Composite membranes were fabricated by incorporating carboxylic acid nanogels (CANs) into a Pebax MH 1657 matrix for potential applications in CO2/CH4 (or N-2) separation. CANs were synthesized by precipitation 'polymerization of N-isopropylacrylamide (NIPAM), acrylic acid (AA) monomer and N,N '-methylenebisacrylamide (BIS) crosslinker. The incorporation of CANs simultaneously tailored favorable water environment and increased CO2 transport sites within the membranes. With increasing CANs loading, the homogeneously dispersed CANs in the Pebax matrix offered more favorable water environment and the additional carboxyl group sites for efficient CO2 transport, thus constructing interconnected preferential CO2 transport passageways. The Pebax-CANs-30 composite membrane exhibited the highest gas separation performance, with the CO2/CH4 and CO2/N-2 selectivities of 33 and 85 respectively, and a CO2 permeability of 2026 Barrer (1 Barrer=10(-10) cm(3) (STP) cm/(cm(2) s cmHg), well surpassing the Robeson's upper bound limit reported in 2008. This effort will give rise to a new option to tailor water environment and increase CO2 transport sites for fabricating high-performance CO2 separation membranes. (C) 2015 Elsevier B.V. All rights reserved.