Langmuir 2008, 24, 5877-5887
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Characterization of Highly Porous Polymeric Materials with Pore Diameters Larger than 100 nm by Mercury Porosimetry and X-ray Scattering Methods C. C. Egger,*,† C. du Fresne,‡ V. I. Raman,§ V. Schädler,| T. Frechen,§ S. V. Roth,⊥ and P. Müller-Buschbaum# Lennard Jones Laboratories, Keele UniVersity, Keele, ST5 5BG Staffordshire, U.K., BASF Nederland B.V. InnoVatielaan 1, NL-8466 SN Nijehaske, The Netherlands, BASF Aktiengesellschaft, GKD/F-B001, D-67056 Ludwigshafen, Germany, BASF Corporation, 1609 Biddle AVenue, Wyandotte, Michigan 48192, HASYLAB at DESY, Notkestr. 85, 22603 Hamburg, Germany, and Physik Department LS E13, TU Mu¨nchen, James-Franck-Str. 1, 85747 Garching, Germany ReceiVed January 21, 2008. ReVised Manuscript ReceiVed March 3, 2008 Highly porous polymeric materials with pore sizes ranging from 100 nm to 1µm are a very challenging class of materials not only to prepare synthetically (due to the high capillary pressures generated upon solvent removal) but also to characterize structurally. Through the examples of three different types of porous compounds synthesized in our laboratory (i) high-density melamine-based “MF-hd” with monomodal pore diameters around 500-900 nm, (ii) low-density melamine-based “MF-ld” with bimodal pore size distribution and average diameters around 2.3 µm and 350 nm, (iii) highly porous polyurethane “PU” with monomodal pore sizes around 150 nm, we confirm the limitations of mercury porosimetry as a means to investigate the architecture of materials with very high porosity (>80 vol %) and low compressive strength. Instead, a combination of high-resolution scanning electron microscopy and small-angle and ultrasmall-angle X-ray scattering (SAXS and USAXS, respectively) studies of these three types of materials helps in determining both the network and the pore structures. This work elucidates the need and applicability of the SAXS/USAXS techniques in characterizing such porous materials. For instance, the polyurethane specimens can only be quantitatively characterized by scattering techniques, the results of which are corroborated by high-resolution scanning electron microscopy observations.
Introduction Materials have often been developed alongside new techniques necessary to characterize them, and this statement is particularly true for porous materials requiring the characterization of both their porous network (volume, pore size and shape) and also their wall surfaces (surface area, functionality). Further, porous materials are a very challenging class of materials owing to the versatility of their syntheses (which usually vary greatly with pore size) but, more importantly for the scope of this paper, owing to their characterization involving very different techniques according to the length scale to investigate. Characterization of porous materials was historically carried out by adsorbing and condensing a gas into the porous network and by measuring the resulting volume change.1,2 Mathematical models and computation have since then allowed the analysis of pore size and shape, traditionally using nitrogen as the adsorbate,3 although other gases have been used.4 Adsorption of liquids in the porous network can also be carried out, though less frequently, as the resulting surface areas are not always reliable.3 Further, the immersion microcalorimetry technique developed by Denoyel * To whom correspondence should be addressed. Telephone:+44 1782 58 3337. Fax: +44 1782 58 2378. E-mail:
[email protected]. † Keele University. ‡ BASF Nederland B.V. Innovatielaan 1. § BASF Aktiengesellschaft. | BASF Corporation. ⊥ HASYLAB at DESY. # TU Mu¨nchen. (1) Zsigmondy, R. Z. Anorg. Chem. 1911, 71, 356. (2) Anderson, J. S. Z. Phys. Chem. 1914, 88, 211. (3) Sing, K. S. W. AdV. Colloid Interface Sci. 1998, 76-77, 3–11. (4) Sing, K. S. W.; Williams, R. T. Part. Part. Syst. Charact. 2004, 21, 71–79.
et al.5,6 based on the interaction between a solid adsorbent and a liquid can also provide information such as surface area, surface chemistry, and wettability of the adsorbent, though it is essentially limited to micro- and mesoporous materials.7 Even more sophisticated techniques such as magnetic resonance imaging, based on the relaxation of protons, and microfocus X-ray imaging have recently emerged with the work of Rigby et al.8,9 These tomographic techniques provide maps of porous spatial distribution, but despite their potential these are still limited to rather large pore sizes (>10 µm) and to chemically homogeneous specimens. In order to access pore size distribution (PSD) in hierarchically porous materials, these methods need to be combined with more standard measurements, such as mercury porosimetry.10 With materials with pores on the 10-100 µm length scale (e.g., polyurethane foams), a new photon density wave technique has recently been used to access the structural features, but only an average pore diameter can be obtained and no information such as a PSD has yet been possible.11 However, a standardized and less expensive technique is used on a daily basis by experimentalists to characterize the materials (5) Denoyel, R.; Rouquerol, F. In Handbook of Porous Solids; Schuth, F., Sing, K. S. W., Weitkamp, J., Eds.; Wiley-VCH: New York, 2002; Vol. 1, pp 276-308. (6) Denoyel, R.; Beurroies, I.; Vincent, D. J. Therm. Anal. Calorim. 2002, 70(2), 483–492. (7) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W. Pure Appl. Chem. 1985, 57, 604. (8) Rigby, S. P.; Gladden, L. F. J. Catal. 1998, 173, 484–489. (9) Rigby, S. P. Catal. Today 1999, 53, 207–223. (10) Rigby, S. P.; Fletcher, R. S.; Raistrick, J. H.; Riley, S. N. Phys. Chem. Chem. Phys. 2002, 4, 3467–3481. (11) Engelhard, S.; Kumke, M. U.; Lo¨hmannsro¨ben, H.-G. Anal. Bioanal. Chem. 2006, 384, 1107–1112.
10.1021/la800197p CCC: $40.75 2008 American Chemical Society Published on Web 04/29/2008
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they prepare in the laboratory. Since 1945, structural characterization through penetration of a nonwetting liquid within a porous network has been possible using mercury.12 In this technique, mercury is forced into a material by applying pressure, which leads to the intrusion of the large pores first and then of the smaller ones at very high pressures, for example, 3.7 nm at 400 MPa. In the case of nitrogen sorption, it is the opposite: the small pores (micropores, i.e., pores below 2 nm) are first filled and then the larger pores are filled up with nitrogen at higher pressures through capillary condensation. The drawback of this technique comes from the maximum relative pressure achievable, which limits the size of the pores accessible (upper limit around 50-100 nm). On the other hand, mercury pressure can be increased to almost infinity and small pores should in theory always be accessible provided the materials are stable (commercially available intrusion porosimeters conveniently assess pore diameters in the range of 1000 µm down to 3.7 nm). Consequently, the nitrogen sorption and mercury intrusion techniques are comparable only for a very limited range of pore sizes. In general, characterization of porous materials with porosities of less than 80 vol % and pore diameters ranging from a few nanometers to about 100 nm has become well-accepted and of little challenge to the experimentalist when simple structural parameters such as average pore diameter, specific pore volume, and specific surface area are to be measured. This pore size window (i.e., 1-100 nm) and porosity (
