Grafting poly(amino acid)s onto cellulose nanofibers for developing biodegradable superabsorbent materials

Superabsorbent materials, mostly based on polyacrylate and its derivatives, are designed to absorb large amounts of water or other liquids, with absorption capacities of up to 100–1000 times their dry weight. Cellulose, the most abundant polysaccharide in nature, is an appealing candidate for replacing conventional non-degradable acrylate superabsorbents because it exhibits high water affinity while remaining water-insoluble. Although the absorption behavior of cellulose can be enhanced by surface modification strategies, conventional cationic and anionic modification strategies often result in relatively low surface charge density. We therefore propose a strategy to enhance the water absorption performance of cellulose nanofibers (CNFs) by grafting polyelectrolytes with a higher density of polar functional groups, such as carboxyl and amine groups. To maintain biodegradability, we grafted two different poly(amino acid)s, ε-poly-L-lysine and γ-polyglutamic acid, onto TEMPO-oxidized CNF surfaces via EDC/NHS- or DMTMM-mediated COOH/NH₂ coupling reactions. The final material properties will be tuned by applying different molecular weights and concentrations of poly(amino acid)s. This strategy may enable the development of eco-friendly absorbent materials for applications such as water-absorbing packaging, soil and agricultural amendments, and hygiene products.