Capillary wave phenomena at the air interface of aqueous dispersions of a linear polystyrene-poly(ethylene oxide) diblock copolymer

Aqueous dispersions of a linear diblock copolymer of polystyrene and poly(ethylene oxide) that forms well-defined micelles have been investigated by surface light scattering. The concentrations examined range from below the critical micelle concentration (3 x 10(-5) g mL(-1)) to well, above it (1 x 10(-4) g mL(-1)). Correlation functions of the scattered light have been analyzed to provide capillary wave frequency and damping, surface tension, dilational modulus, and dilational viscosity. The surface tension gradually decreases with time for all dispersion concentrations, but after 24 h all have the same surface tension. This is interpreted to mean that the surface concentrations of each dispersion are identical. The equilibrium dilational moduli increase as the bulk concentration increases, and this is attributed to differences in the subsurface stratified layer structure. From the frequency dependence of real and imaginary dilational moduli, the mechanism suggested for the approach to equilibrium is the breakdown of micelles followed by the adsorption of individual copolymer molecules at the surface after diffusing through the brushlike layer of poly(ethylene oxide) at the surface and then the re-formation of aggregated structures on the surface.