15650 DOI: 10.1021/la1028806 Langmuir 2010, 26(19), 15650–15656 Published on Web 08/30/2010 pubs.acs.org/Langmuir © 2010 American Chemical Society Micropore Analysis of Polymer Networks by Gas Sorption and 129 Xe NMR Spectroscopy: Toward a Better Understanding of Intrinsic Microporosity Jens Weber,* ,† Johannes Schmidt, ‡ Arne Thomas, ‡ and Winfried B€ ohlmann § † Department of Colloid Chemistry, Max-Planck-Institute of Colloids and Interfaces, Science Park Golm, D-14424, Potsdam, Germany, ‡ Technische Universit € at Berlin, Englische Strasse 20, 10587 Berlin, Germany, and § Faculty of Physics and Geosciences, University of Leipzig, Linn estr. 5, D-04103 Leipzig, Germany Received July 20, 2010. Revised Manuscript Received August 17, 2010 The microporosity of two microporous polymer networks is investigated in detail. Both networks are based on a central spirobifluorene motif but have different linker groups, namely, imide and thiophene units. The microporosity of the networks is based on the “polymers of intrinsic microporosity (PIM)” design strategy. Nitrogen, argon, and carbon dioxide were used as sorbates in order to analyze the microporosity in greater detail. The gas sorption data was analyzed with respect to important parameters such as specific surface area, pore volume, and pore size (distribution). It is shown that the results can be strongly model dependent and swelling effects have to be regarded. 129 Xe NMR was used as an independent technique for the estimation of the average pore size of the polymer networks. The results indicate that both networks are mainly ultramicroporous (pore sizes < 0.8 nm) in the dry state, which was not expected based on the molecular design. Phase separation and network defects might influence the overall network morphology strongly. Finally, the observed swelling indicates that this “soft” microporous matter might have a different micropore size in the solvent swollen/filled state that in the dry state. Introduction Microporous polymers, that is, polymers possessing permanent pores with sizes smaller than 2 nm, gained increasing interest during the past decade. Various potential applications have been suggested, for example, gas storage or separation, dye sorption or as catalyst support. 1-5 A lot of progress has been achieved in the synthesis of microporous polymers; however, the analysis of their porosity, especially the determination of their pore size distribu- tion (PSD), is far less developed, and thus, various open questions remain. As the knowledge of the pore size and the PSD is of crucial importance for the performance of the materials in various applications, a much better understanding of this parameter is necessary. Whether a polymeric material is microporous or not is typi- cally decided on the basis of nitrogen adsorption/desorption isotherms which are measured at 77 K. IUPAC definitions can help to determine if the material is microporous, mesoporous, or both. 6 Although nitrogen sorption has generally been proven to be an extremely versatile tool in the analysis of porous materials, it has several drawbacks when employed for microporous organic materials. The softness of polymeric materials in comparison to inorganic materials such as zeolites can lead to swelling effects. These properties are visible as significant hysteresis of the adsorp- tion/desorption isotherms in the low pressure regime. Further- more, it was reported several times that micropore analysis measurements by nitrogen sorption can last some days. 7,8 There- fore, it is questionable if the obtained isotherms really reflect the equilibrium state. These influences can cause severe effects on the PSD as determined by classical methods based on nitrogen sorp- tion. Further complication arises from the fact that not all pores are accessible to nitrogen. For example, it was shown for micro- porous carbons that nitrogen could be unable to detect very small and narrow micropores which were accessible for other probe molecules (e.g., carbon dioxide). 9 It is a tempting task to explore the porosity of microporous polymers in more detail. Therefore, the results of gas sorption should be compared to results obtained by different techniques. Microporous polymers can also be regarded as polymers of ultrahigh free-volume; that is why the application of well-known methods such as positronium annihilation lifetime spectroscopy (PALS), 129 Xe NMR spectroscopy, and modeling are suitable to investigate the free-volume of microporous polymers. 10 Indeed, some comparative investigations have already been performed on the microporous polymer PIM-1 11 by employing PALS, modeling methods, and nitrogen sorption. 12-14 However, the main focus was laid on PALS or modeling, and no detailed analysis of the nitrogen adsorption/desorption isotherms was presented. Of high importance is furthermore the question on the effec- tiveness of “molecular design”. While the size of the connecting *To whom correspondence should be addressed. Telephone: þþ49-331- 5679569. Fax: þþ49-331-5679502. E-mail: jens.weber@mpikg.mpg.de. (1) Thomas, A.; Kuhn, P.; Weber, J.; Titirici, M.; Antonietti, M. Macromol. Rapid Commun. 2009, 30, 221–236. (2) McKeown, N. B.; Budd, P. M. Macromolecules 2010, 43, 5163–5176. (3) Cooper, A. I. Adv. Mater. 2009, 21, 1291–1295. (4) McKeown, N. B.; Budd, P. M. Chem. Soc. Rev. 2006, 35, 675–683. (5) Tsyurupa, M. P.; Davankov, V. A. React. Funct. Polym. 2006, 66, 768–779. (6) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603–619. (7) Ritter, N.; Antonietti, M.; Thomas, A.; Senkovska, I.; Kaskel, S.; Weber, J. Macromolecules 2009, 42, 8017. (8) Ghanem, B.; McKeown, N.; Budd, P.; Selbie, J.; Fritsch, D. Adv. Mater. 2008, 20, 2766–2771. (9) Lozano-Castell o, D.; Cazorla-Amor os, D.; Linares-Solano, A. Carbon 2004, 42, 1233–1242. (10) Yampolskii, Y. P. Russ. Chem. Rev. 2007, 76, 59–78. (11) Budd, P. M.; Ghanem, B. S.; Makhseed, S.; McKeown, N. B.; Msayib, K. J.; Tattershall, C. E. Chem. Commun. 2004, 230–231. (12) Staiger, C. L.; Pas, S. J.; Hill, A. J.; Cornelius, C. J. Chem. Mater. 2008, 20, 2606–2608. (13) Heuchel, M.; Fritsch, D.; Budd, P. M.; McKeown, N. B.; Hofmann, D. J. Membr. Sci. 2008, 318, 84–99. (14) Budd, P. M.; McKeown, N. B.; Ghanem, B. S.; Msayib, K. J.; Fritsch, D.; Starannikova, L.; Belov, N.; Sanfirova, O.; Yampolskii, Y.; Shantarovich, V. J. Membr. Sci. 2008, 325, 851–860.