Atomistic simulations of the surfaces of thin films of random copolymers

Abstract

Amorphous thin films of styrene-butadiene and styrene-butadiene-acrylonitrile copolymers containing a small amount of acrylic acid are generated using molecular dynamics and energy minimization simulations. The structure of the films shows an interior region of bulk density and a thin outer surface layer where the density falls to zero. The surface layer over which the density changes is approximately 5 A in thickness. The film mass density at its center of mass is close to the bulk values. The orientation function parameter of the backbone carbon-carbon bonds, averaged over the ensemble of representative cells, shows a random nature in the dense region and an inclination towards an orientation somewhat along the plane of the surface layer in the external parts of the film. The styrene rings are mostly located in the bulk region, with random spatial and orientation distributions. The polar groups in the acrylonitrile segment of the SBA films are mostly found in the surface region, where they contribute to the marked enhancement in the surface energy for SBA films compared to SB films. The surface energy values deduced from the models are systematically slightly lower than independent estimates.