Pf. Dewhurst et al., ORGANIZATION OF DISPERSIONS OF A LINEAR DIBLOCK COPOLYMER OF POLYSTYRENE AND POLY(ETHYLENE OXIDE) AT THE AIR-WATER-INTERFACE, Macromolecules, 31(22), 1998, pp. 7851-7864
The spontaneous adsorption of a highly monodisperse associative block
copolymer, poly(styrene-b-ethylene oxide) from bulk aqueous solution t
o the air-solution interface has been studied using static surface ten
siometry and specular neutron reflectometry. The bulk association beha
vior 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 sc
attering suggest a closed association model for the aggregation and a
hydrodynamic radius of 165 as observed for dispersion concentrations g
reater than 0.1 mg mL(-1). Static surface tension data showed that the
copolymer displayed surface activity even at very low bulk concentrat
ions. Selected concentrations have been investigated in detail, coinci
ding with specific regions of the closed association regime. Neutron r
eflectometry was used to determine the structure normal to the air-wat
er 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 s
ubphase and become detectable by neutron reflectometry. At increasing
concentrations above the CMC ''structuring'' of the polystyrene-rich r
egions occurs within the space explored by the poly(ethylene oxide) ch
ains. The layering of the hydrophobic polystyrene blocks is considered
a result of clustering of spherical micelles in the surface region, t
he onset of which coincides with an apparent phase transition in the s
urface tension variation with hulk concentration data.