Line broadening in the carbon-13 NMR spectra of cross-linked polymers

The exp 13 C NMR spectra of cross-linked poly(sodium acrylate) in a swollen gel have been examined. As has been reported elsewhere, the line widths of the resonances increase with the degree of cross-linking. The origin of the broadening has been examined in some detail. Spectra have been obtained at six different field strengths, and a number of relaxation and "hole burning" experiments have been performed. All of the results are consistent with the proposition that the increased width is due to unresolved chemical shift structure arising from the presence of the cross-links rather than from shorter relaxation times associated with decreased polymer mobility. Graphs, spectra. 13 ref.--AA