Computer simulation study of the global phase behavior of linear rigid Lennard-Jones chain molecules : comparison with flexible models

摘要

The global phase behavior ~i.e., vapor-liquid and fluid-solid equilibria! of rigid linear Lennard-Jones~LJ! chain molecules is studied. The phase diagrams for three-center and five-center rigid modelmolecules are obtained by computer simulation. The segment-segment bond lengths are L5s, sothat models of tangent monomers are considered in this study. The vapor-liquid equilibriumconditions are obtained using the Gibbs ensemble Monte Carlo method and by performingisobaric-isothermal NPT calculations at zero pressure. The phase envelopes and critical conditionsare compared with those of flexible LJ molecules of tangent segments. An increase in the criticaltemperature of linear rigid chains with respect to their flexible counterparts is observed. In the limitof infinitely long chains the critical temperature of linear rigid LJ chains of tangent segments seemsto be higher than that of flexible LJ chains. The solid-fluid equilibrium is obtained by Gibbs–Duhemintegration, and by performing NPT simulations at zero pressure. A stabilization of the solid phase,an increase in the triple-point temperature, and a widening of the transition region are observed forlinear rigid chains when compared to flexible chains with the same number of segments. Thetriple-point temperature of linear rigid LJ chains increases dramatically with chain length. Theresults of this work suggest that the fluid-vapor transition could be metastable with respect to thefluid-solid transition for chains with more than six LJ monomer units.