A Model Experiment for Piezoelectric Relaxations in Polymers

Abstract

Based on mechanical and electrical three-element models for piezoelectric polymers, mathematical expressions for the piezoelectric stress-constant, e, and strain-constant, d, are derived as functions of the elastic and dielectric constants in a piezoelectric crystalline phase and a nonpiezoelectric amorphous phase, and the volume fraction of the piezoelectric phase. If no relaxations exist in the piezoelectric phase, apparent piezoelectric relaxations are caused only by elastic and dielectric relaxations in the amorphous phase.Model experiments for a two-phase polymer are made using oriented silk fibers as the piezoelectric phase and an adhesive layer of epoxy resin or poly(vinyl alcohol) as the amorphous phase. In the epoxy resin case the elastic relaxation prevails over the dielectric relaxation, which accounts for the decrease of e-constant and the increase of d-constant with increasing temperature observed for the silk—epoxy resin system. In the poly(vinyl alcohol) case the dielectric relaxation dominates the elastic relaxation, which explains the increase of both e- and d-constants with the increase of temperature for the silk—poly(vinyl alcohol) system.