Understanding the cross-linking reactions in highly oxidized graphene/epoxy nanocomposite systems

Graphene oxide (GO) was produced using acidic graphite oxidation and dispersed within an epoxy matrix using a solvent-based technique, to give nanocomposites containing up to 2wt% of GO. Transmission and scanning electron microscopy revealed a fine dispersion of graphitic sheets which alters the nanocomposite's fractured surface morphology, while Fourier transform infrared spectroscopy revealed an excess of epoxide groups in the system, which are associated with the included GO. These additional moieties react with hardener amine groups and, consequently, displace the reaction stoichiometry away from the optimum. The result of this is a change in the network architecture and, in particular, the introduction of epoxy-terminated branches, which modify the dielectric relaxation. During post-curing, hydroxyl groups on the GO surface react with residual epoxide groups through etherification reactions, to give a marked increase in the glass transition temperature. These reactions lead to increased interfacial interactions between the GO and the matrix, which contribute to an increase in tensile performance. In addition, post-curing also reduces the defect content within the GO lattice which, in turn, increases the electrical conductivity, dielectric permittivity and low frequency losses of the system. Associated chemical pathways are proposed.