Crystal Transformation, Piezoelectricity, and Ferroelectric Polarization Reversal in Poly(Vinylidene Fluoride)

Abstract

The mechanisms for the II-I crystal transformation in poly(vinyl-idene fluoride) (PVDF) by various procedures were studied with a PSPC (position sensitive proportional counter) X-ray system. Simultaneous X-ray and stress-strain relationship measurements during a drawing procedure revealed that the crystal transformation from Form II to Form I always initiates at the deformation stage where a necking was completed at the center of tensile samples, thus suggesting that a heterogeneous stress distribution in the sample plays a critically important role. High pressure X-ray experiments on a heating process exhibited that this polymer transforms from Form II to folded-chain Form I and then extended-chain Form I crystals before melting. The II-I crystal transformation was also observed to proceed with an activation energy of 30 kcal/mol on an annealing procedure at 4000 kg/cm2. Furthermore, a uniaxial compressional deformation and a drawing at high pressures were observed to cause this II-I crystal transformation. These phenomena were utilized to prepare the Form I samples with a high degree of crystal perfection and to improve considerably their piezoelectric properties. Finally, ferroelectric polarization switching experiments were carried out for Form I crystal films in wide ranges of temperature and pressure. The switching current behavior at atmospheric pressure changed remarkably at about −50°C which coincides well with the reported glass transition temperature.