Solid-state F MAS and H→F CP/MAS NMR study of the phase transition behavior of vinylidene fluoride-trifluoroethylene copolymers: 1. Uniaxially drawn films of VDF 75% copolymer

The changes in the phase structures and molecular mobility caused by the ferroelectric-paraelectric phase transition of vinylidene fluoride (VDF) and trifluoroethylene (TrFE) copolymer, P(VDF₇₅/TrFE₂₅), were analyzed using variable temperature (VT) solid-state ¹⁹F MAS and ¹H→¹⁹F CP/MAS NMR spectroscopy. The CF₂ signal of the VDF chain sequence and the CHF signal at the head-to-head linkage of VDF-TrFE sequence showed higher frequency shift in the temperature range 43-92 °C, whereas no change was found for the CHF signal at the head-to-tail linkage of VDF-TrFE up to 92 °C. Hence, VT ¹⁹F MAS spectra revealed that the VDF-TrFE head-to-tail sequence is the most stable part in polymer chains against trans-gauche conformational exchange motions below the phase transition temperature (Curie temperature, T_c) on heating. However, all chain sequences including TrFE units undergo conformational exchange at around T_c. The phase transition behavior is clearly recognized in the ¹⁹F spectral shapes, in which the broad signals of the ferroelectric immobile phase disappeared between 115 and 119 °C. In addition, T_1ρ^F for all peaks decreased to a unique value (ca. 20 ms) at 119 °C, indicating that uniform molecular motion accompanied by a full chain rotation occurred at the temperature. The significantly longer T_1ρ^F for all peaks (ca. 20 ms) in the paraelectric phase (119 °C) than that in the amorphous domain (<4 ms) at ambient temperature supports the conclusion that there is restricted rotational motion of polymer chains around the chain axis in the paraelectric phase. On cooling from 119 to 85 °C, a gradual decrease in gauche conformers in the paraelectric phase was confirmed by the low-frequency displacement of CF₂ signals in VDF sequences accompanied by slight decreases in T₁^F and T_1ρ^F. The phase transition was observed between 85 and 77 °C on cooling, in which the characteristic signals of the paraelectric phase disappeared, the T_1ρ^F values of all peaks quickly increased, and the broad crystalline signals abruptly appeared at 77 °C.