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Structure and dynamic of dense suspensions of soft colloids with tunable short-range attraction

Abstract : Soft colloids are widely used as rheological additives in formulations as diverse as paint, coatings, and cosmetics. In this work we present a novel class of pH responsive colloidal microgels with controllable softness and tunable attractive interparticle interactions. Microgels are crosslinked polyelectrolyte networks swollen by water. The elasticity of individual microgels is tuned at will through the crosslink density which depends on the amount of multifunctional monomers introduced during the synthesis. Alkyl groups induce short range attractions between microgels, whose strength depends on the composition and the architecture of the hydrophobic chains. This system is thus a valuable candidate to elucidate the relation between the macroscopic properties and the strength of the interaction potential when it is varied from soft repulsive to soft attractive. This question is central to the behavior of materials like reversible gels or protein gels. We use non linear rheology, static and dynamic light scattering and video microscopy coupled to high speed Particle Image Velocimetry (PIV) to connect the particle scale properties to the macroscopic behavior of the suspensions in the liquid, glass and jamming regime.In a first part, the interparticle potential is purely repulsive and its strength is varied using the crosslink density as a control parameter. Upon increasing the polymer concentration, highly crosslinked suspensions successively encounter well defined glass and jamming transitions, which delimit the dilute, entropic glass, jammed glass, and dense glass regimes. In the jammed regime the rheological properties support recent predictions and simulations for ideal elastic spheres randomly packed at high volume fraction. The glass-like structure of the suspensions with short range spatial correlations is responsible for non-iridescent structural colors which shift continuously from red to blue when the concentration is increased. In contrast, weakly crosslinked microgels exhibit novel mechanical and flow properties with unprecedented scaling properties, which suggest that they belong to a different class of disordered materials that we termed ‘ultrasoft’ colloids.In the second part we investigate the effects of short range attractions on the static and dynamic properties of soft microgels. The comparison with purely repulsive microgels with exactly the same composition, crosslink density, and particle size highlights the specificities of associative microgels which have different interparticle attractive potential. In the jammed glass regime, attractive interactions are responsible for flow heterogeneities -a competition between wall slip and/or shear-banding- controlled by the strength of attractive interactions. To rationalize the rheology of associative suspensions, we propose a two-state model involving two characteristic time scales associated to the relaxation of the associations and cage opening, respectively. These results establish a link between the strength of short-range attractions and the macroscopic flow properties. Finally, we present two applications where associative microgels are used as in novel formulations as blends with the non associative and as emulsions stabilizers.
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Maddalena Mattiello. Structure and dynamic of dense suspensions of soft colloids with tunable short-range attraction. Material chemistry. Université Paris sciences et lettres, 2018. English. ⟨NNT : 2018PSLET047⟩. ⟨tel-03360697⟩

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