Theory of radiation induced relaxor behavior of poly(vinylidene fluoride-trifluoroethylene) copolymers

Abstract

We propose a theory of the dynamic dielectric response in the organic ferroelectrics polyvinylidene fluoride (PVDF), poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)] copolymer before and after irradiation by fast electrons. For this purpose, we use our random field formalism. In the adopted model we consider the earlier polymers as the disordered ferroelectrics in a mixed ferroglass phase (FG) with a coexistence of polar short- and long-range order regions. Short-range order regions correspond to amorphous layers, while long-range order regions appears in the crystalline phase of all-trans conformation. In our formalism, we account for the change of VDF content as well as for the influence of the irradiation, by variation of the content of electric dipoles and other random field sources. The downward shift of the paraferroelectric phase transition temperature and gradual transformation of the mixed FG phase into a dipole glass state with the increase of the irradiation dose was shown to be related to the self-consistent change of parameters of random field distribution function. We were able to fit the low-temperature dynamic dielectric response related to the amorphous phase by Vogel–Fulcher (V–F) law with parameters close enough to those observed in the experiment. The coincidence between calculated and measured behavior (namely, almost complete disappearance of high-temperature maxima, corresponding to the ferroelectric phase transition, their submergence into low-temperature maxima and V–F law for the low-temperature maxima of absorption) of the irradiated samples speaks in favor of irradiation induced relaxor behavior of P(VDF/TrFE) copolymers. We also discuss the physical reasons for the choice of the parameters of material necessary to fit our theory to the experiment.