Polyimides Containing Biphenyl and Tröger’s Base Units for Gas Separation Membranes

Two diamine monomers containing biphenyl and Tröger's base units were synthesized, and each diamine that differed by the presence of two −CH3 groups was reacted with commercially 4,4′-hexafluoroisopropylidene diphthalic anhydride (6FDA) for obtaining two polyimides of intrinsic microporosity (PIM-TB-PI-1 and PIM-TB-PI-2) with structural elements that ensured the formation of contorted and semirigid chains. Both PIM–PIs were soluble in polar aprotic solvents, including tetrahydrofuran and chloroform, showing high thermal stability (Td > 500 °C) and high glass transition temperatures (Tg > 300 °C). The surface area and the distance between polymer chains were determined by using N2 adsorption/desorption and wide-angle X-ray diffraction (XRD) techniques, respectively. PIM-TB-PI-1 showed a surface area of 81 m2 g–1 and d-spacing of 5.9 Å, while PIM-TB-PI-2 had slightly higher values (SA = 109 m2 g–1 and d-spacing = 6.3 Å). PIM-TB-PI-1 and PIM-TB-PI-2 form dense films that exhibited pure gas permeability coefficients that are 1 order of magnitude higher than Matrimid while they present lower perm-selectivities than the former. This result is related to chain rigidity in PIM-TB-PIs that maintains a distribution of microvoids that are large enough for these gases to diffuse through the interconnected free volume, and the TB units in the main chain of both polymers increase steric hindrance for gas permeability and diffusion. The higher diffusion coefficients of PIM-TB-PI-2, as compared to PIM-TB-PI-1 are related to the presence of −CH3 groups near the imide group, preventing an efficient packing of the polymer chains.