Polarization of poly(vinylidene fluoride) by application of breakdown fields

Abstract

Films of poly(vinylidene fluoride) have been polarized to a high level of piezoelectric activity by application of high electric fields which would normally cause breakdown in the polymer. The high field poling is accomplished by interposition between the two metal electrodes of a dielectric plate which severely limits the current passing through the polymer film during local discharges. Fields with strengths up to an estimated value of 11 MV/cm were applied to 25-μm-thick biaxially oriented films at room temperature over a period of a few minutes to a few hours without incidence of major breakdowns. The piezoelectric e31 (=e32) coefficient of the poled film increases steadily with the applied field, reaching a value of 32 mC/m2 at the estimated field strength of 11 MV/cm. X-ray and ir analyses of the films before and after poling show that the polarization is accompanied by transformation of the nonpolar α to the polar α phase, and further to the β phase depending on the field strength. However, the α to β conversion is only partially complete even at the highest field used, indicating that further polarization may be achieved with still higher field strengths.