MONTE-CARLO SIMULATIONS OF THE FREE-SURFACE OF POLYMER MELTS

We have conducted off-lattice Monte Carlo simulations of the free surface of a bead-spring homopolymer system. Three different monodisperse systems were considered, and they were comprised, respectively, of chains of length 20, 50 and 100. In all cases a relatively sharp interface, spanning ca. 2-3 times the segment diameter, separates the liquid from vacuum. We show that the Gibbs dividing plane, defined relative to the density of polymer segments, represents a hypothetical sharp boundary that separates regions where polymer chains assume melt-like characteristics, from those corresponding to the collapsed coil shapes expected for the chains on the vacuum side. As with the simulations of melt chains near hard walls we find that the density of end segments are enhanced relative to their bulk value at the Gibbs surface. Similarly, the middle segments are depleted in the immediate vicinity of the Gibbs surface. We have compared several other properties on the segmental and chain length scales and find in all cases that properties obtained on the liquid-side of the Gibbs surface are qualitatively similar to the trends obtained for off-lattice melts near hard walls, although important differences exist at higher temperatures. These results serve to reemphasize that the properties of chains in the vicinity of the free surface of a melt are affected significantly by the relatively small penetration of chain segments into the adjoining vacuum.