Crystalline structures and phase transition of the ferroelectric P(VDF-TrFE) copolymers, a neutron diffraction study

Neutron investigations have been performed in order to study the crystalline structure of the ferroelectric and paraelectric phases of random copolymers of vinylidene fluoride and trifluoroethylene P(VDF-TrFE) and to investigate the influence of co-monomer composition. Structural information in the crystal is essential to understand the mechanism of the phase transition. In the ferroelectric phase, the increase of TrFE content from 20% to 40% clearly affects the crystalline order and leads to an anisotropic expansion of the lattice. At high temperature, complementary studies of diffraction and diffuse scattering, on oriented and deuterated samples, allow us to specify a structural description of the highly disordered paraelectric phase and to propose a model describing the conformational disorder of the chain. From these results we present a thermodynamical analysis of the ferroelectric to paraelectric transition based on the Landau theory which enables to evaluate the different energetic contributions to the structural instability in the crystalline regions of the polymer.