Gas diffusivity, solubility and permeability in polysulfone–poly(ethylene oxide) random copolymer membranes

In this study polysulfone–poly(ethylene oxide) (PSF–PEO) random copolymers were synthesized using a polycondensation reaction via nucleophilic substitution. The PSF–PEO random copolymer membranes were prepared by varying PEO content in a glassy PSF matrix to see the effects of PEO segments on the change of gas transport properties. The PEO content varied from 5 mol% (9 wt.%) to 20 mol% (31 wt.%). The gas diffusivity, solubility and permeability of light gases (e.g., He, H2, O2, N2, CO2 and CH4) in these random copolymer membranes were measured as a function of PEO content in PSF, and discussed in relation to the changes in gas transport behavior, including physical and structural properties. Originally we expected that the incorporation of amorphous, flexible PEO segments in a stiff, glassy polymer such as PSF would lead to an increase in gas permeabilities, particularly CO2 permeability because PEO segments with polar ether oxygen units is known to have good affinity with CO2 molecules, thereby resulting in high level of CO2 sorption. However, all gas permeabilities, including CO2 permeability, were unexpectedly reduced with the increase of PEO content (∼31 wt.%) and the permselectivities of gas pairs of interest (e.g., CO2/H2, CO2/N2 and CO2/CH4) were not as high as we anticipated. PSF–PEO random copolymers synthesized in this study do not have well-phase separated structures, that is, each component can be compatible in this copolymer system, leading to no distinct large PEO domains in the PSF–PEO copolymer membranes. As a result, the effect of PEO domains on the gas permeation properties in the PSF–PEO copolymer membranes might be relatively small.