OFF-LATTICE MONTE-CARLO SIMULATIONS OF POLYMER MELTS CONFINED BETWEEN 2 PLATES .2. EFFECTS OF CHAIN-LENGTH AND PLATE SEPARATION

In an earlier paper,2 we presented a Monte Carlo simulation technique in the canonical ensemble to model the static properties of polymer melts confined between two impenetrable plates. In this paper we consider the effects of variations in polymer chain length and plate separation on the detailed, microscopic conformations of polymer chains in the film. Three plate separations, corresponding roughly to RG (the unperturbed radius of gyration of chains), 2RG, and 10RG, were examined for chains with 100 connected beads (i.e., nb = 100). Also, polymers of three different nb's, 50,100, and 200, were studied at the same plate separation, 51 segment diameters, to examine the effects of changes in polymer chain length. We find that the influence of a surface on chain conformations in a polymer melt are restricted to only those segments that are confined to within approximately twice the segment diameter from the surface, independent of chain length and plate separation. All other segments assume random conformations and orientations. This feature, as well as the detailed comparison of the “train sequences” at the surfaces, is in surprisingly good agreement with the predictions of the mean-field lattice theory. The bead-density profile, as obtained from the simulations, exhibits enhanced packing near the surfaces that is apparently independent of plate separation and chain length. The only effect of decreasing plate separation is manifested in more symmetric chain shapes due to an increased presence of bridging conformations. On the other hand, the preference of chain ends to be located at the surface is enhanced with increasing chain length but remains unaffected by plate separation. Implications of these results on the surface forces in a melt and the variation of surface tension of a polymer melt with molecular weight are also discussed. © 1990, American Chemical Society. All rights reserved.