Synthesis and In Situ Gelation Behavior of Thermoresponsive Poly(N-isopropylacrylamide)/Chitosan Microgels

Thermoresponsive microgels composed of poly(N-isopropylacrylamide) (PNIPAM) and chitosan (CS) were synthesized by soap-free emulsion polymerization of N-isopropylacrylamide monomer in the presence of CS, using an anionic radical initiator and a chemical crosslinker. The resulting PNIPAM/CS microgels contracted in an aqueous phase at elevated temperature, exhibiting a negatively thermoresponsive nature with a volume phase transition temperature (VPTT) of about 34.6 °C. The microgel dispersions with a relatively high polymer content gelated when being heated across their VPTT due to their attractive hydrophobic interactions. We systematically investigated this thermo-induced in situ gelation behavior by small-deformation oscillatory rheological measurements within the linear viscoelastic region. The dynamic temperature ramp sweeps revealed that the gelation temperature was greatly influenced by the polymer content of the microgel dispersion, decreasing with increasing polymer content. The isothermal time sweeps showed that the gelation occurred more quickly at either higher polymer content or higher isothermal heating temperature. The relaxation dynamics for the as-formed gel were also studied by isothermal frequency sweeps. It was found that the gels exhibited a frequency-independent storage modulus in the lower frequency range, and the greater the polymer content of the microgel dispersions, the more elastic the gels that formed.