Anomalous diffusion of polystyrene from an attractive substrate based on all-atom simulation.

The diffusion of polystyrene (PS) polymer chains from a hydroxy (-OH)-terminated Si surface with different grafting densities φG is studied based on all-atom simulation. Our particular attention is paid to the impact of the attractive substrate on the diffusive and configurational properties of PS. Our simulation results uncover a very novel and unexpected modification to polymer diffusion with the increment of φG, namely, the diffusion is slowed down most significantly from a substrate with moderate grafting densities, while in lower or full grafting cases, the diffusive dynamics is even facilitated rather than retarded. The underlying mechanism is investigated in terms of energy and conformational change in detail. Surprisingly, we obtain a consistent scenario for diffusion. Under moderate grafting densities, the energy required to be overcome for diffusion is relatively large. In addition, PS chains are more likely to be in a stretched configuration subject to a slower relaxation. These facts can account for the hindered diffusion. While under lower or full grafting densities, the energy required for diffusion becomes even smaller than the ungrafted situation. Also, PS chains prefer a shrinking configuration undergoing faster relaxation. Consequently, the diffusion of PS is reasonably promoted.