Effects of amidation on gas permeation properties of polyimide membranes

Abstract

The effects of amidation on gas separation properties of polyimide dense films were investigated. Using 6FDA-durene and 6FDA-durene/mPDA (50:50) as the examples, the amidation was performed by immersing these polyimide dense films in a 10% (w/v) N,N-dimethylaminoethyleneamine (DMEA) hexane solution for a certain period of time at ambient temperature. FTIR spectra indicate the intensities of the characteristic peaks of imide group decrease, while those of amide groups increase with increasing the immersion time. Gas permeabilities of the modified polyimides to He, O2, N2, CO2 and CH4 were measured at 35°C and the results suggest that the proposed amidation lowers the gas permeabilities of all gases, while improves the gas permselectivities of He/N2 and O2/N2. Experimental results also imply that the effects of amidation on gas permselectivity strongly depend on the polyimide chemical structure. For the gas pairs of CO2/N2 and CO2/CH4, the DMEA modified 6FDA-durene/mPDA (50:50) shows enhanced gas permselectivities, whereas the DMEA modified 6FDA-durene exhibits worse gas permselectivities. Since the amidation by DMEA and the chemical cross-linking modification by p-xylene diamine display similar effects on gas separation properties of modified polyimides, one may conclude that the changes of imide groups to amide groups in the cross-linking process has remarkable effects on gas separation properties. The changes in gas separation properties were discussed in terms of diffusion coefficients and solubility coefficients. © 2002 Elsevier Science B.V. All rights reserved.