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 co nfined to spherical objects of sizes in the colloidal range. We have measur ed the static structure factor of microgel spheres dispersed in good solven ts, differing in size, crosslink density and swelling behaviour, by light s cattering and small angle neutron scattering. Comparing with theoretical ca lculations for polydisperse hard spheres we show how the interactions in hi ghly concentrated dispersions of spherical microgel colloids are determined by an interplay of the relative length of dangling polymer ends at the sur face (determined by the average crosslink density and the particle size) an d the swelling ratio (determined by the average crosslink density and the s olvent quality). In addition the swelling ratio controls the volume fractio n 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 o f 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.