Chemical cross-linking modification of 6FDA-2,6-DAT hollow fiber membranes for natural gas separation

A simple and practical chemical cross-linking method has been demonstrated to make 6FDA-2,6-DAT asymmetric hollow fibers more resistant to plasticization by immersing them into a p-xylenediamine or m-xylenediamine/methanol solution for a short period of time at ambient temperature. FTIR spectra confirm that chemical cross-linking reactions take place between xylenediamine and imide groups of 6FDA-2,6-DAT and form amide groups. The effects of cross-linking modifications on gas separation performance and the resistance to plasticization characteristics are examined by using both pure and CO2/CH4 mixed gas tests. Permeances of all gases tested decrease with an increase in the degree of cross-linking, while CO2/CH4 permselectivity varies in a narrow range. 6FDA-2,6-DAT hollow fibers show favorable resistance to plasticization once the cross-linking reaches a certain degree. XRD spectra indicate almost no changes on the average intersegmental distance of polymer chains after cross-linking modifications, strongly indicating the cross-linking modifications likely protect nodule integrity from CO2-induced swelling and restrict polymer chain vibration for diffusion jumps. In addition, we found that m-xylenediamine has a similar cross-linking effectiveness as p-xylenediamine on 6FDA-2,6-DAT hollow fibers, both yield hollow fiber membranes with comparable CO2/CH4 selectivity and permeance.