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Precise Actuation of Bilayer Photomechanical Films Coated with Molecular Azobenzene Chromophores Ziyi Liu, Rong Tang, Dandan Xu, Jian Liu,* Haifeng Yu* Bilayer photomechanical films are fabricated by depositing one layer of molecular azobenzene chromophores onto flexible low-density polyethylene substrates. The photoinduced bending and unbending behavior of five azobenzene derivatives including azobenzene, 4-hydroxyazobenzene, 4-((4-hydroxyphenyl)diazenyl)bezoitrile, 4-((4-methoxyph-enyl)diazenyl)phenol, and 4-(phenyldiazenyl)phenol is systematically studied by considering the incident light intensity and the thickness of the coated chromophore layers. Precise control of photoinduced curling of the bilayer film is successfully achieved upon irradiation with two beams of UV light, and the curled films can be recovered by thermal relaxation in the dark. The easily fabricated bilayer films show fast photomechanical response, strong photoinduced stress, and stability similar to crosslinked polymeric films. 1. Introduction Photoactive azobenzene (AB) derivatives can undergo reversible isomerization between an extended trans form
Dr. Z. Liu, Dr. R. Tang, Dr. D. Xu State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials Southwest University of Science and Technology Mianyang 621010, China Prof. J. Liu School of Materials Science and Engineering Southwest University of Science and Technology Mianyang, 621010, China E-mail: [email protected] Prof. H. Yu Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871, China E-mail: [email protected] Prof. H. Yu Key Laboratory of Polymer Chemistry and Physics of Ministry of Education Peking University Beijing 100871, China Macromol. Rapid Commun. 2015, 36, 1171−1176 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
(10 Å) and a shorter cis form (5.5 Å) by UV and visible light exposure (or spontaneously thermal relaxation), respectively.[1–4] The photoisomerization synchronously converted light energy into mechanical stress by the deformation of molecular shape. Then the development of various types of molecular machines using the photomechanical effect of azobenzene chromophores has been intensively studied in the past decade. Koshima et al. adopted light-induced trans-cis isomerization of 4-(dimethylamino) azobenzene and 4-aminoazobenzene molecular crystals as actuator, which can be reversibly driven by light.[5,6] However, the configuration transition at molecular lever was very weak and difficult to be detected.[6] Incorporating azobenzene moieties into polymeric networks may amplify the photomechanical effect.[7–10] Lopez et al. dissolved azobenzene dyes into liquid-crystalline elastomers (LCEs), and the resultant LCEs showed large mechanical deformation in response to light irradiation.[11] But the azobenzene dispersed LCEs show only 2D motions such as expansion or contraction. Photoinduced 3D motions, for example, bending, twisting, and rotation, are expected from the viewpoint of practical applications.[12–20] To fulfill 3D macrolevel motion, single-domain orientational LCE films were fabricated through a compact proceeding by Ikeda et al.[14] Typically, glass slabs were
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initially spin-coated with polyimide (PI) and baked, followed by rubbing treatment to yield alignment layers. Then monomer, crosslinker, and initiator were sandwiched between the PI-coated glass plates and polymerization at the LC temperature. The resultant films were removed from the plates, thus producing freestanding LCE films with homogenously aligned mesogens.[14,21–23] Such single layers of LCE films show insufficient mechanical strength, difficult controllable shape and size in fabrication and actuation processes.[24] Then bilayer films were developed in the same group by thermal compression bonding of one single photoactive LCE layer and one unstretched low-density polyethylene (LDPE) film.[16,24] In contrast to the single-layer LCE film, the bilayer film showed excellent photoresponsive and mechanical properties simultaneously.[24,25] The bending movement of the film is usually induced by an asymmetric contraction or expansion of the azobenzene chromophore layer,[26] whereas the polymeric network contributes amplification of the deformation of chromophores. That is, a small amount of azobenzene molecules may generate macroscopically motions as that of the azobenzene polymer. However, the azobenzene dispersed in poly(methyl methacrylate) (PMMA) hybrid composite did not show any bending but contraction motions upon light exposure.[11,27] One reason is that the azobenzene was not aligned in one direction. In this study, we report one macrolever actuating film by use of azobenzene molecular crystals as the photoactive layer. In contrast to previously two-step laminating LCE film, a simple method was developed to fabricate bilayer film by direct casting azobenzene molecules on LDPE film. It is expected that the bilayer film shows unique photomechanical behaviors under the irradiation of actinic light.
2. Experimental Section 2.1. Fabrication of Bilayer Films All the azobenzene compounds (97%) were bought from Aladdin Industrial Corporation of Shanghai and used without further purification, and their chemical structures and abbreviation are shown in Figure 1a. First, a one one commercialized LDPE film (Shengda-FB29, Weifang Shengda Technology Incorporated Co.) was carefully polished with silicon carbide sandpaper (1000 grit) in one direction. Second, the polished LDPE was supersonically washed in deionized water and dried in nitrogen. Third, the LDPE film was immersed into azobenzene solutions of ethanol, and then it was perpendicularly picked up. The bilayer film was obtained upon evaporation of solvent and annealing in an oven at 40 °C for 24 h. The film was cut into samples with various aspect ratios before use. The thickness of the azobenzene layer was controlled by the solution concentration and the impregnation time.
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Figure 1. a) Chemical structure of the azobenzene molecules used in this study and the macroscopic view of the bilayer film. b) Bending movements of ABOH bilayer film (15 mm × 3 mm × 38 μm) upon UV light irradiation (365 nm, 20 mW cm−2) and the unbending in the dark.
2.2. Characterization A 365 nm UV-LED was used as the light source for photoirradiation. Ultraviolet–visible (UV–vis) absorption spectra were measured using UV–vis EVOLUTION 300 spectrophotometer. Lightinduced motions of the bilayer film were recorded by one Sony digital camera. The bending angle was analyzed with adobe Illustrator CS5 software, which was used to determine the tip displacement angle between the starting position and the tip end position of the film.
3. Results and Discussion 3.1. Preparation and Characterization of the Bilayer Film The bilayer film was prepared via a very simple and scalable method modified from a previous report.[28] By immersing the pretreated LDPE film into azobenzene solutions, the solution was adsorbed on the film surface due to the capillary action of the polish-induced microgrooves (as
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Precise Actuation of Bilayer Photomechanical Films Coated with Molecular Azobenzene Chromophores
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(a)
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Bending angle (deg)
60
(b)
90 60
1 2 Irradiation time (s)
3
5 mW/cm2 20 mW/cm2 30 mW/cm2
30 0 0
(c)
DHAB ABOH ABMeO ABCN AB
30
0 0
3.2. Photoinduced Bending and Unbending of the Bilayer Film
1 2 Irradiation time (s)
3
90
Bending angle (deg)
Figure 1b gives several photographs of one bilayer film on photodriven motion and relaxation in dark. The simply fabricated film showed a similar photoinduced bending and unbending behaviors as those of homogeneously aligned polymeric LCE films.[31] Upon irradiation of UV light, the 4-hydroxy-azobenzene (ABOH) bilayer film fast bent to direction of the light source. It rapidly bent to an angle of 90° within 2 s irradiation. The photomechanical behaviors of the bilayer film can be explained with the so-called “bimetal mechanism.”[5] It is well known that azobenzene chromophores undergo trans-cis isomerization under UV irradiation. Although unpolarized UV light was used and selective photoisomerization of azobenzenes did not happen, anisotropic volume contraction of the active layer was induced because the ABOH molecules in the bilayer film had been in ordered distribution with their transition moments along the microgrooves as shown in Figure 3a. Whereas, another side of LDPE film always exhibited photoinert upon photoirradiation, resulting in the occurrence of the photoinduced contraction of the whole bilayer film along the polishing direction. When the UV light was turned off, the bent bilayer film fast returned to the initial flat state in dark. In contrast to the trans-cis isomerization leading to the contraction in volume of the film, the fast cis-trans isomerization caused the expansion of the photoactive layer, and thus the unbending of the whole bilayer film.[24] Five kinds of azobenzene derivatives were used to investigate the photomechanical behaviors of the bilayer films, as shown in Figure 2a. All the bilayer film with the same thickness was irradiated under UV light with
90
Bending angle (deg)
shown in Figure S1d, Supporting Information). When the film was vertically elevated from the solution, molecular crystals were formed with azobenzene moieties aligned along the microgrooves upon evaporation of the solvent, producing uniformed bilayer film with a good quality (Figure 1). The ordered distribution and regular arrangement of azobenzene molecular crystals was confirmed upon observation with polarizing optical microscope (POM). As shown in Figure S1, Supporting Information, the film showed dark field when it was placed in a direction parallel to the polarizer direction.[29] When the film was rotated by 45°, it demonstrated bright field. As contrast, the depositing of azobenzene solution on unpolished LDPE film caused typical crystallites in random distribution upon POM observation (Figure S1e, Supporting Information). Thus, the microgroove on the polished surface of LDPE film guided the orientation of azobenzene molecules, leading to an homogenous alignment of photoactive moieties onto the film.[30]
60
42 µm 35 µm 28 µm
30
0 0
1 2 Irradiation time (s)
3
Figure 2. a) The photomechanical response (bending angle) for a series of azobenzene derivatives deposited bilayer film (15 mm × 3 mm × 38 μm) under UV light irradiation (365 nm, 20 mW cm−2). The photomechanical response (bending angle) as a function of b) exposure light intensity and c) PE thickness.
the identical intensity. The bilayer film coated with nonsubstituted AB molecules showed the weakest photomechanical effect. The film coated with hydroxyl substituted ABOH reached the maximum bending angle of 90°. As contrast, the film coated with methoxyl substituted molecules (4-((4-methoxyph-enyl)diazenyl)phenol
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Table 1. Characterization and the bending behaviors of differently substituted azobenzene molecule deposited bilayer film.
Compounds AB ABOH
Max bending Bending speedb) [deg s−1] stressb) [MPa]
4,4′ positions substitution
Max absorption peaka) [nm]
Max bending angleb) [deg]
H, H
317
14.0
0.18
6.4
0.79
Order parameter
H, –OH
345
90.0
0.28
75.0
0.46
OH, –OCH3
367
73.0
0.25
22.8
0.03
ABCN
OH, –CN
369
26.0
0.21
10.8
0.26
DHAB
OH, OH
358
88.0
0.30
62.8
0.87
ABMeO
a)In
ethanol solution; b)Bilayer film size, 15 mm × 5 mm × 38 μm. UV light radiation at 365 nm, 20 mW m−2.
interaction may affect the contraction force diffusion. (ABMeO)) just obtained the maximum bending angle of Table 1 lists the physical characterizations and the light 26°. Furthermore, in the bending motion, the bigger the response of these five azobenzene derivatives. Accordbending angle of bilayer films, the faster the bending ingly, one possible explanation is that the azobenzenes speed. Typically, the pseudo-stilbene-type azobenzene are physically deposited on the LDPE substrates, thus the offers the advantages of fast trans-cis isomerization and molecular polarity huge affected the interface interaction strong dipolar interactions. In solution, the unsubstituted and then the bending rates. AB clearly underwent photoisomerization according to Obviously, the photomechanical behaviors of the the UV–vis absorptions changes. In contrast, di-substibilayer film such as bending speed and displacetuted 4-(phenyldiazenyl)phenol (DHAB) did not change ment angle should be affected by the intensity of the upon UV irradiation neither in solution nor in a solid irradiation light. When using the light intensity of crystal (Figure S3, Supporting Information). However, 5 mW cm−2, the bending speed of an ABOH-coated bilayer this phenomenon does not suggest that no photoisomerization occurred upon UV light irradiation on polar groups film was relatively slow and only a small bending angle substituted derivatives. Koshima similarly observed no was obtained, as shown in Figure 2b. With increasing the changes of the UV–vis absorptions of azobenzene crysintensity of the incident UV light, the photoresponsive tals, they explained as the photoisomerization occurred speed increased and the photoinduced deformation was near the crystal surface.[6] Another reason may be that enhanced. A bending angle of 90° was achieved within 3 s at 20 mW cm−2. When the UV light of 30 mW cm−2 was the trans-to-cis photoisomerization and cis-to-trans thermal isomerization recovery achieved in a short time used, the photoresponsive speed increased significantly scale which far shorter than the measuring time. Furtherand the maximum bending angle approximately reached more, the supported LDPE layer is photoinert, the bending and unbending of the whole film were beneficial from the directional contraction of photoactive azobenzene layer. The order parameters of all the bilayer films were calculated from their polarized UV–vis absorption spectra and the results are summarized in Table 1.[26] Among them, AB and DHAB exhibited the highest values, which are also higher than that of most of azobenzene nematic liquid crystals.[25,26] An order parameter of 0.03 was obtained for ABMeO, indicating almost no alignment in the bilayer film and leading to a bad photomechanical property. In addition to the ordering of the Figure 3. a) Schematic illustration the molecular distributions and the bending behavior azobenzene chromophores, the interof the bilayer film upon light incident form photoactive side or inert LDPE side. b) Curing facial interaction also greatly influof azobenzene bilayer film (15 mm × 5 mm × 38 μm) by controlling the incident UV radiaences the photoresponsive feature of tion (365 nm, 20 mW cm−2). The arrows indicate the light irradiation direction and its the bilayer film, since the interfacial exposure position on the film.
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Precise Actuation of Bilayer Photomechanical Films Coated with Molecular Azobenzene Chromophores
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(a)
(b)
3.3. Photocontrol of Curling and Unwinding of the Bilayer Film
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0.2
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0.0 0
(c)
300 600 Irradiation time (s)
900
0.3 Light off
Stress (Mpa)
Considering the light-active property, the fabricated bilayer film is Janus structure, since its one side is photosensitive and another side is photoinert. To further study the mechanism of the photoresponsive behaviors of the bilayer films, both sides of the film were irradiated with UV light, as shown in Figure 3a. The film bent toward the irradiated direction of the actinic light when they were irradiated from the photoactive layer side, as shown in Figure 3b. Interestingly, the bending direction was changed when the film was irradiated with an incident light from the LDPE side. The film showed photoinduced deformation bending away from the actinic light source, just like the photoresponsive behaviors of LCEs with homeotropicaligned mesogens.[14] That is to say, the Janus-like bilayer film showed photoinduced bending to the photoactive layer side whatever it was irradiated from either side. This result confirms that the bending of the bilayer film should be benefited from the directional contraction of photoactive azobenzene layer.[24] This result inconsistent to a single layer of photomechanical LCEs with homogenously aligned mesogens film, the bending direction of which depends on the alignment direction of chromophores[32] or the incident light polarization.[33] Taking advantage of the interesting photomechanical behaviors and the good mechanical performance of the LDPE film, we can easily design complex movement by directly using the bilayer film. For instance, precise control of curling and extending of the film was conveniently achieved upon photoirradiation of the combination of two UV light beams, as shown in Figure 3b. Upon the first light exposure, film bent toward the actinic light source direction by irradiation from the side of the photoactive
0.3 Stress (Mpa)
to 90° within 1.5 s. However, when the light intensity increased to 100 mW cm−2, the strong UV light caused a strong thermal effect, leading to heat-caused contraction and permanent damage on the surface of the photoactive film. To study the effect of the structure of the bilayer film on its photoresponsive behaviors, several bilayer films were fabricated with changing the thickness of LDPE film. For comparison, the thickness of the photoactive layer was controlled at about 10 ± 1 μm. As shown in Figure 2c, the bending angle reached to 90° when the thickness of the LDPE film was 28 μm. With further increasing film thickness of PE film, the bending angle and speed are significantly decreased. When the thickness of the PE film reached to 60 μm, the photoisomerization-induced contraction of the azobenzene molecules cannot provide sufficient driving force to cause the bending of the whole LDPE substrate.
0.2
0.1
0.0 420
Light on
425 430 435 440 Irradiation time (s)
445
Figure 4. a) Schematic illustration of the experimental setup for measuring the photoinduced stress generated from the bilayer bending and unbending upon irradiation with UV light (365 nm, 20 mW cm−2) (5 cm × 5 mm × 38 μm). b) Photoinduced stress of the bilayer film upon UV irradiation and c) a partial enlarged diagram of the photoinduced stress.
layer, and then a second light exposure on the film from the side of the LDPE layer. Obviously, the second irradiation also caused the film to bend towards the photoactive side and further enhanced the bending degree of crook, resulting in precise control of curling of the film
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(Figure 3b). Compared to the bending and unbending, a controllable curing could be advantageous for artificial hands and microrobots that are capable of particular manipulations.[13] For the viewpoint of applications, the photoinduced force is one of the most importance factors for photomechanical materials. We quantitatively studied the photomechanical properties of the present bilayer film. These experiments were performed using 50 mm × 5 mm × 38 μm samples. The film was fixed at both ends and using an initial load of 0.01 MPa to film in a flat state. When UV light reached to the film surface, the film bending stress rapidly increased over 0.2 MPa. After turning the UV light off, the film stress rapidly recovered in the initial state, as shown in Figure 4. Typically, small molecules show low mechanical strength contrasting to that of polymer materials. However, the fabricated bilayer film showed strong photomechanical force output from the small molecularly deposited film. Furthermore, the film showed good antifatigue ability after many cycle of light exposure (Figure 4b,c), suggesting that the simple deposited film has similar strength and stability as that of the polymeric film.[24]
4. Conclusions In summary, a series of bilayer photomechanical films were successfully fabricated by casting molecular azobenzene chromophores onto flexible LDPE substrate. They showed reversible photoinduced bending and unbending behaviors as that of LCE polymers. Both the light intensity and the thickness of the photoactive layer showed great influence on the photoresponsiveness of the bilayer films. Good ordering of chromophores upon the polishing treatment and the polar substituent in azobenzenes were beneficial for improvement of the photomechanical response. More interestingly, the bilayer films showed unique photoinduced bending direction, enabling us to design photodriven device. Multibeam irradiations induced curing provides a new strategy to precisely actuatoring photomechanical film. Compared to the bending and unbending, a controllable curing could be advantageous for artificial hands and microrobots that are capable of particular manipulations.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements: This work was financially supported by the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (no. 11zxfk26), the Youth Innovation Research Team of Sichuan for Carbon Nano
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materials (grant number 2011JTD0017) and National Natural Science Foundation of China (51322301, 51372211). Received: March 23, 2015; Published online: May 12, 2015; DOI: 10.1002/marc.201500177 Keywords: actuators; photomechanics
azobenzenes;
photoactive
materials;
[1] S. Li, Y. Zhou, C. Yu, F. Chen, J. Xu, New J. Chem. 2009, 33, 1462. [2] L. Wang, H. Dong, Y. Li, C. Xue, L.-D. Sun, C.-H. Yan, Q. Li, J. Am. Chem. Soc. 2014, 136, 4480. [3] Y. Wang, Q. Li, Adv. Mater. 2012, 24, 1926. [4] Q. Li, Intelligent Stimuli Responsive Materials: From WellDefined Nanostructures to Applications (Ed: Q. Li), John Wiley & Sons, New Jersey 2013. [5] H. Koshima, N. Ojima, H. Uchimoto, J. Am. Chem. Soc. 2009, 131, 6890. [6] H. Koshima, N. Ojima, Dyes Pigment. 2012, 92, 798. [7] H. F. Yu, C. Dong, W. Zhou, T. Kobayashi, H. Yang, Small 2011, 7, 3039. [8] T. Ube, T. Ikeda, Angew. Chem. Int. Ed. 2014, 53, 10290. [9] T. Kim, L. Zhu, R. O. Al-Kaysi, C. J. Bardeen, ChemPhysChem 2014, 15, 400. [10] S. Tsoi, J. Zhou, C. Spillmann, J. Naciri, T. Ikeda, B. Ratna, Macromol. Chem. Phys. 2013, 214, 734. [11] M. Camacho-Lopez, H. Finkelmann, P. Palffy-Muhoray, M. Shelley, Nat. Mater. 2004, 3, 307. [12] K. M. Lee, T. J. Bunning, T. J. White, Adv. Mater. 2012, 24, 2839. [13] K. D. Harris, R. Cuypers, P. Scheibe, C. L. van Oosten, C. W. M. Bastiaansen, J. Lub, D. J. Broer, J. Mater. Chem. 2005, 15, 5043. [14] M. Kondo, Y. Yu, T. Ikeda, Angew. Chem. Int. Ed. 2006, 45, 1378. [15] T. Ikeda, J. Mamiya, Y. Yu, Angew. Chem. Int. Ed. 2007, 46, 506. [16] M. Yamada, M. Kondo, R. Miyasato, Y. Naka, J. Mamiya, M. Kinoshita, A. Shishido, Y. Yu, C. J. Barrett, T. Ikeda, J. Mater. Chem. 2009, 19, 60. [17] D. H. Wang, K. M. Lee, Z. Yu, H. Koerner, R. A. Vaia, T. J. White, L.-S. Tan, Macromolecules 2011, 44, 3840. [18] H. Zeng, D. Martella, P. Wasylczyk, G. Cerretti, J. C. Lavocat, C. H. Ho, C. Parmeggiani, D. S. Wiersma, Adv. Mater. 2014, 26, 2319. [19] H. K. Bisoyi, Q. Li, Acc. Chem. Res. 2014, 47, 3184. [20] T.-H. Lin, Y. Li, C.-T. Wang, H.-C. Jau, C.-W. Chen, C.-C. Li, H. K. Bisoyi, T. J. Bunning, Q. Li, Adv. Mater. 2013, 25, 5050. [21] C. Li, C. W. Lo, D. Zhu, C. Li, Y. Liu, H. Jiang, Macromol. Rapid Commun. 2009, 30, 1928. [22] F. Greco, V. Domenici, A. Desii, E. Sinibaldi, B. Zupancˇicˇ, B. Zalar, B. Mazzolai, V. Mattoli, Soft Matter 2013, 9, 11405. [23] J. Mamiya, Polym. J. 2013, 45, 239. [24] M. Yamada, M. Kondo, J. Mamiya, Y. Yu, M. Kinoshita, C. J. Barrett, T. Ikeda, Angew. Chem. Int. Ed. 2008, 47, 4986. [25] H. Yu, J. Mater. Chem. C 2014, 2, 3047. [26] H. Yu, T. Ikeda, Adv. Mater. 2011, 23, 2149. [27] X. Ye, M. G. Kuzyk, Opt. Commun. 2014, 312, 210. [28] H. Jiang, C. Li, X. Huang, Nanoscale 2013, 5, 5225. [29] J. Liu, M. Wang, M. Dong, L. Gao, J. Tian, J. Microsc. 2011, 244, 144. [30] O. Yaroshchuk, Y. Zakrevskyy, S. Kumar, J. Kelly, L. C. Chien, J. Lindau, Phys. Rev. E 2004, 69, 011702. [31] F. Cheng, Y. Zhang, R. Yin, Y. Yu, J. Mater. Chem. 2010, 20, 4888. [32] A. Priimagi, A. Shimamura, M. Kondo, T. Hiraoka, S. Kubo, J. Mamiya, M. Kinoshita, T. Ikeda, A. Shishido, ACS Macro Lett. 2011, 1, 96. [33] N. Tabiryan, S. Serak, X. M. Dai, T. Bunning, Opt. Express 2005, 13, 7442.
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Communication
Precise Actuation of Bilayer Photomechanical Films Coated with Molecular Azobenzene Chromophores Ziyi Liu, Rong Tang, Dandan Xu, Jian Liu,* Haifeng Yu* Bilayer photomechanical films are fabricated by depositing one layer of molecular azobenzene chromophores onto flexible low-density polyethylene substrates. The photoinduced bending and unbending behavior of five azobenzene derivatives including azobenzene, 4-hydroxyazobenzene, 4-((4-hydroxyphenyl)diazenyl)bezoitrile, 4-((4-methoxyph-enyl)diazenyl)phenol, and 4-(phenyldiazenyl)phenol is systematically studied by considering the incident light intensity and the thickness of the coated chromophore layers. Precise control of photoinduced curling of the bilayer film is successfully achieved upon irradiation with two beams of UV light, and the curled films can be recovered by thermal relaxation in the dark. The easily fabricated bilayer films show fast photomechanical response, strong photoinduced stress, and stability similar to crosslinked polymeric films. 1. Introduction Photoactive azobenzene (AB) derivatives can undergo reversible isomerization between an extended trans form
Dr. Z. Liu, Dr. R. Tang, Dr. D. Xu State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials Southwest University of Science and Technology Mianyang 621010, China Prof. J. Liu School of Materials Science and Engineering Southwest University of Science and Technology Mianyang, 621010, China E-mail: [email protected] Prof. H. Yu Department of Materials Science and Engineering College of Engineering Peking University Beijing 100871, China E-mail: [email protected] Prof. H. Yu Key Laboratory of Polymer Chemistry and Physics of Ministry of Education Peking University Beijing 100871, China Macromol. Rapid Commun. 2015, 36, 1171−1176 © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
(10 Å) and a shorter cis form (5.5 Å) by UV and visible light exposure (or spontaneously thermal relaxation), respectively.[1–4] The photoisomerization synchronously converted light energy into mechanical stress by the deformation of molecular shape. Then the development of various types of molecular machines using the photomechanical effect of azobenzene chromophores has been intensively studied in the past decade. Koshima et al. adopted light-induced trans-cis isomerization of 4-(dimethylamino) azobenzene and 4-aminoazobenzene molecular crystals as actuator, which can be reversibly driven by light.[5,6] However, the configuration transition at molecular lever was very weak and difficult to be detected.[6] Incorporating azobenzene moieties into polymeric networks may amplify the photomechanical effect.[7–10] Lopez et al. dissolved azobenzene dyes into liquid-crystalline elastomers (LCEs), and the resultant LCEs showed large mechanical deformation in response to light irradiation.[11] But the azobenzene dispersed LCEs show only 2D motions such as expansion or contraction. Photoinduced 3D motions, for example, bending, twisting, and rotation, are expected from the viewpoint of practical applications.[12–20] To fulfill 3D macrolevel motion, single-domain orientational LCE films were fabricated through a compact proceeding by Ikeda et al.[14] Typically, glass slabs were
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initially spin-coated with polyimide (PI) and baked, followed by rubbing treatment to yield alignment layers. Then monomer, crosslinker, and initiator were sandwiched between the PI-coated glass plates and polymerization at the LC temperature. The resultant films were removed from the plates, thus producing freestanding LCE films with homogenously aligned mesogens.[14,21–23] Such single layers of LCE films show insufficient mechanical strength, difficult controllable shape and size in fabrication and actuation processes.[24] Then bilayer films were developed in the same group by thermal compression bonding of one single photoactive LCE layer and one unstretched low-density polyethylene (LDPE) film.[16,24] In contrast to the single-layer LCE film, the bilayer film showed excellent photoresponsive and mechanical properties simultaneously.[24,25] The bending movement of the film is usually induced by an asymmetric contraction or expansion of the azobenzene chromophore layer,[26] whereas the polymeric network contributes amplification of the deformation of chromophores. That is, a small amount of azobenzene molecules may generate macroscopically motions as that of the azobenzene polymer. However, the azobenzene dispersed in poly(methyl methacrylate) (PMMA) hybrid composite did not show any bending but contraction motions upon light exposure.[11,27] One reason is that the azobenzene was not aligned in one direction. In this study, we report one macrolever actuating film by use of azobenzene molecular crystals as the photoactive layer. In contrast to previously two-step laminating LCE film, a simple method was developed to fabricate bilayer film by direct casting azobenzene molecules on LDPE film. It is expected that the bilayer film shows unique photomechanical behaviors under the irradiation of actinic light.
2. Experimental Section 2.1. Fabrication of Bilayer Films All the azobenzene compounds (97%) were bought from Aladdin Industrial Corporation of Shanghai and used without further purification, and their chemical structures and abbreviation are shown in Figure 1a. First, a one one commercialized LDPE film (Shengda-FB29, Weifang Shengda Technology Incorporated Co.) was carefully polished with silicon carbide sandpaper (1000 grit) in one direction. Second, the polished LDPE was supersonically washed in deionized water and dried in nitrogen. Third, the LDPE film was immersed into azobenzene solutions of ethanol, and then it was perpendicularly picked up. The bilayer film was obtained upon evaporation of solvent and annealing in an oven at 40 °C for 24 h. The film was cut into samples with various aspect ratios before use. The thickness of the azobenzene layer was controlled by the solution concentration and the impregnation time.
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Figure 1. a) Chemical structure of the azobenzene molecules used in this study and the macroscopic view of the bilayer film. b) Bending movements of ABOH bilayer film (15 mm × 3 mm × 38 μm) upon UV light irradiation (365 nm, 20 mW cm−2) and the unbending in the dark.
2.2. Characterization A 365 nm UV-LED was used as the light source for photoirradiation. Ultraviolet–visible (UV–vis) absorption spectra were measured using UV–vis EVOLUTION 300 spectrophotometer. Lightinduced motions of the bilayer film were recorded by one Sony digital camera. The bending angle was analyzed with adobe Illustrator CS5 software, which was used to determine the tip displacement angle between the starting position and the tip end position of the film.
3. Results and Discussion 3.1. Preparation and Characterization of the Bilayer Film The bilayer film was prepared via a very simple and scalable method modified from a previous report.[28] By immersing the pretreated LDPE film into azobenzene solutions, the solution was adsorbed on the film surface due to the capillary action of the polish-induced microgrooves (as
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Precise Actuation of Bilayer Photomechanical Films Coated with Molecular Azobenzene Chromophores
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(a)
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Bending angle (deg)
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90 60
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5 mW/cm2 20 mW/cm2 30 mW/cm2
30 0 0
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DHAB ABOH ABMeO ABCN AB
30
0 0
3.2. Photoinduced Bending and Unbending of the Bilayer Film
1 2 Irradiation time (s)
3
90
Bending angle (deg)
Figure 1b gives several photographs of one bilayer film on photodriven motion and relaxation in dark. The simply fabricated film showed a similar photoinduced bending and unbending behaviors as those of homogeneously aligned polymeric LCE films.[31] Upon irradiation of UV light, the 4-hydroxy-azobenzene (ABOH) bilayer film fast bent to direction of the light source. It rapidly bent to an angle of 90° within 2 s irradiation. The photomechanical behaviors of the bilayer film can be explained with the so-called “bimetal mechanism.”[5] It is well known that azobenzene chromophores undergo trans-cis isomerization under UV irradiation. Although unpolarized UV light was used and selective photoisomerization of azobenzenes did not happen, anisotropic volume contraction of the active layer was induced because the ABOH molecules in the bilayer film had been in ordered distribution with their transition moments along the microgrooves as shown in Figure 3a. Whereas, another side of LDPE film always exhibited photoinert upon photoirradiation, resulting in the occurrence of the photoinduced contraction of the whole bilayer film along the polishing direction. When the UV light was turned off, the bent bilayer film fast returned to the initial flat state in dark. In contrast to the trans-cis isomerization leading to the contraction in volume of the film, the fast cis-trans isomerization caused the expansion of the photoactive layer, and thus the unbending of the whole bilayer film.[24] Five kinds of azobenzene derivatives were used to investigate the photomechanical behaviors of the bilayer films, as shown in Figure 2a. All the bilayer film with the same thickness was irradiated under UV light with
90
Bending angle (deg)
shown in Figure S1d, Supporting Information). When the film was vertically elevated from the solution, molecular crystals were formed with azobenzene moieties aligned along the microgrooves upon evaporation of the solvent, producing uniformed bilayer film with a good quality (Figure 1). The ordered distribution and regular arrangement of azobenzene molecular crystals was confirmed upon observation with polarizing optical microscope (POM). As shown in Figure S1, Supporting Information, the film showed dark field when it was placed in a direction parallel to the polarizer direction.[29] When the film was rotated by 45°, it demonstrated bright field. As contrast, the depositing of azobenzene solution on unpolished LDPE film caused typical crystallites in random distribution upon POM observation (Figure S1e, Supporting Information). Thus, the microgroove on the polished surface of LDPE film guided the orientation of azobenzene molecules, leading to an homogenous alignment of photoactive moieties onto the film.[30]
60
42 µm 35 µm 28 µm
30
0 0
1 2 Irradiation time (s)
3
Figure 2. a) The photomechanical response (bending angle) for a series of azobenzene derivatives deposited bilayer film (15 mm × 3 mm × 38 μm) under UV light irradiation (365 nm, 20 mW cm−2). The photomechanical response (bending angle) as a function of b) exposure light intensity and c) PE thickness.
the identical intensity. The bilayer film coated with nonsubstituted AB molecules showed the weakest photomechanical effect. The film coated with hydroxyl substituted ABOH reached the maximum bending angle of 90°. As contrast, the film coated with methoxyl substituted molecules (4-((4-methoxyph-enyl)diazenyl)phenol
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Table 1. Characterization and the bending behaviors of differently substituted azobenzene molecule deposited bilayer film.
Compounds AB ABOH
Max bending Bending speedb) [deg s−1] stressb) [MPa]
4,4′ positions substitution
Max absorption peaka) [nm]
Max bending angleb) [deg]
H, H
317
14.0
0.18
6.4
0.79
Order parameter
H, –OH
345
90.0
0.28
75.0
0.46
OH, –OCH3
367
73.0
0.25
22.8
0.03
ABCN
OH, –CN
369
26.0
0.21
10.8
0.26
DHAB
OH, OH
358
88.0
0.30
62.8
0.87
ABMeO
a)In
ethanol solution; b)Bilayer film size, 15 mm × 5 mm × 38 μm. UV light radiation at 365 nm, 20 mW m−2.
interaction may affect the contraction force diffusion. (ABMeO)) just obtained the maximum bending angle of Table 1 lists the physical characterizations and the light 26°. Furthermore, in the bending motion, the bigger the response of these five azobenzene derivatives. Accordbending angle of bilayer films, the faster the bending ingly, one possible explanation is that the azobenzenes speed. Typically, the pseudo-stilbene-type azobenzene are physically deposited on the LDPE substrates, thus the offers the advantages of fast trans-cis isomerization and molecular polarity huge affected the interface interaction strong dipolar interactions. In solution, the unsubstituted and then the bending rates. AB clearly underwent photoisomerization according to Obviously, the photomechanical behaviors of the the UV–vis absorptions changes. In contrast, di-substibilayer film such as bending speed and displacetuted 4-(phenyldiazenyl)phenol (DHAB) did not change ment angle should be affected by the intensity of the upon UV irradiation neither in solution nor in a solid irradiation light. When using the light intensity of crystal (Figure S3, Supporting Information). However, 5 mW cm−2, the bending speed of an ABOH-coated bilayer this phenomenon does not suggest that no photoisomerization occurred upon UV light irradiation on polar groups film was relatively slow and only a small bending angle substituted derivatives. Koshima similarly observed no was obtained, as shown in Figure 2b. With increasing the changes of the UV–vis absorptions of azobenzene crysintensity of the incident UV light, the photoresponsive tals, they explained as the photoisomerization occurred speed increased and the photoinduced deformation was near the crystal surface.[6] Another reason may be that enhanced. A bending angle of 90° was achieved within 3 s at 20 mW cm−2. When the UV light of 30 mW cm−2 was the trans-to-cis photoisomerization and cis-to-trans thermal isomerization recovery achieved in a short time used, the photoresponsive speed increased significantly scale which far shorter than the measuring time. Furtherand the maximum bending angle approximately reached more, the supported LDPE layer is photoinert, the bending and unbending of the whole film were beneficial from the directional contraction of photoactive azobenzene layer. The order parameters of all the bilayer films were calculated from their polarized UV–vis absorption spectra and the results are summarized in Table 1.[26] Among them, AB and DHAB exhibited the highest values, which are also higher than that of most of azobenzene nematic liquid crystals.[25,26] An order parameter of 0.03 was obtained for ABMeO, indicating almost no alignment in the bilayer film and leading to a bad photomechanical property. In addition to the ordering of the Figure 3. a) Schematic illustration the molecular distributions and the bending behavior azobenzene chromophores, the interof the bilayer film upon light incident form photoactive side or inert LDPE side. b) Curing facial interaction also greatly influof azobenzene bilayer film (15 mm × 5 mm × 38 μm) by controlling the incident UV radiaences the photoresponsive feature of tion (365 nm, 20 mW cm−2). The arrows indicate the light irradiation direction and its the bilayer film, since the interfacial exposure position on the film.
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3.3. Photocontrol of Curling and Unwinding of the Bilayer Film
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Considering the light-active property, the fabricated bilayer film is Janus structure, since its one side is photosensitive and another side is photoinert. To further study the mechanism of the photoresponsive behaviors of the bilayer films, both sides of the film were irradiated with UV light, as shown in Figure 3a. The film bent toward the irradiated direction of the actinic light when they were irradiated from the photoactive layer side, as shown in Figure 3b. Interestingly, the bending direction was changed when the film was irradiated with an incident light from the LDPE side. The film showed photoinduced deformation bending away from the actinic light source, just like the photoresponsive behaviors of LCEs with homeotropicaligned mesogens.[14] That is to say, the Janus-like bilayer film showed photoinduced bending to the photoactive layer side whatever it was irradiated from either side. This result confirms that the bending of the bilayer film should be benefited from the directional contraction of photoactive azobenzene layer.[24] This result inconsistent to a single layer of photomechanical LCEs with homogenously aligned mesogens film, the bending direction of which depends on the alignment direction of chromophores[32] or the incident light polarization.[33] Taking advantage of the interesting photomechanical behaviors and the good mechanical performance of the LDPE film, we can easily design complex movement by directly using the bilayer film. For instance, precise control of curling and extending of the film was conveniently achieved upon photoirradiation of the combination of two UV light beams, as shown in Figure 3b. Upon the first light exposure, film bent toward the actinic light source direction by irradiation from the side of the photoactive
0.3 Stress (Mpa)
to 90° within 1.5 s. However, when the light intensity increased to 100 mW cm−2, the strong UV light caused a strong thermal effect, leading to heat-caused contraction and permanent damage on the surface of the photoactive film. To study the effect of the structure of the bilayer film on its photoresponsive behaviors, several bilayer films were fabricated with changing the thickness of LDPE film. For comparison, the thickness of the photoactive layer was controlled at about 10 ± 1 μm. As shown in Figure 2c, the bending angle reached to 90° when the thickness of the LDPE film was 28 μm. With further increasing film thickness of PE film, the bending angle and speed are significantly decreased. When the thickness of the PE film reached to 60 μm, the photoisomerization-induced contraction of the azobenzene molecules cannot provide sufficient driving force to cause the bending of the whole LDPE substrate.
0.2
0.1
0.0 420
Light on
425 430 435 440 Irradiation time (s)
445
Figure 4. a) Schematic illustration of the experimental setup for measuring the photoinduced stress generated from the bilayer bending and unbending upon irradiation with UV light (365 nm, 20 mW cm−2) (5 cm × 5 mm × 38 μm). b) Photoinduced stress of the bilayer film upon UV irradiation and c) a partial enlarged diagram of the photoinduced stress.
layer, and then a second light exposure on the film from the side of the LDPE layer. Obviously, the second irradiation also caused the film to bend towards the photoactive side and further enhanced the bending degree of crook, resulting in precise control of curling of the film
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(Figure 3b). Compared to the bending and unbending, a controllable curing could be advantageous for artificial hands and microrobots that are capable of particular manipulations.[13] For the viewpoint of applications, the photoinduced force is one of the most importance factors for photomechanical materials. We quantitatively studied the photomechanical properties of the present bilayer film. These experiments were performed using 50 mm × 5 mm × 38 μm samples. The film was fixed at both ends and using an initial load of 0.01 MPa to film in a flat state. When UV light reached to the film surface, the film bending stress rapidly increased over 0.2 MPa. After turning the UV light off, the film stress rapidly recovered in the initial state, as shown in Figure 4. Typically, small molecules show low mechanical strength contrasting to that of polymer materials. However, the fabricated bilayer film showed strong photomechanical force output from the small molecularly deposited film. Furthermore, the film showed good antifatigue ability after many cycle of light exposure (Figure 4b,c), suggesting that the simple deposited film has similar strength and stability as that of the polymeric film.[24]
4. Conclusions In summary, a series of bilayer photomechanical films were successfully fabricated by casting molecular azobenzene chromophores onto flexible LDPE substrate. They showed reversible photoinduced bending and unbending behaviors as that of LCE polymers. Both the light intensity and the thickness of the photoactive layer showed great influence on the photoresponsiveness of the bilayer films. Good ordering of chromophores upon the polishing treatment and the polar substituent in azobenzenes were beneficial for improvement of the photomechanical response. More interestingly, the bilayer films showed unique photoinduced bending direction, enabling us to design photodriven device. Multibeam irradiations induced curing provides a new strategy to precisely actuatoring photomechanical film. Compared to the bending and unbending, a controllable curing could be advantageous for artificial hands and microrobots that are capable of particular manipulations.
Supporting Information Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements: This work was financially supported by the Open Project of State Key Laboratory Cultivation Base for Nonmetal Composites and Functional Materials (no. 11zxfk26), the Youth Innovation Research Team of Sichuan for Carbon Nano
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materials (grant number 2011JTD0017) and National Natural Science Foundation of China (51322301, 51372211). Received: March 23, 2015; Published online: May 12, 2015; DOI: 10.1002/marc.201500177 Keywords: actuators; photomechanics
azobenzenes;
photoactive
materials;
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