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

Abstract

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, τ, overall volume fraction of A block, fA, and segregation strength, χN, 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 τ 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.

Design, System, Application

Block copolymers (BCPs) self-assemble into a variety of nanostructures, such as lamellae, hexagonal-packed cylinders, and double gyroid, which in turn enable engineering of materials with desired transport and mechanical properties. The morphology formed by a given BCP is highly dependent on its design in terms of the monomer chemistry, number of blocks (diblock to multiblock), block lengths, and block sequence. As compared to diblock copolymers, multiblock copolymers have been studied to a smaller extent due to their larger design parameter space. However, it is noteworthy that a handful of computational studies of multiBCPs have uncovered novel nanostructures and phase behavior not seen in the well-studied diblock copolymers. In this study, we focus on linking the design of pentablock copolymers (pentaBCPs) to their morphology in the melt state using self-consistent field theory (SCFT) and molecular dynamics (MD) simulations. Our goal is to identify design rules for experimentalists looking to broaden phase windows of transport-friendly morphologies, such as double gyroid. The combination of theory and simulation allows for faster screening of design parameter space using SCFT as compared to MD simulations and quantification of chain conformations using MD simulations especially when getting distribution of chain conformations from SCFT is non-trivial.