Stabilization of the C15 Phase in Binary Blends of Asymmetric AB2 Linear-Branched Block Copolymers and B-Homopolymers

Two sets of binary blends were prepared by mixing core-selective B-homopolymers with either symmetric AB₂ or asymmetric AB₁(B₂) linear-branched block copolymers, aiming to unravel the architectural effects on the formation of complex structures in polymer blends. The spatial distribution of homopolymers depends critically on both the copolymer architecture and homopolymer size. For short homopolymers, symmetric AB₂ blends exhibit a cylinder-to-lamellae transition as the homopolymer loading increases, while their asymmetric counterparts follow a transition sequence from spheres to cylinders and then to double gyroids. When long homopolymers are added, symmetric AB₂ block copolymers retain their initial cylindrical structures, whereas the Frank–Kasper σ and C15 phases are stabilized in the asymmetric systems. These precisely defined blends eliminate molecular uncertainties arising from molecular weight distribution and compositional variations. Our results underscore the significant impact of architectural symmetry on homopolymer partitioning that leads to distinct equilibrium morphologies, providing key insights into rational structural engineering via simple blending.