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

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 melting temperature (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 melting temperature, 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.