Polarisation and dielectric properties of fatigued ferroelectric polymer films

When ferroelectric materials are subjected to repeated reversal of polarisation, the magnitude of the reversed polarisation is observed to decrease. This phenomenon is referred to as fatigue and is generally ascribed either to space charge effects or to the pinning of domain walls at defects. Polyvinylidene fluoride (PVDF) and its copolymers with trifluoroethylene, in 65/35 and 75/25 mol% proportions were studied. The polymers were fatigued by exposing them to an alternating field with amplitude above the coercive field, while measuring the D-E hysteresis loops. Samples containing only the p crystal form showed a reduction in switched polarisation during the hysteresis loop measurement, while those containing a mixture of the a and p phases did not. In addition, dielectric spectra were measured over the range from 10^-2 Hz to 10⁺⁵ Hz. The major change in the dielectric spectra of the fatigued samples was a reduction in the dielectric loss at the lowest measured frequencies. We speculate that this is due to charge trapping