By changing the ratio of resin to hardener, a series of epoxy resin samples has been produced with differing network structures and different retained chemical functionalities. The resulting materials were characterized by thermal analysis, dielectric spectroscopy, DC conductivity, and DC and AC breakdown strength measurements, to explore the effect of network structure and chemical composition on molecular dynamics and electrical properties. Differential scanning calorimetry showed that the glass transition temperature is primarily determined by the crosslinking density and indicates that, under the range of conditions employed here, side reactions, such as etherification or homopolarization, are negligible. Conversely, changes in DC conductivity with resin stoichiometry appear to occur as a result of changes in the chemical content of the system, rather than variations in network structure or dynamics. Specifically, we suggest that the DC conductivity is markedly affected by the residual amine group concentration in the system. While DC conductivity and DC breakdown appear broadly to be correlated, AC breakdown results indicated that this parameter does not vary with changing stoichiometry, which suggests that the AC and DC breakdown strengths are controlled by different mechanisms.

A range of nanocomposites based on a polyethylene polymer and hexagonal boron nitride (hBN) filler have been explored in this study. The dielectric properties of the nanocomposites, which consisted of 2, 5, 10, 20 and 30 wt% of hBN, have been compared to the dielectric properties of the unfilled polyethylene blend. Scanning electron microscopy revealed that the hBN was uniformly distributed in the polyethylene matrix, although large amounts of agglomerates were present in the nanocomposites containing more than 10 wt% of hBN. The incorporation of hBN into polyethylene resulted in a highly disordered morphology in comparison with the unfilled polyethylene, in which this effect was more pronounced with increasing hBN content. This is consistent with the increasing crystallisation temperature as the hBN content increases, as shown by differential scanning calorimetry, where the hBN acted as a highly effective nucleating due to the strong interactions between the polyethylene and the hBN. This strong interaction is again reflected in the thermal decomposition temperature which similarly increases with increasing hBN content. The study demonstrates the remarkable electrical properties of the prepared nanocomposites, where the breakdown strength monotonically increased as a function of hBN content, even with a very high 30 wt% of hBN. The improvement in electrical properties, even at high hBN concentrations, is contradictory to the reported results in the literature and is mainly attributed to the hydrophobic surface of the hBN particles.

The addition of nanofillers can have a significant influence on the resin stoichiometry of thermosetting polymer systems. Based on differential scanning calorimetry (DSC) results, it is estimated that the inclusion of 2 and 5 wt% of silicon nitride nanofiller displaces the resin/hardener stoichiometry of an epoxy/amine network by 6.5 and 18%, respectively. Dielectric spectroscopy results confirm the above findings, in that the spectra of the nanocomposite samples were found to be equivalent to the spectra of unfilled samples when the above stoichiometric effect was taken into account. Therefore, this study provides clear evidence that the presence of a nanofiller can directly and significantly affect the curing process of an epoxy network. Consequently, this should always be considered when introducing nanofillers into thermosetting matrices. These results indicate the presence of covalent bonding between the nanoparticles and the surrounding polymer and, therefore, provide an opportunity to explore the influence of this bonding on the molecular dynamics of the polymer layer around the particles. However, the obtained DSC and dielectric spectroscopy results suggest that, in the system considered here, either the covalent bonding does not have an appreciable influence on the segmental dynamics of the polymer, as revealed by these techniques, or that the thickness of the affected layer is less than 1 nm and therefore too small to be distinguished from experimental uncertainties.

This study proposes a new model for describing the electrical behavior of nanocomposites. Unlike other models in the literature, this model has concentrated on the role of an interphase layer within the boundaries of nanoparticles. The experimental part investigates this role by filling an epoxy matrix with two types of surface-modified silicon nitride nanofiller: (a) the particles were dried at 200 o C, and (b) the particles were calcinated at 1050 o C. Electrical characterization showed that the epoxy which was filled with the calcinated particles has considerably better dielectric performance. Given that thermal and dielectric spectroscopy results demonstrate that the matrix molecular dynamics and polar content are comparable for all the investigated samples, the variations in the dielectric performance point to the particle interphase as an essential reason. As shown by infrared spectroscopy, the complex surface chemistry of the dried particles suggests a particle interphase with a high concentration of localized electronic states, which may enhance charge transport through hopping/tunnelling conduction. On the other hand, calcinating the particles results in a particle interphase with wider band gap, which may work as an energy barrier for charge movement. Consequently, this study highlights the paramount importance of particle interphase for designing dielectric properties of nanodielectrics.

The effect of water absorption on the dielectric response of polyethylene/hexagonal boron nitride nanocomposites has been studied by dielectric spectroscopy. The nanocomposites have been prepared with hBN concentrations ranging from 2 wt % to 30 wt %. Fourier Transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA) revealed a very small amount of hydroxyl groups on the surface of hBN. Mass loss measurements showed that the nanocomposites did not absorb any water under ambient and dry conditions while there was some water absorption under wet conditions. The dielectric spectroscopy results showed a broad relaxation peak, indicative of different states of water with water shells of different thickness, which moved to higher frequencies with increasing water content. However, the dielectric losses were significantly lower than the losses reported in the literature of nanocomposites under wet conditions. In addition, all the absorbed water was successfully removed under vacuum conditions which demonstrated that the interactions between the water and the nanocomposites were very weak, due to the hydrophobic nature of the hBN surface. This is a highly useful property, when considering these materials for applications in electrical insulation.