Clustering of soft colloids due to polymer additives

We examine theoretically the structural behaviour of dilute solutions of multiarm star polymers under the addition of smaller homopolymer chains. The approach is based on effective interactions, for which the star-polymer centres and the centres of mass of the linear chains are used as effective coordinates. We find that addition of linear chains first leads to a softening of the star-polymer repulsions and, at higher chain concentrations, to the formation of stable, multi-star clusters. We accompany the theoretical approach with dynamical light scattering measurements in real systems, finding agreement between theory and experiment. We rationalize our findings by deriving the chain-induced, one-component effective potential between stars, which features an attractive well followed by a repulsive barrier. We discuss the dependence of these characteristics on the size and concentration of the homopolymer additives and relate the present system to recent models that also display cluster formation.