Journal of Membrane Science 187 (2001) 47–55 Phase behavior and mechanism of membrane formation for polyimide/DMSO/water system Jong Hak Kim a , Byoung Ryul Min a , Jongok Won b , Hyun Chae Park b , Yong Soo Kang b,∗ a Department of Chemical Engineering, Yonsei University, Seoul 120-749, South Korea b Polymer Physics Laboratory, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, South Korea Received 8 November 1999; received in revised form 11 May 2000; accepted 6 November 2000 Abstract A macrovoid-free, sponge-like porous membrane was prepared from polyimide/DMSO/water system, whereas a finger-like membrane was obtained from a polyimide/NMP/water system by phase inversion. The sponge-like membrane structure was investigated by the thermodynamics and kinetics of the phase separation process. The rates of phase separation for both systems were similar, but their phase diagrams were significantly different. The distinct features of the phase diagram for the polyimide/DMSO/water system are: (1) extremely narrow miscibility gap, (2) close gelation point to the binodal curve, and (3) almost parallel tie-line passing through the gelation point to the polymer–solvent axis. These properties hinder the phase separation process to proceed further, i.e. the growth of the polymer-lean phase is, thus frozen or stopped in the early stages. Therefore, the membrane morphology will be determined at an earlier stage in the polyimide/DMSO/water system than polyimide/NMP/water, resulting in a macrovoid-free, sponge-like membrane. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Phase inversion; Porous membrane; Polyimide; Membrane structure; Thermodynamics 1. Introduction Phase inversion is a well-known process for prepar- ing a variety of asymmetric membranes [1–3]. In this process, a homogeneous polymer solution consisting of a polymer and an adequate solvent with or with- out an additive is cast on a glass plate, and immersed in a coagulation bath. The diffusive exchange of sol- vent and nonsolvent introduces liquid–liquid phase separation, i.e. the formation of a polymer-rich and a polymer-lean phase in the casting solution lowers ∗ Corresponding author. Tel.: +82-2-958-5362; fax: +82-2-958-6869. E-mail address: yskang@kist.re.kr (Y.S. Kang). the Gibbs free energy of mixing. The successive so- lidification of the phase-separated solution leads to a porous, asymmetric structure. Thus, the morphology and performance of membranes depend strongly on the thermodynamics as well as kinetics of the phase separation process [4–6]. It is well known that systems exhibiting high mutual affinity of solvent with nonsolvent yield finger-like structures because of instantaneous phase separation [3,7–9]. Several researchers have re- ported that membranes prepared with DMSO/water as well as NMP/water showed a finger-like struc- ture, irrespective of the polymer type [8–12]. On the contrary, we have obtained a typical sponge-like mem- brane from a polyimde/DMSO/water system unlike 0376-7388/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0376-7388(00)00648-7