FdddPhase Boundary of Polystyrene-block-polyisoprene Diblock Copolymer Melts in the Polystyrene-Rich Region

We investigated the Fddd phase in the polystyrene (PS) rich region or f PI < 0.5 of the phase diagram for polystyrene- block -polyisoprene (SI) diblock copolymers by using small-angle X-ray scattering and transmission electron microscope, where f PI is a volume fraction of polyisoprene (PI) in a SI diblock copolymer. Five SI diblock copolymers which have different f PI values were prepared, in the range of 0.369 < f PI < 0.392 by living anionic polymerization. We found the Fddd phase as an equilibrium phase in the range of 0.370 ≤ f PI ≤ 0.373 and 19.5 < χ N < 21.1 where χ and N are, respectively, the Flory–Huggins interaction parameter between PS and PI, and polymerization index of the diblock copolymer. The Fddd phase boundary in PS-rich region is much narrower than that in PI-rich region or f PI > 0.5 in terms of f PI and χ N . The asymmetry of the Fddd region between PS-rich and PI-rich region can be explained by the conformational asymmetry between PS and PI chains.