Adsorption Properties of Triblock Copolymers at Solid Surfaces
Abstract: The interfacial behaviour of nonionic triblock copolymers of the type poly(ethylene oxide-tetrahydrofurane-ethylene oxide) has been systematically examined at different types of surfaces. It is shown how the chemical nature of the substrate greatly influences the adsorption behaviour of block copolymers. Adsorption at hydrophilic surfaces resulted in the formation of micellar-like surface aggregates, whereas the same molecules were shown to form a monolayer at hydrophobic surfaces. It is further shown how the adsorbed layer characteristics changes the forces measured between copolymer covered surfaces. Rapidly repeated force curves were found to be perfectly reproducible for all systems studied. The relaxation time of the adsorbed layers is, therefore, relatively short. At hydrophobic surfaces the copolymers exhibit a mixed pancake-to-brush transition as a function of increased concentration. This transition is also reflected in both the adsorption kinetics and the interaction patterns. The main features of the force versus distance curve can be predicted from the adsorption isotherm. The copolymers form surface aggregates when adsorbed at a hydrophilic silica surface. The relatively strong interaction between the PEO blocks and the surface leads to a compositional discrepancy between bulk and surface aggregates. For copolymer samples with high average molecular weights, competitive adsorption between individual high molecular weight copolymers and surface aggregates was observed. Adsorbed molecules of the high molecular weight fraction restricted the formation of surface aggregates. Interaction forces between two copolymer-covered silica surfaces change from attraction to repulsion as a function of increased copolymer concentration. At low concentrations, PEO chains of copolymers tethered in the surface aggregates form bridges between the surfaces from relatively large distances. At higher concentrations, no free space exists at the surface and bridging can not occur, the interaction becomes purely repulsive for all surface-surface separations during both approach and separation.
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