Conformation of perfluoropolyethers in the ultrathin liquid films on solid flat surfaces - effect of polar interaction [hard disks]

Applying the reptation algorithm to a simplified perfluoropolyether (PFPE) Z and Zdol (qualitative) bead-spring off-lattice polymer model an NVT Monte Carlo simulation has been performed to simulate the bulk and the ultrathin film polymer conformation. Bead-bead Lennard-Jones (LJ) potential is used in the bulk and with this substrate-bead LJ potential is added in the ultrathin film condition. In addition, dipole-dipole polar interaction potential is adopted at the two ends of Zdol. Simulation is performed with similar molecular weights of Z and Zdol under the assumption of room temperature and pressure condition. 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. The calculated average radius of gyration (Rg) is nearly equal to the experimental value both being in 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 nano-meters of film thickness a substrate surface at the bottom and a top restriction at film heights of different nano-meters are introduced keeping the maximum volume fraction similar to that of the bulk. The results will be important for the design of the ultrathin liquid films of PFPE.