Rheological study on the thermoinduced gelation behavior of poly(N-isopropylacrylamide-co-acrylic acid) microgel suspensions

Submicron-sized thermoresponsive poly(N-isopropylacrylamide-co-acrylic acid) microgels were synthesized by soap-free emulsion polymerization. The physical state of the microgel suspensions, in a wide range of polymer concentrations (1.1-7.1 wt %), transformed from fluid to gel when the temperature was elevated across their volume phase transition temperature at pH=3.0. Such thermoinduced gelation behavior was studied in detail by small-deformation oscillatory rheological measurements within the linear viscoelastic region. It was found that the gelation temperature was strongly affected by the polymer concentration, decreasing as the polymer concentration increased. The gelation kinetics showed that the suspension gelled more quickly at either larger polymer concentration or higher isothermal heating temperature. In an isothermal frequency sweep for the as-formed gels, both storage and loss moduli, G and G, exhibited a power-law behavior, that is, G() approximate to (0.2-0.4) and G() approximate to (0.46-0.50) within the frequency range where G dominates G. In addition, the elasticity of the gels, which results from the attractive particle bonds, increased markedly with increasing polymer concentration. (c) 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45259.