Interrelationship between solubility and nuclear spin–lattice relaxation time in cellulose derivatives with solid-state NMR

Cellulose is insoluble in water and most of the ionic liquids, but due to the substitution of hydroxyethyl, hydroxypropyl, and methoxy groups with the glucose residue, the cellulose derivatives Hydroxyethyl Cellulose (HEC) and Hydroxypropyl Methyl Cellulose (HPMC) become soluble in cold as well as hot water. The aim of the present investigations is to probe the origin of the enhanced solubility in cellulose derivatives HEC, and HPMC at atomic scale resolution by applying ¹³C cross-polarization magic angle spinning (CP-MAS) SSNMR experiment, ¹³C two-dimensional phase-adjusted spinning sideband (2DPASS) CP-MAS SSNMR experiment, and site specific ¹³C spin–lattice relaxation time measurements. The local-correlation time is also calculated for HEC and HPMC by using values of spin–lattice relaxation time and principal components of CSA parameters. The ¹³C spin–lattice relaxation time and ¹³C local correlation time are drastically decreased in HEC, and HPMC compared to cellulose, suggesting a notable increase in the motional dynamics of cellulose derivatives. Hence, the motional dynamics is enhanced in cellulose derivatives due to esterification, and is one of the reasons of enhanced solubility. The microscopic parameter, the nuclear spin–lattice relaxation rate of polysaccharide is interrelated with the solubility, a macroscopic property.