Preparation of thermally rearranged poly(benzoxazole-co-imide) membranes containing heteroaromatic moieties for CO2/CH4 separation

Enhanced gas separation and mechanical properties of fluorene-based thermal rearrangement copolymers

A series of thermal rearrangement (TR) copolymer membranes were prepared by the copolymerization of 9,9-bis(3-amino-4-hydroxyphenoxyphenyl) fluorene (BAHPPF), 9,9-bis(3-amino-4-hydroxyphenyl)fluorene (BAHPF) and 2,2'-bis(3,4'-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), followed by thermal imidization and further thermal rearrangement. The effects of molar ratio of diamines on the structure and properties of copolymer membranes were studied. The copolymer precursors CP-4:6 and CP-5:5 exhibited excellent mechanical properties. The mechanical properties of precursor membranes rapidly decreased with the increase of thermal treatment temperatures, but the tensile strength of TRCP-4:6 still reached 21.2 MPa. In general, the gas permeabilities of TR copolymers increased with the increase of BAHPF content. Comparatively, TRCP-3:7 and TRCP-4:6 showed higher gas permeabilities, coupled with high O2/N2 and CO2/CH4 selectivities. Especially, the H2, CO2, O2, N2 and CH4 permeabilities of TRCP-4:6 reached 244.4, 269.0, 46.8, 5.20 and 4.60 Barrers respectively, and the selectivities for CO2/CH4 and O2/N2 were 58.48 and 9.00, which exceeded the 2008 upper bound. Therefore, these TR copolymer membranes are expected to be one of the candidate materials for gas separation applications.