Heteroaggregation studies of mixed cationic co-polymer/anionic homopolymer microgel dispersions

Two colloidal microgels-one a cationic co-polymer microgel (poly(N-isopropylacrylamide)-co-4-vinylpyridine), synthesised using 25% (w/w) of 4-vinylpyridine, and the other an anionic poly(N-isopropylacrylamide) homopolymer microgel-were prepared by surfactant-free emulsion polymerisation methods. Interactions between the two microgels have been examined as a function of their relative concentrations at 20 and at 45 °C, pH and electrolyte concentration. Turbidity and transmission electron microscopy measurements show the dispersibility of the mixed microgel system to be dependent upon both environmental temperature and relative concentration. Although mixed microgels aggregate at 20 °C in deionised water, the presence of excess cationic microgel confers stability. Where stable dispersions were observed at temperatures above the volume phase transition temperature (VPTT) of both microgels, the behaviour was interpreted in the context of charge reversal and particle adsorption mechanisms. Microgel mixtures that maintained dispersibility above the LCST were found to do so via the formation of hetero-macroparticles displaying a net overall charge reversal. Heteroaggregation is shown to be influenced by the relative concentration of microgel particles, pH and electrolyte concentration at both 20 and 45 °C.