Polymeric membranes comprised of poly(amidoamine) (PAMAM) dendrimer immobilized in a poly(ethylene glycol) (PEG) network exhibit an excellent CO2 separation selectivity over H-2. The CO2 permeability increases with PAMAM dendrimer concentration in the polymeric membrane and becomes 500 times greater than H-2 permeability when the dendrimer content was 50 wt % at ambient conditions (5 kPa of CO2 partial pressure). However, the detailed morphology of the membrane has not been discussed. The immiscibility of PAMAM dendrimer and PEG matrix results in phase separation, which takes place in a couple of microns scale. Especially, laser scanning confocal microscope captures a 3D morphology of the polymeric blend. The obtained 3D reconstructions demonstrate a bicontinuous structure of PAMAM dendrimer-rich and PEG-rich phases, which indicates the presence of PAMAM dendrimer channel penetrating the polymeric membrane, and CO2 will preferentially pass through the dendrimer channel. In addition, Fourier transform of the 3D reconstructions indicates the presence of a periodic structure. An average size of the dendrimer domain calculated is 2-4 mu m in proportion to PAMAM dendrimer concentration. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012