Gas permeation properties of hydroxyl-group containing polyimide membranes

A series of hydroxyl-group containing polyimides (HPIs) were prepared in order to investigate the structure-gas permeation property relationship. Each polymer membrane had structural characteristics that varied according to the dianhydride monomers. The imidization processes were monitored using spectroscopic and thermog-ravimetric analyses. The single gas permeability of He, H-2, CO2, O-2, N-2 and CH4 were measured and compared in order to determine the effect of the polymer structure and functional -OH groups on the gas transport properties. Surprisingly, the ideal selectivity Of CO2/CH4 and H-2/CH4 increased with increasing level of -OH incorporation, which affected the diffusion of H-2 or the solubility of CO2 in HPIs. For H-2/CH4 separation, the difference in the diffusion coefficients of H-2 and CH4 was the main factor for improving the performance without showing any changes in the solubility coefficients. However, the solubility coefficient of CO2 in the HPIs increased at least four fold compared with the conventional polyimide membranes depending on the polymer structures. Based on these results, the polymer membranes modified with -OH groups in the polymer backbone showed favorable gas permeation and separation performance.