1Department of Chemical Engineering, Amir Kabir University of Technology, Tehran, I.R. IRAN
2Faculty of Engineering, Shahed University, Tehran, I.R. IRAN
Based on Gibbs-Duhem law and using UNIFAC-FV computer algorithm, the activity of polymer component in polymer-solvent systems is determined. The variation of activity of components with concentration at different temperatures are used to find the equilibrium concentration, UCST, and LCST for the systems. Also by generalization of the method, Gel-Liquid equilibrium is studied in some three component systems: polymer-solvent-solvent..