Ferroelectric and antiferroelectric transitions in random copolymers of vinylidene fluoride and trifluoroethylene

Abstract

Copolymers of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) containing 50–80% former undergo a ferroelectric-to-paraelectric transition and then melt. The transition entropies determined by thermal measurements are shown to be consistent with the conformational entropies of the paraelectric and molten phases that are comprised of statistical combinations of TT, TG and TG' bonds. As the VDF-content exceeds 80%, melting overtakes the transition into the paraelectric phase. TrFE-rich copolymers containing 30–50% VDF exhibit an additional phase that is shown to be antiferroelectric-like on the basis of nonlinear dielectric investigation. Ferroelectricity tends to be obscured with increasing TrFE content and disappears at PTrFE. The dependence of the transition behavior on the composition of VDF and TrFE is discussed phenomenologically. It is shown that the most essential role is played by the cohesive energy of the ferroelectric phase that decreases with increasing TrFE content due to its asymmetry and bulkiness. The resulting increase in the Gibbs energy induces appearance of the paraelectric as well as antiferroelectric phases as the VDF content is further decreased.