Chemical modification of polymers of intrinsic microporosity (PIMs) for gas separation membranes.

Polymers of Intrinsic Microporosity (PIMs) are glassy polymers which owe their high free volume, and hence high permeability, to their relatively inflexible, contorted macromolecular backbones.1, 2 Typically, they comprise fused ring sequences interrupted by spiro-centres. The archetypal membrane-forming PIM, referred to as PIM-1,3-5 attracted attention as a membrane material when it was shown to surpass the 1991 Robeson upper bound6 for certain gas pairs, and it contributed to the 2008 revision of the upper bound.7 In recent years there has been significant research on PIMs, aimed at tailoring the selectivity for a variety of separations and at improving the ageing behaviour. A variety of approaches has been taken, including (1) bespoke monomer synthesis, (2) chemical post-modification of precursor polymers, (3) thermal or ultraviolet treatment of precursor polymers and (4) the addition of inorganic, metal-organic or purely organic materials to form mixed matrix membranes. We shall focus on the chemical post-modification of PIM-1. The conversion has been reported of the nitrile groups in PIM-1 to carboxylic acid,8, 9 tetrazole,10 amidoxime,11 thioamide,12 and amine13 functionality. Subtle changes in chemical functionality, particularly with groups capable of strong interactions such as hydrogen bonding, can dramatically influence the amount and distribution of free volume within the polymer, and hence have a profound effect on the transport properties. In the case of amine-PIM-1, the incorporation of a primary amine enhances the affinity for carbon dioxide, as demonstrated by adsorption experiments at 273 K and 298 K, despite a loss of internal surface area as measured by nitrogen adsorption at 77 K. The high uptake of CO2 may arise either from specific interaction with amine groups, or through strong adsorption in very small pores, or a combination of the two. In gas permeation, penetrant-polymer interactions can have contrasting effects, enhancing transport through increased sorption, but potentially inhibiting diffusion and hence penalising transport. In amine-PIM-1, the interaction with CO2 is sufficiently strong that diffusion, and hence permeability, is substantially reduced, but high permeabilities are obtained for small, non-interacting gases. Thus, counter-intuitively, for the H2/CO2 gas pair there is a change from permselectivity towards CO2 in PIM-1 to permselectivity towards H2 in amine-PIM-1. References 1.P. M. Budd and N. B. McKeown, Polym. Chem. 1, 63 (2010). 2.N. B. McKeown and P. M. Budd, Macromolecules 43, 5163 (2010). 3.P. M. Budd, K. J. Msayib, C. E. Tattershall, B. S. Ghanem, K. J. Reynolds, N. B. McKeown and D. Fritsch, J. Membr. Sci. 251, 263 (2005). 4.P. M. Budd, N. B. McKeown, B. S. Ghanem, K. J. Msayib, D. Fritsch, L. Starannikova, N. Belov, O. Sanfirova, Y. Yampolskii and V. Shantarovich, J. Membr. Sci. 325, 851 (2008). 5.P. M. Budd, E. S. Elabas, B. S. Ghanem, S. Makhseed, N. B. McKeown, K. J. Msayib, C. E. Tattershall and D. Wang, Adva. Mater. 16, 456 (2004). 6.L. M. Robeson, J. Membr. Sci. 62, 165 (1991). 7.L. M. Robeson, J. Membr. Sci.320, 390 (2008). 8.N. Du, G. P. Robertson, J. Song, I. Pinnau and M. D. Guiver, Macromolecules 42, 6038 (2009). 9.J. Weber, N. Du and M. D. Guiver, Macromolecules 44, 1763 (2011). 10.N. Du, H. B. Park, G. P. Robertson, M. M. Dal-Cin, T. Visser, L. Scoles and M. D. Guiver, Nature Materials 10, 372 (2011). 11.H. A. Patel and C. T. Yavuz, Chem. Commun. 48, 9989 (2012). 12.C. R. Mason, L. Maynard-Atem, N. M. Al-Harbi, P. M. Budd, P. Bernardo, F. Bazzarelli, G. Clarizia and J. C. Jansen, Macromolecules 44, 6471 (2011). 13.C. R. Mason, L. Maynard-Atem, K. W. J. Heard, B. Satilmis, P. M. Budd, K. Friess, M. Lanc, P. Bernardo, G. Clarizia and J. C. Jansen, Macromolecules, Accepted (2014).