Viscoelasticity of dense suspensions of thermosensitive microgel mixtures undergoing colloidal gelation†
Dense suspensions of temperature (T)-sensitive poly(N-isopropyl acrylamide) (N) and poly(N-isopropyl methacrylamide) (NM) microgel mixtures with different volume transition temperatures (TNc and TNMc, respectively; TNc < TNMc) 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 < TNc, 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 > TNc 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 (TB; TB > TNc) due to the competition between the repulsive interparticle interactions from the packing effect and electrostatic force, and the attractive interactions from the hydrophobicity. The TB in N/NM mixture suspensions shifts to a higher value with a decrease in N content in the mixtures (XN), accompanied by a discontinuous-like change at a specific value of XN (XN*). The TB at every value of XN 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 TB at XN* reflects the change in the network-like flocculation particles, from only attractive N microgels in the high XN regime, to the attractive N and NM microgel mixtures in the moderate XN 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.