Novel polymer ferroelectric behavior via crystal isomorphism and the nanoconfinement effect

In contrast to the comprehensive understanding of novel ferroelectric [i.e., relaxor ferroelectric (RFE) and antiferroelectric] behavior in ceramics, RFE and double-hysteresis-loop (DHL) behavior in crystalline ferroelectric polymers have only been studied in the past fifteen years. A number of applications such as electrostriction, electric energy storage, and electrocaloric cooling have been realized by utilizing these novel ferroelectric properties. Nonetheless, fundamental understanding behind these novel ferroelectric behaviors is still missing for polymers. In this feature article, we intend to unravel the basic physics via systematic studies of poly(vinylidene fluoride-co-trifluoroethylene) [P(VDF-TrFE)]-based terpolymers, electron-beam (e-beam) irradiated P(VDF-TrFE) copolymers, and PVDF graft copolymers. It is found that both the crystal internal structure and the crystal–amorphous interaction are important for achieving the RFE and DHL behaviors. For the crystal internal structure effect, dipole switching with reduced friction and nanodomain formation by pinning the polymer chains are essential, and they can be achieved through crystal repeating-unit isomorphism (i.e., defect modification). Physical pinning [e.g., in P(VDF-TrFE)-based terpolymers] induces a reversible, electric field-induced RFE↔FE phase transition and thus the DHL behavior, whereas chemical pinning [e.g., in e-beam irradiated P(VDF-TrFE)] results in the RFE behavior. Finally, the crystal–amorphous interaction (or the nanoconfinement effect) results in a competition between the polarization and depolarization local fields. When the depolarization field becomes stronger than the polarization field, a DHL behavior is observed. Obviously, the physics for ferroelectric polymers is different from that for ceramics/liquid crystals and can be largely attributed to the long-chain nature of semicrystalline polymers. This understanding will help us to design new ferroelectric polymers with improved properties and/or better applications.