Manipulating polyimide nanostructures via cross-linking for membrane gas separation

Cross-linking has been widely utilized to modify polyimide nanostructures for membrane gas separations, such as increasing size-sieving ability and diffusivity selectivity for H₂-CO₂ and CO₂-CH₄ separation and improving resistance to plasticization derived from CO₂ and heavy hydrocarbons for CO₂-CH₄ and C₃H₆-C₃H₈ separations. However, there is a lack of fundamental understanding of the relationship between crosslinked structure and membrane gas-separation properties. This chapter critically reviews the effect of cross-linking on polymer physical properties (such as glass transition temperature, T_g) and current strategies adopted to cross-link polyimides for membrane gas separation. The information is synthesized to elucidate the effect of cross-linking on T_g and cross-linking density in polyimides, which is then used to interpret the changes of gas permeability and selectivity. The benefits of crosslinking in improving gas-separation properties are also illustrated in Robeson’s upper bound plots for H₂-CO₂, CO₂-CH₄, and C₃H₆-C₃H₈ separations.