Morphotropic Phase Boundary in Graft Poly(Vinylidene Fluoride-co-Tetrafluoroethylene) With High Curie Temperature

Morphotropic phase boundary (MPB) has been demonstrated in various ferroelectric polymers near which piezoelectric coefficient d₃₃ is substantially enhanced. However, all these ferroelectric polymers contain expensive trifluoroethylene while the Curie temperature T_c remains low, limiting their practical applications. Here, MPB is explored in trifluoroethylene-free poly(vinylidene fluoride-co-tetrafluoroethylene) copolymer by grafting design. We find that grafting induces order-to-disorder structural evolution mediated by the formation of MPB reminiscent of composition- and irradiation-induced MPB in trifluoroethylene-contained ferroelectric polymers. Meanwhile, we show that local conformational disorder is pinned by head-head defects inherent to tetrafluoroethylene, which is essential to maintain high T_c. Consequently, markedly improved d₃₃ of -41.6 pC N^-1 and high T_c of over 130°C are concurrently achieved in graft poly(vinylidene fluoride-co-tetrafluoroethylene) copolymers, outperforming currently existing ferroelectric polymers with high T_c. These findings add a trifluoroethylene-free member into MPB family, which offers a solution to address the long-standing inverse d₃₃-T_c relationship in ferroelectric materials.