A blend composed of a high-density polyethylene and a low-density polyethylene was subjected to thermo-oxidative ageing at 160 °C, under conditions where oxygen diffusion effects lead to a non-uniform distribution of chemical defects throughout the specimens. The existence of highly and lightly oxidised regions was demonstrated by Fourier transform infrared spectroscopy and confocal Raman microprobe spectroscopy and the effect of these on the dielectric behaviour of the system was investigated. Dielectric spectroscopy revealed a broad loss peak at frequencies above 1 Hz that increases in strength with increasing ageing time, which we associate with the motion of polar chain segments within the polymer. For ageing times below 3 h, samples exhibited reduced conductivity under a constant electric field compared with the initial unaged system; above 3 h, the conductivity was found to increase monotonically with ageing time and corresponded to the appearance of an additional low frequency (<1 Hz) dielectric loss process and marked migration of space charge into the bulk away from the relevant physical electrode. This change in behaviour was found to occur abruptly and the final distribution of space charge was found to correlate well with the distribution of ageing inducing chemical defects within the system. From these observations, the effects of chemical defects on charge transport dynamics are discussed.

The properties of novel cable insulation systems will rely critically upon the morphology of the material. Here, a blend of high and low-density polyethylene (PE) was processed in order to generate three sets of samples with different morphologies. The influence of thermo-oxidative ageing at 120°C was then considered. The resulting chemical changes included the introduction of unsaturation and oxygen-containing groups and were determined by antioxidant consumption and oxygen permeability. Such chemical defects were found to be concentrated in the fraction of each system that was molten at 120°C and, consequently, served to inhibit recrystallisation following ageing. The resulting spatial distribution of charge trapping sites was therefore strongly dependent on morphology. The electrical conductivity of each system varied non-monotonically with ageing: short times reduced the conductivity; a rapid increase in conductivity over five orders of magnitude occurred beyond a critical ageing threshold. Despite the pronounced structural differences between the morphologically distinct sets of samples, all exhibited comparable conductivity values beyond this threshold, implying that while charge transport is strongly influenced by chemical factors, crystallinity is relatively unimportant. This experimental finding appears at odds with theoretical studies of the electronic states in crystalline and amorphous PE.

A blend of high and low density polyethylene was aged at 160 °C in air and the impact of the chosen aging protocol on local chemistry, crystallinity and charge transport dynamics was considered. The aging conditions were chosen in order to exploit oxygen diffusion effects, such that the resulting systems could be considered as bi-layer specimens, containing two regions: a uniform lightly aged layer and a more spatially varying highly aged layer, which vary in the concentrations of aging-related defects such as carbonyl groups and unsaturation. For aging periods up to 3 h, little space charge was found within both the highly and lightly aged layers. However, after aging for about 3.5 h, an abrupt change in behavior was observed, whereby charges move rapidly through the highly aged layer, accumulating at the interface with the lightly aged layer. Sample melting behavior, as determined by differential scanning calorimetry, was found to depend on aging time, as a result of impeded crystallization and retarded reorganization kinetics. We suggest that this abrupt change in charge transport behavior is a consequence of the local concentration of chemically related trapping sites exceeding some critical threshold. The consequence of the resulting space charge distribution is a dramatic increase in the local electric field across the lightly aged layer and a consequent reduction in the overall DC breakdown strength. However, while further aging exacerbates these space charge effects, counter to expectations, the breakdown strength then recovers somewhat, suggesting a change in the underlying mechanism of electrical failure.

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