A modified NRTL equation for the calculation of phase equilibrium of polymer solutions

A local composition model based on the lattice theory, which is a modified non-random two-liquid (NRTL) equation, is developed for the representation of the Helmholtz free energy of polymer solutions. A simple temperature dependence of the model parameters is introduced to account for the oriented interactions without changing the general formulation of the free energy of mixing. The model provides a flexible thermodynamic framework for both correlating and predicting the phase equilibrium of polymer solutions. Special attention is paid to the representation of phase behavior of homologous polymer solutions and systems with or without oriented interactions.