Concentrated sterically stabilized dispersions and their flocculation by depletion layers

Abstract

Model systems of polystyrene latex dispersions (R=175 nm) containing grafted polyethylene oxide chains (M _w =2000) were prepared using dispersion polymerisation. Their properties were investigated using steady-state shear stress-shear rate and oscillatory measurements. The viscosity-volume fraction ϕ results were fitted to the hard-sphere model of Dougherty and Krieger. This showed a reduction in adsorbed layer thickness with increase in ϕ. The viscoelastic results showed a gradual increase of the elastic response with increase in ϕ. At a critical ϕ value, G′ (storage modulus) becomes greater than G″ (loss modulus) indicating the onset of interaction between the steric layers. G′ - ϕ curves were converted to G′ - h (interparticle distance) curve to illustrate the analogy with the directly measured force (F) distance curves (using mica sheets with adsorbed graft copolymer). The F - h curves could be also converted to G′ - h curves and a comparison could be made with the curves obtained from rheological measurements. Although the values of G′ obtained from the F - h curves were different from these obtained from rheology, the trend was the same; this illustrates the value of rheological measurements for studying steric interaction. — The above concentrated sterically stabilised dispersions were then flocculated by adding a "free" (non-adsorbing) polymer (polyethylene oxide or hydroxyethyl cellulose). The flocculation was followed using rheological measurements. As a result of the presence of depletion layers, flocculation became significant above a critical volume fraction of free polymer ϕ ⁺_p . The results were analysed using the available theories on depletion flocculation and a comparison was made between the free energy of depletion and the energy of separation of particles calculated from rheology.