Using molecular simulation techniques based on a coarse-grained, bead-spring model, we examined the static and dynamic properties of linear perfluoropolyethers (PFPEs) in a nanoscale lubricant film on a solid wall. The conformation of the PFPEs, as predicted by the anisotropic radius of gyration, exhibits an oblate structure near the wall, but recovers a spherical shape as the distance from the wall increases. The density profile of the functional end groups for the PFPE molecules shows a characteristic oscillation as a function of the distance from the wall, indicating molecular layering. We also used the simulated surface morphology to examine the PFPE film roughness. Our preliminary dynamic simulations indicate that the wall interaction produces an anisotropy in the self-diffusion coefficient.

No abstract

From specific heat measurements in high quality H2@C60 samples performed over a broad temperature range, we obtain the smallest yet observed splitting of rotational energy sublevels of encapsulated single H2 molecules, 0.1-0.2 meV, in the nearly spherical potential well provided by highly isotropic C60 cages. Additionally, we find evidence of the quantized oscillation state of isolated H2 in the C60 cage. The minuscule splitting indicates that H2@C60 provides unprecedented opportunities to study free-molecule quantum dynamic properties.

No abstract

In this article, the thermodynamic stability of ultrathin perfluoropolyether (PFPE) films were examined via experimentation and molecular simulation. Using measured surface energies of PFPE films and a virgin carbon overcoat, a mesoscopic model using the Gibbs free energy change of spreading PFPE films was developed, yielding a film stability diagram as a function of film thickness, surface polarity of the carbon overcoat, and molecular weight of the PFPEs. We predict that a lower PFPE molecular weight and a lower surface polarity of the carbon overcoat produce a less stable film. This stability analysis was enhanced with off-lattice Monte Carlo simulations, which were applied for the first time to examine PFPE film nanostructures. The simulation results produced oblate conformations for the PFPE molecules near the surface, layered polar endgroup orientations, and a rougher surface morphology for films with less stable conditions, as predicted by the Gibbs free energy model.