pubs.acs.org/Langmuir © 2010 American Chemical Society
Thermoresponsive Inverse Opal Films Fabricated with Liquid-Crystal Elastomers and Nematic Liquid Crystals† Guanglong Wu, Yin Jiang, Dan Xu, Hong Tang, Xiao Liang, and Guangtao Li* Key Lab of Organic Photoelectronic and Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, PR China Received September 17, 2010. Revised Manuscript Received November 1, 2010 Liquid-crystal elastomers together with nematic liquid crystals have been used as inverse opal materials to fabricate thermoresponsive photonic crystal directly. In the vicinity of the phase-transition point of the mixture, the photonic band gaps of such inverse opal films exhibited a strong temperature dependence. As the molar ratio of liquid-crystal elastomers and nematic liquid crystals changed, the character of their PBGs also changed with increasing temperature. Such a temperature-tuning effect in the photonic band gap should be of great interest in thermal switches and thermal sensors.
Introduction Photonic crystals (PCs) have recently generated a great amount of interest because of their potential applications as sensors to detect chemical and biological species, active photonic crystals to control light propagation in response to various external perturbations, and dynamic optical switches for displays and smart windows.1-10 They are characterized by a periodic dielectric structure that leads to the diffraction of light and to the occurrence of photonic band gaps (PBGs). Light of certain wavelengths cannot propagate in any dimension inside the photonic band gap crystal. Among many possible applications, a tunable photonic band gap in PCs is of great value because it allows fine control of the photonic properties externally. The photonic band structure of a given PC structure can be tuned by changing the lattice constants and the refractive indices of the dielectrics through controlling the temperature, electric field, or magnetic field. Thus, various materials can be employed to obtain tunable PCs, such as † Part of the Supramolecular Chemistry at Interfaces special issue. *Corresponding author. Fax: (þ86) 10-6279-2905. E-mail:
[email protected]. edu.cn.
(1) Busch, K.; L€olkes, S.; Wehrspohn, R. B.; F€oll, H. Photonic Crystals; WileyVCH: New York, 2004. (2) Ozin, G.; Arsenault, A. Nanochemistry: A Chemical Approach to Nanomaterials; RSC Publishing: Cambridge, U.K., 2005. (3) Yablonovitch, E. Phys. Rev. Lett. 1987, 58, 2059. (4) Krauss, T. F. Nat. Mater. 2003, 2, 777. (5) Akahane, Y.; Asano, T.; Song, B.-S.; Noda, S. Nature 2003, 425, 944. (6) Arsenault, A. C.; Puzzo, D. P.; Manners, I.; Ozin, G. A. Nat. Photon. 2007, 1, 468. (7) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829. (8) Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534. (9) Hu, X. B.; An, Q.; Li, G. T.; Tao, S. Y.; Liu, J. Angew. Chem. 2006, 118, 8325. (10) Hu, X. B.; Huang, J.; Zhang, W. X.; Li, M. H.; Tao, C. A.; Li, G. T. Adv. Mater. 2008, 20, 4074. (11) Figotin, A.; Godin, Y. A.; Vitebsky, I. Phys. Rev. B 1998, 57, 2841. (12) Kee, C. S.; Kim, J. E.; Park, H. Y.; Lim, H. Phys. Rev. B 2000, 61, 15523. (13) Ha, Y. K.; Kim, J. E.; Park, H. Y.; Kee, C. S.; Lim, H. Phys. Rev. B 2002, 66, 075109. (14) Puzzo, D. P.; Arsenault, A. C.; Manners, I.; Ozin, G. A. Angew. Chem. 2008, 120, 1. (15) Busch, K.; John, S. Phys. Rev. Lett. 1999, 83, 967. (16) Leonard, S. W.; Mondia, J. P.; van Driel, H. M.; Toader, O.; John, S.; Busch, K.; Birner, A.; G€osele, U.; Lehmann, V. Phys. Rev. B 2000, 61, R2389. (17) Kee, C. S.; Lim, H.; Ha, Y. K.; Kim, J. E.; Park, H. Y. Phys. Rev. B 2001, 64, 085114. (18) Kubo, S.; Gu, Z.-Z.; Takahashi, K.; Ohko, Y.; Sato, O.; Fujishima, A. J. Am. Chem. Soc. 2002, 124, 10950.
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ferroelectric materials,11 ferromagnetic materials,12 semiconductors,13 gels,14 and liquid crystals.15-20 Liquid-crystal elastomers (LCEs) are weakly cross-linked networks that contain anisotropic moieties incorporated into the polymer backbone (main-chain LCEs) or are attached to the polymer backbone via a flexible spacer (side-chain LCEs).21-24 The coupling of liquid-crystal (LC) order and rubber elasticity results in an anisotropic polymer network that can respond to a variety of external stimuli, such as temperature,25-27 light,28-30 magnetic fields,31 and electric fields.32 Their ability to change their shape significantly and reversibly makes these materials interesting candidates for various applications such as sensors, actuators, and artificial muscles.33,34 However, liquid-crystal elastomers were seldom used as inverse opal materials for photonic crystals. The process of infiltrating the ordered assembly of spheres with a precursor capable of solidification and then removing the template yields a macroporous inverse opal material with a closepacked arrangement of air spheres. Inverse opal materials have been demonstrated to exhibit photonic crystal properties, such as the presence of optical stop bands and tunable color changes. Because of its good miscibility, low phase-transition temperature, and large positive dielectric anisotropy, nematic liquid (19) Kubo, S.; Gu, Z.-Z.; Takahashi, K.; Fujishima, A.; Segawa, H.; Sato, O. J. Am. Chem. Soc. 2004, 126, 8314. (20) Kubo, S.; Gu, Z.-Z.; Takahashi, K.; Fujishima, A.; Segawa, H.; Sato, O. Chem. Mater. 2005, 17, 2298. (21) Zentel, R. Angew. Chem., Int. Ed. Engl. 1989, 28, 1407. (22) de Gennes, P. G.; Hebert, M.; Kant, R. Macromol. Symp. 1997, 113, 39. (23) Warner, M.; Terentjev, E. M. Liquid Crystal Elastomers; Oxford University Press: Oxford, U.K., 2003. (24) de Gennes, P. G.; Prost, J. The Physics of Liquid Crystals, 2nd Ed.; Calderon: Oxford, U.K., 1974. (25) Li, M. H.; Keller, P.; Yang, J. Y.; Albouy, P. A. Adv. Mater. 2004, 16, 1922. (26) Thomsen, D. L.; Keller, P.; Naciri, J.; Pink, R.; Jeon, H.; Shenoy, D.; Ratna, B. R. Macromolecules 2001, 34, 5868. (27) Wermter, H.; Finkelmann, H. e-Polym. 2001, 13, 1. (28) Li, M. H.; Keller, P.; Li, B.; Wang, X.; Bruner, M. Adv. Mater. 2003, 15, 569. (29) Yu, Y.; Nakano, M.; Ikeda, T. Nature 2003, 425, 145. (30) Ikeda, T.; Mamiya, J.; Yu, Y. Angew. Chem., Int. Ed. 2007, 46, 506. (31) Ge, J.; He, L.; Goebl, J.; Yin, Y. J. Am. Chem. Soc. 2009, 131, 3484. (32) Spillmann, C. M.; Ratna, B. R.; Naciri, J. Appl. Phys. Lett. 2007, 90, 021911. (33) Lehmann, W.; Skupin, H.; Tolksdorf, C.; Gebhard, E.; Zentel, R.; Kruger, P.; Losche, M.; Kremer, E. Nature 2001, 410, 447. (34) Shenoy, D. K.; Thomsen, D. L.; Srinivasan, A.; Keller, P.; Ratna, B. Sens. Actuators, A 2002, 96, 184.
Published on Web 11/30/2010
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Figure 1. Chemical structure of mesogenic monomer MP, 5CB, and a cross linker.
Figure 2. SEM images of (a) the SiO2 opal film and (b) the polymer inverse opal film. Scheme 1. Schematic Procedure for the Preparation of Inverse Opal Films Based on Liquid-Crystal Elastomers and Nematic Liquid Crystalsa
a
(A) Close-packed colloidal crystals composed of silica particles on a glass substrate; (B) infiltration of the reactant mixture into the colloidal template, followed by polymerization; (C) polymer opal film after the removal of silica particles; and (D) model of a liquid-crystal elastomer and 5CB in an inverse opal film.
crystal 4-cyano-4-n-pentylbiphenyl (5CB) has often been used to infiltrate the voids of photonic band gap crystals to fabricate thermotunable or electrically responsive photonic crystals.35,36 However, it seldom been acted as inverse opal materials, which will utilize its dielectric anisotropy more effectively. Here, a liquidcrystal elastomer together with 5CB is introduced into photonic crystal to act as inverse opal materials, and its optical properties as controlled by the temperature were described. (35) Yoshino, K.; Shimoda, Y.; Kawagishi, Y.; Nakayama, K.; Ozaki, M. Appl. Phys. Lett. 1999, 75, 932. (36) Meng, Q.-B.; Fu, C.-H.; Hayami, S.; Gu, Z.-Z.; Sato, O.; Fujishima, A. J. Appl. Phys. 2001, 89, 5794.
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Figure 3. Reflection spectra for polymer inverse opal films with different contents of 5CB as a function of temperature.
Experimental Part Materials. The structures of mesogenic monomer MP, 5CB, and diacrylate cross-linker 1,6-hexanediol diacrylate used in this study are shown in Figure 1. 1,6-Hexanediol diacrylate, azo-bisisobutyronitrile (AIBN), and 5CB were purchased from Acros. All solvents and chemicals are of reagent quality and were used without further purification unless noted. Mesogenic monomer MP was synthesized as described in the literature.37 (37) Shivaev, V. P.; Kostromin, S. G.; Plate, N. A. Eur. Polym. J. 1982, 18, 651.
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Figure 4. SEM images of polymer inverse opal films for (a) MP/5CB = 5:1, (b) MP/5CB = 2:1, and (c) MP/5CB = 1:1 at 99 °C. Scheme 2. Schematic Illustration of the Mechanism for the Change in the Inverse Opal Film Based on Liquid Crystal Elastomers and 5CB with Increasing Temperaturea
a
The polymer opal film at (A) 25 and (B) 99 °C.
Figure 5. POM images for an MP/5CB = 1:1 film at different temperatures.
Measurements. The morphology and microstructure of silica colloidal crystals and opal films were observed with a Hitachi S-6700 field-emission scanning electron microscope. The samples used in the SEM measurements were coated with gold to render them conductive. Typical acceleration voltages for SEM observations were 5-10 kV. 1H NMR spectra were recorded on a JEOLJNMECS 300 MHz instrument using tetramethylsilane as an internal reference. For UV-vis spectra, an Olympus BX51 M fiber optic spectrophotometer coupled to an optical microscope was used. The mesomorphic properties and phase-transition behavior were examined with a polarizing optical microscope (POM; Olympus, BX51) equipped with a Liakam hot stage (CI 94). Fabrication of Inverse Opal Films. Scheme 1 illustrates the preparation of inverse opal films based on liquid-crystal elastomers. Monodisperse silica particles were synthesized through sol-gel chemistry by the St€ ober method and then self-assembled onto a clean glass slide by a vertical deposition method in anhydrous ethanol to form colloidal crystal templates. MP, 5CB, Langmuir 2011, 27(4), 1505–1509
Figure 6. Temperature dependence of the reflection peak wavelength of the polymer opal films with different molar ratios of MP to 5CB. Table 1. Phase-Transition Temperatures of the Polymer Inverse Opal Films MP/5CB (molar ratio)
TNIS/°C
TNIG/°Ca
1:1 34.0 48.6 2:1 34.0 59.4 5:1 34.0 59.9 a Measured by POM, heating rate 1 °C/min. The N-I transition temperatures of the gel and surrounding solvent (5CB) are denoted as TNIG and TNIS, respectively.
AIBN, and 1,6-hexanediol diacrylate were dissolved in chloroform solution. The mixture was bubbled with nitrogen for 10 min. The silica colloidal crystals were sandwiched between two PMMA plates that were unidirectionally rubbed before used. They were then immersed in the monomer mixture, and the resulting homogeneous monomer mixture infiltrated the voids of colloidal silica DOI: 10.1021/la1037124
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Figure 7. Photographs of the inverse opal films for MP/5CB = 5:1 at (a) 25 and (b) 99 °C, MP/5CB = 2:1 at (c) 25 and (d) 99 °C (d), and MP/5CB = 1:1 at (e) 25 and (f) 99 °C.
crystals. Once the colloidal crystals became transparent, freeradical polymerization was conducted at 70 °C for 4 h. The sandwich structure was immersed in 1% HF solution for 5 h to separate double slides and completely remove the silica colloids. The structures of the inverse opal films were observed by scanning electron microscopy (SEM).
Results and Discussion A side-chain LC network was prepared by radical polymerization of the mesogenic acrylate monomer MP (Figure 1) with 1,6-hexanediol diacrylate as the cross linker. AIBN and lowmolecular-mass liquid crystal 5CB were employed as the initiator and the nonreactive miscible solvent, respectively. The crosslinker concentration was 10 mol % in the feed. The nonreactive nematic solvent was mixed with the reactive monomer to broaden the temperature range of the nematic phase, and 5CB was chosen as a nonreactive miscible solvent because of its good miscibility and large positive dielectric anisotropy. The mixing molar ratio of the reactive mesogen and nematic solvent was 5:1, 2:1, and 1:1, respectively. The topographical character of the PC films was investigated by scanning electron microscopy (SEM). Figure 2 shows SEM images of the opal structure made from SiO2 spheres and also the 1508 DOI: 10.1021/la1037124
polymer inverse opal structure film. The SEM picture illustrates that the colloid spheres are in a face-centered-cubic arrangement with a closel-packed plane (111) oriented parallel to the substrate and contain an interconnected structure of tetrahedral and octahedral voids. The photonic crystal exhibits beautiful opalescent colors when irradiated with white light and clear diffraction peaks depending on the incident angle of the light beam. In the image of the inverse opal film based on liquid-crystal elastomers, a hexagonal structure, which is derived from the FCC opal structure, can be observed. This confirms that the opal structure was successfully replicated. The content of 5CB has not had any effect on the topographical character of the polymer inverse opal film. Figure 3a-c show the reflection spectra for polymer inverse opal films with different contents of 5CB as a function of temperature. As can be seen, the continuous blue shift of the Bragg peak was observed with increasing temperature in all three reflection spectra. The intensity of the reflection spectra also decreased probably because the period porous structure of the inverse opal films collapses with increasing temperature. This can be verified by SEM images of polymer inverse opal films at 99 °C (Figure 4). For the MP/5CB = 5:1 film, the peak position blue shifted about 28 nm; that is, it changed from 517 to 489 nm. However, for the MP/5CB = 2:1 and MP/5CB = 1:1 films, they Langmuir 2011, 27(4), 1505–1509
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blue shifed 68 and 114 nm, respectively. That is, the wavelength by which they blue shifted increased drastically with increasing 5CB content. This phenomenon can be illustrated by the Bragg diffraction law. The peak position due to the optical stop band can be estimated using the Bragg diffraction equation at normal incidence38 pffiffiffiffiffiffiffiffi λ ¼ 2 2=3d111 ðnpoly 2 f þnair 2 ð1 - f ÞÞ1=2 where λ is the wavelength of light, d111 is the lattice contant, npoly and nair are the refractive indices of polymer and the medium in the voids, respectively, and f is the volume fraction of polymer in the opal. It was clearly observed that the changes in d111 and npoly will lead to changes in the position of reflection peaks when nair and f remain constant. Because of the irreversible lattice changes in the inverse opal films, the reversibility of the reflection shift was not good. When the temperature was decreased to room temperature, the PBG position changed little. Scheme 2 illustrates the mechanism for the change in the inverse opal film based on liquid-crystal elastomers and 5CB with increasing temperature. Although the PMMA plates were unidirectionally rubbed before used, the inverse polymer opal films are polydomain liquid-crystal films are the same as a result of the space restriction of SiO2 particles. However, they consisted of many microsized domains of liquid-crystal moieties aligned in one direction in each domain below the transformation temperature of the nematic phase and the isotropic phase. When the temperature was close to the nematic-to-isotopic (N-I) transition, the volume decreased and the lattice structure of the polymer opal film also changed, which led to decreases in d111 and npoly of the photonic crystal. Because 5CB was not linked directly to the polymer by covalent bonds, it could move more freely than mesogens of liquid-crystal elastomers. Moreover, because of the large positive dielectric anisotropy and low transformation temperature of 5CB, the increase in 5CB content will lead to a decrease in the nematic-to-isotopic transition temperature of the inverse opal film. As a result, the reflection peaks shifted drastically toward shorter wavelength with the increase in 5CB content when the temperature increased. The mesomorphic properties were examined by means of POM, and some representative optical textures are shown in Figure 5. For the MP/5CB = 1:1 film, schlieren texture was observed at 32.8 °C, which is below the nematic-isotropic transition temperature of 5CB. With increasing temperature, the schlieren texture disappeared gradually. When the film was heated to above (38) Park, S. H.; Xia, Y. N. Langmuir 1999, 15, 266.
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the nematic-isotropic transition temperature for a mixture of MP and 5CB, the anisotropy of the film disappeared completely. Figure 6 show how the peak positions of the polymer inverse opal films depend on the temperature with different molar ratios of MP to 5CB. It can be seen from this image that the PBG position moves to shorter wavelength with increasing temperature. When the temperature is below the N-I transition temperature (TNIG), the PBG position remains constant with a gradual increase in temperature, and when the temperature is close to TNIG, the PBG position changes drastically. TNIG depends on the molar ratio of MP to 5CB. With increasing 5CB content, TNIG decreased. For MP/5CB (5:1, 2:1, and 1:1) films, PBG peaks could be seen for TNIG = 59.9, 59.4, and 48.6 °C, respectively (Table 1). The change in the reflection spectra can be visualized directly by the change in the color of the film with increasing temperature. Figure 7 shows photographs of the inverse opal films at different temperatures. The color diffracted from the pristine surface of this inverse opal film at normal incidence was green (Figure 7e). When the temperature was increased to 99 °C, its color changed from green to blue (Figure 7f) because of the decrease in the lattice constant and the refractive indices of the inverse opal film, as illustrated in Scheme 2. This result suggests that the switching of the optical properties can be realized by the changes in the lattice constants and the refractive indices of the inverse opal film, which may lead to thermoresponsive photonic crystals.
Conclusions Thermoresponsive liquid-crystal elastomers and nematic liquid crystals can be manipulated to form an inverse opal film by take advantage of their large volume-phase transition. By varying the temperature, the lattice constant together with the refractive indices and therefore the wavelength of the resultant Bragg peak can be tuned continuously. To the best of our knowledge, this work is the first report that a liquid-crystal elastomer together with a nematic liqud crystal was introduced directly into the photonic crystal as the inverse opal material. When the molar ratio of MP/5CB = 1:1, a large Bragg peak blue shift (more than 100 nm) has been realized with increasing temperature. We believe that this thermoresponsive system may find application in optical devices and sensors. Acknowledgment. We gratefully acknowledge financial support from the National Science Foundation of China (nos. 50873051 and 20533050), MOST (2007AA03Z307), and the Transregional Project (TRR61).
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