Stability of the Fddd Phase in Diblock Copolymer Melts

The stability of the Fddd phase as an equilibrium phase in diblock copolymer melts was examined by using small-angle X-ray scattering and transmission electron microscopy. After 2 days of annealing at 150 °C where Fddd was found in a previous study (Takenaka et al. Macromolecules 2007, 40, 4399) for the poly(styrene- block -isoprene) (S−I), the Fddd structure still survived. The thermoreversibility in the order−order transitions (OOTs) between lamella (L) and Fddd and between gyroid (G) and Fddd was investigated. The long-time annealing at 150 °C induced the transformation from L and G to Fddd, indicating that Fddd is more stable than L and G at 150 °C. Fddd transformed into L and G, respectively, by annealing at 130 and 170 °C. These results supported that the OOTs between L and Fddd and between G and Fddd are thermoreversible. The stability of the Fddd structure after 2 days of annealing and the confirmation of the thermoreversibility in OOTs clarified that the Fddd phase exists as an equilibrium phase in S−I diblock copolymer melts.