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

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, ¾, 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.