Double Cross-Linked Chitosan Composite Films Developed with Oxidized Tannic Acid and Ferric Ions Exhibit High Strength and Excellent Water Resistance.

There is tremendous scientific interest in developing biodegradable films through facile and versatile strategies. Although extensive studies on the preparation of chitosan films have been conducted, the reported results commonly present low mechanical strength and weak water resistance. In the present study, high strength and significantly water resistance single-cross-linked chitosan-oxidized tannic acid (SC-CS/OTA) composite films and double cross-linked chitosan/oxidized tannic acid/Fe (DC-CS/OTA/Fe) composite films were created through a Schiff base reaction and metal coordination. As a result, the optimal tensile strength of SC-CS/OTA composite films and DC-CS/OTA/Fe composite films was 35.92 and 209 MPa, respectively. Notably, when compared with other chitosan-based films, the tensile strength of DC-CS/OTA/Fe composite films was approximately three times stronger. Moreover, the water vapor permeability (WVP) values of the films with Fe(0.66 ± 0.03 × 10 g/m·h·Pa) was lower than that of films without Fe (1.33 ± 0.01 × 10 g/m·h·Pa). More importantly, WVP values of the DC-CS/OTA/Fe composite films were 3-4 orders of magnitude lower than those of chitosan films previously reported. The SC-CS/OTA composite films (96.69%) and DC-CS/OTA/Fe composite films (99.06%) also presented high DPPH radical scavenging activity. Furthermore, SC-CS/OTA and DC-CS/OTA/Fe hydrogels were also prepared. This work can be widely applied in the food, biomedical science, and wastewater treatment fields.