Surface Properties of Monolayers of Amphiphilic Poly(ethylene oxide)-Poly(styrene oxide) Block Copolymers

The surface properties of poly(styrene oxide)-poly(ethylene oxide) block copolymers EO12SO10, EO10SO10EO10, and EO137SO18EO137 at the air-water (a/w) and chloroform-water (c/w) interfaces have been analyzed by Tracker drop tensiometry, Langmuir film balance, and atomic force microscopy (AFM). The kinetic adsorptions for the block copolymer solutions at both interfaces were determined by the pendant drop technique. At the a/w, the polymer adsorption is reduced as the polymer hydrophobicity increases. Measurements of the interfacial rheological behavior of the copolymers showed that the adsorption layers at the a/w interface manifest obvious solid-like properties in the whole accessible frequency range, whereas a viscous fluid-like behavior is displayed at the c/w interface. The differences observed in the monolayer isotherms obtained for the three copolymers arose from their different hydrophobic/hydrophilic block ratios and block lengths. In this regard, copolymer EO137SO18EO137 displays an adsorption isotherm with the four classical regions (pancake, mushroom, brush, and condensed states), whereas for EO12SO10 and EO10SO10EO10 copolymers, only two regions were observed. The topographic images of copolymer films were obtained by AFM in noncontact mode. Surface circular micelles are observed at the two surface transfer pressures studied. A micelle size decrease and an increase in monolayer thickness are observed with an increase in transfer pressure. At the largest transfer pressure used, elongated aggregates were observed. Aggregation numbers derived from AFM pictures were in fair agreement with those obtained for micelles in solution, and they became larger as the SO weight fraction increases at a certain deposition pressure. This can be a consequence of stronger attractive interactions between the SO blocks to avoid contact with the solvent.