The effect of crystallinity on observed polarization hysteresis loops in polyvinylidene fluoride

Abstract

It is demonstrated experimentally that electric fields cause conversion of alpha to beta crystals and a growth of beta crystals in PVDF (polyvinylidene fluoride). The former process leads to an increase in sample polarization as measured by external circuitry. Since beta crystals are polar and alpha crystals are not, this increased polarization is to be expected. The field-induced growth of beta crystals appears to lead to a decrease in sample polarization, but the cause for this is not so readily apparent. A detailed model for polarization reversal in ferroelectric polymers, which includes the contribution of the depolarizing field within the crystals, predicts that the length-to-width ratio of the crystals will have an effect on the observed hysteresis loops. Thus, a possible explanation of the reduction in macroscopic polarization is that the field-induced growth of beta crystals alters the length-to-width ratio of the lamellar crystals, thus reducing the depolarizing field within the crystal.<>