Hydrothermal Gelation of Pure Cellulose Nanofiber Dispersions

The gelation of cellulose nanofibers (CNFs) through conventional cross-linking or reprecipitation requires the use of additives. Here, for the first time the gelation of pure CNF dispersions has been achieved solely by mechanical disintegration and a hydrothermal process without chemical modification. Different concentrations of cellulose powder were dispersed in water, following which these CNF dispersions were subjected to hydrothermal treatment at 160 °C for different lengths of time in a sealed reactor. Self-sustaining hydrogels with no discoloration were obtained. The chemical properties and crystal structures of the CNFs were essentially unchanged following hydrothermal treatment. Although the molecular mass of the cellulose was slightly reduced, the polymer density of cellulose at the same hydrodynamic radius was unchanged by hydrothermal treatment. Hence, chemically cross-linked structures did not form during the hydrothermal process. Instead, physical network structures developed within the CNF hydrogels, which increased the mechanical strength. Moreover, this network structure and therefore the strength of the hydrogel could be improved by increasing the mechanical disintegration time or the cellulose concentration of the CNF dispersion. To ensure sufficient hydrothermal gelation, it was important that the CNF dispersion had a storage modulus greater than 20 Pa.