Phenomenological aspect of a ferroelectric vinylidene fluoride/trifluoroethylene copolymer

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Abstract
The phase transition and polarization reversal in a copolymer of 65 mol% vinylidene fluoride and 35 mol% trifluoroethylene are examined in the light of phenomenological theory. The free energy function expanded in even powers of the polarization are determined from the remanent polarization, the transition point and the reciprocal permittivity. The linear coefficient takes a value common to order-disorder type ferroelectrics. The negative sign of the first nonlinear coefficient indicates that the transition is of first order. The transition causes an anomaly in the equilibrium permittivity. The kinetic coefficient, on the other hand, shows no anomaly which means that the potential barrier for molecular motions is independent of the transition. The free energy function predicts much larger coercive field than observation. This leads to an alternative mechanism for practical polarization reversal. Observed switching curves are well reproduced by assuming a random nucleation of reversed domains and their subsequent two-dimensional growth with accelerated velocity.