Effects of cations on the sorting of oppositely charged microgels

Two kinds of thermally sensitive oppositely charged polymeric microgels, poly(N-isopropylacrylamide-co-sodium acrylate) (NIPAM-co-SA) and poly(N-isopropylacrylamide-co-vinylbenzyl trimethylammonium chloride) (NIPAM-co-VT), have been prepared by dispersion polymerization. Their temperature-induced aggregation was studied by use of laser light scattering. In the absence of a salt, NIPAM-co-SA and NIPAM-co-VT microgels in the mixture do not aggregate but shrink upon the heating. The addition of monovalent cations does not affect the aggregation. In the presence of divalent metal ions, the heating leads to the association of NIPAM-co-SA microgels via the complexation of metal ions and carboxyl groups on the microgel surface when the temperature approaches their lower critical solution temperature (LCST approximately 32 degrees C). Further heating over the LCST results in the adsorption of NIPAM-co-VT microgels on the NIPAM-co-SA aggregate via the electrostatic attraction so that a core-shell structure with a NIPAM-co-SA core and a NIPAM-co-VT shell forms, resembling the biological cell-sorting.