EFFECT OF SOLVENT QUALITY ON PURE AND MIXED BRUSHES

We investigate, theoretically and experimentally, the effect of solven t quality on pure, one-component, and two-component brushes. Both mean -field and self-consistent field models are developed, and their predi ctions compared to surface forces experiments on polystyrene and mixed polystyrene-polybutadiene brushes. We find that in pure brushes the s tructure and interactions between surfaces are highly sensitive to sol vent quality. In a good solvent, the brush is extended and the interac tions are repulsive, while in a poor solvent the brush is ''collapsed' ' and the interactions are strongly attractive. Mixing two incompatibl e 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 interact ions between surfaces carrying a 1:1 mixed brush of styrene-butadiene are repulsive even in hexane, a poor solvent for polystyrene. Theoreti cally, we calculate that incorporating only 10% of an equi-molecular-w eight chain into a collapsed brush can eliminate attractive interactio ns between surfaces.