Thermal rearranged poly(benzoxazole)/polyimide blended membranes for CO2 separation

The exceptional CO2 permselectivity of thermal rearranged (TR) poly(benzoxazole) (PBO) membranes from α-hydroxy-polyimide precursors is well known. As a consequence of the TR process, the rigidity of the membrane increases, which can negatively impact the mechanical strength of the membrane. This is overcome here through blends of TR PBO with a polyimide that does not undergo TR. Within the blend membranes, the non-TR polyimide provides additional flexibility to the membrane and hence increases its mechanical strength. The TR polyimide precursor is based on 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) and 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB), while the non-TR polyimide is based on 6FDA and 2,3,5,6-tetramethyl-1,4-phenylenediamine (4MPD). The diamine moiety HAB undergoes TR with 6FDA to form the benzoxazole within the blend membrane, while 4MPD does not undergo thermal rearrangement. Blend membranes of varying TR PBO/polyimide compositions were tested for gas separation of CO2 from CH4. The permeabilities of both gases were a function of blend concentration, and they decreased as 4MPD–6FDA content in the blend increased. For CO2 this was attributed mainly to decreasing diffusivity because of increasing membrane density, while CH4 permeability decrease followed the measured decrease in CH4 solubility. Importantly, the 75% TR PBO/polyimide blend membrane produced a CO2/CH4 permselectivity that was comparable with the current Robeson upper bound while maintained flexibility.