Determination of\textit{ Fddd} phase boundary in polystyrene-\textit{block}-polyisoprene diblock copolymer

We previously reported the discovery of a novel bicontinuous microdomain structure with \textit{Fddd} symmetry in polystyrene-\textit{block}-polyisoprene (SI) diblock copolymer. In this study, we investigated the phase behavior of eight SI diblock copolymer samples having slightly different compositions (0.627$\le f_{PI} \quad \le $0.653) by SAXS and TEM to determine the phase boundary of the \textit{Fddd} structure in the phase diagram of SI. The SI having the lowest $f_{PI}$ (= 0.627) showed only disorder-lamella (L) transition but no order-order transition. The SI having the largest $f_{PI}$ (= 0.653) showed disorder-gyroid (G)-L transition with decreasing temperature, but did not show \textit{Fddd} phase. The other six SI samples having $f_{PI}$ values between these two exhibited disorder-G-\textit{Fddd}-L transition with decreasing temperature. Consequently, we could determine the compositional region where \textit{Fddd} phase is thermally stable, which is in good agreement with that predicted by SCFT.