Aging and polarization phenomena in PE under high electric fields

The influence of high electrical dc fields (>20 kV/mm) on aging, polarization and on the morphology of polyethylene (PE) is discussed. Infrared and positron annihilation spectroscopy measurements as well as capacitance measurements tend to suggest that the polymer morphology is modified by high fields. We show that the accelerated electrical aging characteristics of PE are linked directly to the morphology changes induced by the field. Below a so-called critical field, the activation volume of the aging process is dependent on the field-induced strain. Above the critical field, the amorphous phase is deformed significantly, and weak van der Waals bonds are broken, leading to another, faster, aging regime. There is an excellent agreement between the proposed model and experimental data obtained with various PE samples. The possible relation between the submicrocavity formation proposed in our aging model and various polarization measurements is discussed. It is our contention that strong charge injection occurs only after submicrocavity formation, i.e. after weak bond breakage. As is well known, the polarization currents obtained under high fields are controlled by space charges. It seems that the wave packets and the negative resistance observed at >100 kV/mm in PE are associated with a steady state in field-induced defect formation. This suggests that space charges are related to the formation of submicrocavities and, therefore, are a consequence, not a cause, of high field aging.