On/off switching of structural color by using multi-bilayered films containing copolymers having azobenzene and biphenyl side groups

Abstract

In this paper, the change in structural color of multi-bilayered films fabricated by stacking copolymers, PMAz–PMBP (m : n), consisting of azobenzene and biphenyl side chain groups, and polyvinyl alcohol (PVA) alternately, was investigated. The multi-bilayered films consisting of polyacrylates having an azobenzene side chain group and PVA were found to reflect a light of specific wavelength, and to show reversible change in the reflection intensity by thermal annealing and irradiation with non-polarized ultra violet (UV) light. However, a long irradiation time was required for the change in reflection intensity of the multi-bilayered films. Therefore, we synthesized PMAz–PMBP (m : n), consisting of azobenzene and biphenyl side chain group having no absorbance over 300 nm, and the PMAz–PMBP (m : n)/PVA multi-bilayered films were fabricated. All the multi-bilayered films showed reversible change in the reflection intensity by thermal annealing and irradiation with non-polarized UV light. The re-coloration speed of PMAz–PMBP (75 : 25)/PVA 20-multibilayered film was faster than others fabricated in this study. It is related to penetration depth of light into the films. The effect of the introduction of the biphenyl side groups in the copolymers on the response behavior of the change in the structural color will be discussed.

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Introduction

Structural coloration has attracted great attention due to its potential applications in optical materials.^1,2 The structural coloration is basically caused by interaction between light and periodic nanostructured-materials,¹ and light absorption with dyes or pigments is not required for the structural coloration. Consequently the structural color is extremely resistant to degradation such as oxidation. One can find structural coloration of various materials in nature, and the structural colors are iridescent and very durable: e.g. opal, peacock wings, and so on. Therefore, if one can control and modulate the wavelength or intensity of the structural color consisting of artificial nano-structured materials by external stimuli,^3,4 the materials will be a candidate for optical applications. Actually, many researchers have focused on the wavelength shift of the structural color by external stimuli toward to the applications for color display and sensors.^4,5 However, there are few studies on the appearance and disappearance (on/off switching) of structural color. In addition to the wavelength shift of structural color, the on/off switching of the structural color will contribute to create a new material, for example, the on/off switchable structural color material is expected to be suitable as a substitute for paper, because of its visibility and readability owing to higher contrast.

Recently, we have reported photochemical on/off switching of the reflection intensity of an artificial structural color material which was fabricated by alternative spin coating of polyacrylates having an azobenzene side chain group and polyvinyl alcohol on a glass substrate, giving multi-bilayered films having precisely controlled layer thickness.^6,7 In such multi-bilayered films, the reflection peak wavelength, λ, depended on the layer thickness of two layers and refractive indices of stacked materials according to Bragg diffraction equation as follows:⁸

where a is the film thickness of each bilayer, n₁, n₂ and d₁, d₂ are the refractive indices and the layer thicknesses of stacked materials 1 and 2 in each layer, respectively, and m is the diffraction order integer. In addition, in the multi-bilayered films, the refractive index of the stacked material also affect the peak reflection intensity, R, of the multi-bilayered films as following equation for the normal incidence of light:⁹where n_H and n_L are the high and low refractive indices of the stacked materials in each bilayer, respectively, n_S is the refractive index of the substrate and q is the number of bilayers. Based on eqn (2), when the number of bilayers is fixed, the reflection intensity will be increased with the increase in the difference between n_H and n_L (n_H > n_L, different state). In contrast, the reflectance becomes extremely low when n_H, n_L and n_S are nearly equal (n_H = n_L = n_S, equal state). In our study, we achieved reversible on/off switching of reflection by switching between different and equal state of refractive indices through the change in the molecular orientation between random and out-of-plane orientations.^6,7

These multi-bilayered films could be fabricated on a flexible substrate.⁷ Therefore, the multi-bilayered film is expected to be the substitute material for a paper, and self-standing reflective color display device without backlight. However, long time irradiation was required for the switching of the reflection of the multi-bilayered films, because of higher absorbance in the UV region due to the azobenzene chromophores in the multi-bilayered films. In this work, therefore, we synthesized polyacrylate copolymers bearing azobenzene and biphenyl side chain groups to improve the switching behavior of the reflection intensity.

Experimental

Materials

Monomers having azobenzene and biphenyl side chain groups, MAz6Ac and MBP6Ac (Scheme 1), were synthesized according to the manner reported earlier (for details see the ESI†).^10,11 The structure and purity of the monomers were identified by NMR and elemental analysis as follows.

Scheme 1 Chemical structures of monomers, MAz6Ac and MBP6Ac, and copolymers, PMAz–PMBP (m : n), consisting of azobenzene and biphenyl side groups used in this study.

M̲A̲z̲6̲A̲c̲: 6-[4-(4-methoxyphenylazo)phenoxy]hexylacrylate. Mp 96–97 °C. ¹H NMR (400 MHz, CDCl₃; δ, ppm): 1.42–2.19 (m, 8H), 3.67 (t, J = 6.0 Hz, 2H), 3.89 (s, 3H), 6.42 (d, J = 17.6 Hz, 1H), 6.95–7.90 (m, 8H). Elemental analysis. Calcd for C₂₂H₂₆N₂O₄ (%): C, 69.09; H, 6.85; N, 7.32. Found: C, 69.05; H, 6.91; N, 7.30.

MBP6Ac: 4-[6-(acryloyloxy)hexyloxy]-4-methoxybiphenyl. Yield: 47% as solid. Mp 89–91 °C. ¹H NMR (400 MHz, CDCl₃; δ, ppm): 1.43–1.85 (m, 8H), 3.84 (s, 3H), 3.97–4.00 (t, 12.7 Hz, 2H), 4.16–4.19 (t, 13.7 Hz, 2H), 5.80–5.82 (d, 10.8 Hz, 1H), 6.09–6.15 (m, 17.6 Hz, 1H), 6.37–6.42 (d, 15.6 Hz, 1H), 6.93–6.99 (m, 4H), 7.44–7.48 (m, 4H). Elemental analysis. Calcd for C₂₂H₂₆O₄ (%): C, 74.55; H, 7.39. Found: C, 74.43; H, 7.43.

Copolymerization of MAz6Ac and MBP6Ac was carried out as follows. The total amount of 2.0 g of two monomers and 20 mg of AIBN were dissolved in 20 mL of DMF in a glass tube. After bubbling nitrogen through the solution, the tube was sealed and shaken in a water bath at 60 °C for 48 h. The reaction mixture was poured into excess methanol. The precipitate was collected and dissolved in THF and poured into methanol again. After repeating this reprecipitation cycle several times, the copolymer was collected and dried in vacuum. The chemical structure of the copolymers, PMAz–PMBP (m : n), used in this study is shown in Scheme 1.

Properties of PMAz–PMBP (m : n)

Molecular weights, M_n and M_w, of PMAz–PMBP (m : n) were determined by gel permeation chromatography (GPC; Jasco 870-UV detector at 254 nm, Shodex KF-804F column, THF as eluent). Thermal properties of PMAz–PMBP (m : n) were examined by means of differential scanning calorimetry (DSC; Seiko SSC-5020) and polarizing optical microscopy (POM; Olympus BHSP polarizing microscope; Mettler FP90 and FP82 hot stage and controller). DSC measurements were carried out by heating at a rate of 10 °C min⁻¹. Photoisomerization behaviors of PMAz–PMBP (m : n) were studied by monitoring changes in absorption spectra with an UV/Vis spectrometer (Shimazu UV-1600) at room temperature under irradiation of UV or visible (Vis) light on their solutions in chloroform and solid films. The solid films of PMAz–PMBP (m : n) were prepared on a glass or quartz substrate by spin coating method using cyclohexanone as solvent. The photoisomerization of azochromphores between trans- and cis-forms was carried out by using a 500 W high pressure mercury lamp (Ushio SX-UI 5000) equipped with an adequate cut filter (Sigma UTVAF-36U or Sigma SCF-50S-42L) for UV (365 nm) or Vis (436 nm) light irradiation.

Fabrication of multi-bilayered film

Multi-bilayered films were fabricated by alternative spin coating of PMAz–PMBP (m : n) and PVA solutions on a glass substrates (25 × 25 mm). Spin coating was typically performed as follows: spin rate was 3400 rpm for 60 s with 4.5 wt% of copolymers in cyclohexanone and 2.5 wt% of PVA in water. In the case of multi-bilayered film prepared by using with PVA and PMAz–PMBP (100 : 0), the thickness of PVA and PMAz–PMBP (100 : 0) layer was ca. 87 nm and 95 nm, respectively. The multi-bilayered film was found to show the reflection band at around 560 nm. The reflection spectra of the multi-bilayered films were measured with a spectrometer (Ocean optics USB2000) with or without thermal annealing (80 °C) and irradiation of UV light with a 500 W high pressure mercury lamp.

Results and discussion

Properties of the copolymers are shown in Table 1. The numbers in the abbreviations, m and n, represent the ratios of MAz6Ac and MBP6Ac in the copolymers. The ratios were determined by means of ¹H NMR spectroscopy (see ESI as Fig. S1†). Nematic and smectic phases were observed for all the copolymers (POM images of PMAz–PMBP (100 : 0) we showed in Fig. S2†).

Table 1 Molecular weights, and thermal properties of the copolymers

Abbreviation	Content ratio (m : n)	Mn	Mw/Mn	Thermal properties^a (°C)
a G, S, N and I represent glassy, smectic, nematic and isotropic phases, respectively.
PMAz–PMBP (100 : 0)	100 : 0	7800	1.42	G 67, S 94, N 135, I
PMAz–PMBP (90 : 10)	90 : 10	7500	1.67	G 73, S 99, N 137, I
PMAz–PMBP (75 : 25)	75 : 25	7900	1.47	G 67, S 100, N 133, I
PMAz–PMBP (60 : 40)	60 : 40	7600	1.71	G 70, S 96, N 133, I
PMAz–PMBP (40 : 60)	40 : 60	7200	1.45	G 72, S 104, N 132, I

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Photoisomerization behaviors of azobenzene side chain group of the copolymers in solution state were investigated by irradiation with UV and Vis lights. Fig. 1 shows the changes in absorption spectra of PMAz–PMBP (75 : 25) in chloroform. Before light irradiation, PMAz–PMBP (75 : 25) in chloroform shows a strong absorption band at around 360 nm and a weak absorption band at around 450 nm those are respectively corresponding to π–π* transition and n–π* transition of the trans-form of the azobenzene chromophores. The UV light irradiation (365 nm, 5 mW cm⁻²) on the chloroform solution caused the decrease in the absorbance at 360 nm as well as the increase in absorbance at 450 nm, indicating the trans-to-cis photoisomerization occurred (Fig. 1A). By following Vis light irradiation (436 nm, 80 mW cm⁻²), the cis-to-trans reverse photoisomerization was brought about as shown in Fig. 1B. Other PMAz–PMBP (m : n) in chloroform also showed a similar photoisomerization behavior by irradiation with UV and Vis lights (Fig. S3 in ESI†).

Fig. 1 Changes in absorption spectra of PMAz–PMBP (75 : 25) in chloroform by irradiation with UV light (365 nm, 5 mW cm⁻²) (A) and following Vis light (436 nm, 80 mW cm⁻²) (B).

In our previous work, PMAz–PMBP (100 : 0) was found to exhibit the transformation from the random orientation to the out-of-plane orientation in the solid film by annealing in the smectic phase.⁶ The change in the molecular orientation of PMAz–PMBP (75 : 25) in the solid film is shown in Fig. 2. Upon annealing at 80 °C, the absorbance at 350 nm was significantly decreased, indicating the formation of the out-of-plane orientation state (Fig. 2A). Polarized absorption spectra of the PMAz–PMBP (75 : 25) film after annealing at an incident angle of 40° to the normal of the film were measured by rotating the polarization plane of the monitor light (Fig. 2B, inset). The π–π* absorption band at the rotation angle of 0° was slightly shifted to shorter wavelength (A₀), whereas at 90° a weak π–π* band was observed (A₉₀). It is indicated that thermal annealing in the smectic phase caused not only to the formation of the out-of-plane orientation, but also to the formation of H-aggregates of the azobenzene chromophores.

Fig. 2 Changes in absorption spectra of PMAz–PMBP (75 : 25) solid film by thermal annealing (80 °C, 10 min) (A) and irradiation with UV light (365 nm, 28 mW cm⁻²) (C). Polarized absorption spectra at an incident angle of 40° to the normal of the film by rotating the polarization plane of the monitored light (A₀ and A₉₀) (B) and change in polar plots of the PMAz–PMBP (75 : 25) film at 333 nm at an incident angle of 40° by annealing at 80 °C and irradiation of UV light (circle: after annealing, square: after UV light irradiation) (D).

On the other hand, the absorbance around 350 nm and 450 nm increased by following UV light irradiation as shown in Fig. 2C. The change in the absorbance may be attributed not only to transformation from the out-of-plane orientation to the random orientation, but also to the trans-to-cis photoisomerization. Based on the comparison of this absorbance with that of the random orientation, about 30–40% of the cis-form is required for the transformation through the disorganizing effect of the cis-form.

Fig. 2D shows the transformation of the molecular orientation by annealing and following UV light irradiation. The disappearance of the angle dependence by UV light indicates clearly the transformation from the out-of-plane orientation to the random orientation through the disorganizing effect of the cis-form (square in Fig. 2D). Other films also showed reversible transformation of molecular orientation between out-of-plane orientation and random orientation state by annealing and UV light irradiation (Fig. S4 in ESI†).

Combination of thermal annealing and UV light irradiation caused not only the change in the molecular orientation between the out-of-plane and photo-induced random states, but also the modulation of the refractive index of the copolymer layer. The ordinary and extraordinary refractive indices of PMAz–PMBP (100 : 0), n_o and n_e, are estimated to be 1.48 and 1.81 respectively by using wedge cell.¹² By using the refractive indices, the refractive index of random orientation state (n_av) is estimated to be 1.60. Therefore, it is expected that the refractive index of PMAz–PMBP (100 : 0) will be controlled between 1.60 and 1.48 by thermal annealing and photoirradiation through the change in the molecular orientation.

Fig. 3 shows the absorption spectra of the copolymers and MBP6Ac in chloroform. In spite of the same concentration of the copolymers in chloroform, the absorbance around at 360 nm was decreased with the decrease in the content of azobenzene side chain groups in the copolymers, while the absorbance in the range of 270–300 nm was increased.

Fig. 3 Absorption spectra of PMAz–PMBP (m : n) and MBP6Ac in chloroform solution. (Solid line) PMAz–PMBP (100 : 0), (dotted line) PMAz–PMBP (75 : 25), (dot-and-dash line) PMAz–PMBP (40 : 60), (two-dot line) MBP6Ac.

Fig. 4A shows the absorption spectra of PMAz–PMBP (m : n) fresh films prepared on a quartz substrate. It should be noted that not only decrease absorbance around 360 nm but also shift of absorption maxima were observed. Absorption maxima were shifted from 341 to 360 nm with increasing the content of biphenyl side groups. As explained above in Fig. 2, the polarized π–π* absorption band at the rotation angle of 0° (A₀) was shifted to shorter wavelength by thermal annealing, it is indicated that thermal annealing caused the formation of H-aggregates of the azobenzene chromophores. In spite that, as shown in Fig. 4B, the shift of the A₀ was suppressed with increasing the content of biphenyl side groups. Namely, the higher the content of biphenyl side groups in the copolymers, the more difficult the formation of H-aggregates becomes.

Fig. 4 Changes in absorption spectra of PMAz–PMBP (m : n) solid film (A) and polarized absorption spectra after thermal annealing (80 °C, 10 min) at an incident angle of 40° to the normal of the film by rotating the polarization plane of the monitored light (A₀) (B).

Furthermore, based on the results in Fig. 4B, it is likely that the molecular orientation upon the annealing in smectic phase is influenced by incorporation of biphenyl groups in the copolymers. To evaluate the molecular orientation behavior, the order parameter, S, of the out-of-plane orientation was estimated according to the following equation by using the absorbances, A₀ and A₉₀.¹³

Table 2 shows the change in the order parameter as a function of content ratio in PMAz–PMBP (m : n). The order parameters were slightly decreased, however, introducing biphenyl side chain groups in the copolymers does not strongly affect to form out-of-plane orientation.

Table 2 Change in the order parameter and refractive indices as a function of molecular ratio in PMAz–PMBP (m : n)

Abbreviation	Order parameter^a	Refractive index^b
a The order parameter was evaluated from eqn (3).b The refractive indices of PMAz–PMBP (m : n) solid film on silicon substrate were measured by ellipsometry.
PMAz–PMBP (100 : 0)	0.61	1.613
PMAz–PMBP (90 : 10)	0.57	—
PMAz–PMBP (75 : 25)	0.54	1.601
PMAz–PMBP (60 : 40)	0.47	—
PMAz–PMBP (40 : 60)	0.52	1.593

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On the other hand, refractive indices of the copolymers were decreased with the increase in the content of biphenyl side chain groups in the copolymers. It might affect the reflection intensity of the multi-bilayered films.

The multi-bilayered films were fabricated by alternative spin coating of copolymer cyclohexanone and PVA aqueous solutions on a glass substrate. The multi-bilayered films were found to reflect a light of specific wavelength depending on the thickness and refractive indices of stacked layers according to Bragg equation.

Fig. 5 shows the reflection spectra of the PMAz–PMBP (m : n)/PVA 20-bilayered fresh films. The reflection intensity of the fresh films depends on the difference in the refractive indices between stacked layers and number of bilayers. In the case of PMAz–PMBP (100 : 0)/PVA 20-bilayered fresh film, the reflection intensity was about 90%, because of large difference in the refractive indices of between PVA and PMAz–PMBP (100 : 0) with random orientation. Contrary, the film containing PMAz–PMBP (40 : 60) showed smaller reflection intensity compared to others due to its lower refractive index. In addition, the lower refractive index also contributes to the blue shift of the reflection wavelength.

Fig. 5 Reflection spectra of PMAz–PMBP (m : n)/PVA 20-bilayered films. (Solid line) PMAz–PMBP (100 : 0), (dotted line) PMAz–PMBP (75 : 25), (dot-and-dash line) PMAz–PMBP (40 : 60).

Reflection intensity, R, of the multi-bilayered films can be calculated eqn (2) for the normal incidence of light as described above. Based on eqn (2), when the number of bilayers is fixed, the reflection intensity depends on the refractive index difference between two layers of multi-bilayered films. The multi-bilayered films consisting of copolymers and PVA have high reflection intensity due to large refractive index difference between PVA (n_PVA = 1.50) and copolymers with random orientation state (n_av = 1.60). The refractive index of copolymers can be modulated from n_av = 1.60 to 1.48 which is nearly equal to n_PVA = 1.50 by change in molecular orientation state from random orientation to out-of-plane orientation state. Consequently, the reflection intensity becomes extremely low.

The change in the reflection intensity was observed for all the PMAz–PMBP (m : n)/PVA 20-bilayered films via the reversible change in the molecular orientation between the out-of-plane and photo-induced random orientations by combination of thermal annealing and irradiation with UV light as shown in Fig. 6 (see also Fig. S5†). After fabrication of the PMAz–PMBP (75 : 25)/PVA 20-bilayered film by alternative stacking of copolymer and PVA layers, the fresh film showed the reflection band at around 560 nm. The reflection band was disappeared by annealing at 80 °C for 10 min, leading to the equal state where refractive index of PMAz–PMBP (75 : 25) (n_{(75 : 25)}) was almost the same as n_PVA (n_{(75 : 25)} = n_PVA). Then, the reflection band was recovered by UV light irradiation for 5 min to cause the disorganization of the out-of-plane orientation, resulting in transformation from the equal state to the different state (n_{(75 : 25)} > n_PVA).

Fig. 6 Change in reflection spectra of the PMAz–PMBP (75 : 25)/PVA 20-multibilayered film by thermal annealing (80 °C, 10 min) and irradiation with UV light (λ = 365 nm, 28 mW cm⁻²). (Solid line) fresh film, (dotted line) after annealing and (dot-and-dash line) UV light irradiation.

To evaluate the effect of the introduction of the biphenyl side chain groups in the copolymers on the response behavior of the change in the structural color, the reflection intensities of the multi-bilayered films were plotted as a function of UV light irradiation time in Fig. 7. We noted that the case of the re-coloration by irradiation with UV light. 900 s of UV light irradiation was required for the change in reflection intensity of the PMAz–PMBP (100 : 0) or PMAz–PMBP (90 : 10)/PVA 20-multibilayered film. In contrast, the case of multi-bilayered films containing PMAz–PMBP (75 : 25), PMAz–PMBP (60 : 40) and PMAz–PMBP (40 : 60) which introduced biphenyl side chain groups, was required only 120–300 s UV light irradiation. Irradiation time required for re-coloration was reduced to one third or less.

Fig. 7 Time courses of change in reflectance of the PMAz–PMBP (m : n)/PVA 20-bilayered films by irradiation with UV light (λ = 365 nm, 28 mW cm⁻²).

One of causes is related to penetration depth of light into the film. In order to estimate the absorbance of multi-bilayered films, we fabricated the PMAz–PMBP (m : n)/PVA 10-bilayered films and measured the absorbance of the films. The absorbances of the films at around 350 nm corresponding to π–π* transition of the azobenzene chromophore was decreased with decreasing the relative content of the azobenzene group in the copolymers while the absorbance of the PMAz–PMBP (100 : 0)/PVA 10-bilayered film was much higher than 4, as shown in Fig. 8.

Fig. 8 Absorption spectra of PMAz–PMBP (m : n)/PVA 10-bilayered film. (Solid line) PMAz–PMBP (100 : 0), (dotted line) PMAz–PMBP (75 : 25), (dot-and-dash line) PMAz–PMBP (40 : 60).

Therefore, it is expected that the photoisomerization of azobenzene chromophores at inner and opposite positions in the films to the irradiated side would be very slow, especially for the PMAz–PMBP (100 : 0)/PVA 20-bilayered film. As mentioned above, 30–40 mol% of cis-form in a unit volume is required for the transformation from the out-of-plane orientation to the photo-induced random state. In the case of the PMAz–PMBP (100 : 0)/PVA 20-bilayered film, at near the film surface the transformation to the photochemical random orientation will be faster than others.

However, the amount of the cis-form will be decreased rapidly with the depth of the film from the irradiation side. In contrast, in the case of the PMAz–PMBP (40 : 60)/PVA 20-bilayered the UV light can penetrate deep in the film compared to others. As a result, the photoisomerization of the azobenzene chromophores may be brought about in the whole of the film. It seems, however, that the transformation to the photo-induced random state will occur less often than others, because of low content of the azobenzene side groups. Consequently, the PMAz–PMBP (75 : 25)/PVA 20-bilayered film showed the fastest re-coloration speed among the films.

Therefore, it is expected that the photoisomerization of azobenzene chromophores at inner and opposite positions in the films to the irradiated side would be very slow, especially for the PMAz–PMBP (100 : 0)/PVA 20-bilayered film. As mentioned above, 30–40 mol% of cis-form in a unit volume is required for the transformation from the out-of-plane orientation to the photo-induced random state. In the case of the PMAz–PMBP (100 : 0)/PVA 20-bilayered film, at near the film surface the transformation to the photochemical random orientation will be faster than others. However, the amount of the cis-form will be decreased rapidly with the depth of the film from the irradiation side. In contrast, in the case of the PMAz–PMBP (40 : 60)/PVA 20-bilayered the UV light can penetrate deep in the film compared to others. As a result, the photoisomerization of the azobenzene chromophores may be brought about in the whole of the film. It seems, however, that the transformation to the photo-induced random state will occur less often than others, because of low content of the azobenzene side groups. Consequently, the PMAz–PMBP (75 : 25)/PVA 20-bilayered film showed the fastest re-coloration speed among the films.

Another cause may be related to the formation of H-aggregate in copolymers. As mentioned above in Fig. 4B, the formation of the H-aggregate becomes difficult by increasing the content of the biphenyl side groups in the copolymers. Ichimura reported about the photo-orientation of polyacrylates having p-methoxyazobenzene through different alkylene spacers under visible light irradiation, and revealed that the longer alkyl spacer enhance not only the formation of the H-aggregates of azobenzene chromophores but also suppressing photoorientation.¹⁴

Therefore, in our results, the out-of-plane orientation of PMAz–PMBP (75 : 25) and PMAz–PMBP (40 : 60) in film was easily disorganized with UV light irradiation, as a result, re-coloration speed was faster than PMAz–PMBP (100 : 0)/PVA film.

To demonstrate the influence of re-coloration speed on the patterning, the PMAz–PMBP (m : n)/PVA 20-bilayered films were irradiated through a photo-mask by varying the intensity of UV light. Photographs of the patterned films are given in Fig. 9. No difference in the quality of the patterns was observed when the films were patterned by irradiation with UV light of 30 mW cm⁻² for 3 min (Fig. 9B), whereas the irradiation with UV light of 5 mW cm⁻² for 3 min caused significant change in the contrast of the films as shown in Fig. 9A. The pattern of the PMAz–PMBP (75 : 25)/PVA 20-bilayered films was evidently clear compared to others.

Fig. 9 Photograph of PMAz–PMBP (m : n)/PVA 20-bilayered film after irradiation of different intensity UV light (5 mW cm⁻² for 3 min (A), 30 mW cm⁻² for 3 min (B)) with photomask.

On the basis of eqn (1), the wavelength of the structural color from this type of materials could be controlled easily by changing the layer thickness of the multi-bilayered films. Fig. 10 shows the reflection spectra of PMAz–PMBP (75 : 25)/PVA 20-bilayered films those were fabricated by adjusting the spin rate (1800, 3200, 4800 rpm) of spin coating from the same solutions, PMAz–PMBP (75 : 25) cyclohexanone and PVA aqueous solutions.

Fig. 10 Reflection spectra of the PMAz–PMBP (75 : 25)/PVA structural color 20-multibilayered films (A). Photograph and optical microscope image of the patterned films ((B) and (C)).

After fabrication of the films by the alternating spin coating, the intensities of the films showing blue, green and red structure colors were nearly at 90% as shown in Fig. 10A, and then the thermal annealing and UV light irradiation decreased and increased the reflection intensity of all the films. The results indicate that the resolution of pattern was roughly higher than 90 μm. All the three films showed almost similar photo-response properties such as the response speed and the resolution of the pattern formed on the films by using the photomask.

The UV irradiation time required for the re-coloration is dependent on the light intensity. The more the light intensity rises, the shorter the irradiation time required becomes as shown in Fig. 9. In order to demonstrate the possibility of the film for an alternative to a paper, the coloration of the PMAz–PMBP (75 : 25)/PVA 20-bilayered film was carried out by using UV laser (375 nm, 250 mW cm⁻²). The scan speed was 130 μm s⁻¹. Fig. 11 shows the numbers written in such way by scanning UV laser (please see movie in ESI†). Based on the result, it is expected that the multi-bilayered films have a potential as a recording medium by using pencil with light.

Fig. 11 Photograph of PMAz–PMBP (75 : 25)/PVA red structural color 20-multibilayered film after annealing at 80 °C and irradiation with UV light by using UV laser (λ = 375 nm, 250 mW cm⁻², scan speed was 130 μm s⁻¹). The scale bar is 500 μm.

Conclusions

In this study, the copolymers consisting of MAz6Ac and MBP6Ac were synthesized. The PMAz–PMBP (m : n)/PVA multi-bilayered films were fabricated, and the effect of introduction of biphenyl side groups in the copolymers on the response behavior of the change in the structural color was investigated. The PMAz–PMBP (75 : 25)/PVA 20-multibilayered film showed the fastest re-coloration speed in all the films. Based on the results, response behavior of structural color of PMAz–PMBP (m : n)/PVA multi-bilayered films was related not only the penetration depth of UV light in the film and the amount of azobenzene side group in the PMAz–PMBP (m : n), but also the formation of H-aggregates in copolymers. The patterns of the PMAz–PMBP (m : n)/PVA 20-bilayered film through photomask was evidently clear even low intensity of UV light. Furthermore, it was possible to write the number on the multi-bilayered film by scanning the UV laser at 130 μm s⁻¹. These films will be candidate as recording materials such as a substitute material for paper.

Acknowledgements

This work was supported by JSPS KAKENHI Grant 23350115, and the Core Research for Evolutional Science and Technology (CREST) of the Japan Science and Technology Agency (JST).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra13110c

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