Macro-Tribological Studies of Polymer PLL-g-PEG in Aqueous Lubrication

This work aims to investigate the effects of poly(L-lysine)-g-poly(ethylene glycol) (hereinafter referred to as PLL-g-PEG) on friction and wear conducted on ball-3 plates friction testing machine, and the influences on film formation characteristics in aqueoussenvironment using a ball-disc film measuring apparatus with nanometer resolution based on optical interferometry principle, when PLL-g-PEG was added in HEPES solution. The experimental results show that PLL-g-PEG can reduce friction and wear, mainly due to a layer of hydration with" brushed”structure formed on the rubbing surfaces graftedby polymer PLL-g-PEG; andthe improvement of tribological performance mainly depended on contact stress, entrainment speed and the substrate materials. The " self-healing” behavior of PLL-g-PEG polymer brushwas reported during aqueous lubrication of a tribosystem, which meansthat PLL-g-PEG polymer brush was continuously damaged by peeled off from the surface due to rubbing,at the same time they may quickly adsorb onto the rubbing surface, thereby it can be well explained why PLL-g-PEG polymer brush can effectively improve the tribologicalproperties.It was demonstrated bytheopticalinterference results that a layer of hydrated layer, which was formedassisted by polymer brush with limitedload capacity, may appear in the gap between the steel ball and glass disk grafted by polymer brush by adding PLL-g-PEG in HEPES solution. Although this projective filmis very thin, it can effectively apart from the two rubbing interface in the contact area and then improve the lubrication properties. This research work provides the necessary support for understanding the film forming characteristics and mechanism of water-based lubrication.