Synthesis and Gas Separation of Polyimides Containing Trifluoromethyl, Pyridine and Ester Group

In this study, a novel diamine 4-(2,6-bis(4-(trifluoromethyl)phenyl)pyridine)phenyl-3,5-diaminobenzoate (FPPAB) containing trifluoromethyl, pyridine, and ester groups was synthesized by a three-step organic reaction. Then, four structures of copolyimides were prepared by a one-step polycondensation reaction using diamine (FPPAB), dianhydride 4,4′-(hexafluoroisopropyl)diphthalic anhydride (6FDA), and four commercial diamines. The synthesized copolyimides exhibited high thermal stability, with the glass transition temperature ( T g ) and 5% loss temperature ( T 5% ) in the range of 210°C–256°C and 343°C–452°C, respectively, and X-ray diffraction analysis indicated that the copolyimides featured a d -spacing of 4.90–5.57 Å. They also showed excellent solubility in both high and low boiling point organic solvents. Furthermore, the membranes not only had outstanding optical properties (wavelength range of 80% transmittance = 378–454 nm) but also had excellent hydrophobicity (water contact angle > 91.96°). Notably, the copolyimide membrane with a fluorene structure showed the highest gas permeability ( P (He) = 46.27, P (CO 2 ) = 21.06, P (N 2 ) = 1.57 barrer) and favorable selectivity ( α (He/N 2 ) = 29.55, α (CO 2 /N 2 ) = 13.69). This membrane also exhibited the largest fractional free volume (FFV). Overall, the prepared copolyimides exhibited a well-balanced combination of properties, particularly in gas separation performance, indicating that the properties of copolyimides could be effectively optimized by adjusting the structure of monomers.