AN XPS STUDY OF THE SURFACE-BULK COMPOSITIONAL DIFFERENCES IN SILOXANE-CONTAINING BLOCK COPOLYMERS AND POLYMER BLENDS

X-ray photoelectron spectroscopy (XPS) is used to compare the surface behavior in various binary systems of poly(dimethylsiloxane) with poly(bisphenol A sulfone) and poly(bisphenol A carbonate). All the systems studied show a pronounced surface enrichment in siloxane. At a fixed siloxane concentration in the bulk the lowest extent of surface enrichment is observed in the neat copolymers, next go the blends of copolymers in homopolymers, and the highest extent of surface enrichment is characteristic of the blends of homopolymers. The blends of two copolymers show an unexpected surface behavior: The addition of minor amounts of a siloxane-rich copolymer to another copolymer possessing a much lower siloxane content decreases (rather than increases) the surface siloxane concentration of the latter. A microscopic model is suggested to explain the observed surface behavior. The model involves the formation of a "quasi-two-dimensional" overlayer of additive's macromolecules on the blend surface, with the macromolecules oriented preferentially parallel to the sample surface.