Curie transitions in copolymers of vinylidene fluoride

Abstract

A series of random copolymers of vinylidene fluoride and trifluoroethylene containing 52, 65, 73, and 78 mol % VF₂ has been shown by X-ray and dielectric techniques to undergo Curie transitions. In their ferroelectric phase the copolymer molecules assume structures analogous to the all-trans of β-PVF₂, while in the paraelectric they are partly disordered through introduction of gauche^± bonds. The Curie transitions extend over wide temperature ranges (∼30°C) and exhibit thermal hysteresis in that the ferroelectric → paraelectric transformation occurs at higher temperatures than the reverse. Poled samples have higher Curie temperatures than their unpoled counterparts; for the 78 mol % copolymer, this causes thwarting of the Curie transition by the onset of melting. This suggests that the absence of a distinct Curie transition in β-PVF₂ may be due to prior melting. Extrapolation of our copolymer results to 100 % VF₂ leads to a hypothetical transition at ∼205 °C (which is ∼20°C above the melting point of β-PVF₂); moreover, if such a transition extends over as wide a temperature range as in the copolymers, its earliest crystallographic manifestations might be detectable prior to completion of melting.