Critical lines and phase coexistence of polymer solutions: A quantitative comparison between Wertheim’s thermodynamic perturbation theory and computer simulations

Abstract

We consider the vapor–liquid phase equilibria of binary mixtures of a model bead–spring polymer dissolved in a solvent made of single monomers. We employ Wertheim’s thermodynamic perturbation theory of first order (TPT1), with the properties of the monomer fluid described by the mean spherical approximation and the effect of the attractions considered within a van der Waals one fluid approximation. The results of the TPT1 theory are quantitatively compared to grand canonical Monte Carlo simulations. To this end, we obtain full critical lines all the way from the critical point of the pure polymer to the critical point of the pure monomer. Pressure–composition and pressure–temperature slices are also obtained. At low pressures, we find rather good agreement between simulation and theory for the coexistence liquid densities and compositions. At higher pressures, however, the results are far less satisfactory, as the critical pressures are highly overpredicted.