Immiscibility of large and small symmetric diblock copolymers

We examine binary mixtures of large and small symmetric AB diblock copolymers using self-consistent field theory. In accord with experiment, we find that when the ratio of their polymerization indexes exceeds about 1:5, they become immiscible producing a coexistence between long- and short-period lamellar phases. As in experiment, the short-period phase contains few large diblocks, but the long-period one contains a substantial volume fraction of small diblocks.