Melts of Brush Copolymers: From Microphase Separation to Liquid-Crystalline Order

Abstract

We use coarse-grained molecular dynamics simulations to investigate structural organization in melts of brush copolymers as a function of side-chain grafting density, backbone-side-chain miscibility, and backbone rigidity. At low grafting densities, decreasing miscibility drives microphase separation of backbones and side chains and leads to the formation of multi-backbone domains whose morphology evolves from cylindrical aggregates to lamellar structures as the spacer length between grafting points increases. In this regime, the characteristic wavevector q* corresponding to the peak position in the structure factor decreases with increasing the spacer length, reflecting an increase in the domain size. For semiflexible backbones, microphase separation is accompanied by mutual backbone alignment within the multi-backbone domains. Graft copolymers with miscible rigid backbones and flexible side chains can also undergo microphase separation. In this case, the morphological transformations are entropic in origin and are forced by local packing constraints imposed by the flexibility of backbones and side chains and can be viewed as an emergence of local nematic-like ordering of the backbones in a “solvent” of side chains. At high grafting densities, where multiple side chains are attached to each backbone bead, the backbones become effectively screened by the side chains and the melts display smectic-like organization. In this regime, the structure factor develops higher-order peaks at integer multiples of q*. Analysis of orientational correlations of side chain end-to-end vectors confirms that this ordering corresponds to appearance of layers of oriented side chains separating brush backbones. These results establish how graft copolymer architecture, intermolecular interactions, and backbone rigidity govern the transition from microphase-separated comb-like melts to smectic-like ordering in bottlebrush systems.