Synthesis of nanoporous Torlon® hollow fiber membranes by air-gap spinning method

Nanoporous Torlon® hollow fiber membranes provide a robust platform for versatile separation applications. Conventional fabrication methods, such as additive-assisted or multi-step post-treatment approaches, often face a trade-off between surface pore size and effective surface porosity, as well as increased solvent use and processing complexity. Here, we developed a one-step, additive-free air-gap spinning process using N -methyl-2-pyrrolidone (NMP)/H₂O bore fluids (60–90 wt% NMP) and air-gap distances of 5–80 cm under controlled 25 °C and 30 % RH conditions. At a 5 cm air gap, higher NMP content increased inner-surface porosity, while at 90 wt% NMP, gas transport was governed by an outer-selective Knudsen flow skin (mean pore diameter 4.6 nm). Longer air gaps required lower NMP contents (≤65 wt%) for spinning stability. For fibers spun with 65 wt% NMP, mean pore diameter decreased from 3.0 to 2.1 nm as the air-gap distance increased, attributed to gravity-induced elongational stress. Gas permeance was primarily controlled by effective surface porosity, showing an optimum at 40 cm due to the trade-off between enhanced inner-surface porosity and bulk densification. Mechanical strength remained consistent, while Young's modulus improved with higher draw ratios due to polymer chain orientation. This simplified fabrication route eliminates pore-forming additives and post-treatment steps, reduces solvent use, yields debris-free inner surfaces, and preserves the thermal stability of nanoporous Torlon® hollow fibers for advanced separation applications.