Encyclopedia of Membranes

Living Edition
| Editors: Enrico Drioli, Lidietta Giorno

Highly Permeable Polyimides

  • Mariolino CartaEmail author
Living reference work entry
DOI: https://doi.org/10.1007/978-3-642-40872-4_1967-1
Polyimides are polymers produced by cycloimidization (formation of an imide linkage) between di-anhydrides and diamines via step-growth polymerization (Koros and Fleming 1993; Ghosh and Mittal 1996). Among other useful applications such as materials for electronics, coatings, foam, and fibers, because of their excellent solubility in common low boiling point solvents, thermal stability, and physical properties, they have been extensively studied as gas separation membranes. A typical limitation for their use in this field is due to the fact that they often exhibit high selectivity but low permeability for important gas pairs such as O2 and CO2, usually far below 100 Barrer (1 Barrer = 10−10 cm3(STP) cm cm−2 s−1 cmHg−1). The low permeability is strongly related with the lack of fractional free volume of the material (FFV) because of the free rotation around the imide linkage that allows the polymer to pack densely, limiting the mass transport. Initial successes in enhancing the permeability by increasing the FFV have been achieved with the use of 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA), which is now one of the most common monomers for the formation of high-performance polyimides (Fig. 1), and 2,3,5,6-tetramethyl-1,4-phenylenediamine (4MPDA). The hindrance of the four methyl groups of the di-aniline restricts the rotation around the imide linkage allowing the synthesis of more porous (so less dense) polyimides. The higher porosity facilitates the mass transport of the gases through the pores of the membrane with consequent increase of the permeability (molecular sieving effect), typically over 100 Barrer for O2 and 400 Barrer for CO2 with selectivity \( {\alpha}_{{\mathrm{O}}_2/{\mathrm{N}}_2}=3.5 \) and \( {\alpha}_{{\mathrm{H}}_2/{\mathrm{N}}_2}=16.6 \) (Lin et al. 2000).
Fig. 1

6FDA + 4MPDA-based polyimide (Lin et al. 2000)

A different method to increase the FFV is represented by the formation of hyperbranched polyimides. In this case a triamine monomer, such as 1,3,5-tris(4-aminophenoxy)benzene (TAPOB), is reacted with different di-anhydrides to obtain a highly branched structure (Tsai et al. 2003) (Fig. 2). The polyimides obtained with this technique are insoluble so, to be used as membrane for gas separation, they must be embedded in a support. Typically, it can be prepared by the dispersion of the polyimide with colloidal silica by sol–gel processes, to form a composite material. The most remarkable characteristic of this hyperbranched polymer is that they can be made out of a large variety of A3 + B2 terminal groups so that the synthesis can afford different multifunctional polymers.
Fig. 2

Hyperbranched polyimides (Tsai et al. 2003)

Following the idea of restricting the rotation around the imide link, the concept of polymers of intrinsic microporosity (PIMs), which is based on the polymerization of monomers that possess a site of contortion, was applied to the synthesis of polyimides. In this case, di-anhydrides and/or di-anilines such as the spirobisindane A or the ethanoanthracene B (as shown in Fig. 3) are employed. The site of contortion is represented by the central quaternary carbon in case of A or the methylene bridge for B (Rogan et al. 2014). Although there is still free rotation around the imide linkage, the big hindrance of the bulky substituent, combined with the reduced flexibility given by the site of contortion, allows the formation of high FFV polymers that cannot pack space efficiently in the solid state, leaving interconnected pores of nano-/microdimension (for this reason they are called polymers of intrinsic microporosity).
Fig. 3

Highly permeable polyimides of intrinsic microporosity (Rogan et al. 2014)

Studies on this class of polymers demonstrate that the systematic increase of the rigidity of the structural backbone favors the molecular sieving effect of the material, enhancing both permeability and selectivity for various gas pairs. The resulting PIM-polyimides (PIM-PIs) possess high molecular mass and elevated microporosity, with BET surface areas in the range of 600–700 m2g−1. These features, associated with the high solubility in common organic solvents, allow the preparation of robust thin-film membranes with excellent performance toward important commercial gas pairs, such as O2/N2, H2/N2, and CO2/CH4, with exceptional permeabilities (over 1000 Barrer for O2 and over 7000 Barrer for CO2) accompanied by good selectivities \( {\alpha}_{{\mathrm{O}}_2/{\mathrm{N}}_2}=3.5 \) \( {\alpha}_{{\mathrm{H}}_2/{\mathrm{N}}_2}=11.5, \)and \( {\alpha}_{{\mathrm{CO}}_2/{\mathrm{CH}}_4}=16.1. \) Remarkably, all data points lie above the Robeson upper bounds. With the appropriate choice of the monomers of both di-anhydrides and di-anilines, it is possible to tune the physical properties of these highly permeable polyimides to improve them even further.

References

  1. Ghosh M, Mittal KL (1996) Polyimides: fundamentals and applications. Marcel Dekker, New YorkGoogle Scholar
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© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  1. 1.School of ChemistryThe University of EdinburghEdinburghUK