Thermal rearranged poly(benzoxazole-co-imide) membranes for CO2 separation

Abstract

The thermal rearrangement of α-hydroxyl-polyimide membranes improves gas permselectivity properties compared to the polyimide precursor. By introducing segments within the polymer that do not undergo thermal rearrangement, the gas separation properties of the thermally rearranged membrane can be modified. In particular, polyimides are a class of polymer that have wide gas separation performance, which can be incorporated into thermal rearranged membranes through random copolymers and enable tailoring of the gas separation properties of the resulting membrane, without the need to alter the thermal rearranged moieties. A range of polyimide copolymers based on 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) and diamines 3,3′-dihydroxy-4,4′-diamino-biphenyl (HAB) with 2,3,5,6-tetramethyl-1,4-phenylenediamine (4MPD) or 9,9′-bis (4-aminophenyl) fluorene (FDA) were synthesised, and thermally rearranged into poly(benzoxazole-co-imide) membranes. The diamine moiety HAB undergoes thermal rearrangement to form the benzoxazole segments of the copolymer, while 4MPD or FDA based segments remain as imide segments in the copolymer. These copolymers, in both polyimide and poly(benzoxazole-co-imide) form, were tested as gas separation membranes for CO2 separation from N2 and CH4. Thermal rearrangement of the homo-polymer HAB–6FDA resulted in increased gas permeability compared to the polyimide precursor. With the addition of the 4MPD moiety in the copolymer, it was discovered that the gas permeability, solubility and selectivity of membrane were similar to the homo-polymer HAB–6FDA. This result was attributed to the 4MPD–6FDA segments having similar gas separation properties as the benzoxazole segments. The presence of FDA in the copolymer resulted in a reduction in gas permeability as the FDA to PBO segmental ratio increased. However, the gas selectivity of this poly(benzoxazole-co-imide) was independent of FDA amount. Hence, through the addition of diamine moieties in poly(benzoxazole-co-imide), the gas separation properties of the thermal rearranged membrane can be varied.