Synthesis and Gas Separation Properties of Aromatic Polyimides Containing Noncoplanar Rigid Sites

Four aromatic polyimides were prepared using a conventional two-stage polymerization reaction of diamine monomers containing different noncoplanar rigid sites with commercially available dianhydride 6FDA. All polymers displayed high relative molecular masses, excellent solubilities, and outstanding thermal properties with Tg values higher than 260 °C, which ensured that they were able to be casted as dense films for gas permeation measurement. The comprehensive gas transport properties of four prepared polyimides were higher than conventional aromatic polyimides (e.g., Matrimid 5218 and Kapton), which suggested that the incorporation of the noncoplanar rigid groups into the polymer main chain was beneficial to prevent segment packing as well as to enhance the gas separation properties. The pure CO2 permeability coefficient of the spirobichroman-based polyimide (6FDA-BDA) was 2.9 times as high as that of Matrimid 5218, without sacrificing selectivity. Moreover, the effect of different noncoplanar rigid sites on the gas separation performance of aromatic polyimides was systematically investigated. By measurements of FFV, density, and d-spacing, it has been observed that the incorporation of different noncoplanar rigid sites significantly affected the microstructure, thus affecting the gas transport. For example, because the polyimide containing pendent phenyl groups (6FDA-BDM) had the lowest FFV and the densest segment packing, it exhibited the lowest permeability. Hence, the basic insights to the structure/property relationships of the four polyimides obtained in this work offer a meaningful guide for next-generation gas separation membranes.