THIN-FILMS OF A BINARY POLYMER SYSTEM - POLY(VINYL ACETATE) POLY(DIMETHYLSILOXANE) LAYERS AT THE AIR-WATER-INTERFACE

The surface quasi-elastic light scattering technique (SLS) in parallel with the Wilhelmy plate technique has been used to examine the surface pressure isotherms and surface film viscoelastic properties of poly(vinyl acetate) (PVAc), poly(dimethylsiloxane) (PDMS), and mixtures of the two polymers at various compositions. All systems form stable insoluble films at the air/water interface. The isotherms of the binary system show strong negative deviations from Goodrich's ''area additivity rule,'' but follow the two-dimensional analog of Dalton's law of partial pressures for an ideal gas mixture (''surface pressure additivity rule''), which we used as a fitting function, very closely. All isotherms of the binary system appear to have the same composition-independent maximum collapse pressure far above the collapse pressures of either one-component film. The systems with high PDMS content show an additional plateau at intermediate surface pressures. The surface elasticities, as determined by SLS, are equal or smaller, and the corresponding viscosities are equal or higher, than the values calculated according to the two-dimensional analog of Dalton's law, applied to elasticity and viscosity. We interpret these results in terms of a film structure that deviates from a simple ''flat'' monolayer by forming a bilayer structure even at surface pressures well below the PDMS collapse pressure. This model explains the negative deviations from the area additivity without assuming extraordinarily strong polymer-polymer interactions.