Surface segregation in stereochemically asymmetric polymer blends

The surface composition of binary blends of isotactic and syndiotactic poly(methyl methacrylate) (i-PMMA/s-PMMA blends) solvent-cast in the form of films on glass substrates has been studied by attenuated total reflection Fourier transform (ATR-FTIR) spectroscopy and contact angle measurements. We find that the syndiotactic macromolecules segregate both to the air-side and to the glass-side surface of the blend films, expelling their isotactic counterparts. The observed surface enrichment is interpreted in terms of the difference in the surface activity of the two tactic forms of PMMA, which was established in our earlier paper. Due to the higher surface activity of the syndiotactic polymer, the s-PMMA macromolecules preferentially adsorb at the polymer-air and polymer-glass interfaces during casting of the blend films, in a drive to minimize the interfacial free energy. The blends studied can be related as stereochemically asymmetric blends, that is, blends whose components are composed of the same type of monomer but differ in the configurational main-chain stereochemistry. It is postulated that the phenomenon of surface segregation observed with the i-PMMA/s-PMMA blends may represent a universal feature of stereochemically asymmetric polymer blends.