Bonds breaking and molecular chains straining in the electrical aging of PE and PP

Abstract

It is shown that under high fields, molecular straining becomes significant and that it affects the energy barrier equation for electrical aging of polymers. This implies that at high fields the time to go over the barrier varies not only with the stress (i.e. with the square of field) but also with the square of the stress. Examples are shown along with the corresponding calculated activation volume and the strained activation volume. The physical sense of the two activated volumes is discussed. The Kramers friction coefficient is also included in the life equation and the three main parameters affecting the energy barrier are calculated from various experiments with polyethylene and polypropylene. Discussion of dc and ac aging results are also made. The fact that the activation volume and the friction coefficient vary with the applied stress is illustrated and empirical approaches to determine them are suggested. Implications for other electrical properties are briefly discussed.