Surface Viscoelastic Properties of Spread Films of a Polysilylene−Poly(ethylene oxide) Multiblock Copolymer at the Air−Water Interface

Abstract

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 quasielastic 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.

A viscoelastic regime in dilute hydrophobin monolayers

“…Layers may be more complex than that, and other processes occurring in the adsorbed layer can potentially lead to abnormalities in the determination of the viscoelastic parameters, such as negative surface viscosities. 20,21 Work by Lopez et al 22 showed that the classic hydrodynamic model fails to determine the surface tension and the shear modulus when domains of different viscoelastic properties coexist. This leads to the measurement of apparent negative surface viscosities.…”

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“…Only spread polymer films at the liquid-gas interface have been studied relatively frequently, mainly by means of surface quasi-elastic light scattering (SQELS) (8)(9)(10)(11)(12)(13)(14)(15). This experimental technique usually leads to reasonable values of the surface pressure and the real part of the dynamic surface elasticity, which are comparable with the results of other methods, but the sign of the imaginary component of the surface elasticity proves sometimes to be in contradiction with the second law of thermodynamics (7,10,11,15). Buzza et al showed that an effective two-dimensional negative viscosity could arise if the thickness of the polymer film approaches 1 micrometer (11).…”

“…Buzza et al showed that an effective two-dimensional negative viscosity could arise if the thickness of the polymer film approaches 1 micrometer (11). Such thickness is unreal for most systems and thus the difficulty in determining the surface viscosity probably indicates that the theory of the SQELS method is not completely adequate (4,10,15). The deviation of the transverse surface viscosity from zero in contradiction to the theory is another problem of this method (11).…”

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