Tailor the gas transport properties of network polyimide membranes via crosslinking center structure variation

In this work, a series of symmetrical “H-shaped” tetramine monomers bearing different amounts of side groups and linking units were designed and synthesized via a simple one-step electrophilic substitution reaction of aromatic dialdehyde and aniline. Then, these new designed tetramine monomers using as the crosslinking center were reacted with 6FDA to prepare the network polyimide (PI) membranes. Due to the robust 3D crosslinked structures, all the network PI membranes possessed high rigidity (Tg = 322 ∼ 337 °C), excellent thermal stability (Td,5% =425 ∼ 471 °C), and superior mechanical performance with the tensile strengths ranging from 107.2 to123.0 MPa and elongations of 4.2–5.4%, while the stone-like solubility in common solvents. Interestingly, during the period of tridimensional polycondensation reaction of these designed network PIs, a broad operating flexibility without the risk of gelation was observed, which is beneficial to their actual productions. Furthermore, gas transport results proved that changing the amounts of CH3 side groups at the ortho-positions of the aniline ring and extending their framework’s linking units in the crosslinking center provide viable ways to fine-tune the fractional free volume (FFV) within the polymers, which in turn tailor their gas transport properties efficiently. Specifically, the optimum membrane termed as 6FDA-OHTA showed a huge increment about ∼ 340% for CO2 permeability accompanying with only ∼ 5.6% reduction for CO2/N2 ideal selectivity in comparison with those of the original 6FDA-PTA membrane. We hope this study can open a new avenue to the rational design of network PI membranes to meet various separation needs, especially from the view of crosslinking center structure variation.