Confinement-Induced High Field Antiferroelectric-Like Behavior in a Poly(vinylidene fluoride-co- trifluoroethylene-co- chlorotrifluoroethylene)-graft-Polystyrene Graft Copolymer

摘要

Ferroelectric polymers based on poly(vinylidene fluoride) (PVDF) and its copolymers appears to be a choice for electric energy storage because of their relatively high permittivity (10-20) and high dc breakdown strength (ca. 700 MV/m for a test area of 2 cm2 ). By modifying PVDF with bulky comonomers such as chlorotrifluoroethylene (CTFE) and hexafluoropropylene (HFP), P(VDF-CTFE) and P(VDF-HFP) random copolymers recently have achieved a discharged energy density as high as 17-25 J/cm~3 at ~600 MV/m for millisecond discharge. Nevertheless, ferroelectric polymers have relatively high losses due to the delayed dipole switching with respect to the alternating electric field. For example, the typical dielectric loss at low fields, tanδ, for PVDF and its copolymers is about 0.02, substantially higher than that of the current state-of-the-art capacitor film, biaxially stretched polypropylene (BOPP, tanδ ~ 0.0002). At high electric fields, depending on the polymorphism (β- vs. α-crystals), their ferroelectric losses can be as high as 40-80% because of the ferroelectric hysteresis loops during switching (see Scheme 1A). This ferroelectric loss is also discharge time dependent; an additional 40% loss in discharged energy density when the discharge time decreases from 1 ms to 1 μs.