagents. Thermal reduction is believed to be an eco-friendly method in which no hazardous reagents are used and can be conducted at different temperatures. Thomassin et al.16 fabricated PMMA/GNSs 2


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nanocomposites via dispersion polymerization through a simple thermal reduction of the GO dispersed in PMMA at moderate temperature (210 °C) with high electrical conductivity. Subsequently, various techniques such as solution mixing20, 21, melt blending20, 22, and in-situ polymerization14,

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have been developed to prepare GNSs-based polymer

nanocomposites. Solution mixing and in-situ polymerization usually produce good dispersion, whereas the melt blending shows low dispersion degree.24 Kim et al.20 prepared GNSs/polyurethane nanocomposites by solution mixing, in-situ polymerization, and melt blending; the results revealed that melt blending presented much poorer dispersion than the other two techniques. Wang et al.25 fabricated GNSs/PMMA nanocomposites via in-situ suspension polymerization and reduction of GO using hydrazine hydrate. The resulting nanocomposites presented good thermal stability, high electrical properties, and better mechanical properties at very low GNSs loading compared with the synthetic PMMA. In earlier research, most studies focused on solution mixing or in-situ polymerization for the preparation of GNSs-based polymer nanocomposites; however, the drawbacks of the two techniques are that a relatively large number of organic solvent are used, which is toxic and non-eco-friendly.24 The melt blending technique is versatile and eco-friendly, in which no solvent is used. Given these benefits, the key challenges to obtain high-performance nanocomposites via melt blending are improving the dispersion and interfacial interactions between the polymer matrix and GNSs; thus, this technique has been considered for the most of the prospective industrial applications of GNSs. Miniemulsion polymerization is a very useful strategy for the fabrication of polymer latex made with nanofillers such as GNSs.10,

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This usefulness is attributed to the initial

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miniemulsion droplets that can directly polymerize into polymer particles.27 During the miniemulsion polymerization process, ultrasonication is applied to emulsify efficiently the oil-phase droplets that consist of monomers and GNSs. Polymerization occurs within these droplets, forming polymer/GNSs composite particles that are stabilized by emulsifiers and hydrophobe.12 Compared with other polymerization techniques, miniemulsion polymerization is more advantageous because of the high monomer conversion, high solid content and small particle size of the obtained latexes.28 All these advantages allow this technique to be suitable for preparing polymer/GNSs nanocomposites. In this paper, we report a facile and eco-friendly method for the preparation of polymer nanocomposites with homogeneously dispersed GNSs into the PMMA matrix via miniemulsion process and melt blending. First, the GO was covalently functionalized with hydroxypropyl methacrylate (HPMA) monomer, producing a vinyl-grafted GO. The functionalized GO was pretreated via miniemulsion polymerization in the presence of MMA to obtain a PMMA-g-GO composite particles. Second, the PMMA-g-GO composite particles were blended with neat PMMA at molten state, and the GO was in-situ thermally reduced into the GNSs dispersed in the PMMA matrix during the process. To the best of our knowledge, unlike earlier reports in which either only miniemulsion polymerization or melt blending is employed,12, 22, 26, 27 less reports are available on the fabrication of PMMA/GNSs nanocomposites together, which combines the miniemulsion process and melt blending to broaden the applications of GNSs in polymer industry. The obtained PMMA/GNSs nanocomposites exhibit exfoliated morphology and good dispersion. In addition, significant enhancement of thermal properties and mechanical properties are achieved at low GNSs 4


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loading. 2. EXPERIMENTAL METHODS Raw Materials Natural graphite (400 mesh, 99.5%) was purchased from Tianjin Yongda Chemical Reagent Co., Ltd. (China). Sulfuric acid (H2SO4, 98%), sodium nitrate (NaNO3), Potassium permanganate (KMnO4), hydrochloric acid (HCl, 37%), hydrogen peroxide (H2O2, 30%), sodium chloride (NaCl) and N,N-dimethylformamide (DMF) were of analytical grade and offered by Tianjin Kemiou Chemical Reagent Co., Ltd. (China). Methyl methacrylate (MMA, 99%), Hydroxypropyl methacrylate (HPMA, 99.7%), Dicyclohexylcarbodiimide (DCC), 4-dimethylaminopyridine (DMAP), Sodium dodecyl sulfate (SDS), Polyoxyethy-lene (20) Sorbaitan Monolaurate (Tween-20), 2,2’-azobisisobutyronitrile (AIBN), and hexadecane (HD) were of analytical grade and obtained from Aladdin Reagent Co., Ltd. (China). Poly (methyl methacrylate) (PMMA) (Mw = 5.6×104, Mn = 5.4×104, Mw/Mn = 1.03, Fluka (Shanghai), China) were commercial products and used as received. Preparation of GO GO was synthesized from natural graphite powder using a modified Hummers’ method.29, 30 In a typical procedure, 5.0 g of graphite powder, 5.0 g of NaNO3 and 250 mL of concentrated H2SO4 were mixed and stirred for 20 min in a 2000 mL round flask in an ice bath. Then 30 g KMnO4 was slowly added into the flask in 30 min in the ice bath. Then the mixture was stirred continuously for 48 h, after which 460 mL of deionized water was added dropwise into the suspension within 30 min with stirring. The suspension was further treated with 420 mL of deionized water (50–60 °C) and 100 mL of H2O2 (30%) to reduce residual KMnO4 and 5



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MnO2 to soluble manganese sulfate. The mixture was centrifuged and washed with a solution of 6 wt.% H2SO4/1 wt.% H2O2 to further remove residual KMnO4 and MnO2. Then the product was washed with water several times, dialysed for one week and the GO sample was obtained after freeze-drying for 48 h. Functionalization of GO with HPMA monomer GO was covalently functionalized via an esterification reaction. In a typical reaction, 1.0 g of GO powder was dispersed in 60 mL of DMF, and the suspension was treated by ultrasonication for 60 min. Afterward, 5.0 g of HPMA (34.6813 mmol) was added into the GO suspension and stirred for 30 min. A solution of DMAP (3.4681 mmol, 0.4237 g) and DCC (34.6813 mmol, 7.1731 g) in 20 mL of DMF was added to the GO/HPMA suspension, and then the mixture was stirred for 72 h under an argon atmosphere at room temperature. After the reaction, the mixture was separated by centrifugation and washed with DMF and deionized water several times, and then the product, noted as MGO, was dried at 60 °C under vacuum for 24 h. Synthesis of PMMA grafted GO sheets by miniemulsion polymerization The PMMA-g-GO nanocomposites were prepared by a miniemulsion process, as described in Scheme 1. In typical experiments, the MGO (10.0 wt. % relative to monomer) was added to 10.0 g of MMA monomer and then sonicated for 10 min. Exactly 0.40 g of HD and 0.10 g of AIBN were then dissolved in the suspension. An aqueous surfactant solution (100 mL of deionized water, 0.10 g of SDS, and 0.10 g of Tween-20) was added to the oil phase, and then the mixture was stirred for 30 min for pre-emulsification. After that, the mixture was ultrasonicated in an ice bath for 30 min (600 W output power, 2 s work time, 2 s pause time) 6


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to obtain the miniemulsion. The miniemulsion was then poured into a 250 mL three-neck round-bottomed flask equipped with condenser and stirrer in an oil bath. The reaction was allowed to proceed at 75 °C for 6 h. After polymerization, the resultant was demulsified by NaCl aqueous solution and purified several times with deionized water to remove excess surfactant and NaCl. The obtained product was dried under vacuum at 60 °C for 24 h. A gel fraction measurement was conducted to investigate whether MGO could potentially act as a giant crosslinking point during the miniemulsion process; the detailed procedure is presented in the Supporting Information.

Scheme 1. Strategy for synthesis of PMMA-g-GO. Preparation of PMMA/GNSs nanocomposites Neat PMMA was melt blended with appropriate amount of PMMA-g-GO powder using a Brabender Plasticorder with roller blades at 200 °C, with screw speed at 100 rpm for 10 min. The obtained nanocomposites were hot pressed into 1 mm sheets at 200 °C and 10 MPa for 15 min and cut to the required dimensions for testing. In such a process, the GO was in-situ thermally reduced in the PMMA matrix. To fabricate well-dispersed PMMA/GNSs nanocomposites, a strategy that combines the miniemulsion process and subsequent melt blending was conducted, and the fabrication process of the nanocomposites is shown in Scheme 2. 7



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Scheme 2. Strategy for the preparation of PMMA/GNSs nanocomposites. Characterization and Instruments FTIR analysis was performed using a Spectnlm2000 spectrometer (Perkin Elmer Co. USA). The samples were prepared in potassium bromide pellets. XRD was carried out using a Bruker D8 ADVANCE diffractometer with Cu Κα radiation (λ = 0.15418), conducting a scanning rate of 0.01 °/s. Raman spectra were obtained using a LabRAM-HR laser Raman spectrometer with excitation by 514.5 nm argon laser. X-ray photoelectron spectroscopy (XPS) was measured with a Thermo escalab 250Xi electron spectrometer. AFM observation was recorded with a Multimode Nano 4 instrument in tapping-mode. The aqueous GNSs suspension was spin-coated onto a freshly mica. TEM images were studied using a HITACHI H-7650 instrument at an accelerating voltage of 80 kV. The morphology of GO and the cross-section of the samples were observed using a HITACHI S-3700N SEM at an acceleration voltage of 5.0 kV. Thermogravimetric analysis (TGA) was employed with a Netzsch TG 209 thermo-analyzer instrument, heating from room temperature to 700 °C at a rate of 10 °C/min under N2 atmosphere. DMA of the nanocomposites were measured utilizing a Netzsch DMA 242 at a fixed frequency of 2 Hz from room temperature to 150 °C at a linear heating of 3 °C /min. The tensile strength and elongation at breaking were conducted with an 8


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Instron 3367 instrument at an extension rate of 10 mm/min. Each sample was tested five times for average. 3. RESULTS AND DISCUSSION Structures and Morphologies of graphene and PMMA/GNSs nanocomposites Figure 1 shows the FTIR spectra of GO, MGO, and PMMA-g-GO. As can be seen in Figure 1(a), the characteristic peaks of GO are observed at 3343 cm–1 (stretching vibration of –OH), 1726 cm–1 (stretching vibration of C=O carboxyl), 1623 cm–1 (C=C bonds in remaining sp2 character of graphite),13, 31 and 1368–1063 cm–1 (vibration of C–OH, –COO– and C–O–C)32. However, new characteristic peaks at 2925 cm–1 and 2855 cm–1 for the stretching vibration of C–H in –CH3 and –CH2 are clearly visible in Figure 1(b). The peaks at 1439 cm–1 and 1397 cm–1 correspond to the deformation vibration of –CH3 and –CH2. The typical peak at 1648 cm–1 can be attributed to the C=C stretching vibration of HPMA. The band at 1716 cm–1 is related to the C=O stretching vibration of the ester groups formed by the reaction between -COOH groups situated at the edges of the GO and the –OH groups of HPMA.32, 33 Therefore, it can be concluded that GO nanosheets have been covalently functionalized successfully by HPMA, whereas the formed C=C groups can further polymerize with monomers. As shown in Figure 1(c), after polymerization, the spectra of PMMA-g-GO presents not only the characteristic bands of MGO, but also the bands of PMMA including 3439 cm–1 (–OH), 2992 cm–1, 2947 cm–1, 2845 cm–1, 1484 cm–1, 1448 cm–1, and 1384 cm–1 (C–H vibrations in –CH3 and –CH2), 1733 cm–1 (C=O), and 1274–1067 cm–1 (C–O). In addition, the band at 1645 cm–1 (C=C) disappears. These evidence indicate that MMA monomers have copolymerized with the C=C groups of MGO, resulting in a chemical bond between PMMA nanoparticles and 9



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MGO.

Figure 1. FTIR spectra of (a) GO, (b) MGO and (c) PMMA-g-GO. To verify the in-situ thermal reduction of GO in the polymer matrix during the process, the GNSs were repeatedly reflux extracted from the nanocomposites by a Soxhlet extractor, and the chloroform was used as extractor to remove the polymer materials. The remaining powdered GNSs were dried to a constant weight at 60 °C in a vacuum. XPS was used to investigate the surface chemical composition of GO, MGO, and GNSs. Figure 2a–f display the XPS survey scans and C1s XPS spectra of GO, MGO, and GNSs. As shown in Figure 2(b), the C1s XPS spectrum of GO presents a significant extent of oxidization with three types of carbon atoms: C–C (284.8 eV), C–O (286.5 eV), and C=O (288.7 eV), consistent with the earlier report.15 However, when the HPMA monomer was grafted onto GO, the peak intensities of C–C and C=O are increased (Figure 2(d)) corresponding to the carbon atoms and C=O groups in HPMA. Meanwhile, the decreased intensity of C–O groups originated from MGO, inferring the activation of carboxyl group by DMAP and DCC and subsequently the functionalization of HPMA onto GO.34 Moreover, the proportion of peak at 288.7 eV is 10


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increased, which is attributed to the formation of ester bonds, indicating that HPMA has successfully covalently bonded to GO.34 The C1s XPS spectrum of GNSs (Figure 2(f)) also present these three peaks after the in-situ thermal reduction at 200 °C, corresponding to C–C (284.7 eV), C–O (286.9 eV), and C=O (288.4 eV). However, the intensities of the oxygen groups are remarkably decreased compared with that of GO. These results demonstrate that most of the oxygen-containing groups are eliminated by the in-situ thermal reduction. To further confirm the effective reduction of GO, the electrical conductivity of the nanocomposites were measured using a standard four-probe method. The results (see Figure S4, Supporting Information) show that the neat PMMA is insulating and have a conductivity of ~10−13 S/m. However, the electrical conductivity of the nanocomposites is at least four orders of magnitude higher than the neat PMMA (i.e., with conductivity values of 7.3×10−9 S/m and 6.6×10−6 S/m at 0.5 and 1.5 wt.% GNSs loading, respectively), indicating that the GO was in-situ thermally reduced into GNSs, and the GNSs can form conductive pathway in the PMMA matrix.

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Figure 2. XPS survey scans of (a) GO, (c) MGO, and (e) GNSs; C1s XPS spectra of (b) GO, (d) MGO, and (f) GNSs. XRD was utilized to examine the interlayer distance between the GO sheets and the state of exfoliation of the GO sheets in the nanocomposites. Figure 3 presents the XRD patterns of graphite, GO, MGO, PMMA-g-GO, and PMMA/GO nanocomposites. As shown in Figure 3(A), for graphite, a sharp peak appears at 2θ = 26.62°, corresponding to a d-spacing of 0.335 nm. After oxidation, the sharp peak of graphite disappears, and a new diffraction peak appears at 2θ = 10.95°, representing the diffraction peak of GO; the d-spacing increases up to 0.807 nm from 0.335 nm because of the oxygen-containing groups and the absorbed H2O.35 However, the diffraction peak of MGO at 2θ = 9.93° suggests a d-spacing of 0.889 nm 12


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because of the intercalation by functionalized HPMA between the GO sheets. Almost no apparent peaks are found in the XRD patterns of PMMA-g-GO, indicating that PMMA particles are amorphous, and the GO sheets are exfoliated into a monolayer without re-stacking together. The PMMA/GNSs nanocomposites show broad and weak peaks at approximately 2θ = 13.7° (Figure 3(B)), inferring that the GNSs are exfoliated into individual sheets or few layers in the nanocomposites36, 37 and homogeneously distributed in PMMA matrix.

Figure 3. XRD patterns (A) of (a) graphite, (b) GO, (c) MGO, and (d) PMMA-g-GO; (B) PMMA/GNSs nanocomposites with various GNSs loadings. 13



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Raman spectroscopy was conducted to characterize the structure of graphitic materials.38 Figure 4 shows the Raman spectra of graphite, GO, and GNSs. The Raman features a G band at 1573 cm–1, which gives rise to the active E2g mode of graphite and a D band at 1328 cm–1 that corresponds to the disorder-induced peaks in defective graphite structures.39, 40 In the Raman spectra of GO, the G band shifts to 1587 cm–1 and a strong D band at 1345 cm–1 emerges. The Raman of GNSs also contains D band (1335 cm–1) and G band (1590 cm–1), inferring that the crystallinity of MGO is still good after the in-situ thermal reduction. Nevertheless, the D/G intensity ratio of GNSs (I (D/G) = 1.04) is larger than that of GO (I (D/G) = 0.90), which is ascribed to the decrease in the size of sp2 domains and the increase of in the degree of disorder upon the reduction of GO. 15, 41

Figure 4. Raman spectra of (a) Graphite, (b) GO and (c) GNSs. SEM and TEM measurements were used to visualize the morphology of the GO nanosheets, and the results are shown in Figure 5. Apparently, the completely exfoliated GO nanosheets are observed with an average size of approximately 1 µm. In addition, some GO nanosheets crumple because of very thin thickness (shown in Figure 5(a)). The high-magnification of SEM image (Figure 5(b)) indicates a curled morphology and wavy structure, which are the intrinsic characteristics of GO nanosheets own.1 Figure 5(c) and (d) 14


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present a typical AFM image of GO deposited onto a mica substrate from an aqueous dispersion and the corresponding height profiles of the GO nanosheets. Based on the AFM profiles of GO, the individual GO sheets has an average height of approximately 1.25 nm, which is typical for a single atomic nanolayer.1, 8 Figure 5 (c) also shows that the nanosheets are approximately 1.0 µm in length and about 0.8 µm in width, which agree well with the TEM observation. Furthermore, the TEM images of MGO and PMMA-g-GO are present in the Supporting Information (Figure S3 (a, b)). As shown in Figure S3 (a), the MGO shows a re-stacked few layers structure, which corresponds to the hydrophobicity of MGO after functionalization. Figure S3 (b) shows that PMMA microspheres are grafted to the edges of MGO.

Figure 5. TEM image of GO nanosheets (a) and high-magnification of SEM image of GO (b), Typical AFM image and corresponding height profiles of GO nanosheets (c and d). To observe the dispersion state of GNSs in the PMMA matrix, SEM and TEM observation 15



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were used to examine the morphology of the nanocomposites. Figure 6(a–d) presents the typical TEM and SEM images of the fracture surface of the PMMA/GNSs nanocomposites. The fracture surface of the neat PMMA is relatively smooth (Figure 6(a)), whereas that of the PMMA/GNSs nanocomposites appear to be slightly rough (Figure 6(b)), in which the fluctuant edge structure and tiny protuberances are detected. Furthermore, no GNSs agglomerates could be observed in the composite sample. The homogeneously distributed protuberances suggest that the GNSs are well dispersed in the PMMA matrix.42 As shown in Figure 6 (c, d), the small red arrows point to the recognized GNSs. At low magnification (Figure 6(c)), the GNSs are well dispersed in the PMMA matrix and have good interfacial interaction with the matrix. At high magnification (Figure 6(d)), the GNSs exhibit exfoliated morphology. In addition, the TEM results also show very thin GNSs with approximately 1–5 nm in thickness, corresponding to 1 to 5 layers, which indicate that most of GNSs are exfoliated into monolayer or few layers dispersed in the PMMA matrix.

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Figure 6. Dispersion of GNSs in PMMA matrix: SEM images of fracture surface of PMMA/GNSs, (a) 0 wt.% GNSs loading and (b) 1.5 wt.% GNSs loading; Typical TEM images of PMMA/GNSs nanocomposites at 1.5 wt.% GNSs loading, (c) at low magnification and (d) at high magnification.

Thermal properties of PMMA/GNSs nanocomposites TGA is usually utilized to estimate the thermal behaviors of various materials. Figure 7 shows the TGA and the corresponding differential thermogravimetric (DTG) results for the neat PMMA and PMMA/GNSs nanocomposites with various GNSs loadings. In Figure 7(a), it is clearly that the TGA curve of the nanocomposites shifts up to high temperatures with increasing GNSs loading compared with the neat PMMA. The half-degradation temperature (T0.5) of the nanocomposites is 10.4 °C higher at 1.5 wt.% GNSs loading than that of neat 17



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PMMA. The corresponding results are listed in Table 1. The peak temperature (Tp) of the DTG curve, which indicates the maximum rate of weight loss temperature, was determined (Figure 7 (b)). The Tp of the nanocomposites are 378.9, 383.0, 384.3, and 385.4 °C for 0.50, 0.75, 1.00, and 1.50 wt.% GNSs loading, which are approximately 2.3, 6.4, 7.7, and 8.8 °C higher than that of neat PMMA, respectively. These results demonstrate that the incorporation of GNSs even at low loading (≤1.5 wt.%) can significantly enhance the thermal stability of the nanocomposites. Given the layered structure, the high aspect ratio and high thermal stability of GNSs, its incorporation in polymer matrix can affect the mobilization of polymer chains and significantly improve their thermal stability.43, 44

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Figure 7. (a) TGA and (b) DTG curves for neat PMMA and PMMA/GNSs nanocomposites. The storage modulus and loss angle tangent of the neat PMMA and PMMA/GNSs nanocomposites derived from DMA are presented in Figure 8 and Table 1. As shown in Figure 8(a), the storage modulus of the nanocomposites increases with increasing GNSs loading. This trend is consistent with the results from the tensile testing of the nanocomposites. When the GNSs loading is 1.5 wt.%, the storage modulus reaches up to the maximum (4993 MPa), which is 45% larger than that of neat PMMA (3437 MPa). The glass transition temperature (Tg) is widely measured by tangent delta peak temperature (tan δ) in DMA tests.45, 46 As seen from Figure 8(b), a slight increment in the Tg value (3.2 °C) is observed at 0.5 wt.% GNSs loading compared with the neat PMMA. As the GNSs loading increases, the value of Tg increases from 93.1 °C to 100.6 °C. The incremental changes in the Tg value are caused by the restriction of the mobility of polymer chains, which demonstrates the strong interfacial interaction between PMMA chains and GNSs. In our system, strong interactions between the PMMA-g-GO and PMMA chains exist. After the pretreatment of MGO by the miniemulsion process in the presence of MMA molecules, the MGO nanosheets 19



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are wrapped by PMMA particles, causing the formation of chemical interaction between the polymer chains and GNSs during melt blending easy. Meanwhile, the PMMA particles act as good compatibilizers to prevent the restacking of GNSs, as a result of the good distribution of GNSs in the PMMA matrix (Supporting Information Figure S1), which is crucial in reinforcing the mechanical and thermal properties of polymer nanocomposites.

Figure 8. Plots of dynamic mechanical curves for neat PMMA and PMMA/GNSs nanocomposites: (a) storage modulus; (b) Tan delta.

20


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Table 1. Thermal properties and mechanical properties of the neat PMMA and PMMA/GNSs nanocomposites

a

Tanδ (℃)

SMa(MPa)

7.03

93.1

3437

2.36±0.12

6.13

96.3

3798

54.5±1.9

2.60±0.14

5.33

97.3

4515

384.3

60.3±2.1

2.77±0.17

4.56

98.5

4810

385.4

62.6±2.0

3.26±0.18

3.73

100.6

4993

Sample

T0.5(℃)

Tp(℃)

TSa(MPa)

YMa(GPa) EBa(%)

PMMA

372.1

376.6

45.4±1.3

2.02±0.09

0.50 wt%

375.2

378.9

51.3±1.5

0.75 wt%

379.0

383.0

1.0 wt%

381.7

1.5 wt%

382.5

TS, YM, EB, and SM refer to tensile strength, Young’s modulus, elongation at break and

Storage modulus, respectively.

Mechanical properties of PMMA/GNSs nanocomposites The tensile properties of the neat PMMA and PMMA/GNSs nanocomposites are presented in Figure 9 (a). The detailed results are also listed in Table 1. Apparently, the incorporation of GNSs leads to a marked enhancement in the tensile strength and Young’s modulus even at low GNSs loading (not exceeding 1.5 wt.%). Furthermore, the elongation at the break of the nanocomposites decreases with increasing GNSs loadings. Adding 1.5 wt.% GNSs results in a 37.9% maximum increase in tensile strength and a 61.4% maximum increase in Young’s modulus. These enhanced mechanical properties are attributed to the good dispersion of GNSs in the matrix, GNSs are considered to be the strongest material with superior tensile strength (130 GPa)3 and Young’s modulus (0.25 TPa),47 which is the root of the enhancements. Furthermore, after polymerizing with MMA monomers via miniemulsion 21



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causes a strong interfacial adhesion between GNSs and the PMMA matrix.48 The Halpin–Tsai theory is widely used to predict the modulus of unidirectional or randomly distributed filler-reinforced nanocomposites.49-52 In this study, this theory was utilize to simulate the moduli of the PMMA/GNSs nanocomposites. The moduli for the randomly and unidirectionally oriented GNSs in the PMMA matrix are given by the following equations:  3 1 + ξη LVC 5 1 + 2ηT VC  EC = Em  +  8 1 − ηT VC   8 1 − η LVC

(1)

1 + ξη LVC  EParal = Em    1 − η LVC 

(2)

ηL =

ηT =

( E g Em ) − 1

(3)

( E g Em ) + ξ ( E g Em ) − 1

(4)

( E g Em ) + 2

ξ = 2α g 3 = 2lg 3tg

(5)

where EC and EParal represent the Young’s modulus of the nanocomposites with randomly distributed GNSs and the Young’s modulus of the nanocomposites with GNSs aligned parallel to the surface of sample, respectively. Eg and Em are the Young’s modulus of the GNSs and the polymer matrix, respectively. The ζ can be expressed as Eq. (5), where the symbols α g , lg , and tg refer to the average aspect ratio, length, and thickness of the GNSs. VC is the volume fraction of the GNSs. The Young’s modulus of GNSs is approximately 0.25 TPa47 and that of the PMMA matrix is 2.02 GPa based on experimental data (determined by tensile test). The densities of the PMMA and GNSs are 1.19 and 2.2 g/cm3, respectively. The statistical average values of lg and tg are 1.0 µm and 0.92 nm, respectively, as determined 22


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by the AFM and TEM analyses. By substituting these parameters into Eqs. (1)–(5), two kinds of Young’s modulus are calculated under two hypotheses: (1) GNSs are distributed randomly as a 3D network throughout the polymer matrix and (2) GNSs are aligned parallel to the surface of the nanocomposites.51, 52 The results with GNSs volume fraction of 0.27%, 0.40%, 0.54%, and 0.82% are presented in Figure 9(b). As shown in Figure 9(b), the experimental data derived from the PMMA/GNSs nanocomposites are consistent with the theoretical simulation results for the hypothesis that GNSs are randomly dispersed as 3D network in polymer matrix, thereby inferring that external tensile loads are effectively transferred from the polymer matrix to GNSs via strong interfacial adhesion.50, 53

23



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Figure 9. (a) Typical stress–strain plots of PMMA/GNSs nanocomposites with various GNSs loading. (b) Experimental Young’s modulus of the nanocomposites, calculated data obtained from the Halpin–Tsai theoretical model under the hypothesis that GNSs distributed randomly as a 3D network throughout the polymer matrix, and GNSs aligned parallel (2D) to the surface of the nanocomposites.

4. CONCLUSION In this study, PMMA/GNSs nanocomposites were fabricated using GNSs as reinforcing filler. A simple and eco-friendly method was developed, involving a miniemulsion process, melt blending, and simultaneous in-situ thermal reduction of GO in the PMMA matrix. After the grafting treatment of MGO by miniemulsion, the covalent bond of the PMMA latex particles on the edges of GNSs can effectively prevent re-stacking and aggregation of GNSs, thereby making the GNSs homogeneously dispersed in the PMMA matrix. Significant enhancement of the mechanical properties of PMMA is obtained by incorporating very low GNSs loading, indicating that an efficient load transfer from the PMMA matrix to GNSs through strong 24


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interfacial interactions. The practical Young’s modulus agrees well with the Halpin–Tsai theoretical model, in which GNSs are randomly dispersed as 3D network throughout the polymer matrix. Moreover, the thermal properties of PMMA are remarkably improved because of the barrier effect of GNSs. In summary, an eco-friendly and effective strategy for preparing PMMA/GNSs nanocomposites with excellent performance was developed. This technique can be a promising method to fabricate GNSs-based polymer nanocomposites.

■ ASSOCIATED CONTENT Supporting Information *Detailed gel fraction measurement, schematic of interactions between PMMA-g-GO nanosheets and PMMA macromolecules, TEM images of MGO and PMMA-g-GO, and electrical conductivity data of PMMA/GNSs nanocomposites are given in Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author *Tel. and Fax: 00862087112047. E-mail: [email protected]. Notes The authors declare no competing financial interest.

■ ACKNOWLEDGEMENTS The authors gratefully acknowledge the financial support from the National Natural Science 25



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Foundation of China (No. 50803017) and the Fundamental Research Funds for the Central Universities, SCUT (No. 2013zm0070).

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152x49mm (300 x 300 DPI)


In Situ Thermal Reduction of Graphene Nanosheets Based Poly

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