(252b) Density-Functional Theory for Polymer-Carbon Dioxide Mixtures

We develop a new density-functional theory (DFT) for describing inhomogeneous polymer-carbon dioxide (CO₂) mixtures.  The theory is constructed by combining the bulk Peng-Robinson equation of state (PR-EOS) with the statistical associating fluid theory (SAFT) and the fundamental measure theory (FMT).  The weighted density function from FMT is adopted to extend the bulk excess Helmholtz free energy of PR-EOS to the inhomogeneous cases. The SAFT is used to describe the polymer intra-chain correlations and short-range forces due to weakly polar or association interactions. In addition, quadratic order density expansion is used to account for the long-ranged intermolecular dispersion attractions, with the direct correlation function obtained from a first-order mean-spherical approximation.  We apply our DFT to the interfacial properties of polystyrene-CO₂ and poly(methyl methacrylate)-CO₂ systems. The calculated liquid-vapor interfacial tension is in good agreement with experimental data. Moreover, our DFT captures the oscillatory density profile at the liquid-vapor interface, which is missed in some other versions of density-functional theory.