The stability of ferroelectric polarization of PVDF upon irradiation

Polymeric actuator and sensor materials suffer from a low thermal and tempo ral stability. We report on an attempt to slow down the underlying structur al relaxation in polyvinylidenefluoride (PVDF) films by irradiation with el ectrons of 1.5 MeV. This treatment yielded a dose-dependent crosslinking of the macromolecular chains. It caused a considerable rise of the ferroelect ric remanent and saturation polarization, and the material became a harder ferroelectric. These changes were almost completely and irreversibly lost u pon subsequent annealing. The electron spin resonance (ESR) measurements re vealed that a large number of radicals and trapped electrons remained after irradiation (similar to 1 per crystallite), and that the electric conducti vity 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 completel y by a corresponding increase of the product mu n of the charge carrier mob ility mu and the number density n. Based on a discussion of depolarization effects, we conclude that the change in ferroelectric behavior upon irradia tion 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.