PIEZOELECTRIC RELAXATION IN POLYVINYLIDENE FLUORIDE

The piezoelectric and related properties have been measured for stretched polyvinylidene fluoride as a function of temperature. The piezoelectric relaxation observed around the glass transition temperature Tg is discussed taking the two-phase structure into consideration. The piezoelectric constant e31 (polarization/strain) is positive and increases with temperature. This increase is ascribed to the increase of the electrostrictive constant in noncrystalline regions which couples with the residual polarization located in crystalline and/or noncrystalline regions. The electrostrictive constant changes its sign below Tg from negative to positive with temperature. The piezoelectric activity below Tg is considered to be originated from the intrinsic piezoelectricity, i.e., the rotation of oriented molecular dipoles caused by strain. The residual polarization in the sample due to the dipoles is estimated to be 0.9×10-2 C/m2. The piezoelectric increment around Tg is accounted for by assuming the spatial distribution of the residual polarization due to both dipoles and space charges.