Effects of Polymer-Wall Interactions on Entanglements and Dynamics of Confined Polymer Films.

Using Monte Carlo simulations combined with a geometric primitive path analysis method (Z1 algorithm), we investigate the effects of polymer-wall interactions on the entanglements and dynamics of the polymer films capped between two walls. We introduce a new parameter, the average number of near-neighboring particles of each monomer, to understand the effects of the polymer-wall interactions on the entanglements and dynamics of these confined systems. Our results show that the number of entanglements increases from the attractive polymer-wall interactions to the repulsive polymer-wall interactions. When the film thickness is greater than the bulk chain dimensions, the diffusion coefficient is a slowly decreasing function of the film thickness; however, when the film thickness is smaller than the bulk chain dimensions, the diffusion coefficient is an increasing function of the film thickness. However, for stronger polymer-wall interactions, although the number of entanglements decreases, the average number of the near-neighboring particles rapidly increases, which screens the effect of the disentanglements and thus limits the diffusivity of the polymers. Moreover, our simulations demonstrate that a critical attractive energy exists in the polymer-wall systems, where the diffusion coefficient reaches a maximum value and decreases toward stronger attractions or stronger repulsions. Our simulations provide new insights into the molecular mechanisms of the effects of polymer-wall interactions on the confined polymer films.