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polyacrylonitrile nanocomposites using polyvinyl phenol
Composites Part B 80 (2015) 238e245
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Preparation and properties of reduced graphene oxide/ polyacrylonitrile nanocomposites using polyvinyl phenol Md. Elias Uddin a, Rama Kanta Layek a, Nam Hoon Kim a, David Hui b, Joong Hee Lee a, c, * a Advanced Materials Institute of BIN Technology (BK21 Plus Global), Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea b Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA c Carbon Composite Research Center & Department of Polymer & Nano Science and Technology, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2015 Accepted 4 June 2015 Available online 14 June 2015
Nanocomposites of polyacrylonitrile (PAN) with reduced graphene oxide (rGO) were prepared using a solution mixing technique employing polyvinyl phenol (PVP) as a compatibilizer. The PVP can facilitate composite formation by interacting with both rGO and PAN via p-p and H-bonding respectively. Various amounts of rGO were used to prepare PAN nanocomposites. The cross-sectional morphology of the composite films shows a uniform dispersion of rGO sheets in the PAN matrix. The Fourier transform infrared (FT-IR) studies revealed that good interaction of the rGO/PVP hybrid with PAN. The wide angle xray diffraction (WAXS) study confirms that the rGO sheets were uniformely dispersed as individual sheets in the PAN matrix. Thermogravimetric analysis shows enhanced thermal stability of the composite compared to pure PAN. The tensile strength and elastic modulus of the nanocomposites increased with increasing rGO content. A 102% enhancement in tensile strength and a 62.9% enhancement in elastic modulus were observed in the nanocomposite with 5% rGO. © 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Nano-structures B. Mechanical properties B. Thermal properties D. Thermal analysis
1. Introduction During the last decades, polymer nanocomposites have gained significant interest as light weight nanocomposite materials for a wide variety of applications, such as automobiles, aerospace, packaging materials, coatings, pigments and construction materials. For this purpose, various types of fillers including cellulose nanocrystal, silicate, clay, carbon black, expanded graphite, carbon nanofibers, carbon nanotubes, and graphene have been used to fabricate polymer nanocomposites [1e6]. Among these materials, graphene is the most promising material due to its high aspect ratio and intriguing thermal, mechanical and electrical properties [7]. To develop graphene-based technologies, graphene sheets must be able to be uniformly integrated into the polymer matrix [8]. However, the use of pristine graphene as a filler for composite application is very difficult due to the existence of strong van
* Corresponding author. Advanced Materials Institute of BIN Technology (BK21 Plus Global), Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea. Tel.: þ82 63 270 2342; fax: þ82 63 270 2341. E-mail address: [email protected] (J.H. Lee). http://dx.doi.org/10.1016/j.compositesb.2015.06.009 1359-8368/© 2015 Elsevier Ltd. All rights reserved.
der Waals forces among the graphene sheets, resulting in a strong aggregation potential of the graphene sheets within the polymer matrix [9]. Usually, the aggregation among graphene sheets and weak interfacial interaction between graphene sheets and polymer matrix result in weak properties of such polymer nanocomposites [10]. To overcome this drawback and make rGO sheets compatible with the polymer matrix, surface modification of graphene sheets with a suitable compatibilizer is important as a prospective method to improve the dispersibility of graphene into the polymer matrix [11e13]. Generally, two techniques, (i) covalent [14] and (ii) non-covalent [15], are widely used to enhance dispersion and make graphene compatible with the polymer matrix in order to fabricate high-performance graphene/polymer nanocomposites. Polystyrene-functionalized graphene nanocomposites with polystyrene matrix have been fabricated using the compatiblizing property of graphene functionalized polystyrene with matrix polystyrene [16]. Thermally reduced graphene oxide (rGO) was functionalized using the Bingel reaction to produce carboxylic acid groups on its surface; using the compatiblizing property of this modified graphene, a nanocomposite with epoxy resin was fabricated [17]. Non-covalently-functionalized graphene with pyrene-end poly(glycidyl methacrylate) (Py-PGMA)/epoxy
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nanocomposites were prepared as a thermally conductive nanocomposite using the compatibilizing property of Py-PGMA with epoxy resin [18]. A long alkyl chain-modified graphene/polymethyl methacrylate composite was prepared using the compatiblizing property of alkyl chains with PMMA [19]. The alkyl chain of the alkyl group-modified graphene acts as a very good compatiblizer for the fabrication of high-performance graphene polystyrene nanocomposites [20]. Non-covalently-functionalized graphene with sulphonated polyether ether ketone (SPEEK)/polyvinylidene fluoride (PVDF) nanocomposite were fabricated utilizing the compatibilizing property of SPEEK with PVDF [21]. Though there are few reports of graphene-based polymer nanocomposites using suitable compatibilizer-functionalized graphene, scientists are still facing challenges to disperse individual graphene sheet uniformly into the polymer matrix to produce excellent interfacial stress transfer [22,23]. Polyacrylonitrile (PAN) is a technologically important polymer and a major precursor for the production of carbon fibers. It has significant commercial utility and potential applications in a wide variety of fields. A large number of research efforts have been devoted to various aspects of nanocomposite processing and property development which may have potential application in the fabrication of engineering composites for industrial application. PAN is also used for the construction of packaging and fireretardant material via nanocomposite fabrication [24]. Herein, we prepared graphene-based PAN composites via a solution mixing approach using polyvinyl phenol (PVP) as a compatiblizer to enhance the physical and mechanical properties of the composite films. The rGO shows inhomogeneous dispersion in N, N-dimethylformamide (DMF) and forms an incompatible composite with PAN. Hence, we utilized polyvinyl phenol (PVP) as a compatiblizer to enhance the dispersion of rGO sheets in the PAN matrix. PVP was chosen as a compatiblizer; because it can be interact with rGO via p-p interaction, while phenolic eOH groups of PVP help to interact with PAN to produce high-performance rGO/ PAN nanocomposites. The enhancement of thermal and mechanical properties of graphene/PAN nanocomposite may have potential application in production of high-performance lightweight composite materials.
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mixture was removed from the ice bath and was stirred at 35 C for 6 h. With time, the reaction mixture changed color from black to light brown and formed a thick paste. Then, 92 mL of water was slowly added into, during which the temperature of the reaction mixture increased rapidly. Subsequently, 10 mL of 30% H2O2 was added to the reaction mixture, producing a brown to light yellow color change. The GO was purified by centrifugation with 5% HCl. Finally, GO was obtained by repeat re-dispersion in water and centrifugation and was collected by drying the pellet after centrifugation under vacuum. 2.3. Preparation of rGO 0.1 g GO powder was dispersed in 150 mL water and sonicated for 30 min to make a homogeneous brown solution. Then, hydrazine monohydrate (1 mL) was added drop wise into this aqueous GO solution, which was immersed into an oil bath at 80 C. Then, the reaction mixture was allowed to stir for 12 h. Within a few minutes, the color of the solution changed from brown to black and black particles precipitated from the solution. The resulting black powder was collected by vacuum filtration and drying in a vacuum oven at 60 C for 24 h. 2.4. Preparation of rGO/PVP hybrid PAN nanocomposite (PMG)
2. Experimental section
The nanocomposites were prepared using a solution mixing method. In this typical procedure, 1 g of PAN was dissolved in 10 mL of DMF at 90 C in a culture tube. In another culture tube, an rGO/PVP hybrid solution was prepared by dispersing the required amount of rGO in the presence of a measured amount of PVP via ultrasonication. The resulting rGO/PVP hybrid solution was mixed with PAN solution with constant stirring and allowed to sonicate for 30 min to form a homogeneous solution. Finally, the solution mixture was poured in a Petri-dish and allowed to evaporate at 50 C to produce the PMG films. The films were placed under vacuum at 60 C for 24 h to remove any trapped solvent. The resulting materials were designated as PMG1, PMG3, PMG5, and PMG6, respectively, where the numbers indicates the weight % of rGO/PVP hybrid with respect to polymer. Pure PAN film was also prepared by dissolving the PAN in DMF and evaporating the solvent at 60 C.
2.1. Materials
2.5. Characterization
Natural flake graphite was purchased from SigmaeAldrich Co. (Germany). Sulfuric acid (Showa Chemical Co. Japan, 98 wt %), hydrogen peroxide (Samchun Pure Chemical Co. Ltd, Korea, 35%), and hydrochloric acid (Showa Chemical Co. Japan, 35 wt.%) were used as received. Potassium permanganate (Junsei Chemical Co. Ltd, Japan) and hydrazine monohydrate (TCI, Japan) were used as oxidizing and reducing agents, respectively. PVP and PAN were purchased from SigmaeAldrich, USA and used as received. DMF was received from Showa Chemical Co, Japan and used without purification.
Ultra violet-visible (UVevis) spectroscopy of the GO, rGO, and rGO/PVP hybrid was carried out by dispersing the materials in DMF and recording data using a UVS-2100 SCINCO spectrophotometer. Fourier transform infrared (FT-IR) spectra of the samples were recorded over a wavenumber range of 500e4000 cme1 using a Nicolet 6700 spectrometer (Thermo Scientific, USA). Raman spectra of the samples were recorded at room temperature using a Nanofinder 30 (Tokyo Instruments Co., Osaka, Japan) over the range of 500e3000 cme1. Thermogravimetric analysis (TGA) was carried out at a heating rate of 5 C min1 from 50 to 800 C in a nitrogen atmosphere using a Q50 TGA (TA Instruments, New Castle, Delaware, USA). The cross-sectional morphology of the composite films was observed through field emission scanning electron microscopy (FE-SEM) (JSM-6701F, JEOL, Japan), after each sample was coated with osmium. Wide angle x-ray diffraction (WAXS) analysis of all samples was carried out at room temperature using a D/Max 2500 V/PC (Rigaku Corporation, Tokyo, Japan) from 2q ¼ 5e40 with a scan rate of 2 s/step. The cyclization temperatures were investigated using differential scanning calorimetry (DSC) (Perkin Elmer Pyris1). The tensile properties of the pure PAN and composite films were measured using a universal
2.2. Preparation of graphite oxide (GO) from natural flake graphite GO was synthesized by the oxidation of natural graphite flake using a modified Hummers method [25]. In this typical preparation method, 46 mL of concentrated H2SO4 (95%) was added to a round bottom flask, which was placed in an ice bath to maintain the temperature at 0e5 C during the reaction. Subsequently, 2 g of graphite flake was added slowly into the round bottom flake with constant stirring. Then, 6 g of potassium permanganate was very slowly added to the reaction mixture. After KMnO4 addition, the
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testing machine (LR5K, Lloyd Co., England) according to the ASTM D-412 standard at a strain rate of 50 mm/min at 25 ± 2 C. In order to improve the performance of graphene-based polymer composites, homogeneous dispersion of nanofillers in a specific solvent where the solubility of polymer matrix is very high is essential [19], and there is also a chance to improve the load transfer from polymer to graphene in composites. We chose DMF as a solvent to fabricate rGO/PAN nanocomposites. PAN is highly soluble in DMF, but it is very difficult to disperse the aggregated rGO in DMF, as rGO forms a stable homogeneous black dispersion in the presence of PVP in DMF. Hence, a highly soluble rGO/PAN nanocomposite can be fabricated by mixing rGO/PVP hybrid and PAN solutions and then evaporating the solvent. The solubility of the rGO/PVP hybrid in the presence of PAN composite significantly increases compared to the rGO due to the polymer/solvent synergy effect. Hence, it is expected that the rGO/PVP hybrid will form a high-performance polymer nanocomposite with PAN due to the molecular level dispersion of graphene sheets. The obtained nanocomposite films were characterized by FE-SEM, FT-IR, WAXS, TGA, DSC and mechanical property measurements. Fig. 2. UVevis absorbance spectra of pure GO, rGO, and rGO-PVP dispersions.
3. Results and discussion GO has many oxygen-containing functional groups and is hydrophilic in nature [26]. It is highly dispersible in polar solvents such as water, producing a brown homogeneous solution. The aqueous solution of GO can be treated with hydrazine hydrate to produce rGO [27]. During the reaction, the brown solution changes to a black rGO material [28]. This black rGO forms an aggregate, which precipitates from the aqueous medium. The resultant rGO is not dispersible in polar solvent like DMF, but the rGO/PVP hybrid is easily dispersible by mild sonication due to the compatiblizing nature of PVP with both rGO and DMF. Fig. 1 shows the digital photograph of the dispersions of pure GO, rGO, rGO-PVP, and rGOPVP/PAN in DMF. 3.1. UVevis spectra analysis The UVeVis spectra of GO, rGO, and rGO/PVP hybrid were recorded to illustrate the formation of GO and rGO and also to examine the interaction between rGO and PVP in DMF solution. The UVevis spectra of the samples are presented Fig. 2, where it is clear that GO exhibits a strong peak at ~230 nm, corresponding to the pp* transition of C]C in GO. Additionally, a shoulder-like absorbance peak also appeared at 295 nm due to p-p* transitions [29]. In
Fig. 1. Photographs of the dispersions of (a) pure GO, (b) rGO, (d) rGO-PVP, and (d) rGO-PVP/PAN.
rGO, the peak of the p-p* transition shifted to 280 nm, which indicates that the p-conjugated structure is restored by removing oxygen-containing functional groups [30]. The intensity of the rGO absorbance peak is lower than that of the rGO/PVP hybrid in DMF. This phenomenon indicates that dispersion of rGO is greatly improved within DMF in the presence of PVP [31]. The p-clouds of the PVP benzene rings can interact with p-clouds of rGO rings, and the polar phenolic eOH group interacts with the polar solvent DMF, which enhance the dispersion of rGO in the presence of PVP in DMF [32]. 3.2. FT-IR spectroscopy analysis To examine the formation of GO, rGO, PVP, and rGO/PVP hybrid, FT-IR spectra were collected and are presented in Fig. 3. The FT-IR spectrum of GO shows various types of oxygen-containing functional groups such as broad band at ~3426 cm1 denoting OeH stretching vibration of hydroxyl groups, at ~1730 cm1 for the C]O
Fig. 3. FT-IR spectra of pure GO, rGO, PVP, and rGO-PVP.
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moiety of eCOOH groups, and at ~1626 cm1 for intercalated water molecules/unoxidized graphitic domain. The bands at ~1156 and ~1080 cm1 are ascribed to epoxy and alkoxy stretching, respectively [33,34]. In addition, peaks at ~1399 cm1 and at ~1215 cm1 appeared, which correspond to OeH deformation and CeOH stretching vibration, respectively. The rGO sprectrum does not contain a peak for the epoxide group. However, the hydroxyl group peaks are still observed, though the intensities of these peaks are reduced significantly compared to those of GO [35]. PVP displays characteristic peaks for the aromatic benzene moiety at ~1643 and ~1510 cm1, for phenolic eOH groups at ~3420 cm1 and ~1238 cm1, and at ~823 cm1 for aromatic CeH plane [36]. The rGO/PVP shows the characteristic peaks of both rGO and PVP. The OeH streching peak in rGO/PVP hybrid appeared at ~3438 cm1 due to strong H-bonding interaction between rGO and PVP [37]. FT-IR spectra of pure PAN and the composite films were obtained to investigate the interaction of the rGO/PVP hybrid with PAN and are shown in Fig. 4. The FT-IR spectrum of pure PAN shows a characteristic peak at ~2941 cm1 for the stretching vibration of methylene groups, at ~2244 cm1 for the stretching vibration of nitrile groups, and at ~1497 cm1 for the bending vibration of methylene groups [38]. The rGO/PAN and rGO/PVP hybrid PAN composites show similar peaks similar to those of pure PAN, while no rGO or rGO/PVP hybrid peaks appeared in the composites. In the rGO/PAN composite, the peak position of the nitrile groups remains nearly constant, as in PAN, which indicates poor interaction between the PAN and rGO. However, PAN composites with rGO/PVP hybrid show a significant shift in nitrile group peak position, and PMG5 shows a significant shift from ~2244 cm1 to ~2252 cm1, which indicates strong H-bonding interaction between rGO/PVP hybrids with PAN. 3.3. Raman spectroscopy To investigate the structure of carbon based material, Raman spectroscopy is an important tool. Carbon-based materials show two main bands (D and G) that are the key vibrational modes of Raman spectroscopy. The D band is associated with the degree of
Fig. 4. FT-IR spectra of pure PAN and its composites.
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order/disorder degree due to a breathing k-point phonon of A1g symmetry. The G band, which is assigned to the E2g phonon of sp2 hybrid carbon atoms, is an indicator of the stacking structure [39]. To understand the degree of structural order/disorder of GO, rGO, and rGO/PVP hybrid, Raman spectroscopy was performed, and the results are presented in Fig. 5. In GO, the prominent D and G bands appear at ~1357 cm1 and at ~1579 cm1, respectively, and the intensity ratio (ID/IG) is ~1.04. This indicates formation of sp3 by destruction of the sp2 structure, which produces many defects in the GO [40]. In rGO, both the D and G band positions remained unchanged, but the intensity ratio of these bands (ID/IG ¼ 1.16) was increased compared to that of GO, which indicates that hydrazine treatment reduces GO [41]. The rGO/PVP hybrid shows the same peak position as in rGO but with higher intensity ratio (1.21), demonstrating a strong interaction of rGO with the compatibilizer (PVP) [42]. 3.4. FE-SEM analysis Cross sectional FE-SEM images of the freeze-fracture face of the pure PAN and its composites are presented in Fig. 6 at both low- and high-magnification. It is clear from the figure that the pure PAN film shows a flake-like morphology which is produced during solvent evaporation. The micrometer-size graphene sheets remained well dispersed sheets throughout the PAN matrix in the composite films. From this observation, we assume that a strong interfacial interaction occurs between rGO/PVP hybrids and PAN. In addition, some crystallized PAN was observed on the homogeneously dispersed rGO sheets in the PAN matrix. This phenomenon may be explained by polymer crystallization during solvent evaporation. Actually, in the composite solutions, some PAN chains, which are in close proximity to the rGO surface starts, initiate the nucleation process and form physicochemically-attached PAN crystallites on the graphene surface during solvent evaporation. 3.5. WAXS study To investigate the structures of the composite films, WAXS studies of pure PAN, and the composite films were performed and are presented in Fig. 7. The as-synthesized rGO shows a broad diffraction peak (002) at 2q ¼ ~25 and a small peak (100) at 43 ,
Fig. 5. Raman spectra of pure GO, rGO, and rGO-PVP.
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Fig. 6. FE-SEM images of the cross-sections of (a, d) pure PAN and (b, c, e and f) composites.
the removal of absorbed moisture and residual solvent. The composite films also show approximately the same weight loss in this region. Subsequently, at approximately ~270 C, carbonization occurs and produces a rapid mass loss of 22% due to the removal of non-carbon elements such as ammonia, HCN, CO2, CO, and N2. This carbonization temperature is increased in the composites and is noted at ~298 C for PMG5. Decomposition above ~300 C is attributed to back-bone degradation of pure PAN. In the composites, the PAN back-bone decomposition temperature also increased significantly. Hence, the thermal stability of the PAN composite films with rGO/PVP hybrid is improved extensively at higher temperature. The uniform dispersion and strong interfacial interaction between the rGO/PVP hybrid and PAN cause thermal stability enhancement of the nanocomposite films compared to that of pure PAN, as has been reported in other graphene-based polymer nanocomposite systems. 3.7. DSC analysis
Fig. 7. WAXS results of rGO, pure PAN, and its composites.
which indicate the short-range order of stacked graphene sheets [43]. In pure PAN, a sharp intense peak was noted at 16.89 and a small diffraction peak at 26.0 [44]. The WAXS patterns of composite films show only the diffraction peak for PAN and no characteristic rGO peak, which indicates that rGO sheets are well dispersed in the PAN matrix. 3.6. TGA analysis To examine the thermal stability of the composite films, TGA study of the pure PAN and the composite films was carried out, and the results are presented in Fig. 8 (a). It is clear that pure PAN experiences weight loss with increased temperature at several stages as shown in DTG curves of Fig. 8 (b). The weight loss in the lower temperature region (below 200 C) was ~7 wt. % and this is due to
DSC thermograms of pure PAN and PAN composites with the rGO/PVP hybrid were produced in a nitrogen atmosphere and are presented in Fig. 9. Pure PAN shows a characteristic peak at 309 C for the formation of an extended conjugated ring structure originated due to the cyclization of PAN nitrile groups by a free radical reaction [45]. It is clear from the figure that, in the nanocomposites, the cyclization peak is broader than that of pure PAN. In the composite films, this cyclization peak also shifted to lower temperature compared to that of pure PAN, and the cyclization temperature of PMG5 was 303 C. The homogeneously dispersed rGO is composed of oxygen-containing functional groups, which help to initiate cyclization reaction and therefore shift the cyclization temperature lower temperature compared to that of pure PAN [46]. 3.8. Mechanical properties In order to investigate the mechanical performance of the composite films, tensile tests of the pure PAN and the composite films were carried out, and the resulting stressestrain curve is presented in Fig. 10. The values of different mechanical parameters
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Fig. 9. DSC curves of pure PAN and its composites.
Fig. 8. (a) TGA and (b) DTG curves of pure PAN and its composites.
such as elastic modulus, tensile strength, and strain at break were obtained from the instrument software. Each measurement was repeated five times and the values obtained were used for statistical analysis, which is presented in Table 1. From the figure, it is clear that the mechanical property of PAN is similar to that of rGO1 due to the poor interfacial interaction of the rGO sheets with PAN matrix, which results in a weak load transfer from the graphene to the polymer matrix. However, the PAN composites with rGO/PVP hybrids show a significant enhancement of mechanical properties. It is also clear that tensile strength increases with rGO
Fig. 10. Stress-strain curves of pure PAN, PMG1, PMG3, PMG5, and PMG6.
concentration. This may be due to the aggregation of rGO sheets in the PAN matrix at higher concentration. Therefore, further mechanical property study of the rGO/PVP hybrids with PAN would be quite interesting. It is also obvious from the figure that strain at break decreases with increasing rGO concentration in PMG. Generally, the incorporation of nanofillers stiffens the polymer
Table 1 Mechanical properties of pure PAN and its composite films. Sample
Strain at break (MPa)
PAN rGO1 PMG1 PMG3 PMG5 PMG6
9.2 8.3 6.8 5.0 2.6 2.3
± ± ± ± ± ±
2 3 3 2 2 4
% Decrease in strain at break
Tensile strength (MPa)
e 9.8 26.1 45.7 71.7 75.0
73.4 80.6 90.2 127.2 148.3 147.4
± ± ± ± ± ±
3 4 3 2 4 2
% Increase in tensile strength
Elastic modulus (GPa)
e
3.5 3.6 3.7 3.9 5.7 6.4
9.8 22.9 73.3 102.0 100.8
± ± ± ± ± ±
0.1 0.1 0.2 0.2 0.3 0.3
% Increase in elastic modulus e 2.8 5.7 11.4 62.9 82.9
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matrix, decreasing the strain at break with enhancement of tensile strength. Our PAN composites using the rGO/PVP hybrid also show similar behavior to traditional polymer naocomposites. The PMG5 demonstrated a 102% enhancement in tensile strength compared to pure PAN. The elastic modulus of the composite films also increases progressively in the rGO/PVP hybrid nanocomposites with PAN, and a 62.9% enhancement was observed for the PMG5 sample. This type of mechanical performance of different composite films can be described in terms of nanofiller dispersion and structure-property relationships [47]. The FE-SEM and FT-IR studies show a homogeneous dispersion and interfacial interaction in the PAN composites with rGO/PVP hybrids. This interfacial interaction has a significant role in the transfer of load from graphene to polymer matrix, which enhances the mechanical properties of the matrix.
4. Conclusions In conclusion, we prepared PAN nanocomposites with rGO/PVP hybrid using a solvent cast technique and the compatibilizing properties of PVP with both rGO and PAN. The rGO/PVP hybrid was characterized by solubility study, UVevis, FT-IR, and Raman spectroscopy. The composites were characterized by FE-SEM, FT-IR, WAXS, TGA, DSC, and mechanical studies. FE-SEM analysis provides information about the homogeneous dispersion of rGO sheets in PAN matrix. WAXS, DSC, and FT-IR studies indicate that a strong interfacial interaction occurs between PAN and rGO/PVP hybrids. TGA results indicate thermal stability improvement of the PAN composites compared to pure PAN. Mechanical property measurements show a 102% enhancement in tensile strength and a 62.9% enhancement in elastic modulus for PMG5 compared to pure PAN.
Acknowledgements This study was supported by the Converging Research Center Program (2014M3C1A8048834) and the Basic Research Laboratory Program (2014R1A4A1008140) through the Ministry of Science, ICT & Future Planning. This was also supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Education of Korea (2013R1A1A2011608).
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Contents lists available at ScienceDirect
Composites Part B journal homepage: www.elsevier.com/locate/compositesb
Preparation and properties of reduced graphene oxide/ polyacrylonitrile nanocomposites using polyvinyl phenol Md. Elias Uddin a, Rama Kanta Layek a, Nam Hoon Kim a, David Hui b, Joong Hee Lee a, c, * a Advanced Materials Institute of BIN Technology (BK21 Plus Global), Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea b Department of Mechanical Engineering, University of New Orleans, New Orleans, LA 70148, USA c Carbon Composite Research Center & Department of Polymer & Nano Science and Technology, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 March 2015 Accepted 4 June 2015 Available online 14 June 2015
Nanocomposites of polyacrylonitrile (PAN) with reduced graphene oxide (rGO) were prepared using a solution mixing technique employing polyvinyl phenol (PVP) as a compatibilizer. The PVP can facilitate composite formation by interacting with both rGO and PAN via p-p and H-bonding respectively. Various amounts of rGO were used to prepare PAN nanocomposites. The cross-sectional morphology of the composite films shows a uniform dispersion of rGO sheets in the PAN matrix. The Fourier transform infrared (FT-IR) studies revealed that good interaction of the rGO/PVP hybrid with PAN. The wide angle xray diffraction (WAXS) study confirms that the rGO sheets were uniformely dispersed as individual sheets in the PAN matrix. Thermogravimetric analysis shows enhanced thermal stability of the composite compared to pure PAN. The tensile strength and elastic modulus of the nanocomposites increased with increasing rGO content. A 102% enhancement in tensile strength and a 62.9% enhancement in elastic modulus were observed in the nanocomposite with 5% rGO. © 2015 Elsevier Ltd. All rights reserved.
Keywords: A. Nano-structures B. Mechanical properties B. Thermal properties D. Thermal analysis
1. Introduction During the last decades, polymer nanocomposites have gained significant interest as light weight nanocomposite materials for a wide variety of applications, such as automobiles, aerospace, packaging materials, coatings, pigments and construction materials. For this purpose, various types of fillers including cellulose nanocrystal, silicate, clay, carbon black, expanded graphite, carbon nanofibers, carbon nanotubes, and graphene have been used to fabricate polymer nanocomposites [1e6]. Among these materials, graphene is the most promising material due to its high aspect ratio and intriguing thermal, mechanical and electrical properties [7]. To develop graphene-based technologies, graphene sheets must be able to be uniformly integrated into the polymer matrix [8]. However, the use of pristine graphene as a filler for composite application is very difficult due to the existence of strong van
* Corresponding author. Advanced Materials Institute of BIN Technology (BK21 Plus Global), Department of BIN Convergence Technology, Chonbuk National University, Jeonju, Jeonbuk 561-756, Republic of Korea. Tel.: þ82 63 270 2342; fax: þ82 63 270 2341. E-mail address: [email protected] (J.H. Lee). http://dx.doi.org/10.1016/j.compositesb.2015.06.009 1359-8368/© 2015 Elsevier Ltd. All rights reserved.
der Waals forces among the graphene sheets, resulting in a strong aggregation potential of the graphene sheets within the polymer matrix [9]. Usually, the aggregation among graphene sheets and weak interfacial interaction between graphene sheets and polymer matrix result in weak properties of such polymer nanocomposites [10]. To overcome this drawback and make rGO sheets compatible with the polymer matrix, surface modification of graphene sheets with a suitable compatibilizer is important as a prospective method to improve the dispersibility of graphene into the polymer matrix [11e13]. Generally, two techniques, (i) covalent [14] and (ii) non-covalent [15], are widely used to enhance dispersion and make graphene compatible with the polymer matrix in order to fabricate high-performance graphene/polymer nanocomposites. Polystyrene-functionalized graphene nanocomposites with polystyrene matrix have been fabricated using the compatiblizing property of graphene functionalized polystyrene with matrix polystyrene [16]. Thermally reduced graphene oxide (rGO) was functionalized using the Bingel reaction to produce carboxylic acid groups on its surface; using the compatiblizing property of this modified graphene, a nanocomposite with epoxy resin was fabricated [17]. Non-covalently-functionalized graphene with pyrene-end poly(glycidyl methacrylate) (Py-PGMA)/epoxy
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nanocomposites were prepared as a thermally conductive nanocomposite using the compatibilizing property of Py-PGMA with epoxy resin [18]. A long alkyl chain-modified graphene/polymethyl methacrylate composite was prepared using the compatiblizing property of alkyl chains with PMMA [19]. The alkyl chain of the alkyl group-modified graphene acts as a very good compatiblizer for the fabrication of high-performance graphene polystyrene nanocomposites [20]. Non-covalently-functionalized graphene with sulphonated polyether ether ketone (SPEEK)/polyvinylidene fluoride (PVDF) nanocomposite were fabricated utilizing the compatibilizing property of SPEEK with PVDF [21]. Though there are few reports of graphene-based polymer nanocomposites using suitable compatibilizer-functionalized graphene, scientists are still facing challenges to disperse individual graphene sheet uniformly into the polymer matrix to produce excellent interfacial stress transfer [22,23]. Polyacrylonitrile (PAN) is a technologically important polymer and a major precursor for the production of carbon fibers. It has significant commercial utility and potential applications in a wide variety of fields. A large number of research efforts have been devoted to various aspects of nanocomposite processing and property development which may have potential application in the fabrication of engineering composites for industrial application. PAN is also used for the construction of packaging and fireretardant material via nanocomposite fabrication [24]. Herein, we prepared graphene-based PAN composites via a solution mixing approach using polyvinyl phenol (PVP) as a compatiblizer to enhance the physical and mechanical properties of the composite films. The rGO shows inhomogeneous dispersion in N, N-dimethylformamide (DMF) and forms an incompatible composite with PAN. Hence, we utilized polyvinyl phenol (PVP) as a compatiblizer to enhance the dispersion of rGO sheets in the PAN matrix. PVP was chosen as a compatiblizer; because it can be interact with rGO via p-p interaction, while phenolic eOH groups of PVP help to interact with PAN to produce high-performance rGO/ PAN nanocomposites. The enhancement of thermal and mechanical properties of graphene/PAN nanocomposite may have potential application in production of high-performance lightweight composite materials.
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mixture was removed from the ice bath and was stirred at 35 C for 6 h. With time, the reaction mixture changed color from black to light brown and formed a thick paste. Then, 92 mL of water was slowly added into, during which the temperature of the reaction mixture increased rapidly. Subsequently, 10 mL of 30% H2O2 was added to the reaction mixture, producing a brown to light yellow color change. The GO was purified by centrifugation with 5% HCl. Finally, GO was obtained by repeat re-dispersion in water and centrifugation and was collected by drying the pellet after centrifugation under vacuum. 2.3. Preparation of rGO 0.1 g GO powder was dispersed in 150 mL water and sonicated for 30 min to make a homogeneous brown solution. Then, hydrazine monohydrate (1 mL) was added drop wise into this aqueous GO solution, which was immersed into an oil bath at 80 C. Then, the reaction mixture was allowed to stir for 12 h. Within a few minutes, the color of the solution changed from brown to black and black particles precipitated from the solution. The resulting black powder was collected by vacuum filtration and drying in a vacuum oven at 60 C for 24 h. 2.4. Preparation of rGO/PVP hybrid PAN nanocomposite (PMG)
2. Experimental section
The nanocomposites were prepared using a solution mixing method. In this typical procedure, 1 g of PAN was dissolved in 10 mL of DMF at 90 C in a culture tube. In another culture tube, an rGO/PVP hybrid solution was prepared by dispersing the required amount of rGO in the presence of a measured amount of PVP via ultrasonication. The resulting rGO/PVP hybrid solution was mixed with PAN solution with constant stirring and allowed to sonicate for 30 min to form a homogeneous solution. Finally, the solution mixture was poured in a Petri-dish and allowed to evaporate at 50 C to produce the PMG films. The films were placed under vacuum at 60 C for 24 h to remove any trapped solvent. The resulting materials were designated as PMG1, PMG3, PMG5, and PMG6, respectively, where the numbers indicates the weight % of rGO/PVP hybrid with respect to polymer. Pure PAN film was also prepared by dissolving the PAN in DMF and evaporating the solvent at 60 C.
2.1. Materials
2.5. Characterization
Natural flake graphite was purchased from SigmaeAldrich Co. (Germany). Sulfuric acid (Showa Chemical Co. Japan, 98 wt %), hydrogen peroxide (Samchun Pure Chemical Co. Ltd, Korea, 35%), and hydrochloric acid (Showa Chemical Co. Japan, 35 wt.%) were used as received. Potassium permanganate (Junsei Chemical Co. Ltd, Japan) and hydrazine monohydrate (TCI, Japan) were used as oxidizing and reducing agents, respectively. PVP and PAN were purchased from SigmaeAldrich, USA and used as received. DMF was received from Showa Chemical Co, Japan and used without purification.
Ultra violet-visible (UVevis) spectroscopy of the GO, rGO, and rGO/PVP hybrid was carried out by dispersing the materials in DMF and recording data using a UVS-2100 SCINCO spectrophotometer. Fourier transform infrared (FT-IR) spectra of the samples were recorded over a wavenumber range of 500e4000 cme1 using a Nicolet 6700 spectrometer (Thermo Scientific, USA). Raman spectra of the samples were recorded at room temperature using a Nanofinder 30 (Tokyo Instruments Co., Osaka, Japan) over the range of 500e3000 cme1. Thermogravimetric analysis (TGA) was carried out at a heating rate of 5 C min1 from 50 to 800 C in a nitrogen atmosphere using a Q50 TGA (TA Instruments, New Castle, Delaware, USA). The cross-sectional morphology of the composite films was observed through field emission scanning electron microscopy (FE-SEM) (JSM-6701F, JEOL, Japan), after each sample was coated with osmium. Wide angle x-ray diffraction (WAXS) analysis of all samples was carried out at room temperature using a D/Max 2500 V/PC (Rigaku Corporation, Tokyo, Japan) from 2q ¼ 5e40 with a scan rate of 2 s/step. The cyclization temperatures were investigated using differential scanning calorimetry (DSC) (Perkin Elmer Pyris1). The tensile properties of the pure PAN and composite films were measured using a universal
2.2. Preparation of graphite oxide (GO) from natural flake graphite GO was synthesized by the oxidation of natural graphite flake using a modified Hummers method [25]. In this typical preparation method, 46 mL of concentrated H2SO4 (95%) was added to a round bottom flask, which was placed in an ice bath to maintain the temperature at 0e5 C during the reaction. Subsequently, 2 g of graphite flake was added slowly into the round bottom flake with constant stirring. Then, 6 g of potassium permanganate was very slowly added to the reaction mixture. After KMnO4 addition, the
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testing machine (LR5K, Lloyd Co., England) according to the ASTM D-412 standard at a strain rate of 50 mm/min at 25 ± 2 C. In order to improve the performance of graphene-based polymer composites, homogeneous dispersion of nanofillers in a specific solvent where the solubility of polymer matrix is very high is essential [19], and there is also a chance to improve the load transfer from polymer to graphene in composites. We chose DMF as a solvent to fabricate rGO/PAN nanocomposites. PAN is highly soluble in DMF, but it is very difficult to disperse the aggregated rGO in DMF, as rGO forms a stable homogeneous black dispersion in the presence of PVP in DMF. Hence, a highly soluble rGO/PAN nanocomposite can be fabricated by mixing rGO/PVP hybrid and PAN solutions and then evaporating the solvent. The solubility of the rGO/PVP hybrid in the presence of PAN composite significantly increases compared to the rGO due to the polymer/solvent synergy effect. Hence, it is expected that the rGO/PVP hybrid will form a high-performance polymer nanocomposite with PAN due to the molecular level dispersion of graphene sheets. The obtained nanocomposite films were characterized by FE-SEM, FT-IR, WAXS, TGA, DSC and mechanical property measurements. Fig. 2. UVevis absorbance spectra of pure GO, rGO, and rGO-PVP dispersions.
3. Results and discussion GO has many oxygen-containing functional groups and is hydrophilic in nature [26]. It is highly dispersible in polar solvents such as water, producing a brown homogeneous solution. The aqueous solution of GO can be treated with hydrazine hydrate to produce rGO [27]. During the reaction, the brown solution changes to a black rGO material [28]. This black rGO forms an aggregate, which precipitates from the aqueous medium. The resultant rGO is not dispersible in polar solvent like DMF, but the rGO/PVP hybrid is easily dispersible by mild sonication due to the compatiblizing nature of PVP with both rGO and DMF. Fig. 1 shows the digital photograph of the dispersions of pure GO, rGO, rGO-PVP, and rGOPVP/PAN in DMF. 3.1. UVevis spectra analysis The UVeVis spectra of GO, rGO, and rGO/PVP hybrid were recorded to illustrate the formation of GO and rGO and also to examine the interaction between rGO and PVP in DMF solution. The UVevis spectra of the samples are presented Fig. 2, where it is clear that GO exhibits a strong peak at ~230 nm, corresponding to the pp* transition of C]C in GO. Additionally, a shoulder-like absorbance peak also appeared at 295 nm due to p-p* transitions [29]. In
Fig. 1. Photographs of the dispersions of (a) pure GO, (b) rGO, (d) rGO-PVP, and (d) rGO-PVP/PAN.
rGO, the peak of the p-p* transition shifted to 280 nm, which indicates that the p-conjugated structure is restored by removing oxygen-containing functional groups [30]. The intensity of the rGO absorbance peak is lower than that of the rGO/PVP hybrid in DMF. This phenomenon indicates that dispersion of rGO is greatly improved within DMF in the presence of PVP [31]. The p-clouds of the PVP benzene rings can interact with p-clouds of rGO rings, and the polar phenolic eOH group interacts with the polar solvent DMF, which enhance the dispersion of rGO in the presence of PVP in DMF [32]. 3.2. FT-IR spectroscopy analysis To examine the formation of GO, rGO, PVP, and rGO/PVP hybrid, FT-IR spectra were collected and are presented in Fig. 3. The FT-IR spectrum of GO shows various types of oxygen-containing functional groups such as broad band at ~3426 cm1 denoting OeH stretching vibration of hydroxyl groups, at ~1730 cm1 for the C]O
Fig. 3. FT-IR spectra of pure GO, rGO, PVP, and rGO-PVP.
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moiety of eCOOH groups, and at ~1626 cm1 for intercalated water molecules/unoxidized graphitic domain. The bands at ~1156 and ~1080 cm1 are ascribed to epoxy and alkoxy stretching, respectively [33,34]. In addition, peaks at ~1399 cm1 and at ~1215 cm1 appeared, which correspond to OeH deformation and CeOH stretching vibration, respectively. The rGO sprectrum does not contain a peak for the epoxide group. However, the hydroxyl group peaks are still observed, though the intensities of these peaks are reduced significantly compared to those of GO [35]. PVP displays characteristic peaks for the aromatic benzene moiety at ~1643 and ~1510 cm1, for phenolic eOH groups at ~3420 cm1 and ~1238 cm1, and at ~823 cm1 for aromatic CeH plane [36]. The rGO/PVP shows the characteristic peaks of both rGO and PVP. The OeH streching peak in rGO/PVP hybrid appeared at ~3438 cm1 due to strong H-bonding interaction between rGO and PVP [37]. FT-IR spectra of pure PAN and the composite films were obtained to investigate the interaction of the rGO/PVP hybrid with PAN and are shown in Fig. 4. The FT-IR spectrum of pure PAN shows a characteristic peak at ~2941 cm1 for the stretching vibration of methylene groups, at ~2244 cm1 for the stretching vibration of nitrile groups, and at ~1497 cm1 for the bending vibration of methylene groups [38]. The rGO/PAN and rGO/PVP hybrid PAN composites show similar peaks similar to those of pure PAN, while no rGO or rGO/PVP hybrid peaks appeared in the composites. In the rGO/PAN composite, the peak position of the nitrile groups remains nearly constant, as in PAN, which indicates poor interaction between the PAN and rGO. However, PAN composites with rGO/PVP hybrid show a significant shift in nitrile group peak position, and PMG5 shows a significant shift from ~2244 cm1 to ~2252 cm1, which indicates strong H-bonding interaction between rGO/PVP hybrids with PAN. 3.3. Raman spectroscopy To investigate the structure of carbon based material, Raman spectroscopy is an important tool. Carbon-based materials show two main bands (D and G) that are the key vibrational modes of Raman spectroscopy. The D band is associated with the degree of
Fig. 4. FT-IR spectra of pure PAN and its composites.
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order/disorder degree due to a breathing k-point phonon of A1g symmetry. The G band, which is assigned to the E2g phonon of sp2 hybrid carbon atoms, is an indicator of the stacking structure [39]. To understand the degree of structural order/disorder of GO, rGO, and rGO/PVP hybrid, Raman spectroscopy was performed, and the results are presented in Fig. 5. In GO, the prominent D and G bands appear at ~1357 cm1 and at ~1579 cm1, respectively, and the intensity ratio (ID/IG) is ~1.04. This indicates formation of sp3 by destruction of the sp2 structure, which produces many defects in the GO [40]. In rGO, both the D and G band positions remained unchanged, but the intensity ratio of these bands (ID/IG ¼ 1.16) was increased compared to that of GO, which indicates that hydrazine treatment reduces GO [41]. The rGO/PVP hybrid shows the same peak position as in rGO but with higher intensity ratio (1.21), demonstrating a strong interaction of rGO with the compatibilizer (PVP) [42]. 3.4. FE-SEM analysis Cross sectional FE-SEM images of the freeze-fracture face of the pure PAN and its composites are presented in Fig. 6 at both low- and high-magnification. It is clear from the figure that the pure PAN film shows a flake-like morphology which is produced during solvent evaporation. The micrometer-size graphene sheets remained well dispersed sheets throughout the PAN matrix in the composite films. From this observation, we assume that a strong interfacial interaction occurs between rGO/PVP hybrids and PAN. In addition, some crystallized PAN was observed on the homogeneously dispersed rGO sheets in the PAN matrix. This phenomenon may be explained by polymer crystallization during solvent evaporation. Actually, in the composite solutions, some PAN chains, which are in close proximity to the rGO surface starts, initiate the nucleation process and form physicochemically-attached PAN crystallites on the graphene surface during solvent evaporation. 3.5. WAXS study To investigate the structures of the composite films, WAXS studies of pure PAN, and the composite films were performed and are presented in Fig. 7. The as-synthesized rGO shows a broad diffraction peak (002) at 2q ¼ ~25 and a small peak (100) at 43 ,
Fig. 5. Raman spectra of pure GO, rGO, and rGO-PVP.
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Fig. 6. FE-SEM images of the cross-sections of (a, d) pure PAN and (b, c, e and f) composites.
the removal of absorbed moisture and residual solvent. The composite films also show approximately the same weight loss in this region. Subsequently, at approximately ~270 C, carbonization occurs and produces a rapid mass loss of 22% due to the removal of non-carbon elements such as ammonia, HCN, CO2, CO, and N2. This carbonization temperature is increased in the composites and is noted at ~298 C for PMG5. Decomposition above ~300 C is attributed to back-bone degradation of pure PAN. In the composites, the PAN back-bone decomposition temperature also increased significantly. Hence, the thermal stability of the PAN composite films with rGO/PVP hybrid is improved extensively at higher temperature. The uniform dispersion and strong interfacial interaction between the rGO/PVP hybrid and PAN cause thermal stability enhancement of the nanocomposite films compared to that of pure PAN, as has been reported in other graphene-based polymer nanocomposite systems. 3.7. DSC analysis
Fig. 7. WAXS results of rGO, pure PAN, and its composites.
which indicate the short-range order of stacked graphene sheets [43]. In pure PAN, a sharp intense peak was noted at 16.89 and a small diffraction peak at 26.0 [44]. The WAXS patterns of composite films show only the diffraction peak for PAN and no characteristic rGO peak, which indicates that rGO sheets are well dispersed in the PAN matrix. 3.6. TGA analysis To examine the thermal stability of the composite films, TGA study of the pure PAN and the composite films was carried out, and the results are presented in Fig. 8 (a). It is clear that pure PAN experiences weight loss with increased temperature at several stages as shown in DTG curves of Fig. 8 (b). The weight loss in the lower temperature region (below 200 C) was ~7 wt. % and this is due to
DSC thermograms of pure PAN and PAN composites with the rGO/PVP hybrid were produced in a nitrogen atmosphere and are presented in Fig. 9. Pure PAN shows a characteristic peak at 309 C for the formation of an extended conjugated ring structure originated due to the cyclization of PAN nitrile groups by a free radical reaction [45]. It is clear from the figure that, in the nanocomposites, the cyclization peak is broader than that of pure PAN. In the composite films, this cyclization peak also shifted to lower temperature compared to that of pure PAN, and the cyclization temperature of PMG5 was 303 C. The homogeneously dispersed rGO is composed of oxygen-containing functional groups, which help to initiate cyclization reaction and therefore shift the cyclization temperature lower temperature compared to that of pure PAN [46]. 3.8. Mechanical properties In order to investigate the mechanical performance of the composite films, tensile tests of the pure PAN and the composite films were carried out, and the resulting stressestrain curve is presented in Fig. 10. The values of different mechanical parameters
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Fig. 9. DSC curves of pure PAN and its composites.
Fig. 8. (a) TGA and (b) DTG curves of pure PAN and its composites.
such as elastic modulus, tensile strength, and strain at break were obtained from the instrument software. Each measurement was repeated five times and the values obtained were used for statistical analysis, which is presented in Table 1. From the figure, it is clear that the mechanical property of PAN is similar to that of rGO1 due to the poor interfacial interaction of the rGO sheets with PAN matrix, which results in a weak load transfer from the graphene to the polymer matrix. However, the PAN composites with rGO/PVP hybrids show a significant enhancement of mechanical properties. It is also clear that tensile strength increases with rGO
Fig. 10. Stress-strain curves of pure PAN, PMG1, PMG3, PMG5, and PMG6.
concentration. This may be due to the aggregation of rGO sheets in the PAN matrix at higher concentration. Therefore, further mechanical property study of the rGO/PVP hybrids with PAN would be quite interesting. It is also obvious from the figure that strain at break decreases with increasing rGO concentration in PMG. Generally, the incorporation of nanofillers stiffens the polymer
Table 1 Mechanical properties of pure PAN and its composite films. Sample
Strain at break (MPa)
PAN rGO1 PMG1 PMG3 PMG5 PMG6
9.2 8.3 6.8 5.0 2.6 2.3
± ± ± ± ± ±
2 3 3 2 2 4
% Decrease in strain at break
Tensile strength (MPa)
e 9.8 26.1 45.7 71.7 75.0
73.4 80.6 90.2 127.2 148.3 147.4
± ± ± ± ± ±
3 4 3 2 4 2
% Increase in tensile strength
Elastic modulus (GPa)
e
3.5 3.6 3.7 3.9 5.7 6.4
9.8 22.9 73.3 102.0 100.8
± ± ± ± ± ±
0.1 0.1 0.2 0.2 0.3 0.3
% Increase in elastic modulus e 2.8 5.7 11.4 62.9 82.9
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matrix, decreasing the strain at break with enhancement of tensile strength. Our PAN composites using the rGO/PVP hybrid also show similar behavior to traditional polymer naocomposites. The PMG5 demonstrated a 102% enhancement in tensile strength compared to pure PAN. The elastic modulus of the composite films also increases progressively in the rGO/PVP hybrid nanocomposites with PAN, and a 62.9% enhancement was observed for the PMG5 sample. This type of mechanical performance of different composite films can be described in terms of nanofiller dispersion and structure-property relationships [47]. The FE-SEM and FT-IR studies show a homogeneous dispersion and interfacial interaction in the PAN composites with rGO/PVP hybrids. This interfacial interaction has a significant role in the transfer of load from graphene to polymer matrix, which enhances the mechanical properties of the matrix.
4. Conclusions In conclusion, we prepared PAN nanocomposites with rGO/PVP hybrid using a solvent cast technique and the compatibilizing properties of PVP with both rGO and PAN. The rGO/PVP hybrid was characterized by solubility study, UVevis, FT-IR, and Raman spectroscopy. The composites were characterized by FE-SEM, FT-IR, WAXS, TGA, DSC, and mechanical studies. FE-SEM analysis provides information about the homogeneous dispersion of rGO sheets in PAN matrix. WAXS, DSC, and FT-IR studies indicate that a strong interfacial interaction occurs between PAN and rGO/PVP hybrids. TGA results indicate thermal stability improvement of the PAN composites compared to pure PAN. Mechanical property measurements show a 102% enhancement in tensile strength and a 62.9% enhancement in elastic modulus for PMG5 compared to pure PAN.
Acknowledgements This study was supported by the Converging Research Center Program (2014M3C1A8048834) and the Basic Research Laboratory Program (2014R1A4A1008140) through the Ministry of Science, ICT & Future Planning. This was also supported by the Basic Science Research Program through the National Research Foundation (NRF) funded by the Ministry of Education of Korea (2013R1A1A2011608).
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