Crystallization, field-induced phase transformation, thermally induced phase transition, and piezoelectric activity in P(vinylidene fluoride-TrFE) copolymers with high molar content of vinylidene fluoride

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Abstract
Field‐induced phase transformation and thermally induced phase transition in vinylidene fluoride (VDF) and trifluoroethylene copolymers with VDF content higher than 82 mol% and their ferroelectric and piezoelectric properties have been studied by means of x‐ray diffraction, DSC, D‐E hysteresis, and piezoelectric resonance. It is found that copolymers with VDF content ranging from ∼82 to 90 mol% crystallize under ordinary pressure into thick lamellar single crystals composed of a mixture of α, β, and possibly γ phases. Application of a strong ac electric field transforms these mixed phase crystals into β‐phase crystals completely. The β‐phase crystals are ferroelectric and exhibit strong piezoelectric activity stable up to the meltingtemperature (160–180 °C depending on VDF content); the Curie point coincides with the melting point. Unpoled, mixed phase film has a paraelectric or rotational phase below the meltingtemperature, in which thick lamellar single crystals are grown extensively, whereas β‐phase film prepared by mechanical drawing has no paraelectric phase below the melting point. Definite ferroelectric‐to‐paraelectric phase transition in P(VDF‐TrFE) is inferred to occur only in thick lamellar crystals grown in the paraelectric phase. A phase diagram of P(VDF‐TrFE) is described.