The importance of the interfacial stabilising layer on the macroscopic flow properties of suspensions dispersed in non-adsorbing polymer solution

Abstract

In this paper, the rheological properties, microscopic appearance and macroscopic sedimentation behaviour of 147- and 482-nm polystyrene latices in HEC solutions, bearing different adsorbed poly(ethyleneoxide)–poly(propyleneoxide)–poly(ethyleneoxide) (PEO–PPO–PEO) copolymers are presented and compared with previous results with a 67-nm latex (Langmuir 13 (1997) 2922). The ratio of the steric layer thickness to the particle radius varies from 1:5 to 1:40 for the three latex sizes covering a range of particle softness. The 147-nm latex showed gas, liquid and solid phases, including three phase coexistence with increasing concentrations of HEC. The solid phase was a viscoelastic gel that sedimented slowly and showed initially slow, then faster sedimentation rates for HEC concentrations close to the fluid–gel phase boundary. These properties depended on the adsorbed PEO–PPO–PEO copolymers; the fluid–gel phase boundary moved to lower HEC concentrations with increasing PEO chain length. Oscillatory shear measurements were sensitive to the floc network structure and showed the transition from the fluid to gel phases. The values for the elastic modulus in the gel region were independent of the PEO chain length in the stabiliser implying the presence of similar floc structures with each of the different PEO–PPO–PEO copolymers. Stress relaxation measurements gave long relaxation times (>10 3 s) and showed that the suspensions were viscoelastic fluids with high zero shear viscosities. Higher values were obtained with longer PEO chain lengths. Extrapolated yield stress values showed stronger flocculation with increasing PEO chain length in the adsorbed stabilising layer. There was good correlation between the extrapolated yield stress and the sedimentation velocity, indicating that the collapse of the floc network is governed by the strength of the interparticle bonds (for the same floc structure) and also with the long-time behaviour from the stress relaxation measurements. With the 482-nm latex there was less effect of the adsorbed copolymer both for the rheology and sedimentation behaviour, with the particles behaving more like hard spheres. The transition between hard and soft sphere behaviour, interpreted here as the ratio of the steric layer thickness to the particle radius at which the adsorbed stabilising layer starts to have an effect on the rheology and phase behaviour is estimated to be approximately 1:20.