Chemical cross-linking modification of polyimide/poly(ether sulfone) dual-layer hollow-fiber membranes for gas separation

A novel engineering approach on cross-linking modification of polyimide hollow-fiber membranes is reported. The concept is demonstrated using a dual-layer hollow-fiber membrane structure, where a polyimide {copoly[1,4-durene/1,3-phenylene-2,2-bis(3,4-dicarboxyphenyl) hexafluoropropanediimide] (6FDA-durene/mPDA) (50:50)} is chosen as the outer layer and poly(ether sulfone) (PES) is selected as the inner layer. Chemical cross-linking modification occurs at the outer polyimide layer by immersing the dual-layer hollow fibers in a 5% (w/v) p-xylenediamine/methanol solution at ambient temperature for a short period of time. Fourier transform infrared studies show that chemical cross-linking modification takes place by the formation of amide groups through the reactions between p-xylenediamine and imide groups. The PES inner layer is found to be immune from the proposed chemical cross-linking modification and remains porous and flexible as a supporting layer. Pure gas tests show that chemical cross-linking modification of dual-layer hollow fibers results in a reduction in permeance but significantly enhances the CO₂/N₂ and especially CO₂/CH₄ selectivities. The proposed chemical cross-linking modification also makes the polymer more resistant to plasticization and thus reduces the CO₂-induced increase in gas permeance.