The present invention appreciates that compounds comprising nitrogen-containing moieties that are at least divalent (e.g., urea, urethane, amide, etc.) can be reacted with azides using at least radiation energy to initiate the reaction between at least a portion of the compounds and the azides to form membranes that have surprisingly high selectivities for acid gases relative to nonpolar gases such as hydrocarbons. The membranes are also resistant to CO2 plasticization and have high acid gas flux characteristics. The resultant membranes can be extremely thin (e.g., 10 micrometers or less), which promotes high permeability for the acid gas and can translate into high productivity on a scaled-up, industrial level.