Molecular Structure of Foldable Bottlebrush Polymers in Melts

A bottlebrush polymer consists of a long linear backbone densely grafted with many relatively short side chains. A widely accepted view is that strong steric repulsion among the highly overlapped side chains prestrains the bottlebrush backbone, resulting in low polymer extensibility. However, we recently discovered that in the melt of bottlebrush polymers with highly incompatible side chains and backbone, the backbone collapses to reduce interfacial free energy, regardless of the strong steric repulsion among side chains. Despite this discovery, the molecular structure of these so-called "foldable" bottlebrush polymers and their assemblies remains poorly understood. Here, we present the deterministic relationships among molecular architecture, mesoscopic conformation, and macroscopic properties of foldable bottlebrush polymers. A combination of scaling theory and experiments reveals that as the side chain grafting density decreases, the bottlebrush diameter increases, whereas the bottlebrush end-to-end distance decreases. These behaviors contradict the existing understanding of bottlebrush polymers, which assumes that the backbone and side chains are compatible. Since foldable bottlebrush polymers store lengths that can be released upon large deformations, they offer a way to decouple the intrinsic stiffness-extensibility trade-off in single-network elastomers. These findings provide foundational insights into using foldable bottlebrush polymers as building blocks for designing soft (bio)materials.