Phase behavior of styrene-isoprene diblock copolymers in strongly selective solvents

The phase behavior of polystyrene-polyisoprene diblock copolymers diluted with solvents strongly selective for polyisoprene was investigated using variable-temperature small-angle X-ray scattering. Tetradecane was employed as a model selective solvent; its addition induced lyotropic order-order transitions, with as many as five ordered phases found in solutions of one diblock as tetradecane content was varied. The mesophases observed for tetradecane solutions of 10 different diblocks can be partially collapsed into a single two-dimensional map of segregation strength vs overall polystyrene content, as if the selective solvent simply alters the volume fraction of the soluble block. Among strongly selective solvents (tetradecane, tributylamine, squalane), measurable and even large differences in diblock-solvent phase behavior are revealed, in both the locations of lyotropic order-order transitions and the thermotropic order-disorder transition. The addition of squalane to polystyrene-rich diblocks actually elevates the order-disorder transition temperature relative to the diblock melt.