Unexpected Folding of Bottlebrush Polymers in Melts

Abstract

Bottlebrush molecules are branched polymers with a long linear backbone densely grafted by many relatively short linear side chains. Such a unique molecular architecture enables bottlebrush polymers with properties and functions inaccessible by their linear counterparts. The existing understanding is that, in melts of bottlebrush polymers, the interbackbone distance decreases as the grafting density of side chains becomes smaller. Here, we experimentally discover a behavior opposite to all existing works: the interbackbone distance increases monotonically as the grafting density decreases. To explain these remarkable experimental findings, we develop a theory by accounting for the incompatibility between the backbone and side chains within a bottlebrush molecule. The backbone polymer folds into a cylindrical core with all grafting sites on its surface to reduce interfacial free energy. As the grafting density decreases, the backbone collapses; this process not only increases the diameter of the cylindrical core but also reduces the distance between grafting sites in space, such that the extension of side chains is not alleviated. Our discovery presents a paradigm-shifting understanding of the molecular structure of bottlebrush polymers.

Cylindrical brushes with ionized side chains: Scaling theory revisited

“…46 remain valid in the osmotic regime I (with eqn (21) for f 8 ), classic salt-dominated regime II (with eqn (26) for f 8 ), and quasi-neutral regime V (eqn (55) for f 8 ), but should be modified in the revised salt-dominated regimes III (eqn (35) for f 8 ) and IV (eqn (46) for f 8 ). In contrast to moderate stretching of backbones in the melt of bottlebrush polymers with relatively long spacers, 48 or even backbone folding in melts of bottlebrush polymers with incompatible main and side chains, 49 polyelectrolyte bottlebrushes exhibit strong stretching in solutions due to the long-range electrostatic interactions. 46 The latter affect not only the local structure (distance h between grafts, and the brush thickness D) but also large scale conformations of bottlebrush polyelectrolytes.…”

Bottlebrush polyethylene glycol nanocarriers translocate across human airway epithelium via molecular architecture enhanced endocytosis

“…Analogous to "sausage versus spaghetti," the bottlebrush polymer is essentially a wormlike 'fat' linear polymer [32][33][34][35][36] . The cross-section, or effective monomer size, of the wormlike bottlebrush is about twice the side chain size, 𝑏 𝑒 ≈ 2𝑅 𝑠𝑐 .…”

3D Printable Modular Soft Elastomers from Physically Cross-linked Homogeneous Associative Polymers

“…This may result in uncontrollable cross-linking and mechanical properties. Moreover, in a bottlebrush polymer, the highly overlapping side chains undergo steric repulsion, generating tension that prestrains the bottlebrush backbone. − Consequently, bottlebrush elastomers are often brittle . It is highly desirable to develop DIW printable elastomers that provide modular control over stiffness and toughness and can be printed without the need for solvents or post-print treatment.…”