Synthesis and characterization of AB-crosslinked graft copolymers based on maleilated chitosan and N-isopropylacrylamide

AB-crosslinked graft copolymers based on a maleilated chitosan (MA-CS) and N-isopropylacrylamide (NIPAAm) were successfully synthesized through a two-step reaction. Chitosan was first modified with maleic anhydride to produce MA-CS which contained vinyl carboxylic acid groups. In the second step, NIPAAm monomer was graft-copolymerized onto MA-CS via free radical polymerization initiated by UV radiation. The chemical structures of the synthesized copolymers were confirmed by analyses of the Fourier transform infrared and nuclear magnetic resonance spectra. The grafting efficiency increased with the added amount of NIPAAm monomer and it reached a plateau value of ca. 55%. In the mean time, grafting ratio increased with the added amount of NIPAAm as well, and it had a value of 2.47 when NIPAAm monomer was added five times the amount of MA-CS. A two-stage thermal degradation behavior was observed for the copolymer samples, corresponding to the degradations of chitosan and grafted PNIPAAm chains, respectively. In addition, all copolymer samples exhibited the same glass transition temperature as pure PNIPAAm at 137 °C. All of the experimental results confirmed the AB-crosslinked structure in the formed copolymer of PNIPAAm and MA-CS. The influences of grafting ratio on the phase transition as well as swelling behavior of copolymer gels in various pH buffer solutions at different temperatures were studied.