Equation of State for Macromolecules of Variable Flexibility in Good Solvents: A Comparison of Techniques for Monte Carlo Simulations of Lattice Models

摘要

The osmotic equation of state for the athermal bond fluctuation model on thesimple cubic lattice is obtained from extensive Monte Carlo simulations. Forshort macromolecules (chain length N=20) we study the influence of variouschoices for the chain stiffness on the equation of state. Three techniques areapplied and compared in order to critically assess their efficiency andaccuracy: the repulsive wall method, the thermodynamic integration method(which rests on the feasibility of simulations in the grand canonicalensemble), and the recently advocated sedimentation equilibrium method, whichrecords the density profile in an external (e.g. gravitation-like) field andinfers, via a local density approximation, the equation of state from thehydrostatic equilibrium condition. We confirm the conclusion that the lattertechnique is far more efficient than the repulsive wall method, but we findthat the thermodynamic integration method is similarly efficient as thesedimentation equilibrium method. For very stiff chains the onset of nematicorder enforces the formation of isotropic-nematic interface in thesedimentation equilibrium method leading to strong rounding effects anddeviations from the true equation of state in the transition regime.