Spherical microgel colloids -: Hard spheres from soft matter

While gels are usually considered to be soft materials, we demonstrate that it is possible to model hard sphere behaviour when the gel structure is confined to spherical objects of sizes in the colloidal range. We have measured the static structure factor of microgel spheres dispersed in good solvents, differing in size, crosslink density and swelling behaviour, by light scattering and small angle neutron scattering. Comparing with theoretical calculations for polydisperse hard spheres we show how the interactions in highly concentrated dispersions of spherical microgel colloids are determined by an interplay of the relative length of dangling polymer ends at the surface (determined by the average crosslink density and the particle size) and the swelling ratio (determined by the average crosslink density and the solvent quality). In addition the swelling ratio controls the volume fraction range in which colloidal behaviour is not superposed by the gel character. The latter leads to a loss of structural order due to the deformability of the microgel spheres, an effect that occurs at increasingly lower volume fractions if the swelling ratio increases. To achieve hard sphere behaviour up to very high volume fractions, thus, requires high crosslink densities and small swelling ratios.