ELSEVIER Journal of Membrane Science 89 ( 1994) 37-50 Thermally induced phase separation in ternary crystallizable polymer solutions Hitesh C. Vadalia”, Hwan K. Lee”, Allan S. Myerson=, Kalle Levonb,* ‘Department of Chemical Engineering and bDepartment of Chemistry, Polytechnic University, 6 Metrotech Center, Brooklyn, NY 11201, USA (Received June 7, 1993; accepted in revised form November 19, 1993 ) zyxwvutsrqponmlkjihgfedcbaZYX Abstract Microporous membranes were prepared by the thermally induced phase separation (TIPS) process of a ternary solution of HDPE, ditrydecylphthalate and hexadecane. The equilibrium phase diagram calculations were carried out in terms of a pseudo-binary system, considering the solvent mixture to be one component, to show the effect of the interaction parameter on the phase diagram. The experimental results showed that by varying the composi- tion of the solvent pair, i.e., by varying the interaction parameter systematically, the phase diagrams can be con- trolled successfully. Liquid-liquid phase separation was obtained also below the melting point by controlling the composition of the solvent mixture. It is also shown that the membrane morphology can be controlled with differ- ent solvent compositions when the cooling conditions are kept constant. Key words: Microporous and porous membranes; Thermodynamics 1. Intraduction Thermally induced phase separation (TIPS ) in polymer mixtures is one of the most versatile and widely utilized methods [ zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1 ] to produce a variety of microporous membranes of controlled pore size from the thermoplastic polymers in- cluding polyolefins, condensation and oxidation polymers, copolymers and blends [ 21. The TIPS process may occur by liquid-liquid (GL) phase separation and/or liquid-solid (GS) phase sep- aration [ l-6 1. In L-S separation, kinetics of the polymer crystallization and in some cases that of the sol- vent crystallization play a major role. GL phase *Corresponding author. separation may be induced by reducing the tem- perature when the favorable interaction between the polymer and diluent decreases, or by increas- ing the temperature when the free volume differ- ence increases. A system that undergoes phase separation by decreasing the temperature is de- scribed as an upper critical solution temperature (UCST) type of phase separation. In this study we are concerned with the UCST behavior in a ternary system. The system under consideration contains two solvents and a semicrystalline poly- mer, and L-L phase separation may be coupled with crystallization. The final morphology in a polymer-solvent system depends upon the kinetics as well as the thermodynamics of the phase separations. The equilibrium phase diagram is a good tool for 03767388/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI0376-7388(93)E0207-Z