Gas Transport Behaviors of XTR-PBOI Membranes.The gas permeabilities (P) as well as ideal separation factors (α) for some interesting gas pairs of TR membranes are listed in Table 5. For comparative purposes, the permeability and ideal selectivity for related polybenzoxazoles aPBO and tPBO, from azeotropically and thermally imidized hydroxylhomopolyimide 6FDA-bisAPAF,14 have also been included. As can beseen, the gas permeability of TR-PBO membrane decreased about 1.5 fold while selectivity remained almost unchanged in with previously reported aPBO.14 From PALS results (Table 4), the different cavity size distribution might be the main reason contributing to this effect: smaller cavity sizes and higher intensities for the larger pores were present for TRPBO sample, whereas size and number of the smaller pores was