ABSTRACT
The behavior of polymers near surfaces and in confined environments has been the subject of many studies, both fundamental and applied [1]. Indeed, it is impor tant to understand this behavior for the control of separation processes (filtration, gel chromatography, osmosis, etc.) and oil-recovery processes (drilling muds with added polymers, viscosity control of injected water), among others. From the fundamental point of view, it is interesting to describe how the polymer coils or the polymer networks are affected by the confinement. When the polymers are confined between two surfaces and when the surfaces are close enough, surface forces arise. Their origin is steric most of the time, caused by the repulsion due
to the overlap of chains adsorbed on each surface. The knowledge of these forces is important to control colloid stability or flocculation. Numerous measurements with the surface-force apparatus (SFA) have been reported in the literature for polymers adsorbed onto mica surfaces [2]. In addition to the usual steric repul sion, small attractive forces have been evidenced when the chains are in a “ poor” solvent. These attractive forces are due to monomer-monomer interactions and, in part, to bridging of the surfaces, when the polymer surface coverage is low. Little is known for the case of nonadsorbed polymers, where depletion layers and depletion forces are expected. Depletion forces have been measured in solu tions of polymer like micelles that do not adsorb at the mica surfaces (coated with surfactant of the same charge sign as that of the micelles) [3]. Oscillatory forces have been recently observed with an atomic force microscope in polyelec trolyte solutions. These forces were attributed to the formation of a crystal of coils stabilized by long-range electrostatic forces [4], although they are more likely related to the polymer network, as will be seen in the following with mea surements on freely suspended polymer films.
