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We investigated the complexation of thermoresponsive anionicpoly(N-isopropylacrylamide) (PNiPAM) microgels and cationic$\epsilon$-polylysine ($\epsilon$-PLL) chains. By combining electrophoresis,light scattering, transmission electron microscopy (TEM) and dielectricspectroscopy (DS) we studied the adsorption of $\epsilon$-PLL onto the microgelnetworks and its effect on the stability of the suspensions. We show that thevolume phase transition (VPT) of the microgels triggers a large polyionadsorption. Two interesting phenomena with unique features occur: atemperature-dependent microgel overcharging and a complex reentrantcondensation. The latter may occur at fixed polyion concentration, whentemperature is raised above the VPT of microgels, or by increasing the numberdensity of polycations at fixed temperature. TEM and DS measurementsunambiguously show that short PLL chains adsorb onto microgels and act aselectrostatic glue above the VPT. By performing thermal cycles, we further showthat polyion-induced clustering is a quasi-reversible process: within the timeof our experiments large clusters form above the VPT and partially re-dissolveas the mixtures are cooled down. Finally we give a proof that the observedphenomenology is purely electrostatic in nature: an increase of the ionicstrength gives rise to the polyion desorption from the microgel outer shell.

Overcharging and reentrant condensation of thermoresponsive ionic microgels