Preparation of tough hydrogels based on β-chitin nanofibers via NaOH treatment

Cellulose and chitin exist in nature as highly crystalline nanofibers. Previously, we reported preparing unique hydrogels from cellulose nanofibers by a simple NaOH treatment without use of any specific solvents or cross-linking agents. In the present study, a similar gel preparation was applied to beta-chitin nanofibers extracted from purified squid pen powder. The crystal structure of chitin nanofibers was transformed from beta-chitin to alpha-chitin by NaOH(aq) treatment above 30 wt%. The crystal conversion involving the interdigitation among adjacent nanofibers caused the formation of stable hydrogels with a alpha-chitin nanofiber network. The use of ethanol voided the dissolution during neutralization and enabled preparation of a higher crystalline hydrogel with high mechanical strength. It achieved a Young's modulus of 16.6 MPa, a tensile strength of 7 MPa and a strain at break of 52.2 %, on average. Finally, we note that the shrinkage of the cellulose I and beta-chitin nanofibers in aqueous NaOH solutions was caused by the release of tensile residual stress due to the intracrystalline swelling in NaOH solutions.