A series of copolymer membranes was prepared using polyimide (PI) and hydroxyl-containing polyimide (HPI) precursors. Thermal conversion of the hydroxyl-containing imide group into a benzoxazole structure was performed at the solid state to increase rigidity of the polymer backbone, and thus generate free volume elements to improve gas separation performance of the resultant copolymer membrane. Free volume cavities produced during thermal conversion were easily controlled by varying HPI composition in the copolymer. Evidence of thermal conversion was confirmed using spectroscopic and thermogravimetric analysis. O(2) permeability of copolymer membranes varied from 0.17 Barrer (1 Barrer = 1 x 10(-10) cm(3) (STP) cm/cm(2) s cmHg) to 220 Barrer depending on membrane composition without a significant loss in selectivity. Also, fully converted polybenzoxazole (PBO) membranes showed high CO(2) permeability (1014 Barrer) with a CO(2)/CH(4) selectivity of 24. The copolymer membranes presented here easily overcome the conventional polymeric upper bound limit, and are comparable to the gas separation performance of superior membrane materials such as carbon molecular sieves. The copolymer membrane was also expected to improve the shape properties of the polymer membrane. (C) 2010 Elsevier B.V. All rights reserved.