Cyclic vs Linear Bottlebrush Polymers in Solution: Side-Chain Length Effect

Abstract

Bottlebrush polymers (BBPs) of different architecture are of considerable interest for a broad range of applications, including nanomedicine, electronics, and self-healing materials. Using atomistic molecular dynamics simulations, we investigate and compare the structural and hydration properties of cyclic and linear poly(vinyl alcohol)-graft-poly(ethylene oxide) (PVA-g-PEONsc) BBPs in aqueous solution as functions of PEO side-chain length, Nsc. We find that overall cyclic BBPs are smaller than the corresponding linear BBPs and their shape changes from donutlike to disklike to starlike with increasing side-chain length, while linear BBPs vary in shape from an expanded coil to a rod/cylinder. The radius of gyration of cyclic BBPs increases with an increase of the side-chain length at a somewhat slower rate than the linear BBPs but follows the same scaling Rg ∼ Nsc0.58 in the limit of long side chains. For short grafts, we determine that the persistence length, lp, for both cyclic and linear BBPs increases in a similar manner following an lp ∼ Nsc0.54 dependence. In the long side-chain limit, the persistence length (lp) of cyclic BBP saturates, while for linear BBPs lp strongly increases following the expected scaling relation lp ∼ Nsc15/8. We propose a scaling model that shows that the fraction of side-chains located inside the cyclic BBPs depends on the radius of the backbone ring and significantly decreases with an increase of graft length. For both cyclic and linear BBPs, the hydrogen bonding between PEO side chains and water is somewhat reduced near the backbone, where local chain stretching is observed, while reaching full hydration on the periphery. Overall, the hydration shell within 1 nm of the cyclic BBP backbone is found to be more dynamically stable compared to linear BBPs.