In situ gelation behavior of thermoresponsive poly(N-vinylpyrrolidone)/poly(N-isopropylacrylamide) microgels synthesized by soap-free emulsion polymerization

Poly(N-vinylpyrrolidone)/poly(N-isopropylacrylamide) (PVP/PNIPAM) microgels were successfully synthesized by soap-free emulsion polymerization of N-isopropylacrylamide monomer in the presence of PVP, using an anionic radical initiator and a chemical crosslinker. The dried microgels exhibited nonspherical morphology and submicron size with good monodispersity. In aqueous phase, the microgels shrank when elevating temperature, displaying a negatively thermoresponsive nature with volume phase transition temperature (VPTT) of ca. 33.1 degrees C. The microgel dispersions remained colloidally stable even being heated far above their VPTT but gelled with the addition of electrolyte. Such thermo-induced in situ gelation behavior was studied in detail by small-deformation oscillatory rheological measurements within the linear viscoelastic region. The dynamic temperature ramp sweeps showed that the gelation temperature decreased independently as the polymer content or electrolyte concentration was increased. The isothermal time sweeps were performed to study the gelation kinetics. It was found that the microgel dispersions formed gel more quickly at larger polymer content, larger electrolyte concentration and higher temperature.