Shear and frictional interactions between adsorbed polymer layers in a good solvent

The forces between layers of poly(ethylene oxide) (PEO), of molecular weights M = 37 x 10(3) (PEO37) and M = 112 x 10(3) (PEO112) adsorbed onto smooth, curved solid (mica) surfaces across the good solvent toluene have been determined using a surface force balance (SFB). The SFB used is capable of measuring both normal interactions F-n(D) as a function of surface separation D and, with extreme sensitivity, shear or frictional forces F-s(D,nu (s)) between them as they slide past each other at velocity nu (s). The F-n(D) profiles are closely similar to those measured in earlier studies between adsorbed PEO layers. The shear or frictional forces between the sliding PEO-bearing surfaces are very low up to moderate compressions of the adsorbed layers (local pressures up to ca. 10(5) N m(-2)), corresponding to effective friction coefficients mu (eff) = (F-s/F-n) of order 0.003 or less. This is attributed to the fluid interfacial layer between the adsorbed layers resulting from their weak mutual interpenetration. At higher loads F-s increases markedly, and two forms of behavior are found depending on the PEO molecular weight. For PEO37, a sharp increase in F-s is followed by removal of polymer from within the intersurface gap during sliding, high friction, and adhesion between the surfaces. For the longer PEO112, the initial increase in F-s and in mu (eff) saturates at the highest loads (for the case of mu (eff) even decreasing), indicating that the slip plane has moved from the polymer/polymer midplane to the polymer/ solid interface. The dependence of F-s on the sliding velocity in the high-friction regime is weak, suggesting that at low compressions there is a thinning of the mutual adsorbed-layer-interpenetration region at high nu (s) that offsets the higher viscous dissipation in that region. At the highest loads, when the slip plane has shifted to the mica surface, the weak F-s(nu (s)) dependence is characteristic of sliding friction at solid substrates.