Evidence for Backbone Flexibility of Bottlebrush Polymers Driven by Low Chi Assembly

Bottlebrush polymers are an intriguing class of materials, with unique properties due to their ability to reach large molecular weights without becoming entangled. Bottlebrush block copolymers (BCPs) are the natural extension of the homopolymer brushes, and systems with lamellar morphology have both larger scaling exponents compared to linear BCPs and higher mobility, enabling fast assembly of lamellae with large pitches. In one example, PLA-b-PS-b-PEO bottlebrushes exhibited unusual scaling behavior where an increase in the PEO backbone length resulted in a decrease in the pitch for BCP lamellae. Using soft X-ray reflectivity and near edge absorption fine structure spectroscopy (NEXAFS) measurements we evaluate how each component is distributed throughout the lamellae to determine the origin of this scaling. The presence of the midblock, PS, at the top surface of lamellae oriented parallel to a substrate is confirmed by both techniques and demonstrates that even for densely grafted bottlebrushes the formation of loops, analogous to linear triblocks, is possible. Self-consistent theory calculations match the experimentally determined component distributions and calculated backbone concentrations also provide evidence of backbone flexibility.