Surface and thin film characteristics of poly(tetrafluoroethylene) melts from molecular dynamics simulations

The surface characteristics of thin films of poly(tetrafluoroethylene) (PTFE) melts were investigated by molecular dynamics simulations, employing the recent explicit atomic force field, including partial charges, which was fine-tuned to the experimental PVT data, the chain conformation and the crystal structure (Macromolecules 2003, 36, 5331). Surface tension was calculated from the virial equation of the pressure tensor, taking into full account of the long-range correction terms, in good agreement with experimental data. Compared with polymethylenes of the same chain length, PTFEs have larger vacuum/liquid interface thicknesses concomitant with lower surface tensions. In the surface region, the chain conformations remain unperturbed, but the chain backbone segments tend to be oriented parallel to the surface whereas the chain-end segments tend to be perpendicular to the surface. As compared with polymethylenes, the orientation of PTFE segments in the surface is found to persist deeper into the film due to the larger intermolecular orientational correlation length in PTFE melts than in polymethylene melts.