Observation of \textit{Fddd} structure in SI diblock copolymer/homopolymer blends

We previously reported the discovery of a novel bicontinuous microdomain structure with \textit{Fddd} symmetry in polystyrene-\textit{block}-polyisoprene (SI) diblock copolymer. Then we confirmed the stability of \textit{Fddd} structure and found \textit{Fddd} structure exists as an equilibrium structure in SI diblock copolymer. Also we determined the stable region of \textit{Fddd} structure in the phase diagram of SI diblock copolymers. It is well known that blending homopolymer with diblock copolymer causes order-order transition under wet brush condition. We can anticipate that homopolymer - diblock copolymer blend exhibits \textit{Fddd} structure even though the neat block copolymer does not have \textit{Fddd} region. In this study, we, hence, investigated the phase behaviors of SI diblock copolymer-PS homopolymer blends with various compositions by small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) and explored how blending homopolymer affects the \textit{Fddd} region. Consequently, we could confirm that addition of polystyrene homopolymer (hPS) induces morphology transformation and determine the region of \textit{Fddd} phase in the SI diblock copolymer/hPS blends that is similar to that of SI diblock copolymers we reported.