Wettability and microstructure of polymer surfaces: stereochemical and conformational aspects

Stereoregular poly(methyl methacrylates) (PMMAs) solvent-cast in the form o f films against a glass substrate were employed as model systems for a syst ematic study of the relationship between the molecular structure las charac terized by the stereochemistry and conformations of macromolecules), the fu nctional-group composition, and the wettability of polymer surfaces. The wa ter wettability of a syndiotactic surface was found to be highly sensitive to the polarity of the adjacent phase in the film-casting process, whereas the wettability of an isotactic surface was invariant to the polarity of th e contacting medium. The tacticity-dependent wetting behavior arises from t he difference in the extent of functional-group surface segregation or, in other words, from the different surface activity of the different tactic ve rsions of the polymer. This difference, in turn, is associated with fundame ntal distinctions in the conformational structures energetically allowed fo r the isotactic and syndiotactic configurations of the polymer chain; the s yndiotactic macromolecule is capable of adopting an amphiphilic surface con formation, whereas the energetically allowed conformational structures of t he isotactic macromolecule do not possess amphiphilic character. In view of these findings, the isotactic surfaces: can be regarded as 'ideal' model s urfaces for research on the fundamentals of wetting phenomena. In addition, there is evidence for failure of the basic assumption of the Cassie approa ch, i.e. the assumption of macroscopic chemical heterogeneity, to describe adequately the wetting behavior of isotactic PMMA surfaces.