MOLECULAR-DYNAMICS STUDIES OF LANGMUIR MONOLAYERS OF F(CF2)(20)F

We report the results of molecular dynamics simulations of Langmuir mo nolayers of F(CF2)(20)F using both a united atom model and a recently developed anisotropic united atom model of the molecular force field. A comparison of our simulation results to experimental measures of the structure of Langmuir monolayers and lamellar crystals of F(CF2)(20)F indicates that the anisotropic united atom model provides the better representation. Simulations using the new model at 275 K generate an a zimuthally disordered rotator phase monolayer, while simulations at 15 0 K generate a monolayer phase with azimuthal order. The relative inte nsities of peaks in the in-plane structure function can be used to dis tinguish between the azimuthally ordered and disordered phases. Our si mulations also predict, in agreement with experiment, that at large av erage areas per molecule the monolayer breaks up into islands that hav e a close-packed structure at the center but are disordered at the per iphery. (C) 1996 American Institute of Physics.