ABSTRACT
The surface grafting of polymers in a brush-like conformation is probably one of the most extensively studied approaches to impart lubricity to materials in an aqueous environment. Despite an extremely effective lubricating performance, often reported from controlled tribosystems under mild contacts, engineering applications of this approach have not gained sufficient popularity to date. Polyelectrolytes have shown superior lubricity compared to neutral polymers and may be taken as polymers of choice to enhance the pressure responsiveness of water. Nevertheless, due to the electrostatic interaction with (oppositely) charged substrate surfaces or charge accumulation problems on nonpolar substrate surfaces, it is not straightforward to employ polyelectrolytes as free, easily blending lubricant additives in a base lubricant, water. Several alternative approaches to using polyelectrolytes for aqueous lubrication have recently emerged, such as the use of a polyelectrolyte multilayer (PEM), polyelectrolyte complexation (PEC), and crosslinked systems, which are commonly constructed based on the high reactivity of polyelectrolytes. The resulting polymer coatings display generally higher frictional forces than those in brush conformation, but provide substantially improved antiwear properties, and thus may better fit engineering tribology.
