Stepwise Controlled Immobilization of Colloidal Crystals Formed by

Fukuoka 804-8550, Japan, and Frontier Research Group, Toyota Central R & D Labs, Inc., 41-1, Aza Yokomachi, Oaza Nagakute, Nagakute, Aichi-gun, Ai...
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Langmuir 2005, 21, 4471-4477

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Stepwise Controlled Immobilization of Colloidal Crystals Formed by Polymer-Grafted Silica Particles Kohji Yoshinaga,*,† Kumiko Fujiwara,† Emiko Mouri,† Masahiko Ishii,‡ and Hiroshi Nakamura‡ Department of Applied Chemistry, Faculty of Engineering, Kyushu Institute of Technology, 1-1 Sensui, Tobata, Kitakyushu, Fukuoka 804-8550, Japan, and Frontier Research Group, Toyota Central R & D Labs, Inc., 41-1, Aza Yokomachi, Oaza Nagakute, Nagakute, Aichi-gun, Aichi, 480-1192, Japan Received December 27, 2004. In Final Form: February 17, 2005 Colloidal crystals formed by polymer-grafted silica particles were immobilized by a stepwise procedure consisting of gelation by radical copolymerization followed by solidification by ring-opening radical polymerization. In the first step, the poly(methyl methacrylate) (PMMA)-grafted silica colloidal crystal suspension was incorporated into the gel without altering the crystal structure by copolymerization of cross-linker, 1,2-dimethylacryloyloxyethane (DME) and methyl methacrylate (MMA). In the second step, ring-opening radical polymerization was performed after substituting the solvent with vinylidene-1,3dioxolane. By this two-step procedure, the silica particle array of colloidal crystals was immobilized and made into durable material.

Introduction Its beautiful iridescence has made the colloidal crystal an attractive motif for academic study. The mechanism of the colloidal crystallization and the critical factors that determine the structure have been eagerly studied for several decades.1,2 Because colloidal particles with dissociation groups disperse in aqueous media mainly by the electrostatic repulsion, the interaction can be controlled by the salt concentration and amount of charge on the particles. The solid crystal-like structure appears at a strong interaction regime, while the liquidlike structure appears at a weak interaction regime, although some questions remain unanswered.3,4 Recently, the application of the colloidal crystal to photonic crystals and sensors is an appealing theme pioneered by Asher et al.5-7 because the interparticle distance between colloidal particles in colloidal crystals is comparable to visible wavelength, which is promising for optical material use. As an overview of recent studies, the methods for fabricating photonic crystal are categorized into two types by the method. One is making use of colloidal crystal formation in bulk solution,5-19 and the other is to utilize a closest-packing colloidal crystal array * Corresponding author. Tel./Fax: +81-93-884-3316. E-mail: [email protected]. † Kyushu Institute of Technology. ‡ Toyota Central R & D Labs, Inc. (1) For reviews: (a) Ordering and Phase Transitions in Charged Colloids; Arora, A. K., Tata, B. V. R., Eds.; VCH: New York, 1996. (b) Okubo, T. Acc. Chem. Res. 1988, 21, 281. (2) (a) Aastuen, D. J. W.; Clark, N. A.; Cotter, L. K.; Ackerson, B. J. Phys. Rev. Lett. 1986, 57, 1733. (b) Pusey, P. N., van Mengen, W. Nature 1986, 320, 340. (c) Okubo, T. Prog. Polym. Sci. 1993, 28, 481. (3) Matsuoka, H.; Harada, T.; Yamaoka, H. Langmuir 1994, 10, 4423. (4) Yamanaka, J.; Yoshida, H.; Koga, T.; Ise, N.; Hashimoto, T. Phys. Rev. Lett. 1998, 80, 5806. (5) Asher, S.; Holtz, J.; Liu, L.; Wu, Z. J. Am. Chem. Soc. 1994, 116, 4997. (6) Holtz, J. H.; Asher, S. Nature 1997, 389, 829. (7) Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534. (8) Foulger, S. H.; Jiang, P.; Pattam, A. C.; Smith, D. W.; Ballato, J. Langmuir 2001, 17, 6023. (9) Takeoka, Y.; Watanabe, M. Langmuir 2002, 18, 5977. (10) Lellig, C.; Hartl, W.; Wagner, J.; Hempelman, R. Angew. Chem., Int. Ed. 2002, 41, 102.

formed by deposition on substrates.20-26 In both methods, the inverse-opal structure is also obtained by using a colloidal crystal array as template.9,15,16,19,20,26 A challenging subject for the application as a durable material is the conversion of colloidal crystals in solution to solidlike material without altering the structure. In most cases, the colloidal crystals are formed in aqueous media, and hydrogel is used to control and immobilize the colloidal crystal array.5-11,13,16,18 Asher et al. immobilized the colloidal crystals for the first time5 and changed the lattice distance by pH and ionic strength. Yamanaka et al. used a polyacrylamide gel for immobilization, and they succeeded in constructing a photonic crystal system that covers a wide range of visible light by a mechanical stress gradient.13 Takeoka et al. also applied the polyacrylamide gel to a colloidal crystal system and controlled the color by using the difference of expansion ratio.16 However, from the viewpoint of application of the colloidal crystal as durable material, a hydrogel is not always convenient; it is too soft and too sensitive to temperature and pH to incorporate into optical devices. Here, we report a unique (11) Debord, J. D.; Eustis, S.; Debord, S. B.; Lofye, M. T.; Lyon, L. A. Adv. Mater. 2002, 14, 658. (12) Russsel, W. B. Nature 2003, 421, 490. (13) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shinohara, T.; Sawada, T.; Yonese, M. Langmuir 2003, 19, 977. (14) Gong, T.; Wu, D. T.; Marr, W. M. Langmuir 2003, 19, 5967. (15) Perpall, M. W.; Prasanna, K.; Perera, U.; DiMaio, J.; Ballato, J.; Foulger, S. H.; Smith, D. W., Jr. Langmuir 2003, 19, 7153. (16) Takeoka, Y.; Watanabe, M. Langmuir 2003, 19, 9554 (17) Yoshinaga, K.; Fujiwara, K.; Tanaka, Y.; Nakanishi, M.; Takesue, M. Chem. Lett. 2003, 32, 1082. (18) Yamanaka, J.; Murai, M.; Iwayama, Y.; Yonese, M.; Ito, K.; Sawada, T. J. Am. Chem. Soc. 2004, 126, 7156. (19) Lu, Y.; Mclellan, J.; Xia, Y. N. Langmuir 2004, 20, 3464. (20) Gu, Z.; Kubo, S.; Qian, W.; Einaga, Y.; Tryk, D. A.; Fujishima, A.; Sato, O. Langmuir 2001, 17, 6751. (21) Goldenberg, L. M.; Wagner, J.; Stumpe, J.; Paulke, B. R.; Go¨rnitz. Langmuir 2002, 18, 3319. (22) Im, S. H.; Park, O. O. Langmuir 2002, 18, 9642. (23) Arsenault, A. C.; Mı´guez, H.; Kitaev, V.; Ozin, G. A.; Manners, I. Adv. Mater. 2002, 14, 658. (24) Rugge, A.; Ford, W. T.; Tolbert, S. H. Langmuir 2003, 19, 7852. (25) Cademartiri, L.; Sutti, A.; Calestani, G.; Dionigi, C.; Nozar, P.; Migliori, A. Langmuir 2003, 19, 7944. (26) Fudouzi, H.; Xia, Y. Langmuir 2003, 19, 9653.

10.1021/la0467786 CCC: $30.25 © 2005 American Chemical Society Published on Web 03/31/2005

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Figure 1. Schematic figure of stepwise immobilization. (a) Gelation of PMMA/SiO2 colloidal crystals by copolymerization of MMA and cross-linker. (b) Solidification of PMMA/SiO2 colloidal crystals by ring-opening polymerization.

route to obtain “hard” colloidal crystal material utilizing the formation of colloidal crystal in an organic solvent. The target of our study is to create durable colloidal crystals that can withstand use for a long time. By creating colloidal crystals in an organic solvent, a variety of monomers can be polymerized in the matrix in our system, making it easier to control the degree of hardness of colloidal crystal materials. As was previously reported, various colloidal crystals can be designed in an organic solvent by choosing the kind of polymer grafted and solvent.27,28 We have demonstrated that the ζ potential on polymer-modified silica can be controlled by the addition of salt,28 although basic knowledge on the mechanism and origin of the colloidal crystallization in organic solvent is not sufficient at this stage. Our strategy for immobilizing colloidal crystals consists of three steps as presented in Figure 1: (1) colloidal crystallization in organic solvent,9 (2) gelation of colloidal crystal by radical copolymerization,10 and (3) solidification of colloidal crystal by polymerization of solvent. In this paper, we describe the twostep immobilization of colloidal crystals.

introduced to determine the amount of polymer grafted to silica particle by a TGA-50 (Shimadzu Co. Ltd., Kyoto, Japan) Reflection Spectroscopy. The reflection spectra were measured at the 90° position from the cell surface by a multichannel spectral analyzer, PMA-11 (Hamamatsu Photonics Co. Ltd., Hamamatsu, Japan), with a 150-W halogen lamp. The interparticle distance was determined by the wavelength at the peak-top by combining Bragg’s law and the relationship between lattice constant d and interparticle distance D. The relationship, d ) (2/3)0.5D, holds for close-packed lattice, either face-centered cubic or hexagonal, with hexagonally ordered planes parallel to the interface. The relation between interparticle distance, Dobs, and the wavelength at peaktop, λp, was presented by the following equation by assuming FCC or HCP structure of colloidal crystals:29

Experimental Section

np ) 1.5φ + ns(1 - φ)

1. Materials. Colloidal silica dispersed in water was kindly supplied by Catalysts & Chemicals Co. Ltd., Japan. Ionic impurities were removed by coexisting with the ion-exchange resins over 3 months. Characterization of colloidal silica was carried out by dynamic light scattering (DLS) measurement. The diameter was estimated to be 136 nm, and its monodispersity was confirmed (polydispersity , 0.1). The water solvent was then exchanged to ethanol via centrifugation. Methyl methacrylate monomer purchased from Wako Pure Chemical Industries Ltd. (Osaka, Japan) was used after removal of a polymerization inhibitor and distillation. 3-Mercaptopropyltrimethoxysilane purchased from Tokyo Kasei Kogyo Co., Ltd. (Tokyo, Japan) and 2,2′-azobisisobutyronitrile (AIBN) from Wako Pure Chemical Industries Ltd. (Osaka, Japan) were used without further purification. Commercially available reagent grade solvents were used for colloidal crystal formation without further purification. 2. Measurements. The synthesized polymer and cross-linker were characterized by 1H NMR, AVANCE-400 (Brucker, Germany). The polymer molecular weight and polydispersity were determined by GPC, C-R4A (Shimadzu Co. Ltd., Kyoto, Japan) using TSK gel-3000H. Thermogravimetric analysis (TGA) was (27) Yoshinaga, K.; Chiyoda, M.; Ishiki, H.; Okubo, T. Colloids Surf., A 2002, 204, 285. (28) Yoshinaga, K. Bull. Chem. Soc. Jpn. 2002, 75, 2349.

Dobs )

x38 n

λp p

where Dobs is the interparticle distance in colloid crystal in nanometers, np is the refractive index of solution, and λp is the wavelength at the peak-top. The refractive index of the solution was presented as the average of solute (polymer-grafted colloidal silica) and solvent as follows:

where φ is the volume fraction of colloidal particles and ns is the refractive index of solvent (1.34411 for acetonitrile at 293 K). The value of 1.50 is the refractive index of silica. The change of refractive index by polymerization was not considered in the calculation because the refractive index change cannot be obtained for each step. The error in Dobs by the assumption was estimated to be within 20 nm when λp < 600 nm. The estimation of the size of single crystal was carried out by Scherrer’s equation,30,31

L ) 1/(S1 - S0) where Sx ) 2 sin θ/λx, λ1 is the larger wavelength at half-maximam, λ0 is the smaller wavelength at half-maximum, and θ is the incident angle. The estimation assumes random distribution of a large number of colloidal crystals in the reflection volume. Thus, the application of the formula directly to the system should not be precise, but we also confirmed the agreement of the crystal size by LSM. (29) Hiltner, P. A.; Krieger, I. M. J. Phys. Chem. 1969, 73, 2386. (30) Warren, B. E. X-ray Diffraction; Dover Publications: New York, 1990; Chapter 13. (31) (a) Okubo, T. J. Chem. Soc., Faraday Trans. 1 1986, 82, 3163. (b) Okubo, T.; Okada, S. J. Colloid Interface Sci. 1998, 204, 198.

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Figure 2. Reaction scheme of ring-opening polymerization of vinilidene-1,3-dioxolane proceeded in the solidification step of colloidal crystals. Transmission Electron Microscopy (TEM) and Confocal Scanning Laser Microscopy (LSM). The TEM observation was carried out using a JEM-2000EX (JEOL Co. Ltd., Tokyo, Japan) at 200 keV acceleration. FIB treatment, by the micro-sampling method using a FB-2000A (HITACHI Co. Ltd., Tokyo, Japan), was applied to the samples before TEM observation. For the FIB treatment, a Ga ion beam with 30 keV was used to cut the sample into a micrometer size. The LSM observation was carried out by LSM5 PASCAL (Carl-Zeiss Co. Ltd., Germany). Hardness Examination by Durometer. The hardness of immobilized colloidal crystal was examined with a Type-D durometer, Hardmatic HH-315 (Akashi Co. Ltd., Kanagawa, Japan). 3. Preparation of PMMA-Grafted Silica Particles and Formation of Colloidal Crystals. The polymer-grafted silica was prepared as reported previously.9 The PMMA-grafted silica particles were synthesized as follows: (1) Polymerization of methyl methacrylate in tetrahydrofuran initiated by AIBN are carried out end-capped by 3-mercaptopropyltrimethoxysilane to obtain PMMA-Si(OMe)3. (2) Monodispersed colloidal silica (polydispersity < 0.1) and PMMA-Si(OMe)3 were refluxed overnight after azeotropic distillation. Number-averaged molecular weight of trimethoxysilyl-terminated PMMA and Mw/Mn were estimated to be 6600 and 1.68, respectively, by GPC with a polystyrene standard. The amount of grafted polymer was measured by thermogravimetric analysis: PMMA weight fraction is obtained by the loss of weight by heating to 1073 K. The polymer weight fraction was calculated to be 7-9 wt %, and density of graft polymer at the colloididal silica surface was estimated to be 0.3 molecules nm-2 (1.9 × 104 molecules per particle) at 7 wt % on an assumption of Mn ) 6600. The PMMA-grafted silica particles were dispersed in acetonitrile. After ultrasonic-wave irradiation, the suspension was kept at room temperature. The formation of colloidal crystals was confirmed by iridescence caused by Bragg diffraction from the particle array with constant intervals. The interparticle distance and the size of single crystals were determined by reflection spectroscopy. 4. Gelation of PMMA/SiO2 Colloidal Crystals. The effect of monomer addition on colloidal crystallization was examined, and a suitable monomer for gelation was chosen. The gelation condition was then examined without colloidal crystals, and the optimal condition for gelation was found. Cross-linker, 1,2-dimethacryloyloxyethane (DME) (4-7 wt %), and MMA monomer (ca. 2 mol/L) acetonitrile solution were added to the PMMA-grafted silica colloidal suspension (φ ) 0.08-0.15). The solution was poured into a Pyrex glass cell with 1-10 mm width. AIBN (0.05-0.1 mol/L) was added to the solution as an initiator. Radical polymerization of DME and MMA was carried out by UV irradiation (high-pressure Hg lamp, 500 W) to give the gelation of the suspension.10 2-Hydroxyethyl methacrylate (HEMA) monomer was also used in copolymerization instead of MMA. The condition of colloidal crystals was investigated by reflection spectroscopy, TEM, and LSM. 5. Solidification of PMMA/SiO2 Colloidal Crystals. The obtained gel was solidified by the following two methods: (1) Solidification via Freeze-Drying. The obtained gel was freeze-dried to eliminate acetonitrile, and then the dried gel was dipped into the mixture of cyclic vinyl monomer (4-methylene2-phenyl-1,3-dioxolane or 2,4-dimethylene-1,3-dioxolane) and an initiator of 2,2-dimethoxy-2-phenyl-acetophenone for photopolymerization. These cyclic vinyl monomers were synthesized as previously reported.32 The solution was exposed to UV irradiation to proceed radical polymerization for ca. 18 h. After (32) Yokozawa, T.; Hayashi, R.; Endo, T. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 3739.

Figure 3. The effect of monomer addition on colloidal crystal formation. Critical volume fraction φ0 was plotted with monomer concentration. polymerization, the condition of colloid crystal was investigated by reflection spectroscopy, TEM, and LSM. (2) Solidification by Solvent Substitution. A mixture solution of photopolymerization initiator and cyclic vinyl monomer was prepared. The obtained gel swollen by acetonitrile was dipped into the solution to substitute the solvent. The light irradiation by the Hg lump was carried out on the swollen gel for ca. 18 h. The obtained colloidal crystals were characterized by reflection spectroscopy, TEM, and LSM. The hardness of the material was examined by a durometer.

Results and Discussion 1. Effects of Monomer Addition on Colloidal Crystallization. Addition of an organic compound to colloidal suspension sometimes disturbs the crystallization. As the first step for gelation of colloidal crystals, therefore, we examined three kinds of monomer, MMA, N,N-dimethylacrylamide (DMAAm), and HEMA, to find the optimal monomer for gelation compatible with colloidal crystals. By adding each monomer to the colloidal suspension at various volume fractions of polymer-grafted colloidal silica, the point at which iridescence disappears was examined. Figure 3 shows the change of critical volume fraction φ0 with monomer addition. φ0 is the minimal volume fraction at which colloidal crystals can be formed. Thus, the small value of φ0 indicates the high stability of colloidal crystals. The monomer that shows a low φ0 in the wide range of monomer concentration should be recognized as a suitable monomer for the gelation process. All three monomers showed a φ0 of less than 0.1, throughout the wide range of monomer concentrations. Especially, MMA and HEMA monomers gave lower φ0. 2. Gelation. We searched for the optimal condition for polymer gelation without colloidal crystals. The gelation was preceded by copolymerization of the divinyl monomer, DME, and vinyl monomer initiated by AIBN with UV irradiation. Previously, it was clarified that colloidal crystals were compatible with DME at a concentration under 20 wt %.10 MMA, DMAAm, HEMA, and acrylonitrile (AN) were examined as monomers. The gelation was observed for MMA, DMAAm, and HEMA systems, while

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Table 1. Interparticle Distance and Size of Single Crystal Obtained by Reflection Spectra before gelation

after gelation (fronta)

after gelation (rearb)

run no.

cell width [mm]

φ

DME [mol/L]

AIBN [mol/L]

Dobs [nm]

L [µm]

Dobs [nm]

L [µm]

Dobs [nm]

L [µm]

1 2 3 4 5

10 10 10 2 2

0.2 0.1 0.1 0.1 0.1

0.2 0.3 0.5 0.3 0.3

0.05 0.06 0.06 0.04 0.006

230 280 260 260 270

9 18 20 13 19

220 270

5 5

210 280

1 8

250 280

1 20

250 270

2 10

a

Reflection spectra taken from the irradiated side of the cell. b Reflection spectra taken from the side opposite to the irradiated side.

Figure 4. Iridescence from the colloidal crystal gel immersed in organic solvent.

segregation was observed for the AN system. It seems to be caused by the fact that the polymerization speed of AN was too fast as compared to that of DME. As a result, MMA and HEMA were selected as candidate monomers for gelation of colloidal crystals. The gelation condition with MMA was investigated in detail: The concentration of DME was varied from 2 to 8 wt %, while the MMA concentration was kept at 50 wt %. DME of 3 wt % was found to be the threshold value for the gelation. On the other hand, the MMA concentration was varied from 10 to 50 wt %, while the DME concentration was kept at 5 wt %. The threshold value for gelation was found to be 20 wt % of MMA. The optimal condition for obtaining the gel is concluded to be DEM >5 wt % and MMA of 20-30 wt %. The gelation of colloidal crystals was carried out by copolymerization of DME, and MMA or HEMA. The degree of gel swelling in acetonitrile, that is, the ratio of swelling gel weight to dry gel weight, was calculated to be 180210%. In the HEMA system, however, iridescence was not confirmed after polymerization. In the HEMA system, phase separation occurred during polymerization, which caused the collapse of particle array of colloidal crystals. It should be attributed to the poor solubility of poly(HEMA) in acetonitrile. On the other hand, iridescence was observed in the MMA system, which indicated that the particle array of PMMA/SiO2 colloidal crystals was maintained throughout the gelation process as shown in Figure 4. We searched for the optimal condition for gelation in the MMA system. The detailed structure of colloidal crystals was investigated by the reflection spectra. The representative experimental condition and the result of reflection spectra are summarized in Table 1. The interparticle distance and size of single crystals were ca. 230-270 nm and 1-20 µm, respectively. The effects of φ, volume fraction of colloidal silica, on colloidal crystallization were examined first (RUN 1 and RUN 2 in Table 1). We found that the reflection spectra

Figure 5. Reflection spectra before and after the gelation step at different volume fractions of colloidal silica, φ, with a 10 mm-width cell: (a) RUN 1 (φ ) 0.2), (b) RUN 2 (φ ) 0.1).

became sharper when φ is reduced as indicated by the change from Figure 5a to Figure 5b. At a low volume fraction of the particles, it is plausibly easy to cross-link among loosely packed particles and to preserve the particle array. When the amount of DME was increased as in RUN 3 in Table 1, to create rigid gel, the colloidal crystals were collapsed by polymerization as indicated by the reflection spectra in Figure 6. In these figures, the peaks disappeared after gelation. The peak shift became slightly smaller (Figure 5b), but the spectrum from the front was different from that taken from the rear side, which may be attributed to the inhomogeneous progress of polymerization. To confirm this point, we shortened the optical path, that is, cell width to 2 mm. Under the same condition as shown in Figure 5b, the colloidal crystals were not preserved in a 2 mm cell (RUN 4), being attributable to the effect of volume contraction as shown in Figure 7a. However, we found that the particle array in the colloidal crystals was preserved by reducing the amount of initiator to 1/10 the amount with a 2 mm-width cell. Excellent reflection

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Figure 6. Reflection spectra before and after the gelation step of a large amount of cross-linker with a 10 mm-width cell (RUN 3).

Figure 8. Reflection spectra before and after the solidification step by two methods. (a) Solidification by freeze-dry method. (b) Solidification by solvent substitution.

Figure 7. Reflection spectra before and after the gelation in a 2 mm-width cell: RUN 4 (AIBN ) 0.04 mol/L), (b) RUN 5 (AIBN ) 0.006 mol/L).

spectra were obtained as in Figure 7b. Because of the slow polymerization, the particle array was not disturbed by loose cross-linking. The optimal condition for the gelation of colloidal crystal suspension is controlled by four factors: (1) polymerization rate, which is controlled by the concentration of initiator, (2) volume fraction of PMMA/SiO2, φ, (3) amount of crosslinker, and (4) the optical path. The best condition for gelation of the suspension containing colloidal crystals can be summarized as follows: a low polymerization rate, lower volume fraction of polymer grafted silica particles above φ0, smaller amount of cross-linker, and short optical

path. The combination of PMMA/SiO2, MMA, and DME showed a very small φ0 and a small amount of DME was enough to produce gelation, to make the colloidal crystal gel. The colloidal crystals confined to the gel were further examined by LSM and TEM. The LSM image in Figure 9a shows the size of a single crystal in the gel. Each white fragment is a single colloidal crystal of polymer-grafted silica. The size obtained by reflection spectra agreed with that obtained by LSM. The TEM image of the dried gel in Figure 10a indicated that each silica particle is dispersed in the gel matrix at almost a constant interparticle distance. The result shows that cross-linking between polymer chains takes place along with penetrating among particles. The hydrodynamic radius of PMMA/SiO2 particle estimated by DLS was 65 nm, and the interparticle distance in the gel was estimated to be 230-270 nm by reflection spectra. Thus, a gel with ca. 100 nm thickness should exist between PMMA/SiO2 particles. The interparticle distance on a particle array line obtained from TEM image is about 140-220 nm, which is smaller than the value estimated above. It might be due to shrinking of polymer chains during the drying process. There were some areas without particles, because of heterogeneous drying of the gel (Figure 10a). 3. Solidification of PMMA/SiO2 Colloidal Crystals by Ring-Opening Radical Polymerization. As the next step for further immobilization of colloidal crystal, we carried out ring-opening radical polymerization in the matrix as described in Figure 2 by two methods, freezedrying and solvent substitution. In the present cases, we studied the radical ring-opening polymerization of vi-

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Figure 10. TEM images of immobilized colloidal crystal. (a) Dried gel after the gelation step. (b) After the solidification step by the freeze-drying method. (c) After the solidification step by the solvent substitution method.

Figure 9. LSM images of colloidal crystals before and after the solidification step. (a) Before solidification step. (b) After the solidification step by freeze-drying method. (c) After the solidification step by solvent substitution method.

nylidene-1,3-dioxolane for the solidification of colloidal crystals gel, because of small volume contraction through polymerization. Preliminary studies showed that the polymerization of the two monomers gave a polymer with Mn ) 1000-2000. Via this process, pudding-like gel was turned into solid material. The particle array of colloidal crystal was confirmed to be preserved through ringopening polymerization of the 4-methylene-2-phenyl-1,3dioxolane in the matrix by its iridescence. It was not preserved, however, by polymerization of 2,4-dimethylene1,3-dioxolane. Probably, the destruction of colloidal crystals is due to the segregation between PMMA gel and polyketone formed by the polymerization. Quantitative

estimation on colloidal crystals was performed by reflection spectroscopy. The representative reflection spectra are presented in Figure 8 for the solidified PMMA/SiO2 colloidal crystals for two methods. The interparticle distance and the size of single crystal obtained by the spectra were summarized in Table 2. The interparticle distance did not change in the freeze-drying method, while it became shorter by ca. 15% in the solvent-substitution method. However, the shape of the reflection peak was kept better in the latter case. LSM images taken after freeze-drying and after solvent substitution are presented in Figure 9b and c. The former image showed drastic structural changes when it was compared to Figure 9a taken in the gel state. Large cracks, which were possibly formed during drying, were observed in the dried gel. On the other hand, the image of swollen gel showed almost the same structure as shown in Figure 9a. These images suggested that the solvent-substitution method is a better method for the preservation of colloidal crystals.

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Table 2. Interparticle Distance and Size of Single Crystal Obtained by Reflection Spectra methods for solidification

before solidification Dobs [nm]

after solidification Dobs [nm]

before solidification L [µm]

after solidification L [µm]

freeze-drying solvent substitution

240 250

240 210

7 11

4 5

The TEM images taken after ring-opening polymerization are shown in Figure 10. An inhomogeneous part was observed in the image of the freeze-drying method, while an almost homogeneous structure was confirmed in the image obtained by the solvent-substitution method. As a whole, solvent substitution was clarified to be a better method for solidification. Estimation on the interparticle distance in Figure 10b,c is difficult because more than two particle layers of the colloidal crystal are superimposed. The hardness of solidified colloidal crystal measured with a durometer (type-D) gives an average value of 87. The error was estimated to be within (3. The value 87 is higher than that of general fluorine-containing polymers (50-70)33 such as poly(tetrafloroethylene); we consider that to be hard enough for extended use in an optical device. Conclusion Colloidal crystals from polymer-modified silica could be immobilized by a stepwise procedure. In the first step,

the colloidal crystals formed in organic solvent were incorporated into the gel by copolymerization of a divinyl monomer and MMA. In the second step, the colloidal gel was solidified by ring-opening polymerization of vinylidene-1,3-dioxolane. The two-step method prevents the drastic change in volume and enables the preservation of colloidal crystals. The “hard” colloidal crystals we presented here, in contrast to the “soft” colloidal crystals in gel, should be useful as optical materials. Acknowledgment. This work is supported by a grantin-aid for Scientific Research (No. 14350499) by the Ministry of Education, Culture, Sports, Science, and Technology of Japan, for which our sincere gratitude is due. LA0467786 (33) For example: http://www.tfx.com/products/tubing_materials. htm.