Dipolar glass behaviour of a ferroelectric polymer after irradiation

Abstract

Random copolymers of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) spontaneously crystallize into a ferroelectric structure with a high degree of crystallinity, and in these polymers, the most remarkable effect of irradiation is to induce a structural ferro-to-paraelectric phase transition. Indeed, both the Curie temperature T c and the melting temperature T m decrease with increasing radiation dose. Also the degree of crystallinity decreases and beyond a dose of 4 MGy full amorphization is produced. In the intermediate range (1 MGy < dose < 4 MGy), the ferroelectric structure is no more stable and it is replaced by a conformationally disordered crystalline structure with hexagonal symmetry. At high temperature the disorder is driven by diffusive reorientations of the chain segments, while at low temperature it becomes static. Such a dynamical transition, without symmetry change, presents all the characteristics of an orientational glass transition in a crystal lattice: a thermal expansion anomaly around the static glass transition temperature T g = 270 K, and a typical dielectric dispersion above T g.