A computational exploration of the effect of alignment and aspect ratio on alternating current conductivity in carbon nanofiber-modified epoxy

Carbon nanofiller-modified polymers have been the subject of intense study for years due to their potential use in diverse and far-reaching applications. The effect of nanofiller network parameters on macroscale direct current electrical transport has been thoroughly elucidated by extensive nano-to-microscale modeling. As a result, we now have great insight into how the conductive and piezoresistive properties of nanocomposites can be tailored through judicious control of the underlying nanofiller network. It is also well-known that carbon nanofiller-modified polymers possess frequency-dependent alternating current electrical properties. Even though work has been done to understand the alternating current properties of nanocomposites via experimental characterization and through the development of macroscale equivalent circuit models, much less has been done to understand how macroscale alternating current conductivity depends on microscale effects such as nanofiller alignment and aspect ratio. This is an important knowledge gap because, like direct current conductivity, the underlying nanofiller network ultimately gives rise to macroscale alternating current transport in these materials. To this end, we herein present an alternating current microscale percolation model for carbon filler-based polymer nanocomposites. After calibration against experimental complex impedance data from randomly ordered carbon nanofiber-modified epoxy, this model is used to explore the effect of carbon nanofiber alignment and aspect ratio on alternating current conductivity. These simulations show that alternating current conductivity generally increases with increasing alignment and with aspect ratio; however, the competing effects of alternating current and direct current percolation give rise to substantial variation in alternating current conductivity at low frequencies and with poor percolation. The methodology presented in this article provides a modeling tool by which nanocomposites with highly optimized alternating current properties can be developed through careful control and tailoring of nanofiller network properties for the realization of exotic, next-generation material functionality.