Viscoelasticity of dense suspensions of thermosensitive microgel mixtures undergoing colloidal gelation

Abstract

Dense suspensions of temperature (T)-sensitive poly(N-isopropyl acrylamide) (N) and poly(N-isopropyl methacrylamide) (NM) microgel mixtures with different volume transition temperatures (T^N_c and T^NM_c, respectively; T^N_c < T^NM_c) exhibit a characteristic T-dependent viscoelasticity due to T-induced changes in the type of interparticle interaction as well as the volume fraction of each gel. In the range of T < T^N_c, 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 total volume fraction of the microgels (ϕ). At T > T^N_c 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 a minimum at a characteristic temperature (T_B; T_B > T^N_c) due to the competition between the repulsive interparticle interactions from the packing effect and electrostatic force, and the attractive interactions from the hydrophobicity. The 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 (X_N*). The T_B at every value of X_N agrees approximately with the temperature where the total volume fraction of the attractive hydrophobic microgels is 0.3 regardless of microgel type (N or NM). The discontinuous-like variation in T_B at X_N* reflects the change in the network-like flocculation particles, from only attractive N microgels in the high X_N regime, to the attractive N and NM microgel mixtures in the moderate X_N regime. The requirement of the repulsive electrostatic force with an appropriate strength for the stability of the network-like flocculation is also demonstrated using the PNIPAM-co-fumaric acid (NF) microgel suspensions at various pH.