Fundamental understanding of the material science and theological engineering to fabricate Torlon (R) 4000T-MV and 4000TF hollow fiber membranes with an ultra-thin and defect-free dense-selective layer for gas separation has been revealed. We have firstly investigated the rheology of Torlon (R) 4000T-MV and 4000TF dope solutions, and then determined the effect of temperature-correlated shear and elongational viscosities on the formation of Torlon (R) fibers for gas separation. Interestingly, Torlon (R) 4000T-MV and 4000TF possess different theological characteristics: the elongational viscosity of Torlon (R) 4000T-MV/NMP solution shows strain thinning, while Torlon (R) 4000TF/NMP solution shows strain hardening. The balanced viscoelastic properties of dope solutions, which are strongly dependent on the spinning temperature, have been found to be crucial for the formation of a defect-free dense layer. The optimum theological properties to fabricate Torlon (R) 400OT-MV/NMP hollow fibers appear at about 48-50 degrees C, and the resultant fibers have an O-2/N-2 selectivity of 8.37 and an apparent dense layer thickness of 781 angstrom. By comparison, the best Torlon (R) 4000TF fibers were spun at 24 degrees C with an O-2/N-2 selectivity of 8.96 and a dense layer of 1116 angstrom. The CO2/CH4 selectivity of the above two Torlon (R) variants is 47 and 53.5, respectively. (C) 2008 Elsevier B.V. All rights reserved.