Organization of dispersions of a linear diblock copolymer of polystyrene and poly(ethylene oxide) at the air-water interface

The spontaneous adsorption of a highly monodisperse associative block copolymer, poly(styrene-b-ethylene oxide) from bulk aqueous solution to the air-solution interface has been studied using static surface tensiometry and specular neutron reflectometry. The bulk association behavior of this polymer has been examined using light scattering, and the critical micelle concentration (CMC) was found to be (3.5 +/- 0.4) x 10(-5) g mL(-1). Hydrodynamic radii obtained by quasi-elastic light scattering suggest a closed association model for the aggregation and a hydrodynamic radius of 165 as observed for dispersion concentrations greater than 0.1 mg mL(-1). Static surface tension data showed that the copolymer displayed surface activity even at very low bulk concentrations. Selected concentrations have been investigated in detail, coinciding with specific regions of the closed association regime. Neutron reflectometry was used to determine the structure normal to the air-water interface at these selected concentrations. Well-below the CMC, the copolymer is in a self-similar adsorbed layer, which we attribute to surface micellization. At the CMC the surface excess of the copolymer is increased and the ethylene oxide segments explore deeper into the subphase and become detectable by neutron reflectometry. At increasing concentrations above the CMC "structuring'' of the polystyrene-rich regions occurs within the space explored by the poly(ethylene oxide) chains. The layering of the hydrophobic polystyrene blocks is considered a result of clustering of spherical micelles in the surface region, the onset of which coincides with an apparent phase transition in the surface tension variation with hulk concentration data.