Solid-state 19F MAS and 1H→19F 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, Tc) on heating. However, all chain sequences including TrFE units undergo conformational exchange at around Tc. 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, T1ρF 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 T1ρF 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 T1F and T1ρF. The phase transition was observed between 85 and 77 °C on cooling, in which the characteristic signals of the paraelectric phase disappeared, the T1ρF values of all peaks quickly increased, and the broad crystalline signals abruptly appeared at 77 °C.