NaClO-oxidized cellulose nanofiber/chitosan composite films with improved water resistance and high mechanical...

Cellulose nanofibers (CNFs) are increasingly recognized as sustainable alternatives to petroleum-derived plastics. However, their intrinsic hydrophilicity results in poor water resistance, which severely restricts applications in humid or aqueous environments. Therefore, improving water durability while maintaining mechanical performance remains a key challenge. In this study, we fabricated water-resistant composite films by combining oxidized CNFs (OCNFs), obtained via a simple sodium hypochlorite (NaClO) oxidation introducing carboxyl groups, with chitosan (CS). The OCNF/CS films exhibited markedly enhanced tensile strength (∼50 MPa vs. ≤20 MPa for pristine OCNF) and retained their integrity in water for 24 h, with a swelling ratio of ∼200% compared to ≥900% for OCNF alone. These improvements are attributed to strong ionic interactions between anionic carboxylate groups of OCNFs and cationic ammonium groups of CS, as well as hydrogen bonding within the polymer matrix. Furthermore, post-treatment with sodium hydroxide reinforced the network structure, raising tensile stress to ∼80 MPa. The incorporation of glycerol as a plasticizer significantly improved wet-mechanical strength, with toughness increasing nearly threefold, as a result of suppressed hydration of polysaccharides. This fabrication method, with modifications, provided the ability to produce flexible, large-scale films. Overall, this work demonstrates that exploiting ionic interactions, hydrogen bonding, and simple post-treatments yields fully bio-based composite films with outstanding mechanical robustness, large scalability, and water resistance. These findings highlight the strong potential of OCNF/CS films as eco-friendly packaging materials for moisture-sensitive applications.