Effect of Solvent Quality on Pure and Mixed Brushes

We investigate, theoretically and experimentally, the effect of solvent quality on pure, one-component, and two-component brushes. Both mean-field and self-consistent field models are developed, and their predictions compared to surface forces experiments on polystyrene and mixed polystyrene–polybutadiene brushes. We find that in pure brushes the structure and interactions between surfaces are highly sensitive to solvent quality. In a good solvent, the brush is extended and the interactions are repulsive, while in a poor solvent the brush is “collapsed” and the interactions are strongly attractive. Mixing two incompatible chains significantly reduces the sensitivity of the brush to solvent properties. While one of the chains collapses in a selective solvent, the other chain remains solvated, thus providing long-range repulsive interactions between surfaces. Our experiments show that the interactions between surfaces carrying a 1:1 mixed brush of styrene–butadiene are repulsive even in hexane, a poor solvent for polystyrene. Theoretically, we calculate that incorporating only 10% of an equi-molecular-weight chain into a collapsed brush can eliminate attractive interactions between surfaces.