Effect of Surface Roughness on Structure and Dynamics in Thin Films

We performed Monte Carlo simulations of free-standing, amorphous polyethylene (PE) thin films at 509 K. The three films are constructed from 9, 36, or 144 independent parent PE chains, with 100 carbon atoms per chain. The two-dimensional periodic cross-sectional area of the simulation box is proportional to the number of independent parent chains, with the 144-chain film having an area four times larger than the 36-chain film. All three films have a similar bulk density and a comparable thickness between the two free surfaces. The 144-chain film with the largest periodic surface area has a broader density profile due to the increased roughness of its surfaces. Snapshots of its surfaces along the trajectory indicate dynamic changes in the high and low regions of the rough surfaces. Diffusion of the chains parallel to the free surfaces is suppressed in the 144-chain film, due to increased surface roughness. The tendency of bonds to orient parallel to the free surface is less pronounced in films with higher surface roughness.