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 × 10³ (PEO37) and M = 112 × 10³ (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,ν_s) between them as they slide past each other at velocity ν_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. 10⁵ N m^-2), corresponding to effective friction coefficients μ_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 μ_eff saturates at the highest loads (for the case of μ_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 ν_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(ν_s) dependence is characteristic of sliding friction at solid substrates.