A Critical Temperature–Pressure Window for Attaining a Giant Piezoelectric Voltage Coefficient in Poly(vinylidene fluoride)

Abstract

Polymeric piezoelectric sensors are increasingly important in the context of advancing artificial intelligence and soft robotics. It is known that the electric response to mechanical stress of poly(vinylidene fluoride) (PVDF) increases with increasing fraction of noncentrosymmetric (or “polar”) β and γ crystal forms (X_β+γ), as well as with increasing fraction of extended-chain crystals (F_ECC). Here, we describe a temperature–pressure (T–P) window for achieving both high X_β+γ and high F_ECC through intervention of the high-pressure hexagonal mesophase. Importantly, we show that high X_β+γ and F_ECC can be achieved under considerably milder conditions, 100 °C and 100 MPa below the equilibrium T–P range of the mesophase. By rapidly pressure-quenching the melt significantly below the triple-point temperature, direct melt-crystallization is bypassed, and the system enters a heavily superpressed and supercooled metastable range of the mesophase. This enables the lamellae of the mesophase to grow and thicken, subsequently transforming to largely extended-chain β and γ forms. Thus, a T–P processing window opens up, leading to a marked increase in the piezoelectric response. This way, we achieved a record PVDF piezoelectric voltage constant g₃₃ of 1.35 V·m·N^–1. Moreover, the chain extension involved also raises the melting point of the polymer by ∼30 °C, making the sensors usable at higher temperatures. This study offers guidance for the development of high-sensitivity PVDF-based piezoelectric sensors for applications across a broad temperature range.