Description of Dynamic Dielectric Response in Poly(vinylidene Fluoride-Trifluoroethylene) Copolymers

Abstract

We propose a theory of dynamic dielectric response in organic ferroelectrics PVDF, P(VDF/TrFE) copolymer before and after irradiation by fast electrons. For this purpose, we use random field formalism. In the adopted model we consider above polymers as disordered ferroelectrics in 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 change of VDF content as well as for the influence of irradiation, by variation of content of electric dipoles and other random field sources. The downward shift of para-ferroelectric phase transition temperature and gradual transformation of mixed FG phase into 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 amorphous phase by Vogel-Fulcher (V-F) law with parameters close enough to those observed in the experiment. The coincidence between calculated and measured behaviour (namely, almost complete disappearance of high-temperature maxima, corresponding to ferroelectric phase transition, their submergence into low-temperature maxima and V-F law for low-temperature maxima of absorption) of irradiated samples speaks in favour of irradiation induced relaxor behaviour of P(VDF/TrFE) copolymers. We also discuss the physical reasons for choice of parameters necessary to fit our theory to the experiment.

Keywords:

• Ferroelectric polymers
• relaxors
• dielectric dispersion
• glass transition