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

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 collapsestretching 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 surfacebound 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.