We wish to highlight a long-neglected issue in nanofilm composite (NFC) membranes for gas separation: selective nanofilms (10 μm) due to nanoconfinement and interfacial interactions with substrates. We synthesize three series of poly(ethylene glycol) (PEG)-based copolymers with excellent intrinsic CO2/N2 separation properties. When they are fabricated into NFC membranes with selective layers of 15–95 nm, CO2 permeability decreases dramatically owing to nanoconfinement and the affinity between the copolymers and dopamine-modified gutter layer, while CO2/N2 selectivity remains similar. For example, a copolymer (PEGDA5) synthesized from 95% PEG methyl ether acrylate (PEGMEA) and 5% PEG diacrylate (PEGDA) exhibits CO2 permeability of 420 Barrer for bulk films but only 120 Barrer for a 95 nm layer. Nevertheless, the membrane exhibits CO2 permeance of 1570 GPU and CO2/N2 selectivity of 52 at 25 °C, comparable to state-of-the-art membranes and surpassing Robeson’s upper bound. The nanoscale behaviors elucidated in this study should be useful for designing NFC membranes for important gas separations.
Dong et al. (Mon,) studied this question.