Solid-state F-19 MAS and H-1 -> F-19 CP/MAS NMR study of the phase transition behavior of vinylidene fluoride-trifluoroethylene copolymers: 1. Uniaxially drawn films of VDF 75% copolymer

摘要

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 F-19 MAS and H-1 --> F-19 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 43-92degreesC, whereas no change was found for the CHF signal at the head-to-tail linkage of VDF-TrFE up to 92degreesC. Hence, VT F-19 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 F-19 spectral shapes, in which the broad signals of the ferroelectric immobile phase disappeared between 115 and 119degreesC. In addition, T-1rho(F) for all peaks decreased to a unique value (ca. 20 ms) at 119degreesC, indicating that uniform molecular motion accompanied by a full chain rotation occurred at the temperature. The significantly longer T-1rho(F) for all peaks (ca. 20 ms) in the paraelectric phase (119degreesC) 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 85degreesC, 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 T-1(F) and T-1rho(F). The phase transition was observed between 85 and 77degreesC on cooling, in which the characteristic signals of the paraelectric phase disappeared, the T-1rho(F) values of all peaks quickly increased, and the broad crystalline signals abruptly appeared at 77degreesC. (C) 2004 Published by Elsevier Ltd. [References: 31]