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

Abstract

We have investigated the tribological properties of surfaces with adsorbed poly(L-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG) sliding in aqueous glycerol solutions under different lubrication regimes. Glycerol is a polar, biocompatible liquid with a significantly higher viscosity than that of water. Macrotribological performance was investigated by means of pin-on-disk and mini-tractionmachine measurements in glycerol-PLL-g-PEG-aqueous buffer mixtures of varying compositions. Adsorption studies of PLL-g-PEG from these mixtures were conducted with the quartz-crystal-microbalance technique. The enhanced viscosity of the glycerol-containing lubricant reduces the coefficient of friction due to increased hydrodynamic forces, leading to a more effective separation of the sliding partners, while the presence of hydrated polymer brushes at the interface leads to an entropically driven repulsion, which also helps mitigate direct asperity-asperity contact between the solid surfaces under boundary-lubrication conditions. The combination of polymer layers on surfaces with aqueous phases of enhanced viscosity thus enables the friction to be reduced by several orders of magnitude, compared to the behavior of pure water, over a large range of sliding speeds. The individual contributions of the polymer and the aqueous glycerol solutions in reducing the friction have been studied across different lubrication regimes.

Mesh Size Control of Polymer Fluctuation Lubrication in Gemini Hydrogels

“…In 2011, Nomura et al used an AFM-colloidal probe technique to show a transition in friction coefficient from µ < 0.001 at low shear rates (1-10 µm s -1 ) to µ ~ 0.01 at higher shear rates (10 3 µm s -1 ) between polystyrene brushes in toluene [52]. However, when the polymer brush interface is measured in a macro-tribometer, the resulting lubrication curve resembles the classic Stribeck curve at speeds above 1 mm s -1 [53]. Furthermore, at mN normal loads, the contact pressure at the polymer brush interface is orders of magnitude greater than the Gemini hydrogel interface, which provides an important point of distinction between the two materials [51].…”

Influence of Temperature on the Frictional Properties of Water-Lubricated Surfaces

“…Previous studies of aqueous lubrication of soft contacts involving PDMS showed clear transition of µ to increasing trend with increasing speed in high-speed regime [29,[36][37][38]. However, the base fluids in those studies were mixtures of water and corn syrup [36,37] or glycerol [38], and the viscosities of the fluids were a few orders of magnitude higher than that of water. Even with fluids with much higher viscosity, the transition to fluid-film lubrication was reported to occur from λ is ca.…”

Friction, wear and mechanical behavior of nano-objects on the nanoscale

“…They have been utilized in friction and wear studies as they are simple liquids without additives and their properties are well known. For glycerol, studies have been performed on the macroscale [86]. Dodecane, used with ZnS nanorod additives [25,29,33,46] and Au nano-objects dispersed in water [24,25,33], have resulted in reduced friction and wear on the micro/nanoscale.…”