Viscoelasticity of dense suspensions of thermosensitive microgel mixtures undergoing colloidal gelation.
basic_science · Level V
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- Also identified by DOI 10.1039/c7sm02411h.
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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 and T, respectively; T < T) 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, 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 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<sub>B</sub>; T<sub>B</sub> > T) 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<sub>B</sub> in N/NM mixture suspensions shifts to a higher value with a decrease in N content in the mixtures (X<sub>N</sub>), accompanied by a discontinuous-like change at a specific value of X<sub>N</sub> (X<sub>N</sub>*). The T<sub>B</sub> at every value of X<sub>N</sub> 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<sub>B</sub> at X<sub>N</sub>* reflects the change in the network-like flocculation particles, from only attractive N microgels in the high X<sub>N</sub> regime, to the attractive N and NM microgel mixtures in the moderate X<sub>N</sub> 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.