Conformation analysis and molecular mobility of ethylene and tetrafluoroethylene copolymer using solid-state 19F MAS and 1H → 19F CP/MAS NMR spectroscopy

The changes in the conformation and molecular mobility accompanied by a phase transition in the crystalline domain were analyzed for ethylene (E) and tetrafluoroethylene (TFE) copolymer, ETFE, using variable-temperature (VT) solid-state F-19 magic angle spinning (MAS) and H-1 --> F-19 cross-polarization (CP)/MAS NMR spectroscopy. The shifts of the signals for fluorines in TFE units to higher frequency and the continuing decrease and increase in the TIFF values suggest that conformational exchange motions exist in the crystalline domain between 42 and 145 C. Quantum chemical calculations of magnetic shielding constants showed that the high-frequency shift of TFE units should be induced by trans to gauche conformational changes at the CH2-CF2 linkage in the E-TFE unit. Although the F-19 signals of the crystalline domain are substantially overlapped with those of the amorphous domain at ambient probe temperature (68degreesC), they were successfully distinguished by using the dipolar filter and spin-lock pulse sequences at 145 C. The dipolar coupling constants for the crystalline domain, which can be estimated by fitting the dipolar oscillation behaviors in the H-1 --> F-19 CP curve, showed a significant decrease with increasing temperature from 42 to 145degreesC. This is due to the averaging of H-1-F-19 dipolar interactions originating from the molecular motion in the crystalline domain. The increase in molecular mobility in the crystalline domain was clearly shown by VT T-1rho(F) and H-1 --> F-19 CP measurements in the phase transition temperature range. Copyright (C) 2004 John Wiley Sons, Ltd.