FORMATION OF ULTRATHIN HIGH-PERFORMANCE POLYETHERSULFONE HOLLOW-FIBERMEMBRANES

We have demonstrated, for the first time, that ultrathin skin-layer ho llow-fiber membranes with a skin layer of 474 Angstrom can be prepared using mainly a one-polymer and one-solvent system. This is one of the thinnest skin-layer asymmetric hollow-fiber membranes that have ever been reported in the literature for air and gas separation. This work implies that, in order to yield a high-permeance polyethersulfone (PES ) membrane with a skin layer of approximately 500 Angstrom, the additi on of non-solvents into spinning dopes may not be the pre-condition to form ultrathin skin-layer hollow-fiber membranes for gas separation. The keys to fabricate ultrathin skin-layer hollow-fiber membranes are (1) to control the chemistry of the internal coagulant and the bore-fl uid flow rate and (2) to have a dope exhibiting significant chain enta nglement. The newly developed polyethersulfone (PES) hollow fibers hav e an O-2/N-2 selectivity of 5.80 with a permeance of 9.3 x 10(-6) cc(S TP)/cm(2) s cmHg for O-2 at room temperature. The skin layer thickness was calculated to be 474 Angstrom. These hollow fibers were wet-spun from a 35/75 (weight ratio) PES/N-methyl-pyrrolidone (NMP) dope using water as the external coagulant and 80/20 NMP/H2O as the bore fluid. T he hollow fiber must be coated with a silicone elastomer. This work al so suggests that in order to yield a high-permeance PES membrane with a skin layer of approximately 500 Angstrom, there might not exist a cr itical solvent molar volume when preparing the dope solvent mixture, a s previously suggested by the Permea research group. SEM observation o f skin nodules suggests that the skin layer thickness is less than 700 Angstrom.