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

Poly(vinylidene fluoride) exhibit distinctive ferroelectric property; however, their spontaneous polarization (P_s =60-105 mC/m²) is still inferior to those (>200 mC/m²) of the ceramic counterparts. Here we report an giant P_s (140 mC/m²) 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 279 mC/m², 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/m² 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 between the lamellar crystal and the mobile amorphous fraction. The OAF content was determined to be ~0.28, more than 50% of the amorphous phase. The fundamental knowledge obtained from this study will provide a solid foundation for the future development of PVDF-based wearable electronics and soft robotic applications.