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

Abstract

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(VDF 75/TrFE 25), were analyzed using variable temperature (VT) solid-state 19F MAS and 1H→ 19F CP/MAS NMR spectroscopy. The CF 2 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 1 ρ 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 1 ρ 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 CF 2 signals in VDF sequences accompanied by slight decreases in T 1 F and T 1 ρ 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 1 ρ F values of all peaks quickly increased, and the broad crystalline signals abruptly appeared at 77 °C.