Non-uniformities of electrical polarization in polyacrylonitrile films

Abstract

Persistent electrical polarizations can be established in most polymer films by poling at elevated temperatures. A variety of contributions are now known to make major contributions to this polarization; these include preferred orientation of dipolar groups, asymmetrically displaced ionized species, asymmetrically distributed charged groups created in the polymer molecules themselves, and ions injected from external sources into the polymer solid. As long as characteristic times associated with relaxation of these electrical anisotropies are long compared with the observation period, then the material will effectively retain whatever is the net polarization. It follows, then, that heating these materials will quicken these relaxation processes, and one can stimulate the discharge of these polarizations thermally. Analysis of the temperature-dependence of the overall depolarization of such a polymer specimen will usually reveal that the various contributions to total polarization discharge more or less individually. Some discharge steps will display Arrhenius-type kinetics, corresponding to simple solid state diffusional transport or to the onset of mobility of small groups of atoms in polymer chains; other discharge steps may display WLF-type kinetics, implying that the depolarization event requires mobility of whole segments of polymer backbones.