Properties of piezoelectric poly (vinylidene fluoride) films irradiated by γ-rays

Abstract

Abstract The influence of γ-ray irradiation on short-term and long-term properties of uniaxially stretched and poled PVF2 films has been investigated. Significant improvements were observed in the high temperature retention characteristics of both piezoelectric property and film dimensions in the irradiated samples. Additionally, stress relaxation tests at elevated temperatures revealed that the inverse relaxation (or stress buildup) behavior, commonly observed in the uniaxially stretched polymers, is suppressed markedly by irradiation. However, the film showed a steady decrease in its tensile strength with increasing dose and became embrittled in the direction perpendicular to the initial draw direction as the γ-doze exceeded 50 Mrad. The structural changes in the irradiated films were probed by means of differential scanning calorimetry, X-ray diffraction, and gel fraction analysis. The results indicate that the improvement of thermal stability is associated with the formation of crosslinks in the polymer; the slower piezoelectric decay in the irradiated films is brought about by the crosslinks in the crystalline region which impede the relaxation of molecular dipoles and charge trapping sites, while the enhanced dimensional stability is primarily due to the crosslinking in the amorphous region which resists the action of retractive force induced by the entropic effect of oriented molecules.