Pressure-Induced Polar Phases of Poly(vinylidene fluoride) under Enhanced Dipole–Dipole Interactions by Introducing Poly(vinylidene fluoride-trifluoroethylene)

Abstract

The strong piezoelectric activity and ferroelectric phenomena of polyvinylidene fluoride (PVDF) at high temperatures could be attributable to thick β-lamellae grown in the hexagonal phase under high pressure and high temperature, based on the temperature–pressure phase diagram. However, PVDF inevitably undergoes thermal degradation under such harsh crystallization conditions. To achieve high content polar phases via melt-crystallization under mild conditions, we incorporated a small amount (10 wt %) of poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] into PVDF. This addition introduces enhanced dipole–dipole interactions, which, together with pressure, synergistically induce the formation of pure β/γ phases with different lamellar thicknesses during isothermal crystallization at 190 °C. At low pressure (≤100 MPa), PVDF predominantly crystallizes into thermodynamically stable α folded-chain crystals (α-FCCs). Meanwhile, the enhanced dipole–dipole interactions induce locally ordered trans (T) conformations, giving rise to a small fraction of β-FCCs. When the pressure exceeds 200 MPa, PVDF melt crystallizes through the metastable hexagonal phase (in which the interchain spacing increases), forming thick lamellae of β/γ phases (partially extended-chain crystals, termed β/γ-PECCs) as well as incompletely grown β/γ-FCCs. Simultaneously, higher pressures enhance the effectiveness of dipole–dipole interactions, further promoting the formation of β-FCCs. Remarkably, the 100% β/γ phase (comprising both FCCs and PECCs) is achieved at pressures above 300 MPa. Interestingly, the relative content of β phase with longer trans (T) sequences within the polar phases increases with pressure and reaches 86% at 500 MPa. This behavior contrasts with that of neat PVDF, where α phase content remains more than 17% even at a pressure beyond 300 MPa, highlighting the crucial role of enhanced dipole–dipole interactions.