Enhanced polarization in melt-quenched and stretched poly(vinylidene fluoride-hexafluoropropylene) films

Abstract

β-phase poly(vinylidene fluoride-hexafluoropropylene) (P(VDF-HFP)) copolymer films were prepared by uniaxially stretching solution-cast or melt-quenched samples. Different preparation routes lead to different amounts of the crystalline α and β phases in the films, as detected by means of Fourier-transform infrared spectroscopy and X-ray diffractometry. The β phase is significantly enhanced in melt-quenched and stretched films in comparison to solution-cast and stretched films. This is particularly true for copolymer samples with higher HFP content. The β-phase enhancement is also observed in ferroelectric-hysteresis experiments where a rather high polarization of 58 mC/m2 was found on melt-quenched and stretched samples after poling at electric fields of 140 MV/m. After poling at 160 MV/m, one of these samples exhibited a piezoelectric d33 coefficient as high as 21 pC/N. An electric-field-induced partial transition from the α to the β phase was also observed on the melt-quenched and stretched samples. This effect leads to a further increase in the applications-relevant dipole polarization. Uniaxially stretched ferroelectric-polymer films are highly anisotropic. Dielectric resonance spectroscopy reveals a strong increase of the transverse piezoelectric d32 coefficient and a strong decrease of the transverse elastic modulus c32 upon heating from 20 to 50°C.