Dynamics Transition of Polymer Films Induced by Polymer-Obstacle Entanglements on Rough Surfaces

We systematically investigate the effects of surface roughness on the static and dynamic properties of polymer chains in confined films by using molecular dynamics simulations, in which the roughness is characterized by height and distance of the obstacles. Our simulations demonstrate that near a smooth surface the polymer chains tend to disentangle because of the orientation of the segments near the surface. In contrast, near the rough surfaces, the oriented segments are entangled with the obstacles. Furthermore, the presence of even short obstacles can restrain or reverse the disentanglement tendency. However, the short-lived entanglements between the polymer chains and short obstacles exert only a slight influence on the relaxation and diffusion of the polymer chains. When the height of the obstacles is comparable to the size of polymer chains, the chains near the rough surfaces are totally confined in the array of the obstacles and severely entangled with these obstacles. Moreover, the distance between obstacles directly controls the relaxation and diffusion of the confined polymer chains. As the polymer chains leave the array of obstacles, the influence degree decreases sharply. In addition, the static and dynamic properties recover the bulk nature when the polymer chains are away from the surfaces or obstacles by more than an order of radius of gyration. Our results clarify the effects of the surface roughness on the influence scope and degree of the static and dynamic properties of polymer chains in confined films, which is significant for understanding the physical nature and developing the corresponding applications of confined polymer systems.