Dielectric behavior and ferroelectric transition of copolymers of vinylidene fluoride and trifluoroethylene

Dielectric behavior of copolymers of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) with VDF contents of 52 to 78 mol % has been investigated over wide frequency and temperature ranges with particular reference to the ferroelectric transition. Some related physical properties such as the dynamic tensile modulus, the thermal expansion, and the enthalpy of transition were also studied. The copolymers exhibited three dielectric relaxations γ, β, and Tt in the increasing order of temperature. The γ process is the local mode relaxation of part of frozen molecular chains below the glass transition. The β relaxation is related with micro-Brownian motions of molecular chain backbone in amorphous regions. The Tt relaxation which is featured with a rather narrow distribution of relaxation times is associated with cooperative motions of molecular chain in crystalline regions in the vicinity of the ferroelectric transition. Dielectric measurements under hydrostatic pressure revealed that the anomalous peak of the permittivity at the ferroelectric transition moves to higher temperature with increasing pressure and that the pressure dependence of the transition temperature is considerably large for these copolymers: dTt/dp was 0.314 KMPa−1 for 52.8 mol-% VDF and 0.383 and 0.401 KMPa−1 in heating and cooling process for 64.6 mol-% VDF, respectively. Observed values of dTt/dp were fairly close to those calculated by the Clausius-Clapeyron relation. These results suggest that the nature of ferroelectric transition in the copolymer is of the first order.