Numéro |
J. Phys. II France
Volume 6, Numéro 1, January 1996
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Page(s) | 57 - 76 | |
DOI | https://doi.org/10.1051/jp2:1996102 |
J. Phys. II France 6 (1996) 57-76
Rheology and Friction of Compressed Polymer Layers Adsorbed on Solid Surfaces
Jean-Marie Georges, André Tonck, Jean-Luc Loubet, Denis Mazuyer, Etienne Georges and François SidoroffLaboratoire de Tribologie et de Dynamique des Système U.R.A. C.N.R.S 855, Ecole Centrale de Lyon, 69131 Ecully Cedex, France
(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.
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