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Polymer Gels that Memorize Structures of Mesoscopically Sized Templates. Dynamic and Optical Nature of Periodic Ordered Mesoporous Chemical Gels Yukikazu Takeoka* and Masayoshi Watanabe* Department of Chemistry and Biotechnology, Yokohama National University, 156 Tokiwa-dai, Hodogaya-ku, Yokohama 240-8501, Japan Received February 7, 2002. In Final Form: May 2, 2002
A periodically ordered interconnecting porous structure could be imprinted in thermosensitive hydrogels by using silica colloidal crystals as mesoscopically sized templates. The interconnecting porous structure provides a fast response to the change in temperature for reversible swelling and shrinking of the hydrogels, while the periodically ordered mesoscopical structure endows a structural color to the gel. The structural color imprinted in the gel is quickly synchronized with the change in the volume of the gels. The newly invented mesoporous hydrogels possess optical properties as temperature sensitive smart gels.
Chemical gels can memorize their original macroscopic shape during polymerization. For example, the vessel used can often be a template of the gels. Using this property, researchers have fabricated shape memory gels1-3 that find a use in sensors, actuators, and display devices. Compared to well-known shape memory alloys and polymers, the shape memory gels are advantageous because of their very large deformations and responsiveness to many different stimuli, such as temperature, pH, and solvent composition. Furthermore, the molecular level shape memory in the gels can also be achieved by the molecular imprinting method,4-7 using molecules as templates. The obtained gels possess synthetic recognition sites with predetermined selectivity for various substances and provide a potential alternative to the use of fragile biological antibodies for drug assay. Thus, the so-called template synthesis or the imprinting technique is a commonly used tool in the preparation of functional gels and the control of their physical and chemical nature, which reflects the character of the templates on covering a wide-scale size. Here, the interesting physical properties of periodic ordered mesoporous gels are demonstrated by the use of silica colloidal crystals8-10 ranging in size from the nanometer to micrometer level as templates, and the potential ability for scientific fascination and technological applications are described. The silica colloidal crystals are three-dimensionally regular crystalline arrays of (1) Osada, Y.; Matsuda, A. Nature 1995, 376, 219. (2) Hu, Z.; Zhang, X.; Li, Y. Science 1995, 269, 525-527. (3) Lendlein, A.; Schmidt, A. M.; Langer, R. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 842-847. (4) Vkatakis, G.; Andersson, L. I.; Muller, R.; Mosbach, K. Nature 1993, 361, 645-647. (5) Remcho, V. T.; Tan, J. Z. Anal. Chem. News & Features 1999, 248-255. (6) Alvarez-Lorenzo, C.; Guney, O.; Oya, T.; Sakai, Y.; Kobayashi, M.; Enoki, T.; Takeoka, Y.; Ishibashi, T.; Kuroda, K.; Tanaka, K.; Wang, G.; Grosberg, A. Y.; Masamune, S.; Tanaka, T. Macromolecules 2000, 33, 8693-8697. (7) Shi, H.; Tsai, W.-B.; Garrison, M. D.; Ferrari, S.; Ratner, B. D. Nature 1999, 398, 593-597. (8) Jiang, P.; Bertone, J. F.; Hwang, K. S.; Colvin, V. L. Chem. Mater. 1999, 11, 2132-2140. (9) Donselaar, L. N.; Philipse, A. P.; Suurmond, J. Langmuir 1997, 13, 6018-6025. (10) Miguez, H.; Meseguer, F.; Lopez, C.; Misfsud, A.; Moya, J. S.; Vazquez, L. Langmuir 1997, 13, 6009-6011.
highly monodisperse silica spheres. They have important technological uses as optical filters, switches, and materials with photonic band gaps. It was found that the mesoscopically templated mesoporous gels display dynamic and optical abilities from the three-dimensional shape and the distinctive optical property of the used templates. This approach provides gels with not only improvement in the slow response to external stimuli but also the potential for use in optical-related technologies. A well-known thermosensitive monomer, N-isopropylacrylamide (NIPA), and N,N′-methylenebis(acrylamide) (BIS) were used as the main monomer and cross-linker, respectively, to obtain a gel. First, 1.132 g of NIPA and 0.015 g of BIS were dissolved in a certain amount of 1,4dioxane. The solution was degassed by bubbling nitrogen gas for 30 min before the gelation. A few types of silica colloids with size distributions less than 5% were able to form three-dimensional periodic colloidal crystals by a gravity sedimentation method8 from the dispersions in the Petri dishes. It is known that the silica colloids are organized into a close-packed arrangement with a longrange order, strongly diffracting light at visible and nearinfrared wavelengths.8-10 The stable dried crystals were obtained within 1 week as the water evaporated, and then completely dried in vacuo at 60 °C. These silica-air crystals can serve as mesoscopic templates for the formation of mesoporous polymers. The infiltrated monomer solution in the crystals was polymerized with benzoyl peroxide as a free-radical initiator at 60 °C so as to trap their crystalline order. The obtained gels were soaked in 5% hydrofluoric acid solution for a few hours to remove the silica colloids. The resulting gels were carefully washed with pure water and allowed to reach an equilibrium swelling state in water at 25 °C. The mesoscopic structures of the gels in water were observed by the use of an inverted microscope. Figure 1 shows photos of the swelling state of the mesoporous NIPA gel made by 6 µm diameter silica colloid crystals as the template. The gel clearly has retained the three-dimensional crystalline order of the template. The size of the voids becomes much larger than the used silica colloid when the gel is swollen at 25 °C. These voids are not
10.1021/la020133t CCC: $22.00 © 2002 American Chemical Society Published on Web 06/29/2002
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Figure 1. Micrographs of templated mesoporous gels by silica colloidal crystal. (a) This photo shows both homogeneous (left portion) and mesoporous NIPA gel (right portion). The longrange order of voids is apparent. (b) A hexagonally ordered network can be observed on the cut surface of the mesoporous gel, sliced parallel to the used silica template surface. (c) The underlying plane can be seen through the upper layer of the ordered network.
isolated and extend throughout the gel. Figure 1b and c shows the structural integrity of the sample with hexagonally ordered pores. The lower layer of the microstructure can be observed through the holes of the upper layer. Thus, this interconnecting inner pore structure means that the gel has a large specific surface area. Furthermore, the periodic voids with long-range order ensure the free diffusion of solvents and solutes in the gel; the physical properties such as the refractive index of the diffusible species must be identical to those of the bulk state.
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In pure water, NIPA gel can undergo a thermal volume phase transition from the low-temperature swollen state to the high-temperature shrunken state at about 33 °C.11 It was observed that the pore size could also be changed reversibly depending on the temperature, while the periodic structure can be retained. Such a weak, crosslinked polymer gel can memorize its mesoscopic structure and control the size in response to the external environment. Thus far, the response time of a gel (approximately millimeter size) is slowson the order of several hours. The time it takes for the swelling and shrinking of a gel is governed by a collective diffusion of the polymer network.12 It is proportional to the square of a characteristic size of a gel. It is known that, during the shrinking process, the gel shrinks only in the surface region because of the collective diffusion process. The formation of a dense skin layer, which acts as an impermeable barrier, prevents the gel from shrinking further and causes high osmotic pressure inside the gel; the inner pressure blows up some portion of the dense surface layer like a balloon. Therefore, a porous structure increases the surface areato-volume ratio and allows the solvent to diffuse more rapidly into/out-of the gel, resulting in fast volume changes.13 The kinetics of the shrinking process of a cylindrical gel, 3 mm in diameter, with both mesoporous and homogeneous portions, were monitored. Since the homogeneous portion that behaves as a typical homogeneous gel was polymerized on the silica colloid crystal, the homogeneous portion was connected with the mesoporous portion. In Figure 2, the shrinking time course of the gel is shown when there is a sudden temperature jump from 25 to 40 °C. The gel was placed in a double jacket cell holder with the temperature controlled by circulating water. The temperature within the cell was measured by the use of a digital thermometer embedded within the cell. The time to reach the phase transition temperature (33 °C) in the cell was about 10 s, which is much shorter than the shrinking time of the homogeneous portion of the gel. The surface of the homogeneous portion became opaque at high temperature after a temperature jump: the skin layer at the surface was formed. Following this, the shrinking process stopped for a certain period of time and the balloons appeared. Eventually, it took more than 1 day for the homogeneous portion to reach a completely collapsed state. On the other hand, a blinkingly fast response can be observed in the mesoporous portion. The deswelling of the gel was accomplished within 30 s: the rate is more than 1000 times faster than that of the homogeneous gel. The balloons did not appear during this process. The fast response was attributed to the interconnecting inner pore structure that was generated from silica colloidal crystals. The mesoporous gels obtained by using larger silica particles that are white solids, with a micrometer diameter (4, 6, 10 µm), show opacity (Figures 1 and 2), whereas the homogeneous ones are almost clear. It is known that the high turbidity is caused by the decrease in the intensity of transmitted light due to the scattering of white light. From a classical point of view, the difference in the refractive indexes between two contiguous substances lead to some reflection or scattering at the interface. Therefore, (11) Hirokawa, Y.; Tanaka, T.; Matsuo, E. S. J. Chem. Phys. 1984, 85, 6379-6380. (12) Sato-Matsuo, E.; Tanaka, T. J. Chem. Phys. 1988, 89, 16951703. (13) Zhao, B.; Moore, J. S. Langmuir 2001, 17, 4758-4763.
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Figure 2. Shrinking processes of a NIPA gel with both mesoporous (dark moiety) and homogeneous (transparent moiety) portions. The time courses of the pictures are (a) 0 s, (b) 8 s, (c) 12 s, (d) 16 s, (e) 20 s, (f) 32 s.
the clear-cut outline of the homogeneous gel in water (Figure 2) indicates the obvious distinction in the refractive index between the gel and water. When the dispersed particles, which show a different refractive index from that of a medium, are larger than the wavelength in an illuminated medium, Mie scattering occurs. Mie scattering is not strongly wavelength dependent and produces an almost white glare when a lot of particles are present in a medium. Thus, it also gives us the opacity from the spherical voids filled with water in the mesoporous gel. When using the template from submicron silica particles, however, the gels are almost transparent and iridescent when placed in water below the phase transition temperature. The silica colloidal crystals are also revealed as iridescent, depending on the particle size. Figure 3 shows photographs of a mesoporous NIPA gel by the use of 0.5 µm silica colloidal crystals as the template. This gel reveals a faster response to temperature change. The gel exhibits striking iridescence under white light illumination (Figure 3a-c). The iridescence must be mainly caused by the Bragg diffraction of visible light from the ordered voids regarded as crystallites.14-16 The Bragg diffraction differs from that of ordinary optical gratings in that the crystal is three-dimensional and consists of several successive layers separated by lattice spacing. Also, it is proportional to the number of crystallites in the correct orientation for diffraction. Therefore, when the gel is placed at different orientations toward the applied light, the number of crystallites at all orientations and the lattice spacing of the crystals with respect to the sample can be changed. Hence, the observable brightness and color were also changed. Moreover, the heterochromatic color from the gel must be caused by the presence of the random orientation of diffracting planes (Figure 3b, c). (14) Holts, J. H.; Holtz, J. S.; Munro, C. H.; Asher, S. A. Anal. Chem. 1998, 70, 780-791. (15) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829-832. (16) Liu, L.; Li, P.; Asher, S. A. Nature 1999, 397, 141-144.
The dazzling color was changed depending on the temperature and disappeared when the gels collapsed at high temperature (Figure 3d). The Bragg condition must be shifted to the shorter wavelengths above the transition temperature. The quick optical switching behavior synchronized with the volume change of the gel can be observed reversibly when the temperature is changed. This work demonstrates the size effect of templated spherical voids on the physical properties of the periodically ordered mesoporous gels. The three-dimensional structure of the silica colloidal crystals, which are structurally stable and easily handled as templates, endows the gel with rapid motility and optical properties without pigments and dyes, depending on the particle size. The structural color imprinted in the gel is quickly synchronized with the change in the gel volume. The newly invented mesoporous hydrogels possess optical properties as temperature sensitive smart gels. Also, these mesoporous gels are likely to be interesting materials for promoting a broad range of chemical gel applications, such as quick response chemical sensors and actuators. Catalytic applications may also benefit from the open porous structure.17 Porous soft materials should have enhanced physical and chemical properties because of their large and accessible surfaces. Such materials are potentially valuable in numerous technologies. Moreover, the visible appearance suggests their potential in optical applications. It is well-known that many natural18-20 and man-made objects21 exhibit beautiful iridescent and lustrous colors. Such iridescent colors can be observed (17) Wang, G.; Kuroda, K.; Enoki, T.; Grosberg, A.; Masamune, S.; Oya, T.; Takeoka, Y.; Tanaka, T. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 9861-9864. (18) Tada, H.; Mann, S. E.; Miaoulis, I. N.; Wong, P. Y. Mater. Res. Soc. Symp. Proc. 1998, 489, 173-178. (19) Vukusic, P.; Sambles, J. R.; Lawrence, C. R.; Wootton, R. J. Nature 2001, 40, 36. (20) Lythgoe, J. N. Nature 1971, 205-207. (21) Tsujii, K.; Hayakawa, M.; Onda, T.; Tanaka, T. Macromolecules 1997, 30, 7397-7402.
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Figure 3. Iridescent color in parts a-c resulting from the diffraction and scattering of visible light by the periodic structure of a NIPA gel can be changed by veering the direction of white light. The direction of the incident light relative to the observed direction was changed. (d) The collapsed state of the NIPA gel is almost water-clear.
in a layer of oil on wet pavement or in a soap bubble. These colors are also produced by an object’s surface structure, rather than by incorporated pigment molecules, and they are often referred to as “structural colors”. So are the metallic colors found in some insects18,19 and fish.20 The structural colorations in biological systems can change quickly with the orientation when certain stimuli are applied.19 The gel in this study could be a biomimetic model of biological tissue and iridophore. Work is underway to promote such worthwhile capacities of the gels.
Acknowledgment. This work was supported by Grant-Aids for Scientific Research on Priority Areas of “Molecular Synchronization for Design of New Materials System” (No. 404) and “Dynamic Control of Strongly Correlated Softmaterials” (No. 413) from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. We wish to thank H. Oishi of Nippon Shokubai Co. Ltd. for a technical suggestion to use the silica colloidal suspension. LA020133T
