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

Abstract

A new mesophase in binary blends of A–b–(BA′)3 miktoarm star block copolymers and A homopolymers has recently been discovered experimentally and explored with field-theoretic simulations. This mesophase 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, a deeper understanding of its structural and dynamic-mechanical properties, including its domain connectivity, linear rheological behavior, response to shear, and response to uniaxial tension, is warranted. These properties are explored here using dissipative particle dynamics in three dimensions, for the first time. These simulations establish that the so-called “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 and outweighs the influence of microphase segregation (contributions from the interfacial tension alone) even at the highest homopolymer concentrations we study. Additionally, it appears that the bricks-and-mortar phase is sensitive to the application of sufficiently high shear, leading to nonisotropic mechanical responses, which has ramifications for the processability of such materials. We find that upon application of shear the phase becomes closer in structure 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 observed depends heavily on the material process pathway.

Architectural Design of Block Copolymers

“…Gao et al. proposed that the architecture consisting of multiple AB diblock arms should produce a high ratio of bridging configuration due to the effect of combinatorial entropy. ,, In other words, multiple AB arms have a great tendency to be partitioned into different neighboring domains to maximize the number of possible configurations. Later on, Matsen and co-worker have performed SCFT calculations to confirm that the bridging ratio can be increased close to 100% by increasing the arm number .…”