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

In this paper the influence of the adsorbed PEO-PPO-PEO (PEG, poly(eth ylene oxide); PPO (poly-(propylene oxide!) stabilizing polymer on lati ces is studied. Experiments are presented for small, 67 nm, polystyren e latex with either Synperonic PE/P103, P105, or F108 PEO-PPO-PEO bloc k copolymers adsorbed onto the latex surface at full coverage. Nonadso rbing hydroxyethylcellulose (300 000 molar mass) has been added as the depleting polymer to flocculate the particles, which are at a core vo lume fraction of 0.159. Rheology, sedimentation, and microscopy have b een used to characterize these suspensions. As the polymer concentrati on is increased, phase separation is noted. Firstly a gas-liquid phase transition is noted and viscoelastic gels are formed. Further increas e in polymer concentration results in three phases: gas, liquid, and s olid being in thermodynamic equilibrium. At yet higher concentrations a solid-gas phase transition is noted. It was further noted that the p olymer concentration required to bring about these changes increased a s the stabilizing chain length decreased. Oscillatory shear measuremen ts showed that identical flee 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 s ofter 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 in teraction of soft spheres.