Switchable Piezoelectricity of Polyvinylidene Fluoride Films Induced by Crystal Transition in Shape Memory Process

With the rapid development of wearable self-powered devices, the piezoelectric materials having deformable and switchable characteristics are attracting extensive attention. Herein, the cross-linked polyvinylidene fluoride (cPVDF) was fabricated through an alkali-catalyzed defluorination and chemical cross-linking method by introducing trimethylhexamethylenediamine (THDA). The system filled with 1 wt % THDA (CP1) was proved to possess balanced cross-linking density and crystallinity, which would play a crucial role in achieving a switchable piezoelectric effect. In comparison to pristine PVDF, the cross-linked one exhibited repeatable shape memory characterization due to restrained plastic deformation above the melting transition. Both the shape-fixing and shape-recovery ratios were stably maintained above 90%. More significantly, the thermo-mechanical program also triggered the α-β-α crystal transition accompanied by the variation of conformational entropy. The largest amount of β crystals was produced in the temporary shape, whereas the original and recovery shapes were dominated by α crystals. Such structural transition occurred repeatedly in the successive shape memory cycles, which thereby induced the periodic fluctuation of the piezoelectric constant (d33). For the CP1 sample, its d33 was only about 2 pC/N in the original and recovery shapes but reached up to 9.4 pC/N in the temporary shape. When the latter one was fabricated into a piezoelectric device, alternating voltage and current were generated by performing periodic impact force and were demonstrated to be capable of monitoring some pressure-related motions in real time without an external power supply. Finally, the switchable piezoelectric effect of the CP1 at different shape memory stages was further revealed through its electroactive response to the sinusoidal voltage stimulation. This work offers a special perspective in tailoring piezoelectric performance through the structural transition in shape memory progress, which is of great significance for enriching the types and applications of piezoelectric polymers.