Tribological behavior of ionic polymer brushes in aqueous environment

The surface-tethering of polymers has become a widely used method for improving the surface physicochemical properties, such as wettability, adhesion, and friction, of solid surfaces. An assembly of polymer chains end-grafted to a solid surface at a sufficiently high graft density in a good solvent is generally referred to as a “polymer brush” [1,2]. The graft density is the number of tethered chains at the surfaces per unit area, which largely depends on the preparation process, such as “grafting-to” or “grafting-from.” Over the last decade, various types of well-defined, high-density polymer brushes have been prepared via surface-initiated controlled radical polymerization (CRP). Such brushes are grown from surface initiating sites that are immobilized on solid surfaces or substrates through covalent bonding. As a result, the brush chains are strongly anchored and are hardly detached from the substrate, even in a good solvent as well as under large shear deformation. Therefore, the polymer brushes with nanometer-scale thickness can act as an efficient lubricant in friction.