The effects of chemical modifications on morphology and performance of 6FDA-ODA/NDA hollow fiber membranes for CO₂/CH₄ separation

We have studied the rheological characteristics, membrane formation and CO₂-induced plasticization phenomenon of almost defect-free copoly(4,4′-diphenylene oxide/1,5-naphthalene-2,2′ -bis(3,4-dicarboxylphenyl) hexafluoropropane diimide) (6FDA-ODA/NDA) hollow fiber membranes. Experimental results indicate that the dope rheology changes from a Newtonian to non-Newtonian fluid with elasticity when increasing shear rate. The asymmetric 6FDA-ODA/NDA hollow fiber membranes exhibit plasticization if the feed pressure is greater than 75psi. Chemical cross-linking modifications with p-xylenediamine were conducted at ambient temperature in order to enhance anti-plasticization characteristics of 6FDA-ODA/NDA hollow fiber membranes. Chemical modified membranes show a significant reduction in CO₂-induced plasticization at least up 550psi (37.4atm or 37.9×10⁶Pa) if the immersion time is great 1.5min. However, deteriorations in permeance and permselectivity are clearly observed if the immerse time is greater than 5min. FTIR spectra confirm that the 6FDA-ODA/NDA polyimide can be chemically cross-linked with p-xylenediamine, while wide-angle X-ray diffraction (WAXD) spectra imply the d-space of 6FDA-ODA/NDA membranes slightly decreases with an increase in cross-linking time. SEM pictures show that the outer selective skin as well as substructure becomes denser and thicker after chemical modification. The slight decrease in d-space may account for the permselectivity increase for the cross-linked membranes if the immersion time is short. However, the benefit of d-space reduction is offset by a denser and more compact selective skin and substructure when the immersion is prolonged.