Surface viscoelastic properties of spread films of a polysilylene-poly(ethylene oxide) multiblock copolymer at the air-water interface
Résumé
The viscoelastic properties of a spread layer of a poly(ethylene oxide)-co-poly(methylphenylsilylene) alternating block copolymer at the air-water interface have been obtained by surface quasi-elastic light scattering over a range of surface concentrations at a fixed capillary wavenumber and as a function of wavenumber for two surface concentrations. The frequency and the damping for a fixed capillary wavenumber showed a maximum at a surface concentration of 0.8 mg m-2, where resonance between the capillary and dilational waves of the surface film occurs. The surface viscoelastic parameters, i.e., surface tension, dilational modulus, and dilational viscosity, were obtained from the heterodyne correlation functions of the scattered light by direct spectral fitting. The surface tension obtained from light scattering data showed the same qualitative dependence on surface concentration as that from the surface pressure data; however, the light scattering values were somewhat larger, indicating the presence of relaxation processes. Attempts to determine the nature of the relaxation process were made by obtaining the surface viscoelastic parameters as a function of surface wavenumber at the surface concentration where resonance between the surface modes is evident. However, the frequency dependence of the surface moduli followed none of the expectations for simple models of the relaxation process. Close analysis of the capillary wave frequency and damping as a function of surface wavenumber indicated that capillary and dilational modes were mixed at the resonance condition. The possibility of a splay mode of aggregated silylene blocks contributing to surface wave dynamics has been discussed.