Theoretical estimation of thermodynamic properties of the system PS/PPO on the basis of modified combining rule of Sanchez–Lacombe lattice fluid model

The three scaling parameters described in Sanchez–Lacombe lattice fluid theory (SLLFT), T*, P* and ρ* of pure polystyrene (PS), pure poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) and their mixtures are obtained by fitting corresponding experimental pressure–volume–temperature data with equation-of-state of SLLFT. A modified combining rule in SLLFT used to match the volume per mer, υ* of the PS/PPO mixtures was advanced and the enthalpy of mixing and Flory–Huggins (FH) interaction parameter were calculated using the new rule. It is found that the difference between the new rule and the old one presented by Sanchez and Lacombe is quite small in the calculation of the enthalpy of mixing and FH interaction parameter and the effect of volume-combining rule on the calculation of thermodynamic properties is much smaller than that of energy-combining rule. But the relative value of interaction parameter changes much due to the new volume-based combining rule. This effect can affect the position of phase diagram very much, which is reported elsewhere [Macromolecules 34 (2001) 6291]