Development of Nanocomposites Using Graphene Synthesized by Solvent Exfoliation Method

The electrical conductivity of polymers can be changed by adding conductive fillers, such as, metal fibers, and carbon fibers. The resulting materials can be used as conductive coating, where metals were traditionally used. From the engineering viewpoint, the advantages of conductive composites over metals are light weight, resistance to corrosion and ease of processing. Conductive composite is useful in electronic circuits and packaging applications such as, flexible electronics, organic electronic devices, and printable circuits and antennas. An innovative method of making graphene filled polymer nanocomposites is proposed in this dissertation. Graphene/polymer nanocomposites were obtained by directly exfoliating graphite into graphene using organic solvent, and solution phase mixing with a polymer. The feasibility of this one-pot approach was studied for making graphene/poly(methyl methacrylate) (PMMA) nanocomposite and graphene/polypyrrole (PPy) nanocomposite. To make graphene/PMMA nanocomposite, two kinds of solvents, 1-methyl-2-pyrrolidinone (NMP) and acetonitrile, were investigated to be used to exfoliate graphite. The conductivity of PMMA has been increased after the addiction of graphene. The conductivity of graphene/PMMA nanocomposite reached to 6x10-3 S/cm at graphene concentration of 20 wt%. The oxygen plasma was used to treat the surface of resultant graphene/PMMA film, and was found to be improved the electrical conductivity. Tensile tests conducted on graphene/PMMA nanocomposite suggested that there was no improvement in mechanical properties after adding the graphene additives to PMMA. The graphene/PPy nanocomposite was processed using in-situ polymerization. During polymerization, PPy tends to grow on the surface of graphene sheets. The resultant graphene/PPy nanocomposite has a core-shell structure. Dependence of resistivity of graphene/PPy nanocomposite on temperature was measured and analyzed with the Arrhenius theory. The Raman spectroscopy, x ray diffraction, optical microscopy, and UV-visible spectroscopy were used to characterize the properties of graphene/polymer nanocomposites. To design the optimized composite, electrical conductivity was studied using a theoretical model. The electrical conductivity models based on Monte Carlo and percolation theory were developed for graphene/PMMA nanocomposite. Two dimensional and three dimensional models were developed for the graphene/PMMA nanocomposite. Experimental data of graphene/PMMA were in excellent agreement with three dimensional simulation results, proving that the numerical model developed in this dissertation was accurate, and could be applied to predict the electrical conductivity of the graphene/PMMA nanocomposite. This model provides an insight into the mechanisms of graphene/PMMA nanocomposite and the important factors that affect the electrical conductivity and hence, the model can be used to improve the electrical performance of composites by choosing the size and concentration of graphene nanoparticles. In summary, this study indicated that the innovative one-pot synthesis method using graphene synthesized by direct solvent exfoliation was feasible. The solvent used to exfoliate graphene served as both the exfoliation agent and the surfactant. By adding graphene into PMMA, the electrical conductivity has been increased by the order of 1012 at a grpahene concentration of 20 wt%. However, after adding graphene into PPy, the electrical conductivity decreased due to increase in disorder introduced by graphene sheets. The simulation results of three dimensional hard-core soft-shell modeling was in agreement with the experimental data. This model can be used to predict the electrical conductivity of graphene/PMMA nanocomposite.