Load-Bearing Nanostructures in Composites of Chitosan with Anionic Surfactants: Implications for Programmable Mechanomaterials

Polysaccharides like chitosan (CHT) can sustainably replace synthetic polymers in many material applications, but their native mechanical properties are often subpar. Addition of ionic surfactants like the anionic sodium dodecylsulfate (SDS) can bring about dramatic mechanical enhancements in polysaccharide materials, including those of CHT. At basic pH, CHT is neutral and forms elastic hydrogels, but the cationic nature of CHT at acidic pH enables ionic cross-linking with SDS, leading to viscoelastic hydrogels with superior strength. Thus, SDS:CHT has emerged as a promising platform for spatial and dynamic programming of hydrogels with unique responses to mechanical loads, but the nanoscale origins of their load-bearing mechanisms remain elusive. To address this gap, CHT hydrogel networks were self-assembled at varying pH values and SDS concentrations and mechanically tested using a multiscale modeling pipeline. In addition to yielding self-assemblies with mechanical properties consistent with experimental reports, our methods revealed distinct pH- and SDS-dependent load-bearing mechanisms. We found that basic CHT networks underwent load-dependent crystallization, similar to stretched rubber, while SDS micelles shouldered the load response in acidic SDS:CHT networks by merging into larger micelles. These findings may enable the adaptation of these programming mechanisms for other polysaccharide–surfactant combinations, lead to the improvement of mechanical robustness of existing SDS:CHT applications, and inspire the development of new applications.