Implicit Side-Chain Model for Melt Bottlebrush Block Copolymer Assembly

Densely grafted “bottlebrush” polymers are characterized by strong steric repulsions between side chains that can affect molecular conformations. Block bottlebrush copolymers (BBCPs), consisting of two or more side-chain chemistries, have thus found use in a wide variety of materials applications, due to their ability to rapidly self-assemble into nanoscale structures. However, the grafting density and length of these side chains contribute to a large molecular design space that is cumbersome to experimentally explore them. Advances in computation and theory have made significant progress in modeling block bottlebrush self-assembly, but the individual side-chains in large-scale simulations incur a large computational cost. We recently demonstrated that it is possible to describe a bottlebrush polymer as a linear semiflexible cylinder that does not include explicit side chains. These implicit side-chain (ISC) models have been demonstrated in solutions and homopolymer melts, but their applicability to melt BBCPs requires predictions for the incompatibility χ between different polymer chemistries. In this study, we use large-scale coarse-grained simulations of BBCPs with different architectures to determine the pair interaction potential for the ISC model as a function of χ. Iterative Boltzmann inversion (IBI) is used to determine the pair interaction potential uP,AB(r) from binary bottlebrush blends, which shows a linear deviation from the homopolymer potential uP,h(r) with χN. Simulations of diblock BBCP ordered self-assembly predict ordered lamellar structures for several different architectures and exhibit lamellar spacings that are consistent with experiments on polystyrene (PS) and poly(lactic acid) (PLA) BBCPs. We also show that grafting density has a pronounced effect on the length scale of self-assembled domains, based on comparison between simulation and experiment for three different BBCPs with one, two, and five grafted PLA blocks. This ISC model enables efficient large-scale simulations of BBCP assembly and has potential applications in modeling other nonconventional architectures such as shape-defined bottlebrushes.