Interacting and noninteracting dipole systems in ferroelectric poly(vinylidene fluoride-trifluoroethylene) copolymer

Abstract

Ultrathin films of poly(vinylidene fluoride-trifluoroethylene) copolymers are prepared by a Langmuir–Blodgett technique. These films show ferroelectric properties like hysteresis loops of the polarization and butterfly curves of the ac capacitance versus the applied field. Small signal measurements exhibit a power law of the dielectric permittivity in the frequency domain as well as a power law for the depolarization current in the time domain, respectively. This behavior can be explained by a distribution of relaxation times in the dipole system. From these measurements alone it cannot clearly be distinguished whether this distribution results from the interaction between the dipoles or if the dipoles relax independently from each other with individual relaxation times. To this aim the field reversal experiment which is sensitive to the coupling of dipole systems is introduced. As final result we get that the small signal nonswitching relaxational response resulting in the dielectric permittivity is coupled to the large signal ferroelectric behavior resulting in the hysteresis loops.