Dielectric nonlinearities of P(VDF-TrFE)

The nonlinear dielectric permittivities of copolymers of polyvinylidenfluoride (PVDF) with trifluoroethylene (TrFE) have been measured as a function of temperature. The phase transition from the ferroelectric to the paraelectric phase is characterised by the sign of the third order permittivity in the paraelectric phase. The observed third order permittivity is positive for a copolymer of 70/30 mol% VDF-TrFE, but negative for 56/44 mol-% VDF-TrFE. This shows a change of the nature of the phase transition with the composition. Poling of the material causes a strong increase of the permittivities of even order in the ferroelectric phase, while the linear dielectric constant and the odd order permittivities slightly decrease. In the paraelectric phase, the investigated 56/44 mol-% VDF-TrFE specimens show a non vanishing second order permittivity. This deviation of the inversion symmetry is described by an internal bias field. While the absolute value of the bias field varies for different samples, it is found to be independent of temperature and a previous poling