Rheological investigation of depletion flocculation of concentrated sterically stabilised polystyrene latex dispersions

Abstract

The flocculation of a concentrated polystyrene latex suspension (volume fraction φ = 0.3) containing grafted poly (ethylene oxide) chains, by “free” (non-adsorbing) poly (ethylene oxide) (PEO) was investigated using rheological measurements. Three techniques were used, namely steady-state shear stress-shear rate measurements, shear modulus and oscillatory measurements. From these the following rheological parameters were determined: the Bingham yield value τ β, the Casson yield value τ c, the high frequency modulus G ∞, the complex modulus G*, the storage modulus G′ and the loss modulus G″. All rheological parameters showed a sudden increase above a critical free polymer volume fraction, φ p +, which decreased with increase of molecular weight of the free polymer. The flocculation was reversible. From the Bingham yield value, the energy of separation of flocs into single units, E Sep, was calculated using reasonable values for the number of contact points in a floc and assuming that above the yield point all flocs are broken down into individual units. The values of E Sep were compared with the depletion attraction energy, G dep, which was calculated from Asakura and Oosawa's and Fleer, Scheutjens and Vincent's models, assuming the particles to behave as hard spheres. The results of the calculations showed that G dep deviates from E Sep indicating that the hard-sphere model is not valid for sterically stabilised dispersions with “soft” tails.