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Investigation of Electrical, Mechanical, and Thermal Properties of Functionalized Multiwalled Carbon Nanotubes-Reduced Graphene Oxide/PMMA Hybrid Nanocomposites.

INTRODUCTION

Due to increasing demand of high quality multifunctional materials for various applications, there are tremendous research efforts on the advanced nanostructured composites with interesting properties. Due to exciting physical properties of carbon nanotubes (CNTs) and graphene [1-8], they are considered as ideal nanofillers for composite materials that results improved electrical, mechanical, and thermal properties [9, 10]. Several investigations on high performance CNTs or graphene based polymer composites have been reported [11, 12]. Poor dispersion of CNTs/graphene in polymeric matrices and load transfer at the interface limits their potential applications in polymer composites [13, 14].

For the full realization of physical properties of CNTs/graphene in their respective polymer composites, two main critical issues must be resolved. First one is the dispersion of CNTs/graphene in polymer matrix and the second one is the interfacial interaction between CNTs/graphene and polymer matrix. On this regard, for improved dispersion of these nanofillers (CNTs/graphene) in polymer matrix, various approaches have been adopted and one of them is functionalization of CNTs/graphene surfaces with functional groups [15-18]. The functionalized fillers (CNTs/graphene) establish a chemical interaction with the host polymer in the respective composites. Functionalization also helps in effective load transfer resulting improved electrical, mechanical, and thermal properties as compared with polymer composites without functionalized CNTs or graphene [19-22]. Park et al. [23] have demonstrated enhancement in the mechanical properties of carboxyl functionalized single walled CNTs (SWCNTs) compared with unfunctionalized SWCNTs. Improved tensile modulus of functionalized SWCNT epoxy nanocomposites have been reported by Zhu et al. [24]. Enhancement in the thermal and mechanical properties of multi-walled CNTs (MWCNTs)/poly(trimethylene terephthalate) (PTT) nanocomposites have been demonstrated by Wu et al. [25] in which MWCNTs are hydroxyl functionalized and PTT is acrylic acid grafted. Significant enhancements in the mechanical properties of functionalized SWCNT/poly(vinyl alcohol) (PVA) composites have been obtained by Liu et al. [26]. Dispersion of graphene by using CNTs have been reported by various research groups [27-29]. Li et al. [30] and Kim et al. [31] have studied the effects of hybrid nanofillers on the mechanical and electrical properties. SWCNTs and graphene nanoplatelets (GnPs) hybrid filler provides synergistic effect, which results enhancement in the thermal conductivity of epoxy composites [28]. It is also reported that improved thermal conductivity and mechanical properties are observed by admixture of CNTs and GnPs in polymer compared with single filler composites [32, 33]. Patole et al. [34] have demonstrated that admixture of graphene and CNTs provide enhancements in the thermal and mechanical properties than the polymer matrix. In their method, polymer composites have been prepared by using in-situ microemulsion polymerization. In this process, the presence of monomer in the respective composite is inevitable. Enhanced mechanical (tensile strength and Young's modulus) and thermal stability has been observed in the PVA nanocomposite with reduced graphene oxide (rGO) and treated CNT as filler than the rGO/PVA and treated CNT/PVA composites [35]. In this method, reduction of graphene oxide (GO) and treated CNT have been done by using hydrazine hydrate and hence majority of the functional groups present in treated CNT are eliminated. Hence, here the CNT cannot be treated as really functionalized. Enhancement in mechanical and electrical properties have been seen in f-rGO-CNT polyimide composite which is due to the better dispersion, unique structure of the hybrid fillers and strong interactions between hybrid fillers and the matrix in the composites [36]. Wang et al. [37] have also reported increase in electrical property of the polymer composites containing rGO and CNTs. Kim et al. [38] have discussed effect of reduced graphene oxide platelets/CNT hybrid filler on the dielectric properties of polymer composite film. In most of the hybrid composites, GnP have been used as reinforcements, which have the tendency to agglomerate at higher filler content. Moreover, the detail investigations of electrical, mechanical, and thermal properties of the polymer composites containing CNTs and rGO are yet to be done.

Enhancement in electrical, mechanical, and thermal properties are expected by using functionalized MWCNTs and rGO hybrid nanofillers. In rGO, some functional groups are always present. Therefore, investigations of electrical, mechanical, and thermal properties of hybrid composites prepared by using functionalized MWCNTs and rGO in polymer is reported in this work. The improved properties are due to the synergistic effect of functionalized MWCNTs (f-MWCNTs) and rGO, which is an added advantage of the polymer composites for future applications.

EXPERIMENTAL

Materials

The materials used for this work are nitric acid (HN[O.sub.3]), sulfuric acid ([H.sub.2]S[O.sub.4]), sodium nitrate (NaN[O.sub.3]), potassium permanganate (KMn[O.sub.4]), hydrazine hydrate ([N.sub.2][H.sub.4]), hydrogen peroxide ([H.sub.2][O.sub.2]), N-N dimethyl formamide (DMF), MWCNTs with diameter 15-20 nm, graphite powder (GFG 50), and poly(methyl methacrylate) (PMMA). [H.sub.2]S[O.sub.4] and HN[O.sub.3] were supplied by FINAR reagents. NaN[O.sub.3] and KMn[O.sub.4] were supplied by S D Fine-Chem Limited. [N.sub.2][H.sub.4] and [H.sub.2][O.sub.2] were procured from Sigma-Aldrich, India and DMF was from CDH-Laboratory chemicals India. PMMA having molecular weight of 120,000 g/mol having glass transition temperature about 105[degrees]C was supplied by Sigma Aldrich.

Functionalization of MWCNTs and Composite Preparation

In order to take advantages of covalent functionalization, MWCNTs were treated with acids for surface modifications. For surface modifications, 200 mg of MWCNTs were treated with mixture of [H.sub.2]S[O.sub.4] and HN[O.sub.3] in 3:1 (by volume) ratio and the mixture was sonicated at 60[degrees]C for 5 h [39]. Then the solution was filtered and washed with deionized water followed by ethanol until it becomes neutral. Then the material was dried at 80[degrees]C for 24 h in a vacuum oven. The obtained material was labeled as f-MWCNTs. Modified Hummers method [40] was adopted to prepare GO and rGO was obtained by reducing the GO using hydrazine hydrate followed by washing and drying [41]. The f-MWCNTs-rGO based PMMA hybrid composites (FHC) with varying f-MWCNTs wt% keeping rGO wt% (0.3 wt%) constant were prepared by using solution casting method. In this method PMMA, f-MWCNTs, and rGO were separately dispersed in DMF with the help of ultrasonication and magnetic stirrer. Then the solutions were mixed and again sonicated followed by stirring. At last, the solutions were put into petri dish and then dried in a vacuum oven at 80[degrees]C and -760 Torr. By drying the DMF has been removed below its boiling point [42-44]. Moreover, the confirmation of DMF removal could be ascertain from FTIR. In the present case the absence of peaks of DMF at 1,671 and 644 [cm.sup.-1] corresponding to C=O stretching and C--N in the FTIR spectra of FHC, inferred complete removal of DMF [45, 46].

Similar method was also used to prepare MWCNTs-rGO/PMMA hybrid composite (HC) and PMMA composites with the individual fillers, that is, rGO/PMMA and f-MWCNTs/PMMA composites. The thickness of the obtained composites is approximately 0.25 mm.

Characterizations

The individual composite and hybrid composite films were recovered

for various characterizations. For structural analysis, X-ray diffraction (XRD) analysis was conducted using Rigaku diffractometer with Cu K[alpha] radiation. For chemical bonding configurations, the samples were characterized by Fourier transform infrared spectrometer (FTIR) using a PerkinElmer spectrometer in transmission mode. Raman spectra were obtained by Renishaw Raman microscope with 514.5 nm wavelength of argon-ion laser from 0 to 4,000 [cm.sup.-1]. Morphology characterizations were done by Field emission scanning electron microscopy (FESEM, Nano Nova SEM/FEI) and transmission electron microscopy (TEM) using Tecnai G220 electron microscope. For electrical measurement, resistance-temperature and current-voltage measurement was carried out. For thermal analysis, thermogravimetric (TGA) by a TA Instrument SDT 2960 and differential scanning calorimeter (DSC) measurements by alternating DSC-low temperature (ADSC) instrument at 10[degrees]C/min under nitrogen atmosphere were carried out. Mechanical tensile test was done using ASTM D 3039.

RESULTS AND DISCUSSIONS

XRD Pattern

The diffraction patterns of MWCNTs, f-MWCNTs, and rGO are shown in Fig. 1a. It is observed that rGO shows diffraction peaks at 23[degrees] and 43[degrees] due to (002) and (101) planes, which correspond to the graphitic structure [47]. Similarly, MWCNTs and f-MWCNTs show diffraction peaks at 25.7[degrees] and 43[degrees] due to (002) and (101) plane of hexagonal graphitic structure respectively. Increase in the intensity of diffraction peak at (002) of functionalized MWCNTs has been observed as compared with the pristine MWCNTs. The observed increase in intensity is due to the order MWCNTs floss in the f-MWCNTs [48]. It is evident from the similar kind of XRD patterns of the MWCNTs and f-MWCNTs that the tubular structure of f-MWCNTs is protected with defects created on the outer walls by functionalization. Similarly, Fig. 1b shows the diffraction pattern of PMMA, FHC with different wt% of f-MWCNTs. From Fig. 1b, it is observed that PMMA shows mainly two broad diffraction peaks at 13.5[degrees] and 30[degrees]. The absence of XRD peaks of f-MWCNTs and rGO in all composites are observed since they are highly covered by PMMA. These FHC also show much lower XRD intensity compared with PMMA, may be because of interfacial interaction between f-MWCNTs/rGO and PMMA matrix and reduction in crystalline size of polymer [49].

FTIR Characterization

For the confirmation of the functional groups present on MWCNTs surfaces and composites, FTIR studies have been performed. The FTIR spectra of rGO, MWCNTs, and f-MWCNTs are shown in Fig. 2a. FTIR spectrum of rGO shows the characteristic peaks at 1169 and 1,088 [cm.sup.-1] due to epoxy and carbonyl groups, 1,310 [cm.sup.-1] is attributed to C-O (carboxyl) and 3,418 [cm.sup.-1] is originated from O--H stretching vibration and water molecules present in the material. The peaks at 1,634 and 2,357 [cm.sup.-1] are assigned to C=C stretching vibration and C[O.sub.2] respectively. FTIR spectrum of MWCNTs shows peaks at 2,946 and 2,874 [cm.sup.-1] which are assigned to the C--H asymmetric and symmetric stretching vibrations and the strong broad peak at 3,657 [cm.sup.-1] is assigned to O--H stretching vibrations [50]. The peak at 1,738 [cm.sup.-1] in case of f-MWCNTs is because of acid carbonyl (C=O) stretching [51]. The strong peaks at 2,856 and 2,926 [cm.sup.-1] (Fig. 2a) are due asymmetric and symmetric stretching vibration in f-MWCNTs [51]. Figure 2b shows the FTIR spectra of PMMA, FHC with different f-MWCNTs wt%. The peak at 1,386-1,436 [cm.sup.-1] is owing to the bending vibration of C[H.sub.2] and C[H.sub.3] groups in PMMA. The peak at 1,000-1,260 [cm.sup.-1] corresponds to stretching vibration of C-O-C groups whereas the peak 2,937-3,042 [cm.sup.-1] assign to stretching vibration of C--H, respectively [52]. The peak at 1,719 [cm.sup.-1] is due to the C=O stretching vibration of the PMMA macromolecules. With the incorporations of rGO in PMMA, the peak intensities of PMMA decreases. Further decreases in intensities of FTIR peaks of PMMA have been observed with progressive incorporation of f-MWCNTs in rGOs/PMMA.

Raman Spectra

Figure 3a depicts the room temperature Raman spectra of rGO, MWCNTs, and f-MWCNTs. The typical peaks at 1,339 and 1,576 [cm.sup.-1] assign to D band and G band of rGO and MWCNTs. The D-band arises due to the disorder in the sample [53]. The G peak is a characteristic of crystalline graphite arising due to the stretching modes in the pair of [sp.sup.2] sites [39]. The intensity ratio of [I.sub.D] and [I.sub.G] ([I.sub.D]/[I.sub.G]) gives measure of defects present in the sample and its values are 0.97 and 1.24 for MWCNTs and f-MWCNTs, respectively. Relative increase in D peak intensity in Raman spectra of f-MWCNTs compared with non-functionalized MWCNTs suggests higher degree of disorder structures induced on the surface by functionalization. The observed shift of D and G peaks in Raman spectra of f-MWCNTs as compared with MWCNTs may be attributed to chemical charge transfer due to functionalization of MWCNTs [54].

To study the interaction between filler and matrix in the composites, Raman spectroscopy is also used [55]. Raman spectra of PMMA, rGO/PMMA composite at 0.3 wt% ([R.sub.0.3]), and FHC with different f-MWCNTs wt% are shown in Fig. 3b. The peaks at 2,800-3,100 [cm.sup.-1], 1,720 [cm.sup.-1], 1,450 [cm.sup.-1], and 900-1,300 [cm.sup.-1] are due to C--H stretching, C=O stretching, C--H bending, and C-O stretching as observed in the Raman spectrum of PMMA [52]. Raman spectrum of rGO/PMMA composite shows characteristics peaks of PMMA at 2,800-3,100 [cm.sup.-1] and 1,450 [cm.sup.-1] with reduced intensity. Further decrease in intensity has been observed by the incorporation of 0.05 wt% f-MWCNTs in [R.sub.0.3]. Raman spectra show significant decrease in the peak intensities with progressive incorporation of f-MWCNTs wt%. This decrease in the peak intensity of the PMMA matrix might be due to the increased content of f-MWCNTs in rGO/PMMA and similar result have also been reported by Wang et al. [56]. Moreover, shift in D and G band of FHC have also been observed with increase in f-MWCNTs wt% in [R.sub.0.3]. In addition, peak shifting is observed as compared with HC (Supporting Information Fig. S1). This shift could be due to strong chemical interaction between f-MWCNTs and PMMA [57]. The observed shift in 2D peak with slight increase in intensity in FHC than that of f-MWCNTs and rGO may be due to better dispersion and stress induced strain in the PMMA attached f-MWCNTs and rGO [58].

Morphological Analysis

The morphology of f-MWCNTs and rGO within the PMMA matrix are evidenced by FESEM and TEM micrographs. Figure 4a-c shows FESEM images of the f-MWCNTs-rGO/PMMA films prepared by solution casting technique. It is observed that the rGO flakes and MWCNT are well dispersed in the PMMA matrix. Only partial sections are visible because most parts of rGO sheets and f-MWCNTs are buried inside the PMMA matrix. The PMMA coated f-MWCNTs lying on the rGO surface are seen clearly in FESEM images. Some of the exposed rGOs sheets are folded. There may be agglomeration of f-MWCNTs or restacking of rGO in the composite; however, not observed in the surface morphology of the FESEM micrographs. The TEM images of MWCNTs and f-MWCNTs are presented in the supplementary information. It is observed that the outer surfaces of the MWCNTs are destroyed after acid treatment (see Supporting Information Fig. S2). The TEM images of FHC (0.05, 0.3, and 3 wt% of f-MWCNTs) are shown in Fig. 4d-f. From TEM image, it is observed that the bridging between PMMA grafted f-MWCNT and rGO is established resulting in a self-assembled structure. It is also evident that presence of f-MWCNTs minimizes the restacking of rGO, which results better dispersion of both the fillers in PMMA [59].

Electrical Properties

The chemical functionalization of CNTs increases the dispersability of CNTs. However, the [pi]-bonds (C=C) present in the CNT break to C--C bonds due to acid treatment, reducing electrical conductivity of CNTs [39]. The room temperature electrical conductivities of the FHC with increasing filler (f-MWCNTs) wt% are shown in Fig. 5a. At a certain loading, the fillers form conducting network that results abrupt change of electrical conductivity in the composite and is termed as percolation threshold that follow the power law relation,

[sigma] = [[sigma].sub.0] [(p -[p.sub.c]).sup.t] (1)

where, [sigma] refers to the electrical conductivity of the material, p refers to the filler wt%, [p.sub.c] is the percolation threshold, and t is the critical exponent. The obtained percolation threshold is 0.8 wt% for FHC and is more compared with HC (0.553 wt%). The percolation threshold of FHC is less than that of the f-MWCNTs/PMMA (2.735 wt%) and rGO/PMMA (1 wt%) composites (see Supporting Information Fig. S3). The observed reasonably low percolation value compared with other composites could be ascribed to the better distribution of f-MWCNTs and rGO in PMMA. It is also observed that with increasing wt% of f-MWCNTs in iGO/PMMA the electrical conductivity increases and at 4 wt% f-MWCNTs, the obtained conductivity is 1.21 X [10.sup.-3] S/cm. However, for f-MWCNTs/PMMA and rGO/PMMA composites conductivity is 1.01 x [10.sup.-7] S/cm and 8.24 x [10.sup.-4] S/cm at 4 wt% f-MWCNTs and rGO, respectively.

The electrical conductivity of FHC is more than that of f-MWCNTs/ PMMA and rGO/PMMA composites. This result may be due to efficient transfer of charge carriers by homogeneously distributed one-dimensional (1D) f-MWCNTs and two-dimensional (2D) rGO due to formation of conducting networks in f-MWCNTs-rGO/PMMA composites. The more percolation threshold of FHC than HC is due to the chemical functionalization of MWCNTs, which disturbed the extended [pi] conjugation structure of MWCNTs that result in degradation of electrical properties of MWCNTs.

To address the transport mechanism of the composites, the electrical conductivity has been measured from 60 to 300 K. Generally, Mott VRH model is used in strongly disorder semiconducting materials at low charge carrier densities [60]. According to this model, relationship between conductivity and temperature is given by [61].

[sigma] = [[sigma].sub.0]exp[-[([T.sub.0]/T).sup.Y]] (2)

where, critical exponent [gamma] depends on dimensionality of the system, [[sigma].sub.0] is the electrical conductivity at infinite temperature, and [T.sub.0] is the Mott characteristics temperature.

The graph plotted between ln([rho]) and [T.sup.-1/4] for the samples with 0.5, 1, and 3 wt% f-MWCNTs show linear behavior in the range of temperature (60 K [less than or equal to] T [less than or equal to] 300 K). Thus, the linear fit in the present case suggests three-dimensional (3D) VRH transport mechanism. The resistance of FHC with 0.05 wt% content of f-MWCNTS is very high. Hence, the result has not been presented in the R versus T. The temperature dependence electrical conductivity data is further analyzed by using magneto resistance (MR) measurement, which is defined as MR = [R(H) - R(0)]/R (0), where R(H) and R(0) stand for resistance with and without magnetic field, respectively, as shown in Fig. 5c and d. It is observed that all the composites show negative MR at 77, 50, and 35 K. Generally, the positive MR behavior is predictable for VRH electrical conduction mechanism, which is due to the application of magnetic field results contraction of the wave functions of electrons; whereas the negative MR response is due to the weak localization in nanocarbon and nanocarbon based polymer composites [62-64]. Increase in the resistance at low temperature occurs due to the constructive interference of wave functions because of back scattering (i.e., weak localization). This effect is suppressed on applying magnetic field, which causes derealization resulting decrease in resistance. It is also found that the negative MR in the sample in the VRH regime is not strikingly different from the MR in the weak localization regime by Faran and Ovadyahu [65]. However, in the present case, the resulting negative MR may be due to forward interference effects among various possible hopping paths in a magnetic field [66]. Similar results on CNTs/polymer composites have been reported by different research groups [67, 68].

Mechanical Properties

The mechanical properties of prepared PMMA composite films are determined from tensile measurements. Figure 6a represents the stress-strain graphs of PMMA, [R.sub.0.3], HC with 0.3 wt% MWCNTs, and FHC with different wt% of f-MWCNTs. The tensile strength and Young's modulus obtained for PMMA polymer are 21.19 MPa and 1.42 GPa, respectively. Incorporation of 0.05 wt% f-MWCNTs in rGO/PMMA further enhances the tensile strength and Young's modulus of PMMA. The obtained mechanical properties of FHC have been compared with the HC in which non-functionalized MWCNTs used in rGO/PMMA. More tensile strength and Young's modulus for hybrid composite with f-MWCNTs is observed than that of the hybrid nanocomposite using non-functionalized MWCNTs. These improvements in these properties of FHC is owing to the better dispersion of f-MWCNTs within the PMMA matrix, which permit a larger matrix/filler contact area and efficient load transfer from PMMA matrix to the f-MWCNTs and rGO through the strong chemical interaction. This interaction arises due to the presence of different functional groups on the MWCNTs and rGOs [69]. Enhancement in the tensile strength and Young's modulus of FHC are observed with increase in f-MWCNTs wt% and reaches a maximum at 0.5 wt% f-MWCNTs (28.86 MPa and 2.13 GPa, respectively). Tensile strength and Young's modulus decrease with further addition of f-MWCNTs. It is expected that higher concentration of f-MWCNTs in rGO/PMMA may start agglomeration that might and thereby decrease in these properties. However, the agglomeration or restacking of the f-MWCNTs or rGOs has not been observed in FESEM or TEM micrographs. Other research groups have also reported that decrease in tensile strength and modulus are observed with increase in filler loading [70, 71].

Thermal Properties

Thermal stability of neat PMMA, [R.sub.0.3], HC with 0.3 wt% MWCNTs and FHC has been characterized by TGA. Usually, higher thermal stability is observed in the polymer/CNTs or polymer/graphene composites than that of neat polymer matrix [72, 73]. Increase in thermal stability of PMMA occurs with the addition of rGO in PMMA due to interaction between rGO and PMMA (Fig. 7a). Further increase in the thermal stability of PMMA is observed by addition of f-MWCNTs in rGO/PMMA. Even the thermal stability of FHC with 0.3 wt% f-MWCNTs is comparatively more than that of the HC with same loading of MWCNTs in rGO/PMMA. The functional groups present on MWCNTs surface provide strong interaction to the polymer in the composites. Chain cleavage and radical formation indicate the beginning of thermal degradation of polymer [74]. In the composite, the f-MWCNT is acted as radical scavengers, which delayed the thermal degradation temperature resulting and improved the thermal stability of PMMA [74]. It is also seen that with increase in wt% of f-MWCNTs thermal stability increases but at higher wt% of f-MWCNTs it decreases. The heat concentrated at the probable agglomerated sites of f-MWCNTs at higher wt% in rGO/PMMA increases the heat diffusion and hence faster degradation of polymer which results decrease in thermal stability [75].

Figure 7b shows the DSC curves of PMMA, [R.sub.0.3], HC with 0.3 wt% MWCNTs and FHC. The observed strong endothermic peak approximately at 317[degrees]C is due to the thermal degradation of PMMA polymer constituents. Further increase in the degradation temperature is observed by addition of rGO in PMMA. Progressive incorporation of f-MWCNTs in rGO/PMMA results further increase in the degradation temperature is observed. However, after 0.5 wt% f-MWCNTs loading, degradation temperature decreases but still more than PMMA matrix may be due to agglomeration of f-MWCNTs, which have already been discussed in the TGA analysis. Hence, the DSC results are in accordance with the TGA results.

CONCLUSIONS

Investigated successfully the electrical, mechanical and thermal properties of hybrid composites prepared by using f-MWCNTs and rGO in PMMA. The obtained percolation threshold of FHC is 0.8 wt% with maximum conductivity 1.21 x [10.sup.-3] S/cm at 4 wt% of f-MWCNTs. Increase in the mechanical and thermal properties with progressive incorporation of f-MWCNTs in rGO/PMMA have been observed. Maximum tensile strength (28.86 MPa) and Young's modulus (2.13 GPa) are observed at 0.5 wt% f-MWCNTs. Similarly maximum enhancement in thermal stability of FHC is observed at 0.5 wt% f-MWCNTs. Improved mechanical and thermal properties of f-MWCNTs-rGO/PMMA hybrid composite are observed than that of hybrid composite without functionalization. These improved properties in the present case are because of the synergistic effect of f-MWCNTs-rGO in PMMA and the strong interaction between PMMA and f-MWCNTs-rGO.

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Kadambinee Sa, (1) Prakash Chandra Mahakul, (1) Sunirmal Saha, (1) Prakash Nath Vishwakarma, (1) Karuna Kar Nanda, (2) Pitamber Mahanandia (iD) (1)

(1) Department of Physics and Astronomy, National Institute of Technology, Rourkela, Odisha, India

(2) Materials Research Centre, Indian Institute of Science, Banaalore. Karnataka. India

Additional Supporting Information may be found in the online version of this article.

Correspondence to: P. Mahanandia; e-mail: pitam@nitrkl.ac.in

DOI 10.1002/pen.25084

Published online in Wiley Online Library (wileyonlinelibrary.com).

Caption: FIG. 1. XRD pattern of (a) rGO, MWCNTs and f-MWCNTs, (b) PMMA, FHC with different wt% of f-MWCNTs. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 2. FTIR spectra of (a) rGO, MWCNTs and f-MWCNTs, (b) PMMA, [R.sub.0.3], FHC with different wt% of f-MWCNTs. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 3. Raman spectra of (a) rGO, MWCNTs, f-MWCNTs, (b) PMMA, [R.sub.0.3], FHC with different wt% of f-MWCNTs. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 4. (a-c) FESEM and (d-f) TEM images of FHC with 0.05, 0.3 and 3 wt% f-MWCNTs respectively. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 5. (a) Electrical conductivities as a function of filler wt%, (b) ln(p) versus [T.sup.-1/4] with 0.5, 1, 3 wt% of f-MWCNTs, (c) and (d) MR at 35, 50, 77 K with 0.5 and 3 wt% of f-MWCNTs. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 6. (a) Stress-strain graph, (b) tensile strength and (c) Young's modulus of PMMA, [R.sub.0.3], HC with 0.3 wt% MWCNTs, and FHC with different wt% of f-MWCNTs. [Color figure can be viewed at wileyonlinelibrary.com]

Caption: FIG. 7. (a) TGA and (b) DSC graphs of PMMA, [R.sub.0.3], HC with 0.3 wt% MWCNTs, and FHC with different wt% of f-MWCNTs. [Color figure can be viewed at wileyonlinelibrary.com]
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Title Annotation:polymethyl methacrylate
Author:Sa, Kadambinee; Mahakul, Prakash Chandra; Saha, Sunirmal; Vishwakarma, Prakash Nath; Nanda, Karuna K
Publication:Polymer Engineering and Science
Article Type:Report
Geographic Code:9INDI
Date:May 1, 2019
Words:5790
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