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

Sponge-like and finger-like structures are two distinct membrane structures commonly observed in membranes produced by the wet immersion process. An i ndex phi calculated solely from solubility parameters was defined as an ind icator of the membrane structure. The phi values of four polymers, poly(met hyl metharylate), polysulfone, cellulose acetate, and poly(vinylidene fluor ide), in various solvent-nonsolvent pairs were calculated and compared with the corresponding membrane structures. It was found that the finger-like s tructure often occurred at higher phi values. Although the phi value repres ents mostly the thermodynamics nature of a system, as an index for prior se lection of solvent-nonsolvent pairs for a particular polymer, a general rul e of thumb was developed. Taking 15% polymer concentration and 300 mu m cas ting thickness as a referential casting condition, selecting solvent-nonsol vent pairs with phi values higher than 0.25 is suggested, when a finger-lik e structure is desired. The polymer concentration in the casting solution a nd the casting thickness will also affect the membrane structure. If a high er polymer concentration needs to be used, selection of a polymersolvent-no nsolvent system with a phi value much higher than 0.25 is suggested, or kee ping the casting thickness lower than 300 mu m to obtain a finger-like memb rane structure. (C) 1999 John Wiley & Sons, Inc.