Phase Diagram Calculations of Water/Tetrahydrofuran/Poly (vinyl chloride)Ternary System Based on a Compressible Regular Solution Model

Modified Flory-Huggins theory with concentration dependent binary interactionparameters is often used to predict the phase diagram in a ternary nonsolvent/solvent/polymer system. The interaction parameters in this model areusually determined experimentally which limit the model predictive capability. In thispaper a compressible regular solution (CRS) model was used to predict the phasediagram of a membrane forming water/tetrahydrofuran/poly(vinyl chloride) system, bypure component properties such as, solubility parameter, coefficient of thermalexpansion and hard-core volume. In this respect, the binodal curve, spinodal curve andthe critical point were determined by numerical calculations. Properties of componentsthat were needed for these calculations have been taken from available data in theliterature. Experimental cloud point data were obtained by the titration method of PVCin THF solutions with concentrations reaching 15 wt%. The good agreement betweenthe theoretical binodal and experimental cloud points indicates that this model is apromising method to calculate the theoretical phase diagram for membrane formingsystems, with particular attention to the fact that no adjustable parameters such asbinary interaction parameters should be used for theoretical calculations. The result ofthese calculations revealed that small amounts of water (less than 10%) is needed forliquid-liquid phase separation in water/tetrahydrofuran/poly(vinyl chloride) system.