Porous, Self-Polarized Ferroelectric Polymer Films Exhibiting Behavior Reminiscent of Morphotropic Phase Boundary Induced by Size-Dependent Interface Effect for Self-Powered Sensing

Piezoelectric poly(vinylidene fluoride) (PVDF) and its copolymer, poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)), have attracted considerable attention due to their potential in flexible, biocompatible energy harvesting and sensing devices. However, their limited piezoelectric performance hinders their widespread application. Inspired by the concept of morphotropic phase boundary (MPB) prevalent in high-performance piezoelectric ceramics, we successfully constructed MPB in the piezoelectric polymer P(VDF-TrFE) through size-dependent interface effects. We provided direct structural evidence using atomic force microscopy-infrared spectroscopy (AFM-IR) and significantly improved the piezoelectric performance of P(VDF-TrFE). The emergence of MPB is attributed to the interface effect induced by electrostatic interactions between ZnO fillers and the -CH₂, -CF₂, and -CHF groups in P(VDF-TrFE). This interaction drives a concomitant competition between the all-trans β phase (normal ferroelectric) and the 3/1 helical phase (relaxor), resulting in enhanced piezoelectric responses in the transition region. By coupling the MPB effect with a porous structure, we developed a piezoelectric nanogenerator (PENG) that surpasses the electrical output limitation of current P(VDF-TrFE)-based PENGs. The fabricated PENG exhibits superior piezoelectric outputs (6.9 μW/cm²), impressive pressure sensitivity (2.3038 V/kPa), ultrafast response time (4.3 ms), and recovery time (46.4 ms)─notably, without the need for additional poling treatment. In practical applications, the constructed PENG can efficiently generate characteristic signals in response to various human movements and harvest biomechanical energy. This work offers insight into utilizing interface-induced MPB and proposes a simple, scalable approach for developing high-performance self-polarized piezoelectric polymer films for self-powered sensing systems toward human-machine interaction.