Thermodynamic, structural, and dynamic properties of polymethylene melt sur
faces are studied by molecular dynamics simulations using both an explicit
atom and a united atom model. N-tridecane (C13H28) melt films with a thickn
ess of about 30 Angstrom are studied by NVT-MD simulation method at the tem
peratures from 300 K to 450 K. We obtain stable surface properties such as
surface tension, density profile, order parameter, and diffusivity upon per
forming the simulation on these films for 1 or 2 ns. When compared with exp
eriment, simulations give a reasonable agreement for the surface tension wi
th error of ca. 20%. It is observed that the density of chain-end group (me
thyl) is enhanced near the free surface, while it is depleted in the region
below the surface. The interfacial thickness of the density transition reg
ion defined as liquid density divided by maximum density gradient is estima
ted to be about 5 Angstrom at room temperature. In this interfacial region,
a slight preference for chain segments to orient along the direction paral
lel 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 alon
g the surface normal in the interfacial region. Both the explicit atom and
the united atom model show almost the same thermodynamic and structural pro
perties near the surface. (C) 2001 American Institute of Physics.