Impact of solvation on equilibrium conformation of polymer brushes in solvent mixtures

We have investigated the response of dextran brushes in aqueous dimethyl sulfoxide (DMSO) mixtures under normal compression using the extended surface forces apparatus. The hydrophilic, tethered polymer chains extend significantly in the studied aqueous mixtures and two brush-covered surfaces exhibit repulsive forces upon compression against each other; these attributes are relevant for both anti-biofouling and lubrication properties. An estimation of the height of the dextran brushes with varying solvent quality is made from measurements of the onset of the repulsion: the brush swells significantly in pure water, a decrease in swelling being observed with an increase in DMSO concentration (up to similar to 50 vol%), followed by a more pronounced swelling at higher DMSO concentrations. This behavior is in qualitative agreement with the solubility of bulk dextran polymer in aqueous DMSO solutions. Besides the conformational changes, an increase in the compressibility of the dextran brushes upon increasing the DMSO concentration was observed and explained by the change in the solvation structure of the polymer brush. We describe the equilibrium configuration of the polymer brush under marginal/good solvent conditions with the help of a mean-field approach, considering both solvent quality and a solvation-structure-dependent bond length. This work shows that an appropriate selection of polymer brush and solvent mixture can be used to tune the conformational properties of the polymer brushes at the interface, as well as the consequent repulsive and frictional forces.