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

As in the polyvinylidene-fluoride (PVDF) homopolymer, the crystalline phase of the semi-crystalline poly(vinylidene fiuoride-hexafluoropropylene) (P(VDF-HFP)) copolymer should preferably consist of beta-phase crystals in order to achieve high piezoelectric activity. The authors report an investigation of phase transformations and morphologies in P(VDF-HFP) during and after various processing steps. Sample films were first prepared by solution casting, then melt-pressed and quenched. Some of them were uniaxially stretched at 90 degC with a stretching ratio of 4. Fourier-transform infrared (FTIR) spectroscopy was utilized to analyze crystalline phase transformations during the processes. The piezoelectric d₃₃ coefficient was measured by observing the charge-density changes upon application of a low-frequency force.