Gelation Kinetics of Thermosensitive PNIPAM Microgel Dispersions

In the absence of added salt, thermosensitive poly(N-isopropylacrylamide) (PNIPAM) microgel dispersions are colloidally stable even at temperature well above their volume phase transition temperature (VPTT). However at high ionic strength they gel upon heating. The kinetics of the heat-induced gelation is investigated at various polymer concentrations, temperatures, and NaCl concentrations using small-deformation oscillatory rheological measurements. In situ gelation is achieved by isothermal heating. Gelation times are determined as the point where the storage and loss moduli, G' and G '', crossover. The effects of two dominating factors, polymer concentration, and temperature, on gelation time are investigated. At constant NaCl concentration, the gelation time decreases with increasing polymer concentration. The critical gelation concentration is determined to be ca 0.4 wt.-%. At constant polymer and NaCl concentration, the gelation time decreases with increasing temperature. At each NaCl concentration, the critical gelation temperature is slightly higher than the corresponding VPTT. The gelation kinetics of PNIPAM microgel dispersion can be well-described by the Tobitani-Ross-Murphy model.