Honeycomb-like porous chitosan films prepared via phase transition of poly(N-isopropylacrylamide) during water evaporation under ambient conditions.

Honeycomb-like porous chitosan (CS) films are attractive tools for developing functional materials for filters, catalyses, adsorbents, biomaterials, etc. A simple method for fabricating honeycomb-like porous CS films without special reagents, facilities, and techniques would make them accessible. Here we introduce an easily available method for fabricating honeycomb-like CS films without a strong acid/base, toxic reagents, or special facilities/techniques. An aqueous solution containing CS and poly(N-isopropylacrylamide) (PNIPAm) was allowed to stand at 25 °C to evaporate water. After 3 days, CS-PNIPAm composite films with homogenously phase-separated PNIPAm particles were obtained. The PNIPAm particles were removed by immersion in methanol, and the resulting films dried under reduced pressure to become honeycomb-like porous CS films. The pore size could be varied in the range of 0.5-3.0 µm by altering the CS concentration and the molecular weight of CS where the pore size was reduced under conditions with stronger interaction between CS molecules. We reveal that the key to success with this system is the decrease of lower critical solution temperature (LCST) of PNIPAm with water evaporation. In addition, we confirmed the removed PNIPAm was recyclable in this system. Furthermore, we found this method was also applicable to alginate. The proposed facile method for fabricating honeycomb-like porous polymeric films could provide various functional porous materials.

Facile preparation of surface N-halamine chitin nanofiber to endow antibacterial and antifungal activities.

N-halamine chitin nanofiber (NF) film was prepared by the reaction of chitin NF film with sodium hypochlorite solution to endow the film with antibacterial and antifungal activities. The amount of active chlorine content loaded on the chitin NF film depended on the sodium hypochlorite concentration and reaction time. FT-IR, UV-vis, XRD, and TG analyses showed that the N-H bond was substituted to the N-Cl bond and that the reaction took place at the chitin NF surface. After chlorination, the characteristic nanochitin morphology was maintained. Although the active chlorine content of the film gradually decreased by disassociation of the N-Cl bond, chlorine was rechargeable into chitin NF by another sodium hypochlorite solution treatment. The chlorinated chitin NF film showed strong efficacies against Gram-negative and -positive bacteria of Escherichia coli and Staphylococcus aureus, respectively. Moreover, the films showed 100% and 80% inhibition of spore germination when faced against Alternaria alternata and Penicillium digitatum fungi, respectively.

Thermoresponsive chitosan/N-isopropylacrylamide copolymer through atom transfer radical polymerization.

Regioseletive copolymerization of N-isopropylacrylamide (NIPAM) onto chitosan was achieved by atom transfer radical polymerization (ATRP) by using a regioselective 3,6-di-O-bromoisobutyryl-2-N-phthaloyl chitosan as a macroinitiator. The degree of polymerization (DP) of polyNIPAM onto the chitosan derivative changed by varying the ratio between NIPAM monomer, macroinitiator, ligand, and transition metal. ATRP successfully proceeded and a DP of polyNIPAM up to 110.5 was obtained. The thermal decomposition temperature of the 3,6-di-O-bromoisobutyryl-2-N-phthaloyl chitosan was significantly improved by increasing the DP of the NIPAM component. The polyNIPAM-g-chitosan derivative showed a thermoresponsive property. Although it formed a stable suspension in water at room temperature, it caused a hydrophilic-to-hydrophobic transition at around 32°C, resulting in precipitate formation.