Relaxation processes in surface excess layers of aqueous solutions of poly(ethylene oxide)

The dynamic surface behavior of the surface excess film adsorbed at the air-liquid interface of dilute solutions of poly(ethylene oxide) has been investigated using surface quasielastic light scattering over a q range of 200 to 1300 cm(-1). Molecular weights of 15000, 106000 and 828000 in 0.1% w/w solution in water were investigated. Capillary wave frequencies were close to simple predictions based on the density and surface tension of the solutions, whereas the dampings were higher than predicted, due to the extra surface viscosities of the adsorbed polymer. Light scattering data were further analyzed to yield the four surface viscoelastic parameters which are currently used to model the surface behavior-surface tension, gamma(0), transverse shear viscosity, gamma', dilational modulus, epsilon(0), and dilational viscosity, epsilon'-as a function of the capillary wave frequency. Variations in the dilational modulus and viscosity indicated the presence of a relaxation process in the surface film: this was fitted to a model describing the surface relaxation in terms of diffusion between the bulk solution and a subsurface layer. Including an adsorption barrier in the model enabled the quantitative description of the observed variation in both the dilational modulus and the dilational viscosity, and also provided a rationale for the negative dilational viscosities observed.