Interaction between hydrophilic surfaces in triblock copolymer solution

We report on the interactions between fused quartz surfaces immersed in sol utions of low molecular weight nonionic homopolymers and triblock copolymer s. The polymers studied were a poly(ethylene oxide) (PEO) homopolymer and a triblock copolymer of the type poly(ethylene oxide)-poly(tetrahydrofuran)p oly(ethylene oxide). The surface force measurements were performed by the i nterfacial gauge technique. In water, a strong repulsive force was observed at short surface separations. This force has its origin in electrosteric i nteractions imposed by formation of a gel layer on silica. This interaction was totally suppressed when small amounts of polymers were adsorbed or whe n water was substituted for ethanol. Adsorbed polymers induce a long-range steric repulsion. However, in solutions containing homopolymers, when the d istance between the surfaces decreases below 3 nm, the repulsive interactio n 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 surface s. In presence of small amounts of triblock copolymer surface aggregates, w e observed an attractive interaction at intersurface distances smaller than approximately 10 nm. An adhesion between the surfaces can also bee seen du ring separation of the two surfaces in this region. The attraction is cause d by surface aggregates, which bridge the surface-to-surface gap. When adso rption 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 su rfaces.