Polyimide materials based on pyromellitic dianhydride for the separation of carbon dioxide and methane gas mixtures

The sorption and transport of carbon dioxide and methane in a series of dense polyimide films were examined to evaluate these materials for gas separation applications. The polyimides described in this study are based on the reaction of pyromellitic dianhydride (PMDA) with oxydianiline (ODA), methylene dianiline (MDA), or isopropylidene dianiline (IPDA). The carbon dioxide permeabilities for this series of compounds increased in the order: PMDA-ODA < PMDAMDA<PMDA-IPDA, while the permselectivity increased in the opposite order: PMDA-IPDA < PMDA-MDA < PMDA-ODA. Sorption studies coupled with mixed gas permeability measurements demonstrated that the trends in permeabilities and permselectivities were due primarily to differences in the mobilities of the gases in the various films. The trends observed for permeabilities and selectivities were correlated with trends in the macroscopic densities and average intersegmental distance for each film as measured by wide angle X-ray diffraction. These results indicated that in addition to intrachain motions, interchain spacing differences are contributing, if not dominating, factors in understanding differences in the transport behavior of gas through these polyimide films.