Monte Carlo simulation for the effect of chain stiffness on the free surface polymer melts

The role of chain stiffness on structural and molecular properties for the free surface of polymer melts was examined by Monte Carlo (MC) simulations of the coarse-grained (CG) polyethylene-like chains on the second nearest neighbor diamond ( 2nnd ) lattice. Polyethylene-like model by multiplying the statistical weights with the chain stiffness parameter (k) k ) to represent more flexible (k k = 0.0 and 0.5) and stiffer (k k = 1.5 and k = 2.0) chains than normal polyethylene (k k = 1.0). For the melt-vacuum surfaces of stiffer chains, bulk densities become lower with broader surface profiles. For more flexible chains, monomers at the end/middle position are more segregated/depleted near the surface. Bonds and chains tend to be randomly orientated in the bulk region but exhibit anisotropic orientation near the surface, especially for stiffer chains. For more flexible chains, polymers adopt larger amount of gauche conformation and chain dimensions become smaller with a more compact shape. The main principal axes of polymer coils are preferably oriented parallelly to the surface and the magnitudes of this anisotropy are larger for stiffer chains. Based on intra- and intermolecular energetics, more flexible chains have larger amount of gauche conformation with denser bulk structures but they adopt more trans conformation near the surface.