Single-molecule elasticity of bottlebrush polymers

The elasticity of highly branched polymers characterized by a dense grafting of
side chains, known as bottlebrush polymers, is central to understanding their
physical properties in a wide variety of applications such as soft elastomers,
solution processing, and confinement-induced stretching. Unlike linear polymer chains where the molecular
origin of this extension is well understood, it remains a challenge to connect
the bottlebrush architecture to force-extension behavior. We study single
bottlebrush polymers subjected to a constant pulling force using molecular
simulations and determine force-extension curves as a function of side-chain
length. To understand bottlebrush elasticity at the
single-molecule level, we compare with a parameterized wormlike cylinder
implicit side chain model. We demonstrate the emergence of two distinct modes
of bottlebrush stretching; at low forces, we show that both linear and
non-linear regimes correspond to stretching the overall cylindrical shape of the
molecule, and at high forces, there is specific extension of internal degrees of
molecular freedom corresponding to the bottlebrush backbone. This two-regime
molecular picture provides insight valuable to the molecular design of
bottlebrush materials.