INTERFACIAL SLIDING OF POLYMER-BEARING SURFACES

We have measured simultaneously both the normal forces F-perpendicular to(D) and the shear forces F-parallel to(D) that act between compressed polymer-bearing mica surfaces, a distance D apart, as they slide past each other. We find that for surface-attached polystyrene (PS) brushes in the good solvent toluene the shear forces are extremely weak, over a wide range of shear velocities and at normal loads that correspond to 2L < D < D < 1/6(2L), where L is the height of each brush. Compared with the forces required to slide bare mica surfaces immersed in simple liquids (toluene or cyclohexane) under comparable loads, the brushes reduce the effective friction coefficient (F-parallel to/F-perpendicular to) by two-three orders of magnitude. For mica surfaces bearing adsorbed PS layers in the near-theta-solvent cyclopentane, the forces required to slide the surfaces are very much larger than for the PS brushes in toluene, at similar normal loads and shear velocities. The very different behaviour in the two cases is attributed to the different extents of mutual interpenetration and entanglement of the compressed polymer layers.