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 , CO 2 , O 2 , N 2 and CH 4 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 CO 2 /CH 4 and H 2 /CH 4 increased with increasing level of -OH incorporation, which affected the diffusion of H 2 or the solubility of CO 2 in HPIs. For H 2 /CH 4 separation, the difference in the diffusion coefficients of H 2 and CH 4 was the main factor for improving the performance without showing any changes in the solubility coefficients. However, the solubility coefficient of CO 2 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.