Possible explanation of the Λ-shape anomaly in polymer surface diffusion

We suggest an explanation for the anomalous behavior of the polymer surface-diffusion coefficient D reported by Zhao and Granick [J. Am. Chem. Soc. 126, 6242 (2004)]: D first increased with increasing polymer surface concentration Gamma, then suddenly dropped at a threshold value Gamma(*). Our molecular dynamics simulations show the same behavior. We find that polymers form single-layered pancake structures for Gamma <Gamma(*), while double layers form for Gamma >Gamma(*). The double-layer structures allow the polymers to better adapt to the substrate corrugation. This increases barriers for lateral diffusion and is thus consistent with small values of D at larger Gamma.