Surface viscoelastic properties of spread films of a polysilylene-poly(ethylene oxide) multiblock copolymer at the air-water interface

The viscoelastic properties of a spread layer of a poly(ethylene oxide)-co- poly(methylphenylsilylene) alternating block copolymer at the air-water int erface have been obtained by surface quasielastic light scattering over a r ange of surface concentrations ata fixed capillary wavenumber and as a func tion of wavenumber for two surface concentrations. The frequency and the da mping for a fixed capillary wavenumber showed a maximum at a surface concen tration of 0.8 mg m(-2), where resonance between the capillary and dilation al waves of the surface firm occurs. The surface viscoelastic parameters, i .e., surface tension, dilational modulus, and dilational viscosity, were ob tained 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 th at from the surface pressure data; however, the light scattering values wer e somewhat larger, indicating the presence of relaxation processes. Attempt s 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 evid ent. However, the frequency dependence of the surface moduli followed none of the expectations for simple models of the relaxation process. Close anal ysis of the capillary wave frequency and damping as a function of surface w avenumber indicated that capillary and dilational modes were mixed at the r esonance condition. The possibility of a splay mode of aggregated silylene blocks contributing to surface wave dynamics has been discussed.