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Use of a Self-Assembling Organogel as a Reverse Template in the Preparation of Imprinted Porous Polymer Films Grace Tan,† Mohit Singh,† Jibao He,‡ Vijay T. John,*,† and Gary L. McPherson§ Departments of Chemical and Biomolecular Engineering and Chemistry and Coordinated Instrumentation Facility, Tulane University, New Orleans, Louisiana 70118 Received April 22, 2005. In Final Form: July 17, 2005 The concept of reverse templating of an organogel to form imprinted porous divinylbenzene polymer films with submicrometer channels is demonstrated. The organogel comprising a 1:1 molar ratio of two organogelators, that is, bis(2-ethylhexyl) sodium sulfosuccinate and 4-chlorophenol, was formed in divinylbenzene. The gel was cast as a thin film before UV polymerization of the solvent, and the organogelators were later removed by simple washing with water and isooctane. The integrity of the fiber bundles of the organogel was preserved during polymerization, and an exact hollow replica was obtained after the organogelators were leached away. It is easily possible to imprint gel fiber bundle structures into polymeric films through this technique. The gel can also be formed on macroporous substrates to yield supported thin porous polymeric films. With the incorporation of functional nanoparticles in AOT inverse micelles and hence the organogel, nanoparticle-containing porous polymer films exhibiting luminescence or magnetic properties are envisioned.
Introduction The gelation of organic liquids by nonpolymeric species leads to an interesting class of materials, where noncovalent interactions of relatively low molecular weight compounds at low concentrations promote self-assembly to elongated polymerlike structures to form threedimensional (3D) networks in solution.1 These materials have recently been of much interest for applications encompassing fields such as biomimetics,2 separations,3 templated materials synthesis,4-6 and drug delivery.7 Examples of such nonpolymeric gelators include organometallic complexes,8 steroids,9 alkylamide derivatives,10 and fatty acids.11 Where nonpolar organic solvents are used, gelation is typically due to hydrogen bonding or dispersion interactions that lead to self-assembled networks.12-16 The literature describes a range of orga* Corresponding author: e-mail
[email protected]; phone (504) 865-5883; fax (504) 865-6744. † Department of Chemical and Biomolecular Engineering. ‡ Coordinated Instrumentation Facility. § Department of Chemistry. (1) Abdallah, D. J.; Weiss, R. G. Adv. Mater. 2000, 12, 1237. (2) Hafkamp, R. J. H.; Kokke, P. A.; Danke, I. M.; Guerts, H. P. M.; Rowan, A. E.; Feiters, M. C.; Nolte, R. J. M. Chem. Commun. 1997, 545. (3) Phillips, R. J.; Deen, W. M.; Brady, J. F. J. Colloid Interface Sci. 1990, 139, 363. (4) Loos, M.; Esch, J.; Stokroos, I.; Kellogg, R. M.; Feringa, B. L. J. Am. Chem. Soc. 1997, 119, 12675. (5) Rees, G. D.; Robinson, B. H. Adv. Mater. 1993, 5, 608. (6) Ono, Y.; Nakashima, K.; Sano, M.; Hojo, J.; Shinkai, S. J. Mater. Chem. 2001, 11, 2412. (7) Haering, G.; Luisi, P. L. J. Phys. Chem. 1986, 90, 5892. (8) Terech, P.; Chachaty, C.; Gaillard, J.; Giroud-Godquin, A. M. J. Phys. (Paris) 1987, 48, 663. (9) Terech, P.; Ramasseul, R.; Volino, F. J. J. Phys. (Paris) 1985, 46, 895. (10) Hanabusa, K.; Tange, J.; Taguchi, Y.; Koyama, T.; Shirai, H. J. Chem. Soc., Chem. Commun. 1993, 390. (11) Tachibana, T.; Mori, T.; Hori, K. Bull. Chem. Soc. Jpn. 1980, 53, 1714. (12) Abdallah, D. J.; Weiss, R. G. Langmuir 2000, 16, 352. (13) Lin, Y. C.; Kachar, B.; Weiss, R. G. J. Am. Chem. Soc. 1989, 111, 5542. (14) Geiger, C.; Stanescu, M.; Chen, L.; Whitten, D. G. Langmuir 1999, 15, 2241. (15) Esch, J. H.; Feringa, B. L. Angew. Chem., Int. Ed. 2000, 39, 2263.
nogels formed with single or binary gelator species,17-22 some of which are specifically designed such that they can be polymerized.4,23-25 This paper describes the use of a binary gelator species to form easily removable templates in polymerizable solvents, leaving behind an imprint of the template. We use the binary system of AOT [bis(2-ethylhexyl) sodium sulfosuccinate], 1, and a phenol (e.g., 4-chlorophenol, 2) as an effective gelator of nonpolar organic solvents.26-28 These organogels spontaneously form when 1 and 2 are doped into the solvent at equimolar compositions and at concentrations as low as 0.02 M. Hydrogen-bonding interactions between the sulfosuccinate headgroups of the anionic surfactant (AOT) and the phenol are essential to the formation of the gel.26 Our proposed microstructure of the gel is based on 2.1 nm strands containing motionally restricted and stacked phenols surrounded by AOT molecules.29 These strands aggregate to fibers with an approximate diameter of 10 nm, which undergo a second level of aggregation to fiber bundles 100-200 nm in dimension, which can be readily visualized through atomic force microscopy.29 The strands are arranged in columnar hexagonal order in the fibers (Figure 1b), and a characteristic signature of the gel is the sharp X-ray diffraction (16) Duncan, D.; Whitten, D. G. Langmuir 2000, 16, 6445. (17) Terech, P.; Pasquier, D.; Bordas, V.; Roassat, C. Langmuir, 2000, 16, 4485. (18) Lu, L,; Weiss, R. G. J. Chem. Soc. Chem.Commun. 1996, 17, 2029. (19) Lu, L.; Weiss, R. G. Langmuir 1995, 11, 3630. (20) Inoue, K.; Ono, Y.; Kanekiyo, Y.; Ishi-I, T.; Yoshihara, K.; Shinkai, S. J. Org. Chem. 1999, 64, 2933. (21) Waguespack, Y. Y.; Banerjee, S.; Ramannair, P.; Irving, G. C.; John, V. T., McPherson, G. K. Langmuir 2000, 16, 3036. (22) Ajayaghosh, A.; George, S. J. J. Am. Chem. Soc. 2001, 123, 5148. (23) Wang, G.; Hamilton, A. D. Chem. Eur. J. 2002, 8, 1954. (24) George, M.; Weiss, R. G. Chem. Mater. 2003, 15, 2879. (25) Aoki, K.; Kudo, M.; Tamaoki, N. Org. Lett. 2004, 6, 4009. (26) Xu, X.; Ayyagari, M.; Tata, M.; John, V. T.; McPherson, G. L. J. Phys. Chem. 1993, 97, 11350. (27) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Am. Chem. Soc. 1994, 116, 9464. (28) Tata, M.; John, V. T.; Waguespack, Y. Y.; McPherson, G. L. J. Phys. Chem. 1994, 98, 3809. (29) Simmons, B.; Taylor, C.; Landis, F. A.; John, V. T.; McPherson, G. L.; Schwartz, D. K.; Moore, R. J. Am. Chem. Soc. 2001, 123, 2414.
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tures at its best cannot be avoided. Atomic force microscopy (AFM), an emerging characterization tool widely used in analyzing biological specimens, is perfect for this purpose. This relatively noninvasive imaging technique when performed in the tapping or noncontact mode is able to provide clear imaging of the gel surface structure. In this article, we attempt to show the simplicity of template removal in the AOT + chlorophenol gel system and the similarity of the imprint to the original fiber bundle structure. Furthermore, we attempt to show preservation of the gel structure and retention of gel dimensions up till removal of the template. Experimental Procedures
Figure 1. (a) Chemical structures of bis(2-ethylhexyl) sodium sulfosuccinate (1) and 4-chlorophenol (2). (b) Illustration of the hexagonal packing of gel strands to form a gel fiber.
pattern that is observed at a d spacing of 1.8 nm.29 The phenol organogels undergo sharp, reversible, melting transitions with temperature. Perhaps more interesting is the fact that small amounts of water can irreversibly transform the gel to low-viscosity solutions by breaking the AOT-phenol hydrogen bonding. This simple destruction of the gel phase is exploited here in the removal of the template. The hypothesis is that an organogel film containing a polymerizable, nonpolar solvent such as divinylbenzene can be prepared and then can be polymerized to a film, leaving the template organogel fibers intact. The template can then be easily broken by simple washing with water (which effectively breaks the phenol-AOT hydrogen bonding), and the gel species can be dissolved in a nonpolar hydrocarbon (e.g., isooctane) to leave behind structured pores, demonstrating the imprint of the template. Concepts of reverse templating and imprinting of organogels were first advanced for inorganic sol-gel transcription by Shinkai and co-workers30 and in polymer materials by Mo¨ller and co-workers,31 Weiss and co-workers,32 and Nolte and co-workers.2 Although the papers demonstrating imprinting of organogels in polymer materials are remarkably encouraging, utilizing techniques such as optical microscopy and transmission electron microscopy, the shortfall of these techniques in displaying the gel struc(30) Ono, Y.; Nakashima, K.; Sano, M.; Kanekiyo, Y.; Inoue, K.; Hojo J.; Shinkai, S. Chem. Commun. 1998, 1477. Some recent review articles for readers interested in the use of organogels as templates include (a) Jung, J. H.; Shinkai, S. Top. Curr. Chem. 2005, 248, 223. (b) van Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980. (c) Sanchez, C.; Soler-Illia, G. J. de A. A.; Ribot, F.; Grosso, D. C. R. Chim. 2003, 6, 1131. (d) de Solar-Illia, G. J.; Sanchez, C.; Lebeau, B.; Patarin, J. Chem. Rev. 2002, 102, 4093. (31) Beginn, U.; Keinath, S.; Mo¨ller, M. Macromol. Chem. Phys. 1998, 199, 2379. (32) Gu, W.; Lu, L.; Chapman, G. B.; Weiss, R. G. Chem. Commun. 1997, 543.
Materials. All chemicals used for synthesis were purchased from commercial sources: bis(2-ethylhexyl) sodium sulfosuccinate (AOT) (Aldrich, 98%), 4-chlorophenol (Sigma), divinylbenzene, technical grade (Aldrich, 80%), 2,2′-azobisisobutyronitrile (AIBN) (Aldrich, 98%), and 2,2,4-trimethylpentane (Aldrich). Sample Preparation. AOT (0.2 M) and 0.2 M chlorophenol were dissolved in divinylbenzene in separate vials by ultrasonication until clear isotropic solutions were obtained. The contents of the two vials were mixed together and a white-colored gel formed in a matter of seconds. To prepare the polymerized gel, the preformed gel was melted in a hot water bath before approximately 1 mol % 2,2′-azobisisobutyronitrile (AIBN) initiator was added to it, and the mixture was shaken to ensure complete dissolution. A tiny amount of this melted gel was immediately poured out and deposited as a thin layer on a cleaved mica disk or into an X-ray diffraction (XRD) sample holder. The samples were subsequently UV-polymerized for at least 2 h before characterization by XRD and AFM. (Note: Care should be taken not to overheat the initiator prior to formation of a thin layer of sample on the mica disk, as this would cause polymerization and solidification of the sample before the gel sets. Polymerization can also be conducted in a water bath to prevent the temperature from increasing excessively. However, this would need to be compensated for by a longer polymerization time.) X-ray Diffraction Characterization. A Scintag XRD-2000 equipped with a Cu KR1 radiation source and a Si (Li) detector was used to collect the diffraction data of the organogel. The X-ray tube was operated at -43 kV and 38 mA. A scan range corresponding to a 2θ of 1-10° was performed at a scan rate of 1.00°/min. Cut-Section Transmission Electron Microscopy. The sample was embedded in an epoxy resin and ultramicrotomed into thin sections (approximately 70 nm) with a diamond knife. A thin slice of the microtomed sample is placed on a Formvarcoated copper grid and viewed on a JEOL 2011 transmission electron microscope at an acceleration voltage of 120 kV. Intermittent-Mode Atomic Force Microscopy. We used the magnetic alternating current mode, a form of intermittentmode microscopy (PicoSPM, Molecular Imaging) equipped with a medium-range scanner to image the gels. Type II MacLevers (Molecular Imaging) were operated at their fundamental resonance frequencies of 50-74 kHz. The tip vibration frequency was adjusted to be on the low-frequency side of the resonance peak corresponding to a free air tip amplitude of 70% of the resonance peak height amplitude. Upon engagement with the sample, amplitude distance graphs were adjusted to ensure good contact of the cantilever tip with the surface without causing excessive sample damage. Images for topography, amplitude, and phase were collected simultaneously. A scan rate of 2-3 lines/s was employed. I and P gains were adjusted to give the clearest images without introduction of excessive noise.
Results and Discussion AFM images of the AOT + chlorophenol gel formed in divinylbenzene are illustrated in Figure 2a-c. Figure 2a provides an 8 µm × 8 µm view of the gel network, which mainly consists of 100-200 nm fiber bundles randomly entangling throughout the gel. The higher resolution AFM picture (Figure 2c) reveals the presence of individual fibers
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Figure 2. (a-c) AFM phase images of the unpolymerized organogel at various magnification levels, and (d) the polymerized gel prior to strand removal. Concentrations of AOT and chorophenol in divinylbenzene are 0.2 M each.
in the fiber bundles. Although formed in divinylbenzene for the first time, these AOT + chlorophenol gels are similar to those formed in other nonpolar solvents and described in our earlier work.29 The films of these gels were then polymerized through UV irradiation. Upon polymerization of the solvent, AFM imaging was made easier as adhesive sample-tip interactions diminish with the hardened material. Examination of the phase-mode image in Figure 2d shows the polymerized gel to be similar to that before polymerization, suggesting that the gel fiber structure was not destroyed during the polymerization step. The AOT + chlorophenol gels exhibit a sharp XRD peak at 1.8 nm, the characteristic d spacing for the hexagonal arrangement of individual gel strands to form fibers, as shown in Figure 1b.29 Disruption of the gel strands and loss of gel integrity is clearly indicated by the disappearance of the X-ray diffraction peak. We have made use of this as a diagnosis for gel integrity at various stages of polymerization. While in situ XRD during polymerization has not been carried out, we have subjected the organogel film to XRD at various stages of polymerization. The results are shown in Figure 3 and indicate retention of strands and strand aggregation over the course of polymerization. A small decrease in the d spacing from 1.8 to 1.7 nm is observed, indicating a tightening of the assembly. The next step in generating the imprint is to remove the gel structure. To etch away the gelators after polymerization, a simple washing procedure was performed where the samples were first washed in water, to destroy strand integrity by breaking the AOT-phenol hydrogen bonding, and later with isooctane, to dissolve the AOT
Figure 3. X-ray diffraction of 0.2 M AOT/0.2 M chlorophenol divinylbenzene samples polymerized for different time duration. Inset shows the corresponding diffraction pattern of the washed polymer after all gel species have been removed.
and phenol, which have low solubilities in water. Isooctane, which is miscible with divinylbenzene, also serves to remove the unpolymerized monomeric divinylbenzene in the sample. At the end of this procedure, the XRD of the polymerized film shows a complete absence of the gel diffraction pattern (inset to Figure 3), indicating that the gel structure no longer exists in the film. Figure 4 illustrates the AFM topography images of the washed sample at various magnification levels. The 8 µm × 8 µm image (Figure 4a) reveals that the surface of the
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Figure 4. AFM 2D and 3D topology images of the washed polymer film, revealing the gel fiber imprint at different magnification levels.
polymerized matrix is relatively smooth and full of random channels just like the fiber bundles in the gel prior to strand removal. Close-up images in Figures 4b,c reveal these channels to be of the same dimension as the fiber bundles originally existing in the gel. Figure 4c in particular depicts a ca. 200 nm channel. There is a gradual darkening of color on both sides of the channel depicting a curvature to the channel edge. A three-dimensional version of the topographic image is shown in Figure 4d to aid in visualization of the porous polymer films. Although AFM is an excellent tool for imaging the organogels, it is limited in the aspect that only surface information about the specimen can be obtained. To examine the internal structure of the polymerized film, we followed up our results with cut-section transmission electron microscopy of the porous polymer films (Figure 5). The specimen was microtomed parallel to the surface of the film to provide information on the cross-sectional interior of the thin film. Channels measuring 20-100 nm were observed. The channels as seen in the TEM micrographs run horizontally, diagonally, and also perpendicularly into the polymerized film. The fact that channels exist in the cut-section samples gives evidence that polymerization proceeds throughout the thin film and successful leaching of the gelators is not limited to the film surface. We are able to form AOT + chlorophenol organogels not only in pure solvents but also in mixtures of solvents. At a 1:1 volume ratio of divinylbenzene and isooctane, a transparent gel with a purplish tinge forms, in contrast to gels made with pure divinylbenzene and pure isooctane, which are turbid white in color. Deviation from a 1:1
Figure 5. Cut-section TEM micrograph of the porous polymer film revealing channels running horizontally, diagonally and perpendicularly into polymerized divinylbenzene.
proportion of the solvents yields an increasingly translucent gel. Gels with divinylbenzene as the solvent are relatively weaker than gels with isooctane.29 Hence, replacement of part of the divinylbenzene solvent with isooctane enhances the physical integrity of the gel. When
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smaller area scans to increase the resolution. The retention of gel structure when repeated scans are carried out is evidence of the stability of the polymerized gel to AFM imaging. In Figure 6b, there is evidence that the fiber bundles penetrate through thin layers of poly(divinylbenzene). Indeed, the even smaller area scan shown in Figure 6c illustrates details of the fiber bundles essentially threading through thin polymer layers. Conclusions We have demonstrated an interesting concept of the way organogels held together by physical interactions can generate thin films of porous materials simply by polymerizing the solvent and removing the gel fibers. The novelty of the method is the facile removal of the gel fibers simply by washing to break hydrogen bonds, leaving an imprint that has a high fidelity to the original template. The technique can be exploited in a number of useful applications. Since the gels originate from the AOT reverse micellar system (in a dry state) through the addition of the phenolic component, it is possible to first incorporate biomolecules and/or inorganic nanoparticles into the micelles, translate the micelles to the organogel, and then conduct polymerization of the solvent.33 In this way, it is possible to generate porous polymer membranes with functional materials (e.g., enzymes, nanoparticles) embedded selectively in the pores. By modifying the solvent with mixtures of styrene and divinylbenzene for example, membrane flexibility can be modified. Such applications are aspects of ongoing research. Figure 6. AFM phase images, (a) 15 µm × 15 µm, (b) 2.5 µm × 2.5 µm, and (c) 1 µm × 1 µm, of a polymerized 0.2 M AOT/ 0.2 M chlorophenol organogel formed in a solvent mixture of 50 vol % isooctane and 50 vol % divinylbenzene.
polymerized, these gels, shown in the AFM phase image in Figure 6, display a similar random gel network structure. Figure 6 panels a, b, and c indicate progressively
Acknowledgment. Funding from the National Science Foundation (Grants 0438463 and 0092001) and NASA (NAG-1-02070) is gratefully acknowledged. G.T. thanks Pierre L. Burnside and Limin Liu for helpful discussion. LA051080T (33) Simmons, B.; Li, S.; John, V. T.; McPherson, G. L.; Taylor, C.; Schwartz, D. K.; Maskos, K. Nano Lett. 2002, 2, 1037.
