Giant spontaneous polarization for enhanced ferroelectric properties of biaxially oriented poly(vinylidene fluoride) by mobile oriented amorphous fractions

Abstract

Poly(vinylidene fluoride) (PVDF) and its random copolymers exhibit the most distinctive ferroelectric properties; however, their spontaneous polarization (60–105 mC m⁻²) is still inferior to those (>200 mC m⁻²) of the ceramic counterparts. In this work, we report an unprecedented spontaneous polarization (P_s = 140 mC m⁻²) for a highly poled biaxially oriented PVDF (BOPVDF) film, which contains a pure β crystalline phase. Given the crystallinity of ∼0.52, the P_s for the β phase (P_s,β) is calculated to be 279 mC m⁻², if a simple two-phase model of semicrystalline polymers is assumed. This high P_s,β is invalid, because the theoretical limit of P_s,β is 185 mC m⁻², as calculated by density functional theory. To explain such a high P_s for the poled BOPVDF, a third component in the amorphous phase must participate in the ferroelectric switching to contribute to the P_s. Namely, an oriented amorphous fraction (OAF) links the lamellar crystal and the mobile amorphous fraction. From the hysteresis loop study, the OAF content was determined to be ∼0.28, more than 50% of the amorphous phase. Because of the high polarizability of the OAFs, the dielectric constant of the poled BOPVDF reached nearly twice the value of conventional PVDF. The fundamental knowledge obtained from this study will provide a solid foundation for the future development of PVDF-based high performance electroactive polymers for wearable electronics and soft robotic applications.