Rheological Properties of Dense Suspensions of Thermo-Responsive Microgel Mixtures Undergoing Colloidal Gelation

Abstract

Dense suspensions of thermo-responsive poly(N-isopropyl acrylamide) (N) microgels exhibit a characteristic temperature (T)-dependent viscoelasticity at characteristic temperature (T c) due to T-induced changes in the type of interparticle interaction as well as the volume fraction of microgels[1]. 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 entirely to the packing effect, i.e., a reduction in the volume fraction of the microgels (φ). At T > T c N where the attractive interparticle interactions between dehydrated and hydrophobic microgels emerge, the suspensions show solid-like elastic properties due to the network-like flocculation of the shrunken 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; T B > T c N) due to the competition between the repulsive interparticle interactions from the packing effect and electrostatic force, and the attractive interactions from the hydrophobicity. The present study focuses on the mixtures of thermo-responsive N and poly(N-isopropyl methacrylamide) (NM) microgels with different volume transition temperatures (T c N = 31oC and T c NM = 39 oC, respectively)[2]. The temperature T B in N/NM mixture suspensions shifts to a higher value with a decrease in N content in the mixtures (X N), accompanied by a discontinuous-like change at a specific value of X N. We analyze the T-dependent viscoelasticity of the mixture suspensions with various X N on the basis of the volume fractions of the total microgels (φ m) as well as the hydrophobic microgels (Φ m a). This analysis indicates that (1) the crossover of the φ m dependence of G occurs at a value of φ m at T c N and/or T c NM, and that (2) the values of Φ m a at T B are about 0.3 independently of X N. We also demonstrate the significant impacts of electrostatic repulsive interaction on the colloidal gelation using the NIPAm-co-fumaric acid microgels. At pH 11, the attractive interparticle force through hydrophobic interactions is fully suppressed due to the highly charges surface of the microgels, with the result that neither colloidal gelation nor the crossover of the φ m-dependence of G at T c NF occur. This result indicates that the repulsive electrostatic force with an appropriate strength requires for the stability of the network-like flocculation. [1] S. Minami, T. Watanabe, D. Suzuki, K. Urayama, Polymer Journal, 48, 1079-1086, 2016 [2] S. Minami, T. Watanabe, D. Suzuki, K. Urayama, Soft Matter, 14, 1596-1607, 2018 Figure 1