Oriented Polar Crystals in Poly(Vinylidene Fluoride) Produced by Simultaneously Applying Pressure and Flow

Fabricating oriented polar crystals in poly(vinylidene fluoride) (PVDF) is of significance whether in basic scientific research, or in developing flexible piezoelectric devices. Here, a set of pressuring and shearing devices are utilized to fabricate oriented polar crystals in PVDF. Under 50 and 100 MPa, isotropic α-crystals are formed regardless of imposing a shear rate of 30 s^–1, due to the fast relaxation of molecular chains as the flow is introduced at a high temperature of 190 °C. Intriguingly, oriented polar crystals (β- and γ-crystals) in the form of shish-kebab superstructure are induced by the shear flow (30 s⁻¹) when increasing the pressure to 200 MPa, for the relaxation of molecular chains is retarded by the elevated pressure, and thus, flow-induced long trans conformation can crystallize into oriented polar crystals. Generally, PVDF can crystallize into extended chain crystals (ECCs) under 200 MPa, but unexpectedly, when a shear flow is applied simultaneously, oriented polar crystals composed of folded chain crystals (FCCs) instead of ECCs are generated. These results shed light on the formation of oriented polar crystals in PVDF crystallizing under flow and pressure, and provide new opportunities to fabricate flexible piezoelectric devices with oriented polar crystals.