Piezo-and ferroelectric nanofibers of the polymer poly(vinylidenefluoride) (PVDF) have been widely employed in strain and pressure sensors as well as nanogenerators for energy harvesting. Despite this interest, the mechanism of electromechanical transduction is under debate and a deeper knowledge about relevant piezoelectric or electrostatic properties of nanofibers is crucial to optimize transduction efficiency. Here we prepare PVDF-TrFE nanofibers at different electrospinning conditions. We compare macroscopic electromechanical response of fiber mats with microscopic analysis of single nanofibers performed by piezoelectric and electrostatic force microscopy. Our results show that electrospinning favors the formation of the piezoelectric -phase in the polymer and leads directly to piezoelectric properties that are comparable to annealed thin films. However, during electrospinning the electric field is not strong enough to induce direct ferroelectric domain polarization. Instead we observe accumulation of triboelectric surface charges and trapped space charge in the polymer that explain the electret like macroscopic electromechanical response.
Poly(vinylidene fluoride) (PVDF) displays ferroelectric, piezoelectric and pyroelectric behavior and it is widely used in high-tech applications including sensors, transducers, energy harvesting devices and actuators. The crystallization of this polymer into highly polar phase is desirable but is hard to achieve without applying specific thermo-mechanical treatments. Indeed, fabrication processes directly affect PVDF molecular chain conformation, inducing distinct polymorphs. In this paper, we present the fabrication of PVDF/BaTiO3 composite foams by thermally induced phase separation method (TIPS). Different compositions are tested and characterized. The crystallinity, and in particular the development of electroactive crystal phase is monitored by FTIR, DSC and XRD measurements. Dielectric properties are also evaluated. It turns out that TIPS is a straightforward method that clearly promotes the spontaneous growth of the phase in PVDF and its composite foams, without the need to apply additional treatments, and also significantly improves the degree of crystallinity. BaTiO3 content gives additional value to the development of phase and total crystallinity of the systems. The low permittivity values (between 2 and 3), combined with the cellular morphology makes these materials suitable as lightweight components of microelectronic circuits.
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