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This work was performed to investigate the effects of different nanofillers on the structure and dielectric properties of polypropylene (PP) nanocomposites. This work considered studying three types of nanofillers, namely, magnesium aluminate (MgAl2O4), calcium carbonate (CaCO3), and surface-modified calcium carbonate (CaCO3T). PP nanocomposites containing MgAl2O4, CaCO3, and CaCO3T nanofillers were prepared using a Brabender melt mixer. The samples were melt pressed using a hydraulic laboratory press at 180°C to produce thin-film samples of 100 μm thick. The prepared samples were subjected to Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy (SEM) for chemical and morphological analysis. Meanwhile, the DC breakdown test was carried out to analyzed the dielectric properties of the evaluated samples. The results showed that PP/MgAl2O4 nanocomposites possessed lowered breakdown strength compared to unfilled PP. In contrast, PP/CaCO3 nanocomposites possessed a higher breakdown strength over PP/MgAl2O4 nanocomposites. Meanwhile, PP/CaCO3T nanocomposites possessed the highest breakdown strength among the nanocomposites. Possible mechanisms governing these property changes are discussed.

Polymer nanocomposites have become one of the attractive research areas in exploring new dielectric materials. This is due to their potential dielectric property enhancements as a consequence of nanostructuration. However, the degradation of the dielectric performance of nanocomposites has also been reported and this is can be related to the presence of water within the materials. The main objective of the current work was to investigate the effects of water absorption on the mass change and dielectric response of polyethylene (PE) nanocomposites containing uncalcined and calcined zirconia (ZrO2) nanofillers. The results show that nanocomposites containing uncalcined ZrO2 exhibited a marginal increase in the real permittivity after water immersion. Under similar water immersion periods, nanocomposites containing calcined ZrO2 also demonstrated a marginal increase in the real permittivity. Factors influencing the dielectric response of the nanocomposites after ZrO2 calcination and water immersion are discussed.

The current work reviews the importance of engineering the interface between nanofillers and polymers to achieve unique dielectric properties in nanocomposites. Although many improved dielectric properties of nanocomposites have been attributed to the presence of the interface, the interface can also be an attractive location for water to accumulate, which may otherwise jeopardize the dielectric properties of nanocomposites. Consequently, the use of surface functionalization and calcination techniques in removing water-related moieties on nanofillers is highlighted. Specifically, the effects of nanofiller calcination on two exemplar oxide-based nanocomposite systems, namely, silica-based nanocomposites and zirconia-based nanocomposites, are discussed. Evidence suggests that nanofiller calcination influences not only the water-related chemistry, but also the structure of oxide-based nanofillers. Significantly, for detailed interfacial chemistry of nanofillers to become relevant in engineering the dielectric properties of nanocomposites, effective removal of interfacial water on nanofillers is crucial.

This paper reports on an investigation of the effect of thermal aging on the structure and dielectric properties of unfilled polypropylene (PP) and PP nanocomposites containing 1 wt%, 2 wt%, and 5 wt% of calcium carbonate (CaCO3 ) nanofiller. Three different aging temperatures were considered, i.e., 20°C (unaged), 110°C, and 140°C. After aging, the dielectric strength of the samples was tested by means of DC breakdown testing. The results showed that, although the breakdown strength of PP nanocomposites was lower than that of unfilled PP before aging, the reduction in DC breakdown strength of PP nanocomposites was less pronounced compared with unfilled PP after aging. This indicated that the DC breakdown property of PP nanocomposites was less susceptible to thermal aging.

Nanodielectrics have been regarded as a class of material system that can provide significantly improved chemical, mechanical and dielectric properties over conventional microcomposites. This is due to the presence of a high volume fraction of the interphase between nanoparticles and polymers. However, precise effects of nanodielectrics are not well understood, leading to difficulties in interpreting the dielectric behaviours of nanodielectrics. In the current work, effects of nanoparticle distributions, interparticle distances, nanoparticle sizes, interphase permittivities and interphase thicknesses on the possible electric field variations within a nanodielectric model have been simulated using Finite Element Method Magnetics (FEMM) 4.2. The results demonstrate that different nanoparticle and interphase configurations lead to different effects on the electric field intensity within the nanodielectric model. Mechanisms leading to changes in dielectric properties based on the observed electric field variations are discussed.

The topic of polymer nanocomposites remains an active area of research in the dielectrics community, due to the unique electrical properties that these materials could exhibit. To explain the behaviour of these materials, the importance of clarifying the interfaces between nanoparticles and polymer matrices has been emphasised. However,understanding of the interface in nanocomposites is unsatisfactory and, consequently,many experimental results remain unexplained. This thesis reports on an investigation into a polyethylene nanocomposite system that contains varying amounts of nanosilica that differ with respect to their surface chemistry. The addition of nanosilica, even with different surface chemistries, was found to enhance the nucleation density of polyethylene and perturb the spherulitic development. While less organised lamellar structures would be expected to lead to a lower breakdown strength, this does not appearto be the case for the material systems considered here under alternating current (AC) fields. In addition, nanosilica filled polyethylene was found to absorb significantly more water than unfilled polyethylene, with the consequence that both the permittivity and the loss tangent increase with increasing duration of water immersion. However, appropriate surface treatment of nanosilica reduces the water absorption effect and modifies the dielectric response of the nanocomposites compared with those containing an equivalent amount of untreated nanosilica. Although water absorption may not be a technologically desirable characteristic, the results indicate that water molecules can act as effective dielectric probes of interfacial factors. Meanwhile, the direct current (DC) breakdown strength reduces with the inclusion of increasing amount of nanosilica in the polyethylene, but surface treatment of nanosilica improves the DC breakdown strength with respect to equivalent nanocomposites containing untreated nanosilica. Results from space charge studies reveal increased space charge accumulation in the presence of the untreated nanosilica and, upon surface treatment of the nanosilica, the charge development was suppressed in comparison with nanocomposites containing an equivalent amount of untreated nanosilica. This observation suggests that space charge accumulation and DC failure are related in these systems and it would seem that control of surface chemistry is particularly critical in connection with the use of nanocomposites in DC applications. Finally, the mechanisms underpinning the concept of filler functionalisation in nanocomposites were investigated via the use of different aliphatic chain length silane coupling agents, and the results show that long silane chains enhance the DC breakdown strength of the resulting nanocomposites. The possible further enhancement in DC breakdown strength is also highlighted. Overall, this thesis demonstrates how a nanoparticle’s interface chemistry can affect both the structure and the electrical properties of the resulting nanocomposites, and serves as an important foundation towards the engineering of nanocomposites as the reliable electricalinsulation materials of the future, through the understanding of the interface.