Capillary wave fluctuations of spread films of an amphiphilic graft copolymer at the air-water interface

A graft copolymer with a polymethyl methacrylate backbone and polyethylene oxide grafts was spread at the air-water interface. The polyethylene oxide grafts contained 54 monomer units and were randomly distributed along the backbone. Surface quasi-elastic light scattering was used to determine the frequency and damping of the thermally drive surface waves. Surface visco-elastic moduli were also obtained at the air-water interface from spectral analysis of the correlation functions. The dependence of these parameters on surface concentration of graft copolymer and surface wave frequency exhibits features which are not commensurate with current theory. Dilational waves appear to be destabilized whilst capillary waves become strongly damped. A mixing of the two modes occurs and the frequency behaviour of the damping is symptomatic of this mixing. A qualitative molecular explanation has been attempted using the Alexander-de Gennes brush theory as a model for the organization of the graft copolymer at the air-water interface. It is evident that quantitative description requires development of relevant theory. Copyright (C) 1996 Elsevier Science Ltd.