Forces between surfaces bearing terminally anchored polymer chains in good solvents

Abstract

The adsorption of high polymers modifies the short-range forces between surfaces in liquid media and is widely used to stabilize or flocculate colloidal dispersions1. Previous studies have shown that low surface-coverage by the polymer leads to long-range bridging attraction between the surfaces2, whereas repulsion sets in at high adsorbance in good solvents3,4. In addition, marked hysteresis has been observed for both adsorbed homopolymers and co-polymers on approach and rapid separation of the surfaces3–5. Here we report the direct measurement of forces between terminally anchored, non-adsorbing polymer chains (polystyrene) in good solvents (toluene and xylene). The polymer layers are found to extend some 6Rg (where Rg is the unperturbed radius of gyration of the polymer) out from each surface, substantially further out than adsorbing homopolymers. Monotonically increasing repulsive forces are observed as the surfaces approach each other and there is no evidence of bridging attraction at low-surface coverage. In addition we find that the force law is reproducible and independent of compression/decompression rates, in marked contrast with adsorbed homopolymers.