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Photonic Rubber Sheets with Tunable Color by Elastic Deformation Hiroshi Fudouzi* and Tsutomu Sawada National Institute for Materials Science, 1-2-1 Sengen, Tsukuba 305-0031, Japan ReceiVed August 1, 2005. In Final Form: NoVember 13, 2005 This article describes an elastic silicone sheet with reversible tuning structural color. The sheet has a thin layer of cubic close-packed, ccp, colloidal particles embedded in poly(dimethylsiloxane), PDMS, elastomer. The array of ccp (111) planes diffracts light of selective wavelengths according to Bragg’s law. This is responsible for the structural color of the PDMS sheet. Because the sheet was stretched in the horizontal direction, it was reduced in size in the vertical direction. As a result, the lattice distance of ccp (111) planes decreased, and the reflected wavelength of light shifted to shorter wavelengths. For example, the peak of reflection was tuned from 589 to 563 nm as a function of sheet elongation. The peak position decreased linearly with deformation when the deformation was within 20% of its elongation. Accordingly, the color of the PDMS sheet changed from red to green. When the mechanical strain on the PDMS sheet was released, the peak returned to its original position, and the color of the PDMS sheet also changed back to red. Tuning the color of the PDMS sheet is a reversible and repeatable process. The novel PDMS sheet has the potential to be applied to mechanical strain sensing.
Introduction Colloidal crystalsslong-range-ordered lattices assembled from spherical particlessmay represent a class of ideal candidates for fabricating optical sensors that can be used to monitor, measure, and display environmental variations in terms of color changes, which can be easily visualized by the naked eye. Asher et al. have demonstrated the fabrication of temperature-, pH-, and ionresponsive optical sensors by embedding colloidal crystals in appropriate polymer hydrogels; these are referred to as intelligent sensors.1-4 Remarkable developments in the application of sensing materials using colloidal crystals have been made recently.5-9 The colloidal crystals could also serve as a sensing material to provide a platform for the in situ monitoring of mechanical strains by stretching or compressing. Yoshino et al. were the first to demonstrate that a reflection peak of the colloidal crystal block constructed using polymer spheres shifted to a higher wavelength as a function of the uniaxial compression strain.10 The color of the rigid block was tuned by compressing it at 0.5 MPa. Recently, soft materials, such as colloidal crystals embedded in hydrogels, have been reported to deform more easily by compression strain.11,12 The wavelength of the reflected light was tuned by applying linear compression strain in a wide range * Corresponding author. E-mail:
[email protected]. (1) Weissman, J. M.; Sunkara, H. B.; Tse, A. S.; Asher, S. A. Science 1996, 274, 959-960. (2) Holtz, J. H.; Asher, S. A. Nature 1997, 389, 829-832. (3) Holtz, J. H.; Holtz, J. S. W.; Munro, C. H.; Asher, S. A. Anal. Chem. 1998, 70, 780-791. (4) Haacke, G.; Panzer, H. P.; Magliocco, L. G.; Asher, S. A. Narrow Band Radiation Filter Films. U.S. Patent 5,266,238; Nov 30, 1993. (5) Debord, J. D.; Lyon, L. A. J. Phys. Chem. B 2000, 104, 6327-6331. (6) Debord, J. D.; Eustis, S.; Debord, S. B.; Lofye, M. T.; Lyon, L. A. AdV. Mater. 2002, 14, 658-662. (7) Arsenault, A.; Fournier-Bidoz, S. B.; Hatton, B.; Miguez, H.; Tetrault, N.; Vekris, E.; Wong, S.; Yang, S. M.; Kitaev, V.; Ozin, G. A. J. Mater. Chem. 2004, 14, 781-794. (8) Lee, Y. J.; Braun, P. V. AdV. Mater. 2003, 15, 563-566. (9) Takeoka, Y.; Watanabe, M. AdV. Mater. 2003, 15, 199-201. (10) Yoshino, K.; Kawagishi, Y.; Ozaki, M.; Kose, A. Jpn. J. Appl. Phys., Part 2 1999, 38, L786-L788. (11) Foulger, S. H.; Jiang, P.; Ying, Y. R.; Lattam, A. C.; Smith, D. W.; Ballato, J. AdV. Mater. 2001, 13 (24), 1898-1901. (12) Foulger, S. H.; Jiang, P.; Lattam, A.; Smith, D. W.; Ballato, J.; Dausch, D. E.; Grego, S.; Stoner, B. R. AdV. Mater. 2003, 15, 685-689.
of wavelengths, covering almost the entire visible-light region.13 The tension or compression parallel to the observation direction limits its applications in technology. In contrast, the tuning color obtained by stretching was reported on an inverse opal sheet made of polystyrene.14 In this case, however, the tuning color was not reversible because of the plastic deformation of the sheet. Most recently, Li et al. reported an elastic inverse opal film that enabled the tuning of reflected light wavelengths by stretching and releasing in the IR region.15 Taking a look at the engineering side, the work remains out of reach at the colloidal crystal elastic sheet with tunable visible color by stretching. In this article, we propose a novel structural color material, a photonic rubber sheet, that has characteristics of ordinary rubber (dry, soft, elastic, and durable) and can be fabricated as large sheets by a relatively simple process. Periodically arranged latex particles are embedded in a silicone rubber such that they do not come in contact with each other; this ensures that the interparticle distance can sensitively and reversibly change with the elastic deformation of the rubber matrix. The present material is expected to have wide-ranging potential applications, such as optical sensing of strain and color decoration by strain, under practical conditions. Experimental Section Monodispersed polystyrene (PS) spheres were obtained as aqueous dispersions from Polysciences (Warrington, PA). They were diluted five times with ultrapure water and then used for crystallization. Silicone liquids with viscosities in the range of 0.65-10 cSt were purchased from Dow Corning Toray Silicone (SH-200 series, Tokyo, Japan). The poly(dimethylsiloxane), PDMS, elastomer was obtained from Dow Corning (Sylgard 184, Midland, MI). It was supplied as a kit containing two separate components: the base material (part A) and the curing agent (part B). In our experiments, part A was premixed with silicone liquid SH-200 (0.65 cSt) in a 1:1 ratio (by weight) to reduce the viscosity of the PDMS elastomer precursor. The polyester film was purchased from Art Scientific Co. (25 µm in thickness, cat. no. 07-123-01, Ibaraki, Japan). The film was cut (13) Iwayama, Y.; Yamanaka, J.; Takiguchi, Y.; Takasaka, M.; Ito, K.; Shinohara, T.; Sawada, T.; Yonese, M. Langmuir 2003, 19, 977-980. (14) Sumioka, K.; Kayashima, H.; Tsutsui, T. AdV. Mater. 2002, 14, 12841286. (15) Li, J.; Wu, Y.; Fu, J.; Cong, Y.; Peng, J.; Han, Y. C. Chem. Phys. Lett. 2004, 390, 285-289.
10.1021/la0521037 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/20/2005
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Scheme 1. Preparation Procedure of Photonic Rubber Sheets
into squares (50 mm × 50 mm in area), sequentially cleaned with 2-propanol and ultrapure water, and dried in a stream of nitrogen gas. The surfaces of the substrates were made hydrophilic by treatment with oxygen plasma (Vacuum Device Co., PIB-10, Ibaraki, Japan). The preparing scheme for photonic rubber sheets is shown in Scheme 1. Step 1: Colloidal crystals were fabricated in the form of thin films by drying the aqueous dispersion of PS spheres on polyester film substrates. In a typical procedure, ∼10 mL of the PS dispersion was placed on a piece of the cut film to form a thin film of liquid. The surface of this liquid film was then completely covered with a silicone liquid (SH-200, 10 cSt). Finally, this sample was placed on a bench (under ambient laboratory conditions) for several days to allow the water to evaporate slowly by diffusing through the thin silicone layer. The attractive capillary forces generated during water evaporation directed the 202 nm PS spheres into a longrange-ordered 3D lattice. Step 2: After crystallization, the skin of the silicone liquid was carefully removed from the surface of the colloidal crystal, and the voids among the PS spheres were completely filled with the premixed PDMS precursor through capillary action. The PDMS elastomer was then cured at room temperature for 12 h, followed by additional hardening at 50 °C for 2 h. The composite film was thus fabricated by embedding a 3D lattice of PS spheres in the PDMS matrix with a lattice distance of D1. Step 3: The composite film was swollen with silicon oil, SH-200 (0.65 cSt). The additional space between the PS spheres was filled with additional premixed PDMS elastomer. The lattice distance of the PS array expanded from D1 to D2. On the basis of the surface scanning electron microscopy (SEM) images, the separation between PS spheres was increased by as much as 10%.16 In addition, the composite film was combined with a PDMS elastomer sheet. Step 4: We carefully peeled off the composite film with a PDMS elastomer sheet from the polyester substrate. The elastic sheet with the composite film is defined as a photonic rubber sheet. Photographs were taken using a digital camera (NIKON, Coolpix 885, Tokyo, Japan). SEM images were recorded using a field emission microscope (JEOL, JSM-6500F, Tokyo, Japan). The microscope was operated at an accelerating voltage of 15 kV. All samples for SEM studies were coated with a thin layer of gold (∼50 nm in thickness) before observation. Reflection spectra of the colloidal (16) Fudouzi, H.; Xia, Y. N. AdV. Mater. 2003, 15, 892-896.
Figure 1. Elastic deformation of the colloidal crystal composite film. (A) Concept of reversible tuning lattice distance of polystyrene, PS, array embedded in a poly(dimethylsiloxane), PDMS, elastomer matrix due to stretching and shrinking, (B) SEM images of the surface and cross section of the PS spheres arrayed with cubic close packing in a PDMS elastomer. crystals were obtained using a miniature fiber optic spectrometer (Ocean Optics, USB2000, Dunedin, FL). The incident light was aligned perpendicular to the (111) planes of the colloidal crystal for all optical measurements.
Results and Discussion Figure 1 illustrates the concept of tuning the structural color of a composite film and its microstructure scanning electron microscopy (SEM) image. Three-dimensional arrays of polystyrene, PS, containing polymer spheres embedded in an elastic polymer of poly(dimethylsiloxane), PDMS, were prepared by the same procedure presented in a previous paper.17 The PS spheres were closely packed. Next, they were spaced out by filling and curing additional PDMS elastic polymers. The stop band position was shifted from 557 to 586 nm because of the increase in the center-to-center distance between the PS spheres of the colloidal crystal (i.e., the interplanar spacing). The interplanar spacing expanded from D1 to D2 shown in Scheme 1. This nanoscale gap allows the reversible color change of the films by compressing the lattice distance. When the elastic polymer is stretched by mechanical stress, the lattice constants increase slightly in the horizontal direction but decrease in the vertical direction. Light is selectively diffracted by the array of planes of the PS spheres. The peak position, λ, which is the wavelength of the structural color, is expressed by Bragg’s equation with Snell’s law.
λ ) 2dx(ne2 - sin2 θ)
(1)
In this equation, d is the interplanar spacing of the planes, ne is the effective refractive index, and θ is the angle of incident light. When the PDMS rubber sheet is stretched in the horizontal direction, d is decreased in the vertical direction. The peak position shifts to a shorter wavelength because of the elongation of the PDMS rubber sheet. In contrast, when the mechanical strain is (17) Fudouzi, H.; Xia, Y. N. Langmuir 2003, 19, 9653-9660.
Photonic Rubber Sheets with Tunable Color
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Figure 2. Changes in the structural color of the colloidal crystal film covering the silicone rubber sheet. (A) Elastic deformation when the rubber sheet is stretched from L to L + ∆L by applying mechanical strain. (B) Photographic image of the initial sheet (L). (C) Photographic image of the stretched sheet (L + ∆L).
released, the PDMS rubber sheet returns rapidly to its original condition. This reversible tuning of the spacing causes the change in the structural color of the rubber sheet. Figure 1B shows the SEM images of the top surface and the cross section of the composite constructed with PDMS elastomer and PS spheres. These two images suggest that the lattice of this opal composite exhibits a cubic close-packed, ccp, structure with one set of (111) planes oriented parallel to the surface of the supporting substrate. The SEM images also show that the voids among the PS spheres of 202 nm have been completely filled with the PDMS elastic polymer. Figure 2 shows the changes in the structural color of the colloidal crystal film from red to green by the elongation of the rubber sheet due to mechanical strain. The colloidal crystal film, less than 20 µm in thickness, was formed on a supporting PDMS elastomer sheet, 5 mm in thickness. Figure 2A illustrates the elastic deformation: The length of the rubber sheet changed from the initial value, L, to the stretched length, L + ∆L. Parts B and C of Figure 2 are the photographic images of the initial sheet and the stretched sheet as shown in part A of Figure 2, respectively. The gradients in color of the specimen are due to inhomogeneous spacing out from step 2 to step 3 in Scheme 1. On the initial specimen in Figure 2B, the gradients in color were observed from red to yellow-green. On the stretched specimen in Figure 2C, the gradients in color were observed from yellowgreen to green. The color of the area encircled in Figure 2B changed from red to green by mechanical strain, as shown in Figure 2C. After releasing the mechanical strain on the photonic rubber sheet, the sheet quickly shrank, and its color changed from green to red. Thus, the structural color of the PDMS rubber sheet reverted to its original color. Furthermore, the tuning was reversible and repeatable during the cycle of application and release of mechanical strain. The structural color of the sheet due to Bragg diffraction and its peak position corresponds to the elongation of the photonic rubber sheet. Figure 3 show the relationship between diffraction peak and mechanical strain. Figure 3A shows the reflectance spectrum of Bragg’s diffraction from the ccp (111) planes of the PDMS rubber sheet. The measurement was carried out on the PDMS rubber sheet shown in Figure 2B in the circled area. The
Figure 3. Relationship between the peak positions and elongation of the silicone rubber sheet owing to stretching. (A) Reflectance of Bragg diffraction. (B) Tuning peak position as a function of ∆L/L.
peak position shifted from 590 to 560 nm, and the reflectance intensity decreased gradually as indicated by the arrow. Detecting the peaks at high mechanical strain was difficult. As shown in Figure 3B, the peak positions were plotted as a function of elongation, ∆L/L. Here, the mechanical strain is defined as x ) ∆L/L. Mechanical stress in the x direction, σx, acts on the elastic materials and is expressed as σx ) Ex ) E(∆L/L0) within the elastic range. In this case, E denotes the Young’s modulus. Accompanying this elongation in the x direction are contractions in the z direction. The mechanical strain in the vertical direction, z, is expressed by Poisson’s ratio, υ.
z ) -υ
()
( )
σx ∆L ) -υ E L
(2)
The periodic lattice, d, shown in Figure 1B is proportional to the mechanical strain, z. Furthermore, taking into account eqs 1 and 2, the peak position, λ, is found to be closely related to the elongation of the rubber sheet in the horizontal direction, which is expressed as ∆L/L. Therefore, the peak position shown in Figure 3B linearly decreased as a function of mechanical strain for up to 20%. In this region, the lattice distance of the ccp (111) planes, d, also decreased at the same rate. In contrast, for over 20% of ∆L/L, the position of the peak is approximately the same. This result suggests that the loss of the gap between the ccp (111) planes is due to the arrangement of the PS spheres such that they are in contact with each other, as shown in Figure 1A. In addition, we calculated the value of υ ) 0.22. This value is smaller than that reported for the PDMS sheet, 0.5.18 This difference in values is due to the microstructure of the photonic (18) Lacour, S. P.; Wagner, S.; Huang, Z. Y.; Suo, Z. Appl. Phys. Lett. 2003, 82, 2404-2406.
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Figure 4 shows the tuning color of the PDMS rubber sheet that is reversible and repeatable during cyclic elastic deformation. Figure 4A shows tuning peaks at the initial, stretched, and recovered stages in one elastic deformation cycle. Under mechanical strain, the initial peak shifted to a short wavelength, whereas when the stress was relaxed the stretched peak shifted to the recovered position that is almost the same as the initial position. Figure 4B plots the peak positions by repeating the stretch and release steps for four cycles. A small fluctuation in the peak position was observed, but the tuning peak position is repeatable and reversible. There is no deterioration of the rubber sheet during the stretching and releasing cycle that was conducted several times. The photonic rubber sheet has flexibility and durability to undergo cyclic deformation.
Conclusions In summary, we have demonstrated a silicone rubber sheet that exhibits tunable and reversible structural color due to mechanical strain. This new silicone rubber sheet is termed the photonic elastic rubber sheet, and it is expected to be useful for developing a new mechanical sensor because of its color-tuning capability. ccp polymer spheres were embedded in a silicone elastomer sheet, and the Bragg’s diffraction peak of the photonic rubber sheet was linearly tuned as a function of mechanical strain in elastic deformation. The photonic rubber sheet is valid for up to 20% stretching elongation. Furthermore, the PDMS rubber sheet can be used in mechanical strain sensors that use light of visible wavelengths. The PDMS rubber sheet can be employed in practical applications such as mechanical sensing, strain imaging, and smart sensing. Figure 4. Tuning structural color, which is reversible and repeatable, by elastic deformation. (A) Reversible peak position due to elastic deformation. (B) Tuning peak position by repeating the stretch and release steps on rubbery elasticity.
rubber sheets (i.e., closely packed PS spheres that are embedded in the PDMS elastomer).
Acknowledgment. We thank Ms. Jyunko Imasu for the technical support she rendered during this work. Supporting Information Available: QuickTime movie showing the tuning color in the Table of Contents graphic. This material is available free of charge via the Internet at http://pubs.acs.org. LA0521037
