Advanced Materials Research Vol. 812 (2013) pp 263-266 Online available since 2013/Sep/10 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/AMR.812.263
Electrical Properties of Graphene Filled Natural Rubber Composites SRINIVASARAO Yaragalla1,a RI HANUM Yahaya Subban2,b* CHIN Chan Han2,c NANDAKUMAR Kalarikkal1,d SabuThomas1,e 1
Center for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, 686560 Kerala, India. 2
Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Malaysia.
a
[email protected] [email protected] [email protected] d
[email protected] [email protected]
Key words: Graphene, Natural Rubber, Polymer Composites.
Abstract: Thermally reduced graphene oxide (graphene) filled natural rubber (NR) composites were fabricated by melt mixing method. Dielectric constant, dielectric loss and a.c conductivity data of the NR composites are reported. Highest conductivity of 3 x 10-4 S/m was obtained for the composite with 3 wt. % graphene with initial electrical percolation at a loading of 0.5 wt. %. High conductivity in the composite with 3 wt. % graphene is accounted by its homogeneity as observed in SEM micrographs. Introduction In recent years, graphene filled polymer composites have attracted much attention owing to their unique electric, mechanical and optical properties [1, 2]. Many studies have been reported on preparation of graphene from natural graphite exfoliation and their composites for conductive applications [5, 6, 7]. Graphite contains a mixture of graphene layers which are stacked together by weak Van der Waal’s forces. Hence, it is easy to separate the layers from graphite through exfoliation. Different kinds of graphite fillers such as improved graphene oxide, thermally treated graphite, chemically modified graphene have been used to prepare polymer composites [8, 9]. The major problem with graphene is dispersion. For improving dispersion of graphene, chemical modification is needed. Functional groups introduced during the modification may enhance interfacial interactions between graphene and the polymer matrix giving rise to enhanced properties. Omar et al [9] evaluated microwave and dielectric properties of graphene filled NR composites prepared by two roll mill mixing at microwave frequency region (1-12 GHz) using graphene obtained from Hayzen Engineering Co. However, no a.c conductivity data of the composite was reported. In the present study, graphene fabricated in our laboratory is used for fabrication of graphene-NR composites via melt mixing method. The dielectric constant, dielctric loss and A.C conductivity values are reported. Experimental Graphite oxide was prepared by using graphite powder through previously reported method [10]. Thermal reduction of graphene oxide into graphene was carried out for 60 minutes in a furnace at 200°C. Graphene filled NR composites were prepared by melt mixing method for 10 minutes at 125°C using Haake mixer followed by addition of curatives via two roll mill method at room temperature. The additives used in this process are 5g of ZnO, 2g of stearic acid, 0.5 g of tetra methyl thiuram mono sulphide, 1.5 g of cyclohexyl-2-benzothazolesulfenamide and 2.5 g of sulphur. The total mixing time was around 20 minutes for each composition. After mixing, the samples were compression molded based on their cure time at 160oC.
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The prepared samples were characterised by transeference elctron microscopy using JEOL JEM 2100 for its morphological properties to identify exfoliated graphene. 5mg of graphene was mixed with 10 ml of THF solvent and then sonicated for 5 minutes. A few drops of sonicated solvent (graphene+THF) was cast on TEM grid before running the sample. A.c impedance was carried out using HIOKI Impedance analyzer to determine the electrical properties of the composites in the frequency range of 200 Hz to 1 MHz. The electrically tested samples were in a disc shape with thickness of 2 mm and diameter of 2 cm. Scaning electron microscope was carried out to study the distribution of graphene in the NR matrix using JEOL (JSM-6390) with an accelerating voltage of 15 kV. The samples were broken by using liquid nitrogen and were coated with platinum for analysis. Results and Discussion SEM micrographs of graphite and graphene shown in Fig. 1 clearly indicates the exfoliation of graphite layers into graphene sheets to confirm that graphene has been exfoliated from graphite. This is supported by the TEM image in (c) which shows the exfoliated graphene sheet.
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Fig. 1: SEM images of (a) Graphite (b) Graphene and (c) TEM image of grapheme The dielectric constant, dielectric loss and a.c conductivity of graphene filled NR composites is portrayed in Fig. 2. From Fig. 2, it is clear that the dielectric constant and dielctric loss increased with addition of graphene. This is because of the polarity induced by graphene in the NR matrix. The dielectric constant decreased with increasing frequency and is independent of it for frequencies beyond 104 Hz. This is due to insufficient time to relax the polymer chains at higher frequencies. The a.c conductivity increased with addition of graphene loading. This is due to increase in delocalization of pi electrons of graphene and connectivity of graphene particles in the NR which forms a conducting pathway leading to improvement in the conductivity. The percolation begins at graphene loading of 0.5 wt. % as shown in (d) where the conductivity suddenly increased and gradually increased thereafter. Fig. 3 shows the SEM micrographs of the NR composites. It is obvious from the figure that the composite with 3 wt. % graphene is homogeneous indicating the even distribution of graphene in the NR matrix. This even distribution is believed to have given rise to better conducting pathways giving rise to higher conductivity.
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Fig. 2: (a) Dielectric constant (b) Dielectric loss (c) A.C Conductivity of graphene filled NR composites and (d) A.C Conductivity at 1 MHz
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Fig. 3: SEM images of (a) NR gum and graphene-NR composites with (b) 0.5 wt% (c) 2 wt% and (d) 3 wt% of grapheme
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Conclusion Graphene has been successfully prepared from graphite and used to fabricate grapheme- NR composites through environmentally friendly melt mixing method. Graphene loading of 3 wt. % showed high dielectric constant of more than 8 with conductivity of 3 x 10-4 S/m. SEM images revealed homogeneous distribution of graphene for loading of 3 wt%. To optimize the dielectric and electrical properties of NR composites, more graphene have to be added and functionalization is needed in order to enhance the dispersion of graphene in the NR matrix. Acknowledgement We would like to express our gratitude to CSIR-UGC India and the Malaysian Government via grant No. : 600-RMI/DANA 5/3/RIF (636/2012)) for their financial support. References [1] H. Kim, A. A. Abdala, C. W. Macosko, Graphene/polymer nanocomposites. Macromolecules. 43 (2010) 6515-6530. [2] D. Cai, M. Song, Recent advance in functionalized graphene/polymer nanocomposites. J. Mater. Chem. 20 (2010) 7906-7915. [3] H. Wu, W. Zhao, H. Hu, G. Chen, One-step in situ ball milling synthesis of polymerfunctionalized graphene nanocomposites. J. Mater. Chem. 21 (2011) 8626-8632. [4] K. Wakabayashi, C. Pierre, D. A. Dikin, R. S. Ruoff, T. Ramanathan, L. C. Brinson, J. M. Torkelson, Polymer-graphite nanocomposites: Effective dispersion and major property enhancement via solid-state shear pulverization. Macromolecules. 41 (2008) 1905-1908. [5] X. S. Du, M. Xiao, Y. Z. Meng, Facile synthesis of highly conductive polyaniline/graphite nanocomposites. Eur. Polym. J. 40 (2004) 1489-1493. [6] K. Dutta, S. K. De, Electrical conductivity and optical properties of polyaniline intercalated graphite oxide nanocomposites. J. Nanosci. Nanotechnol. 7 (2007) 2459-2465. [7]A. Omar Al-Hartomy, A. Al-Ghamdi, N. Dishovsky, R. Shtarkova, V. Iliev, I.Mutlay, F. ElTantawy Dielectric and microwave properties of natural rubber based nanocomposites containing graphene. Materials Sciences and Applications, 3 (2012) 453-459. [8] M. L. Chen, C.Y. Park, J. G. Choi, W. C. Oh, Synthesis and characterization of metal (Pt, Pd and Fe)-graphene composites. J. Korean Ceramic Soc. 48 (2011) 147-151.
Progress in Polymer and Rubber Technology 10.4028/www.scientific.net/AMR.812
Electrical Properties of Graphene Filled Natural Rubber Composites 10.4028/www.scientific.net/AMR.812.263
