Is the "Bricks-and-Mortar" Mesophase Bicontinuous?: Dynamic Simulations of Miktoarm Block Copolymer/Homopolymer Blends

A mesophase in binary blends of A-b-(BA')₃ miktoarm star block copolymers and A homopolymers has been reported to consist of aperiodic discrete domains of A embedded in a continuous matrix of B, up to very high concentrations of A. Because of the material's potential as a thermoplastic elastomer, an understanding of its structural and mechanical properties, including its domain connectivity, rheological behavior, response to shear and to uniaxial tension, is warranted.  These properties are explored here using dissipative particle dynamics in three dimensions.  These simulations establish that the "bricks-and-mortar" phase while appearing discrete in two dimensions is bicontinuous. The simulations focusing on dynamics establish that the role of molecular bridging dominates the mechanical behavior, outweighing the influence of microphase segregation even at the highest homopolymer concentrations we study. Additionally, the phase is sensitive to the application of high shear, leading to anisotropic mechanical responses, which has ramifications for processability.   We find that upon application of shear the phase becomes closer to its speculated discrete nature. Molecular simulations on our longest accessible timescales show that the material is unable to relax back to its original structure, suggesting that the morphology depends heavily on the processing pathway.