Cellulose organic solutions: A nuclear magnetic resonance investigation

Abstract

AbstractFour solvents of cellulose have been studied by using 13C‐NMR spectroscopy. All these solvents, N‐methyl morpholine‐N‐oxide, methylamine, hydrazine, and paraformaldehyde (PF), contained dimethyl sulfoxide (DMSO) as a cosolvent. Oligomers of cellulose of DP = 10 soluble in hot DMSO have been used as model compounds. 13C chemical shifts and line shapes show that three of the mentioned solvents are “true solvents” of cellulose. On the other hand, dissolution of cellulose in DMSO‐PF system occurs by the formation of a statistical derivative of cellulose. Enriched 13C bacterial cellulose on C‐1 and C‐6 positions have been used to identify the 13C positions mainly in DMSO‐N‐methyl morpholine‐N‐oxide system. This solvent has been found to be degradative for the macromolecule when the solution is kept at 100°C over a long period. Viscosity measurements show a reduction of the molecular weight in these conditions. Polarimetry indicates that no glucose is present in solution and hence there is a statistical break of the chain. Enriched cellulose solution in DMSO–N‐methyl morpholine‐N‐oxide has been also used for relaxation time (T1) determination both of the solvent and of the enriched carbons of the polymer. Nuclear Overhauser enhancement (NOE) was found to be 1.8 for C‐1 and 2.1 for C‐6 showing that relaxation phenomenon is not purely dipolar. T1 values of 97 and 65 msec are found for C‐1 and C‐6 of cellulose, in good agreement with the values known for polysaccharides. Determination of T1 for the different carbon atoms of the solvent DMSO‐N‐methyl morpholine‐N‐oxide with and without cellulose shows a large reduction of T1 for N‐methyl morpholine‐N‐oxide molecule. This denotes a slower molecular motion of this molecule and a preferential interaction with the cellulose macromolecule.