Field-induced crystallization in highly plasticized poly(vinylidene fluoride) films

Abstract

Abstract : Highly plasticized films of poly(vinylidene fluoride) (PCF2) containing approximately 18% by weight of tricresyl phosphate (TCP) and with initial crystallinites (phase II) of approximately 15% were poled under high vacuum. The poling temperatures were 75 C and 90 C and the poling field was 250 KV/cm. The poling time was varied from 15 minutes to 240 minutes. Weight measurements and X-ray diffraction data of poled films showed that with increasing poling time, (up to 4 hrs) that plasticizer content decreased (to a minimum of 11%) and the crystallinity of the PVF2 films increased (to a maximum of 45%). The field induced crystallization occurred as highly oriented phase I crystals. After poling for 1 hour (at 90 C), a significant percent of the crystalline material was observed to be in the polar phase I form. At longer times, a measure of the relative percent of phase I with respect to the total crystallinity decreased. As expected, the piezoelectric strain coefficient d sub 31, stress coefficient e sub 31, and pyroelectric coefficient P sub y increased with increasing phase I content, reaching maximum values after a 1 hour poling time at 90 C of d sub 31 approx. = 10 PC/N, e sub 31 approx. = 11 mc/m2 and P sub y approx. = 17 micro Cm per deg K. The dynamic mechanical modulus and dielectric constant (measured at 3 Hz) were found to be approx. 1.3 GN/sq m and approx. 14.7 respectively under these poling conditions.