We succeeded in the simulation of microphase separation in the semi-dilute solution of an ultra-high-molecular-weight block copolymer using a neutral solvent and a differentiating non-solvent. The solutions of polystyrene-block-poly(tert-butyl methacrylate) (PS-b-PtBuMA, the weight average molecular weight Mw is 8.2 × 105g/mol, the volume fraction of polystyrene fPS is 0.43) in a mixture of tetrahydrofuran (THF)/water as a good and neutral solvent and a differentiating non-solvent, respectively, were used. Microphase separation was induced by the addition of water into a disordered block copolymer/THF solution. The phase diagram where microphase separation was induced was indicated by means of spectroscopy using UV-vis spectrometer. The role of water in the semi-dulite solution was revealed by the spatial distribution of the solvents simulated by the self-consistent field theory (SCFT) approach. By increase of water content, the volume fraction of PS phase decreased in order to avoid an increase of enthalpy because of a contact between PS and water. THF and water collectively move into the PtBuMA phase and the ratios of water to THF in both phases are almost identical to each other. This means that the solvent mixture of THF and water behaves like strongly selective solvent as a whole.
