A polymeric membrane composed of poly(amidoamine) (PAMAM) dendrimer immobilized in a poly (ethylene glycol) (PEG) network expresses excellent CO2 separation properties over smaller H-2. The preferential CO2 permeation can be explained by specific interaction between CO2 and primary amine of the dendrimer, which enhances CO2 solubility into the polymeric membrane. CO2 forms carbamate with the amines or bicarbonate in the presence of water determined by inverse-gate decoupled C-13 NMR. The resulting carbamate ion pair works to form a quasi-crosslinking, which would suppress H-2 permeation by a CO2-selective Molecular Gate, while bicarbonate ion can be a major moving species to pass through the polymeric membrane. Attenuated total reflection (ATR) indicates the formation of carbamate. Small-angle X-ray scattering (SAXS) reveals increase in scattering intensity under CO2 atmosphere due to the formation of scattering particles, which can be a cluster of the dendrimer-CO2 crosslinks. Tensile testing of the membrane exhibits increase in both Youngs modulus and elongation-to-break by CO2 treatment, suggesting that the crosslinking is reversible and rearrangeable. Differential scanning calorimetry (DSC) also shows an exothermic peak at 120 degrees C, which is associated with dissociation of the crosslinks. (C) 2013 Elsevier B.V. All rights reserved.