Development of asymmetric 6FDA-2,6DAT hollow fiber membranes for CO2/CH4 separation: Part 2. Suppression of plasticization

We have studied the CO2-induced plasticization phenomenon of asymmetric poly(2,6-toluene-2,2-bis(3,4-dicarboxyphenyl) hexafluoropropane diimide) (6FDA-2,6 DAT) hollow fiber membranes for CO2/CH4 applications. Several processing and thermal approaches have been investigated to study their effectiveness to enhance anti-plasticization characteristics. Experimental results indicate that hollow fiber membranes spun at different shear rates and take-up rates cannot effectively suppress the CO2 induced plasticization. Thermally treated 6FDA-2,6 DAT hollow fiber membranes show significant reduction in CO2-induced plasticization. Wide-angle XRD spectra reveal no visible change in d-space after thermal treatment, while solubility data imply no cross-links occurred. Scanning electron microscopy (SEM) pictures illustrate heat treatment results in more compact selective-skin layer and substructure, thus strengthening the anti-plasticization characteristics of hollow fibers. By considering the degree of plasticization, dense-layer thickness, and heat treatment temperature, an optimal temperature of 250 °C (for 5 min) is identified for the heat treatment of 6FDA-2,6 DAT hollow fiber membranes. NMR spectra suggest the cause of forming a highly densified skin after heat treatment is mainly due to chain relaxation and enhanced nodule interaction at elevated temperatures.