Polystyrene (PS) microgel colloids have often been used successfully to model hard sphere behaviour even though the term "gel" invokes inevitably the notion of a more or less soft, deformable object. Here we systematically study the effect of reducing the cross-link density from 1 : 10 (1 cross-link per 10 monomers) to 1 : 100 on particle interactions and "softness". We report on the synthesis and purification of 1 : 10, 1 : 25, 1 : 50, 1 : 75 and 1 : 100 cross-linked PS particles and their characterization in terms of single particle properties, as well as the behaviour of concentrated dispersions. We are able to tune particle softness in the range between soft PNiPAM-microgels and hard PMMA particles while still allowing the mapping of the microgels onto hard sphere behavior with respect to phase diagram and static structure factors. This is mainly due to a rather homogeneous radial distribution of cross-links in contrast to PNiPAM microgels where the cross-link density decreases radially. We find that up to a cross-link density of 1 : 50 particle form factors are perfectly described by a homogeneous sphere model whereas 1 : 75 and 1 : 100 cross-linked spheres are slightly better described as fuzzy spheres. However the fuzziness is rather small compared to typical PNiPAM microgels so that a hard sphere mapping still holds even for these low cross-link densities. Finally, by varying the reaction conditions - changing from batch to semibatch emulsion polymerization and varying the feed rate or by adjusting the monomer to initiator ratio we can tune the fuzziness or significantly alter the inner structure to a more open, star-like architecture.

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SANS measurements in THF-d 6 were carried out at the D11 instrument at ILL, Grenoble with two aliphatic aromatic hyper-branched (hb) samples with nearly the same degree of polymerization (DP ≈ 90), containing hydroxyl (OH) and silyl-ether (SY) terminal groups. The choice of a large region of momentum transfer q permitted probing the global shape and the local structure. The scattering of both samples showed pronounced concentration dependence. Using Zimm's well-known scattering equation the true size and shape of the macromolecules at finite concentration were derived. Agreement of the data and curves at finite concentration with those at c = 0 was obtained. The scattering curves were analyzed on the basis of the Sinha et al.'s refinement of fractals. Complete agreement with the experimental scattering curves was achieved. The apparent anomalous behavior at very large qvalues was shown to arise from the monomer and segmental contributions. Molecular dynamics (MD) simulation of an uniform hb sample (DP 35) revealed the monomer contribution in agreement with the experiment. The discrepancy of the experimental curve at small q-values is shown to arise from the broad molar mass distribution. When Flory's molar mass distribution was combined with the MD simulation data, the experimental results were described well.

Synthesis of colloidal particles which are sterically stabilized in water and at the same time isorefractive in aqueous dispersion has been achieved by copolymerizing a fluoracrylate monomer with a PEG-macromonomer. Colloidal stability against salt addition and freeze-thawing cycles is demonstrated, indicating that the particles consist of a core of fluoroacrylate and a stabilizing surface layer of grafted PEG. To test the performance as a model system for studies of colloidal vitrification in aqueous media, the dynamics of a binary mixture of these particles with a size ratio of 0.8 and a number ratio of 1.3 (small : large particles) were studied in an isorefractive mixture of water and DMSO (≈ 11 mol % DMSO). The characteristic features of a system close to the glass transition were observed in the density density autocorrelation function f(q, τ) as measured by DLS. However, the glass transition occurred at an unexpectedly low volume fraction of 0.262. This discrepancy correlates with an unusually large difference between TEM and DLS radii and is explained by the presence of a rather extended hairy PEG surface layer.

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