Effect of crystalline phase, orientation and temperature on the dielectric properties of poly (vinylidene fluoride) (PVDF)

The effect of crystalline phase, uniaxial drawing and temperature on the real (epsilon') and imaginary (epsilon") parts of the relative complex permittivity of poly (vinylidene fluoride) (PVDF) was studied in the frequency range between 10(2) and 10(6) Hz. Samples containing predominantly alpha and beta phases, or a mixture of these, were obtained by crystallization from a DMF solution at different temperatures. alpha phase samples were also obtained from melt crystallization and from commercial films supplied by Bemberg Folien. Different molecular orientations were obtained by uniaxial drawing of alpha and beta phase samples. The results showed that the crystalline phase exerts strong influence on the values of epsilon' and epsilon', indicating that the alpha(a) relaxation process, associated with the glass transition of PVDF, is not exclusively related to the amorphous region of the polymer. An interphase region, which maintains the conformational characteristics of the crystalline regions, should influence the process decisively. The molecular orientation increased the values of epsilon' for both PVDF phases and modified its dependence with temperature over the whole frequency range studied. The influence of the crystallization and molecular orientation conditions on the dc electric conductivity (sigma(dc)) were also verified. The value of sigma(d)c was slightly higher for samples crystallized from solution at the lowest temperature and decreased with draw ratio. (C) 1999 Kluwer Academic Publishers.