The effect of molecular architecture of polymeric dispersants on the stabilization and rheology of dispersions

In this work, we have investigated the effect of polymer molecular architecture on the stabilization and rheology of dispersions. Well-defined model systems of functionalized polystyrene chains with narrow molecular weight distributions and monodisperse silica particles have been used.;Different sizes of monodisperse silica particles were prepared by the hydrolysis of tetraethylorthosilicate. Different molecular weight linear and bilinear alkoxysilane functionalized-polystyrenes were synthesized by anionic polymerization techniques. Bilinear alkoxysilane-polystyrenes were defined as polystyrenes with alkoxysilane functional groups located exactly in the middle of the chains. The alkoxysilane functionality allowed reaction with surface silanol groups to chemically graft the polystyrene onto silica surfaces.;The effect of molecular weight on dispersion stability, as monitored by the dispersion turbidity as a function of time, showed that the dispersion stability increased with increasing molecular weight up to molecular weights of 10,000 to 30,000, after which little additional effect was found. These critical molecular weights were found to be dependent on particle size and molecular architecture. Bilinear polystyrenes showed better dispersion efficiency than did linear polystyrenes having similar arm molecular weights. The effect of molecular architecture on stabilization diminished as the dispersed medium approached theta conditions for polystyrene.;In dilute suspensions, the deviation of the measured relative viscosity from the value calculated from the Einstein equation could be correlated with the stabilization efficiency of the polymer. The results found for the rheological properties were consistent with results obtained from the stabilization studies. The unstabilized particles exhibited wall-slip and critical shear thinning phenomena, while the rheological properties of stabilized silica suspension showed both shear thinning and shear thickening behavior. The thickness of the polymer surface layer, as calculated from the high shear viscosity of concentrated suspensions, increased with increasing polymer concentration on the silica surface. At high stabilizer surface coverage, the calculated polymer layer thickness indicated that the polymer chains were in a partially extended conformation. At low stabilizer surface coverage, the apparent decrease in stabilizer thickness suggested that inter-mixing of grafted polymer chains between neighboring particles occurred.