The long-standing observations that different amorphous materials exhibit a pronounced enhancement of viscosity and eventually vitrify on compression or cooling continue to fascinate and challenge scientists, on the ground of their physical origin and practical implications. Glass formation is a generic phenomenon, observed in physically quite distinct systems that encompass hard and soft particles. It is believed that a common underlying scenario, namely cage formation, drives dynamical arrest, especially at high concentrations. Here, we identify a novel, asymmetric glassy state in soft colloidal mixtures, which is characterized by strongly anisotropically distorted cages, bearing similarities to those of hard-sphere glasses under shear. The anisotropy is induced by the presence of soft additives. This phenomenon seems to be generic to soft colloids and its origins lie in the penetrability of the constituent particles. The resulting phase diagram for mixtures of soft particles is clearly distinct from that of hard-sphere mixtures and brings forward a rich variety of vitrified states that delineate an ergodic lake in the parameter space spanned by the size ratio between the two components and by the concentration of the additives. Thus, a new route opens for the rational design of soft particles with desired tunable rheological properties.

By employing rheological experiments, mode coupling theory, and computer simulations based on realistic coarse-grained models, we investigate the effects of small, hard colloids on the glassy states formed by large, soft colloids. Multiarm star polymers mimic hard and soft colloids by appropriately varying the number and size of their arms. The addition of hard colloids leads, depending on their concentration, to either melting of the soft glass or the emergence of two distinct glassy states. We explain our findings by depletion of the colloids adjacent to the stars, which leads to an arrested phase separation when the repulsive glass line meets the demixing binodal. The parameter-free agreement between experiment, theory, and simulations suggests the generic nature of our results and opens the route for designing soft-hard colloidal composites with tunable rheology.

SynopsisUpon addition of small nonadsorbing linear polymers, colloidal glasses composed of large hard spheres melt and eventually revitrify into the so-called attractive glass regime. We show that, when replacing the hard spheres by star polymers representing model soft particles, a reentrant gel is formed. This is the result of compression and depletion of the stars due to the action of the osmotic pressure from the linear homopolymers. The viscoelastic properties of the soft dense gel were studied with emphasis on the shear-induced yielding process, which involved localized breaking of elements with a size of the order of the correlation length. Based on these results, a phenomenological attempt was made at describing the universal rheological features of colloid/nonadsorbing polymer mixtures of varying softness. The star gel was found to undergo thermoreversible melting, despite the fact that conventional hard-sphere depletion gels are invariant to heating. This phenomenon is attributed to the hybrid internal microstructure of the stars, akin to a dry-to-wet brush transition, and is characterized by slow kinetics, on the time scale of the osmotic gel formation process. These results may be useful in finding generic features in colloidal gelation, as well as in the molecular design of new soft composite materials. V C 2014 The Society of Rheology. [http://dx

A divergent synthetic scheme was developed for the preparation of high branching functionality hybrid polymers from carbosilane dendrimer substrates and polybutadiene side chains. Carbosilane dendrimers with 32, 64, or 128 peripheral Si-Cl functional groups were first coupled with 1,2-polybutadienyllithium chains having a number-average molecular weight M n ≈ 1000. The polybutadiene-grafted substrates were then hydrosilylated with dichloromethylsilane and reacted with high 1,4-microstructure content polybutadienyllithium chains to generate high branching functionality dendrimer-arborescent hybrids. Three series of hybrid polymers were synthesized containing 1,4-polybutadiene side chains with M n ≈ 1500, 5000, or 30 000. Size exclusion chromatography analysis of the polymers confirmed that a narrow molecular weight distribution was maintained (M w /M n e 1.14). The branching functionality of the arborescent hybrids varied from 140-335, 160-1110, and 360-2830 for the 32-, 64-, and 128-site coupling precursors, respectively. The experimental branching functionalities attained were lower than the theoretical values due to decreased coupling efficiency within each series, in particular for polymers with longer polybutadiene side chains, apparently due to steric limitations in the grafting reaction.

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