Phase behavior and microstructural length scales of a diblock copolymer in the presence of a selective solvent

We employ self-consistent mean-field (SCMF) theory in studying the phase behavior as well as the microstructural domain sizes for a diblock copolymer in the presence of a selective solvent. First we examine the effects of solvent addition on the formation of fcc and bcc packed spheres. As has been found in experiments, the so-called “normal” spheres, i.e., formed by the minority blocks, tend to pack into the bcc array, while the “inverted” spheres formed by the majority blocks favor the fcc packing. Upon increasing the solvent selectivity and/or solvent amount, the formed inverted spheres tend to pack from bcc to fcc. This thermotropic transition of bcc → fcc upon increasing the solvent selectivity is induced by the fact that the intermicellar interactions vary from long-range to short-range via a combination of the solvent exclusion from the cores and an increase in the aggregation number. In analyzing the effects of solvent addition on the microstructural sizes, the SCMF results have successfully captured the crossover behavior of characteristic domain spacing from decreasing with added solvent to increasing by increasing the solvent selectivity. Further, the variation of the characteristic domain spacing when the systems transform to a more curved structure changes from a discontinuous decreasing behavior to even a discontinuous increasing behavior upon increasing the solvent selectivity and/or the formation of inverted structures.