Segregation of chain ends is a weak contributor to increased mobility at free polymer surfaces

Polyethylene thin films are simulated on a high coordination lattice, which is formed by connecting every other site on a diamond lattice. The films present two surfaces to vacuum at 509 K. Each film contains 36 linear or cyclic chains of 50 beads on the coarse-grained lattice, where each bead represents two united-atom carbon atoms. Both linear and cyclic films exhibit bulk density and properties in the interior region, and hyperbolic density profiles at the interfaces. The mobility increases toward the interfaces on the scale of individual beads and chains due to the decrease in density, The increased mobility at the free surfaces is determined more strongly by the low density rather than the segregation of chain ends, since both films exhibit similar characteristics, and there are no chain ends in the film composed of cyclic chains.