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We explore the impact of three water-soluble polyelectrolytes (PEs) on theflow of concentrated suspensions of poly(N-isopropylacrylamide) (PNIPAm)microgels with thermoresponsive anionic charge density. By progressively addingthe PEs to a jammed suspension of swollen microgels, we show that the rheologyof the mixtures is remarkably influenced by the sign of the PE charge, PEconcentration and hydrophobicity only when the temperature is raised above themicrogel volume phase transition temperature $T_c$, namely when microgelscollapse, they are partially hydrophobic and form a volume-spanning colloidalgel. We find that the original gel is strengthened close to the isoelectricpoint, attained when microgels are mixed with cationic PEs, while PEhydrophobicity rules the gel strengthening at very high PE concentrations.Surprisingly, we find that polyelectrolyte adsorption or partial embedding ofPE chains inside the microgel periphery occurs also when anionic polymers ofpolystyrene sulfonate with high degree of sulfonation are added. This givesrise to colloidal stabilization and to the melting of the original gel networkabove $T_c$. Contrastingly, the presence of polyelectrolytes in suspensions ofswollen, jammed microgels results in a weak softening of the original repulsiveglass, even when an apparent isoelectric condition is met. Our study putsforward the crucial role of electrostatics in thermosensitive microgels,unveiling an exciting new way to tailor the flow of these soft colloids andhighlighting a largely unexplored path to engineer soft colloidal mixtures.

Impact of polyelectrolyte adsorption on the rheology of concentrated Poly(N-Isopropylacrylamide) microgel suspensions