Process-directed micro- and macrophase separation in diblock copolymer blends

The equilibrium phase behavior of binary diblock copolymer blends involves a complex interplay between microphase and macrophase separation. We investigate blends of linear diblock copolymers, A₁B₁ (cylinder-forming) and A₂B₂ (cylinder- or lamella-forming), using a combina- tion of self-consistent field theory (SCFT) and single-chain-in-mean-field (SCMF) simulations. When the chain-length asymmetry between the A₁B₁ and A₂B₂ copolymers becomes large, the equilibrium phase diagram exhibits a wide macrophase-separation channel. Strikingly, our simulations reveal a strong pathway dependence within this region: rapid quenching yields a spatially homogeneous structure with narrow cylinder-size distributions and strong hexagonal order, whereas gradual annealing promotes local demixing, resulting in bimodal domain sizes and weaker order. We demonstrate that this process-dependent nonequilibrium behavior can be explained by the distinct evolutions of the system state and free-energy landscape of the blends under quenching or annealing. These findings highlight how different processing conditions can direct nanostructure formation in block copolymer blends, and establish a mechanistic link between processing pathway and the final morphology, thus offering insights into rational design of targeted nanostructured materials.