Rheological studies on sterically stabilized model dispersions of uniform colloidal spheres: II. Steady-shear viscosity

Abstract

Model sterically stabilized poly(methylmethacrylate) (PMMA) particles were dispersed in silicone fluids of different viscosities, at concentrations up to 45% by volume. The particles were uniform in size, with core diameters ranging from 170 to 620 nm. Silicone viscosities ranged from 8 to 450 cP. Thickness of the steric stabilizing layer was estimated to be 7 nm. Rheological behavior of the different dispersions fell into two distinct categories, referred to here as “gelled” and “fluid.” Gelled dispersions exhibited time-dependent thixotropy, yield stresses, and high-shear Newtonian viscosities. They were obtained when the molecules of the dispersing medium were comparable in length to average spacings between particle surfaces. Gelation is attributed to elastic-steric interactions between the stabilizing fragments and the molecules of the stabilizing medium. The fluid dispersions, obtained with larger particles dispersed in silicone fluids of lower molecular weight, showed moderate low-shear viscosities even at temperatures so low that the media are highly viscous. When the thickness of the stabilizing layer was included in calculating volume fractions, the fluid dispersions obeyed a principle of corresponding rheological states which had previously been demonstrated for dispersions of noninteracting spheres.