Monte Carlo simulation in the semi-grand canonical ensemble as a 'thermodynamic' reverse Monte Carlo technique, with application to a polymer melt

摘要

We describe the use of Monte Carlo simulation in the semi-grand canonical ensemble (SGMC) to analyse and interpret experimental data for nonequilibrium states of matter such as glasses and many processed polymer melts. Experiments that provide information about atomic-level ordering are amenable to this approach. Closure of the inverse problem of determining the structural detail from limited data is achieved by selecting the lowest-free-energy ensemble of configurations that reproduces the experimental data. The free energy is calculated using the thermodynamic potential of the appropriate semi-grand canonical ensemble defined by the experimental data. To illustrate the method we examine uniaxially oriented polyethylene melts of average chain length C-78. The simulation results are analysed for features not explicitly measured, such as the density, torsion angle distribution, and free energy, to understand more fully the underlying features of these non-equilibrium states.