Chem. Mater. 1996, 8, 927-935
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Aluminosilicate Nanocomposite Materials. Poly(ethylene glycol)-Kaolinite Intercalates James J. Tunney and Christian Detellier* Ottawa-Carleton Chemistry Institute, Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada Received November 8, 1995. Revised Manuscript Received January 15, 1996X
This paper reports the first example of the direct intercalation of an organic polymer into the interlamellar spaces of kaolinite. Poly(ethylene glycols) (PEG 3400 and PEG 1000) were intercalated into kaolinite by displacing dimethyl sulfoxide (DMSO) from the DMSOkaolinite intercalate (Kao-DMSO). This was done directly from the polymer melt at temperatures between 150 and 200 °C. XRD showed that the intercalated oxyethylene units were arranged in flattened monolayer arrangements, such that the interlayer expansion was 4.0 Å. Infrared analysis of Kao-PEG 3400 supported the assignment of a trans conformation to at least a portion of the (O-CH2CH2-O) groups of the PEG polymer while 13C CP and DD/MAS NMR indicated that the polymer was intercalated intact and was more constrained in the interlamellar spaces of kaolinite than it was in its bulk form. TGA/DSC analysis revealed that the complete decomposition of the organic component of the oxyethylene-based organokaolinites did not occur until greater than 1000 °C. The formulas were estimated to be Al2Si2O5(OH)4(-OCH2CH2-)0.77(DMSO)0.27 and Al2Si2O5(OH)4(-OCH2CH2-)0.99(DMSO)0.17, respectively for the cases of PEG 1000 and PEG 3400. The calculated formulas for both samples are consistent with one unit of oxyethylene or DMSO per Al2Si2O5(OH)4 unit. Given the known density of hydroxyl groups on the aluminol surface, a polymer conformation where ethyleneoxy groups repeat every 2.8 Å with the oxygens all facing toward the hydroxyl surface should be favored. It is suggested that the polymer adopts a conformation TG2T′G2′, in which the repeat unit per ethyleneoxy group is 2.94 Å, and all the oxygens are lined up on one side of the chain. The tension resulting from the imperfect fit could be relaxed by a certain degree of trans conformations.
Introduction Over the past few years, there has been considerable interest in the intercalation of poly(ethylene oxide) (PEO) and other polymers in layered structures.1-19 The two-dimensional confinement of PEO in an inorganic lattice can lead to certain desirable anisotropic ionic conduction properties. Some of the layered materials which have been intercalated with PEO, or its smaller X Abstract published in Advance ACS Abstracts, February 15, 1996. (1) Ozin, G. A. Adv. Mater. 1992, 4, 612-649. (2) Parfitt, R. L.; Greenland, D. J. Clay Miner. 1970, 8, 305-315. (3) (a) Ruiz-Hitzky, E. Adv. Mater. 1993, 5, 334-340. (b) RuizHitzky, E.; Jimenez, R.; Casal, B.; Mariquez, U.; Santa Ana, A.; Gonzalez, G. Adv. Mater. 1993, 5, 738-741. (4) Aranda, P.; Ruiz-Hitzky, E. Chem. Mater. 1992, 4, 1395-1403. (5) Aranda, P.; Ruiz-Hitzky, E. Acta Polym. 1994, 45, 59-67. (6) Ruiz-Hitzky, E.; Aranda, P. Adv. Mater. 1990, 2, 545-547. (7) Aranda, P.; Galvan, J. C.; Casal, B.; Ruiz-Hitzky, E. Electrochim. Acta 1992, 37, 1573-1577. (8) Lagadic, I.; Le´austic, A.; Cle´ment, R. J. Chem. Soc., Chem. Commun. 1992, 1396-1397. (9) Lemmon, J. P.; Lerner, M. M. Chem. Mater. 1994, 6, 207-210. (10) (a) Vaia, R. A.; Ishii, H.; Giannelis, E. P. Chem. Mater. 1993, 5, 1694-1696. (b) Burnside, S. D.; Giannelis, E. P. Chem. Mater. 1995, 7, 1597-1600. (c) Carrado, K. A.; Thiyagarajan, P.; Elder, D. L. In Synthesis of Microporous Materials: Zeolites, Clays, and Nanostructures; Ocelli, M. L., Kessler, H., Eds.; Marcel Dekker: New York, in press. (d) Carrado, K. A.; Thiyagarajan, P.; Elder, D. L. Clays Clay Miner., in press. (e) Lan, T.; Pinnavaia, T. J. Chem. Mater. 1994, 6, 2216-2219. (f) Lan, T.; Kaviratna, D.; Pinnavaia, T. J. Chem. Mater. 1995, 7, 2144-2150. (11) Ortiz-Avila, C. Y.; Clearfield, A. Inorg. Chem. 1985, 24, 17731778. (12) Costantino, U.; Marmottini, F. Mater. Chem. Phys. 1993, 35, 193-198. (13) Liu, Y. J.; DeGroot, D. C.; Schindler, J. L.; Kannewurf, C. R.; Kanatzidis, M. G. Chem. Mater. 1991, 3, 992-994.
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molecular weight analogue poly(ethylene glycol) (PEG), include zirconium phosphate,11,12 V2O5,13 MPS3 (M ) Mn, Cd),8 MoS2,3,9 and smectite clays.2,4-7,10 The conformations that PEO adopts in the confined spaces of these inorganic matrixes are quite variable. For example, PEO intercalated in V2O5 is thought to be in a straight-chain conformation,13 whereas in MPS38 and MoS23,9 the PEO chains are thought to adopt a bilayer arrangement. Finally, in some smectite clays, the PEO was hypothesized to form helixes, in which the interlayer cation is found within the helix.4 The preparation of two-dimensional inorganic-polymer nanocomposites generally involves either polymer intercalation from solution or intercalation of a suitable monomer and its subsequent polymerization. Workers at Toyota, for example, have prepared a number of nanoscale polymer-clay hybrid composites using these techniques.14-17 Many of these composite hybrids were found to exhibit much improved mechanical and thermal properties. Recently, workers have started looking at alternate preparation methods, where the polymer is inserted directly into the layered material without the use of a solvent. Giannelis and co-workers have prepared a series of two-dimensional nanostructures (14) Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci. A: Polym. Chem. 1993, 31, 983-986. (15) Usuki, A.; Kojima, Y.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. J. Mater. Res. 1993, 8, 1179-1184. (16) Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Fukushima, Y.; Kurauchi, T.; Kamigaito, O. J. Mater. Res. 1993, 8, 1185-1189. (17) Kojima, Y.; Usuki, A.; Kawasumi, M.; Okada, A.; Kurauchi, T.; Kamigaito, O. J. Polym. Sci. A: Polym. Chem. 1993, 31, 1755-1758.
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based on the direct polymer melt intercalation of organically modified smectites.10a,b A PEO-smectite nanocomposite with a XRD pattern identical with the PEO-smectite materials prepared via the solution intercalation method4 was one of the materials prepared. Very recently, hectorite was hydrothermally crystallized in the presence of a polymer solution,10c,d yielding a polymer-clay nanocomposite. Clay-reinforced epoxy nanocomposites were prepared, forming exfoliated nanocomposites, with well-separated clay layers in a continuous polymer matrix.10e,f The direct incorporation of PEO or any other polymer into the structure of kaolinite has never been reported in the literature. Sugahara et al. have nonetheless succeeded in preparing kaolinite-polyacrylonitrile18 and kaolinite-polyacrylamide19 nanocomposites by first intercalating the appropriate monomer into the interlayers and then inducing polymerization by thermal treatment. For kaolinite, it has been shown that it is possible to prepare two distinct ethylene glycol phases of kaolinite, Kao-EG 9.4 Å and Kao-EG 10.8 Å.20 The Kao-EG 10.8 Å phase was the more weakly bound intercalated phase, while the Kao-EG 9.4 Å phase was attributed to an EG unit covalently bound to interlayer aluminol surface of kaolinite. A polytype of kaolinite, halloysite, was also reported to intercalate not only EG but also diethylene glycol (DiEG) and triethylene glycol (TriEG).21 From these data, it was logical to assume that larger molecules based on the oxyethylene unit, of which EG was the simplest unit, may also intercalate into the structure of kaolinite. With this idea in mind, it was attempted to intercalate larger oxyethylene-based molecules into kaolinite. In this paper, the preparation and characterization of two oxyethylene-based organokaolinites are reported, kaolinite PEG-1000 (Kao-PEG 1000) and Kao-PEG 3400. This is the first example of the direct intercalation of a polymer into kaolinite. Experimental Section Chemicals. All chemicals used were of reagent-grade quality and were not further purified unless otherwise specified. Well-crystallized kaolinite from Georgia (labeled KGa1) was obtained from the Source Clay Repository of the Clay Mineral Society (Department of Geology, University of Missouri). This was purified based on previously reported sedimentation techniques,22-25 and the 1000 °C) to ensure the complete combustion of the nanocomposite. The chemical formula for samples 1 and 6 were calculated on the basis of the calcination weight losses at 1100 °C for the samples. The OEU/DMSO ratio for 1 and 6 were estimated to be approximately 2.8 and 5.9, respectively, from the CP/MAS NMR data, and the calcination weight losses were 29.1 and 29.4%, respectively. Using these data, the formula for sample 1 was estimated to be Al2Si2O5(OH)4(-OCH2CH2-)0.77(DMSO)0.27 and, similarly, sample 6 was estimated to be Al2Si2O5(OH)4(-OCH2CH2-)0.99(DMSO)0.17. The calculated formula for both samples is consistent with one unit of oxyethylene or DMSO per Al2Si2O5(OH)4 unit. This stoichiometry is similar to that which one obtains for Kao-DMSO where TGA measurements indicate that for every Al2Si2O5(OH)4 unit there is close to 1.0 intercalated DMSO molecule.27,55 Given the large uncertainty associated with the quantitative CP/MAS NMR data under the conditions of this study, the ratio OEU/DMSO should be taken only as a crude approximation. On the basis of recently published unit-cell dimensions,39 the surface area of one interlamellar face of one Al4Si4O10(OH)8 unit is calculated to be 23.0 Å2. The approximate area, based on CPK models, that one side of a flattened oxyethylene unit occupies is about 14 Å2, so that to a first approximation there remains some void space in the interlamellar regions of kaolinite. One can expect that the main driving force for the intercalation reaction results from an orientation of the oxygens of the polymer chains such that they hydrogen bond with the hydroxylated aluminate surface of kaolinite. To obtain maximum hydrogen-bonding interaction, the polymer should adopt a conformation such that its oxygens match up with the surface hydroxyl groups (64) Sanchez-Soto, P. J.; Justo, A.; Perez-Rodriguez, J. L. J. Mater. Sci. 1994, 29, 1276-1283. (65) Perez-Maqueda, L. A.; Perez-Rodriguez, J. L.; Scheiffele, G. W.; Justo, A.; Sanchez-Soto, P. J. J. Therm. Anal. 1993, 39, 1055-1067.
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of the kaolinite. This means that the distances between oxygens in the polymer chain be roughly comparable to the distances between hydroxyl groups in kaolinite. The interhydroxyl distance is about 2.8 Å as determined recently by a Rietveld refinement of the kaolinite structure.66 It is then expected that a polymer conformation where ethyleneoxy groups repeat every 2.8 Å with the oxygens all facing toward the hydroxyl surface would be particularly favorable. This type of polymer conformation has been described previously in a polyethylene oxide-HgCl2 complex.67,68 It has been given the designation PEO-HgCl2 type II complex and has been found to be nearly in the form TG2T′G2′; that is -CH2 -(- O - CH2 - CH2 - O - CH2 - CH2-)n73° -167° -81° -73° 167° 81°
The angle values indicated are those determined in the study of the PEO/HgCl2 complex. These are given here only as an indication of a possible conformation of this Kao-PEO intercalate. With this conformation, the repeat unit per ethyleneoxy group is 2.94 Å, and all the oxygens are lined up on one side of the chain. The tension resulting from the imperfect fit could be relaxed by a certain degree of trans conformations. These conformations are shown by IR (see above) to be present. Conclusion It has been shown that it is possible to intercalate PEG into kaolinite by displacing DMSO from KaoDMSO. These include PEG 1000 and PEG 3400 with the presence of some residual DMSO molecules. XRD showed that the intercalated oxyethylene units were (66) Bish, D. L.; Von Dreele, R. B. Clays Clay Miner. 1989, 37, 289296. (67) Takahashi, Y.; Sumita, I.; Tadokoro, H. J. Polym. Sci., Polym. Phys. 1973, 11, 2113-2122. (68) Yokoyama, M.; Ishihara, H.; Iwamoto, R.; Tadokoro, H. Macromolecules 1969, 2, 184-192.
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arranged in flattened monolayer arrangements, such that the interlayer expansion was 4.0 Å. Infrared analysis of Kao-PEG 3400, supported the assignment of a trans conformation to at least a portion of the (OCH2CH2-O) groups of the PEG polymer while 13C DD/ MAS NMR indicated that the polymer was intercalated intact and was more constrained in the interlamellar spaces of kaolinite than it was in its bulk form. TGA/DSC analysis revealed that the complete decomposition of the organic component of the oxyethylenebased organokaolinites did not occur until greater than 1000 °C. A small weight loss was observed for the air atmosphere TGA/DSC runs associated with the two exothermic transitions between 990 and 1040 °C. This weight loss was thought to be the result of the release of carbonaceous material trapped within a metakaolinite matrix during the structural reorganization of metakaolinite which takes place above 1000 °C. This work represents the first example of the direct intercalation of a polymer into the interlamellar surface of kaolinite, and this material may prove to have interesting intercalation properties, since both kaolinite and PEO are known to intercalate various alkali salts. A Kao-PEG salt complex would represent a secondgeneration nanocomposite material, whereby within the rigid inorganic kaolinite lattices, the intercalated PEG polymer could in turn aid in the complexation of the salt. This material would then be a suitable candidate to act as an anisotropic solid electrolytic material. Materials derived from the intercalation of crown ether into the structure of kaolinite may also fulfill this role.69 Acknowledgment. The Natural Sciences and Engineering Research Council of Canada (NSERCC) is gratefully acknowledged for its continuous financial support. We thank Dr. Glenn Facey (University of Ottawa) for recording the solid-state 13C NMR spectra. CM9505299 (69) Tunney, J. J., Detellier, C., to be published.
