Effects of solvent immiscibility on the phase behavior and microstructural length scales of a diblock copolymer in the presence of two solvents

We employ self-consistent mean-field theory to study the phase behavior and the microstructural sizes of AB diblock copolymers in the presence of a neutral solvent S1 and a slightly B-selective solvent S2. In particular, the effects of copolymer volume fraction phiC, the solvent ratio, and the immiscibility parameter between two solvents chiS1S2, are examined. We find that increasing chiS1S2 not only enlarges the ordered microphase region in the concentrated solutions, but also induces a less concentrated homogeneous solution to form an ordered structure and even undergo a macrophase separation. This is due to the fact that increasing chiS1S2 enhances the preferentially of S1 for A and S2 for B and, thereafter, the effective segregation between A and B. Hence, we observe that the structural length results obtained by varying chiS1S2 resemble a consequence of varying the solvent selectivity in the diblock copolymer solutions when only one solvent is added. For example, when chiS1S2 is small, the domain spacing decreases with decreasing phiC while at larger values of chiS1S2, it first shows a decreasing trend and then an increasing behavior with decreasing phiC.