Strategies to Create Electrically Conductive Polymer/Graphene Composites

Abstract

Conductive polymer composites, typically constructed by melt compounding conductive fillers into a polymer matrix, enjoy specialized applications such as electrostatic discharge protection. Graphene nanoplatelets (GNPs) exhibit high inherent electrical conductivity and geometric anisotropy, thus require much lower loading (< 1 wt%) in a polymer matrix to achieve electric percolation while preserving good melt processability. However, due to their relative high cost, it is desirable to further reduce GNP loading while enhancing the polymer/GNP composite electrical conductivity. In this thesis, I demonstrate two formulation strategies to attain conductive polymer composites by controlling GNP localization in cocontinuous polymer blends using both miscible and immiscible systems. For the miscible system, poly(methyl methacrylate) (PMMA) and poly(styrene-co-acrylonitrile) (SAN) blends are selected. By first compounding PMMA, SAN, and GNP together at lower temperature and then inducing PMMA/SAN spinodal decomposition by heating, I create spatially regular, cocontinuous domains where GNPs preferentially localize within the thermodynamically preferred SAN-rich phase and form conductive networks. I develop a quantitative transmission electron microscopy (TEM) image analysis method to quantify both the polymer domain size and GNP localization. Dielectric measurements show that quiescent annealing improves particle connectivity of the GNP network, leading to further enhancement in electrical conductivity to ~ 10^[-8] S/cm at 1 wt% GNP concentration. For the immiscible system, polylactide/poly(ethylene-co-vinyl acetate) (PLA/EVA) blends are selected. PLA/GNP masterbatches are melt compounded with the EVA homopolymer. Since GNPs preferentially wet the EVA phase, they transfer from PLA to EVA but become kinetically trapped at the interface, as confirmed by electron microscopy. I achieve an ultra-low percolation threshold of 0.048 wt% GNPs and obtain blends with electrical conductivities of ~ 10^[-5] S/cm at 0.5 wt% GNP concentration. Rheology, in-situ dielectric measurements, and TEM imaging after nonlinear shearing and extensional deformations all show that interfacial GNP network remains at the PLA/EVA interface. Moreover, high electrical conductivity is maintained during a wide range of melt compounding times, between 2–10 minutes. In addition to cocontinuous blends, this thesis also addresses practical challenges related to homopolymer-based conductive composites. The effect of electric field-induced conductivity enhancement and dielectric breakdown due to electrical treeing formation within EVA/GNP composites is studied through in-situ measurement of the electrical conductivity. Furthermore, the relationship between shear rheology, filler dispersion, and electrical conductivity of industrially produced conductive polymer composites is studied. These analytical techniques allow for understanding of composite characteristics, enabling industrial partners to quickly determine which conductive fillers are best suited for the construction of conductive polymer composites.