Simulation of polyethylene thin films on a high coordination lattice

We present the results of some film simulations of polyethylene (PE) on a coarse-grained lattice, which is termed as the second nearest neighbor diamond (2nnd)lattice. Monte Carlo simulations of PE melts have been performed on the 2nnd lattice by including short-and long-range interactions. Films can be obtained from equilibrated bulk snapshots by increasing one periodic side to infinity. The presence of attractive long-range interactions besides the repulsive ones gives a cohesive nature to the film. PE films, which contain up to 108 chains of C-99 and have thicknesses of more than 100 Angstrom between the two surfaces, can be produced and equilibrated on the 2nnd lattice. In these films, the density profiles are hyperbolic, with end beads being more abundant than the middle beads at the interface. There are orientational preferences at the interface on the scale of individual bonds and whole chains. The center of mass distribution of the chains exhibits oscillatory behavior. A comparison of films with different thicknesses, which contain different number of chains, does not indicate any significant differences in local and global equilibrium properties. At lower temperatures, the interfaces get sharper and the orientational preferences are more pronounced. Surface energies close to experimental values are calculated directly from the on-lattice energetics. It is also possible to reverse map Equilibrated snapshots from the lattice back to the atomistic model and minimize their energy. The energetics of the resulting film snapshots in continuous space seem to be in agreement with the experimental data on PE.