Sliding friction between polymer-brush-bearing surfaces: crossover from brush-brush interfacial shear to polymer-substrate slip.

摘要

A model is presented for the shear (or frictional) forces F_s between two surfaces a distance D apart as they slide past each other while bearing mutually compressed polymer brushes, on the assumption that sliding takes place at the brush-brush interface. The predictions of the model for the rapid increase in F_s at increasing compressions are in reasonable agreement with experiments on polystyrene brushes immersed in toluene over two decades in F_s. At higher compressions (smaller D) the experimental shear forces increase only slowly, and diverge from the calculated ones which continue to increase rapidly; at the same time the form of the shear force response at these higher compressions reverts from a viscous-like one to a stick-slip behaviour. These observations strongly indicate that at sufficiently high compressions the plane of slip crosses over from the brush-brush interface to the polymer-solid surface.