Triblock copolymer lubricant films under shear: Effect of molecular cross-linking
Résumé
The normal interaction and the behavior under shear of mica surfaces covered by a triblock copolymer of poly( L-lysine)-b-polydimethysiloxane-b-poly( L-lysine) ( Plys(40)-b-PDMS40-b-Plys(40)) before and after cross-linking reaction with two dicarboxylic acids were studied, combining the capabilities of the surface forces apparatus and atomic force microscopy (AFM). At low pH values, this copolymer spontaneously adsorbed on the negatively charged mica surfaces from aqueous solutions as a consequence of the positive charge of the polylysine moieties, forming an extremely smooth boundary layer. This smooth layer displays a very small resistance to shear under small loads, exhibiting outstanding lubrication properties. Nevertheless, the fusion of two contacting layers can be induced by compression at a certain pressure, a process that causes a marked increase in the friction forces. Cross-linking of the adsorbed polymer molecules by covalent bond formation with dicarboxylic acids increases the mechanical stability of the adsorbed layers and hinders the fusion of the boundary layers under pressure but impairs its lubrication properties to a certain extent.