Interfacial Control of Pebax/PDMS-b-PEO Composite Membranes via Air Plasma Treatment for Enhanced CO2/N2 Separation

Pebax selective layers contain hydrophilic polyethylene oxide (PEO) segments. These polar chains do not spread uniformly on hydrophobic polydimethylsiloxane (PDMS) intermediate layers. This poor wetting makes defect-free composite membranes difficult to obtain for CO2/N2 separation. Although air plasma (AP) treatment could enhance surface hydrophilicity and promote Pebax adhesion, excessive treatment duration inevitably leads to an over-thickened selective layer and the formation of a dense SiOx barrier, severely compromising gas permeability. To address this trade-off, this study proposes a synergistic strategy combining AP surface engineering with precise blade-gap adjustment to control the Pebax selective layer thickness. PDMS-b-PEO was introduced into the PDMS intermediate layer to mitigate plasma-induced permeability loss through PEO segment migration. Systematic characterization revealed that AP treatment primarily improves Pebax coating through hydrophilic modification rather than etching effects. The optimized membrane, treated with AP for 20 s at a PDMS:PDMS-b-PEO ratio of 1:1, achieved a CO2 permeance of 1711.6 GPU and a CO2/N2 selectivity of 44.4, representing an optimal balance between permeability and selectivity. Density functional theory (DFT) calculations further elucidated that the Pebax/PDMS-b-PEO blend network forms “synergistic polar regions,” enhancing CO2 adsorption energy from −20.77 kJ/mol to −26.26 kJ/mol while leaving N2 adsorption unchanged, thereby increasing the CO2/N2 adsorption selectivity by 68.2%. These findings demonstrate that interfacial engineering combined with thickness control offers a facile and effective route for high-performance CO2 separation membranes.