pubs.acs.org/Langmuir © 2010 American Chemical Society
Micropore Analysis of Polymer Networks by Gas Sorption and 129Xe 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. 129Xe 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 distribution (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 typically 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 adsorption/desorption isotherms in the low pressure regime. Furthermore, it was reported several times that micropore analysis *To whom correspondence should be addressed. Telephone: þþ49-3315679569. Fax: þþ49-331-5679502. E-mail:
[email protected].
(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.
15650 DOI: 10.1021/la1028806
measurements by nitrogen sorption can last some days.7,8 Therefore, 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 sorption. Further complication arises from the fact that not all pores are accessible to nitrogen. For example, it was shown for microporous 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), 129Xe 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-111 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 effectiveness of “molecular design”. While the size of the connecting (8) Ghanem, B.; McKeown, N.; Budd, P.; Selbie, J.; Fritsch, D. Adv. Mater. 2008, 20, 2766–2771. (9) Lozano-Castello, D.; Cazorla-Amoros, 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.
Published on Web 08/30/2010
Langmuir 2010, 26(19), 15650–15656
Weber et al.
Article
units between the nodes has a crucial influence on the observable pore size in the case of crystalline porous materials (MOFs, COFs, i.e., thermodynamically stable materials),15-17 it is not clear if they have the same impact in the case of amorphous polymeric networks. As those are synthesized under kinetic control, it could be expected that the details of the phase separation process (solvent, temperature, etc.) also influence the resulting morphology and pore size. The aim of the present paper is to provide a detailed micropore analysis of two polymer networks which have a comparable architecture but differ in the synthetic pathway and finer details of the linking units. The sorption of various gases (nitrogen, argon, and carbon dioxide) is investigated, and different models are employed in order to determine the PSD. Additionally, 129Xe NMR spectroscopy is used to provide an independent result on the pore size. The obtained results will be critically discussed using different methods and models.
Experimental Section Spirobifluorene based polymer networks were synthesized as described elsewhere.18,19 All presented data were collected using the same batch of the respective material. Gas sorption isotherms were collected using an Autosorb-1 MP instrument equipped with a multigas option (Quantachrome Instruments). High purity gases were used for gas sorption experiments. Nitrogen and argon isotherms were measured at 77 K, and the carbon dioxide sorption at 273 K. The polymer samples were degassed at high vacuum and 150 °C for >8 h prior to measurements. Data evaluation was performed using the AS1WinTM software from Quantachrome Instruments. For the 129Xe NMR experiments, the polymer materials were outgassed under vacuum for 48 h at 100 °C. After the samples were cooled to room temperature, xenon gas with a known pressure was condensed by cooling the material with liquid nitrogen. Finally, the NMR glass tubes were flame-sealed. The obtained samples were measured at a resonance frequency of 138.29 MHz on a Bruker MSL 500 spectrometer. The 129Xe NMR spectra were acquired using a 90° pulse length of 5.9 μs, a recycle delay of 4 s, and magic angle spinning (MAS) with a rotation frequency of 4 kHz. Typically, 1000 scans were performed to obtain a good signal to-noise ratio. At least 4000 scans were accumulated at xenon loading pressures lower than 40 kPa. The xenon chemical shifts were referenced to the chemical shift of xenon gas extrapolated to zero pressure.
Figure 1. (a) Chemical structure of the polymer network repeating units. (b) Two 3D views on the repeating unit of SpPAT; arrows in the left-hand view indicate the connection points.
Results and Discussion Two 9,90 -spirobifluorene based microporous networks were synthesized according to previously developed procedures.18,19 The first network, a poly(imide) (SpPI), has imide linkages between the spirobifluorene centers which introduces a certain amount of polarity. The second network has dithienylene linkages connecting the spirobifluorene cores and can be regarded as a rather nonpolar poly(arylene thienylene) (SpPAT). The chemical structure of the networks is depicted in Figure 1, showing the comparable architecture. The 9,90 -spirobifluorene introduces a 90° kink which prevents space efficient packing (PIM principle). In both cases, these structure defining units are connected by a (15) Hunt, J.; Doonan, C.; LeVangie, J.; Cote, A. P.; Yaghi, O. M. J. Am. Chem. Soc. 2008, 130, 11872–11873. (16) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed 2004, 43, 2334– 2375. (17) Yaghi, O.; O’Keeffe, M.; Ockwig, N.; Chae, H.; Eddaoudi, M.; Kim, J. Nature 2003, 423, 705–714. (18) Weber, J.; Antonietti, M.; Thomas, A. Macromolecules 2008, 41, 2880– 2885. (19) Schmidt, J.; Weber, J.; Epping, J.; Antonietti, M.; Thomas, A. Adv. Mater. 2009, 21, 702–705.
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Figure 2. Nitrogen (left-hand side) and argon (right-hand side) adsorption/desorption isotherms of SpPI (upper part) and SpPAT (lower part). Closed symbols belong to the adsorption branch; open symbols represent the desorption branch. See the Supporting Information for a logarithmic plot.
linker of comparable length and stiffness. Figure 1b also shows 3D models of the repeating unit of SpPAT. Gas Sorption Isotherms and Data Evaluation Procedure. Initially, the porosity of both networks was screened by nitrogen sorption (see Figure 2). Both networks have a comparable apparent specific surface area of ∼550 m2 g-1 as determined by the Brunauer-Emmett-Teller (BET) method under consideration of recently proposed consistency criteria.20 Furthermore, the nitrogen sorption isotherms confirmed the absence of significant mesoporosity which would complicate the analysis. At high (20) Walton, K. S.; Snurr, R. Q. J. Am. Chem. Soc. 2007, 129, 8552–8556.
DOI: 10.1021/la1028806
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Weber et al. Table 1. Surface Areas of the Networks As Determined by Various Methods sample
SBET (N2)
SQSDFT (N2)a
SNLDFT (N2)a
SNLDFT (Ar)a
SNLDFT (CO2)b
SpPI 547 634 546 590 460 SpPAT 544 512 440 407 524 a NLDFT/QSDFT: slit pores, cumulative surface area of pores with R < 3 nm. b Cumulative surface area of pores with R < 1 nm.
Figure 3. Carbon dioxide (right-hand side) adsorption/desorption isotherms of SpPI (upper part) and SpPAT (lower part) measured at 273 K. Closed symbols belong to the adsorption branch; open symbols represent the desorption branch.
relative pressures (p/p0 > 0.95), a strong nitrogen uptake was monitored, which can be attributed solely to nitrogen condensation in the interstitial voids of the primary polymer particles (see Figures S8 and S9 in the Supporting Information for scanning electron microscopy (SEM) micrographs of the polymer networks). Argon sorption at 77 K yielded adsorption/desorption isotherms of comparable shape in the low and intermediate pressure regime (see Figure 2). However, the pressure regime of p/p0>0.95; that is, the argon condensation in interstitial voids was not assessed. In both cases, a significant low pressure hysteresis was observed upon desorption. The reasons and implications of the observed hysteresis will be discussed below. Carbon dioxide adsorption/desorption isotherms of both networks were measured at 273.15 K (see Figure 3). Generally, only a very small hysteresis was seen upon desorption. While the total CO2 uptake of both networks is comparable (45-50 cm3 g-1 STP), the isotherm shape differs significantly. SpPI showed a high uptake of CO2 at low p/p0, but it leveled off toward high relative pressures. In contrast, SpPAT pointed to a more linear CO2 uptake, with an only slightly convex shape at relatively high pressure. The consequences of this difference will be manifested in the results of the data evaluation as described below. Finally, the measurements of CO2 sorption were much faster than the sorption measurements of N2 or Ar at 77K. Typically, complete isotherms were collected within 3 h, while N2 or Ar sorption measurements took between 20 and 60 h. The data obtained by gas sorption experiments were evaluated by different methods with a special focus on the application of microscopic models such as density functional theory (DFT) or grand canonical Monte Carlo (GCMC) models. Generally speaking, DFT models including nonlinear DFT (NLDFT) and quenched solid DFT (QSDFT) are microscopic (molecular level) models of the sorption process which are based on validated intermolecular interaction parameters and certain pore geometries. The measured isotherm is then fitted as a combination of calculated
(21) Dombrowski, R. J.; Hyduke, D. R.; Lastoskie, C. M. Langmuir 2000, 16, 5041–5050. (22) Ravikovitch, P. I.; Vishnyakov, A.; Russo, R.; Neimark, A. V. Langmuir 2000, 16, 2311–2320. (23) Ravikovitch, P. I.; Neimark, A. V. J. Phys. Chem. B 2001, 105, 6817–6823. (24) Neimark, A. V.; Ravikovitch, P. I.; Vishnyakov, A. J. Phys.: Condens. Matter 2003, 15, 347–365. (25) Neimark, A. V.; Lin, Y.; Ravikovitch, P. I.; Thommes, M. Carbon 2009, 47, 1617–1628.
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isotherms in individual pores. A more complete description of the methods can be found in the literature.21-26 We applied various models on the measured isotherms and evaluated the obtained data of the fitted isotherms (PSD, pore volume, surface area). In all cases, models were applied which are developed for carbon materials. In the case of nitrogen and argon sorption isotherms, only the adsorption branch was used for DFT analysis. This is due to the presence of the large hysteresis at low pressure which does not represent the equilibrium state (see below). NLDFT and QSDFT analysis were used under the assumption of slit pores. CO2 sorption measurements were analyzed by a NLDFT model as well as by a GCMC model. The full isotherm was used for evaluation in these cases. The experimental isotherms could be well fitted by the models (see Figures S2-S7 in the Supporting Information). The best fits were obtained from the CO2 sorption analysis (fitting error:
