Dehydrofluorination Induced High Piezoelectric Poly(Vinylidene Fluoride) and Applications

Abstract

The piezoelectric effect in poly (vinylidene fluoride) (PVDF) and its copolymers allows the exchange of mechanical and electrical energy. Combined with their unique properties as soft materials, piezoelectric polymers have attracted great interest in actuating, sensing and energy harvesting applications. The electromechanical coupling capacity of the piezoelectric polymers is governed by their crystalline phase composition. The β-phase of PVDF, which has an all-trans chain conformation, is desired because it has the highest aligned dipole density and piezoelectricity. However, the production and application of β-phase PVDF has been restricted by complex processing methods (i.e. in situ mechanical stretching) and limited thermal stability. Developing a method to prepare a thermally stable high β-phase PVDF homopolymer that does not require mechanical stretching is of great importance for high-performance PVDF devices and has been a long-lasting challenge in the PVDF industry. This dissertation details investigations into the structure-property relationships in piezoelectric fluoropolymers and the development of a novel and versatile method to synthesize high β-phase PVDF with improved piezoelectricity and thermal stability. Initially, the effects of molecular structural defects on the crystallization behavior of PVDF are studied through molecular simulations. It is demonstrated that defects consisting of carbon-carbon double bonds on the PVDF backbone can effectively reduce the relative conformational energy of β-phase thus leading to preferential β-phase crystallization. Secondly, a novel dehydrofluorination method is developed and optimized in this work to induce the discussed molecular defects into PVDF in order to promote β-phase formation without mechanical drawing. It is demonstrated that after chemical modification by the developed method, PVDF with β-phase fraction of over 80% can be directly produced through solution casting. Further research is then performed to thoroughly investigate both the direct and inverse piezoelectric effects of this dehydrofluorinated PVDF. Record-breaking piezoelectric strain coefficients are observed from dehydrofluorinated PVDF which achieves as much as a 107% increase in the piezoelectric strain coefficient d33 and a 40% increase in d31, compared with conventional drawn PVDF. Furthermore, the dehydrofluorination method removes the restrictions encountered during high-temperature processing of conventional drawn PVDF. The improved thermal stability of dehydrofluorinated PVDF allows it to recrystallize in the β-phase from any temperature below 210 ˚C which allows a variety of polymer processing methods to be used for PVDF device fabrication. Lastly, this work evaluates the performance of dehydrofluorinated PVDF in piezoelectric applications. Two novel additive manufacturing methods are developed to fully utilize the unique properties of dehydrofluorinated PVDF: a direct-writing method and an electrospin-assisted 3D printing method. Based on these methods, energy harvesters and actuators are fabricated using dehydrofluorinated PVDF and their performance is evaluated. The power density of a dehydrofluorinated PVDF-based stretching mode energy harvester is shown to reach 34.80 mW/cc, which exceeds previously reported PVDF-based energy harvesters and is almost five times higher than the power density of similar devices based on conventional drawn PVDF. By integrating dehydrofluorinated PVDF using novel additive manufacturing methods, the advances described in this dissertation provide new approaches to the development of future piezoelectric devices. This dissertation will serve to disseminate a novel and versatile method of preparing high β-phase PVDF with excellent piezoelectricity and improved thermal stability for the future development of high-performance piezoelectric devices.