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) bl ends on an aqueous subphase (pH = 2.0) have been studied over the whole sur face concentration (0 less than or equal to Gamma less than or equal to col lapse) and blend composition (mole fraction of PVAc x(1) = 0-1) ranges. Bes ides a classical surface tension technique, a surface light scattering by t hermally excited capillary waves spectrometer has been used. The results ob tained include the static surface pressure Pi and elasticity epsilon(st), a s well as the dynamic dilatational elasticity epsilon(0)(omega) and viscosi ty kappa(omega) as a function of frequency omega. The analysis of the Pi vs Gamma curves point out that the aqueous-air interface behaves as a good so lvent for the monolayers of PVAc-rich blends, while it is a near-Theta solv ent for P4HS-rich blends. The critical exponent nu calculated from the stat ic results in the semidilute regime is in excellent agreement with the valu es obtained from kappa(omega) for all the samples. The relaxation times obt ained from the dynamic data are compatible with the description of the mono layers as 2-D gel-like systems. In the concentrated regime it is found that epsilon(st) > epsilon(0), which suggests the existence of diffusive proces ses out of the plane of the interface.