Binary and ternary mixtures of microgel particles, hard spheres and non-adsorbing polymer in non-aqueous solvents

Crosslinked polystyrene (PS) particles were dispersed in diisopropyl adipate, a non-volatile, good solvent for PS. Depletion attractions between particles were induced by adding linear PS, with a polymer/particle size ratio of 0.3. For hard particles, with a high crosslink density, the colloid-polymer mixtures displayed phase separation in agreement with predictions for hard sphere - polymer mixtures. For similar sized but weakly crosslinked, soft microgel particles however, a significantly higher concentration of linear PS needed to be added to observe phase separation. This is because the non-adsorbing polymer can penetrate the soft microgels which weakens the depletion interaction.

In ternary mixtures of the hard and soft particles and linear PS, confocal microscopy reveals that mixed particle networks are formed. Upon adding soft particles to a hard particle gel network, there is only modest variation in the flow properties, with the moduli decreasing somewhat, yet viscosity increasing. It is argued that the effect of an increase of volume fraction is offset by a reduction in depletion interaction strength. This demonstrates that, in these ternary mixtures, microgels act like particles rather than polymer depletants; yet microgel addition allows high volume fraction polymer - colloid mixtures to be made whilst avoiding the viscosity increase that would normally result.