The rheology of concentrated suspensions of depletion-flocculated latex particles

The effect of adding non-adsorbing polymer, poly(acrylic acid) (PAA), on the rheological properties of concentrated electrostatically-stabilised polystyrene latex suspensions was investigated. The extrapolated yield stress of the depletion-flocculated suspensions was studied as a function of the concentration and the molecular weight of the free polymer. Specifically, the Bingham yield value was observed to increase with either an increase in the weight fraction of the polymer or through the use of a longer chain polyacid. Unlike the sterically stabilised colloidal suspensions studied in the literature, at high polymer concentrations these charge-stabilised latex particles exhibited plateauing in the yield stress for each polyacid. The energy of separating particles within an aggregate into single units was calculated using the yield stress and average particle co-ordination number. In contrast to what was commonly done in the literature, the latter was not assumed to be constant but estimated from the aggregate mass fractal dimension as obtained from static light scattering techniques. It was found that the separation energy showed similar trends to those in the yield stress with both varying the polymer concentration and molecular weight. These trends also bear remarkable similarities to those in the depth of the secondary potential energy well as measured using atomic force microscopy (AFM).