AN EXPERIMENTAL-STUDY OF A MODEL COLLOID-POLYMER MIXTURE

We report an experimental study of the phase behaviour of a model hard sphere colloid + non-adsorbing polymer mixture - sterically stabilised PMMA (radius = 217 nm) and polystyrene (r(H) = 12.8 nm) in cis-decalin. It was found that the main effect of adding polymer was to expand die fluid-crystal coexistence region of the colloidal suspension, which spans 0.494 <phi <0.545 (phi = colloid volume fraction) at zero polymer concentration. The separated dilute and dense colloidal phases showed marked polymer partitioning, giving rise to strong osmotic compression of the colloidal crystal phase. These findings are shown to compare favourably with a recent statistical mechanical model [Lekkerkerker et al. Europhys. Left. 20 (1992) 559)] ; possible sources of discrepancies are discussed. In addition the presence of a small amount of polymer incolloidal glasses (phi > 0.58) was shown to induce crystallization. At high enough polymer concentrations crystallization was inhibited at all phi and a > state was obtained. Finally, the effect of temperature on the phase behaviour is reported. Increasing temperature lowers the amount of polymer needed to cause fluid-crystal phase separation as well as transition into the > state.