Thickness Dependence of Macrovoid Evolution in Wet Phase-Inversion Asymmetric Membranes

Following an inspiration from Vogrin et al. ( J. Membr. Sci . 2002, 207, 139), we have investigated the thickness dependence of macrovoid evolution during the phase inversion of asymmetric flat membranes and observed, for the first time, a critical structure-transition thickness, L c, which indicates a transition of the membrane morphology from a spongelike to a fingerlike structure with an increase in membrane thickness. Below L c, the membranes show a fully spongelike cross-sectional structure, whereas above L c, the membranes exhibit a mainly fingerlike macrovoid structure. Two binary solutions are employed in this study: 20 wt % polyethersulfone/ N -methyl-2-pyrrolidone (NMP) and 20 wt % BTDA−MDI/TDI co-polyimide/NMP solutions. Their corresponding critical thicknesses, L c, are 1.5 ± 0.4 and 11 ± 2 μm, respectively.