Effect of chemical defects on electrostriction-enhanced piezoelectric property of poly(vinylidene fluoride) via high-power ultrasonication

Abstract

Among all ferroelectric polymers, poly(vinylidene fluoride) (PVDF)-based polymers exhibit the best piezoelectric properties and thus are promising for sensors, actuators, and energy harvesters in flexible/wearable electronics and soft robotics. Despite decades of research effort, the structure-property relationship is still unclear for ferroelectric polymers, and their piezoelectric performance is often limited to ∼30 pC/N. In this study, we report the effects of chemical defects [i.e., the head-to-head and tail-to-tail (HHTT) sequence] and high-power ultrasonication on the piezoelectric performance of PVDF. Two PVDF homopolymers with different HHTT contents were studied. The PVDF with a lower HHTT content (4.3 %) exhibited a higher melting temperature (Tm, denoted as HMT), whereas that with a higher HHTT content (5.9 %) exhibited a lower Tm (denoted as LMT). In addition to the primary crystals (PCs) and the isotropic amorphous fraction, wide-angle 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.