Applying the reptation algorithm to a simplified perfluoropolyether (PFPE) bead-spring off lattice polymer model, an NVT Monte Cairo simulation has been performed to simulate the bulk and 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. Molecular weights of 3840 and 1700 g/mol for Fomblin Z (Ausimont) are considered with 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 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 thick 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. Rg and Rgz decrease and Rgx and Rgy increase (here, the xy plane is taken parallel to the substrate surface and z denotes the height 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 molecular weight. Effect on density variation due to conformation change is also revealed in the results.
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