Equation of state for polymer systems

We developed an equation of state for systems containing chain-like molecules. The equation consists of three contributions. As a reference, we first adopted the equation for hard-sphere-chain fluids (HSCF) derived through the r-particle cavity-correlation function of sticky hard spheres. The HSCF equation has a simple form similar to the Carnahan–Starling equation and has been satisfactorily tested by computer-simulation results for chain fluids with a wide range of chain length. Secondly, for the perturbation, we used the square-well (SW) equation. Thirdly, for molecules with specific oriented interactions such as hydrogen bonding, we introduced a contribution from chemical association (ASS). The corresponding expression for association comes naturally from the sticky model used in deriving the reference equation. The ASS term was well tested with computer-simulation results for associated molecules. The equation of state was widely tested by fitting experimental vapor–liquid equilibrium data. For liquid–liquid equilibria, we adopted a lattice model developed previously as a high-pressure limit that virtually serves as a mixing rule.