Structure and properties of polymethylene melt surfaces from molecular dynamics simulations

Thermodynamic, structural, and dynamic properties of polymethylene melt surfaces are studied by molecular dynamics simulations using both an explicit atom and a united atom model. N-tridecane (C13H28) melt films with a thickness of about 30 Angstrom are studied by NVT-MD simulation method at the temperatures from 300 K to 450 K. We obtain stable surface properties such as surface tension, density profile, order parameter, and diffusivity upon performing the simulation on these films for 1 or 2 ns. When compared with experiment, simulations give a reasonable agreement for the surface tension with error of ca. 20%. It is observed that the density of chain-end group (methyl) is enhanced near the free surface, while it is depleted in the region below the surface. The interfacial thickness of the density transition region defined as liquid density divided by maximum density gradient is estimated to be about 5 Angstrom at room temperature. In this interfacial region, a slight preference for chain segments to orient along the direction parallel to the surface is observed with practically no difference in the chain conformation from the bulk value. The molecular diffusivity along the film surface is enhanced by a factor of ca. 3 compared with the diffusivity along the surface normal in the interfacial region. Both the explicit atom and the united atom model show almost the same thermodynamic and structural properties near the surface. (C) 2001 American Institute of Physics.