Surface visco-elastic modes in a spread film of a block copolymer
A linear diblock copolymer of polymethyl methacrylate and poly-4-vinyl pyridine quaternised with ethyl bromide has been spread as a thin film at the air/water interface and the properties of the capillary waves obtained using surface quasi-elastic light scattering. The data have been analysed for surface visco-elastic parameters on the basis of the complete absence of any transverse shear viscosity in the spread film. A resonance between capillary and dilatational modes was observed at a block copolymer surface concentration of 0.8 mg m-2. At this surface concentration the frequency dependence of the surface tension, dilatational modulus and dilatational viscosity exhibited behaviour which suggested that spread film could be represented as a Maxwell fluid with a relaxation time of ca. 3 µs. This Maxwell fluid model also described the dependence of an ‘apparent’ relaxation time of the dilatational mode on the surface concentration for a capillary wave of fixed wavenumber. A comparison of the observed dispersion behaviour (damping as a function of capillary wave frequency) with that predicted by theoretical forms of the dispersion equation, showed that there was no need to include a postulated coupling factor. This observation concurred with the modest dimensions of the surface region occupied by the quaternised vinyl pyridine blocks.