Ferroelectric polarization in stretched piezo- and pyroelectric poly(vinylidene fluoride-hexafluoropropylene) copolymer films

Abstract

After suitable preparation, films of poly(vinylidene fluoride-hexafluoropropylene) [P(VDF-HFP)] show strong piezo- and pyroelectric effects. We investigated the polarization buildup in stretched P(VDF-HFP) copolymer films, cast from a dimethylsulfoxide/acetone solution. The films were poled under varying or constant electric fields. We observed a hysteresis of the polarization as a function of the electric field during poling of the stretched P(VDF-HFP) copolymer films with varying electric fields. From the hysteresis curve, a polarization of 24 mC/m2, and a coercive field of 100 MV/m were determined. The polarization values are lower than those of PVDF films cast from solution and stretched under the same conditions. It seems that HFP comonomer leads to a decrease in the amount of switchable dipoles. Nevertheless, the P(VDF-HFP) copolymer is very attractive because of its high piezo- and pyroelectric coefficients as well as its better chemical and thermal stability and its predicted longer lifetime. A rather uniform polarization across the whole sample thickness was found in spatially resolved measurements of the piezo- and pyroelectric properties. For comparison with PVDF and for structural information, we also report and discuss infrared transmission as well as dielectric response.