Composite membranes were fabricated by incorporating poly(N-isopropylacrylamide) nanohydrogels (NHs) into Matrimid (R) 5218 matrix to improve the separation performance for CO2/CH4 and CO2/N-2 mixtures. The membranes were characterized by a Courier transform infrared spectrometer (FT-IR). scanning electron microscopy (SEM), tensile Lest, dynamic mechanical analysis (DMA), X-ray diffraction (XRD), positron annihilation lifetime spectroscopy (PALS), the static contact angle and water content measurement. The incorporation of nanohydrogels increased the fractional free volume of the composite membranes, water uptake and water retention capacity. The composite membranes displayed better performance than the pure Matrimid (R) membrane. The nanohydrogels homogeneously embedded in the Matrimid (R) matrix acted as water reservoirs to not only provide more water for dissolving CO2, but also construct interconnected CO2 transport passageways. The as-prepared Matrimid (R)/NIHs-20 composite membrane showed CO2/CH4 and CO2/N-2 selectivities of 61 and 52 with a CO2 permeability of 278 Barrer, surpassing or being close to the 2008 Robeson upper bound lines. (C) 2014 Elsevier B.V. All rights reserved.