The Stability of Ferroelectric Polarization of PVDF upon Irradiation

Polymeric actuator and sensor materials suffer from a low thermal and temporal stability. We report on an attempt to slow down the underlying structural relaxation in polyvinylidenefluoride (PVDF) films by irradiation with electrons of 1.5 MeV. This treatment yielded a dose-dependent crosslinking of the macromolecular chains. It caused a considerable rise of the ferroelectric remanent and saturation polarization, and the material became a harder ferroelectric. These changes were almost completely and irreversibly lost upon subsequent annealing. The electron spin resonance (ESR) measurements revealed that a large number of radicals and trapped electrons remained after irradiation (~1 per crystallite), and that the electric conductivity increased from 0.11 pS/m in the un-irradiated but poled sample to 1.1 pS/m at 450 kGy. The increase in conductivity with dose is caused completely by a corresponding increase of the product [Mu][Eta] of the charge carrier mobility [Mu] and the number density [Eta]. Based on a discussion of depolarization effects, we conclude that the change in ferroelectric behavior upon irradiation mainly is caused by the activity of those charges which are implanted or created by irradiation, and that the majority of them are annihilated by thermal treatment. However, the reduction of polarization with time, or at elevated temperature, is considerably slowed down due to the existence of crosslinks.