(The) effect of shear flow on the dispersion stability of sterically-stabilized dispersions

Abstract

The shear stability of the sterically-stabilized dispersions was studied theoretically and experimentally. The steric interaction was calculated from Scheutjens and Fleer``s theory. At the restricted equilibrium in which the ploymer can not escape from the gap between the plates and the amount of polymer is constant, we could obtain the following general trends. In poor solvents there is an atraction at intermediate separation and a strong repulsion when the surfaces approach each other closely. In good solvents the same behaviors are found when the adsorbed polymer amount is low, but at high coverage the minimun disapperas and the surfaces repel each other over the whole range of interparticle separations. The attaction is caused by briding and by an osmotic pressure in bad solvents, whereas the repulsion orignates from the osmotic pressure due to the high segment concentration and from the entropical restriction of polymer in a narrow space. At a given solvent quality, the free energy of interaction is determined mainly by the adsorbed polymer amount. Long chains at a low concentration have the same effects as short chains at much higher concentrations at the same amount. Since it is much easier to obtain a high adsorbed amount with high molecular weight polymer, lon chains are better stabilizers that short ones. Theoretical predictions of the shear stability were carried out from the trajectory analysis involving hydrodynamic interaction, Van der Waals attraction, electrical double-layer repulsion, and steric interaction. In the absence of the steric interaction, the shear stability is strongly affected by the double-layer repulsive forces. The stability ratio is found to be approximately proportional to $G^{0.818}$ when the doublelayer repulsive forces are negligible. In the presence of the steric interaction the behaviors of shear stability are found to be similar to the case without the steric interaction when the adsorbed polymer is low, but at high adso...