Shear behavior of adsorbed poly(ethylene oxide) layers in aqueous media

We use a surface force balance to study shear interactions between adsorbed poly(ethylene oxide) layers (M_w = 150 000 or 170 000) adsorbed onto mica in 0.1 M KNO₃ aqueous solution. The shear forces increase with increasing compression of the layers or with increasing shear rates (at a constant compression), though at high compressions or shear rates the shear stress σ_s appears to saturate. This behavior is attributed to frictional effects that are dominated by viscous dissipation (and possible weak monomer-monomer complexing) between the mutually sliding layers at low compressions or shear rates, and by a substrate-slip mechanism at the highest compressions and shear rates. The shear behavior of adsorbed PEO layers in aqueous electrolyte at the higher compressions or shear rates can be well understood in terms of the attachment mechanism of PEO segments, via ion ligands to the charged substrate (as elucidated earlier, J. Am. Chem. Soc. 2005, 127, 1104).