Dynamic Friction by Polymer/Surfactant Mixtures Adsorbed on Surfaces

A cationic polyelectrolyte was adsorbed on mica from highly concentrated solutions. The friction and surface force behaviors of the adsorbed layers in aqueous media were studied using a new homemade surface force apparatus (SFA). The long-range repulsions produced by the pure cationic polymer at low salt concentration indicate that the chains are in an extended conformation. The addition of anionic surfactant or of salt condenses the cationic polymer chains as evidenced by the much shorter range of the repulsions. These forces are, for both conformations, a combination of steric and double-layer forces. During sliding, the friction forces produced by the adsorbed layers increase monotonically with the load. A strong dependence of these forces on the sliding speed is noticeable for the extended conformations, while the dependence vanishes in the coiled conformations. This study shows the important role of the conformational state of adsorbed polymer chains on their tribological properties.