Electrostriction-enhanced piezoelectric property of poly(vinylidene fluoride) via high-power ultrasonication*

Despite decades of research, the structure-piezoelectric property relationship is still unclear for ferroelectric polymers, such as poly(vinylidene fluoride) (PVDF); therefore, their piezoelectric coefficients (-d₃₃ and d₃₁) are often limited to ~30 pC/N. Recently, we report electrostriction-enhanced piezoelectricity in PVDF, taking advantage of the head-to-head and tail-to-tail (HHTT) defects (usually 3-6 mol.%). After high-power ultrasonication, the piezoelectric performance of PVDF can be significantly enhanced. Two PVDF homopolymers with different HHTT contents were studied. The PVDF with a lower HHTT content (4.3%) exhibited a higher melting temperature (T_m, denoted

as HMT), whereas that with a higher HHTT content (5.9%) exhibited a lower T_m (denoted as LMT). In addition to the primary crystals (PCs) and the isotropic amorphous fraction, X-ray diffraction also suggested the presence of the oriented amorphous fraction (OAF) and secondary crystals (SCs), which are important in enhancing the piezoelectricity for PVDF. Intriguingly, the LMT PVDF exhibited higher piezoelectric performance than the HMT PVDF, because it had a higher OAF/SC content. In addition, high-power ultrasonication was shown to effectively break relaxor-like SCs off from the PCs, further enhancing the piezoelectric performance. That is, the inverse piezoelectric coefficient d₃₁ reached as high as 76.2 pm/V at 65 ^◦C for the ultrasonicated LMT PVDF. The insight from this study will enable us to design better piezoelectric PVDF polymers for practical electromechanical applications.