Improved piezoelectricity in thick lamellar β-form crystals of poly(vinylidene fluoride) crystallized under high pressure

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Abstract
Poly(vinylidene fluoride) (PVDF) was crystallized under high pressures on the basis of the pressure‐temperature (P‐T) phase diagram obtained recently, and piezoelectric and ferroelectric properties of the resulting films were evaluated. Crystallization in the metastable hexagonal phase appearing at high pressure and temperature below the triple point yielded a film comprising extended chain lamellar crystals of a mixture of β and γ forms, the latter of which is transformed into β form by poling at an elevated temperature (120 °C). The poled film exhibits strong piezoelectric effect persistent up to the melting temperature (205 °C). The electromechanical coupling factor kt for the thickness extensional piezoelectric effect is 0.27 at 25 °C, the largest value ever found in PVDF. The results of studies on morphology, structure, and thermal properties are also described.