Concentration profile near the surface of polymer mixtures: a Monte Carlo study

Abstract

The concentration profile of a symmetrical binary (AB) polymer mixture confined between two ‘neutral’ repulsive walls is studied by Monte Carlo 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 coexisting with each other, in each kind of domain the minority component being enriched at the surface. In contrast to simple Ising lattice gas models of mixtures, the steepest gradient in the volume fraction profile 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) are briefly discussed.