Influence of diamine moieties on the gas permeation performances of polyimide: perspectives from experiment and simulation

This work reported the gas permeation performances of three 6FDA-based polyimides, including 6FDA-DABA, 6FDA-ODA and 6FDA-TFDB, which contain polar amide, flexible ether, and bulky -CF3 groups in diamine units, respectively. The effect of diamine units on the microstructure and gas transmission performance of polyimides was detailly researched via positron annihilation, X-ray diffractogram and molecular simulations, with respect to chain packing, hydrogen bonding, free volumes and movement of molecular chains. As the diamine units changed from amide to ether and -CF3, the gas transport coefficient of polyimides enhanced, and the gas transport selectivity (including H-2/N-2, O-2/N-2, CO2/CH4 and CO2/N-2) decreased. With the introduction of amide to diamine moieties, a large number of hydrogen bonds were formed in 6FDA-DABA, which improved the chains packing and decreased free volumes and chains movement. Consequently, the 6FDA-DABA showed the lowest permeation coefficients. Bulky -CF3 in the diamine greatly hindered chain stacking, which increased free volume, promoted chains movement and endowed 6FDA-TFDB with the highest permeation coefficients. The decreasing permeation selectivity from 6FDA-DABA to 6FDA-ODA and 6FDA-TFDB was primarily ascribed to the reduction of diffusive selectivity, which was stemmed from the improved free volume and chains mobility.

Automated summary

This study investigated the gas permeation performances of three 6FDA-based polyimides with different diamine units - 6FDA-DABA, 6FDA-ODA, and 6FDA-TFDB. The introduction of amide, ether, and -CF3 groups in the diamine units had an impact on the microstructure and gas transmission performance of polyimides. The research findings showed that the gas transport coefficient increased while the gas transport selectivity decreased with the change of diamine units.