The subject of this doctoral project was the investigation of the lubrication properties and mechanisms of aqueous solutions of copolymers that form highly solvated brush-like monolayers on sliding surfaces. The brush-like copolymer, poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG), has been used in this thesis. PLL-g-PEG is water soluble and fully biocompatible. PLL-g-PEG shows a unique adsorption behavior on oxide surfaces. The positively charged PLL backbones are electrostatically attracted to negatively charged oxide surfaces, thereby forcing the PEG side chains into a brush-like structure. As shown in this thesis, the brush-like structure allows the fixation of a large number of solvent molecules and, therefore, provides a lubricant film at the tribo-interface. The electrostatic interaction between the PLL backbone and the oxide surface permits rapid re-adsorption, or "self-healing", of the copolymer when the adlayer is detached under tribological stress. This has been demonstrated to offer a clear advantage over covalently bound boundary lubricants (e.g. SAMs) which cannot exhibit such a self-healing effect, at least not on the same time scale. A systematic and controlled variation of the surface properties of PLL-g-PEG is offered through the variation of the polymer architecture. Architectural features of PLL-g-PEG include adjustable parameters such as backbone and side-chain length as well as side-chain density. Instrumental techniques, such as optical waveguide-lightmode spectroscopy (OWLS), ellipsometry, and quartz crystal microbalance with dissipation monitoring (QCM-D) have been employed to examine both the adsorption behavior of PLL-g-PEG on silica substrate surfaces and the amount of solvent absorbed in the PEG brush. The tribological methods comprised of pin-on-disk, mini-traction machine (MTM), ultrathin-film interferometry, and colloidal-probe lateral force microscopy (LFM). The self-healing mechanism of the polymer coating was investigated by the complementary approaches of pin-on-disk tribometry and ex situ fluorescence microscopy. These measurements have demonstrated that the adsorption of PLL-g-PEG polymers onto metal oxide surfaces strikingly reduces the interfacial friction measured in aqueous solutions. The relevant structural features of these copolymers include the properties required to promote adsorption onto the substrate, polymer architecture and solvation at the polymer-liquid interface. Strategies leading to an increase in the surface PEG density, in the brush-height, and in the amount of solvent absorbed in the polymer brush result in a significant improvement in the lubricating properties. The results of the present thesis highlight the opportunity for controlling the tribological properties at interfaces immersed in aqueous solutions through the design and introduction of adsorbed copolymer structures. This thesis provides a comprehensive understanding of the tribological properties and mechanisms of lubrication of surface-bound brush-like copolymers in aqueous environments.
Swiss Federal Institute of Technology Zurich