Molecular simulation of structural and surface properties of poly(ethylene-ran-propylene) thin films

The effect of comonomer composition on structural and surface properties of poly(ethylene-ran-propylene) thin films at various ethylene fraction were studied by Monte Carlo simulation of coarse-grained copolymer model on the second nearest neighbor diamond (2nnd) lattice. Intramolecular and intermolecular interactions of poly(ethylene-ran-propylene) were treated by the Rotational Isomeric State (RIS) model and the Lennard–Jones potential, respectively. The relative bead densities in the bulk region of films were increased as a function of ethylene fraction. Thin films become denser with sharper surface for copolymer with larger ethylene portion. End beads of copolymer chains were more abundant and the bonds tend to be oriented in the perpendicular direction near the free surface. More anisotropic orientation was observed for films with larger ethylene content. The radius of gyration component of copolymer chains in the perpendicular direction to the film surface was monotonically decreased, while the parallel component had no noticeable changed as a function of displacement from the film center of mass. Molecular size (components of the radius of gyration) and molecular shape (acylindricity) were apparently changed as a function of ethylene fraction. For molecular orientation, the largest axis was aligned preferably in parallel direction to the film surface, and relatively changed to more random isotropic orientation for less ethylene content.