High-Temperature Dielectric Relaxation of Form III Poly(vinylidene fluoride)

Abstract

The dielectric relaxation for form III poly(vinylidene fluoride) prepared by isothermal crystallization was observed at high temperatures. The values of ε′ and ε″ were very large at high temperatures, ε′ underwent almost a tenfold increase when the temperature was raised from 150°C toward the melting point of the sample (198°C). The sharp drop in ε′ beyond the melting point was even more striking. Also, at 100 Hz, ε″ reached a very sharp maximum at about 180°C. The maximum value of ε″ increased with the degree of crystallinity. The activation energy estimated for the high temperature relaxation was about 150 kcal mol⁻¹. The dielectric relaxation, whose strength was about 2.8×10⁴, may be due to the crystalline region. It was also made clear that the relaxation is due to the form III crystallites according to mechanical and DSC measurements. There are two possibilities for the mechanism responsible for the relaxation: (1) the orientational motion of crystallites accompanying the applied ac electric field; (2) the motion of the domain walls of the form III polar crystallites which bring about a hysteresis on the polarization-electric field diagram.