Crystal phase transition dependence of the energy storage performance of poly(vinylidene fluoride) and poly(vinylidene fluoride‐hexafluoropropene) copolymers

Abstract

ABSTRACTLarge‐scale mechanical stretching has been performed to modify the crystal phase structures of the pristine poly(vinylidene fluoride) (PVDF) and its copolymer poly(vinylidene fluoride‐hexafluoropropylene) [P(VDF‐HFP)] with various molar contents HFP in an attempt to improve their energy storage performances. It is found that the physical stretched PVDF and P(VDF‐HFP) 95.5/4.5 mol % films have a phase transition from the nopolar α‐phase to highly polar β‐phase, which is different from the P(VDF‐HFP) films with relative high HFP molar contents (α to γ phases). The following results show that the phase transition in these PVDF‐based polymers has a significant effect on their dielectric and energy storage performances. On account of the reformation of the crystalline property and elimination of the impurity defects, an ultra‐high breakdown electric field of ∼900 MV/m has been obtained in all the stretched samples. Consequently, the higher discharged energy densities of 27.1 and 27.7 J/cm3 are calculated from the D–E loops of the β‐PVDF and β‐P(VDF‐HFP) 95.5/4.5 mol % films, respectively. Regarding their excellent discharging energy density of ∼10 J/cm3 under 600 MV/m for thousands of times, the stretched PVDF and its copolymer P(VDF‐HFP)s are promising candidates for high power capacitors applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018, 135, 46306.