Interaction between hydrophilic surfaces in triblock copolymer solution

We report on the interactions between fused quartz surfaces immersed in solutions of low molecular weight nonionic homopolymers and triblock copolymers. The polymers studied were a poly(ethylene oxide) (PEO) homopolymer and a triblock copolymer of the type poly(ethylene oxide)-poly(tetrahydrofuran)poly(ethylene oxide). The surface force measurements were performed by the interfacial gauge technique. In water, a strong repulsive force was observed at short surface separations. This force has its origin in electrosteric interactions imposed by formation of a gel layer on silica. This interaction was totally suppressed when small amounts of polymers were adsorbed or when water was substituted for ethanol. Adsorbed polymers induce a long-range steric repulsion. However, in solutions containing homopolymers, when the distance between the surfaces decreases below 3 nm, the repulsive interaction levels off. Here the force remains more or less constant with decreasing surface-to-surface distance. Adsorbed polymer molecules are in this region to a large extent expelled from the gap between the two approaching surfaces. In presence of small amounts of triblock copolymer surface aggregates, we observed an attractive interaction at intersurface distances smaller than approximately 10 nm. An adhesion between the surfaces can also bee seen during separation of the two surfaces in this region. The attraction is caused by surface aggregates, which bridge the surface-to-surface gap. When adsorption is increased above a certain value, this attraction vanishes and the interaction becomes purely repulsive at all surface-to-surface distances. When ethanol was used as solvent instead of water, the interaction did not show long-range patterns but the copolymers induced adhesion between the surfaces.