The rheology of Torlon® solutions and its role in the formation of ultra-thin defect-free Torlon® hollow fiber membranes for gas separation

Fundamental understanding of the material science and rheological engineering to fabricate Torlon® 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® 4000T-MV and 4000TF dope solutions, and then determined the effect of temperature-correlated shear and elongational viscosities on the formation of Torlon® fibers for gas separation. Interestingly, Torlon® 4000T-MV and 4000TF possess different rheological characteristics: the elongational viscosity of Torlon® 4000T-MV/NMP solution shows strain thinning, while Torlon® 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 rheological properties to fabricate Torlon® 4000T-MV/NMP hollow fibers appear at about 48-50 °C, and the resultant fibers have an O2/N2 selectivity of 8.37 and an apparent dense layer thickness of 781 Å. By comparison, the best Torlon® 4000TF fibers were spun at 24 °C with an O2/N2 selectivity of 8.96 and a dense layer of 1116 Å. The CO2/CH4 selectivity of the above two Torlon® variants is 47 and 53.5, respectively.