PROPERTIES OF POLY(ETHYLENE OXIDE)-POLY(BUTYLENE OXIDE) DIBLOCK COPOLYMERS AT THE INTERFACE BETWEEN HYDROPHOBIC SURFACES AND WATER

The interactions between molecules of a low molecular weight diblock c opolymer of poly(ethylene oxide) (E) and poly(butylene oxide) (B), B8E 41, at hydrophobic surfaces were investigated experimentally by using two surface force techniques and ellipsometry. Extended mean-field the ory was employed to describe the adsorption of EB diblock copolymers a t 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 so lution in a ''brushlike'' or at least stretched structure. The adsorpt ion 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 ster ic repulsion increased with increasing bulk copolymer concentration, w hereas the concentration of an inert salt (KBr, up to 0.1 M) did not i nfluence the measured steric interaction. Upon dilution the block copo lymer slowly dissolved, which resulted in a less long-range steric for ce, and under a high force the layers were squeezed out from between t he 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 i nteraction parameters entering the mean-field model were fitted to rep roduce adsorption isotherms of the diblock copolymer and of two triblo ck copolymers of different architectures. Calculations were performed for mondisperse and polydisperse diblock copolymers. The agreement bet ween theory and experiement was improved when the molecular polydisper sity (M-m/M-mu = 1.1) of the sample was taken into account. In particu lar, polydispersity led to predicted adsorption isotherms that are mor e of the high affinity type and more sensitive to low volume-to-surfac e area ratio and to the interaction between surfaces starting at a lon ger separation. Among the polymer components, it is those with the lar gest B block that adsorb preferentially, which leads to an increased a mount adsorbed and forces the E chains to adopt more extended conforma tions.