Polymeric membrane materials containing ethylene oxide (K) groups show promising applications in CO2 separation processes for clean energy supply and environmental remediation, but the fabrication of defect-free thin membrane possessing high CO2 permeance and CO2/gas selectivity is still a challenge. In this work, the thin film composite (TFC) membranes containing EO groups were developed by interfacial polymerization (IP) with trimesoyl chloride and commercial diamines named diethylene glycol bis(3-aminopropyl) ether (DGBAmE) and diaminopolyethylene glycol (DAmPEG). The TFC membranes prepared with DGBAmE and DAmPEG were named EO-3 and EO-21 according to the length of EO units, respectively. The structure and separation performance of membranes with short EO units and long EO units were compared, and the concentrations of monomers used in IP were adjusted to optimize the structure and separation performance of membranes. The results show that the structure and separation performance of EO-21 are hardly optimized by simply increasing the concentration of DAmPEG, because in IP process, the long EO units of DAmPEG not only decrease the diffusion rate of DAmPEG from aqueous phase to organic phase, but also hinder the diffusion of DAmPEG through the nascent film for further reaction, which results in the extremely low crosslinked density of EO-21 and the deterioration of gas selectivity at high feed pressure. In contrast, the structure and separation performance of EO-3 are conveniently adjusted by changing concentrations of monomers due to the much shorter EO units of DGBAmE which are beneficial for the diffusion and reaction of DGBAmE in IP process. EO-3 prepared with optimized monomer concentrations displays attractive CO2 permeance and CO2/gas selectivity, which shows promising applications in syngas and natural gas purification and flue gas CO2 capture. (C) 2013 Elsevier B.V. All rights reserved.