Mechanical Properties and Preparing Processes of the TEMPO-Oxidized Cellulose Nanofibers Hydrogels

Aqueous TEMPO-oxidized cellulose nanofiber (TOCN) dispersions can be converted to hydrogels by adding an acid or polyvalent metal salt solution. Here we report the mechanical properties of the TOCN hydrogels in terms of gelling additives. The mechanical properties of the hydrogels were significantly affected by the added amount and chemical structure of the additives. The breaking strength of the hydrogels became higher with a decrease in pH value of the gels, suggesting that surface-carboxylated TOCNs were more strongly aggregated at pH <4. At the same pH 2.8, the gel strength became higher when the valence of added metal ions was higher; the hydrogels of physically aggregated TOCNs were further reinforced by cross-linking of TOCNs with polyvalent metal ions. Moreover, the composite hydrogels of TOCN and water-soluble carboxymethylcellulose (CMC) were prepared by adding aluminum acetate to aqueous TOCN/CMC mixtures. The strength of the composite hydrogels had unique properties, different from the conventional composite theory. These results suggest that in the TOCN/CMC composite hydrogels, specific interactions or entangled structures are present between TOCNs and CMC molecules.