Piezo-to-Piezo (P2P) Conversion: Simultaneous $\beta$-Phase Crystallization and Poling of Ultrathin, Transparent and Freestanding Homopolymer PVDF Films via MHz-Order Nanoelectromechanical Vibration
An unconventional yet facile low-energy method for uniquely synthesizing neat poly(vinylidene fluoride) (PVDF) films for energy harvesting applications through piezo-to-piezo (P2P) conversion is reported. In this novel concept, the nanoelectromechanical energy from a piezoelectric substrate is directly coupled into another polarizable material (i.e., PVDF) during its crystallization to produce a micron-thick film that not only exhibits strong piezoelectricity, but is also freestanding and optically transparent - properties ideal for its use for energy harvesting, but which are difficult to achieve through conventional synthesis routes. In particular, we show that the unprecedented acceleration ($\mathcal{O}$($10^{8}$ m s$^{-2}$)) associated with the nanoelectromechanical vibration in the form of surface reflected bulk waves (SRBWs) facilitates preferentially-oriented nucleation of the ferroelectric PVDF $\beta$-phase, while simultaneously aligning its dipoles to pole the material through the SRBW's intense native evanescent electric field ($\mathcal{O}$($10^{8}$ V m$^{-1}$)). The resultant neat (additive-free) homopolymer film synthesized through this low voltage method requiring only $\mathcal{O}$(10 V) - orders-of-magnitude lower than the energy-intensive conventional poling methods utilising high kV electric potentials - is shown to possess a 76% higher macroscale piezoelectric charge coefficient ($d_{33}$), together with a similar improvement in its power generation output, when compared to the gold-standard commercially-poled PVDF films of similar thicknesses.