THIN-FILM BEHAVIOR OF POLY(METHYL METHACRYLATES) .8. MONOLAYER BEHAVIOR OF MIXTURES OF POLY(METHYL METHACRYLATE) AND POLY(ISOBUTYL METHACRYLATE)

In view of recent observations of stereocomplexation processes in mixed monolayers of isotactic and syndiotactic poly(methyl methacrylate) (i-PMMA and s-PMMA) monolayers of mixtures of isotactic PMMA and syndiotactic poly(isobutyl methacrylate) (s-PiBMA) were studied in search of an alternative system exhibiting stereocomplexation at the air-water interface. An anomalous transition observed in the isotherms of these mixtures is shown not to correspond to such a stereocomplexation process, but rather to a pressure-induced mixing, which is responsible for a suppression of the crystallization of the i-PMMA component in the monolayer. Miscibility is also observed (even without a required surface pressure buildup) for mixtures of s-PiBMA and s-PMMA. The large effects observed indicate that mixing must occur up to close to the segmental level. The observation that random copolymers of these monomers exhibit smaller effects than homopolymer mixtures suggests that the two-dimensional character of the monolayer, prohibiting random segmental mixing of the homopolymer mixtures, is favorable with respect to the adhesive interactions. For the mixtures of s-PiBMA and i-PMMA, clear molecular weight effects are found, both with respect to the kinetics of the lateral interdiffusion process upon mixing, as well as in terms of the thermodynamics of the mixing process.