Hollow fiber formation using Lewis acid: Base complex in the polymer solution

In the present work, hollow fiber formation was investigated by using Lewis acid:base complexes in the polymer solution. The studied systems consisted of poly(ether sulfone); N-methyl-2-pyrrolidone (NMP) as solvent and Lewis base, and acetic, propionic, n-caproic, and adipic acids as additives and Lewis acids. Bore liquid was formed by water/NMP solutions, as well as vaseline; whereas water was used as external precipitation bath. The spinneret and precipitation bath distance (i.e., air gap) also varied. The membranes were characterized by scanning electron microscopy and pure gas permeation tests. The influence of the complex dissociation rate was observed mainly when a reduction to water inflow from the bore liquid to the polymer solution occurred, because in this case, dissociation rate was inhibited. Therefore, stability was favored in the sublayer for a longer period, allowing macrovoid formation. These results are also evidenced by the permeability coefficients, which increased as fast as the polymer solution reaches water (i.e., the kinetics of precipitation was accelerated). (C) 20012001 John Wiley & Sons, Inc.