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 2 wt% 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 γ 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.

A range of modified amine- and anhydride-cured epoxy systems based upon diglycidyl ether of bisphenol A was produced, through the systematic incorporation of moieties termed functional network modifiers (FNMs) that serve to change the network structure in controlled ways. Here, the chosen FNM was trimethylolpropane triglycidyl ether (TTE). The resulting materials were characterized by Fourier transform infrared spectroscopy, thermal analysis, dielectric spectroscopy and measurements of direct current conduction. A progressive reduction in the glass transition temperature of the modified samples was seen with increasing TTE, which is interpreted in terms of changes in the network architecture of the resin. The molecular origins of the dielectric γ and β relaxation processes are proposed. The observed increase in conduction seen exclusively with increasing TTE content in the amine-cured systems is considered in terms of the chemistry of the FNMs, variations in free volume, changes in molecular dynamics and residual unreacted groups retained from the curing reaction. Specifically, we relate the observed increase in conduction to the presence of unreacted amine groups.

A synthetic route for the production of graphene oxide is described, in which the commonly used potassium permanganate (KMnO₄) is replaced by chromium trioxide (CrO₃) as the oxidizing agent. Raman spectroscopy, thermogravimetric analysis and X-ray photoelectron spectroscopy demonstrate that the product is characterized by a reduced level of oxidation and a reduced defect content, compared to conventional graphene oxide (GO). We therefore term the product moderately oxidized graphene oxide (mGO). In comparison with GO, it is shown that when introduced into an epoxy matrix, mGO offers significant potential benefits. These include: excellent compatibility with the epoxy matrix leading to a low percolation threshold for electrical conductivity (~ 0.5 vol%); an associated increase in electrical conductivity of about eight orders of magnitude; no adverse influence on the epoxy curing reactions; potentially simplified material processing strategies.

The potential of polymer nanocomposites as dielectric materials is an active area of research, due to the promising electrical properties that may result from nanostructuration. Recently, the effect of water absorption in nanocomposites has been considered and it is found that nanocomposites can absorb significantly more water than the equivalent unfilled polymer when exposed to humid environmental conditions. This is due, supposedly, to the presence of the nanoparticle-matrix interface, which could be a preferred location for the aggregation of water molecules. Nevertheless, from an application perspective, the presence of water is not welcomed as it might negatively affect the overall electrical properties. This paper reports on a dielectric spectroscopy investigation into polyethylene systems that contain different amounts of nanosilica with different surface chemistries. Results indicate that the permittivity and loss tangent of nanocomposites are higher than that of the unfilled polyethylene, especially in the low frequency range. Furthermore, water absorption tests show that nanocomposites do absorbed significantly more water than unfilled polyethylene, with the consequence that both the permittivity and loss tangent increase with increasing amounts of water. However, appropriate surface treatment of nanosilica was found to reduce the water absorption effect and to modify the dielectric response of nanocomposites compared with those containing untreated nanosilica. While water absorption may not be a technologically desirable characteristic, our results suggest that water molecules can act as effective dielectric probes of important factors.