Process-Directed Self-Assembly of Copolymer Blends: I. Micro- and Macrophase Separation

The phase behavior of binary diblock copolymers features a complex interplay between microphase and macrophase separation. Using a combination of self-consistent field-theory (SCFT) and single-chain-in-mean-field (SCMF) simulations, we investigate the self-assembled morphologies of binary blends composed of two linear diblock copolymers, A₁B₁ and A₂B₂, both in equilibrium and under processing conditions such as sudden quenching and gradual annealing. We focus on cylinder- and lamella-forming copolymers, where the equilibrium phase diagrams exhibit wide macrophase-separation channels when the length asymmetry between short A₁B₁ and long A₂B₂ copolymers becomes sufficiently large. Notably, our particle-based simulations uncover a strong dependence of the resulting morphology on the processing pathway used to reach the same thermodynamic state point within the macrophase-separation channel. Quenching produces a homogeneous mixture of A₁B₁ and A₂B₂ copolymers, a narrow cylinder-size distribution, and stronger hexagonal order, whereas annealing induces local demixing of the two copolymers, yielding a bimodal size distribution and weaker hexagonal order. This process-dependent behavior originates from the complex free-energy landscape of the blends. Overall, our study provides insights into the structure-processing-property relationships in block copolymer systems and contributes to the development of rational processing strategies for targeted nanostructures.