The dielectric and piezoelectric properties of vinylidene fluoride-trifluoroethylene copolymers

Abstract

The dielectric and piezoelectric properties of vinylidene fluoride (VDF) -trifluoroethylene (TrFE) copolymers with molar compositions of 56:44, 70:30 and 80:20 VDF:TrFE have been studied. Ferroelectric to paraelectric phase transitions have been observed by measurement of dielectric constant as a function of temperature. The transition temperature increases from 70°C to 121°C with increasing VDF content. Optimisation of piezoelectric coefficient with respect to poling field, on thin (30-80 μm) solvent cast films, produces d31 = d32 values of 21 pCN-1 for the 56:44 VDF:TrFE copolymer, 17 pCN-1 for the 70:30 and 10 pCN-1 for the 80:20. The effect on the piezoelectric properties of ageing the films at 70°C for periods of up to 24 hours are reported and a comparison made between the properties of solvent-cast and pressed films. The potential of VDF:TrFE copolymers for piezoelectric applications is discussed.