FORCES BETWEEN POLYMER-BEARING SURFACES UNDERGOING SHEAR

ADSORBED or grafted polymers are used to control phenomena such as colloidal stability, fluid flow and the tribological properties of surfaces. Forces between polymer-bearing surfaces have been studied comprehensively over the past decade 1-10, but little is known about such forces in shear. These are intimately related to the dynamic as well as the equilibrium properties of the surface-attached chains. We have constructed a device that measures directly the forces that act between surfaces bearing polymer layers in a liquid medium as they slide past each other. For the case of mica sheets bearing end-grafted chains of polystyrene in toluene (a good solvent), we find that, as the surfaces move parallel to each other, there is a marked change in the normal forces between them. These become increasingly repulsive at higher velocities. The effect occurs only above certain critical shear rates, probably related to relaxation dynamics of the end-grafted chains themselves. Our findings have direct implications for the properties of polymeric lubricants, and for the rheological behaviour both of stabilized dispersions and of multi-phase polymeric systems.