Characterization of cellulose ethers by 13C NMR, 1. Studies of high molecular weight polymers in solution and in the solid state

Carbon‐13 NMR was used to investigate the structure of a cellulose derivative, ethyl‐O‐(hydroxyethyl)cellulose (EHEC). Analysis of high molecular weight samples in solution showed that the line widths of the main chain carbon resonances are very sensitive to the concentration of the solution. Above a certain concentration, excessive broadening occurs due to gel formation, which makes a quantitative evaluation of the degree of substitution difficult. If the concentration is decreased below the critical value for gel formation, the use of NMR parameters suitable for quantitative measurements (no NOE and long pulse intervals) leads to excessively long accumulation times. Spectra of solid samples obtained by cross‐polarization magic angle spinning (CP‐MAS) NMR exhibit better defined resonances of the main‐chain carbons than do the spectra of solutions with concentrations above the critical value for gel formation. Variable contact time experiments with a specific sample indicated that a suitable contact time should be 400 μs. The quantitative evaluation of the total degree of substitution by CP‐MAS NMR yielded values in agreement with those obtained by GC‐analysis.