Crystallographic changes characterizing the Curie transition in three ferroelectric copolymers of vinylidene fluoride and trifluoroethylene: 1. As-crystallized samples

Copolymers of vinylidene fluoride/trifluoroethylene of molar composition $\text{65}\text{35}$, $\text{73}\text{27}$ and $\text{78}\text{22\%}$ respectively, are ferroelectric and undergo a Curie transition to the paraelectric state at high temperatures. In contrast to the irregular structure found earlier for the $\text{52}\text{48}\text{mol}\text{\%}$ copolymer, the structures of these three compositions in the low-temperature state are all well ordered and analogous to that of β-poly(vinylidene fluoride): they consist of molecular chains in a polar trans conformation whose order is improved with increasing vinylidene fluoride content, packed pseudo-hexagonally in unit cells whose dimensions decrease with increasing vinylidene fluoride content. In their paraelectric phase, the chains assume a partly disordered conformation consisting of irregular TG, T? and TT sequences and are packed on an expanded pseudo-hexagonal lattice. The Curie transitions were found to occur over a broad temperature range, encompassing ～30°C, and in the case of the $\text{78}\text{22}\text{mol}\text{\%}$ copolymer to extend into the melting region; they were also found to exhibit hysteresis by occurring at much lower temperatures upon cooling than upon heating.