Shear in surface gel of associative polymer

The normal and shear forces between adsorbed layers of telechelic associative polymer poly(ethylene oxide) end-terminated with octadecyl groups have been studied using a surface force balance. The normal forces profiles measured for polymer monolayers and surface gels were similar to those reported for the same polymer in earlier studies. The shear forces between the polymer-covered surfaces were very weak up to compression of ca. 0.27 of the original thickness of the polymer layers, corresponding to the effective friction coefficient mu < 0.04. Weak frictional forces were attributed to osmotic repulsion between poly(ethylene oxide) chains and shear thinning of the gelled polymer. At higher polymer compression the shear forces gradually increased due to compression-forced increase in interpenetration and entanglement of the macromolecules adhering to the two opposite surfaces and interactions between the octadecyl groups. Shear thinning was also observed for long-time shear and for shear velocity v(s) exceeding 600 nm/s. For strongly compressed polymer and v(s) < 600 nm/s the dependence of frictional forces on the shear velocity was weak, caused by a thinning of the interpenetration zone between the sheared polymer layers and/or a transition of the slip plane to the polymer-substrate interface.