Sliding Dynamics of Ring on Polymer in Rotaxane: A Coarse-Grained Molecular Dynamics Simulation Study

Rotaxane is a topological supramolecule in which a ring molecule is threaded onto an axial chain. A unique feature of rotaxane is that the ring can slide along the chain. In this study, we investigate the sliding dynamics of a ring along an axial polymer in rotaxanes with different ring sizes and axial chain conformations using coarse-grained molecular dynamics simulations and considering the effect of the axial-chain fluctuation on the sliding motion. When the inner diameter of the ring is smaller than or close to the outer one of the chain, the sliding diffusion coefficient of the ring in rotaxane increases with the ring size, irrespective of whether the axial chain is fixed in space or fluctuates. In contrast, when the ring size is larger than the chain diameter, the diffusion behavior is quite different. The ring on the fixed chain diffuses freely in one dimension, whereas the ring on the fluctuating axial chain slides along the axial chain faster than that on the fixed chain. These results indicate that the sliding dynamics is dominated not only by the diffusion of the ring but also by the local fluctuating motion of the axial polymer. We conclude that the ring and axial polymer do not move independently, and the coupling effect between them is important for understanding the sliding motion in rotaxane.