Fluidization of Highly Concentrated Colloidal Dispersions by Tailoring of Attractive Interactions

Abstract

Mode coupling theory (MCT) predicts fluid states of colloidal dispersions at particle volume fractions ϕ well above the hard sphere (HS) colloidal glass transition due to weak attractive interactions among particles. This opens a versatile, new route to manufacture highly concentrated, freely flowing dispersions with narrow particle size distribution. Our investigations are based on two model systems: polystyrene (PS)-microgel particles suspended in an isorefractive organic solvent and an aqueous polymer dispersion based on a well-stabilized, commercial polymer latex. Both systems exhibit hard sphere type flow behavior with a divergence of zero-shear viscosity at ϕ = 0.58. Suspensions were fluidized via addition of non-adsorbing polymers to the continuous phase, thus introducing weak depletion attraction among particles. The index-matched microgel system was used to study phase behavior as a function of particle and polymer concentration as well as polymer to particle size ratio and particle rigidity. A tight correlation between structural relaxation times from dynamic light scattering (DSL) experiments and rheological data was found. Fluid states were observed at particle loadings close to ϕ = 0.7 and a minimum viscosity has been achieved at polymer concentrations below the overlap concentration c*. Low viscosity values at particle loadings beyond ϕ = 0.58 could so far only be obtained for dispersions with bi- or multimodal particle size distribution. Flow curves obtained here for monomodal dispersions fluidized due to weak attractive interactions are similar to those of commercial dispersions with broad particle size distribution, demonstrating the competitive strength of the new concept. Sharply monodisperse aqueous polymer dispersions were used to demonstrate that beyond the predictions of MCT, also densely packed, crystalline suspensions can be fluidized upon adding small amounts of non-adsorbing polymer. A microfluidic flow channel attached to an inverted fluorescence microscope was used to study the true flow profiles of suspensions doped with size-matched fluorescent tracer particles. Reducing the range of weak depletion attraction by reducing the size of free, non-adsorbing polymer extended the fluidized region to even higher particle loadings of about ϕ = 0.72—in qualitative agreement with MCT predictions. However, an increase of the microgel crosslink density from 1:50 to 1:10 reduced the fluidized region significantly to about the effect observed with hard sphere-like PMMA dispersions. Particle softness and osmotic deswelling are discussed as possible origins of the exceptionally effective depletion fluidization in case of 1:50 crosslinked microgels. To enable similar studies combining DLS and rheology on model systems which are closer to aqueous, technical dispersions, perfluoroacrylate particles sterically stabilized with polyethylene-glycol (PEG) chains have been synthesized and characterized. A first study indicates that such dispersions can be refractive index matched in aqueous media and undergo a glass transition, thereby exposing dynamics in DLS which are quite analogous to that seen in so far studied model systems. The potential of fluidizing such dispersions at high particle loading by addition of free PEG or other depletants will be systematically explored in future work.