Robust Self-Standing Chitin Nanofiber/Nanowhisker Hydrogels with Designed Surface Charges and Ultralow Mass Content via Gas Phase Coagulation

Partially deacetylated alpha-chitin nanofibers/nanowhiskers mixtures (DEChNs) and. TEMPO-oxidized alpha-chitin nanowhiskers (TOChNs) that had positive and negative charges, respectively, were transformed into hydrogels with mass concentrations of 0.2, 0.4, 0.6, 0.8, and 1.0% under ammonium hydroxide or hydrochloric acid "gas phase coagulation". To the best of our knowledge, 0.2% is the lowest mass content reported for the successful preparation of physical self-standing hydrogels based on chitin nanofibers/nanowhiskers. The even and uniform coagulation under "gas phase" is one of the key aspects of preparing hydrogels with quite low mass content. The storage modulus achieved the highest value of 8.35 and 3.73 KPa for DEChN and TOChN hydrogels, respectively, at the mass concentration of 1.0%, and these are known to be the highest values reported in the literature for hydrogels at the same mass concentration of chitin nanofibers/nanowhiskers. The equilibrium swelling ratio (ESR) of both DEChN and TOChN hydrogels decreased with increasing mass content at neutral pH. As the pH increased from 2 to 10, the swelling degree of DEChN hydrogels decreased from 268 to 130, whereas the swelling degree of TOChN hydrogels increased from 128 to 242. Additionally, due to the electrostatic attraction between the hydrogels and dyes, DEChN hydrogels had significant adsorption of Reactive Blue 19, while TOChN hydrogels had effective adsorption of Basic Green 4. The different pH-dependent swelling behavior and adsorption affinity of the DEChN and TOChN hydrogels were related to their designed opposite surface charges corresponding to the surface amino groups on the DEChNs and carboxyl groups on the TOChNs.