Highly soluble fluorinated polyimides with promising gas transport performance and optical transparency

Abstract

In this study, a novel fluorinated diamine monomer 2,2-bis(4-(4-aminophenylsulfonyl)phenyl)hexafluoropropane (BASA) was designed and synthesized. Then, a series of polyimides were obtained via one-pot high-temperature polycondensation of BASA with commercial aromatic dianhydrides, i.e., 4,4′-(4,4′-isopropylidenediphenoxy)bis(phthalic anhydride) (BPADA), 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), 4,4′-oxydiphthalic anhydride (ODPA). The structures of the monomer and polymers were characterized by FT-IR and NMR. The intrinsic viscosity (η_inh) of the polymers is in the range of 0.46–0.58 dL g⁻¹. The polymers show good thermal stability with the glass transition temperature (T_g), 5% loss temperature (T_5%) and 10% loss temperature (T_10%) in the range of 246–296 °C, 530–540 °C and 545–560 °C, respectively. They also present perfect solubility in high- and low-boiling-point solvents, excellent optical properties (cut-off wavelength = 322–357 nm), high hydrophobicity (water contact angle = 89.4–96.4°) and promising gas transport properties. X-ray diffraction analysis indicated that the polymers feature a d-spacing of 5.36–5.43 Å. The gas permeability test showed that BASA-6FDA exhibits the highest gas permeability (P(CO₂) = 15.00, P(O₂) = 3.67, P(N₂) = 0.85, and P(He) = 32.56 barrer, respectively) and reasonably good selectivity (α(O₂/N₂) = 4.31, α(He/N₂) = 38.26, α(CO₂/N₂) = 17.62 and α(He/CO₂) = 2.17) at 4 bar and 35 °C. The fractional free volume distribution of the polymers was simulated using molecular mechanics and molecular dynamics to relate with experimental gas separation parameters such as permeability and selectivity. These polymers could be applied to the field of optics and gas separation.