Lubrication Properties of a Brushlike Copolymer as a Function of the Amount of Solvent Absorbed within the Brush

Abstract

The shear forces between poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG)-modified SiO2 tribopairs have been measured with colloidal-probe, lateral force microscopy (LFM) and related to the mass of solvent absorbed within the brushlike structure of immobilized PEG chains. The amount of solvent (per unit substrate area) absorbed within the tethered, brushlike polymer, referred to as areal solvation, Ψ, appears to be of importance in determining the lubrication properties of the tethered polymers. In this study, the degree of solvation was varied by choosing different solvents (aqueous buffer solution, methanol, ethanol, and 2-propanol) and was determined by a technique that combines the results of quartz crystal microbalance (QCM-D) experiments and optical waveguide lightmode spectroscopy (OWLS). The highest degree of solvation was measured for aqueous buffer solutions, and a progressive decrease in solvation of PLL-g-PEG was observed in moving from methanol to ethanol to 2-propanol. A concomitant increase in the measured shear force was observed with this decrease in solvation. The lubrication mechanism of the PLL-g-PEG-coated SiO2 tribopair is discussed in terms of solvation and solvent quality and compared with the lubrication mechanism of the corresponding tribopair where only one surface is coated with the polymer brush.

Direct Measurement of Normal and Shear Forces between Surface-Grown Polyelectrolyte Layers

“…Measurements of the frictional forces between neutral polymer brushes have shown them to have remarkable lubricating properties [1][2][3][4] . These properties have been attributed to the very limited interpenetration between opposing polymer brushes even at quite high compressions suggested by some theoretical studies [5][6][7] .…”

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

“…Along with polymer architecture [11], the quality of the solvent surrounding the polymer brush is an important parameter for determining both adsorption kinetics and lubrication properties [16][17][18]. For end-grafted polymers in poor solvents, the cohesive forces between polymer molecules (both inter-and intrachain) or polymers and surface dominate, resulting in a dense collapsed structure of the polymer when adsorbed on the surface (pancake structure).…”

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.…”