Long-wavelength dynamics of spread films of poly(methyl methacrylate) and poly(ethylene oxide) at the air/water interface

Abstract

Surface quasi-elastic light scattering data for poly(methyl methacrylate)(PMMA) and poly(ethylene oxide)(PEO) films spread at the air/water interface have been interpreted using simple phenomenological models. PMMA behaves as a Voigt solid, whereas PEO can be described as a Maxwell fluid. However, the surface waves for the spread PEO film exhibit mode mixing at a surface concentration of ca. 0.6 mg m^–2 which is where the polymer penetrates the subphase. Although negative dilational viscosities are observed and the dependence of the normalised damping coefficient of the capillary waves on surface wave number is commensurate with such dilational viscosities being the source of mode mixing, large positive values of the transverse viscosity are also evident. Such large transverse viscosities were originally cited as the cause of mode mixing. However, the dispersion curves of the normalised damping coefficient do not reproduce the experimental observations for the spread PEO films. It is speculated that the negative dilational viscosities may be the result of the current absence of appropriate theory which would lead to a more accurate dispersion equation.