Article Cite This: Macromolecules XXXX, XXX, XXX-XXX
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Photomechanical Motion of Liquid-Crystalline Fibers Bending Away from a Light Source Zhangxiang Cheng,† Shudeng Ma,‡ Yihe Zhang,*,† Shuai Huang,‡ Yuxuan Chen,‡ and Haifeng Yu*,‡ †
Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid Waste, National Laboratory of Mineral Materials, School of Materials Sciences and Technology, China University of Geosciences, Beijing 100083, P. R. China ‡ Department of Materials Science and Engineering, and Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Engineering, Peking University, Beijing 100871, P. R. China S Supporting Information *
ABSTRACT: A series of photomechanical fibers was fabricated with a thermal drawing method by using liquid-crystalline random copolymers containing azobenzene and biphenyl groups in side chain. After being post-cross-linked under mild conditions, these fibers showed photoinduced bending motion away from the light source even though homogeneous alignment of mesogens was observed along the drawing direction. This abnormal photoinduced deformation of the obtained fibers is far different from previously reported light-directed motions about liquid-crystalline fiber and film materials. The interesting photomechanical deformation can be ascribed to the surface volume expansion caused by photoisomerization of azobenzene moieties. Then the photoinduced bending behaviors of these fibers containing different azobenzene concentrations and cross-linking densities were systematically investigated, suggesting that the location of photoresponsive azobenzene played an important role in deciding their photomechanical behaviors. This provides one convenient way of controlling over the photoinduced bending direction through the location of light-active moieties in side chain or cross-linker. In addition, irradiation of visible light accelerated the recovery of bent fibers. These fibers possess quick response, large deformation, and good thermal stability, indicating their promising applications for smart devices and energy conversion devices.
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INTRODUCTION Azobenzene- (AZ-) containing cross-linked liquid-crystalline polymers (AZ-CLCPs) are a fascinating class of photoresponsive materials due to their dramatic changes in size and shape caused by the photoisomerization of AZ moieties.1−7 Under irradiation of actinic light, AZ-CLCP materials often show photoinduced deformations such as reversible contraction or expansion,8−10 bending or unbending11−14 and twisting or coiling,15−22 which have been realized by many groups. Thus, researches on photoinduced deformation of AZ-CLCP materials, which can directly convert light energy into mechanical work, have been flourishing owing to their promising applications as motors,23 oscillators,24−26 inchworms,19,27 and optical pendulum generators28 and in biomimetic motion,13,14,29,30 and so on. In photoresponsive AZ-CLCP material system, there are two kinds of photoisomerization-induced effects accompanying the transition of one AZ moiety from its trans state to the less stable cis state. The first effect is the reduction of the order parameter of systems when the cis isomer forms upon photoirradiation of actinic light.31−33 Generally, the conformation of polymer main chains strongly couples with the alignment of AZ mesogens in the side chains. As a result, the photoinduced molecular cooperative motion of LC molecules has been utilized to magnify the structural change of AZ © XXXX American Chemical Society
molecules and the alignment change of the mesogens directed by light. Thus, the diversification in polymer-chain conformation can be amplified into macroscopic deformation, which is often visible to the naked eye. A lot of AZ-CLCP film and fiber materials of Ikeda type (a high concentration of AZ groups) have been reported to demonstrate photoinduced bending toward the light source based on this effect.31−36 They also revealed that the photomechanical motion of AZ-CLCP materials was influenced by several factors, such as crosslinking density,34 film thickness, and AZ concentration.35 Particularly, the initial alignment of photoactive mesogens strongly affects the photomechanical behavior of AZ-CLCPs.36 The second famous effect is the free-volume expansion caused by the trans-to-cis photoisomerization since the cisisomers of AZ groups often need a larger free volume than the trans-ones.37 For instance, in a homogeneously aligned CLCP material system, the film with a low concentration of AZ moieties was reported to bend away from the light source due to surface volume expansion.38 Barrett et al. have demonstrated this volume-expansion effect by using ellipsometry and neutron reflectometry when they studied the photomechanical properties of amorphous non-cross-linked AZ-containing polyReceived: August 11, 2017
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DOI: 10.1021/acs.macromol.7b01741 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. Materials used in this paper and fabrication process for fibers. (a) Chemical structures of the monomers (M6AB2 and M6BP3OH) and the cross-linker (HDI) used to fabricate CLCP fibers and (b) Schematic illustration of fabrication process of the CLCP fibers.
mers.39,40 Broer and co-workers also reported that dualwavelength light exposure can create a large surface deformation of an AZ-CLCP coating (Broer type: a low concentration of AZ groups),37,41,42 exhibiting a striking volume change upon photoirradiation. Till now, there are seldom researches to elucidate the influence of the structure on the photoresponsive behavior of these AZ-CLCP materials. Recently, white et al. adopted bluegreen light to induce oscillation motion of AZ-CLCP film (Ikeda-type) as well as direct bending motion of monodomain aligned AZ-CLCPs systems (Broer-type).24,43,44 However, it is difficult to identify these two photoisomerization-caused effects in the AZ-CLCP materials, which could determine the bending direction of the photomechanical motion. In this study, we aim to explain the effect of the reduction of mesogenic ordering and the free-volume expansion on the photomechanical motion of AZ-CLCP fibers. We will show unusual photoinduced deformation of AZ-CLCP fibers prepared by post-cross-linking of a series of LC random copolymers with bridging hexamethylene diisocyanate (HDI). The study on photoinduced bending behaviors of AZ-CLCP fibers containing different AZ concentrations and cross-linking densities indicates that the location of AZ has an important influence on the photomechanical behaviors of AZ-CLCPs. In addition, it also offers one superior approach to achieve photoresponsive CLCPs fibers in mild condition.
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Table 1. Nomenclature and Feed Ratios of the Two Monomers for Synthesis of a Series of AZ-Containing Random Copolymers sample
M6AB2 (mol %)
M6BP3OH (mol %)
AIBN (1 wt %)
P91 P82 P73 P55 P46 P37
90 80 70 50 40 30
10 20 30 50 60 70
1 1 1 1 1 1
evacuate−thaw cycles, and the polymerization was carried out in anisole at 70 °C for 48 h. Then the random copolymers were precipitated in methanol, collected by filtration, and dried in a vacuum oven. The synthesis details and characterizations are provided in Figures S2−S8. Fabrication of the AZ-CLCP Fibers. As shown in Figure 1b, the AZ-CLCP fibers were prepared by the following steps.29,46,47 A small amount of one AZ-containing random copolymer was heated up to 155 °C (higher than the clearing point) on a glass substrate placed on a hot stage. The un-cross-linked fibers were fabricated by quickly drawing out from the soft copolymer by using tweezers, which were then immersed in a solution of the cross-linker HDI to undergo crosslinking at 50 °C for 2 days. After being washed with ethyl acetate to get rid of the residual HDI and dried at ambient temperature, a series of freestanding CLCP fibers with various AZ concentrations and crosslinking densities were obtained. Characterization. 1H NMR (DRX-500, Bruker) spectra was used to characterize the structure of the copolymers, and the molar ratio of the two LC components calculated by value of integral of random copolymers. The molecular weight and the polydispersity index were measured by gel permeation chromatography (GPC; Waters1515, Waters) with tetrahydrofuran (THF) as the eluent at a flow rate of 1.0 mL/min. Thermal properties of the LC monomers and the random copolymers were characterized by differential scanning calorimetry (DSC, DSC 8000, PerkinElmer) at a heating and cooling rate of 10 °C/min. The phase behavior of the random copolymers and the optical anisotropy of the AZ-CLCP fibers were measured by using a polarized optical microscope (POM; ZEISS, Axio Scope.A1) with a hot stage. The mesogenic alignment in the AZ-CLCP fibers was confirmed by a polarized UV−vis absorption spectrometer (Lambda 750, PerkinElmer).
EXPERIMENTAL SECTION
Synthesis of the Random LC Copolymers. The AZ-containing random LC copolymer was synthesized through free radical polymerization of the two LC monomers (M6AB2 and M6BP3OH) with a small amount of 2, 2-azobis(isobutyronitrile) (AIBN) as the initiator. The chemical structure and thermal properties of these two molecules are shown in Figure 1a. The monomer M6BP3OH was homemade with a high yield, and its 1H NMR spectrum and corresponding structural assignment are given in Figure S2. The AZcontaining monomer M6AB2 was synthesized according to our previously reported processes,45 which is also provided in Supporting Information (SI). To prepare a series of random LC copolymers, these two LC monomers were mixed with different molar feed ratios as shown in Table 1. The mixtures were degassed by three freeze− B
DOI: 10.1021/acs.macromol.7b01741 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules Photoinduced Bending Behaviors of AZ-CLCP Fibers. The samples for photomechanical studies were prepared by cutting the obtained AZ-CLCP fibers into a size of 10 mm (L) × 0.3 mm (D). The photoinduced bending motion of the AZ-CLCP fibers was brought about upon photoirradiation with 365 nm UV light (HTLD-4 II, Shenzhen Height-LED Optoelectronic Tech Co. Ltd.). The photodeformed fibers were then heated or exposed to 530 nm visible light to revert to the initial shape. These processes were in situ recorded with one video camera.
(Figure S7), suggesting the formation of the nematic LC phase.45 Properties of the AZ-CLCP Fibers. The un-cross-linked AZ-CLCP fibers were fragile at room temperature but became soft when they were dipped into the HDI solution for several hours. To verify whether the cross-linking was completed after the cross-linking reaction, both the virgin and the post-treated fibers were immersed in THF solution, as depicted in Figure 2b. The cross-linked fibers exhibited stable in organic solvents and no obvious expansion was observed, whereas the virgin fiber was completely dissolved in THF. Moreover, the peaks corresponding to the phase transition (Tni) of the random copolymers disappeared after the cross-linking reaction (Figure S8). These different phenomena indicate that the cross-linking reaction between the hydroxyl groups in the side chain of the copolymer and the isocyanates at both ends of HDI really occurred even at mild condition. The average diameter of the AZ-CLCP fibers was approximately 300 μm, which can be obtained from Figure 2c and the cross-sectional image (Figure S9). In addition, the chemical cross-linking reaction had little effect on the size of the fibers. Then the mesogenic alignment in CLCP fibers was checked by observation with one polarizing optical microscope (POM) and measurement of polarized UV−vis absorption spectra. As shown in Figure 2c, the lowest transmittance was observed when the fiber axis was parallel to one of the polarizer (P) or the analyzer (A), whereas the highest transmittance appeared when the fiber was rotated by 45°. From the POM observation and the polarized absorption spectra (Figure S10), it is reasonable to infer that highly ordered orientation of mesogens was achieved in the fabrication process, which is similarly to that of previously reported CLCP fibers.29,46,47 Obviously, the mesogens were preferentially aligned along the fiber axis, which is also the drawing direction in the fabrication process. After the chemical cross-linking treatment, there was almost no change in the mesogenic alignment, which is very important for the following study on photomechanical motion. Photomechanical Motion of the AZ-CLCP Fibers. The photoinduced deformation of the AZ-CLCP fibers were examined by using an optical setup shown in Figure 3a. To
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RESULTS AND DISCUSSION Properties of the Random Copolymers. The numberaverage molecular weight (Mn) and the molecular-weight distribution (Mw/Mn) of a series of AZ-containing random copolymers are summarized in Table 2. Because of the similar Table 2. Summary of the Properties of the Obtained Random LC Copolymersa sample
Mn
PDI
Tg (°C)
Tni (°C)
P91 P82 P73 P55 P46 P37
16 700 13 100 17 600 17 000 17 300 16 900
1.97 1.93 2.13 1.96 1.97 2.01
89 75 88 99 86 80
144 138 144 146 151 155
a
Tg is the glass transition temperature, and Tni is the nematic LC to isotropic phase transition temperature (or the clearing point).
molecular structure of methacrylate, the two LC monomers showed very similar polymerization capability, leading to their composition in the random copolymer is almost the same as the feed molar ratio upon free radical polymerization. Their mesomorphic properties were investigated by the measurement of DSC and observation of POM. According to DSC curves (Figure S6), all the random copolymers exhibit higher phasetransition temperatures (above 130 °C) in both the heating and cooling processes. In addition, a glass transition temperature at about 80 °C was observed. These thermal properties are also summarized in Table 2. All the random copolymers show typical Schlieren texture when it was heated up to about 130 °C
Figure 2. (a) Photograph of the free-standing AZ-CLCP fiber (P73). (b) Uncross-linked fiber (vial 1) and cross-linked fiber (vial 2) were dissolved in THF, showing that cross-linking reaction was realized through the simple immersion in the cross-linker HDI. (c) POM images of AZ-CLCP fibers with a drawing direction parallel to one of the polarization direction and angle between the drawing direction and polarization direction of either polarizer was 45°. A, analyzer. P, polarizer. C
DOI: 10.1021/acs.macromol.7b01741 Macromolecules XXXX, XXX, XXX−XXX
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Figure 3. Photomechanical motion of the AZ-CLCP fibers. (a) Experimental setup for study the photoinduced bending of AZ-CLCP fiber upon irradiation with one light source of UV-LED. Size of the fiber: 10 mm (L) × 0.3 mm (D). (b) Series of snapshots of maximum bending of the AZCLCP fibers at different temperatures. When the temperature of the fibers was set below 80 °C, long-time photoirradiation did not cause any significant deformation to the fibers.
Figure 4. Relationship between the temperature and photoinduced bending motion of AZ-CLCPs fibers: (a) upon UV irradiation, the time required for the fibers to bend to the maximum angle at different temperatures; (b) after the light is turned off, the time required for the fiber to return to the initial state.
carried out this light-directed experiment, one side of the fiber was fixed on a glass substrate put on a hot state, and the other side was irradiated with UV light (365 nm, 192 mW/cm2) at elevated temperatures. Because of their Tgs (Table 2), both the un-cross-linked and the cross-linked fibers showed no response at room temperature, and prolonged exposure did not cause any significant deformation. The photoinduced changes were not observed until the temperature was increased above 80 °C. The un-cross-linked fibers became soft at this temperature and then quickly melting upon UV irradiation. Such photosoft and melting behaviors of the random LC copolymers were ascribed to the occurrence of the photoinduced phase transition from LC to isotropic phase when the temperature was higher than Tg, which has been recently reported in AZ-containing LC polymer materials.48 Far differently, the cross-linked AZ-CLCP fibers exhibited interesting photomechanical motion, as shown in Movie S1. Upon irradiation of UV light, the cross-linked fiber began to deform, bending away from the light source, and then quickly reached the maximum bending angle. Further exposure did not enhance this photoinduced deformation. Figure 3b shows the photoinduced bending behaviors of the fibers performed at different temperatures when the best morphing effect was observed.
Temperature Effect on the Photomechanical Motion. It must be mentioned here that the bending rate of the AZCLCP fiber linearly increased with the elevated temperature until it reached a crucial point where the photoinduced deformation became less controllable. This critical temperature was about 140 °C, as shown in Figure 4a. Interestingly, the bending rate of the fiber at 130 °C was slower than that obtained at 120 °C (Figure 4a). It can be ascribed to the faster thermal relaxation of AZ moieties from their cis-isomers to trans-ones at a higher temperature. In addition, it was also found that the maximum bending angle was almost the same when the temperatures was above 80 °C, but which is higher than the bending angle at 80 °C (Figure 3b). This can be explained by the Tg of the random copolymers which is close to 80 °C (Figure S6 and Table 2). Obviously, the segmental motion of the copolymer was frozen at the temperatures below its Tg, and the photoisomerization of AZ groups in the side chain did not bring about strong effect on the polymer main chain. As a result, the light-induced deformation to the AZCLCP system was too small to be detected. Effect of Wavelength of Light on the Photomechanical Motion. Recently, Broer et al. reported interesting enhancement of surface morphing in LC coating by using two-wavelength 365 and 455 nm exposure comparing to oneD
DOI: 10.1021/acs.macromol.7b01741 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules wavelength 365 nm exposure.37,42 Light at 455 nm is usually adopted to promote the back conversion to the AZ transisomers and the related oscillatory dynamics of the isomerization reaction.24,42 It is a pity that this boosting effect was not obvious in the present AZ-CLCP fiber system upon photoirradiation of two-wavelength light. Considering the periodic microstructures in Broer’s LC coating, the accelerated backisomerization can elegantly amplify the pressure gradient between the two stripe-patterned areas with different crosslinked densities.37 Such enhanced pressure gradient has been used to enhance the surface relief in holographic grating.49 This is very different from the present AZ-CLCP fibers. Although a gradient in cross-linking density could be formed upon the post-cross-linking treatment, no effect can be acquired because the gradient in cross-linking density should be circularly symmetric and no periodic structure was achieved even the back-isomerization of AZs can be facilitated. Like the photomechanical motion of other AZ-CLCP material systems, the present AZ-CLCP fibers also showed good reversibility due to the existence of the cross-linking structure. Turning off UV light, the bent fibers completely reverted to the initial flat state when they were set at a higher temperature or being irradiated with visible light. Figure 4b demonstrates that the recovering rate increases monotonically with temperature, but it took several minutes to recover when the temperature was lower than 110 °C. This recovery process of the bent AZ-CLCP fibers was greatly accelerated with the aid of exposure to visible light at 530 nm. As shown in Movie S7, it took ca. 15 s for the bent AZ-CLCP fiber to restore to its initial state upon visible-light irradiation compared with ca. 60 s in the case without photoirradiation at 100 °C. Mechanism of the Photomechanical Motion. In an AZCLCP material system (Ikeda type) with homogeneously aligned mesogens, the current mechanism is based on the reduction of mesogenic ordering when the cis-AZ moieties are photoinduced (Figure 5a).32 While illuminated with UV light, the contraction of materials is asymmetric due to the gradient in the cis-isomer concentration, causing the material to bend toward the light source. Obviously, these effects cannot explain the opposite bending direction of the present AZ-CLCP fibers. In previous work,11−13,21,23,27,32−35,47,50 the overall photo-
mechanical response of related material systems has one common feature: the CLCP films or fibers, which bend toward the light source, have one photoresponsive AZ group in the cross-linker. Some of the other factors, such as the concentration of AZ, cross-linking densities, do not affect the bending direction of the CLCP materials. Recently, Ikeda et al. have demonstrated that the effect of AZ photoisomerization in CLCP side chain is insignificant on photomechanical properties.35 Therefore, we postulate that whether the AZ moiety is in the cross-linker or not plays an important role in determining the bending direction of the AZ-CLCP fibers. As shown Movie S6 and Figure S11, the reduction of mesogenic ordering was not observed in the process of photoinduced bending of the present AZ-CLCP fibers under photoirradiation. Although UV light easily causes trans-to-cis isomerization of AZ groups in CLCP side chain, the phase transition from an LC to isotropic phase has not been observed, which is different from the photomechanical motion of Ikedatype CLCP materials.11−13,21,23,27,32−35,47,50 The monomethacrylate AZ monomer is only linked in the side chain of the cross-linked LC network, and photoisomerization process of AZ groups should be decoupled from the LC network. Therefore, the conformation of polymer main chain has no obvious change because of the photochemical reaction of AZs in side change exerting less force on the LC network. A more plausible explanation is the free volume effect of AZ photoisomerization, which should be responsible for this unusual bending behavior of the present CLCP fibers. It has been discussed by Barrett, Priimagi, and Broer et al. that an AZ group generally needs free space for its isomerization to occur upon UV irradiation.37−42 Because of the large molar extinction coefficient of the AZ chromophores, UV irradiation can induce trans-to-cis isomerization only in the surface region of the AZCLCP fibers, causing an expansion along the horizontal direction or the fiber axis direction. Thus, the local volume in the photoirradiated side of the fiber increases, whereas the back or the unirradiated side of the fiber remained unchangeable since no photoisomerization occurs. This results in the bending motion of the fibers away from the light source, like the way shown in Figure 5b. Although a gradient in cross-linking density could be formed upon the post-cross-linking treatment,51 it should not influence the bending direction because the gradient in cross-linking density should be circularly symmetric. Upon irradiation of UV light, the transition of azobenzene located on the LC network from the trans to cis state occurred only in the surface area of the fiber, and the corresponding volume expansion in the irradiated area is the driven force for photoinduced bending away from the light sources. To further characterize this photoinduced expansion behavior, both ends of the CLCP fiber were fixed in a hot stage. Such a fiber exhibited deformation toward the light source, as shown in Movie S8 and Figure 6. Similarly, this photoinduced deformation is thermally reversible upon irradiation of visible light at 530 nm. In a homogeneously aligned CLCP material system with a low concentration of AZ chromophores, Priimagi et al. reported that the film containing AZs as side chains bent away from the light source due to surface volume expansion.38 They demonstrated that if the AZ concentration was increased to 20 mol % or higher, the samples bent toward the light source (at least under high-intensity irradiation) irrespective of the location of the AZ moieties. Far differently, the present AZCLCP fibers bend away from the light source even though they
Figure 5. Schematic illustration of the different photoinduced deformation of CLCP materials with AZ groups (a) in the crosslinker and (b) in the side chain. The present AZ-CLCP fibers show the photomechanical motion of part b. E
DOI: 10.1021/acs.macromol.7b01741 Macromolecules XXXX, XXX, XXX−XXX
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Figure 6. Photoresponsive properties of the CLCP (P55) fiber with both ends fixed on a hot stage upon UV irradiation. The fiber exhibited deformation toward the light source.
possess a high AZ concentration (>70%, Table 1). To further validate our point, we prepared a series of AZ-CLCP fibers with different AZ concentrations and cross-linking densities to study their photomechanical behaviors. The photoinduced bending behaviors of these CLCP fibers are in situ recorded and given in Movies S2, S3, S4, and S5, respectively. Their photoresponsive results are summarized in Table 3. All the AZ-CLCP fibers
Figure 7. UV−vis absorption spectrum of the random copolymer (P73, feed ratio = 7:3) in THF, before irradiation (a) and after UV irradiation for 1 s (b) and for 2 s (c), and after subsequent heating for 30 s at 130 °C (d) and at 110 °C (e).
Table 3. Summary of the Photomechanical Motion of a Series of AZ-CLCP Fibers with Different AZ Concentrations and Cross-Linking Densitiesa
fibers triggered by the AZ photoisomerization, both the photoresponsive rate and the recovery of the photomechanical motion were accelerated at the higher temperature (Figure 4). In addition, another way to accelerate the back-isomerization of AZ is irradiation of visible light, which also contributes to the enhancement of reversible photomechanical motion of the fibers (Movie S7).
cross-linked samples
feed ratio (M6AB2:M6BP3OH) photoinduced motion
1
2
3
4
5
6
P91 9:1
P82 8:2
P73 7:3
P55 5:5
P46 4:6
P37 3:7
N
SB
BA
BA
BA
BA
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CONCLUSION In summary, we have demonstrated a novel class of photoresponsive CLCP fibers fabricated with a series of AZcontaining random copolymers by using a simple process of post-cross-linking in mild condition. The AZ-CLCP fibers exhibited a large, quick and reversible photomechanical motion with bending direction away from the light source. Their photoinduced bending behaviors were systematically studied with AZ-CLCP fibers containing different AZ concentrations and cross-linking densities. This interesting photoinduced deformation can be ascribed to the volume expansion caused by AZ photoisomerization upon UV irradiation, which provides one convenient way of controllable photomechanical motion through the location of AZ moieties in side chains or crosslinkers. The present material system will have promising applications for smart devices and energy conversion devices.
a
N: unbending; BA: Bending away from the light source; SB: slightly bending.
exhibited the same light-responsive feature; i.e., they bent away from the light source with only one exception of sample 1. No obvious photomechanical motion was observed for sample 1 with a very high AZ concentration (90 mol %) even though the fiber was heated up to a high temperature or irradiated with a high intensity of UV light. It can be ascribed to that the penetration of the incident light is too low in the high AZconcentration polymer, resulting in the ratio of AZ isomerization of the fiber lower than that of the others.34 To eliminate the influence of geometry on the photomechanical motion, this AZ-CLCP material in a conventional cantilever state was also fabricated. Such a cantilever has the same responsive feature, as shown in Movie S9, Movie S10, and Figure S12. It bent away from the UV light source and recovered upon visible light irradiation. Besides, it is well-known that visible light and heating can accelerate the back-isomerization of AZ moieties from cis to trans isomers. As shown in Figure 7, one random copolymer P73 without cross-linking was chosen to study the thermal effect on the back-isomerization. In the UV−vis absorption spectra of the copolymer before UV irradiation, the maximum absorption peak at 360 nm is owing to the π−π* transition of AZs, the other peak at about 460 nm is attributed to the n-π* transition of AZ (Figure 7a). Upon light irradiation, the photoisomerization was quickly induced and the photostationary state was acquired after 2 s of photoirradiation (Figure 7c). Then the back-isomerization of AZ was thermally induced, and the peak at about 460 nm increased obviously at 110 °C (Figure 7d) and 130 °C (Figure 7e). Since the photoinduced bending of fibers was acquired by the surface expansion of
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01741. Experimental details and characterization data (Schemes S1 and S2 and Figures S1−S12) (PDF) Movie S1, photoinduced bending behavior of the AZCLCP fiber fabricated by random copolymer 3 (n(M6AB2):n(M6BP3OH) = 7:3) (AVI) Movie S2, photoinduced bending behavior of the AZCLCP fiber fabricated by random copolymer 2 (n(M6AB2):n(M6BP3OH) = 8:2) (AVI) Movie S3, photoinduced bending behavior of the AZCLCP fiber fabricated by random copolymer 4 (n(M6AB2):n(M6BP3OH) = 5:5) (AVI) F
DOI: 10.1021/acs.macromol.7b01741 Macromolecules XXXX, XXX, XXX−XXX
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(9) Li, M. H.; Keller, P.; Li, B.; Wang, X.; Brunet, M. Light-Driven Side-On Nematic Elastomer Actuators. Adv. Mater. 2003, 15, 569− 572. (10) Lv, J.; Liu, Y.; Wei, J.; Chen, E.; Qin, L.; Yu, Y. Photocontrol of Fluid Slugs in Liquid Crystal Polymer Microactuators. Nature 2016, 537, 179−184. (11) Ikeda, T.; Nakano, M.; Yu, Y.; Tsutsumi, O.; Kanazawa, A. Anisotropic Bending and Unbending Behavior of Azobenzene LiquidCrystalline Gels by Light. Adv. Mater. 2003, 15, 201−205. (12) Yu, H.; Ikeda, T. Photocontrollable Liquid-Crystalline Actuators. Adv. Mater. 2011, 23, 2149−2180. (13) Zeng, H.; Wani, O. M.; Wasylczyk, P.; Kaczmarek, R.; Priimagi, A. Self-regulating Iris Based on Light-actuated Liquid Crystal Elastomer. Adv. Mater. 2017, 29, 1701814. (14) Wani, O. M.; Zeng, H.; Priimagi, A. A Light-driven Artificial Flytrap. Nat. Commun. 2017, 8, 15546. (15) Zheng, Z.; Li, Y.; Bisoyi, H. K.; Wang, L.; Bunning, T. J.; Li, Q. Three-Dimensional Control of the Helical Axis of a Chiral Nematic Liquid Crystal by Light. Nature 2016, 531, 352−356. (16) Yu, Y.; Maeda, T.; Mamiya, J.; Ikeda, T. Photomechanical Effects of Ferroelectric Liquid-Crystalline Elastomers Containing Azobenzene Chromophores. Angew. Chem., Int. Ed. 2007, 46, 881− 883. (17) Liu, Z.; Tang, R.; Xu, D.; Liu, J.; Yu, H. F. Precise Actuation of Bilayer Photomechanical Film Coated with Molecular Azobenzene Chromophores. Macromol. Rapid Commun. 2015, 36, 1171−1176. (18) Wie, J. J.; Lee, K. M.; Smith, M. L.; Vaia, R. A.; White, T. J. Torsional Mechanical Responses in Azobenzene Functionalized Liquid Crystalline Polymer Networks. Soft Matter 2013, 9, 9303−9310. (19) Zeng, H.; Wani, O. M.; Wasylczyk, P.; Priimagi, A. Light-driven, Catepillar-Inspired Miniature Inching Robot. Macromol. Rapid Commun. 2017, 1700224. (20) Lu, X.; Guo, S.; Tong, X.; Xia, H.; Zhao, Y. Tunable Photocontrolled Motions Using Stored Strain Energy in Malleable Azobenzene Liquid Crystalline Actuators. Adv. Mater. 2017, 29, 1606467. (21) Ube, T.; Kawasaki, K.; Ikeda, T. Photomobile Liquid-Crystalline Elastomers with Rearrangeable Networks. Adv. Mater. 2016, 28, 8212−8217. (22) Iamsaard, S.; Asshoff, S. J.; Matt, B.; Kudernac, T.; Cornelissen, J. J. L. M.; Fletcher, S. P.; Katsonis, N. Conversion of Light into Macroscopic Helical Motion. Nat. Chem. 2014, 6, 229−235. (23) Yamada, M.; Kondo, M.; Mamiya, J.; Yu, Y.; Kinoshita, M.; Barrett, C. J.; Ikeda, T. Photomobile Polymer Materials: towards Light-Driven Plastic Motors. Angew. Chem., Int. Ed. 2008, 47, 4986− 4988. (24) White, T. J.; Tabiryan, N. V.; Serak, S. V.; Hrozhyk, U. A.; Tondiglia, V. P.; Koerner, H.; Vaia, R. A.; Bunning, T. J. A High Frequency Photodriven Polymer Oscillator. Soft Matter 2008, 4, 1796−1798. (25) Serak, S.; Tabiryan, N.; Vergara, R.; White, T. J.; Vaia, R. A.; Bunning, T. J. Liquid Crystalline Polymer Cantilever Oscillators Fueled by Light. Soft Matter 2010, 6, 779−783. (26) Lee, K. M.; Smith, M. L.; Koerner, H.; Tabiryan, N.; Vaia, R. A.; Bunning, T. J.; White, T. J. Photodriven, Flexural−Torsional Oscillation of Glassy Azobenzene Liquid Crystal Polymer Networks. Adv. Funct. Mater. 2011, 21, 2913−2918. (27) Yamada, M.; Kondo, M.; Miyasato, R.; Naka, Y.; Mamiya, J.-i.; Kinoshita, M.; Shishido, A.; Yu, Y.; Barrett, C. J.; Ikeda, T. Photomobile Polymer Materials - Various Three-Dimensional Movements. J. Mater. Chem. 2009, 19, 60−62. (28) Tang, R.; Liu, Z.; Xu, D.; Liu, J.; Yu, L.; Yu, H. Optical Pendulum Generator Based on Photomechanical Liquid-Crystalline Actuators. ACS Appl. Mater. Interfaces 2015, 7, 8393−8397. (29) Gelebart, A. H.; Mc Bride, M.; Schenning, A. P. H. J.; Bowman, C. N.; Broer, D. J. Photoresponsive Fiber Array: toward Mimicking the Collective Motion of Cilia for Transport Applications. Adv. Funct. Mater. 2016, 26, 5322−5327.
Movie S4, photoinduced bending behavior of the AZCLCP fiber fabricated by random copolymer 5 (n(M6AB2):n(M6BP3OH) = 4:6) (AVI) Movie S5, photoinduced bending behavior of the AZCLCP fiber fabricated by random copolymer 6 (n(M6AB2):n(M6BP3OH) = 3:7) (AVI) Movie S6, upon UV light irradiation, the structural anisotropy of the AZ-CLCP fiber fabricated by random copolymer 3 was not destroyed (AVI) Movie S7, visible light accelerating the recovery process of the AZ-CLCP fiber (AVI) Movie S8, photoinduced bending behavior of the AZCLCP fiber fabricated by random copolymer 4 (n(M6AB2):n(M6BP3OH) = 5:5) (AVI) Movie S9, photoinduced bending behavior of the AZCLCP in a conventional cantilever geometry fabricated by random copolymer 3 (n(M6AB2):n(M6BP3OH) = 7:3) (AVI) Movie S10, recovery behavior of the AZ-CLCP cantilever shown in Movie S9 (AVI)
AUTHOR INFORMATION
Corresponding Authors
*(H.Y.) E-mail:
[email protected]. *(Y.Z.) E-mail:
[email protected]. ORCID
Yihe Zhang: 0000-0002-1407-4129 Haifeng Yu: 0000-0003-0398-5055 Author Contributions
Z.C. and S.M. contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was sponsored by the National Natural Science Foundation of China (Grant Nos. 51322301, 51573005, 51572246)
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REFERENCES
(1) Bandara, H. M. D.; Burdette, S. C. Photoisomerization in Different Classes of Azobenzene. Chem. Soc. Rev. 2012, 41, 1809− 1825. (2) Bisoyi, H. K.; Li, Q. Light-Driven Liquid Crystalline Materials: From Photo-Induced Phase Transitions and Property Modulations to Applications. Chem. Rev. 2016, 116, 15089−15166. (3) Li, Q. Intelligent Stimuli-Responsive Materials: From Well-Defined Nanostructures to Applications; John Wiley & Sons: 2013. (4) Bisoyi, H. K.; Li, Q. Light-Directing Chiral Liquid Crystal Nanostructures: From 1D to 3D. Acc. Chem. Res. 2014, 47, 3184− 3195. (5) Seki, T. Light-directed Alignment, Surface Morphing and Related Processes: Recent Trends. J. Mater. Chem. C 2016, 4, 7895−7910. (6) Bushuyev, O. S.; Aizawa, M.; Shishido, A.; Barrett, C. J. ShapeShifting Azo Dye Polymers: Towards Sunlight-Driven Molecular Devices. Macromol. Rapid Commun. 2017, 1700253. (7) Priimagi, A.; Barrett, C. J.; Shishido, A. Recent Twists in Photoactuation and Photoalignment Control. J. Mater. Chem. C 2014, 2, 7155−7162. (8) Finkelmann, H.; Nishikawa, E.; Pereira, G. G.; Warner, M. A New Opto-Mechanical Effect in Solids. Phys. Rev. Lett. 2001, 87, 015501. G
DOI: 10.1021/acs.macromol.7b01741 Macromolecules XXXX, XXX, XXX−XXX
Article
Macromolecules
Induces Reversible Solid-to-Liquid Transitions. Nat. Chem. 2017, 9, 145−151. (49) Yu, H.; Okano, K.; Shishido, A.; Ikeda, T.; Kamata, K.; Komura, M.; Iyoda, T. Enhancement of Surface-Relief Gratings Recorded in Amphiphilic Liquid-Crystalline Diblock Copolymer by Nanoscale Phase Separation. Adv. Mater. 2005, 17, 2184−2188. (50) Zhang, Y.; Xu, J.; Cheng, F.; Yin, R.; Yen, C.-C.; Yu, Y. Photoinduced Bending Behavior of Crosslinked Liquid-Crystalline Polymer Films with a Long Spacer. J. Mater. Chem. 2010, 20, 7123− 7130. (51) van Oosten, C. L.; Corbett, D.; Davies, D.; Warner, M.; Bastiaansen, C.W. M.; Broer, D. J. Bending Dynamics and Directionality Reversal in Liquid Crystal Network Photoactuators. Macromolecules 2008, 41, 8592−8596.
(30) Gelebart, A. H.; Mulder, D. J.; Varga, M.; Konya, A.; Vantomme, G.; Meijer, E. W.; Selinger, R. L. B.; Broer, D. J. Making Waves in a Photoactive Polymer Film. Nature 2017, 546, 632−636. (31) Yu, Y. L.; Nakano, M.; Ikeda, T. Directed Bending of a Polymer Film by Light - Miniaturizing a Simple Photomechanical System Could Expand its Range of Applications. Nature 2003, 425, 145. (32) Harris, K. D.; Cuypers, R.; Scheibe, P.; Van Oosten, C. L.; Bastiaansen, C. W. M.; Lub, J.; Broer, D. J. Large Amplitude LightInduced Motion in High Elastic Modulus Polymer Actuators. J. Mater. Chem. 2005, 15, 5043−5048. (33) Ube, T.; Ikeda, T. Photomobile Polymer Materials with Crosslinked Liquid-Crystalline Structures: Molecular Design, Fabrication and Functions. Angew. Chem., Int. Ed. 2014, 53, 10290−10299. (34) Lee, K. M.; Koerner, H.; Vaia, R. A.; Bunning, T. J.; White, T. J. Relationship between the Photomechanical Response and the Thermomechanical Properties of Azobenzene Liquid Crystalline Polymer Networks. Macromolecules 2010, 43, 8185−8190. (35) Kondo, M.; Sugimoto, M.; Yamada, M.; Naka, Y.; Mamiya, J.; Kinoshita, M.; Shishido, A.; Yu, Y.; Ikeda, T. Effect of Concentration of Photoactive Chromophores on Photomechanical Properties of Crosslinked Azobenzene Liquid-Crystalline Polymers. J. Mater. Chem. 2010, 20, 117−122. (36) Kondo, M.; Yu, Y. L.; Ikeda, T. How Does the Initial Alignment of Mesogens Affect the Photoinduced Bending Behavior of LiquidCrystalline Elastomers. Angew. Chem., Int. Ed. 2006, 45, 1378−1382. (37) Liu, D.; Bastiaansen, C. W. M.; den Toonder, J. M. J.; Broer, D. J. Photo-Switchable Surface Topologies in Chiral Nematic Coatings. Angew. Chem., Int. Ed. 2012, 51, 892−896. (38) Priimagi, A.; Shimamura, A.; Kondo, M.; Hiraoka, T.; Kubo, S.; Mamiya, J.-I.; Kinoshita, M.; Ikeda, T.; Shishido, A. Location of the Azobenzene Moieties within the Cross-linked Liquid-Crystalline Polymers can Dictate the Direction of Photoinduced Bending. ACS Macro Lett. 2012, 1, 96−99. (39) Tanchak, O. M.; Barrett, C. J. Light-Induced Reversible Volume Changes in Thin Films of Azo Polymers: the Photomechanical Effect. Macromolecules 2005, 38, 10566−10570. (40) Yager, K. G.; Tanchak, O. M.; Godbout, C.; Fritzsche, H.; Barrett, C. J. Photomechanical Effects in Azo-Polymers Studied by Neutron Reflectometry. Macromolecules 2006, 39, 9311−9319. (41) Liu, D.; Bastiaansen, C. W. M.; den Toonder, J. M. J.; Broer, D. J. Light-Induced Formation of Dynamic and Permanent Surface Topologies in Chiral−Nematic Polymer Networks. Macromolecules 2012, 45, 8005−8012. (42) Liu, D.; Broer, D. J. New Insights into Photoactivated Volume Generation Boost Surface Morphing in Liquid Crystal Coatings. Nat. Commun. 2015, 6, 8334. (43) White, T. J.; Serak, S. V.; Tabiryan, N. V.; Vaia, R. A.; Bunning, T. J. Polarization-Controlled, Photodriven Bending in Monodomain Liquid Crystal Elastomer Cantilevers. J. Mater. Chem. 2009, 19, 1080− 1085. (44) Lee, K. M.; Koerner, H.; Vaia, R. A.; Bunning, T. J.; White, T. J. Relationship between the Photomechanical Response and the Thermomechanical Properties of Azobenzene Liquid Crystalline Polymer Networks. Macromolecules 2010, 43, 8185−8190. (45) Yu, H.; Dong, C.; Zhou, W.; Kobayashi, T.; Yang, H. Wrinkled Liquid-Crystalline Microparticle-Enhanced Photoresponse of PDLCLike Films by Coupling with Mechanical Stretching. Small 2011, 7, 3039−3045. (46) Naciri, J.; Srinivasan, A.; Jeon, H.; Nikolov, N.; Keller, P.; Ratna, B. R. Nematic Elastomer Fiber Actuator. Macromolecules 2003, 36, 8499−8505. (47) Yoshino, T.; Kondo, M.; Mamiya, J.; Kinoshita, M.; Yu, Y.; Ikeda, T. Three-Dimensional Photomobility of Crosslinked Azobenzene Liquid-Crystalline Polymer Fibers. Adv. Mater. 2010, 22, 1361− 1363. (48) Zhou, H.; Xue, C.; Weis, P.; Suzuki, Y.; Huang, S.; Koynov, K.; Auernhammer, G. K.; Berger, R.; Butt, H. J.; Wu, S. Photoswitching of Glass Transition Temperatures of Azobenzene-containing Polymers H
DOI: 10.1021/acs.macromol.7b01741 Macromolecules XXXX, XXX, XXX−XXX
