The structure-property relationship of poly(vinylidene difluoride)-based polymers with energy storage and loss under applied electric fields

This paper systematically examines the family of poly(vinylidene difluoride) (PVDF)-based fluoropolymers, including homo-, co-, and terpolymers containing vinylidene fluoride (VDF), trifluorethylene (TrFE), and chlorotrifluoroethylene (CTFE) units, with the objective of tuning the polymer chain conformation and crystal structure in order to identify the most suitable polymer for energy storage (capacitor) applications. All polymers have high molecular weight, uniform composition distribution, semicrystallinity, and high purity. They were prepared by a borane/oxygen control radical initiator in a homogeneous solution at ambient temperature. The resulting polymers were solution-cased, then melt-conditioned into uniform (defect-free) thermoplastic thin films (thickness 10-20 μm). The combination of thermal, dielectric, and uni- and bipolar charge displacements reveals their polarization profiles, which are dependent on chain conformation, crystal phase, crystal size, Curie temperature, and ac and dc fields. Ferroelectric VDF/TrFE copolymers, having all-trans chain conformation and polar β-phase crystals, exhibiting huge remnant polarization, are not suitable for energy storage (capacitor) applications. Some poled PVDF homopolymer and VDF/CTFE copolymers with γ-phase crystals show potential for dc (not ac) powered applications. The most suitable polymer is the VDF/TrFE/CTFE terpolymer having a TTTG chain conformation, small polar γ-phase crystals, relaxed dielectric properties, and a Curie transition at near ambient temperature, providing both ac and dc powered capacitors with a balance of properties, high-energy density and low-energy loss.