Controlled Structure Evolution of Graphene Networks in Polymer Composites

Exploiting graphene's exceptional physical properties in polymer composites is a significant challenge because of the difficulty in controlling the graphene conformation and dispersion. Reliable processing of graphene polymer composites with uniform and consistent properties can therefore be difficult to achieve. We demonstrate distinctive regimes in morphology and nanocomposite properties, achievable through systematic control of shear rate and shear history. Remarkable changes in electrical impedance unique to composites of graphene nanoplatelets (GNPs) are observed. Low shear rates <= 0.1 s(-1) break up the typical GNP agglomerates found in graphene composites, partially exfoliate the GNPs to few-layer graphene, and reduce orientation, enhancing electrical conductivity in the composite materials, whereas at higher shear rates GNP orientation increases and the conductivity reduces by four orders of magnitude, as the graphene filler network is broken down. The structure of the composite continues to evolve, reflected in further changes in conductivity, after the shear force has been removed and the process temperature maintained. This work provides critical insights for understanding and controlling GNP orientation and dispersion within composites and will have important consequences in the industrial processing of graphene polymer composites via the informed design and choice of processing conditions.