Simultaneous thermally stimulated luminescence and conductivity in low-density polyethylene

Abstract

Thermally stimulated luminescence (TSL) and conductivity (TSC) in low-density polyethylene samples were measured simultaneously, while the sample was being heated to room temperature after X-ray irradiation at -190 degrees C. The TSC curves of samples from which absorbed air had been removed showed five peaks, labelled C1-C5 in order of increasing temperature, while the TSL glow curves showed only three peaks (L1-L3). The effects on these peaks of (i) immersing the samples in fuming nitric acid, (ii) annealing them in vacuum, and (iii) exposing them to an oxygen/ozone mixture, were investigated. It is concluded that the C1 and L1 peaks are associated with the same traps, these traps being formed by the polymer chain themselves in the chain-fold regions of the sample, and being broken up by the onset of side-chain motion. The same is true for the C2 and L2 peaks, except that a different set of traps and 'crankshaft' motion are involved. There is no obvious correlation between the L3 peak and any of the TSC peaks; the L3 traps are also formed in the chain-fold regions, and are broken up by the glass transition. The C3, C4 and C5 traps are probably structural defects of various kinds in the crystalline regions of the samples, from which electrons escape by thermal excitation. When the samples contain absorbed air, a hitherto unreported peak is observed in the TSC curve, corresponding to the TSL 'gas' peak.