Poly(vinylidene fluoride) (PVDF) films with various crystal phases (a, b, and c phases) and varied crystallinities were fabricated via different processes. The influence of the crystalline properties, such as the crystallinity and crystal phases, on the breakdown strength and dielectric and energy storage properties of the films were studied. Under low electric field, the dielectric constant was governed by the crystallinities of the films, and the dielectric loss was more related to the polarity of their crystal phases. Under high electric field, the high polarity of the crystal phases favored high-maximum, remnant, and irreversible polarization of the films. The lower crystallinity of the films with the same crystal phases led to a higher maximum and remnant polarization but a lower irreversible polarization. Under direct-current electric field, the discharged energy efficiency was mainly dominated by the polar nature of crystal phases. Under an electric field below 300 MV/m, the discharged energy density and energy loss of the three kinds of films were rather close, regardless of the phase transition. When the electric field was over 300 MV/m, the overall discharged energy density was dominated by the practical breakdown strength. c-PVDF with a proper crystallinity and crystal grain size is expected to realize an energy density over 10 J/cm 3 under an electric field over 400 MV/m.
Poly(methyl methacrylate) (PMMA) was introduced into poly(vinylidene fluoride) (PVDF) via a solution blending process, and a series of PVDF/PMMA blends were obtained in an effort to reduce the energy loss of pure PVDF. The effects of the composition and thermal treatment on the properties of the polymer blends were carefully studied. The results show that the introduction of PMMA led to a lower crystallinity and a smaller crystal size of PVDF for its dilution effect. As a result, the dielectric constant and energy storage density of the polymer blends were slightly reduced. Meanwhile, the phase transition of the PVDF crystals from the a phase to the b phase happened during the quenching of the blend melt to ice-water; this was also observed in the untreated or annealed blends with PMMA contents over 50 wt %. Compared with the a-PVDF, the PVDF crystals in the b phase possessed a lower melting temperature, a higher dielectric constant, and a lower dielectric loss. The addition of PMMA reduced the energy loss of PVDF significantly, whereas the energy storage density decreased slightly. The optimized blend film with about 40 wt % PMMA and PVDF in the b phase showed a relative high energy storage density and the lowest energy loss.
High discharged energy density observed in poly(vinylidene fluoride) (PVDF) based copolymers has attracted considerable research interests in the past years. Crystalline properties exhibit great influence on their dielectric and energy storage properties. To understand how crystalline properties influence the energy storage properties of PVDF, PVDF films with three different crystal forms are investigated in this paper. It is shown that γ-PVDF is allowed to work under higher electric fields than α- and β-PVDF in the absence of phase transition in α-PVDF and early polarization saturation in β-PVDF. Consequently, γ-PVDF exhibits the highest energy density of 14 J/cm3 under 500 MV/m electric field.
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