Phase transition and properties of a ferroelectric poly(vinylidene fluoride-hexafluoropropylene) copolymer

The phase transition, ferroelectric, and piezoelectric properties of a poly(vinylidene fluoride-hexafluoropropylene) [PVDF/HFP (10 mol %)] copolymer have been investigated. A rapid quenching process led to the formation of the ferroelectric beta phase in the PVDF/HFP (10 mol %). An irreversible crystal phase transition from the beta phase to the alpha and gamma phases at a temperature from 120 to 130 degrees C, which is below its melting point 159 degrees C, was observed through x-ray diffraction (XRD) experiments. Stretching the melt-quenched material or annealing the material at a temperature below the crystal phase transition temperature resulted in a significant increase in the beta phase. Based on the XRD results, the stretched samples had oriented beta phase, but the amount of beta phase is less than that in the annealed samples; however, our ferroelectric property measurements showed that the stretched samples had a higher polarization than the annealed samples. A theoretical analysis was conducted to clarify the effects of the polymer chain orientation in order to explain our ferroelectric measurement results. Assuming the dipoles are rigid and ignoring the effects of the internal electric field, it was found that the polarization of a randomly oriented PVDF-based polymer is about two-thirds of that of a completely oriented polymer when the volume fractions of beta phase are same. According to the analysis, polymer chain orientation significantly affects the ferroelectric polarization due to their determinative role on the distribution of the dipole polarization direction, although there is no polarization along the polymer chain direction. (C) 2005 American Institute of Physics.