Polymer Composites Comprising Single-Atomic-Layer Graphenic Conductive Inclusions and Their Unusual Dielectric Properties

In this study, we report the properties of the epoxy polymer composites, comprising reduced graphene oxide (RGO) in the form of the single-atomic-layer sheets. This structure is different from composites comprising multilayer RGO flakes and RGO aggregates, typically described in the literature. Viscosity of the uncured liquid resin increases by 390% after introducing 0.4% GO and increases by 4700% after its subsequent in situ reduction. The latter is explained by the reorganization of the original liquid crystalline structure of the GO-epoxy formulations with GO reduction. At the filling fractions >0.1%, the single-atomic-layer RGO flakes are assembled into clusters, where they alternate with a thin resin layer. This structure is also responsible for very unusual dielectric behavior of the cured solid composites. From one side, the real part of the complex permittivity reaches relatively high values at extremely low filling fractions: 14 at 0.1% and 60 at 0.4% RGO content. At the same time, the permittivity dispersion is accompanied with the well-pronounced symmetrical loss peaks on the imaginary part functions, which is typical for low permittivity materials. Such dielectric behavior is difficult to interpret in the frames of any single existing model. The high permittivity values strongly evidence for the Maxwell-Wagner interfacial polarization, even though the shape of the loss peaks would be better interpreted by α- and/or β-relaxation in neat solid polymers. The single-atomic-layer character of RGO affords a high interfacial area, which, in turn, translates to high capacitance and high permittivity. The relaxation time and activation energy, calculated from the temperature dependence experiments, suggest that the RGO clusters, but not individual RGO flakes, serve as conductive inclusions. The extremely long relaxation times are due to the charge transfer between the individual RGO flakes within the clusters. The striking difference between the newly prepared composites and control samples comprising multilayer RGO particles exemplifies the unique structure of our materials.