σ Phase Formed in Conformationally Asymmetric AB-Type Block Copolymers

The stability of various spherical phases formed in conformationally asymmetric AB diblock and architecture asymmetric AB_m miktoarm block copolymers is investigated using self-consistent field theory. Both the conformational and architecture asymmetries are unified into a parameter of conformationally asymmetric degree, ε. We find that a complex spherical phase, the σ phase, becomes stable and its phase region expands between bcc and hexagonal phases as increasing ε. Only for large conformational asymmetry, for example, ε = 9 (or m = 3), the A15 phase becomes stable in the region between the σ phase and the hexagonal phase and its phase region terminates at the intermediate segregation region. Compared with the σ phase, the A15 phase has more favorable interfacial energy by enabling the formation of larger spherical domains, and therefore, it becomes more stable in the region of more symmetric volume fraction and stronger segregation.