Forces between mica surfaces bearing layers of adsorbed polystyrene in cyclohexane

Abstract

Direct measurements of the forces acting between polymer surface phases adsorbed at interfaces have only recently been reported^1–3. These were carried out in good solvents and showed that repulsive forces were acting between the adsorbed layers; these forces increased monotonically as the surfaces approached each other. In certain conditions, however, for example, those leading to the flocculation of sterically stabilized colloidal systems, one expects the surfaces bearing the adsorbed phases to attract before repelling each other^4,5. I have measured the forces acting between two curved mica surfaces immersed in cyclohexane at 24 °C (a worse than θ solvent at this temperature), each bearing a surface layer of adsorbed polystyrene (M_w = 6 × 10⁵). No forces are observed at surface separations larger than about three radii of gyration of the polymer; on closer approach a strong attraction develops between the surfaces, changing to an ultimate repulsion as the surfaces approach closer than about one radius of gyration. Between times of a few minutes and several hours the forces are stable, well behaved and reproducible.