Microphase-separated morphology controlled polyimide graft copolymer membranes for CO2 separation

Abstract

We report microphase-separated structured, 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA)–2,4,6-trimethyl-1,3-diaminobenzene (DAM) polyimide (PI)-based PI-POEM graft copolymer membranes with various poly(ethylene glycol)methyl ether methacrylate (POEM) grafting ratios for efficient CO2 separation. The brominated PI (PI-Br) was used as a macroinitiator to form amphiphilic microphase-separated PI-POEM graft copolymers via atom transfer radical polymerization (ATRP). The proton nuclear magnetic resonance (1H NMR) spectra demonstrate the chemical structure of PI-POEM graft copolymers with controllable POEM concentrations. The PI-POEM graft copolymers exhibit modified mechanical properties with a higher elongation at break and more hydrophilic membrane surface properties because of the relatively rubbery and hydrophilic POEM side chain addition. Notably, the PI-POEM graft copolymers exhibit microphase-separated morphology-controllable characteristics based on transmission electron microscope (TEM) images and X-ray scattering (small- and wide-angle) spectra. These structural properties enable the PI-POEM graft copolymer membranes to develop carbon dioxide (CO2)-gas-favorable transport characteristics with a high CO2 affinity. The PI-POEM graft copolymer membranes can exhibit controllable separation performance depending on the POEM concentrations. Consequently, the PI-POEM(3:1) graft copolymer membrane improves simultaneously in permeability and selectivity compared with the pure PI membrane. Thus, the PI-POEM(3:1) graft copolymer membrane exhibits CO2 permeability of 1,220 Barrer with selectivities of 27.5 and 23.5 for CO2/CH4 and CO2/N2, and H2 permeability of 810 Barrer with selectivities of 16.2 and 15 for H2/CH4 and H2/N2.