Monte Carlo simulation of molecular and structural properties of random copolymer thin films.

Abstract

Molecular and structural properties of random copolymer thin films were studied by Monte Carlo simulation of coarse-grained copolymer model on the high coordination lattice. Random copolymer thin films with 50% comonomer fraction with varied interaction strength between comonomer units were studied. Intramolecular interaction was represented by Flory's rotational isomeric state (RIS) model of polyethylene (PE). The non-bonded interactions were treated by Lennard-Jones potential with different parameter sets for comonomer units. When the interaction strength of comonomer is increased, the density of thin films is higher in the inner region and significantly dropped near the free surface along the normal direction of the film. Higher bulk densities and narrower interfacial thicknesses are observed for random copolymer with stronger bead interaction. End beads were segregated at the free surface region and tend to orient perpendicularly to the surface. Copolymer size and shape were significantly changed as a function of bead interaction. For orientation of the whole chain, the largest molecular axis tends to orient along the film surface, but changed toward random orientation for weaker bead interaction. The intra-chain energies were decreased at the surface region while the non-bonded energies were increased. The stronger the comonomer bead attraction, the more change in energetics across the thin film.