Selection criteria for solvent and coagulation medium in view of macrovoid formation in the wet phase inversion process

Abstract

Sponge-like and finger-like structures are two distinct membrane structures commonly observed in membranes produced by the wet immersion process. An index Φ calculated solely from solubility parameters was defined as an indicator of the membrane structure. The Φ values of four polymers, poly(methyl metharylate), polysulfone, cellulose acetate, and poly(vinylidene fluoride), in various solvent-nonsolvent pairs were calculated and compared with the corresponding membrane structures. It was found that the finger-like structure often occurred at higher Φ values. Although the Φ value represents mostly the thermodynamics nature of a system, as an index for prior selection of solvent-nonsolvent pairs for a particular polymer, a general rule of thumb was developed. Taking 15% polymer concentration and 300 μm casting thickness as a referential casting condition, selecting solvent-nonsolvent pairs with Φ values higher than 0.25 is suggested, when a finger-like structure is desired. The polymer concentration in the casting solution and the casting thickness will also affect the membrane structure. If a higher polymer concentration needs to be used, selection of a polymer-solvent-nonsolvent system with a Φ value much higher than 0.25 is suggested, or keeping the casting thickness lower than 300 μm to obtain a finger-like membrane structure. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1495–1502, 1999