The Effect of the Temperature Dependence of the Dielectric Constant on Thermally Stimulated Currents in Polymers

A dipolar theory of thermally stimulated currents taking into account the thermal dependence of the equilibrium polarization has been tested in the glass transition range of a series of polymers selected for their well-resolved relaxation spectra and the absence of any significant bulk conductivity (triblock, diblock, and star-shaped styreneisoprene copolymers). Systematic studies of the effect exercized on thermally stimulated polarization currents (TSPC) and thermally stimulated depolarization currents (TSDC) by varying parameters such as field strength, electrode material, field polarity, and type of heating-cooling sequence, have been carried out and full qualitative agreement with the predictions of the theory has been found. In particular, these results imply that the variations of equilibrium polarization occurring during the nonisothermal steps of TSDC and TSPC experiments cannot be a priori neglected, as is still commonly done in classical treatments of experimental data.