LUBRICATION FORCES BETWEEN SURFACES BEARING POLYMER BRUSHES

We have studied both the equilibrium and the hydrodynamic lubrication forces that act in the normal direction between mica surfaces in toluene when bearing polystyrene chains anchored to each surface by a zwitterionic group at one end. The quasistatic force-distance profiles reveal the long-ranged repulsion characteristic of steric interactions between the extended brush-like layers in the good solvent and provide a measure of the brush thickness L in close agreement with earlier studies of such brushes. Dynamic measurements of the surface forces, where the surface separation D is varied sinusoidally in time, show two regimes: at D > 2.5L, the hydrodynamic forces are characteristic of Newtonian flow of liquid with the viscosity of bulk toluene, but with a shear plane shifted a distance L(H) from each mica surface (qualitatively similar to earlier observations with adsorbed homopolymer layers). We find that L almost-equal-to L(H). When the layers are compressed (D < 2L), the hydrodynamic forces are dominated by flow of solvent through the confined polymer layers in the gap. They may be described by an ''effective viscosity'' which increases as D decreases, varying in fair quantitative accord with a recent model. At the highest compressions (D < L/2) the confined polymer layers behave in a solid-like manner, suggesting that the confinement between the surfaces greatly increases molecular relaxation times.