Stability analysis and molecular simulation of nanoscale lubricant films with chain-end functional groups

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. (C) 2002 American Institute of Physics.