Influence of poling and annealing on the nonlinear dielectric permittivity of PVDF-TRFE copolymers

The linear and second order permittivities of as-prepared and annealed, poled and unpoled PVDF-TRFE 56/44 mol% copolymer film are investigated as a function of temperature. In contrast to annealed PVDF-TRFE, the linear permittivity of unannealed PVDF-TRFE and its temperature dependence is not influenced by poling. This effect can be explained by the existence of domain walls in the crystalline regions of annealed samples, while the smaller crystallites in unannealed samples are always in a single domain state. Thus the poling degree dependence of the linear permittivity of annealed samples may therefore be an effect of domain wall motion. The second order permittivity is a measure of the remanent polarization. Samples poled before the first annealing exhibit a small persistent polarization that remains stable in the paraelectric phase, in contrast to samples which remain unpoled before annealing. This non-switchable polarization is unchanged after several heating cycles and even after poling in the opposite direction. An explanation for this fixed polarization are non-switchable dipoles which form an intermediate phase between the crystalline and the amorphous phase.