CONCENTRATION PROFILE NEAR-THE-SURFACE OF POLYMER MIXTURES - A MONTE-CARLO STUDY

The concentration profile of a symmetrical binary (AB) polymer mixture confined between two 'neutral' repulsive walls is studied by Monte Ca rlo simulation of the bond fluctuation model, using a chain length N = N-A = N-B = 32 and a distance of D = 20 lattice spacings between the walls. Choosing volume fractions and temperatures such that one stays inside the coexistence curve, one has A-rich and B-rich domains coexis ting with each other, in each kind of domain the minority component be ing enriched at the surface. In contrast to simple Ising lattice gas m odels of mixtures, the steepest gradient in the volume fraction profil e does not occur right at the surface, but a few layers away from the wall. Possible reasons for the observed flatness of this concentration profile near walls (which agrees with recent experimental findings) a re briefly discussed.