Effects of film viscosity on electric field-induced alignment of graphene flakes in B-stage graphene-epoxy composite films

Due to the excellent properties of graphene such as high electron mobility, high thermal conductivity, and low gas permeability, graphene-polymer composites have been widely studied for several applications. Moreover, due to the high aspect ratio of graphene, aligned graphene-polymer composites have been also studied by using various aligning techniques. Especially, electric field-induced alignment of graphene flakes has been investigated since the orientation can be easily tailored. Because low viscosity polymer matrix is desired for better alignment by an electric field, many researchers demonstrated carbon nanotube and graphene alignment by applying electric field at liquid-like polymer composites. However, this method has disadvantages in terms of postprocessing. On the other hand, it was demonstrated by our research group that aligned B-stage graphene-epoxy composite films were achieved by applying electric field at higher temperature where the viscosity of the composite films could be reduced without curing reaction. It was also shown that the curing reaction prohibits the graphene flake alignment due to the viscosity increase. Since viscosity is one of the most important factors for efficient alignment, and the viscosity varies temperature, the effects of viscosity change on electric-field induced alignment of graphene need to be investigated. In this study, effects of time and temperature on the film viscosity and electric-field induced graphene flake alignment of B-stage graphene-epoxy composite films were investigated. Film viscosity was measured as a function of temperatures, and isothermal viscosity was also measured with times at constant temperature in order to confirm the curing reaction. Electric field was applied at various times and temperatures. The aligned B-stage graphene-epoxy composite films were characterized by cross-sectional morphology and electrical resistivity.