A method for experimental evaluation of phenomenological coefficients in media with dielectric relaxation.

We show a method to measure experimentally the phenomenological coefficients and to verify some inequalities which occur in a thermodynamical model for dielectric relaxation proposed by one of us in some previous papers. Assuming a sinusoidal form for induction vector D (extensive variable: cause), the electric field (intensive variable: effect) inside the system which depends on unknown phenomenological coefficients, has been obtained by integration. Then we compare it with a similar form of the electric field obtained by experimental considerations, where well known experimentally determinable coefficients appear. The comparison of these two forms, together with the introduction of an experimentally determinable relaxation time, allows us to determine univocally the above-mentioned phenomenological coefficients as functions of experimentally determinable coefficients only. This allows us to verify the aforementioned inequalities. Moreover, a condition for the applicability of Kluitenberg-Ciancio theory is determined. Finally we carry out dielectric measurements on PMMA and PVC at different frequencies and fixed temperature in order to obtain the phenomenological coefficients as functions of the frequency.