Rheology and Friction of Compressed Polymer Layers Adsorbed on Solid Surfaces

(Received 9 June 1995, received in final form 29 September 1995, accepted 3 October 1995)

Abstract
The rheology and the friction of adsorbed layers of poly-isoprene between two cobalt surfaces were investigated with a recently developed molecular tribometer. Each poly-isoprene layer is obtained by adsorption of the polymer from a semi-dilute solution of cis 1-4 poly- isoprene in 2,4, dicyclohexyl-2-methylpentane, which is a small hydrocarbon molecule and a good solvent of the poly-isoprene at 23 °C. The aim of these experiments is the mechanical characterizations of the compressed film formed by the contact of the two adsorbed layers, under both normal approach and sliding conditions. The rheological behaviour of adsorbed layers is studied with the normal approach of a smooth sphere on a plane. An "hydrodynamic layer" is detected on each surface, whose thickness is smaller than the thickness of each polymer layer adsorbed on the cobalt surface. During the compression process, the solvent molecules are repelled from the polymer network. When the separation distance becomes very small, the layers are formed by a compressed polymer "mesh" not connected. The mean "mesh" size is lower than the one corresponding to the "rubber" plateau of a poly-isoprene melt. During friction testing, the film thickness was accurately measured by variations of the sphere-plane capacitance. The film thickness variations follow those of the friction force and is the sum of two contributions. One is a thickness decrease due to creep of the layers themselves. Another is a very small increase of the interfacial thickness between the two layers, which was found to be dependent of the sliding speed. A "pinning" regime, where the application of a shear results in ordered polymer chains and reduced friction, is found for high pressure and low speed. The "pinning " regime corresponds to the macroscopic "static" friction regime.