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

ABSTRACT Morphotropic phase boundary (MPB) has been demonstrated in various ferroelectric polymers near which piezoelectric coefficient d ₃₃ 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 d ₃₃ 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 d ₃₃ ‐ T _c relationship in ferroelectric materials.