Effects of chemical structure on gas transport properties of polyethersulfone polymers

Compared to polysulfone (PSF), the effects of chemical structure on gas separation performance of polyethersulfone (PES) are rarely in-depth studied. To explore the potential of various PES polymers as gas separation membranes, PES containing four different backbone structures have been synthesized. They are polyphenylsulfone (PPSU), poly trimethyl benzene ethersulfone (TPESU), polyethersulfone (PESU) and hydrophilic polyethersulfone (HPESU). Both wide-angle X-ray diffraction (XRD) and positron annihilation life-time spectroscopy (PALS) data indicate that PPSU and TPESU have bigger d-space, pore size and fractional free volume (FFV) values than PESU and HPESU. Among the four PES, PPSU stands out with the highest O2 and CO2 permeability of 1.61 and 9.13 Barrer at 35 °C, respectively, because the two additional aryl groups in PPSU contributes to high segmental motions. HPESU has the lowest gas permeability but shows the highest CO2/N2 and CO2/CH4 selectivity of 34.9 and 34.6, respectively, possibly because of the affinity between its polyethylene oxide units and CO2. The addition of trimethyl benzene groups into TPESU not only increases all gas permeability but also increases selectivity of some gas pairs. It has the highest O2/N2 selectivity of 6.0 and the 2nd highest CO2/CH4 selectivity of 33.8. Its O2 and CO2 permeability are 1.33 and 5.74 Barrer at 35 °C, respectively. In contrast, PESU has low gas permeability and average selectivity because of its linear ethersulfone chain structure. In addition, the diffusivity ratio between Henry and Langmuir modes as a function of chain structure has been investigated. The diffusivity ratio is always smaller than 32%. This study may provide useful insights to design better PES polymers for gas separation.