Unexpected crossover dynamics of single polymer in a corrugated tube

We present molecular dynamics study of a generic (coarse-grained) model for single-polymer diffusion confined in a corrugated cylinder. For a narrow tube, i.e., diameter of the cylinder $\delta < 2.3$, the axial diffusion coefficient $D_{||}$ scales as $D_{||} \propto N^{-3/2}$, with chain length $N$, up to $N \approx 100$ then crosses over to Rouse scaling for the larger $N$ values. The $N^{-3/2}$ scaling is due to the large fluctuation of the polymer chain along its fully stretched equilibrium conformation. The stronger scaling, namely $N^{-3/2}$, is not observed for an atomistically smooth tube and/or for a cylinder with larger diameter.