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

The effect of crystalline phase, uniaxial drawing and temperature on the re al (epsilon') and imaginary (epsilon") parts of the relative complex permit tivity of poly (vinylidene fluoride) (PVDF) was studied in the frequency ra nge 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 obta ined from melt crystallization and from commercial films supplied by Bember g Folien. Different molecular orientations were obtained by uniaxial drawin g of alpha and beta phase samples. The results showed that the crystalline phase exerts strong influence on the values of epsilon' and epsilon', indic ating that the alpha(a) relaxation process, associated with the glass trans ition of PVDF, is not exclusively related to the amorphous region of the po lymer. An interphase region, which maintains the conformational characteris tics of the crystalline regions, should influence the process decisively. T he molecular orientation increased the values of epsilon' for both PVDF pha ses and modified its dependence with temperature over the whole frequency r ange studied. The influence of the crystallization and molecular orientatio n 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.