Acetate-Functional Thermally Rearranged Polyimides Based on 2,2-Bis(3-amino-4-hydroxyphenyl)hexafluoropropane and Various Dianhydrides for Gas Separations

Three acetate-functional polyimides based on 2,2-bis(3-amino-4-hydroxy-phenyl)hexafluoropropane (APAF) and dianhydrides with different linking groups (-O-, -CO-, and -C(CF 3 ) 2 -) were prepared, from which films were prepared and thermally rearranged (TR) to form poly(benzoxazole) dense membranes. The polymers were characterized by 1 H nuclear magnetic resonance, Fourier transform infrared, differential scanning calrimetry, thermogravimetric analysis-mass spectrometry, and wide-angle X-ray diffraction. Transport properties of the TR membranes were investigated, and effects of the linking group and TR conversion rate on the membrane performance were discussed. Among the TR polymers studied, those bearing larger linking groups have higher d -spacing, fractional free volume, and gas permeability. The membrane permeability to gases increases with TR conversion rate and follows the order H 2 > CO 2 > O 2 > N 2 > CH 4, except that CO 2 surpasses H 2 in APAF-6FDA-TR450. The CO 2 /CH 4 separation performance of all of the TR polymers studied surpasses Robeson’s 1991 upper bound ( J. Membr. Sci. 1991, 62 (2), 165–185), and APAF-6FDA-TR450 is most favorable, the performance of which is closest to the 2008 upper bound ( J. Membr. Sci. 2008, 320 (1–2), 390–400).