We describe the preparation of hairy nanospheres by grafting of polystyrene macromonomer chains onto polyorganosiloxane microgels. Our strategy was to obtain special surface-modified colloidal particles, which can be blended with linear polymer chains without depletion demixing found for standard colloid-polymer mixtures. For this purpose, the molecular weight of the polymer hairs and of the linear chains has been varied between 4000 and 19 000 g/mol. In all cases, the number of hairs per single particle with core radius about 10 nm exceeded 150. Studying the internal structure of mixtures of these hairy spheres with linear polymer chains by transmission electron microscopy and small-angle X-ray scattering, we identified homogeneous systems, i.e., suppression of depletion demixing, in case the molecular weight of the polymer hairs on the particle surface is at least as large as the molecular weight of the single polymer chains. This result is interpreted in terms of a new depletion model, taking into account the surface roughness of colloidal spheres with a surface of polymer hairs, and the probability of single polymer chains to partially penetrate into this hairy particle surface.
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 hardsphere 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.
We have investigated diffusion and thermal diffusion properties of light-absorbing colloidal polyorganosiloxane microgels containing tiny nanometer-sized gold clusters dispersed in toluene. Transient holographic gratings allow for very subtle perturbations in the linear regime where Soret feedback is negligible. Gold-doped colloids of different size and crosslink ratios show different Soret coefficients but identical thermal diffusion coefficients D(T). Undoped colloids tend to aggregate, but a consistent interpretation is obtained if an identical D(T) is assumed for the doped, the undoped, and the aggregated particles. Previously reported Soret feedback measurements on similar systems incidentally yielded comparable Soret coefficients. We show, however, that they suffer from strong convective perturbations.
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