Unusual self-diffusion behaviors of polymer adsorbed on rough surfaces

We investigate the diffusion dynamics of a single polymer strongly adsorbed on surfaces in an extremely broad chain length and surface roughness by means of molecular dynamics simulations. Our simulations demonstrate that with the increase in chain length, the diffusion dynamics of polymer chains exhibits three regimes: the Rouse dynamics with D similar to N-1 when the lateral size of polymer chains is smaller than a half of distance between obstacles on rough surfaces; the reptationlike dynamics with D similar to N(-1.)5 and tau(r) similar to N-3 when the obstacles inhibit the freely Rouse behavior of polymer chains; and the quasi-Rouse dynamics with D similar to N-1 and similar to tau(r) similar to N-2.5 when the height of obstacles is smaller than twice the vertical size of polymer chains, where D, N, and tau(r) are the diffusion coefficient, chain length, and end-to-end vector relaxation time of polymer chains, respectively. The long chains have sufficient conformation entropy to form loops to hop over short obstacles, which could dramatically reduce the confinement from obstacles on the rough surfaces and changes the diffusion and relaxation dynamics of polymer chains from the reptationlike dynamics to the quasi-Rouse dynamics. Our results reveal the whole diffusion dynamics of polymer chains strongly adsorbed on rough surfaces and clarify the corresponding transition mechanism, which is significant for the understanding of the physical nature and the development of the corresponding applications.