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 (ε′) and imaginary (ε″) parts of the relative complex permittivity of poly (vinylidene fluoride) (PVDF) was studied in the frequency range between 102 and 106 Hz. Samples containing predominantly α and β phases, or a mixture of these, were obtained by crystallization from a DMF solution at different temperatures. α 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 α and β phase samples. The results showed that the crystalline phase exerts strong influence on the values of ε′ and ε″, indicating that the αa relaxation process, associated with the glass transition of PVDF, is not exclsively 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 ε′ 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 (σdc) were also verified. The value of σdc was slightly higher for samples crystallized from solution at the lowest temperature and decreased with draw ratio.