An NMR relaxation and spin diffusion study of cellulose structure during water adsorption

Abstract

The goal of this paper is a systematic investigation of changes in the supramolecular structure of cellulose during its water uptake. The main attention is concentrated on the analysis of the mechanism of dispersion of microfibrils by proton NMR relaxation techniques. Spin diffusion NMR experiments made it possible to estimate the linear dimensions of the surface thickness of cellulose crystallites and the average depth of micropores that are formed between elementary fibrils, as well as the character of the filling of micropores during adsorption. It has been shown that when the relative water content gradually increases to 7–8%, water molecules occupy the space between cellulose microfibrils, which is accompanied by an increase in the pore sizes and their specific surface area and a simultaneous decrease in the degree of crystallinity. Upon acquiring a free induction decay signal, a magic sandwich echo sequence was used, due to which the accuracy and information value of the results were considerably improved.