Preferential Solvation and Its Effect on the Lubrication Properties of a Surface-Bound, Brushlike Copolymer

Abstract

We have investigated the collapse−stretching transition of a surface-bound, brushlike copolymer, poly(l-lysine)-graft-poly(ethylene glycol) (PLL-g-PEG), and the consequence of such transitions on the frictional properties of this coating. The frictional properties of the interface have been measured by colloidal-probe lateral force microscopy (LFM) in liquid environments on the nanoscale. The collapse−stretching transition has been induced through the systematic variation of the chemical composition of the binary solvent mixture comprised of an aqueous buffer solution and 2-propanol. The influence of solvent composition on the polymer conformation was monitored by comparing measurements conducted with optical waveguide lightmode spectroscopy (OWLS) and quartz crystal microbalance with dissipation monitoring (QCM-D). The combined approach employing QCM-D and OWLS has allowed the quantification of the mass of solvent molecules absorbed in the brushlike structure of PLL-g-PEG and has revealed a significant preferential solvation effect. This study has demonstrated preferential solvation of a surface-bound polymer and the role of such solvation in maintaining the favorable lubricating properties of a PEG brush when exposed to mixtures of good and poor solvents.

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

“…The solid lines in figure 5 show fits to the measured steric forces using equation (4). Only force profiles measured with the surfaces approaching one another were used for the fitting.…”

“…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

“…Several studies have been conducted to understand the effect of solvent quality on the structure and stability of brushes [18,19]. The structural changes and preferential solvation of polymer brushes have been studied in detail [20][21][22] by varying the solvent quality using binary solutions, which contain varying volume fractions of good and bad solvents in the solution. Müller et al [22] studied the frictional properties of adsorbed PLL-g-PEG polymers on silica surfaces using colloidal-probe AFM for binary mixtures of water and 2-propanol.…”

“…The structural changes and preferential solvation of polymer brushes have been studied in detail [20][21][22] by varying the solvent quality using binary solutions, which contain varying volume fractions of good and bad solvents in the solution. Müller et al [22] studied the frictional properties of adsorbed PLL-g-PEG polymers on silica surfaces using colloidal-probe AFM for binary mixtures of water and 2-propanol. They observed little or no variation in the frictional properties of the brushes until the critical volume fraction of / = 0.85 (2-propanol) is reached, beyond which the friction increases remarkably with even a slight increase in the volume fraction of the solvent.…”

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.…”