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 compress ed polymer-bearing mica surfaces, a distance D apart, as they slide pa st each other. We find that for surface-attached polystyrene (PS) brus hes in the good solvent toluene the shear forces are extremely weak, o ver a wide range of shear velocities and at normal loads that correspo nd to 2L < D < D < 1/6(2L), where L is the height of each brush. Compa red with the forces required to slide bare mica surfaces immersed in s imple liquids (toluene or cyclohexane) under comparable loads, the bru shes reduce the effective friction coefficient (F-parallel to/F-perpen dicular to) by two-three orders of magnitude. For mica surfaces bearin g adsorbed PS layers in the near-theta-solvent cyclopentane, the force s 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 differe nt extents of mutual interpenetration and entanglement of the compress ed polymer layers.