Rheology of Aqueous Latices with Adsorbed Stabilizer Layers

Abstract

The rheology of two aqueous latices containing adsorbed stabilizer layers is studied and compared with the results from nonaqueous dispersions with chemically grafted stabilizer layers. For this purpose poly(butyl acrylate–styrene) particles are used; the stabilizer consists of a mixture of two surfactants, an ionic one and a nonionic one. The difference between the two latices resides in the length of the hydrophilic part of the surfactants. It is verified that the stabilization is essentially steric rather than electrostatic. Viscosities and dynamic moduli have been measured at various volume fractions. The evolution of the plateau values of the storage modulus with volume fraction is used to calculate the interaction potential. Its shape closely resembles those obtained earlier for nonaqueous dispersions with a similar stabilizer layer thickness. From the interparticle potential an effective hard sphere diameter can be derived. If the limiting Newtonian viscosities are plotted versus volume fraction as calculated from the effective hard sphere diameter, the curves superimpose on the available curves for chemically grafted, nonaqueous dispersions. For thick adsorbed layers, extrapolation of the potential to larger distances can become questionable for the computation of the effective hard sphere diameter. The dynamic moduli also obey the same scaling relations as those found for the chemically grafted systems. Using samples with different stabilizer layer thicknesses, a suggested scaling for the hydrodynamic thickness could be verified. It can be concluded that adsorption of a stabilizer layer can lead to a rheological behavior identical to that of chemical grafting and that the detailed rheological properties can be predicted in a similar fashion.