Correlating gas permeability and morphology of bio-based polyether-block-amide copolymer membranes by IR nanospectroscopy

The gas permeability of polymer membranes is determined by their nanoscale morphology, which strongly depends on the membrane fabrication. Here, we demonstrate how puzzling gas permeability and fabrication-dependent nanoscale morphology of polymer membranes can be elucidated by infrared (IR) nanospectroscopy based on elastic IR scattering at an atomic force microscope tip. Specifically, we fabricated membranes of PEBAX® RNEW – a bio-based polyether-block-amide copolymer – by solvent casting and extrusion, achieving unprecedented CO2 permeability and CO2/N2 selectivity for the solvent cast membranes. For the extruded membranes, however, we found an about 50% reduced CO2 permeability, which could not be explained by differential scanning calorimetry and conventional IR spectroscopy. In contrast, IR nanospectroscopy revealed a highly crystalline polyether oxide (PEO) surface layer on the extruded membranes, not observed for the solvent cast membranes. Annealing of the extruded membrane at 110 ºC transformed the crystalline into amorphous PEO layers, as confirmed by IR nanospectroscopy, yielding a gas permeability close to that of the solvent cast membrane. We thus attribute the dramatic gas reduction of the extruded membrane to its highly crystalline surface layer. Thus, studying polymer morphology by IR nanospectroscopy provides valuable information for better understanding the local gas permeability properties of polymer membranes.