Highly mobile oriented amorphous fraction in semicrystalline ferroelectric polymers and its unique contribution to electrostrictive and piezoelectric properties*

Piezoelectric polymers hold great potential for various electromechanical applications, but only show low performance, with |d₃₃| < 30 pC/N. We prepared a highly piezoelectric polymer (d₃₃ = -62 pC/N) based on a biaxially oriented poly(vinylidene fluoride) (BOPVDF, crystallinity = 0.52). After unidirectional poling, macroscopically aligned samples with pure β crystals were achieved, which showed an unprecedentedly high spontaneous polarization (P_s) of 140 mC/m². Given the theoretical limit of P_s = 188 mC/m² for the neat b crystal, the high P_s could not be explained by a simple two-phase model (i.e., the crystalline and the amorphous phases). Instead, we deduce that a significant amount (at least 0.25) of an oriented amorphous fraction (OAF) must be present between these two phases. Experimental data suggest that the mobile OAF resulted in the negative and high d₃₃ for the poled BOPVDF. The plausibility of this conclusion was supported by molecular dynamics simulations.