Effects of adsorption of low-molecular-weight triblock copolymers on interactions between hydrophobic surfaces in water

in this work, we report on the interaction forces between hydrophobed silica surfaces immersed in polymer solutions. The polymers studied were a series of poly(ethylene oxide)-polytetrahydrofuranpoly(ethylene oxide) (PEO-PTHF-PEO) triblock copolymers and a poly(ethylene oxide) homopolymer. The interaction forces were measured by the interfacial gauge technique. We show how the interactions are changed by the adsorbed state of the copolymers. This depends on both the copolymer concentration and the adsorption time. Above a critical surface coverage, the interaction between approaching surfaces at first shows a steric repulsion due to overlap of the adsorbed polymer layers. This repulsion increases as the distance between the surfaces decreases. In this regime the energy-distance curve could be accounted for by the theory of grafted polymer brushes of de Gennes. However, for small surface-to-surface distances the interaction curves do not follow this prediction. Instead, the repulsion stabilized at a more or less constant level with decreasing intersurface separation. Finally, however, hard wall contact was established between the two surfaces. We infer that adsorbed copolymers to a large extent are expelled from the gap between the surfaces in this small surface-to-surface distance range. The force needed to expel copolymers from the intersurface gap was shown to be equal to the surface pressure at the solid-liquid interface. We also studied the influence of the rate of approach and the separation of the surfaces on the energy-distance curves The process of expelling polymers from the surface-to-surface gap was shown to depend on the velocity of the approaching surfaces and the surface coverage. For high approach rates and/or large surface coverages, the lateral mobility of the polymers was such that it inhibited the expulsion process of polymers from the gap. However, rapidly repeated force curves, measured at a constant rate, and successively, were found to be perfectly reproducible.