A comparison between integral equation theory and molecular dynamics simulations of dense, flexible polymer liquids a

Recently we (J.G.C. and K.S.S.) formulated a tractable "reference interaction site model" (RISM) integral equation theory of flexible polymer liquids. The purpose of this paper is to compare the results of the theory with recent molecular dynamics simulations (G.S.G. and K.K.) on dense chain liquids of degree of polymerization N = 50 and 200. Specific comparisons were made between theory and simulation for the intramolecular structure factor ω̂(k) and the intermolecular radial distribution function g(r) in the liquid. In particular it was possible to independently test the assumptions inherent in the RISM theory and the additional ideality approximation that was made in the initial application of the theory. This comparison was accomplished by calculating the intermolecular g(r) using the simulated intramolecular structure factor, as well as, ω̂(k) derived from a freely jointed chain model. The RISM theory results, using the simulated ω̂(k), were found to be in excellent agreement, over all length scales, with the g(r) from molecular dynamics simulations. The theoretical predictions using the "ideal" intramolecular structure factor tended to underestimate g(r) near contact, indicating local intramolecular expansion of the chains. This local expansion can be incorporated into the theory self consistently by including the effects of the "medium induced" potential on the intramolecular structure.