The local dynamics of water and its effect on the segmental mobility in the amorphous phase of polyamide 6,6 have been investigated. At 300 K and below, where the polymer segments undergo no significant motion (displacements below 2 Å, no reorientation) in the time scale of the simulation, water molecules exhibit mostly orientational freedom, and a bimodal distribution of reorientation times is observed. As the temperature is increased, an increasing fraction of the water molecules also experience hopping behavior between polyamide 6,6 cavities, but true diffusion on a nanosecond time scale is observed only above 400 K. The analysis of amide and methylene group reorientation clearly shows lubrication of the intermolecular amide−amide hydrogen bond, detected above 350 K only at the time scale of 2 ns. This phenomenon goes beyond a simple free volume effect, as is shown through the analysis of the local hydration of individual amide groups. The motion of inner methylene segments is also affected by water, but in a more limited extent. The enhancement of their mobility with water content is probably an indirect effect of the lower reorientation times of neighboring amide groups.

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.