Effect of the Steric Stabilizing Layer on the Phase Behavior and Rheology of Small Polystyrene Latex Dispersions Induced by Changes to the Concentration of Nonadsorbing Hydroxyethylcellulose

Abstract

In this paper the influence of the adsorbed PEO−PPO−PEO (PEO, poly(ethylene oxide); PPO (poly(propylene oxide)) stabilizing polymer on latices is studied. Experiments are presented for small, 67 nm, polystyrene latex with either Synperonic PE/P103, P105, or F108 PEO−PPO−PEO block copolymers adsorbed onto the latex surface at full coverage. Nonadsorbing hydroxyethylcellulose (300 000 molar mass) has been added as the depleting polymer to flocculate the particles, which are at a core volume fraction of 0.159. Rheology, sedimentation, and microscopy have been used to characterize these suspensions. As the polymer concentration is increased, phase separation is noted. Firstly a gas−liquid phase transition is noted and viscoelastic gels are formed. Further increase in polymer concentration results in three phases: gas, liquid, and solid being in thermodynamic equilibrium. At yet higher concentrations a solid−gas phase transition is noted. It was further noted that the polymer concentration required to bring about these changes increased as the stabilizing chain length decreased. Oscillatory shear measurements showed that identical floc structures were formed with each of the stabilizing polymers while yield stress measurements revealed stronger flocculation with the higher molecular weight stabilizers, in accord with the observed phase behavior. These observations are a result of softer interactions with the suspensions bearing the shorter molecular weight chains. The experimental yield stress values are compared with the VEEJ (Vincent, Edwards, Emmett, Jones) theory for the depletion interaction of soft spheres.