Rheology of water-in-water suspensions formed with lightly sulfonated ionomers

Abstract

The formation and rheological properties of water-in-water suspensions formed by mixing a dilute nonaqueous solution containing a lightly sulfonated ionomer with an aqueous solution are described. The spheres formed via this process are composed of a very thin (approximately 2000 Å), ionically crosslinked gel membrane which separates the continuous aqueous phase from the encapsulated aqueous phase. The membrane itself is composed of the lightly sulfonated ionomer (i. e., sulfonated polystyrene) swollen with the nonaqueous solvent. Interestingly, surface tension measurements indicate that the sulfonated ionomer in this nonaqueous solvent has no significant interfacial activity. Viscometric measurements confirm that aqueous solvents containing these spheres have considerably enhanced viscosity even in the presence of high concentrations of a salt or acid. Thixotropic behavior is observed at low shear rates, whereas Newtonian behavior is observed at higher rates of shear (> 40 sec⁻¹). Cessation of stress reverts the viscosity to its initial value. Dilution studies show that the streamlines in the flowing exterior aqueous phase cause circulation of the aqueous fluid within the sphere. These results point to the fluid-like characteristics of the gel membrane, since transmission of the stress across the membrane is not dramatically inhibited. Comparison of the low shear rate data with the Mooney equation support these conclusion.