Molecular structure of foldable bottlebrush polymers in melts

A bottlebrush polymer has a long linear backbone densely grafted with many relatively short side chains. Classical understanding is that strong steric repulsion among highly overlapped side chains pre-strains the bottlebrush backbone, making bottlebrush polymers of low extensibility. We recently discovered that for bottlebrush polymers with highly incompatible backbone and side chains, the backbone collapses to reduce interfacial free energy regardless of the highly overlapped side chains. However, understanding the molecular structure of this so-called foldable bottlebrush polymer and its assemblies remains incomplete. Here, we report the deterministic correlations between molecular architecture, mesoscopic conformation, and macroscopic properties of foldable bottlebrush polymers in melts. A combination of theory and experiments reveals that as the side chain grafting density decreases, the bottlebrush diameter increases, whereas the bottlebrush end-to-end distance decreases. Both behaviors contradict the understanding of conventional bottlebrush polymers, which assume the backbone and side chains are compatible. Since foldable bottlebrush polymers store lengths that can be released upon large deformations, when being as network strands, they allow for decoupling the intrinsic stiffness-extensibility trade-off in unentangled single-network elastomers. These findings help establish the foundational science of using foldable bottlebrush polymers as a platform for soft (bio)materials design and innovation.