Effect of nonadsorbed polymer on the stability of weakly flocculated suspensions

The stability of weakly flocculated, nonaqueous suspensions of aluminum oxide with adsorbed poly-(methyl methacrylate) (PMMA) layers as a function of nonadsorbed PMMA volume fraction was studied through rheological measurements and optical microscopy. In the absence of nonadsorbed polymer, suspensions exhibited strong non-Newtonian shear-thinning behavior. Interparticle potential energy calculations yielded a secondary minimum of a few kT for this system. The degree of shear thinning, low shear relative viscosity, and characteristic floe size were observed to decrease with increasing nonadsorbed polymer over the entire concentration range studied. Improved suspension stability was observed at free polymer concentrations as low as 0.59 g/L, nearly 2 orders of magnitude below the estimated onset of depletion restabilization. These observations were attributed to the presence of a repulsive barrier (estimated to be of the order of kT or higher) arising prior to the exclusion of nonadsorbed species from the gap region. Kinetic effects stemming from energy barrier considerations appear to dominate colloidal interactions in weakly flocculated systems in the presence of nonadsorbed polymer.