Morphology of binary mixtures of polystyrene-Mocfe-isoprene) (SI) having different compositions /ps of the polystyrene block in SI and total molecular weights was investigated on the solution-cast film with toluene as a neutrally good solvent. In the range of the composition /ps = 0.35-0.69 covered, the two SI copolymers were found to be totally miscible on the molecular level at all compositions, forming a single microdomain morphology, if their molecular weight ratio is smaller than about 5. They were found to be only partially miscible, if their molecular weight ratio is greater than about 10, forming macroscopically phase-separated coexisting lamellar microdomains with different domain spacings. In the partially miscible pairs, the copolymer with the larger molecular weight was found to solubilize the copolymer with the smaller molecular weight up to about 30 wt %, but the copolymer with the smaller molecular weight was found to hardly solubilize the copolymer with a larger molecular weight. A new but unidentified bicontinuous ordered microdomain structure was found for a miscible pair.
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