Structure and dynamics of hairy spherical colloids in a matrix of nonentangled linear chains

We describe the structure and dynamical behavior of a homogeneous mixture o f hairy nanospheres, i.e., polyorganosiloxane microgels grafted with polyst yrene chains, and linear polystyrene chains. Previously, it has been found that such mixtures are only homogeneous if the molecular weight of the poly mer hairs is at least as large as that of the linear matrix chains. Otherwi se, the depletion mechanism typical for most colloid-polymer mixtures leads to a phase separation. It indicates that the length of the polymer hairs w ith respect to the length of the matrix chains has an important effect on s tructure and dynamics of the colloid-polymer blends. Here we demonstrate th at if the chains are much shorter than the hairs, the colloidal hair corona is strongly swollen by the matrix polymer, leading to a long-range soft in terparticle repulsion ("wet brush"). If hairs and chains are comparable in length, the corona shows much less volume swelling, leading to a short-rang e repulsive potential similar to hard-sphere systems ("dry brush"). As a co nsequence, wet brush mixtures exhibit a liquid-solid transition, as identif ied by dynamic mechanical measurements, at much lower particle weight fract ion in comparison to dry brush systems. These studies are compared with our recent work on copolymer micelles with corona and matrix chains of much hi gher molecular weight.