Structural Colored Balloons Responsive to pH Change - Langmuir

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Structural Colored Balloons Responsive to pH change Kenji Higashiguchi, Jun Imai, and Kenji Matsuda Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b00607 • Publication Date (Web): 25 Apr 2016 Downloaded from http://pubs.acs.org on May 5, 2016

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Structural Colored Balloons Responsive to pH change Kenji Higashiguchi,*,†,‡Jun Imai,† and Kenji Matsuda*,† †

Department of Synthetic Chemistry and Biological Chemistry, Graduate School of Engineering, Kyoto University, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan.

‡

PRESTO, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan.

E-mail: [email protected] & [email protected]

RECEIVED DATE

Abstract: Structural colored balloons (SCBs) composed of poly(4-vinylpyridine-co-styrene) (P4VPPS) exhibited pH-controlled structural color change by the presence and absence of p-toluenesulfonic acid. The diameter of the SCBs increased and decreased under acidic and neutral conditions, respectively. The different colors were exhibited at different pH supposedly resulted from a change in the shell thickness not only due to the change in the diameter of the SCBs, but also due to the uptake of p-toluenesulfonic acid to the pyridyl side chain of P4VP-PS.

KEYWORDS

Structural color, Microcapsule, Poly(4-vinylpyridine)

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Introduction Structural color is the result of optical interference through diffraction, reflection, and refraction when the size of an object is comparable to the wavelength of visible light.1-4 The color is dependent on the optical path length and is strongly affected by the refractive index and size of the object, and the incident angle of light. Soft matter, which includes liquid crystals, gels, colloidal crystals, and polymers, can be used to tune structural color5-18 because external stimuli can change the size, periodic structure, and refractive index of these materials. Therefore, these materials are expected not only to change exhibiting colors, but also to be applicable to tunable optical devices. Polyvinylpyridine exhibits response of volume19,20 and solubility21-23 change to the change of environmental pH. Therefore, a polymer containing polyvinylpyridine can be used to control the structural color by the control of the environmental pH.24,25 Structural colored balloons (SCBs)26 are sub-millimeter capsules that exhibit structural color on their shell surface in water. The shell thickness is required to be a few hundred nanometers to develop structural color by optical interference.27 A polymer used for the shell film of SCB is required to dissolve in dichloromethane for the sake of preparation by the double-surface emulsion method. A polymer that has a high refractive index is desirable for strong reflectance. We have previously reported the responsivenesses of the structural color of SCBs composed of polystyrene (PS)28 and poly(vinyl cinnamate) (PVCi).29 In the case of PS, the SCBs did not respond to typical external stimuli, although response to solvent was observed.28 The mechanism for the change in structural color was the change in the shell thickness associated with expansion and shrinkage of SCBs due to an influx of the outer solvent by osmotic pressure and the outflow of the inner solvent via cracks generated in the shell film. The color change was reproduced using spectral simulations and CIE chromatograms on the assumption that the shell thickness changes along with the SCB diameter without changing total amount of the shell material of SCB. When PVCi was employed, the responsiveness to solvent was suppressed by irradiation with UV light due to a photocrosslinking reaction.29 The control of the structural color is significant to obtain the desired color. In particular, the responsiveness of the structural color to the 2


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external stimuli is important to obtain micrometer-sized sensing materials. In this study, a random copolymer of poly(4-vinylpyridine)-polystyrene (P4VP-PS) was employed for the shell film of SCBs to control the structural color by the control of environmental pH. The concept for the change in structural color of SCBs is illustrated in Figure 1. P4VP undergoes protonation under acidic conditions at the pyridyl side chain from the outer surface, which results in an increase in the total amount of shell material by the addition of counter anions. The subsequent change in diameter can be categorized into two types: a large change in diameter that is expansion by osmotic pressure and shrinkage by surface tension, as previously reported,28 and a small change in diameter due to swelling and de-swelling of the polymer film under respective acidic and neutral conditions, as shown in Figure 1. i)

ii)

iii)

incident light 2da Di

D

2di Li

Da

2d Li

acid

La

acid 2L: expanding

2L: constant

acid x

y

TolSO3 H

x

neutral

2L: expanding

2L: shrinking

iv)

- TolSO3 H N

y

N H SO3

Dn

2dn Ln

Figure 1. Schematic illustration of change in the SCB diameter 2L, the thickness of the shell D, and the structural color of SCBs composed of P4VP-PS random copolymer under pH control. Ionization of the shell film occurs by the addition of acidic aqueous solution containing p-toluenesulfonic acid (TolSO3H) and the amount of shell material increased without a change in diameter (i to ii). Solvation of the shell film also occurs from the outer surface of the SCB. When the shell film is adequately solvated, the balloon starts to expand (ii to iii). Repetitive change in the diameter is achieved by homologous deformation of the SCB structure under pH switching (between iii and iv) with the change in the amount of shell material. The difference in the light path 2d, shown as the bold line, has been discussed in further detail in the literature.26 The subscripts “n” and “a” indicate neutral and acidic states, respectively. The gray color represents the ionized regions. 3


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Experimental General. The gelatin and the solvents used in this study were commercially available. The styrene monomer and 4-vinylpyridine monomer were also commercially available and were purified by extraction of the inhibitor using 1 N NaOH aqueous solution prior to polymerization. Gel permeation chromatography (GPC) analysis was carried out using a Shodex, KD-804 GPC column (eluent: dimethyl formamide (DMF)) and HPLC system (Hitachi, LaChrom Elite) to estimate the polystyrene-reduced molecular weight of the obtained P4VP-PS random copolymers, as shown in Figure S1 of the Supporting Information (SI). 1H NMR spectra were recorded on 500 MHz (JEOL JMN-ALPHA 500) instruments. Samples were dissolved in CD2Cl2. The refractive index was measured using an Abbe refractometer (Atago, DR-M4) with a thermostatic bath at 20 °C. Preparation of Poly(vinyl-4-pyridine-co-styrene) (P4VP75; P4VP:PS = 3:1). To a mixture of styrene (3.00 g, 28.8 mmol) and 4-vinylpyridine (9.00 g, 85.6 mmol) was added a solution of AIBN (190 mg, 1.16 mmol) in THF (2.0 mL). After the mixture was degassed by five freeze-evacuate-thaw cycles and placed under an inert nitrogen atmosphere, the vessel was immersed in an oil bath at 60 °C. After 16 h, the resulting solution was cooled to room temperature. The polymer was immersed in CH2Cl2 for 1 day to dissolve. The resulting solution was slowly poured into an excess of ether at room temperature to precipitate the copolymer. After separation by decantation and drying under vacuum at room temperature, P4VP75 was obtained as a white powder (6.3 g, 52%). P4VP75: 1H NMR (500 MHz, CD2Cl2, ): 1.44 (br, 12H), 6.46–7.09 (br, 11H), 8.27 (br, 6H); Elemental analysis Calcd (P4VP:PS = 74:26): H 6.98%, C 83.14%, N 9.88% Found: H 7.05%, C 82.87%, N 9.97%; GPC: Mn = 2.66×105; Mw = 4.81×105, Mw/Mn = 1.8; IR (neat film): 564, 703, 763, 822, 993, 1068, 1220, 1415, 1452, 1493, 1556, 1597, 1938, 2853, 2926, 3023, 3064 cm-1; RI: n486 = 1.606, n546 = 1.596, n589(D) = 1.593, n656 = 1.588. Preparation of Poly(vinyl-4-pyridine-co-styrene) (P4VP25; P4VP:PS = 1:3). The same method was applied to the polymerization. Styrene (9.00 g, 85.6 mmol), 4-vinylpyridine (3.00 g, 28.8 mmol), and AIBN (189 mg, 1.16 mmol) were used to obtain P4VP25 as a white powder (6.96 g, 58%). 4


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P4VP25: 1H NMR (500 MHz, CD2Cl2, ): 1.44 (br, 12H), 6.50–7.18 (br, 17H), 8.23 (br, 2H); Elemental analysis Calcd (P4VP:PS = 29:71): H 7.44%, C 88.67%, N 3.89%, Found: H 7.74%, C 88.28%, N 3.73%; GPC: Mn = 2.92×105; Mw = 6.16×105, Mw/Mn = 2.1. Preparation of SCBs. SCBs were prepared by a double-surface emulsion method,30 which is a modification of a previously reported method.26,28 P4VP75 and P4VP25 were selected for measurement of the responsiveness to pH. First, three types of solution were prepared as follows: (1) 3 g of gelatin was dissolved in 100 mL of water at 40 °C and cooled to room temperature to form a gelatin aqueous solution () (2) 400 mg of P4VP-PS copolymer was dissolved in 9.7 g (7.3 mL) of dichloromethane to produce a 4 wt% P4VP-PS copolymer solution named as P4VP-PS organic solution (); (3) 5 g of gelatin was dissolved in 500 mL of water at 40 °C and cooled to room temperature to produce a 1 wt% solution named as dilute gelatin aqueous solution (). The gelatin aqueous solution () was added to the P4VP-PS organic solution () and stirred vigorously to generate an oil/water emulsion (first emulsification). The first emulsion was subsequently poured into the dilute gelatin aqueous solution () with stirring at 500 rpm and a water/oil/water doublesurface emulsion was obtained (second emulsification). It should be noted that mixing was first conducted at room temperature, and the temperature was then gradually increased to 40 °C over 4 h to completely remove dichloromethane, as described in the literature.26,28,30 After the mixture was cooled to room temperature, the dispersion was washed many times with pure water, so that microcapsules composed of P4VP-PS copolymer were obtained as SCBs with different diameters. All of the SCBs were then separated according to size using appropriate sieves to obtain SCBs with various structural colors. SCBs with diameters, 2L, of around 200 m and with a reddish color were selected in order to carry out the experiment under the uniform conditions.28 Optical Microscopy. Observations of the diameter and color changes in SCBs under pH switching were conducted using an optical microscope with transmission geometry (Nikon, ECLIPSE LV100) 5


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coupled with a CCD camera (Zeiss, AxioCam MRc), as shown in Figure S2 (SI). The objective lens used was a Nikon Plan-Fluor 20×/0.45. Both diameter and color of the balloon were obtained from optical images by intervals of 10-30 seconds. The CIE color space (xy) was determined as follows: any places of the colored region were picked up by software (Photoshop, Adobe) and the RGB values were exchanged to the CIE color space using the equations described in the literature.31 pH Switching. The preparation of the solvents was performed as follows. Pure water was prepared by the ion-exchange method and used as pH 7 water. The acidified water (pH 3 p-toluenesulfonic acid (TolSO3H) in water) was prepared by dissolving p-toluenesulfonic acid monohydrate (ca. 0.25 mmol) in pure water (250 mL). The pH of the solution was measured using a pH meter (Horiba, F-53). The exchange of solvent surrounding the SCBs was conducted as follows. An SCB with surrounding water was picked up from a vial with a fine pipette and placed into a flow-cell (5.0 mm thick) under a stereomicroscope (Nikon, SMZ-2B). Two syringes charged with neutral (pH 7) and acidic (pH 3) water were connected to a three-way stopcock via tubes. Another short tube (diameter: 0.3 mm and length: 10.5 cm) was connected to the three-way stopcock and to the flow cell, by which the acidic/neutral solutions were supplied. pH switching was conducted by changing the fluidic path. The solvent was introduced using a syringe pump (KD Scientific, KDS100) at 0.17 mL/min. The delay time required for the exchange of solvent inside flow cell was determined using water solution of Rhodamine B dye. The change in absorption upon exchange was monitored using a fiberoptic spectrometer (Hamamatsu, PMA-12), as shown in Figure S3 (SI). The increase of absorbance began at 4 min and adequately (80%) exchanged after 10 min. When the tube between stopcock and the flow cell had been filled in advance, the delay time was 3 min. The obtained delay times were used as trigger time for the pH switching experiment and the correction has been included in Figures 3, 5, 6, 7, and S9.

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Scanning electron microscopy.

Scanning electron microscopy (SEM; JEOL, JSM-6010LV)

observations were conducted to obtain the thickness of the shell films from 3D images of the SCB cross-sections. An SCB was frozen with liquid nitrogen and broken using a razor, followed by drying at room temperature. The dried SCB was coated with Au (ca. 20 nm thick) under vacuum using an ion sputterer (Sanyu, SC-70f). SEM observation was performed under an accelerating voltage of 20 kV. The change in the volume of P4VP75 under acidic condition was confirmed by SEM using a spincoated film. The sample was prepared as follows. A CH2Cl2 solution (10 wt%) of P4VP75 was spincoated on a cover glass. The sample was divided in two by freeze-fracture and one part was immersed in a pH 3.0 aqueous solution of p-toluenesulfonic acid for 2 h. Both samples were subsequently dried at 100 °C under vacuum and divided again by freeze-fracture for observation at a new cross-section because deformation was observed near the ridgeline. IR measurement. The protonation of P4VP75 was confirmed by IR measurement using a cast polymer film. The sample was prepared as follows: A CH2Cl2 solution (2 wt%) of P4VP75 was cast on a Teflon sheet substrate, dried at 100 °C under vacuum, and then removed from the substrate. The polymer sheets were immersed into acidic aqueous solution for 1 h and then dried at 100 °C under vacuum. The polymer sheet was then set in the transmission path of an IR spectrometer (Jasco, FT/IR4200).

Results and Discussion Preparation of SCBs Composed of P4VP-PS Copolymer. The random copolymer of poly(vinylpyridine) and PS (P4VP-PS) was employed as a material to investigate the pH responsiveness of SCBs. The P4VP-PS copolymer was obtained by polymerization of the mixed monomers (4vinylpyridine and styrene) in THF at 60 °C with AIBN. The ratio of P4VP in the copolymer was almost the same as the ratio in the monomers determined by 1H NMR and elemental analysis. The refractive index was obtained as nD = 1.593, which is almost the same as that of PS.32 SCBs composed of the P4VP-PS copolymer were prepared using a method similar to that previously used for PS,26,28 i.e., the 7


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double-surface emulsion method,30 using an aqueous solution of gelatin and a dichloromethane solution of the copolymer, as described in the Experimental section. The copolymers with various ratios of P4VP to PS were used to prepare SCBs. However, SCBs were not obtained for the case of P4VP:PS = 1:0 (e.g., pure P4VP), because the first emulsion was not formed. The molecular weight affects the responsiveness to stimuli, as previously reported.28 Therefore, SCBs were prepared from P4VP75 (P4VP:PS = 3:1) and P4VP25 (P4PV:PS = 1:3) copolymers with similar molecular weight Mw = 4.81×105 and 6.16×105, respectively. The obtained SCBs exhibited several structural colors depending on the shell thickness. Figure 2 shows optical micrographs and SEM images of the capsule structure of an SCB composed of P4VP75. The observed thickness D was 1.34 m. The SCB has a dull peacock green color, which indicates the diffraction order was m > 4 and the calculated shell thickness D was 1.25-1.40 m.28 The calculated shell thickness reproduced well the observed thickness.

Figure 2. Micrographs of an SCB composed of P4VP75. Optical micrograph of (a) an intact SCB in water, and (b) a broken SCB in air. The optical microscope was set to 20× magnification with unpolarized transmitted light. The diameter was measured as 2L = 160 m. (c) SEM micrograph of a dried SCB and (d) magnified cross-section of the SCB shell indicated by the yellow circle in (c). The shell thickness without the Au sputter-coating was measured as D = 1.34 m.

Responsiveness to the Change of Solvent. The SCBs composed of P4VP-PS showed response to organic solvent, as reported in our previous work.28 The response of P4VP75 to aqueous organic 8


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solvent (acetone/water = 2/8) was as follows. The SCB which was solvated by the aqueous organic solution expanded by osmotic pressure because the interior of capsule was composed of gelatin aqueous solution having ionic property. Subsequent shrinking occurred by outflow via a crack due to shrinkage of the polymer film in the aqueous organic solvent by surface tension. During these processes, the total amount of polymer did not change because the polymer shell did not dissolve in the aqueous organic solvent. Therefore, the shell thickness and structural color changed along with the change in diameter, as shown in Figure 3. The difference of the structural color in circumferential direction, e.g. the lower right at 157 min, was caused by inhomogeneity of shell thickness. SCBs had slightly dispersed shell thickness originating from the preparation process. An SCB showing homogeneous color was selected as much as possible.

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Figure 3. (a) Micrographs of an SCB composed of P4VP75 in acetone/water = 2/8 without acid. (b) Change in the diameter, (L−Li)/Li, of the SCB. (c) Partial CIE chromatogram showing the change in the developed color. The black dotted line indicates the simulated interference color in the case of a singlelayer film. The optical path length31,33 2d, increased in the clockwise direction from 730 nm (yintercept) to 1040 nm (x-intercept).

Responsiveness to the Acidic Aqueous Solution. The pH responsiveness of the SCB composed of P4VP-PS was observed with respect to the change in diameter and color. A flow cell with a three-way stopcock to supply two solvents, water and an aqueous solution of p-toluenesulfonic acid, were employed for pH control, as illustrated in Figure 4. The monovalency and high acidity were required for easy analysis of responsiveness and low vaporizability was also significant to exclude damage for instruments. 10


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SCB pH 7 (water) pH 3 (TolSO3 H in water)

Figure 4. Method for pH control of the solvent around the SCBs. Two syringe pumps and a three-way stopcock were employed to control the pH of the solvent surrounding the SCBs. When stepwise pH change was required, as in the experiment shown in Figure 5, one syringe was exchanged. The change in diameter and structural color of the SCBs was observed using optical microscopy under transmitted light. The pH switching time was corrected, as described in Experimental section.

When P4VP75 was used, the SCBs showed clear response to acidification in water without using acetone, as shown in Figure 5a. At first, the diameter of the SCBs did not change at pH 7. Subsequent acidification to pH 3.4 resulted in only a very slight expansion of the SCBs. After 100 min at pH 3.4, the SCBs started to expand slowly. Finally, the SCBs expanded in stronger acid at pH 3.0. Infrared spectroscopy (IR) measurements showed the decrease of the peaks of pyridine ring and the increase of the peaks of pyridinium ring, suggesting the ionization of the pyridyl side chains at pH 3.0, as shown in Figure S4 (SI).34,35 The change in the amount of material using a spin-coated polymer film is shown in Figure S5 (SI). SCBs were not dissolved at pH 3 in aqueous solvent as shown in Figure 5a and S6a, however the SBC was dissolved in acidic solvent (pH 3) included small amount of acetone as shown in Figure S7 (SI). In contrast, the acid responsiveness of the SCBs prepared from P4VP25 was lower than that for P4VP75; the SCB did not expand until pH 2.2, as shown in Figure 5b. The reason for this is considered to be the low ratio of pyridyl side chains that involves in diffusivity of the solvent by protonation. The SCBs composed of only polystyrene (PS, P4VP0; P4VP:PS = 0:100) hardly responded to acid in water. When mixed solvent of acetone/water was employed, the responsiveness appeared.28 11


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(b)

50 40

pH 7 3.4

Diameter change / %

(a)

Diameter change / %

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25 20

pH 7 3.0

2.6

2.2

1.4

150

200

15 10 5 0 0

Time / min

50

100

250

300

Time / min

Figure 5. Change in the diameter, (La−Ln)/Ln, of SCBs composed of (a) P4VP75 and (b) P4VP25 under stepwise pH control. Optical images of these SCBs with structural color change are shown in Figure S6 (SI).

Structural Color and Diameter Change under Acidic Aqueous Solution. Figure 6 showed the color and the diameter change of an SCB composed of P4VP75 by pH control. The first stage (0-7 min, indicated with blue arrow in Figure 6b) showed structural color change from yellowish orange to red purple (Figures 6a). The color change is plotted on a CIE chromatogram3,31 (Figure 6c, blue circles) and the shell thickness is calculated to be changed from ca. 540 to 600 nm.26 In this stage, the diameter of the SCB expanded only slightly by 0.1%. The color change is not likely due to the change in the SCB diameter. If the color change at the first stage was achieved via a change in the diameter, the change in color would require ca. 15% reduction in the diameter. The origin of the color change is considered to be due to the ionization and solvation by the surrounding acidic solution. When initial solvation was conducted with the acetone/water mixed solvent under neutral conditions, the structural color did not change significantly until expanding. This is presumably because the effect of the increase in shell thickness by swelling of the polymer film was cancelled by the effect of the decrease in the refractive index by inclusion of the solvent (Figures 3 (between 0 and 27 min) and S9 (SI, for 240 min before acidification). In contrast, P4VP75 absorbed not only solvent but also p-toluenesulfonate anions under acidic conditions, and therefore the effect of the cancellation is not distinctive. 12


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When the shell was swollen enough by solvent, color change along with diameter change28 appeared. The SCB expanded between 7–19 min (second stage, indicated with green arrow in Figure 5b) and shrunk between 19–80 min (third stage, indicated with red arrow in Figure 5b). The behaviors of the second and third stage of the SCB were almost the same as the response in the acetone/water mixed solvent, thus the mechanism was estimated to the osmotic pressure and surface tension of shell polymer. The color change at the second and third stages is also shown in the CIE chromatogram in Figure 6c. The developed colors were reproduced by the optical path length estimated from the diameter on the assumption that the total amount of the SCB shell does not change.

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Figure 6. (a) Micrographs of an SCB composed of P4VP75 showing considerable change in the diameter and color development by pH control. (b) Change in the diameter, (La−Ln)/Ln, of the SCB with respect to pH. White and gray regions represent the pH of surrounding water as pH 7 and 3, respectively. The continual shrinking of diameter until 180 min was shown in Figure S8 (SI). (c) Partial CIE chromatogram showing the development of the SCB color. Blue, green, and red circles, which correspond to the colored arrows in (b), indicate color change without diameter change (stage 1, 0–7 min), expanding (stage 2, 7–19 min), and shrinking (stage 3, 19–80 min), respectively. The black dotted line indicates the simulated interference color in the case of a single-layer film. The optical path length31,33 2d, increased in the clockwise direction from 730 nm (y-intercept) to 1040 nm (x-intercept).

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Repetitive Structural Color and Diameter Change under pH Switching.

The SCBs showed

repetitive change in diameter and structural color when the surrounding solvent was switched between acidic and neutral. Figure 7 shows that the change in diameter can be categorized into two types, one requiring a long time (>60 min) with a large change in diameter (>20%) as described in Figure 5 and another exhibiting a small (ca. 10%) repetitive change in diameter but a fast response (ca. 10 min) by pH switching. The responsiveness for pH was different between under slow expansion and shrinkage. Under slow expansion process, the diameter of SCB did not change at pH 7. In contrast, under the slow shrinkage process, the SCB showed fast shrinking at pH 7 as described below. The structural color change was also observed under pH switching. When the first acidification was conducted at 0 min, the color was changed until 6 min due to an increase in the shell thickness without expansion. The color change subsequently reversed as a result of a decrease in the shell thickness by expansion. As a result, the same red color as the initial (0 min) red color was developed at 23 min. When the surrounding solvent was neutralized at 28 min, the change in diameter almost stopped because the deionized shell polymer became hardened and solvent diffusion was decreased. At the third cycle started at 72 min, a crack was supposed to generate judging from the first shrinking at pH 7 due to outflow via the crack. In the fourth cycle or later, the SCB showed a tendency toward slow shrinking and repetitive change in the diameter caused by pH change. The reason is considered to be enlargement or contraction of the capsule structure with the change in the amount of shell material. In the process of SCB diameter change by pH switching shown in the lower row of Figure 7a, the developed color was also changed. When two SCBs with the same diameter in different pHs were compared, the developed color was different. As discussed in the previous section, the ionization and solvation by the acidic solution should cause the increase of the shell thickness and the change in structural color. The cancellation between the effects of increase of shell thickness and decrease of refractive index, which is observed in the case of swelling by solvent, should not be distinctive. Therefore, the colors were considered to be different for SCBs with the same diameter in different pHs. At 61 and 72 min, the diameter was 2L = 253 m, but the colors of SCB were green and blue, 15


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respectively, and the shell thicknesses estimated from the structural color change were D = ~430 nm (2d = 750 nm) and D = ~400 nm (2d = 700 nm), respectively. This phenomenon occurred not only with slow expansion process of the SCB, but also for the slow shrinking SCB. The pair at 155 and 205 min, which had same diameter (2L = 263 m) but different colors of blue and green, were estimated to have shell thicknesses of D = ~390 nm (2d = 690 nm) and D = ~420 nm (2d = 740 nm).

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Figure 7. (a) Micrographs of an SCB composed of P4VP75 that exhibits a repetitive change in diameter and color development by pH switching. The upper row shows the initial color change without a change in diameter and then subsequent expansion. The lower row shows selected images for comparison. The images at 61 and 72 min show SCB with the same diameter (2L = 253 m) but different colors under slow expansion process. During slow shrinkage process, the SCB showed similar behavior at 155 and 205 min, where the diameter was the same (2L = 263 m) but the colors were different. CIE chromatograms of the lower row SCBs are summarized in Figure S10 (SI). (b) Changes in the diameter, (La−Ln)/Ln, of the SCB by change in the pH. White and gray regions represent the pH of the surrounding solvent at pH 7 and 3, respectively.

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Conclusions. SCBs that are responsive to environmental pH change were prepared from P4VP-PS copolymer and the change in the structural color was studied. Upon acidification the SCBs showed the initial expansion and the subsequent shrinkage in diameter along with the structural color change. The structural color change was explained by the change in shell thickness associated with the change in the diameter and by the change in the total amount of polymer originating from the uptake and release of counter ion. The change in the diameter of SCBs upon sequential switching of pH was classified into two types, one showing a large change with a slow response and another exhibiting a small repetitive change with a fast response by pH switching. These results are expected to lead to the development of smart coloring materials based on the structural color.

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 the Ministry of Education, Culture, Sports, Science and Technology of Japan (MEXT), and the PRESTO program of the Japan Science and Technology Agency (JST).

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