Effect of cross-link density on re-entrant melting of microgel colloids

We investigated concentrated suspensions of newly synthesized highly cross-linked (1:10) polystyrene (PS) microgel particles dispersed in a good, iso-refractive organic solvent that allowed for a detailed study using light scattering techniques. Inducing depletion attraction by addition of non-adsorbing (PS) polymer, we analyzed the re-entrant melting behavior of the particle system. We compared our results with that of previous studies on polystyrene microgel particles with a lower cross-linking ratio (1:50) and on PMMA suspensions. In contrast to the (softer) 1:50 microgels where an unexpectedly huge re-entry effect was found with fluid samples extending up to a volume fraction phi approximate to 0.69, the re-entry region here extends only up to phi approximate to 0.61 and the attractive glass transition line is found at significantly lower polymer concentrations. Instead we find a closer similarity of the re-entry behavior with that observed for hard sphere-like PMMA particles. Whereas the highest volume fractions up to which fluid states could be observed are comparable, the polymer concentration needed to induce re-entrant melting and the formation of attractive glass is shifted to higher polymer concentrations. While the latter effect may be attributed to the non-ideal behavior of the free polymer, the extension of the fluid region with respect to colloid volume fraction appears to depend sensitively on system details like particle softness or particle size distribution. (C) 2011 Elsevier B.V. All rights reserved.