Polyethylene glycols have received worldwide attention as a highly permeable CO2‐philic polymer in carbon dioxide separation applications. In this study, we investigated the influence of a low molecular weight polyethylene glycol (PEG 200) on physicochemical, morphological, and gas separation properties of Matrimid®5218 as a novel polymer blend. Both symmetric and asymmetric Matrimid flat sheet membranes with 0–20 wt.% PEG were prepared via a dense film‐casting method. The miscibility of blends at low PEG concentrations (3–5 wt.%) was confirmed by differential scanning calorimetry (DSC) and polarized light microscopy (PLM). Moreover, the blends were partially miscible at higher PEG concentrations (10–20 wt.%). Matrimid/PEG molecular interactions were further studied by FTIR and XRD analysis. SEM images indicated an impressive influence of PEG on the symmetric structure of Matrimid membrane. An asymmetric structure with a dense thin skin layer and a highly porous sub‐layer was observed for the blends containing 10–20 wt.% PEG. The CO2 permeability and CO2/CH4 selectivity of the best‐yield CO2‐selective blend membrane (Matrimid/PEG (95:5)) incvreased about 25% (from 7.68 to 9.62 Barrer) and 15% (from 35 to 40) compared to pure Matrimid, respectively. Furthermore studying the plasticization pressures of the membranes revealed that the more PEG content in blend the less the plasticization pressure obtained. © 2015 Society of Chemical Industry and John Wiley & Sons, Ltd

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