Convenient folding-hot-pressing fabrication and enhanced piezoelectric properties of high β-phase-content poly(vinylidene fluoride) films

Poly(vinylidene fluoride) (PVDF) is the most attractive piezoelectric polymer for application in flexible sensors. To attain excellent piezoelectric properties, a substantial amount of spontaneous polar β-phase content is highly desired. Nevertheless, the current reported manufacturing methods to increase β-phase contents are inconvenient and complex, hindering progress in PVDF's application. This work proposes a folding-hot-pressing method to fabricate high β-phase-content PVDF films. Structural characterization indicates that the films have α and β phases and the folding-hot-pressing process transforms the α phase into the β phase. Due to the 97.5% β-phase content and aligned structure, a piezoelectric constant of 20 pC/N is achieved in the three-times folded film. Furthermore, the process method enhances the tensile strength (126.2 MPa) of the films, with a low Young's modulus (0.87 GPa) remaining, making the films applicable for flexible piezoelectric sensors. Additionally, sensors based on the achieved films were assembled and applied for human physiological activity monitoring. This work offers a scalable new melt-processing strategy for developing high-performance PVDF-based piezoelectric composite films for wearable electronic devices.