Membrane technology is extensively applied to gas separation. For this application, high gas permeability and gas pair selectivity are the two most important criteria for choosing membrane materials. See, e.g., P. Bernardo, et al., Ind. Eng. Chem. Res., 48, 2009, 4638 and V. Abets, et al., Adv. Eng. Mater. 8, 2006, 328.
Cross-linked aromatic polyimide materials have received much attention for use in gas separation. Methods to cross-link aromatic polyimides include ultraviolet irradiation of benzophenone-containing polyimide chains, thermal treatment of polyimide chains containing acetylene end groups, and cross-linking polyimide chains by small molecules with multiple reactive groups. See H. Kita, et al., J. Membrane. Sci. 87, 1994, 139; Y. Xiao, et al., J. Membrane. Sci., 302, 2007, 254; and C. Staudt-Bickel et al., J. Membr. Sci., 155, 1999, 145. These methods tend to increase chain packing and inhibit intra-segmental and inter-segmental mobility among chains, resulting in improved gas pair selectivity but sacrificing gas permeability.
There is a need for improved polyimide membranes that can be used in gas purification with both high gas permeability and gas pair selectivity.