Conformation of perfluoropolyethers in ultrathin liquid films on solid flat surfaces-effect of polar interaction

Abstract

A reptation algorithm applied to a simplified bead-spring off-lattice polymer model an NVT Monte Carlo simulation for perfluoropolyether (PFPE) Z and Zdol (qualitative) has been performed to simulate the bulk and the ultrathin film polymer conformations. A bead-bead Lennard-Jones (LJ) potential is used in the bulk, and a substrate-bead LJ potential is added in the ultrathin film condition. In addition, a dipole-dipole polar interaction potential is adopted at the two chain ends of Zdol. The simulation is performed with similar molecular weights of PFPE Z and Zdol under the assumption of room temperature and pressure condition. A maximum volume fraction of approximately 80%-82%, typical of a melt at room condition, is considered for both the bulk and the ultrathin film condition. It is observed that the calculated average radius of gyration (Rg) is nearly equal to the experimental value for the bulk situation. The average components of Rg in the x, y, and z directions Rgx, Rgy, and Rgz are almost equal to each other in the bulk denoting the average shape of a polymer as a sphere. In the ultrathin films of several nanometers of film thickness, a substrate surface at the bottom and a top restriction at film heights of different thicknesses are introduced, keeping the maximum volume fraction similar to that of the bulk. Rg and Rgz decrease and Rgx and Rgy increase (here, xy plane is taken parallel to the substrate surface and z denotes the distance from the same surface) in comparison to their bulk values with the decrease of film thickness initiated at a specific ultrathin film thickness, which depends on the polar (Zdol) or nonpolar interaction. The results will be important for the design of the ultrathin liquid films of PFPE.