Piezo-to-piezo (P2P) conversion: simultaneous β-phase crystallization and poling of ultrathin, transparent and freestanding homopolymer PVDF films via MHz-order nanoelectromechanical vibration

Abstract

An unconventional yet facile low-energy method for uniquely synthesizing neat poly(vinylidene fluoride) (PVDF) films for energy harvesting applications by utilizing nanoelectromechanical vibration through a ‘piezo-to-piezo’ (P2P) mechanism is reported. In this concept, the nanoelectromechanical energy from a piezoelectric substrate is directly coupled into another polarizable material (i.e., PVDF) during its crystallization to produce an optically transparent micron-thick film that not only exhibits strong piezoelectricity, but is also freestanding—properties ideal for its use for energy harvesting, but which are difficult to achieve through conventional synthesis routes. We show, particularly through in situ characterization, that the unprecedented acceleration associated with the nanoelectromechanical vibration in the form of surface reflected bulk waves (SRBWs) facilitates preferentially-oriented nucleation of the ferroelectric PVDF β-phase, while simultaneously aligning its dipoles to pole the material through the SRBW's intense native evanescent electric field . The resultant neat (additive-free) homopolymer film synthesized through this low voltage method, which requires only —orders-of-magnitude lower than energy-intensive conventional poling methods utilizing high kV electric potentials, is shown to possess a 76% higher macroscale piezoelectric charge coefficient d₃₃, together with a similar improvement in its power generation output, when compared to gold-standard commercially-poled PVDF films of similar thicknesses.