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Highly transparent and stretchable hydrogels with rapidly responsive photochromic performance for UV-irradiated optical display devices
Reactive and Functional Polymers 138 (2019) 88–95
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Reactive and Functional Polymers journal homepage: www.elsevier.com/locate/react
Highly transparent and stretchable hydrogels with rapidly responsive photochromic performance for UV-irradiated optical display devices ⁎
T
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Lin Guan, Yongqi Yang, Fei Jia , Guanghui Gao
Polymeric and Soft Materials Laboratory, School of Chemical Engineering, Advanced Institute of Materials Science, Changchun University of Technology, Changchun 130012, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogels Photochromic Flexible devices Display devices
Photochromic hydrogels have attracted considerable attention owing to their potential application in optical devices. However, the weak mechanical properties severely limited their applications. Here, a novel photochromic hydrogel with high transparency, high stretchability and rapid photochromic responsiveness has been successfully designed and prepared by introducing a redox photochromic material (Mo) into the hybrid hydrogel. The photochromic hydrogel with rapid photochromic response achieved discoloration process within only 3 s and the fading process could be easily performed under the air environment. Moreover, the ability to rewrite and erase optical information have been implemented on the photochromic hydrogels. Therefore, this strategy of photochromic hydrogel would contribute to design a new generation of optical display devices.
1. Introduction Photochromic materials, which reversibly change their color in response to light, have been widely investigated in many fields such as displays [1,2], optical information storage [3,4] and optical switches [5,6] in recent decades. Organic photochromic materials include spiropyrans [7–10], diarylethenes [11–13] and azobenzenes [14–16]. However, high manufacturing cost and poor reversibility severely limited mass production and applications of photochromic materials. Inorganic photochromic materials such as titanium dioxide [17–19] and tungsten trioxide [20–22]. These inorganic photochromic materials have been extensively investigated owing to their stable chemical stability and low cost. Among various types of inorganic photochromic materials, polyoxometalates (POM), which undergo potential light inducing electron and proton transfer processes, have excellent photostability and photochromic properties [23,24]. For instance, as a kind of polyoxometalate, ammonium molybdate (Mo) has been widely used because of good stability and low cost, meanwhile its photochromic mechanism has also been clarified. That is, Mo (VI) was colorless. When UV light irradiated to Mo (VI), the reduced Mo (V) was obtained by capturing the electron, thereby achieving color transition from colorless to green or blue. In the air environment, oxygen could capture electron from Mo (V) again, resulting in oxidized Mo (VI) with a colorless state. During the color transition, the stable photochromic property of Mo
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was due to the fact that its structure did not change [25]. Hydrogels are soft materials which can hold large amounts of water as 3D network structure. Due to their excellent flexible and stretchable properties, hydrogels are promising candidates for many applications, such as display devices, wearable equipments and sensors [26–29]. However, the weak mechanical properties of conventional hydrogels limited their applications. Therefore, a lot of novel hydrogel systems have been developed by designing idiographic structure in recent years, such as double network hydrogels [30,31], hydrophobic association hydrogels [32,33], nanocomposite hydrogels [34,35], macromolecular microsphere composite hydrogels [36], ionically crosslinked hydrogels [37,38] and hybrid crosslinking hydrogels [39,40]. Among them, hybrid crosslinking hydrogels are regarded as the effective approach to obtain tough hydrogels. Hybrid crosslinking hydrogels are composed of two crosslinked methods: covalent crosslinking and noncovalent crosslinking. The covalent crosslinking stabilized the skeleton construction of the hydrogels and the noncovalent crosslink provided a mechanism for energy dissipation for achieving toughness and good self-recovery [40,41]. In our previous work, we synthesized a photochromic hydrogel by covalent and ionic crosslinking, which had high transparency and rapid light responsiveness. Nonetheless, poor mechanical properties of hydrogels still could not be solved [42]. Therefore, other hybrid crosslinking combinations were attempted to prepare a photochromic hydrogel with excellent mechanical properties. In this work, we proposed a strategy to prepare hybrid crosslinking
Corresponding authors. E-mail addresses: [email protected] (F. Jia), [email protected] (G. Gao).
https://doi.org/10.1016/j.reactfunctpolym.2019.03.003 Received 25 December 2018; Received in revised form 1 March 2019; Accepted 3 March 2019 Available online 05 March 2019 1381-5148/ © 2019 Published by Elsevier B.V.
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2. Experimental
intensity of 150 mM cm−2) to illuminate the photochromic hydrogels and set specific UV irradiation time and UV irradiation intensity. The height of the UV light irradiation source to the samples were set as 12 cm. Simultaneously, the patterned hydrogels were exposed to UV light though different masks to produce patterns. The masks manufacturing method was described as follows: We firstly printed the designed patterns on the tape with a printer, and then attached another layer of tape to it without bubbles, and finally obtained masks with designed patterns. Moreover, the samples fading experiments were performed by placing the hydrogels in a sealed box containing the soaked cotton to prevent it from losing water. Simultaneously, we set a specific UV light to illuminate the HPAAm-Mo3 hydrogel for cycle test (UV light irradiation time T = 30 s, UV light irradiation intensity I = 100%).
2.1. Materials
2.4. Ultraviolet test
Acrylamide (AAm, 99.0%), N, N′-Methylene bis-(acrylamide) (MBA, 99.0%), Lauryl methacrylate (LMA, 95%), potassium persulfate (KPS, 99.5%), N, N, N′, N′-Tetramethylethylenediamine (TEMED, ≥99.5%), and ammonium molybdate [(NH4)6Mo7O24·4H2O] (Mo, 99.0%) were purchased from Aladdin Industrial Corporation. Dodecyl dimethyl betaine (BS-12, 30.0 ± 1%) was supplied by Zhengzhou Sansan Rihua Co. Ltd. Deionized water (18.2 Ω cm resistivity at 25 °C) was used in the experiment.
The UV–visible absorbance spectrum and transmittance spectra were measured via a UV–vis-NIR spectrophotometer (Agilent Technologies, Cary 5000). The scanning wavelength was recorded in the range of 400 nm–800 nm. The scanning speed was 2000 nm·min−1. All the simples were cut into a square shape with the length of 3 cm, width of 2 cm and thickness of 2 mm.
2.2. Preparation of the photochromic hydrogels
To measure the mechanical properties of photochromic hydrogels, the tensile measurements (SHIMADZU, model AGS-X, 100 N, Japan) were performed on the simples at a speed of 100 mm min−1 at ambient temperature. Among them, all the hydrogels were cut into dumbbell shapes of 30 mm in length, 4 mm in width and 2.5 mm in thickness. For reproducibility, each sample needed to be measured for 5 times and the results were averaged. Then the fracture stress and the fracture elongation were recorded. Moreover, the loading-unloading cycles of hydrogels were also measured by the above-mentioned tensile tester and the dissipated energy were calculated by the area between loadingunloading curves. In order to prevent the hydrogel from drying out, we used a humidifier to moisturize it during the stretching test.
hydrogels with photochromic properties. Dodecyl dimethyl betaine (BS12) was used as a surfactant in this system. Simultaneously, BS-12 contained positive charges, which could combine with Mo by electrostatic interaction. Moreover, we employed the synergistic effect of chemical crosslinking and hydrophobic association to enhance the mechanical properties of the photochromic hydrogels. The resulting hydrogel had high transparency, ductility, puncture resistance, and could undergo photochromic behavior under UV light for only 3 s. Simultaneously, the hydrogel has successfully implemented rewritable and erasable optical information. Therefore, the photochromic hydrogel could be considered as a promising candidate for erasable optical displays, artificial intelligence systems and wearable flexible devices.
2.5. Tensile measurement
Firstly, BS-12 (1.665 g) and LMA (50 μL) were dissolved in the deionized water (18 mL) with continuous magnetic stirring at 40 °C for 3 h. Subsequently, AAm (6 g), MBA (0.002 g) and ammonium molybdate were separately added to the above solution and stirred for 15 min, and then KPS (0.03 g) and TEMED (0.1 mL) as redox initiators were added into the system and stirred continuously for 1 min. Finally, the uniform solution was poured into the glass mold (90 × 60 × 3 mm3) with a silicone spacer, and carried out at 40 °C for 3 h to obtain the hybrid hydrogels (HPAAm-Mo). Structure and formation mechanism of the photochromic hydrogels was shown in Scheme 1 and all the recipes of the hybrid hydrogels were shown in Table 1. For comparison, the preparing process of the physical hydrogel (PPAAm-Mo3) was similar with HPAAm-Mo3 but the MBA was removed to eliminate chemical cross-linking. Simultaneously, the chemical hydrogel (CPAAm-Mo3) was fabricated under the same condition without any LMA.
3. Results and discussion 3.1. Mechanical properties of hydrogels Tensile tests were carried out on a serious of photochromic hydrogels with different amounts of Mo. As shown in Fig. 1a, with the increase of Mo content, the mechanical strength of the photochromic hydrogels decreased. It was well known that LMA micelles served as crosslinking centers for hydrophobic association, which could significantly enhance the mechanical properties of hydrogels. However, as
2.3. Photochromic behavior of hydrogels All the samples were treated by the UV light. We used the UV curing machine (Intelli-ray 600, Uvitron International, maximum light
Scheme 1. Structure and formation mechanism of photochromic hydrogels. 89
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Table 1 Recipes for photochromic hydrogels. Samples
AAm/g
LMA/mL
BS-12/g
Mo/g
MBA/g
KPS/g
TMEDA/mL
H2O/ mL
HPAAm-Mo1 HPAAm-Mo2 HPAAm-Mo3 HPAAm-Mo4
6 6 6 6
0.05 0.05 0.05 0.05
1.665 1.665 1.665 1.665
0.7095 0.946 1.1825 1.409
0.002 0.002 0.002 0.002
0.03 0.03 0.03 0.03
0.1 0.1 0.1 0.1
20 20 20 20
Fig. 1. (a) Tensile curves of different content of HPAAm-Mo hydrogels; (b) Tensile curves of PPAAm-Mo3 hydrogel, CPAAm-Mo3 hydrogel and HPAAm-Mo3 hydrogel; (c) Tensile loading-unloading curves of PPAAm-Mo3, CPAAm-Mo3, HPAAm-Mo3 hydrogels; (d) The corresponding stress and dissipated energy; (e) Five successive loading-unloading cycles of HPAAm-Mo3 hydrogel; (f) The corresponding stress and dissipated energy. 90
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3.2. Photochromic behavior of hydrogels
the Mo content increased, the stability of the LMA micelles was destroyed by the electrostatic interaction between the surfactant and Mo, thereby reducing the mechanical properties of the photochromic hydrogels. From Fig. 1b, the fracture stress and fracture strain of the hybrid crosslinked HPAAm-Mo3 hydrogel were 130 kPa and 2900%. However, the fracture stress was only 6 kPa and the elongation was 1600% for the physical crosslinked PPAAm-Mo3 hydrogel and the pure chemical crosslinked CPAAm-Mo3 hydrogel without hydrophobic segments was brittle and fractured easily at a relatively low strain. Its fracture stress and fracture strain were 35 kPa and 2050%, respectively. Therefore, the HPAAm-Mo3 hydrogels survived much higher strength than the single crosslinked hydrogels due to the synergistic effect of physical crosslinking and chemical crosslinking. Moreover, the tensile loading-unloading tests at a stain of 1000% for PPAAm-Mo3, CPAAm-Mo3 and HPAAm-Mo3 hydrogels were shown in Fig. 1c and the corresponding stress and dissipated energy were calculated in Fig. 1d. Compared with PPAAm-Mo3 and CPAAm-Mo3 hydrogels, the tensile strength and dissipated energy of HPAAm-Mo3 hydrogels were significantly increased, which indicated that HPAAmMo3 had an outstanding energy dissipation mechanism due to the synergistic effect between physical crosslinking and chemical crosslinking. In addition, the successive loading-unloading cycles at a stain of 1000% were conducted for HPAAm-Mo3 hydrogels and the corresponding stress and dissipated energy were calculated in Fig. 1e,f. After the first cycle at a strain of 1000%, the drawing cycle was repeated for four times under same manner without any spare time. As could be seen from Fig. 1f, the dissipated energy decreased suddenly after the first cycle, which indicated that the internal network structure of HPAAmMo3 hydrogel was destroyed and could not recover immediately. However, there was no large change in tensile strength and dissipation energy after the subsequent four cycles of stretching. These data indicated that the HPAAm-Mo3 hydrogels had outstanding self-recovery property and anti-fatigue property, which greatly promoted the development of applications for flexible display devices. In order to prove the excellent mechanical properties of the photochromic hydrogel, we have carried out various exhibitions. From Fig. 2a, HPAAm-Mo3 could easily lift a kilogram of weight. The sheetlike hydrogel could be blown into a large, thin balloon, indicating that the hydrogel was capable of bearing an extension force and had excellent ductility (Fig. 2b). Moreover, the HPAAm-Mo3 hydrogel was punctured without any damage or penetration, which proved excellent puncture resistance and good recovery performance of the hydrogels in Fig. 2c. The above exhibition showed that the photochromic hydrogels had good mechanical properties as well as puncture resistance and ductility.
The transmittance is a significant parameter for optical display device which requires visualization. All the hydrogels before UV irradiation exhibited high transparency (Fig. 3a). Fig. 3b exhibited the UV–vis transmittance spectra of original hydrogels before UV irradiation. The transmittance could reach above 85% in the range of 400 nm to 800 nm, which directly proved that the hydrogels had high transparency. In addition, the color of hydrogels after UV irradiation were gradually deepened with the increase of the Mo content in Fig. 3c, and the absorbance measurement of different contents hydrogels after UV irradiation was also carried out in Fig. 3d. The UV–vis absorption spectra of photochromic hydrogels after 60 s under the UV irradiation showed that the peak intensity at 620 nm and 740 nm was significantly enhanced with the increase of the Mo content. To investigate the photochromic performance of hydrogels, the influence of UV light irradiation time on photochromic behavior was firstly discussed. The UV–vis absorption spectra measured by using a UV–Vis-NIR spectrometer. The absorption curves of the photochromic hydrogels with increasing illumination time were shown in Fig. 4a and the absorption peak intensity showed an obvious alteration within 1 min. In addition, the fitting curve of the absorbance peak strength at the wavelength of 740 nm at different UV irradiation time was shown in Fig. 4b, which indicated that the HPAAm-Mo3 hydrogels had rapid photochromic responsibility. Meanwhile, the color of HPAAm-Mo3 hydrogels gradually deepened as UV light irradiation time increased in Fig. 4c. Moreover, the photochromic behavior of hydrogels was also affected with UV light irradiation intensity. As the UV-irradiation intensity increased from 0% to 100%, the absorbance intensity of HPAAm-Mo3 hydrogels increased accordingly in Fig. 5a. The fitting curve of the absorbance peak strength at the wavelength of 740 nm with different UV irradiation intensity indicated that the HPAAm-Mo3 hydrogels had excellently controlled photochromic behavior in Fig. 5b. Simultaneously, as the UV light irradiation intensity increased, the color of the HPAAm-Mo3 hydrogels gradually deepened from light green to dark green in Fig. 5c. Therefore, the degree of coloration of the photochromic hydrogels could be precisely controlled by adjusting the intensity of the UV light. 3.3. Fading behavior of hydrogels The fading behavior of photochromic hydrogel was also discussed. As shown in Fig. 6a, with the fading time increased, the photochromic HPAAm-Mo3 hydrogel gradually became transparent in an air environment and faded completely within 6 h. This phenomenon was
Fig. 2. Mechanical exhibitions of HPAAm-Mo3 hydrogels: (a) lifting up a steel block of 1 kg; (b) blowing nitrogen ball; (c) stabbing and rapid recovery. All samples used for display were colored with Rhodamine B. 91
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Fig. 3. (a) Transparency display of different contents hydrogels; (b) Corresponding visible light transmittance of different contents hydrogels; (c) Exhibition of different contents hydrogels after UV irradiation (UV light irradiation time T = 60 s, UV light irradiation intensity I = 100%); (d) Corresponding absorbance spectra of different contents hydrogels after UV irradiation (T = 60 s, I = 100%).
Fig. 4. (a) Absorbance spectra of HPAAm-Mo3 hydrogels after different UV irradiation times (UV light irradiation intensity I = 100%); (b) Fitting curve of absorbance at 740 nm (I = 100%); (c) Exhibitions of HPAAm-Mo3 hydrogels after different UV irradiation times (I = 100%).
fading time was shown in Fig. 6c, which also implied that the HPAAmMo3 hydrogel had excellent fading behavior. From Fig. 6d, the fading process was reproducible and still had good photochromic behavior and transparency after five cycles (6 h per cycle).
consistent with the absorption curve of the photochromic HPAAm-Mo3 hydrogel (Fig. 6b). It could be seen from the UV absorbance measurement that the absorbance peak strength of the HPAAm-Mo3 hydrogel gradually decreased with the fading time increased, and the absorbance characteristic peak disappeared entirely at 6 h, which proved that the photochromic hydrogel faded completely. Moreover, the fitting curve of the absorbance peak strength at the wavelength of 740 nm with the 92
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Fig. 5. (a) Absorbance spectra of HPAAm-Mo3 hydrogels after different UV irradiation intensity (UV light irradiation time T = 30 s); (b) Fitting curve of absorbance at 740 nm (T = 30 s); (c) Exhibitions of HPAAm-Mo3 hydrogels after different UV irradiation intensity (T = 30 s).
addition, the patterns on the photochromic hydrogels gradually returned to the initial colorless transparent state within 6 h in an air environment. As could be seen from Fig. 7c, repeated erasing and rewriting could be easily achieved in HPAAm-Mo3 hydrogels. The word “CCUT” was shown as the first image and then spontaneously faded to the original transparent state under the air environment. Subsequently, the second pattern was recorded on the same hydrogel and still returned to the transparent state. This successful demonstration of rewrite process suggested the good repeatable recording information for the HPAAm-Mo3 hydrogels. Consequently, rapid and excellent
3.4. Display function of photochromic hydrogels Different patterns could be achieved on hydrogels based on the photochromic properties of HPAAm-Mo3 hydrogels. The schematic diagram of patterning process was presented in Fig. 7a. Firstly, the masks with different patterns attached to the surface of the HPAAmMo3 hydrogels and then irradiated for 60 s under 100% intensity of UV light. After removing the masks, the photochromic hydrogels with different patterns were obtained. Fig. 7b demonstrated the patterns of butterfly and birds on the HPAAm-Mo3 hydrogels after UV light. In
Fig. 6. (a) Fading exhibitions of HPAAm-Mo3 hydrogel after UV irradiation (UV light irradiation time T = 30 s, UV light irradiation intensity I = 100%); (b) Absorption spectrum of fading process of HPAAm-Mo3 hydrogel after UV irradiation (T = 30 s, I = 100%); (c) Fitting curve of absorbance at 740 nm (T = 30 s, I = 100%); (d) Absorbance cycle of photochromic and fading processes at 740 nm. 93
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Fig. 7. (a) schematic diagram of patterning process of HPAAm-Mo3 hydrogels by irradiation with UV light through a mask; (b) photographs of HPAAm-Mo3 hydrogels marked with different patterns; (c) The process of rewritable imaging on the HPAAm-Mo3 hydrogel by using UV light. The blue regions indicated the patterns displayed after irradiation with UV light (UV light irradiation time T = 30 s, UV light irradiation intensity I = 100%). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
photochromic properties and the diverse patterns design have greatly facilitated the development of functional photochromic devices and flexible devices for visual display. 4. Conclusions In conclusion, we have successfully designed and synthesized a novel photochromic hydrogel, which exhibited excellent photochromic behavior, high transparency, stretchability, rapid photochromic responsiveness and self-recovery property. The fracture stress of hydrogels could reach up to 130 kPa and the fracture elongation achieve about 3000%. Moreover, this hydrogel could observe discoloration within only 3 s, which proved its rapid photochromic responsibility. At the same time, the fading process could be easily achieved in the air environment. Therefore, we have used the hydrogel as a new recording media for optical information display, successfully implementing the functions of rewritable and erasable optical information on hydrogels. We anticipated that the photochromic hydrogels would be widely utilized in flexible and stretchable devices for visual display. Conflicts of interest The authors declare no competing financial interests. Acknowledgements This research was supported by grants from National Natural Science Foundation of China (Nos. 51703012 and 51873024), Science and Technology Department of Jilin Province (No. 20180101207JC), and Education Department of Jilin Province (No. JJKH20181027KJ). 94
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Contents lists available at ScienceDirect
Reactive and Functional Polymers journal homepage: www.elsevier.com/locate/react
Highly transparent and stretchable hydrogels with rapidly responsive photochromic performance for UV-irradiated optical display devices ⁎
T
⁎
Lin Guan, Yongqi Yang, Fei Jia , Guanghui Gao
Polymeric and Soft Materials Laboratory, School of Chemical Engineering, Advanced Institute of Materials Science, Changchun University of Technology, Changchun 130012, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrogels Photochromic Flexible devices Display devices
Photochromic hydrogels have attracted considerable attention owing to their potential application in optical devices. However, the weak mechanical properties severely limited their applications. Here, a novel photochromic hydrogel with high transparency, high stretchability and rapid photochromic responsiveness has been successfully designed and prepared by introducing a redox photochromic material (Mo) into the hybrid hydrogel. The photochromic hydrogel with rapid photochromic response achieved discoloration process within only 3 s and the fading process could be easily performed under the air environment. Moreover, the ability to rewrite and erase optical information have been implemented on the photochromic hydrogels. Therefore, this strategy of photochromic hydrogel would contribute to design a new generation of optical display devices.
1. Introduction Photochromic materials, which reversibly change their color in response to light, have been widely investigated in many fields such as displays [1,2], optical information storage [3,4] and optical switches [5,6] in recent decades. Organic photochromic materials include spiropyrans [7–10], diarylethenes [11–13] and azobenzenes [14–16]. However, high manufacturing cost and poor reversibility severely limited mass production and applications of photochromic materials. Inorganic photochromic materials such as titanium dioxide [17–19] and tungsten trioxide [20–22]. These inorganic photochromic materials have been extensively investigated owing to their stable chemical stability and low cost. Among various types of inorganic photochromic materials, polyoxometalates (POM), which undergo potential light inducing electron and proton transfer processes, have excellent photostability and photochromic properties [23,24]. For instance, as a kind of polyoxometalate, ammonium molybdate (Mo) has been widely used because of good stability and low cost, meanwhile its photochromic mechanism has also been clarified. That is, Mo (VI) was colorless. When UV light irradiated to Mo (VI), the reduced Mo (V) was obtained by capturing the electron, thereby achieving color transition from colorless to green or blue. In the air environment, oxygen could capture electron from Mo (V) again, resulting in oxidized Mo (VI) with a colorless state. During the color transition, the stable photochromic property of Mo
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was due to the fact that its structure did not change [25]. Hydrogels are soft materials which can hold large amounts of water as 3D network structure. Due to their excellent flexible and stretchable properties, hydrogels are promising candidates for many applications, such as display devices, wearable equipments and sensors [26–29]. However, the weak mechanical properties of conventional hydrogels limited their applications. Therefore, a lot of novel hydrogel systems have been developed by designing idiographic structure in recent years, such as double network hydrogels [30,31], hydrophobic association hydrogels [32,33], nanocomposite hydrogels [34,35], macromolecular microsphere composite hydrogels [36], ionically crosslinked hydrogels [37,38] and hybrid crosslinking hydrogels [39,40]. Among them, hybrid crosslinking hydrogels are regarded as the effective approach to obtain tough hydrogels. Hybrid crosslinking hydrogels are composed of two crosslinked methods: covalent crosslinking and noncovalent crosslinking. The covalent crosslinking stabilized the skeleton construction of the hydrogels and the noncovalent crosslink provided a mechanism for energy dissipation for achieving toughness and good self-recovery [40,41]. In our previous work, we synthesized a photochromic hydrogel by covalent and ionic crosslinking, which had high transparency and rapid light responsiveness. Nonetheless, poor mechanical properties of hydrogels still could not be solved [42]. Therefore, other hybrid crosslinking combinations were attempted to prepare a photochromic hydrogel with excellent mechanical properties. In this work, we proposed a strategy to prepare hybrid crosslinking
Corresponding authors. E-mail addresses: [email protected] (F. Jia), [email protected] (G. Gao).
https://doi.org/10.1016/j.reactfunctpolym.2019.03.003 Received 25 December 2018; Received in revised form 1 March 2019; Accepted 3 March 2019 Available online 05 March 2019 1381-5148/ © 2019 Published by Elsevier B.V.
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2. Experimental
intensity of 150 mM cm−2) to illuminate the photochromic hydrogels and set specific UV irradiation time and UV irradiation intensity. The height of the UV light irradiation source to the samples were set as 12 cm. Simultaneously, the patterned hydrogels were exposed to UV light though different masks to produce patterns. The masks manufacturing method was described as follows: We firstly printed the designed patterns on the tape with a printer, and then attached another layer of tape to it without bubbles, and finally obtained masks with designed patterns. Moreover, the samples fading experiments were performed by placing the hydrogels in a sealed box containing the soaked cotton to prevent it from losing water. Simultaneously, we set a specific UV light to illuminate the HPAAm-Mo3 hydrogel for cycle test (UV light irradiation time T = 30 s, UV light irradiation intensity I = 100%).
2.1. Materials
2.4. Ultraviolet test
Acrylamide (AAm, 99.0%), N, N′-Methylene bis-(acrylamide) (MBA, 99.0%), Lauryl methacrylate (LMA, 95%), potassium persulfate (KPS, 99.5%), N, N, N′, N′-Tetramethylethylenediamine (TEMED, ≥99.5%), and ammonium molybdate [(NH4)6Mo7O24·4H2O] (Mo, 99.0%) were purchased from Aladdin Industrial Corporation. Dodecyl dimethyl betaine (BS-12, 30.0 ± 1%) was supplied by Zhengzhou Sansan Rihua Co. Ltd. Deionized water (18.2 Ω cm resistivity at 25 °C) was used in the experiment.
The UV–visible absorbance spectrum and transmittance spectra were measured via a UV–vis-NIR spectrophotometer (Agilent Technologies, Cary 5000). The scanning wavelength was recorded in the range of 400 nm–800 nm. The scanning speed was 2000 nm·min−1. All the simples were cut into a square shape with the length of 3 cm, width of 2 cm and thickness of 2 mm.
2.2. Preparation of the photochromic hydrogels
To measure the mechanical properties of photochromic hydrogels, the tensile measurements (SHIMADZU, model AGS-X, 100 N, Japan) were performed on the simples at a speed of 100 mm min−1 at ambient temperature. Among them, all the hydrogels were cut into dumbbell shapes of 30 mm in length, 4 mm in width and 2.5 mm in thickness. For reproducibility, each sample needed to be measured for 5 times and the results were averaged. Then the fracture stress and the fracture elongation were recorded. Moreover, the loading-unloading cycles of hydrogels were also measured by the above-mentioned tensile tester and the dissipated energy were calculated by the area between loadingunloading curves. In order to prevent the hydrogel from drying out, we used a humidifier to moisturize it during the stretching test.
hydrogels with photochromic properties. Dodecyl dimethyl betaine (BS12) was used as a surfactant in this system. Simultaneously, BS-12 contained positive charges, which could combine with Mo by electrostatic interaction. Moreover, we employed the synergistic effect of chemical crosslinking and hydrophobic association to enhance the mechanical properties of the photochromic hydrogels. The resulting hydrogel had high transparency, ductility, puncture resistance, and could undergo photochromic behavior under UV light for only 3 s. Simultaneously, the hydrogel has successfully implemented rewritable and erasable optical information. Therefore, the photochromic hydrogel could be considered as a promising candidate for erasable optical displays, artificial intelligence systems and wearable flexible devices.
2.5. Tensile measurement
Firstly, BS-12 (1.665 g) and LMA (50 μL) were dissolved in the deionized water (18 mL) with continuous magnetic stirring at 40 °C for 3 h. Subsequently, AAm (6 g), MBA (0.002 g) and ammonium molybdate were separately added to the above solution and stirred for 15 min, and then KPS (0.03 g) and TEMED (0.1 mL) as redox initiators were added into the system and stirred continuously for 1 min. Finally, the uniform solution was poured into the glass mold (90 × 60 × 3 mm3) with a silicone spacer, and carried out at 40 °C for 3 h to obtain the hybrid hydrogels (HPAAm-Mo). Structure and formation mechanism of the photochromic hydrogels was shown in Scheme 1 and all the recipes of the hybrid hydrogels were shown in Table 1. For comparison, the preparing process of the physical hydrogel (PPAAm-Mo3) was similar with HPAAm-Mo3 but the MBA was removed to eliminate chemical cross-linking. Simultaneously, the chemical hydrogel (CPAAm-Mo3) was fabricated under the same condition without any LMA.
3. Results and discussion 3.1. Mechanical properties of hydrogels Tensile tests were carried out on a serious of photochromic hydrogels with different amounts of Mo. As shown in Fig. 1a, with the increase of Mo content, the mechanical strength of the photochromic hydrogels decreased. It was well known that LMA micelles served as crosslinking centers for hydrophobic association, which could significantly enhance the mechanical properties of hydrogels. However, as
2.3. Photochromic behavior of hydrogels All the samples were treated by the UV light. We used the UV curing machine (Intelli-ray 600, Uvitron International, maximum light
Scheme 1. Structure and formation mechanism of photochromic hydrogels. 89
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Table 1 Recipes for photochromic hydrogels. Samples
AAm/g
LMA/mL
BS-12/g
Mo/g
MBA/g
KPS/g
TMEDA/mL
H2O/ mL
HPAAm-Mo1 HPAAm-Mo2 HPAAm-Mo3 HPAAm-Mo4
6 6 6 6
0.05 0.05 0.05 0.05
1.665 1.665 1.665 1.665
0.7095 0.946 1.1825 1.409
0.002 0.002 0.002 0.002
0.03 0.03 0.03 0.03
0.1 0.1 0.1 0.1
20 20 20 20
Fig. 1. (a) Tensile curves of different content of HPAAm-Mo hydrogels; (b) Tensile curves of PPAAm-Mo3 hydrogel, CPAAm-Mo3 hydrogel and HPAAm-Mo3 hydrogel; (c) Tensile loading-unloading curves of PPAAm-Mo3, CPAAm-Mo3, HPAAm-Mo3 hydrogels; (d) The corresponding stress and dissipated energy; (e) Five successive loading-unloading cycles of HPAAm-Mo3 hydrogel; (f) The corresponding stress and dissipated energy. 90
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3.2. Photochromic behavior of hydrogels
the Mo content increased, the stability of the LMA micelles was destroyed by the electrostatic interaction between the surfactant and Mo, thereby reducing the mechanical properties of the photochromic hydrogels. From Fig. 1b, the fracture stress and fracture strain of the hybrid crosslinked HPAAm-Mo3 hydrogel were 130 kPa and 2900%. However, the fracture stress was only 6 kPa and the elongation was 1600% for the physical crosslinked PPAAm-Mo3 hydrogel and the pure chemical crosslinked CPAAm-Mo3 hydrogel without hydrophobic segments was brittle and fractured easily at a relatively low strain. Its fracture stress and fracture strain were 35 kPa and 2050%, respectively. Therefore, the HPAAm-Mo3 hydrogels survived much higher strength than the single crosslinked hydrogels due to the synergistic effect of physical crosslinking and chemical crosslinking. Moreover, the tensile loading-unloading tests at a stain of 1000% for PPAAm-Mo3, CPAAm-Mo3 and HPAAm-Mo3 hydrogels were shown in Fig. 1c and the corresponding stress and dissipated energy were calculated in Fig. 1d. Compared with PPAAm-Mo3 and CPAAm-Mo3 hydrogels, the tensile strength and dissipated energy of HPAAm-Mo3 hydrogels were significantly increased, which indicated that HPAAmMo3 had an outstanding energy dissipation mechanism due to the synergistic effect between physical crosslinking and chemical crosslinking. In addition, the successive loading-unloading cycles at a stain of 1000% were conducted for HPAAm-Mo3 hydrogels and the corresponding stress and dissipated energy were calculated in Fig. 1e,f. After the first cycle at a strain of 1000%, the drawing cycle was repeated for four times under same manner without any spare time. As could be seen from Fig. 1f, the dissipated energy decreased suddenly after the first cycle, which indicated that the internal network structure of HPAAmMo3 hydrogel was destroyed and could not recover immediately. However, there was no large change in tensile strength and dissipation energy after the subsequent four cycles of stretching. These data indicated that the HPAAm-Mo3 hydrogels had outstanding self-recovery property and anti-fatigue property, which greatly promoted the development of applications for flexible display devices. In order to prove the excellent mechanical properties of the photochromic hydrogel, we have carried out various exhibitions. From Fig. 2a, HPAAm-Mo3 could easily lift a kilogram of weight. The sheetlike hydrogel could be blown into a large, thin balloon, indicating that the hydrogel was capable of bearing an extension force and had excellent ductility (Fig. 2b). Moreover, the HPAAm-Mo3 hydrogel was punctured without any damage or penetration, which proved excellent puncture resistance and good recovery performance of the hydrogels in Fig. 2c. The above exhibition showed that the photochromic hydrogels had good mechanical properties as well as puncture resistance and ductility.
The transmittance is a significant parameter for optical display device which requires visualization. All the hydrogels before UV irradiation exhibited high transparency (Fig. 3a). Fig. 3b exhibited the UV–vis transmittance spectra of original hydrogels before UV irradiation. The transmittance could reach above 85% in the range of 400 nm to 800 nm, which directly proved that the hydrogels had high transparency. In addition, the color of hydrogels after UV irradiation were gradually deepened with the increase of the Mo content in Fig. 3c, and the absorbance measurement of different contents hydrogels after UV irradiation was also carried out in Fig. 3d. The UV–vis absorption spectra of photochromic hydrogels after 60 s under the UV irradiation showed that the peak intensity at 620 nm and 740 nm was significantly enhanced with the increase of the Mo content. To investigate the photochromic performance of hydrogels, the influence of UV light irradiation time on photochromic behavior was firstly discussed. The UV–vis absorption spectra measured by using a UV–Vis-NIR spectrometer. The absorption curves of the photochromic hydrogels with increasing illumination time were shown in Fig. 4a and the absorption peak intensity showed an obvious alteration within 1 min. In addition, the fitting curve of the absorbance peak strength at the wavelength of 740 nm at different UV irradiation time was shown in Fig. 4b, which indicated that the HPAAm-Mo3 hydrogels had rapid photochromic responsibility. Meanwhile, the color of HPAAm-Mo3 hydrogels gradually deepened as UV light irradiation time increased in Fig. 4c. Moreover, the photochromic behavior of hydrogels was also affected with UV light irradiation intensity. As the UV-irradiation intensity increased from 0% to 100%, the absorbance intensity of HPAAm-Mo3 hydrogels increased accordingly in Fig. 5a. The fitting curve of the absorbance peak strength at the wavelength of 740 nm with different UV irradiation intensity indicated that the HPAAm-Mo3 hydrogels had excellently controlled photochromic behavior in Fig. 5b. Simultaneously, as the UV light irradiation intensity increased, the color of the HPAAm-Mo3 hydrogels gradually deepened from light green to dark green in Fig. 5c. Therefore, the degree of coloration of the photochromic hydrogels could be precisely controlled by adjusting the intensity of the UV light. 3.3. Fading behavior of hydrogels The fading behavior of photochromic hydrogel was also discussed. As shown in Fig. 6a, with the fading time increased, the photochromic HPAAm-Mo3 hydrogel gradually became transparent in an air environment and faded completely within 6 h. This phenomenon was
Fig. 2. Mechanical exhibitions of HPAAm-Mo3 hydrogels: (a) lifting up a steel block of 1 kg; (b) blowing nitrogen ball; (c) stabbing and rapid recovery. All samples used for display were colored with Rhodamine B. 91
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Fig. 3. (a) Transparency display of different contents hydrogels; (b) Corresponding visible light transmittance of different contents hydrogels; (c) Exhibition of different contents hydrogels after UV irradiation (UV light irradiation time T = 60 s, UV light irradiation intensity I = 100%); (d) Corresponding absorbance spectra of different contents hydrogels after UV irradiation (T = 60 s, I = 100%).
Fig. 4. (a) Absorbance spectra of HPAAm-Mo3 hydrogels after different UV irradiation times (UV light irradiation intensity I = 100%); (b) Fitting curve of absorbance at 740 nm (I = 100%); (c) Exhibitions of HPAAm-Mo3 hydrogels after different UV irradiation times (I = 100%).
fading time was shown in Fig. 6c, which also implied that the HPAAmMo3 hydrogel had excellent fading behavior. From Fig. 6d, the fading process was reproducible and still had good photochromic behavior and transparency after five cycles (6 h per cycle).
consistent with the absorption curve of the photochromic HPAAm-Mo3 hydrogel (Fig. 6b). It could be seen from the UV absorbance measurement that the absorbance peak strength of the HPAAm-Mo3 hydrogel gradually decreased with the fading time increased, and the absorbance characteristic peak disappeared entirely at 6 h, which proved that the photochromic hydrogel faded completely. Moreover, the fitting curve of the absorbance peak strength at the wavelength of 740 nm with the 92
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Fig. 5. (a) Absorbance spectra of HPAAm-Mo3 hydrogels after different UV irradiation intensity (UV light irradiation time T = 30 s); (b) Fitting curve of absorbance at 740 nm (T = 30 s); (c) Exhibitions of HPAAm-Mo3 hydrogels after different UV irradiation intensity (T = 30 s).
addition, the patterns on the photochromic hydrogels gradually returned to the initial colorless transparent state within 6 h in an air environment. As could be seen from Fig. 7c, repeated erasing and rewriting could be easily achieved in HPAAm-Mo3 hydrogels. The word “CCUT” was shown as the first image and then spontaneously faded to the original transparent state under the air environment. Subsequently, the second pattern was recorded on the same hydrogel and still returned to the transparent state. This successful demonstration of rewrite process suggested the good repeatable recording information for the HPAAm-Mo3 hydrogels. Consequently, rapid and excellent
3.4. Display function of photochromic hydrogels Different patterns could be achieved on hydrogels based on the photochromic properties of HPAAm-Mo3 hydrogels. The schematic diagram of patterning process was presented in Fig. 7a. Firstly, the masks with different patterns attached to the surface of the HPAAmMo3 hydrogels and then irradiated for 60 s under 100% intensity of UV light. After removing the masks, the photochromic hydrogels with different patterns were obtained. Fig. 7b demonstrated the patterns of butterfly and birds on the HPAAm-Mo3 hydrogels after UV light. In
Fig. 6. (a) Fading exhibitions of HPAAm-Mo3 hydrogel after UV irradiation (UV light irradiation time T = 30 s, UV light irradiation intensity I = 100%); (b) Absorption spectrum of fading process of HPAAm-Mo3 hydrogel after UV irradiation (T = 30 s, I = 100%); (c) Fitting curve of absorbance at 740 nm (T = 30 s, I = 100%); (d) Absorbance cycle of photochromic and fading processes at 740 nm. 93
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Fig. 7. (a) schematic diagram of patterning process of HPAAm-Mo3 hydrogels by irradiation with UV light through a mask; (b) photographs of HPAAm-Mo3 hydrogels marked with different patterns; (c) The process of rewritable imaging on the HPAAm-Mo3 hydrogel by using UV light. The blue regions indicated the patterns displayed after irradiation with UV light (UV light irradiation time T = 30 s, UV light irradiation intensity I = 100%). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
photochromic properties and the diverse patterns design have greatly facilitated the development of functional photochromic devices and flexible devices for visual display. 4. Conclusions In conclusion, we have successfully designed and synthesized a novel photochromic hydrogel, which exhibited excellent photochromic behavior, high transparency, stretchability, rapid photochromic responsiveness and self-recovery property. The fracture stress of hydrogels could reach up to 130 kPa and the fracture elongation achieve about 3000%. Moreover, this hydrogel could observe discoloration within only 3 s, which proved its rapid photochromic responsibility. At the same time, the fading process could be easily achieved in the air environment. Therefore, we have used the hydrogel as a new recording media for optical information display, successfully implementing the functions of rewritable and erasable optical information on hydrogels. We anticipated that the photochromic hydrogels would be widely utilized in flexible and stretchable devices for visual display. Conflicts of interest The authors declare no competing financial interests. Acknowledgements This research was supported by grants from National Natural Science Foundation of China (Nos. 51703012 and 51873024), Science and Technology Department of Jilin Province (No. 20180101207JC), and Education Department of Jilin Province (No. JJKH20181027KJ). 94
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