Possible explanation of polymer surface diffusion anomaly

Surface diffusion of polymer is studied with molecular dymanics simulation. Our simulation result shows the non-monotonic behavior of the polymer surface-diffusion coefficient D as a function of surface coverage. D first increased with increasing polymer surface concentration, then suddenly drops at a critical value. This finding shows striking similarities with the experimental result [1]. We found that the strong reduction of D above the threshold concentration is related to the spontaneous formation of double-layered structures, whose bottom layers manage to lock into the registry of the substrate much better than the single-layer pancakes that form at smaller concentration. This increases the barriers for lateral diffusion and is thus consistent with small values of D at larger concentration. \newline \newline [1] J. Zhao and S. Granick, J. Am. Chem. Soc. 126, 6242 (2004).