Rheology of a Miscible Polymer Blend at the Air-Water Interface. Quasielastic Surface Light Scattering Study and Analysis in Terms of Static and Dynamic Scaling Laws

Monolayers of poly(vinyl acetate) (PVAc) + poly(4-hydroxystyrene) (P4HS) blends on an aqueous subphase (pH = 2.0) have been studied over the whole surface concentration  (0 ≤ Γ ≤ collapse) and blend composition (mole fraction of PVAc x₁ = 0−1) ranges. Besides a classical surface tension technique, a surface light scattering by thermally excited capillary waves spectrometer has been used. The results obtained include the static surface pressure Џand elasticity ε_st, as well as the dynamic dilatational elasticity ε₀(ω) and viscosity κ(ω) as a function of frequency ω. The analysis of the  Π vs Γ curves point out that the aqueous-air interface behaves as a good solvent for the monolayers of PVAc-rich blends, while it is a near- solvent for P4HS-rich blends. The critical exponent v calculated from the static results in the semidilute regime is in excellent agreement with the values obtained from κ(ω) for all the samples. The relaxation times obtained from the dynamic data are compatible with the description of the monolayers as 2-D gel-like systems. In the concentrated regime it is found that ε_st > ε₀, which suggests the existence of diffusive processes out of the plane of the interface.