On the influence of morphology and chemical defects on charge transport dynamics in polyethylene: thermal ageing and concentration gradient

Abstract

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.