Synthesis, characterization, and gas separation properties of novel fluorinated co-polyimides with bulky side groups

A series of random 6FDA-MFPAP/BAMF co-polyimides with different MFPAP:BAMF molar ratios (2:1, 1:1, 1:2, and 1:3) were synthesized from a novel diamine, 4-(3-methoxy-4-(difluoromethoxy)-phenyl)-2,6-bis(4-aminophenyl)pyridine (MFPAP); a commercial fluorene diamine (BAMF); and a dianhydride, 4,4'-(hexafluoroiso-propyl)diphthalic anhydride (6FDA). The prepared co-polyimides exhibited outstanding thermal performances ( T g > 345 °C, T d > 509 °C), good optical properties (λ cut-off > 342 nm, λ 80%  > 410 nm), and hydrophobic properties (contact angle >91.8°). They also showed excellent solubility in both high and low boiling point organic solvents . The pure gas (He, CO 2 , O 2, and N 2 ) permeability of the co-polyimide membranes was measured using the constant volume-variable pressure method. The gas permeabilities of the membranes were superior to those of the commercial Kapton, Matrimid and P84, where the He permeability coefficient of 6FDA-MFPAP/BAMF (2:1) was 55.39 barrer. This can be reasonably explained and supported by the fractional free volume and molecular chain spacing of the polyimides . In addition, the gas transport performances of the copolymers have significantly improved over their corresponding homopolymers (6FDA-MFPAP and 6FDA-BAMF). And the 6FDA-MFPAP/BAMF co-polyimides with 2:1 and 1:1 M ratios of MFPAP/BAMF had the best gas separation performance for the gas pairs He/CO 2 and O 2 /N 2 , respectively, which was close to the Robeson upper limit in 1991.