The role of additives on dope rheology and membrane formation of defect-free Torlon^® hollow fibers for gas separation

The high demands on high performance membranes for energy, water and life science usages provide the impetus for membrane scientists to search for a comprehensive understanding of membrane formation from molecular level to design membranes with desirable configuration and separation performance. This pioneering work is to elaborate the importance of polymer rheology on hollow fiber formation and reveal the integrated science bridging polymer fundamentals such as polymer cluster size, shear and elongational viscosities, molecular orientation, stress relaxation to membrane microstructure and separation performance for gas separation. Torlon^® poly(amide imide) was employed in this study with various solvent/nonsolvent additives. The effects of additives on polymeric cluster size, hydrogen bonding and dope rheology during the phase inversion have been examined. It is found that hydrogen bonding and strain-hardening characteristics play very important roles in dope rheology and membrane separation performance. Torlon^® possesses strong hydrogen bonds with NMP/water mixtures, the addition of a small amount of water enlarges polymer cluster size, strengthen molecular network (i.e., strain hardening) and facilitate macrovoid-free morphology. However, strong hydrogen bonding may retard chain unfolding during spinning, induce faster relaxation for highly oriented dense-selective skin, and thus reduce gas-pair selectivity. By adjusting dope chemistry and spinning conditions with balanced solubility parameters and dope rheology, we have developed defect-free Torlon^® hollow fiber membranes with an O₂/N₂ selectivity of 8.55 and an ultra-thin layer of 488 Å simply using water as the additive. Fibers spun from dopes containing other additives have the optimal O₂/N₂ selectivity varying from 7.69 to 9.97 at 25 ± 2 °C, and the dense layer thickness varying from 500 Å to 2000 Å. Their corresponding mixed-gas O₂/N₂ selectivity for compressed air varies from 7.12 to 9.00.