Self-consistent field theory and coarse-grained molecular dynamics simulations of pentablock copolymer melt phase behavior

SJ Park and T Myers and V Liao and A Jayaraman, MOLECULAR SYSTEMS DESIGN & ENGINEERING, 9, 1235-1253 (2024).

DOI: 10.1039/d4me00138a

Block copolymer (BCP) self-assembly leads to nanostructured materials with diverse ordered morphologies, some of which are attractive for transport applications. Multiblock AB copolymers are of interest as they offer a larger design parameter space than diblock copolymers allowing researchers to tailor their self-assembly to achieve target morphologies. In this study, we investigate the phase behavior of symmetric AxByAzByAx and BxAyBzAyBx pentablock copolymers (pentaBCPs) where A and B monomers have the same statistical segment length. We use a combination of self-consistent field theory (SCFT) calculations and molecular dynamics (MD) simulations to link the polymer design parameters, namely the fraction of middle block volume to the volume of all blocks of same type, tau, overall volume fraction of A block, fA, and segregation strength, chi N, to the equilibrium morphologies and the distributions of chain conformations in these morphologies. In the phase diagrams calculated using SCFT, we observe broader double gyroid windows and the existence of lamellar morphologies even at small values fA in contrast to what has been seen for diblock copolymers. We also see a reentrant phase sequence of double gyroid -> cylinder -> lamellae -> cylinder -> double gyroid with increasing tau at fixed fA. The chain conformations adopted in these morphologies are sampled in coarse- grained MD simulations and quantified with distributions of the chain end-to-end distance and fractions of chains whose middle (A or B) and end (A or B) blocks remain within domains of same chemistry (A or B). These analyses show that the pentaBCP chains adopt "looping", "bridging", and "hybrid" (both looping and bridging) conformations, with a majority of the chains adopting the hybrid conformation. The spatial distributions for each of the blocks in the pentaBCPs show that blocks of the same type in a chain locally segregate within the same domains, with shorter blocks segregating towards the domain boundaries and longer blocks filling the domain interior. This combined SCFT-MD approach enables us to rapidly screen the extensive pentaBCP design space to identify design rules for transport-favorable morphologies as well as verify the chain conformations and spatial arrangements associated with the theory predicted reentrant phase behavior. Using a combined theory- simulation approach we rapidly screen a large polymer design space to identify rules for desired morphologies as well as the chain conformations associated with the theory-predicted phase behavior.

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