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 of hairy nanospheres, i.e., polyorganosiloxane microgels grafted with polystyrene chains, and linear polystyrene chains. Previously, it has been found that such mixtures are only homogeneous if the molecular weight of the polymer hairs is at least as large as that of the linear matrix chains. Otherwise, the depletion mechanism typical for most colloid-polymer mixtures leads to a phase separation. It indicates that the length of the polymer hairs with respect to the length of the matrix chains has an important effect on structure and dynamics of the colloid-polymer blends. Here we demonstrate that 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 interparticle repulsion ("wet brush"). If hairs and chains are comparable in length, the corona shows much less volume swelling, leading to a short-range repulsive potential similar to hard-sphere systems ("dry brush"). As a consequence, wet brush mixtures exhibit a liquid-solid transition, as identified by dynamic mechanical measurements, at much lower particle weight fraction in comparison to dry brush systems. These studies are compared with our recent work on copolymer micelles with corona and matrix chains of much higher molecular weight.