Properties of Poly(ethylene oxide)−Poly(butylene oxide) Diblock Copolymers at the Interface between Hydrophobic Surfaces and Water

Abstract

The interactions between molecules of a low molecular weight diblock copolymer of poly(ethylene oxide) (E) and poly(butylene oxide) (B), B8E41, at hydrophobic surfaces were investigated experimentally by using two surface force techniques and ellipsometry. Extended mean-field theory was employed to describe the adsorption of EB diblock copolymers at planar surfaces as well as the forces between surfaces with adsorbed diblock copolymers. It is the hydrophobic poly(butylene oxide) block that anchors the diblock copolymer at the hydrophobic surface with the water-soluble poly(ethylene oxide) block protruding in the aqueous solution in a “brushlike” or at least stretched structure. The adsorption kinetics demonstrate that two adsorption regimes exist, one which is transport-limited and the other at higher adsorption where a slower branch due to crowding effects at the surface exists. Only monotonic repulsive steric forces between the diblock copolymer-coated surfaces were observed in the surface force measurements. The range of the steric repulsion increased with increasing bulk copolymer concentration, whereas the concentration of an inert salt (KBr, up to 0.1 M) did not influence the measured steric interaction. Upon dilution the block copolymer slowly dissolved, which resulted in a less long-range steric force, and under a high force the layers were squeezed out from between the surfaces. The adsorbed layer thickness obtained in the experiments varied with solution volume-to-surface area ratio. This is interpreted as being caused by the polydispersity of the diblock copolymer. The interaction parameters entering the mean-field model were fitted to reproduce adsorption isotherms of the diblock copolymer and of two triblock copolymers of different architectures. Calculations were performed for mondisperse and polydisperse diblock copolymers. The agreement between theory and experiement was improved when the molecular polydispersity (Mm/Mn = 1.1) of the sample was taken into account. In particular, polydispersity led to predicted adsorption isotherms that are more of the high affinity type and more sensitive to low volume-to-surface area ratio and to the interaction between surfaces starting at a longer separation. Among the polymer components, it is those with the largest B block that adsorb preferentially, which leads to an increased amount adsorbed and forces the E chains to adopt more extended conformations.