Solvent-Responsive Structural Colored Balloons - ACS Publications

The structural colored balloons (SCBs) consisting of polymer microcapsules showed several structural colors developed by optical thin-layer interferen...
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Solvent-Responsive Structural Colored Balloons Kenji Higashiguchi,* Masafumi Inoue, Tomohiro Oda, and Kenji Matsuda* Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan S Supporting Information *

ABSTRACT: The structural colored balloons (SCBs) consisting of polymer microcapsules showed several structural colors developed by optical thin-layer interference. The SCBs were prepared using a mixture of low- and high-molecularweight polystyrene to give solvent responsiveness. When the surrounding solvent was transferred from water to the acetone/ water mixture using a flow cell, the SCBs swelled at first and shrunk subsequently. The gradual color change of the SCBs was observed along with the size change. The color change accompanying the size change was successfully reproduced by assuming that the total amount of polymer in the thin film does not change. The swelling rate was rationalized by the diffusion of solvent through the shell polystyrene film to the inside of the balloons.

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temperature11 and pH12 can release internal content, so that many practical applications are being proposed.10 In this study, the solvent responsiveness was given to the SCBs. It is expected that the shell thickness of the microcapsule decreases when the microcapsule swells, as in the case of latex balloons. Therefore, the structural color of the SCBs also changes when the surrounding solvent penetrates through the shell film as shown in Figure 1. Generally, the PS shell of the

INTRODUCTION Structural colors of objects are caused by optical effects such as interference, diffraction, and reflection when the size range of the objects is comparable to the optical wavelength range.1 Some natural products2 such as opal and butterfly were known to develop the structural color. Structural color, which differs from absorption, fluorescence, and emission, is regarded as an important coloring technique. Recently, structural color is applied to environment-resistant coloring such as metal oxide on metal surface.3 Structural colors have advantages in high fatigue resistance, a wide range of colors, a metallic luster, and applicability to colorless materials. The conventional type of structural color made of inorganic materials3,4 and polymers5 is static and does not change from initial preparation. On the other hand, dynamic development of structural color was achieved by using soft matter such as polymer,6 gels,7 and liquid crystals.8 The developing color strongly depends on the refractive index and the size of periodic structure. Soft matter can change the chemical entity and the periodic structure; therefore, by introducing the appropriate system, its structural color can be easily controlled. Balloon-shaped microstructures made of polystyrene (PS) microcapsules, reported by Ikkai, showed structural color, and they are called structural colored balloons (SCBs).9 PS was used as the material for shell films because its refractive index was higher than that of the surrounding water. The diameters of SCBs are a few hundred micrometers, and their shell thicknesses are about a few hundred nanometers. When the radius of the balloon is substantially larger than the shell thickness, the wavelength of the developing color, λ, depends on the shell thickness and angle of incident light. Thus, structural colors could appear on the fringe of the balloons depending on the shell thickness under the transmitted light. Submicrometer- and micrometer-sized hollow spheres and vesicles are attracting interests due to their potential applications in many fields, such as drug delivery systems and catalysts.10 Especially, the hollow spheres that are sensitive to © 2012 American Chemical Society

Figure 1. The concept of the change of the structural color by swelling the SCB. The optical path difference can change due to the swelling accompanied by the size change of the thickness of the shell of the SCB. The difference of the light path 2d shown as bold line has been discussed in further detail in the literature.9 L and D are the radius and thickness of the shell, respectively. The subscript 0 indicates the initial state.

SCBs shows no permeability to water. However, PS which was solvated by the acetone/water mixture is estimated to transmit the solvent.13 The inside portion of the balloon is composed of not only water but also gelatin. Because of the ionic property of gelatin, osmotic pressure is generated when the shell film is permeable to the solvent, and the structural colors of SCBs were controlled by the surrounding solvent. The relationship between the developing color and the change of thickness accompanying the change of radius will be discussed. Received: December 2, 2011 Published: March 1, 2012 5432

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EXPERIMENTAL SECTION

Materials. PS that have a high molecular weight (labeled as n = 1600−1800, measured Mw = 4.89 × 105, Wako) and low molecular weight (labeled as Mw = 50 000, measured Mw = 4.33 × 104, Polysciences), gelatin, and dichloromethane are commercially available. The purchased PSs and mixed PS were analyzed by Shodex KF-404HQ GPC column and HITACHI Lachrom system as shown in Figure S1 of the Supporting Information. Preparation of Structural-Colored Balloons. SCBs were prepared by the method of double-surface emulsion14 that is modified from the method reported by Ikkai.9 First, three types of solution were prepared as follows: (1) 0.3 g of gelatin was dissolved in 10 mL of water at 80 °C and cooled to room temperature to form gelatin aqueous solution α; (2) 120 mg of polystyrene mixture (i.e., 108 mg of low-molecular-weight PS and 12 mg of high-molecular-weight PS forming a 9:1 mixture) was dissolved in 3.9 g of dichloromethane to produce a 3 wt % PS solution, namely, polystyrene organic solution β; (3) 1.5 g of gelatin was dissolved in 150 mL of water at 40 °C and cooled to room temperature to produce a 1 wt % solution, namely, gelatin aqueous solution γ. The gelatin aqueous solution α was added to polystyrene organic solution β and stirred vigorously (1350 rpm) to generate an oil/water emulsion (first emulsification). Subsequently, the first emulsion was poured into the gelatin aqueous solution γ with stirring at 500 rpm, and water/oil/water double-surface emulsion was obtained (second emulsification). It should be noted that mixing was first carried out at room temperature, and then, the temperature was gradually increased to 40 °C over 4 h to remove dichloromethane completely, as described in the literature.9 After the mixture was cooled to room temperature, the dispersion was washed many times with pure water, so that PS microcapsules were obtained as structural-colored balloons with different sizes. All the balloons were purified using appropriate sieves. Optical Microscope. Using a Nikon SMZ-2B stereomicroscope, one SCB was chosen and moved to flow cell whose thickness is 5.0 mm, and thus, only one particle was measured. Acetone/water was premixed and the solvent was introduced using KdScientific KDS100 syringe pump at 0.17 mL/min. Direct observation of SCBs in pure water was carried out in transmission geometry using Nikon ECLIPSE LV100 coupled with Nikon DS-Fi1 CCD camera. The objective lenses were Nikon PlanFluor 10×/0.30 and 20×/0.50. The interference reflectance spectra of the SCB surface were measured under an optical microscope by using a miniature fiber-optic spectrometer Ocean Optics S2000 with a 600 μm optical fiber. SEM. Scanning electron microscopy was carried out using a Hitachi S-4700 to obtain a 3D image of the SCB cross section. SCB was taken out from flow cell, rapidly frozen under liquid nitrogen, and lyophilized at −15 °C under low pressure. The dried SCB was Aucoated under vacuum using Sanyu ion spatter SC-70f. The deposited metal thickness is 20 nm. The observation was performed under 5 kV accelerating voltage.

Figure 2. Optical micrographs of SCBs prepared with various mixtures PS having low and high molecular weight. The ratio of the low- to high-PS is (a) 99:1, (b) 9:1, (c) 7:3, (d) 5:5, (e) 1:9, and (f) 0:10.

high-molecular-weight PS being 9:1, SCBs were obtained along with shards. The shards were supposed to be obtained as follows: the SCBs were stirred vigorously by stirrer bar during preparation of SCBs, so that almost all SCBs had been collided and the SCBs with low elastic durability were broken into some pieces. SCBs were also formed using mixtures of low- and highmolecular-weight PS with ratios of 7:3 and 0:10. In these cases, deformed balloons were obtained with the decreasing number of the shards because the elasticity of the shell was relatively high by the addition of high-molecular-weight PS; thus, a larger amount of high-molecular-weight PS gives more mechanically durable SCBs. The appearance of the purified SCBs is similar each other for different molecular weight of PS, which means that the molecular weight of PS scarcely affect the shell thickness. However, the concentration of PS significantly affected the shell thickness. The concentration of PS was varied from 2 to 5 wt % using the 9:1 mixture of low- and high-molecular-weight PS. In the case of 2 wt % concentration, no SCBs were obtained. In the cases of 3−5 wt %, SCBs were obtained but their film thickness differed considerably, which was confirmed on the basis of their respective structural colors, as described below. We used 3 wt % dichloromethane solution of 9:1 mixture ratio of low- and high-molecular-weight PS mainly for controlling the shell thickness in solvent mixture (shown below). Microscopy. The prepared SCBs showed various structural colors that are caused by optical thin-layer interference. To determine the developing color of the SCBs, an SCB was separated using a fine pipet and observed under an optical microscope. The SCB appeared green, and its diameter was measured to be 210 μm as shown in Figure 3a. Additionally, the color of pale purple was observed just inside of green color. The reflection spectrum of the SCB was also measured as shown in Figure 3b. There were three reflectance peaks in the spectrum, and the central peak at 528 nm corresponds to green. The other peaks at λ = 440 and 655 nm correspond to the purple color just inside of green color. The SCB was observed by SEM after lyophilizing, as shown in Figure 3c,d. The diameter of the dried SCB was found to be 209 μm, and it was almost the same as the diameter measured by an optical

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RESULTS AND DISCUSSION Preparation of SCBs. SCBs with diameter of a few hundred micrometers and shell thickness of a few hundred nanometers were prepared by the method of double-surface emulsion14 using an aqueous solution of gelatin and dichloromethane solution of PS, as described in the literature.9 SCBs were prepared using various mixtures of PS having low and high molecular weight as shown in Figure 2. In the case in which only the low-molecular-weight PS or the mixture with the ratio of low- to high-molecular-weight PS being 99:1 were used, no SCB but PS particles were formed because of the low stability of the double-surface emulsion, i.e., the dichloromethane solution of low-molecular-weight PS has low viscosity, thus the thin layer of dichloromethane deformed by the surface tension. In the case of the mixture with the ratio of low- to 5433

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reflection maxima at 440 and 655 nm correspond to m = 3 (calcd 432 nm) and m = 2 (calcd 648 nm). 2d = mλ

micrograph. This means that the structure of the SCB did not change after lyophilizing. The shell thickness was also measured and was found to be 750 nm. The reflection maximum λ was calculated using theoretical equation.9,15 Constructive interference occurs when the difference of light path 2d which derived from some geometrical equations was equal to a half-integer number of wavelength as shown in eq 1 (m = 0, 1, 2, 3, ...)

(2)

The structural color changed with the incident angle. From the reflection maxima, the average angle of incidence of light from a plane light source was estimated to be 0° under bright-field illumination. On the other hand, the dark-field illumination has the large incident angle. An example is illustrated in Figure S4, which shows a balloon having different colors under bright-field and dark-field illumination. Solvent Exchange. Upon exposure to acetone and water mixtures,16 SCBs showed time-dependent changes in diameter and changes of color after a certain period. The method of exchange of the solvent surrounding SCBs is described in Figures S5 and S6. However, the responsiveness to the solvent surrounding SCBs fairly depended on the molecular weight of the polymers; i.e., the SCBs composed of low-molecular-weight PS had high responsiveness to the solvent (Figure S7). The rate of size change also depended on the ratio of the solvent mixture. The diameters of some SCBs in various solvents were plotted against time, as shown in Figure S8. In all cases, SCBs swelled initially and shrunk subsequently. The point at which swelling stops and shrinking starts scarcely depended on the ratio of the solvent mixture; it appeared to be random. On the other hand, the swelling and shrinking rates depended on the ratio of solvent mixtures. In the case of acetone/water mixture ratio of 5/5, SCBs showed rapid shrinking. On the other hand, in the case of acetone/water mixture ratio of 3/7, both the swelling and shrinking rates were very low. Thus, the solvent mixture with acetone/water ratio of 4/6 was mainly used as the environment solvent for the measurement of the response of SCB. Swelling. The swelling of SCB was examined in detail for the SCB that showed considerable swelling. The colors of SCB measured by microscope were analyzed based on the diameter and the calculated shell thickness as shown in Figure 4. There was an induction period of 20 min after exchange of the surrounding solvent. The balloon started to swell after 20 min, and after 130 min, the balloon showed rapid swelling and

Figure 3. (a) Optical micrograph of the SCB at 20× magnification under unpolarized transmitted light, (b) reflectance spectrum of the interference light on the surface of the SCB, (c) SEM image of the lyophilized SCB, and (d) cross section of SCB shell magnified around yellow circle of (c). The shell thickness without coated metal was measured as 750 nm.

2d = λ(2m + 1)/2

(m = 0, 1, 2, 3, ...)

(1)

where m represents the interference order. The reflection maxima λ was calculated as 518 nm at m = 2 as shown in Figure S3. The colors appeared just inside of the fringe can be reproduced by assuming the difference of light path is equal to an integer number of wavelength using eq 2. The observed

Figure 4. (a) Micrographs of an SCB showing considerable swelling and development of color as a result of swelling. The black sphere is a PS-filled sphere, which is used as a scale and mark. (b) The changes in diameter (open circles) and the expected shell thickness (blue solid squares). (c) Calculated interference color considering multiple interference, and (d) the magnified image of (a); the y-axis was aligned by the expected shell thickness using D0 = 870 nm. The ratio of the low- to high-PS was 9:1, and the ratio of acetone/water was 4/6. 5434

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Figure 5. (a) Micrographs of an SCB showing considerable shrinking and developing color by shrinking. (b) Changes in the diameter (open circles) and the expected shell thickness (blue solid squares). (c) Calculated interference color considering multiple interference. (d) The magnified image of (a) between 48 and 172 min; the y-axis was aligned by the expected shell thickness. (e) SEM images of the final state of the shrunk SCB. (f) Cross section of SCB shell magnified around yellow circle of (e). The ratio of the low- to high-PS was 9:1, and the ratio of acetone/water was 4/6.

mixture with a ratio of 4/6 (Figure 5). Its behavior was different from the first balloon as it slightly swelled at first and then shrunk considerably. Its structural color changed from violet to peacock green. The final state of the SCB was preserved by additional transfer into pure water. The conserved SCB was lyophilized and observed by SEM as shown in Figure 5e,f. The measured diameter was 183 μm, and it approximately agreed with the diameter obtained by the optical image (188 μm). The surface was rougher than that of the initial SCB, shown in Figure 3c, because of the deformation due to shrinking. However, the shell film thickness could increase without folding of the film. This is important for the smooth color change caused by the shrinking. This means that the PS film solvated17 sufficiently to change the thickness, as described in the Mechanism section. The shell thickness of the SCB was measured from the magnified SEM image and found to be 1384 nm (Figure 5f). The film thickness corresponding to each diameter was calculated as mentioned above. Since the total amount of polymer in the shell obtained experimentally, the change in the shell thickness with the diameter could be accurately calculated. For each shell thickness the developing colors were calculated. When the thick shell is employed, multiple colors overlap from the multiple interference order as in the case of swelling. The computer-generated interference color of the PS film is also shown in Figure 5c. The agreement between the observed and calculated colors of each balloon means that the calculated shell thickness was correct. Therefore, the color change is due to the change in the shell thickness with a change in the size of balloons. Mechanism. When solvent penetrates into inside of the balloons through the PS film, the rate depends on the diffusion rate in the PS film. Herein, this flow is described by Fick’s first law,18 which relates the diffusive flux to the concentration field.19

significant color change. The shell thickness of the balloon was expected to decrease with the swelling. The shell thickness was calculated with the assumption that the total amount of polymer composing the shell film scarcely changed by solvent exchange because of the low solubility of PS in the acetone/ water solvent.13 Therefore, the shell thickness decreased with increasing the diameters, and consequently, the structural color also changed. The initial diameter 2L0 was determined to be 168 μm from the micrograph. Therefore, for the given initial shell thickness D0, the shell thickness D for each observed radius L can be calculated by using the relationship 4πL02D0 = 4πL2D, and the developing color for the each observed radius L can be estimated. The interference order m decreases as the shell thickness decreases. For thicker shell, multiple colors overlap from the different interference orders. The candidates of initial thickness D0 were 340−360 nm (m = 0), 600−660 nm (m = 1), 840−940 nm (m = 2−3), and 1120−1220 nm (m = 3−4) considering the observed reddish-purple color at 0 min and the simulated interference color (Figure 4c).17 Taking into account that the greenish blue was observed twice by swelling at 135 and 180 min, the initial thicknesses of 340−360 nm (m = 0) and 600−660 nm (m = 1) were excluded because in the range of 100−1350 nm, fine blue color appeared only twice around 700 and 400 nm (Figure S9). In the case of 1120−1220 nm (m = 3−4), the calculated color does not agree with the observed color change. Thus, the initial shell thickness D0 was expected as 840−940 nm (m = 2−3). Optimization of D0 was performed by comparing the color at the largest balloon (at 180 min) because the thinner shell develops the color of the smaller interference order, so that the developing color gets simple and gets easier to compare. All the calculated colors can reproduce the observed colors by use of the optimized D0 = 870 ± 15 nm (Figure 4c,d). In this experiment, the shell thickness was not experimentally measured but could be determined as a unique solution of the simulation of size change. Microcapsules that change their sizes are important for materials transport system. Therefore, the estimation of shell thickness using only interference color is significant for evaluating the size change. Shrinking. In the case of another balloon, the surrounding solvent was also transferred from water to the acetone/water

J = δ(t )(C(t )/D(t ))

(3)

Here, J, δ, C, and D are the diffusion flux, diffusion coefficient, concentration of inner solution of the capsule, and thickness of the shell, respectively. The shell thickness D can be represented as MPS/S, where S (= 4πL2) is the area of shell surface and MPS 5435

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the PS film. Then, water was squeezed out because of the surface tension of the solvated PS. Then, the balloon shrunk without folding.

represents the polymer volume of the shell. The inner concentration of the capsule C can be represented as m/V, where V (= (4/3)πL3) is the balloon volume and m represents the amount of gelatin. Thus, the flux J, which is the diffusion rate per area, is represented as (3m/MPS)δ(t)L−1. The increase of the balloon volume dV/dt, which corresponds to the amount of solvent flowing through the entire shell surface per unit time, is described as JSαm, where αm represents the molar volume. Here, the increase in the volume dV/dt is equal to 4πL2(dL/ dt), so that the following equation is obtained: 4πL2(dL /dt ) = (3m /MPS)δ(t )L−1(4πL2 α m)

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CONCLUSIONS Polymer microcapsules that showed several structural colors originating from optical thin-layer interference was prepared using a mixture of low- and high-molecular-weight polystyrene to give the solvent responsiveness. When solvent was transferred from water to the acetone/water mixture with a ratio of 4/6 using a quartz flow cell, the diameters of balloons increased and their colors changed with time, e.g., from reddish purple to several other colors. The color change accompanying the size change was successfully reproduced by assuming the total amount of polymer in the thin film does not change. In the case of shrinking which occurred subsequently after swelling, the color change occurred in the opposite direction. The swelling rate was rationalized by the diffusion of solvent through the shell PS film to the inside region of the balloons. This result shows the new directions of development of smart coloring materials based on the structural color.

(4)

Thus, the radius of the balloon L is described as eq 5, where A represents 3mαm/MPS and a1 is the integration constant. 1 2 L =A 2

∫ δ(t ) dt + a1

(5)

If the diffusion coefficient δ(t) is constant, the rate of swelling decreases upon time. But in our case, the radius L increases acceleratedly, suggesting accelerated increase of δ(t) due to the gradual solvation of PS film. Therefore, an exponential function is employed as eq 6 for the function of the diffusion coefficient δ(t). δ1(t ) = Q exp[(t /t1) ln(δmax /Q )]

δ2(t ) = δmax

(t < t1)

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S Supporting Information *

(6)

(t > t1)

Figures S1−S11. This material is available free of charge via the Internet at http://pubs.acs.org.

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(7)

where δmax represents the diffusion coefficient of completely solvated PS and Q is the appropriate coefficient. t1 represents the boundary between state during solvation and completely solvated states; therefore, δ1(t1) = δ2(t1) = δmax. Equations 6 and 7 are substituted into eq 5, and the time-dependent changes in the radius are obtained as eqs 8 and 9. In these equations, B is the integration constant. ⎡

⎛t ⎞⎤ δ exp⎜ ln max ⎟⎥ + B Q ⎠⎥⎦ ⎝ t1 ⎣ ln(δmax /Q )

L12(t ) = 2AQ ⎢ ⎢

t1

(t < t1)

⎡ ⎤ t1 L 2 2(t ) = 2A δmax ⎢t − t1 + ⎥+B ln(δmax /Q ) ⎦ ⎣

ASSOCIATED CONTENT

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research (No. 23107535) on the Innovative Areas: “Fusion Materials” (Area No. 2206) from MEXT.

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(t > t1)

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By least-squares fitting to the observed data (Figure 4b), t1, δmax, A, Q, and B were optimized as 175 min, 947 μm2 min−1, 0.1, 3.12 μm2 min−1, and 7.26 × 103 μm2, respectively. The obtained t1 (= 175 min) is close to the time of cracking; therefore, the diffusion coefficient δ(t) may not reach the maximum value at the time of cracking. The fitted curve is shown in Figure 4b as a solid line. The agreement between the obtained and calculated values indicates that the assumption that solvent penetrated inside the balloons because of osmotic pressure is correct. It should be emphasized that the increase of δ(t) occurs acceleratedly. When a linear function was employed for the increasing function of δ(t), the fitting curve disagreed as shown in Figure S11. The mechanism of size change can be summarized as follows: initially, water could not penetrate the PS film, but after the introduction of acetone, the PS film was solvated, and solvent could be transported between the outside and inside regions of the balloons through the solvated film because there exists an osmotic pressure. Thus, the balloon swelled and the thickness of the shell film decreased acceleratedly. Finally, the shell cracked due to the limitation of mechanical durability of 5436

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dx.doi.org/10.1021/la3006234 | Langmuir 2012, 28, 5432−5437