Rheology of Dense Suspensions of Thermoresponsive Microgel Mixtures Undergoing Colloidal Gelation

Dense suspensions of thermoresponsive poly(N-isopropylacrylamide) (N) and poly(N-isopropylmethacrylamide) (NM) microgels with different transition temperature (T_c^N < T_c^NM) exhibit a characteristic temperature (T)-dependent viscoelasticity due to T-induced changes in the type of interparticle interaction as well as the volume fraction of microgels. In the range of T < T_c^N, where the swollen microgels with repulsive interparticle interactions are densely packed, the equilibrium modulus (G) decreases upon heating due to a reduction in the volume fraction of the microgels (φ). At T > T_c^N where the attractive interparticle interactions between hydrophobic microgels emerge, the suspensions show solid-like properties due to the network-like flocculation of microgels (colloidal gelation), even when φ becomes considerably lower than the threshold for randomly close packing. The T-dependence of G shows minimum at a characteristic temperature (T_B), and T_B shifts to a higher value with a decrease in N content in the mixtures. We discuss the contributions of packing degree effect and interparticle interaction effect to viscoelasticity. [1]
[1] S. Minami, T. Watanabe, D. Suzuki, K. Urayama, Soft Matter, 14, 1596-1607, 2018