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Abstract

In this work, we have explored the application of poly(L-lysine)-g-poly(ethylene glycol) (PLL-g-PEG) as an additive to improve the lubricating properties of water for metal-oxide-based tribo-systems. The adsorption behavior of the polymer onto both silicon oxide and iron oxide has been characterized by optical waveguide lightmode spectroscopy (OWLS). Several tribological approaches, including ultra-thin-film interferometry, the mini traction machine (MTM), and pin-on-disk tribometry, have been employed to characterize the frictional properties of the oxide tribo-systems in various contact regimes. The polymer appears to form a protective layer on the tribological interface in aqueous buffer solution and improves both the load-carrying and boundarylayer-lubrication properties of water.

Macrotribological Studies of Poly(L-lysine)-graft-Poly(ethylene glycol) in Aqueous Glycerol Mixtures

“…The coefficient of friction values in the absence of polymer appear to converge at 150 mm/s irrespective of the viscosity of the operating fluid, suggesting that asperity contacts between surfaces are no longer occurring; the frictional forces originate only from viscous dissipation within the fluid film, and thus the coefficient of friction at the interface diminishes to a very low value. PLL-g-PEG, when adsorbed from HEPES onto glass and steel surfaces, has been observed to reduce the coefficient of friction at the sliding interface [12]. The adsorption of PLL-g-PEG polymers from viscous HEPESglycerol solutions has been observed by means of the QCM technique (Fig.…”

“…Adsorption of the poly(L-lysine)-graftpoly(ethylene glycol) (PLL-g-PEG) copolymer has been extensively used [10,11] as a facile approach for the attachment of water-compatible polymer brushes to surfaces. Studies have been conducted to understand the lubrication properties of these polymer brushes both at macroscopic [11,12] and at nanoscopic scales [13,14] in aqueous environments. PLL-g-PEG contains a positively charged polypeptide backbone that adsorbs spontaneously via electrostatic interactions onto several metal oxide surfaces, such as TiO 2 , Nb 2 O 5 , and SiO 2 at neutral pH.…”

Aqueous Lubrication of SiC and Si3N4 Ceramics Aided by a Brush-like Copolymer Additive, Poly(l-lysine)-graft-poly(ethylene glycol)

“…Past approaches have included modifying the surface charge by adjusting the pH of the lubricant [16] or using additives such as ionic liquids [17]. An alternative, promising approach for aqueous lubrication of ceramics is to use water-soluble brush-like copolymers, which have recently been applied to several oxide-based tribosystems [18][19][20][21][22][23][24][25][26][27][28][29][30]. Poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) has a polycationic backbone that adsorbs onto negatively charged surfaces, such as SiO 2 under neutral pH conditions.…”

“…Under certain types of abrasive tribocontact, such as sliding contact in the presence of macroscopic asperities, PLL-g-PEG performs best when an excess amount of copolymer is present in the base aqueous lubricant. Although the copolymer layer is easily rubbed away during tribocontact, due to its relatively weak but reversible electrostatic attachment, the excess copolymer in the vicinity of the surface can rapidly readsorb onto the surface due to fast surface adsorption kinetics, reforming, or ''self-healing'' the lubricating brush film [18][19][20][22][23][24][25][26][27][28][29].…”

“…Past work on the adsorption [18-20, 24, 32-35] and tribological properties [18][19][20][22][23][24][25][26][27][28][29] of PLL-g-PEG on oxide surfaces, especially SiO 2 , suggest that PLL-g-PEG is a promising candidate as an aqueous lubricant additive. To investigate the possible benefits of PLL-g-PEG as an aqueous lubricant additive, adsorption tests using ellipsometry (ELM) and X-ray photoelectron spectroscopy, as well as tribological tests at a variety of speeds, using a pin-on-disc tribometer, have been carried out.…”