Fddd Structure in Polystyrene-block-polyisoprene Diblock Copolymer/Polystyrene Homopolymer Blends

We have induced Fddd structure in polystyrene-block-polyisoprene diblock copolymer (SI) which does not have Fddd phase by adding polystyrene (hPS) homopolymer. We prepared the SI/hPS blend samples in the range of 0.627 < ϕ_PI < 0.646 and investigated the phase boundary of Fddd phase by using small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), where ϕ_PI is volume fraction of polyisoprene (PI) in SI/hPS. We found the Fddd phase as an equilibrium phase in the range of 0.631 ≤ ϕ_PI ≤ 0.641 and 26.6 < χN < 30.7, where χ and N are respectively the Flory–Huggins interaction parameter between PS and PI and polymerization index of the SI. The Fddd phase boundary in the SI/hPS is similar to that in the neat SI block copolymer since the effect of the addition of the hPS with the low molecular weight on the phase diagram corresponds to that of the same change of PS volume fraction in the neat block copolymer.