Polymers of intrinsic microporosity and thermally rearranged polymer membranes for highly efficient gas separation

In the last decades, many novel polymeric materials have been considered to prepare highly energy-efficient gas separation membranes. There is a continuous search to overcome the trade-off relationship between the gas permeability and selectivity to improve the membrane separation performance during long-term operation. One of the most important research fields of molecular design and polymer chemistry is the synthesis of new polymers with a highly microporous structure, referred to as polymers of intrinsic microporosity (PIMs) and thermally rearranged (TR) polymers. The major challenge of PIMs and TR polymers, required for their application in membrane gas separation, is achieving a high permeability and selectivity, which remain stable over prolonged periods and under all possible operating conditions. Limits of their use can be low mechanical stability, sophisticated and challanging synthesis methods, physical aging of PIMs, etc. For TR polymers, identifying the optimum temperature in the thermal treatment process is challenging, and the preparation of thin-film TR membranes from PBO precursor via conventional methods is difficult. Moreover, the productivity of the membrane modules and the adhesion between the support/polymer and nanomaterials/polymer need to be improved in mixed matrix membranes (MMMs). Also, the formation of defects in the membrane structure is the main challenge to overcome for the fabrication of thin-film composites (TFCs). In this manuscript, we carefully review the molecular design and the properties and performance of PIMs and TR materials as energy-efficient membranes for various gas separations, emphasizing CO2/CH4 and CO2/N-2. Different methods for synthesizing PIMs and TR polymers are described, and the resulting polymers are evaluated for gas separation. Structure designing for the synthesis of PIMs and TR membranes and the influence of different chemical structures of monomers for creating chain rigidity by non-planar groups, and tuning the angle of contorted centres, are investigated. Moreover, the presence of functional groups and other substituents, such as fluorinated, sulfonated, carboxylic, tetrazole, and other groups, are investigated. Different strategies for membrane preparation and tailoring of their properties, such as cross-linking, co-polymerization, blending with other polymeric or organic and inorganic materials, are discussed as well.