EFFECT OF ARCHITECTURE IN THE SURFACE SEGREGATION OF POLYMER BLENDS

We examine the surface segregation behavior of binary polymer blends w ith architecturally asymmetric components. Particular attention is giv en to molten blends containing linear and branched polymers composed o f the same type of monomer. If the branches are sufficiently long and the branch points (joints) dilute, we argue that the surface enrichmen t behavior has universal features that are independent of the chemical identity of monomers. We identify entropic and enthalpic contribution s to the surface potentials acting on end, joint, and middle monomers of each species and show that these can be incorporated into a linear response formalism for the prediction of surface enrichment. Applicati ons to linear-comb and linear-star mixtures are explicitly demonstrate d, and scaling relations for the integrated surface enrichment and enr ichment length scale are identified. We also identify a topological. e nrichment mechanism for blends that contain loops, e.g. linear-ring mi xtures. Surprisingly, a dilute amount of ring polymer added to a linea r melt is found at a surface at precisely twice the bulk concentration , independent of ring length. This is argued to be a natural consequen ce of the reflecting boundary conditions that are imposed on polymer p ropagators for nearly incompressible melts at impenetrable surfaces.