JUNCTION POINT MOTION FROM P-31 NMR LINE-SHAPES AND RELAXATION-TIMES IN POLY(PROPYLENE GLYCOL) URETHANE NETWORKS

31P NMR line shapes and spin-lattice relaxation times in the lab (T1p) and rotating (T1pp) frames have been measured over a wide range of temperatures for a series of network polymers with molecular weight between cross-links (Mc) ranging from 400 to 3000. The networks are formed from α,ω-dihydroxypoly(propylene glycol) and tris(4-isocyanatophenyl)thiophosphate. The line shapes are interpreted in terms of a diffusional motion model; the fit is reasonably good for short correlation times. The discrepancy between calculated and observed minimum relaxation times indicates the inadequacy of an isotropic motional model especially for lower Mc samples. Some computation of factors to correct for the nonisotropic motional case are included. Comparison with previous13C relaxation data shows that the31P motional frequency lags the segmental motion by a factor of 3 and 5 for the Mc 1000 and Mc 3000 cases, respectively. Correlation times obtained from the line-shape fitting and from the relaxation times do not agree; the former is about 3 times longer. This discrepancy may be attributed to the inadequacy of the relaxation model of isotropic averaging of the chemical shift anisotropy or to the over-simple motional model of the line-shape fitting, or both. © 1990, American Chemical Society. All rights reserved.