Development of asymmetric 6FDA-2,6 DAT hollow fiber membranes for CO₂/CH₄ separation. 1. The influence of dope composition and rheology on membrane morphology and separation performance

We have determined the effects of dope composition, rheology and process conditions on membrane morphology and gas separation performance of asymmetric poly(2,6-toluene-2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane diimide) (6FDA-2,6 DAT) hollow fiber membranes for air and CO₂/CH₄ applications. Two spinning dope systems were prepared; one is a binary system in N-methyl-2-pyrrolidone (NMP), the other is a ternary system in a mixture of NMP and ethanol (EtOH). Both dopes exhibit non-Newtonian and shear-thinning power-law characteristics. Experimental data show that the shear stress within the spinneret significantly changes the skin and cross-section morphology and separation performance. The degree of changes is more severe in a highly viscous binary system than in an EtOH containing ternary system. For the 6FDA-2,6 DAT/NMP system, both permeances of O₂, N₂, CO₂ and CH₄ and selectivities of O₂/N₂ and CO₂/CH₄ decrease slightly with an increase in shear rate. While for the 6FDA-2,6 DAT/NMP/EtOH system, similar to the previous case, permeances of all testing gases decrease with increasing shear rate because of shear-induced orientation. However, very interestingly and contradictory to the binary dope system, the CO₂/CH₄ selectivity increases with increasing shear rate. The difference in selectivity trend is probably due to the fact that the ternary system has EtOH as a non-solvent additive which shortens the precipitation path and reduces the degree of vigorousness during the phase inversion process. Scanning electron microscopy (SEM) pictures show fibers spun from the viscous binary system tend to have more shear-induced defects in high shear rates. In addition, the elongational stress induced by the take up unit has a tendency to induce molecular orientation as well as to create skin surface defects.