Low-Cost, Rapidly Responsive, Controllable, and Reversible

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A low-cost, rapidly-responsive, controllable and reversible photochromic hydrogel for display and storage Yongqi Yang, Lin Guan, and Guanghui Gao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00235 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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ACS Applied Materials & Interfaces

A low-cost, rapidly-responsive, controllable and reversible photochromic hydrogel for display and storage Yongqi Yang, Lin Guan, Guanghui Gao* Polymeric and Soft Materials Laboratory, School of Chemical Engineering, and Advanced Institute of Materials Science, Changchun University of Technology, Changchun, 130012, P. R. China. E-mail: [email protected] (Guanghui Gao)

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ABSTRACT

Traditional optoelectronic devices without stretchable performance could be limited for substrates with irregular shape. Therefore, it is urgent to explore a new generation of flexible, stretchable and low-cost intelligent vehicles as visual display and storage devices, such as hydrogels. In the investigation, a novel photochromic hydrogel was developed by introducing negatively charged ammonium molybdate as a photochromic unit into polyacrylamide via ionic and covalent crosslinking. The hydrogel exhibited excellent properties of low cost, easy preparation, stretchable deformation, fatigue-resistance, high transparence, and second order response to external signals. Moreover, the photochromic and fading process of hydrogels could be precisely controlled and repeated under the irradiation of UV light and exposure of oxygen at different time and temperature. The photochromic hydrogel could be considered applied for artificial intelligence system, wearable health care device, and flexible memory device. Therefore, the strategy for designing a soft photochromic material would open a new direction to manufacture flexible and stretchable devices. KEYWORDS : hydrogels, photochromism, flexible devices, patterning, memory devices


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Introduction The emergence and development of wearable devices would provide new direction in some areas such as artificial intelligence system and wearable health care device.1 However, the wearable equipment is still limited to glasses and watches. This is the fact that critical devices, such as display devices and memory devices, cannot be flexible and stretchable. A soft and stretchable device like skin is highly desirable for substrates with irregular shape.2–7 Hydrogels can be as soft as tissues, and as stretchable as elastomers.8–12 Thereby, they are considered to be next ideal generation carriers for future soft wearable equipment. Recent investigations exhibited some flexible and stretchable hydrogels as potential devices for conductors13–15, displays16,17, sensors18,19, capacitors20–24, transistors25–27 and batteries28,29. In particular, the display device as an important medium for human-computer interaction has been widely designed and produced and relies on light-emitting or color-changing. So some lightemitting substances (zinc sulfide30–32) or color-changing substances (4,4-bipyridines33–35, spiropyrans36–39 and diarylethenes40) were introduced into hydrogels to realize visual display. However, hydrogels modified by zinc sulfide require an alternating current with high-frequency, high-voltage, which is not conducive to miniaturization of wearable equipment. Moreover, hydrogels assisted by color-changing substances cannot avoid toxicity and high cost. Therefore, it is essential to explore easily responsive, non-toxic and inexpensive alternatives to achieve the above performance. Polyoxometalate (POMs) is an inorganic material with reversible photochromic property. For example, ammonium molybdate (Mo7) is cheap and mass-produced, and its photochromic mechanism has been clearly revealed. Briefly, when Mo7 was irradiated by the ultraviolet light, Mo(VI) in Mo7 was reduced to Mo(V) by capturing the electron, exhibiting a photochromic


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appearance from colorless to blue. Subsequently, oxygen can again capture the electron from Mo(V), which was oxidized to Mo(VI). As a result, the blue color would be recovered to colorless41. During the process, the spatial structure of Mo7 was not changed, thereby, the photochromic property was very stable. Consequently, it was envisioned that the incorporation of Mo7 into the hydrogel network would endow hydrogels with photochromic ability. The photochromic hydrogels as flexible and stretchable devices would be applied for artificial intelligence system, wearable health care device and flexible memory device. In the investigation, a novel photochromic hydrogel was explored by the introduction of negatively charged ammonium molybdate into polyacrylamide via ionic and covalent crosslinking. The hydrogel exhibited excellent properties of low cost, easy preparation, stretchable deformation, fatigue-resistance, high transparence, and second order response to external signals. Moreover, the photochromic and fading process of hydrogels could be measured repeatedly under the irradiation of UV light and exposure of oxygen. The photochromic hydrogels exhibited good photochromic properties. It had a faster photochromic rate than the most commonly reported spiropyran hydrogels42-45. And spiropyran-based hydrogels could fade in visible light, whereas POM-based hydrogels remained stable under visible light. More importantly, due to the mechanochromism properties of spiropyran, this meant that the external force could destroy the displayed images. In contrast, the POM-based hydrogel remains stable under external force, which was more suitable for tensile display. Besides, compared to the previous POM-based materials, the photochromic efficiency of this POM-based hydrogel has been significantly improved from several minutes to as fast as 2s. It was envisioned that the photochromic hydrogel could be potentially applied as flexible and stretchable devices for visual


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display and storage. Therefore, the strategy for designing a soft photochromic material would open next generation of flexible and stretchable devices. Experimental Section Materials: Acrylamide (AAm, 99.0%), N,N′-Methylenebis(acrylamide) (MBA, 99.0%), [2(Methacryloyloxy)ethyl] trimethyl ammonium chloride solution (DMC, 75% aqueous), potassium persulfate (KPS, 99.5%), and N,N,N’,N’-tetramethylethylenediamine (TMEDA, 99.5%) were supplied by Shanghai Aladdin Reagent Co. Ltd. Ammonium molybdate [(NH4)6Mo7O24·4H2O] (Mo7, 99.0%) was supplied by Tianjin Kaida Chemical Plant. Deionized water (18.2 MΩ·cm resistivity at 25°C) was used in the experiment. Preparation of hydrogels: PAAm-Mo0 was prepared as follows: AAm (120 mM, 8.53 g), MBA (0.24 mM, 37 mg) and KPS (0.6 mM, 162 mg) were firstly dissolved in water (30 mL). Then, TMEDA (0.6 mM, 90.6 µL) was added to the previous solution and stirred for 30 s. The resulting solution was poured into molds and carried out at 35°C for 2 h to obtain PAAm-0 hydrogels. PAAm-Mo1 was prepared as follows: AAm (120 mM, 8.53 g), MBA (0.24 mM, 37 mg), DMC (3 mM, 0.833 g) and KPS (0.6 mM, 162 mg) were dissolved in water (30 mL). Then, TMEDA (0.6 mM, 90.6 µL) was added to the solution and stirred for 30 s. The resulting solution was poured into molds and carried out at 35°C for 2 h to obtain precursor hydrogels. The precursor hydrogels were soaked in Mo7 solution (0.0167 mol·L-1) for 3h to obtain PAAm-Mo1 hydrogels (shown in Scheme 1). PAAm-Mo2 and PAAm-Mo3 hydrogels were prepared as the previous procedure. The corresponding amount of DMC was 1.67 g (6 mM) and 2.5 g (9 mM), and the concentration of Mo7 was 0.0334 mol·L-1 and 0.05 mol·L-1, respectively. The contents of each component were shown in Table 1 The concentration of Mo7 in an aqueous medium is determined by the ratio of DMC in the hydrogel at a molar ratio of 1: 6 for Mo7/DMC.


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Scheme 1. Preparation of photochromic hydrogels and principle of photochromic.

Table 1. Recipes of the hydrogels H2O (mL)

AAm (mM)

MBA (mM)

DMC (mM)

KPS (mM)

TMEDA (mM)

Mo7 aqueous (mol·L-1)

PAAm-Mo0

30

120

0.24

0

0.6

0.6

0

PAAm-Mo1

30

120

0.24

3

0.6

0.6

0.0167

PAAm-Mo2

30

120

0.24

6

0.6

0.6

0.0334

PAAm-Mo3

30

120

0.24

9

0.6

0.6

0.0500

Photochromic exhibition of hydrogels: The samples for photochromic experiment were treated by the ultraviolet light. The UV-light irradiation source was obtained from a microprocessor controlled light curing system (Intelli-ray 600, maximum light intensity of 150 mw/cm2, Uvitron International). The height of the sample from the light source was set as 20 cm. The fading experiment of hydrogel were carried out in a sealed plastics bag under the atmosphere of air or oxygen. During the experiment, 2 or 3 drops of water was added into the plastics bag to prevent desiccation of the hydrogel samples. The photochromic process and the fading process were briefly described in Scheme 1


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Absorbance and transmittance measurement: The UV-Vis absorbance and transmittance spectra were measured via a UV-visible spectroscopy (Varian Cary 50). The scanning range was set from 800 nm to 200 nm. The scanning speed was 4800 nm·min-1. All samples were fixed on the sample rack and the thickness was set as 2.5 mm. Mechanical test: Tensile measurement was performed on a tensile tester (SHIMADZU, model AGS-X, 100N, Japan) at room temperature, with a constant velocity of 40 mm·min-1. All hydrogels were cut into a dumbbell shape with length of 30 mm, gauge length of 12 mm, width of 4 mm, and thickness of 3 mm. For tensile loading–unloading cyclic tests, the hydrogel samples were stretched to 50% strain and then returned to the initial length with a constant velocity of 1000 mm·min-1. The cycle number was set as 1000 times. Results and Discussions Photochromic behavior of hydrogels In the photochromic hydrogels, the photochromic unit Mo7 was "bound" to the hydrogel networks by the electrostatic interaction with DMC. This could limit the free movement of Mo7 and enhance the mechanical properties of hydrogels. Since the ratio of Mo7 to DMC is 1: 6, the concentration of Mo7 solutions were set to 1/6 of the DMC concentration in the hydrogels. All original hydrogels exhibited high optical transparency without any color in Figure 1a. The visible light transmittance measurement also indicated that four hydrogels exhibited a high transmittance with over 93%–97% in the range of 400-800 nm although the transparency of hydrogels showed a slight decrease with the increase of Mo7 (Figure 1b).


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Figure 1. (a) Transparency exhibition of photochromic hydrogels, (b) corresponding visible light transmittance of hydrogels (c) Photos of all photochromic hydrogels after different UV irradiation times (UV light irradiation power P = 100%), (d) absorbance measurement of all photochromic hydrogels after irradiation with UV light (P = 100%, UV light irradiation time T = 30s), (e) absorbance measurements of PAAm-Mo3 hydrogels after different UV irradiation times (P = 100%), (f) absorbance at 740 nm (P = 100%), (g) photos of PAAm-Mo3 hydrogels after different UV irradiation power (T = 15s), (h) absorbance measurement of PAAm-Mo3 hydrogels after different UV irradiation power (T = 15 s) and (i) absorbance at 740 nm (T = 15 s)


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Subsequently, the photochromic behavior of hydrogels was measured through the irradiation of UV light. It could be seen clearly from Figure 1c that PAAm-Mo0 without any Mo7 had no photochromic appearance. However, PAAm-Mo1, PAAm-Mo2 and PAAm-Mo3 exhibited photochromic behavior after only 5 seconds, and the darkness of color increased with the increase of Mo7 concentration and irradiation time. From the result of UV-vis spectrum, the peak intensity at 620 nm and 740 nm increased significantly with the increase of Mo7 concentration for hydrogels irradiated by UV after 30 seconds (Figure 1d), indicating an extremely rapid photochromic response. Moreover, the UV-vis absorbance peaks of PAAm-Mo3 gradually increased at the different irradiation time ranging from 0 s to 30 s (Figure 1e). The corresponding maximum absorbance intensity at a wavelength of 740 nm exhibited an excellent “S” curve profile fitting with the irradiation time (Figure 1f). The results indicated completely controllable photochromic behavior for hydrogels via fixing the irradiation time. In addition, the intensity of the UV light could also have an effect on the photochromic behavior of hydrogels. From Figure 1g, the color darkness of PAAm-Mo3 gradually increased with the increase of the UV light intensity after irradiation for 15 seconds. The corresponding absorbance intensity increased with the increase of the irradiation levels ranging from 0% to 100% in Figure 1h. Similarly, a curve could be fitted between the UV-irradiation intensity and the absorbance peak strength at the wavelength of 740 nm, indicating that the photochromic behavior of hydrogels could be precisely controlled by adjusting the irradiation levels in Figure 1i. Fading behavior of hydrogels The photochromic hydrogels were able to fade gradually in the air. For example, allochroic PAAm-Mo3 was placed in the bag with air and the blue color would completely fade into


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colorless after 6.5 hours in Figure 2a. Moreover, the external atmosphere significantly influenced the fading rate of photochromic hydrogels. The effect of ambient light on the fading process was also observed and the result indicated that the fading behavior was not changed for photochromic hydrogels even the sample was placed in the dark field (Figure 2b). However, it was found from Figure 2c that the fading rate significantly increased under the atmosphere of oxygen comparing to air, indicating that the fading behavior of photochromic hydrogels would be controlled by oxidation process.


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Figure 2. (a) The fading time of the PAAm-Mo3 hydrogels after photochromism (P = 100%, T = 30s), (b) the fading time of PAAm-Mo3 hydrogels under different lighting conditions after photochromism, (c) in different gas atmospheres, (d) under different temperature conditions (e) the relationship between the fading rate and the temperature of PAAm-Mo3 hydrogels after photochromism and (f) repeatable photochromism of PAAm-Mo3 hydrogels. (Photochromic process: P=100%, T=30s)


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Expect oxidation, it was also found that temperature was one of the main factors to control the fading process of photochromic hydrogels. The higher the temperature, the faster the fading rate (Figure 2d). When the temperature was raised to 40°C, the fading time was shortened to 2.25 h. The fastest fading time of hydrogel was only 0.5 h at temperature of 80°C. Moreover, it could be seen clearly that the fading rate was quantitatively fitted to temperature in Figure 2e, indicating the fading behavior was precisely controlled by setting temperature. It is worth mentioning that the fading hydrogel could have an iterative photochromic behavior and change color again under the irradiation of UV light for 30 s in Figure 2f. The hydrogels contained two types of crosslinking: chemical crosslinking and ionic crosslinking. Chemical crosslinking provided a stable network of hydrogels and ionic crosslinking had a synergistic effect on the network. Since Mo7 was "bound" to the PAAm network by the electrostatic interaction with DMC, Mo7 could be closely linked to the amino groups of AAm units. This facilitated the formation of hydrogen bonds between Mo7 and AAm units. According to the principle of photochromism, the formation of hydrogen bonds migh be the reason for the fast response of the photochromic hydrogels. During photochromic process, the rate was controlled precisely by UV irritation time and light intensity. Under the same illumination time, the degree of photochromism is related to the concentration of Mo7. The greater the concentration of Mo7, the darker the photochromic color. During the fading process, the oxygen concentration could have a significant impact on the fading rate, indicating that the fading process required the participation of oxygen. This phenomenon fit in with its fading mechanism.41 It should be noted that the higher the temperature, the faster the fading rate. Mainly, oxygen in the air diffused into the hydrogels as a fast rate and the chemical reaction was accelerated at high temperature.


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Visual display and storage Based on the photochromic property, different patterns could be drawn in the body of hydrogels. For example, a mask covered the surface of hydrogels and then was irradiated by UV light for 30 s. After removing the mask, the hydrogels would display corresponding visual badges in Figure 3a. Moreover, the photochromic hydrogel also exhibited multiply reproducible different patterns when the sample was treated by switching the environmental conditions between UV light and oxygen. As can be seen from Figure 3b, the three cycles for patterning and erasing images indicated excellent repeatable recording information for photochromic hydrogels. Correspondingly, the transmittance of hydrogels at the wavelength of 740 nm was measured in Figure 3c during the photochromic and fading process. The hydrogel transparency was almost unchanged for at least 5 cycles, exhibiting stable molecular structure of hydrogels. In addition, the QR 2-dimensional code recorded in the hydrogel could be easily recognized by smartphone in Figure 3d, endowing hydrogels with a potential ability as a device for information storage. To realize permanent information storage, some strategies were attempted including directly freezing at -20°C and vacuum-drying at 80°C. As can be seen from Figure 3(e,f), the photochromic hydrogels treated by freezing or vacuum-drying could store visual information for at least 180 days, possibly preventing diffusion of oxygen. To our surprise, the photochromic hydrogels after thawing or swelling could still have the repeatable ability of patterning and erasing without any influence.


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Figure 3. (a) Patterning process of PAAm-Mo3 hydrogels, (b) repeatable pattern writing and erasing of PAAm-Mo3 hydrogels, (c) measurement of transmittance at 740 nm during repeatable writing and erasing, (d) the process of writing a QR code on a PAAm-Mo3 hydrogel, and QR code was recognized by the smartphone, (e) Long-term preservation of the pattern by freezing, (f) by vacuum drying. (Photochromic process: P=100%, T=30s) (g) the process of cyclic stretching after PAAm-Mo3 hydrogel had been patterned, and (h) stress measurement during cyclic stretching. (Photochromic process: P=100%, T=30s)


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The fatigue-resistant property of photochromic hydrogels was also considered in the practical applications. For the patterned hydrogel in Figure 3g, the sample could undergo stretchable deformation with the stain of 50% for at least 1000 cycles and still remain its original state. The corresponding mechanical strength was measured and exhibited an invariable level in Figure 3h. Therefore, the mechanically photochromic hydrogels would open more possibilities for flexible display and storage as wearable devices. Storing binary data For data storage, a high storage density means that more data can be stored in hydrogels with the same size. A mask with “CCUT” was printed as small as possible and the character width was around 180-190 µm by optical microscopy in Figure 4a. Subsequently, the hydrogel sample was patterned by the mask and exhibited the corresponding character with a width of 240-250 µm in Figure 4b, indicated that the photochromic hydrogel could be acted as binary memory devices to store visual information with a unit size of over 250 µm. Moreover, another efficient pattern in the hydrogel was explored to increase the storage density for UV-light signals. For optical storage devices, each unit could store only 1-bit as Single-Level Cell (SLC). For example, the conventional optical storage information was denoted by "0" for the unit without irradiation and the photochromic unit was denoted as "1" to realize binary storage. However, it was found that the NAND Flash storage device could achieve 2 or more bits in each unit. Inspired by the memory rule of the NAND Flash, possible 2 bits Multi-Level Cell (MLC) could be realized in the hydrogels. Since the gray level of photochromic hydrogels could be adjusted by changing UV-irradiating time and quantitatively detected by UV-vis measurement. As can be seen from Figure 4(c,d), the blank unit without irradiation was denoted as "00" and the hydrogel unit with irradiation for 5s was denoted as "01". According to the rule, the unit for 10s was


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denoted as "10", and the unit for 15s was denoted as "11". The corresponding absorbance intensity was measured in Figure 4d. For these units with different gray levels, the 2-bits data could be store in one unit as MLC. The properties endowed photochromic hydrogels as potential optical storage materials. For example, when using 1-bit SLC storage, storing four letters required 32 units based on American Standard Code in Figure 4e, while the same four letters need only 16 units by using 2-bit MLC storage in Figure 4f. The results indicated that the photochromic hydrogels would be potentially considered as non-volatile storage devices with higher density by recognizing different gray levels.

Figure 4. (a) The photo of micron-scale mask, (b) PAAm-Mo3 hydrogels were patterned at micron-scale (Photochromic process: P=100%, T=15s), (C) the principle of 2bit MLC storage devices, (D) the comparison of storage density between 1bit SLC and 2bit MLC storage devices


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Optical filter function of the photochromic hydrogels The hydrogels could exhibit photochromic behavior in the presence of UV irradiation, meaning the possible resistance of light. As can be seen from Figure 5a, PAAm-Mo0 and PAAm-Mo3 covered a GF254 silica gel plate, which could emit green light with a UV lamp. Under the visible light, both hydrogels exhibited excellent transparency. However, under the UV light of 254 nm, the region covered by PAAm-Mo0 had a green light, indicated that PAAm-Mo0 had no protection for UV-light. For the region covered by PAAm-Mo3, however, the excellent UV-light resistance was observed. Moreover, the photochromic hydrogels were also found to prevent the penetration of red laser light (650nm, 5mW) as laser-resistant materials. As can be seen from Figure 5b, the red laser could easily penetrate the transparent photochromic hydrogels. After the hydrogel was irradiated with UV light for 30 s, the red laser light could not be seen through the allochroic hydrogels, indicated that the laser was almost completely absorbed by hydrogels. Subsequently, the transmittance of photochromic hydrogels after UVirradiation for 30 s was measured and the intensity was less than 0.2% in Figure 5c, indicating that the absorption level for the red light was more than 99.8%.


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Figure 5. (a) UV protection performance of PAAm-Mo3 hydrogel, (b) performance of fed laser protection (Photochromic process: P=100%), (c) absorbance measurement of PAAm-Mo3 hydrogels in the red-light area after photochromism (Photochromic process: P=100%, T=30s), (d) PAAm-Mo3 hydrogel as an optical filter application (Photochromic process: P=100%).


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Then, a simulation experiment was designed by using the photochromic hydrogels as light filters. The photochromic hydrogel was placed in front of the digital camera and the focus point was concentrated on the hydrogel and the background, respectively. When the hydrogels were treated under the irradiation for 0 s, 15 s and 30 s in Figure 5d, the blue color gradually increased, indicating that the absorbent ability of UV-treated hydrogels for red light was gradually enhanced. However, the background clarity was not affected. When the local part of photo was enlarged to 800%, the details still maintained excellent sharpness and the scene was not deformed. As a result, the hydrogels exhibited excellent adjustable optical properties and could be considered as adjustable optical filters. Conclusions In summary, a novel photochromic hydrogel with high transparency, rapid responsibility and fatigue resistance was successfully prepared. The hydrogel exhibited excellent photochromic behavior and could change the color under the irradiation of UV light for only 2 s. The photochromic intensity could be precisely adjusted by setting the illumination time. In addition, the allochroic hydrogel would gradually fade after placed in the oxygen or air atmosphere and the fading rate would be accelerated by temperature. Moreover, the hydrogel could undergo reduplicative photochromic process with different pattern and be stretched repeatedly for more than 1000 times under the stain of 50% without any damage. Even the patterned image in the photochromic hydrogel could be stored permanently by freezing or vacuum-drying. As a result, the stretchable, low cost, fatigue-resistant, rapidly responsive photochromic hydrogel would be widely utilized in artificial intelligence system, wearable device, flexible visual display and optical storage device.


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Supporting Information. Supporting Information is available free of charge on the ACS Publication website: Preparation of the mask and the hydrogels, ATR-FTIR spectra, absorbance and transmittance mesurements, stretching test, optical images, SEM images. ACKNOWLEDGMENT This research was supported by a grant from National Natural Science Foundation of China (NSFC) (Nos. 51703012 and 51473023). REFERENCES (1)

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TOC Figure


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Low-Cost, Rapidly Responsive, Controllable, and Reversible

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