Reduction of frictional forces between solid surfaces bearing polymer brushes

Abstract

THE use of lubricants to reduce friction and wear between rubbing surfaces has been documented since antiquity^1–3. Recent approaches have focused on boundary lubrication by surfactant-like species coating the surfaces, whereby the friction between them is replaced by the weaker forces required for shear of adhesive contacts between the surfactant layers^3,4. An alternative approach is to tether polymer chains to the surfaces by one end which, when swollen by a solvent, then act as molecular ‘brushes’ that may facilitate sliding. The normal forces between sliding brush-bearing surfaces have been previously investigated^5,6, but the lateral forces, which are the most important from the point of view of lubrication, are harder to measure. Here we report the measurement of lateral forces in such a system. We find a striking reduction in the effective friction coefficients μ_b between the surfaces to below our detection limit (μ_b < 0.001), for contact pressures of around 1 MPa and sliding velocities from zero to 450 nm s⁻¹. We believe that this effect is due to the long-ranged repulsion, of entropic origin, between the brushes, which acts to keep the surfaces apart while maintaining a relatively fluid layer at the interface between them.