Bottlebrush Polymers Fold 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 inter-backbone distance decreases as the grafting density of side chains becomes smaller. Here, we experimentally discover a behavior opposite to all existing works: the inter-backbone distance increases monotonically as the grafting density decreases. To explain the remarkable experimental findings, we develop a theory by accounting for the incompatibility between side chains and the backbone polymer. 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 becomes more collapsed; 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 sides is not alleviated. Our discovery presents a paradigm-shifting understanding of the molecular structure of bottlebrush polymers.



Keywords: Bottlebrush polymers; X-ray scattering; scaling theory