Preparation, physicochemical and stability studies of chitosan-PNIPAM based responsive microgels under various pH and temperature conditions

This study describes the synthesis, characterization and detailed physicochemical investigation on the stimuli-responsive behaviour of chitosan-poly (N-isopropylacrylamide) [chitosan-PNIPAM] copolymer microgels. Six different compositions of dual-responsive chitosan-PNIPAM based microgels were synthesized by soapless-emulsion free radical copolymerization method in an aqueous medium at 70 degrees C. The chemical composition, swelling/de-swelling, volume phase transitions, electrical properties, colloidal stability, and physicochemical behaviour under various pH/temperature conditions were investigated using Fourier transform infrared spectroscopy (FTIR), dynamic light scattering (DLS), zeta-potential, and rheology. Various functional groups present in the copolymer microgels were confirmed by FTIR analysis. The hydrodynamic diameter (D-h), volume phase transition temperature (VPTT), and swelling/de-swelling behaviour of microgels, under different pH and temperature conditions, in aqueous media were investigated using DLS measurements. Zeta potential measurements were used to evaluate the stability, electrical and electro-kinetic properties of the gels at various pH/temperatures. Likewise, the influence of temperature and chemical composition of microgels on the particle behaviour was investigated through rheological studies. A transition from sol-to-gel state at temperature beyond 50 degrees C was also noticed in all hydrogel samples. On the basis of results obtained, we observed that variables such as NIPAM/chitosan ratio, amount of cross-linker, and temperature/pH not only affect the above mentioned properties but also affect the dual-sensitivity and stability of the microgels. Most of the chitosan-PNIPAM particles remained soluble and maintained good stability without significant sedimentation in water through a wide range of pH (pH approximate to 2-8) for about 3 months at room temperature.