STERIC AND BRIDGING FORCES BETWEEN SURFACES FEARING ADSORBED POLYMER - AN ATOMIC-FORCE MICROSCOPY STUDY

Direct measurement of the forces of interaction between a 10 mu m radius sphere of zirconia with a flat polished zirconia ceramic plate in aqueous electrolyte solution has been performed. The interaction profiles of these surfaces after the addition of a high molecular weight (M(w) - 750 000) polyacrylic acid were also recorded. The surface coverage of the polymer adsorbed at the solid/aqueous interface was controlled by varying the added polymer concentration and by varying the equilibration time of the polymer with the two surfaces at large separations. At a long equilibration times (24 h) and higher polymer concentration (30 mu g/L) a repulsive interaction force was observed between the surfaces upon their approach, this repulsive force beginning at twice the radius of gyration and increasing monotonically as the separation is decreased. At a lower polymer concentration (10 mu g/L) and short equilibration time (<1 h) the polymer was seen to bridge between the sphere and the flat plate. At the scan rates employed here, for the approach and separation of the surfaces, the bridging polymer remained adsorbed to both surfaces tethering them together.