The present invention appreciates that compounds comprising ester linkages and nitrogen-containing moieties that are at least divalent (e.g., urea, urethane, amide, etc.) can be crosslinked with azides to form membranes that are resistant to CO2 plasticization, that are selective for acid gases relative to nonpolar gases such as hydrocarbons, and that have high acid gas flux characteristics. The resultant membranes have stable structure and stable separation properties at higher temperatures and pressures. The membranes are compatible with many industrial processes in which polar gases are separated from nopolar gases. In an exemplary mode of practice, the membranes can be used to separate acid gases from the hydrocarbon gases in natural or non-acid gas components of flue gas mixtures (e.g., N2, O2, etc.).