NIRâVisâUV Light-Responsive Actuator Films of Polymer-Dispersed

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NIR-VIS-UV Light-Responsive Actuator Films of PolymerDispersed Liquid Crystal/Graphene Oxide Nanocomposites Zhangxiang Cheng, Tianjie Wang, Xiao Li, Yihe Zhang, and Haifeng Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b09676 • Publication Date (Web): 23 Nov 2015 Downloaded from http://pubs.acs.org on November 29, 2015

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ACS Applied Materials & Interfaces

NIR-VIS-UV Light-Responsive Actuator Films of Polymer-Dispersed Liquid Crystal/Graphene Oxide Nanocomposites Zhangxiang Cheng1, Tianjie Wang2, Xiao Li2, Yihe Zhang1* and Haifeng Yu2*

1 School of Materials Science and Technology, China University of Geosciences, Beijing 100083, P. R. China. Email: [email protected] 2 Department of Material Science and Engineering, College of Engineering and Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, Peking University, Beijing 100871, China Email: [email protected]

KEYWORDS: Photomechanical materials, polymer-dispersed liquid crystals, liquid crystal and nanocomposites, light-responsive actuator, GO nanocomposites

ABSTRACT: To take full advantage of sunlight for photomechanical materials, NIR-VIS-UV light-responsive actuator films of polymer-dispersed liquid crystal (PDLC)/graphene oxide (GO)

ACS Paragon Plus Environment

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nanocomposites were fabricated. The strategy is based on phase transition of LCs from nematic to isotropic phase induced by combination of photochemical and photothermal processes in the PDLC/GO nanocomposites. Upon mechanical stretching of the film, both topological shape change and mesogenic alignment occurred in the separated LC domains, enabling the film to respond to NIR-VIS-UV light. The homo-dispersed GO flakes act as photo-absorbent and nanoscale heat source to transfer NIR or VIS light into thermal energy, heating the film and photothermally inducing phase transition of LC microdomains. By utilizing photochemical phase transition of LCs upon UV-light irradiation, one azobenzene dye was incorporated into the LC domains, endowing the nanocomposite films with UV-responsive property. Moreover, the lightresponsive behaviors can be well controlled by adjusting the elongation ratio upon mechanical treatment. The NIR-VIS-UV light-responsive PDLC/GO nanocomposite films exhibit excellent properties of easy fabrication, low cost, good film-forming and mechanical features, promising their numerous applications in the field of soft actuators and optomechanical systems driven directly by sunlight.

1. INTRODUCTION One of the main focuses in the rapid developing area of advanced functional materials is the rational design and efficient fabrication of novel stimuli-responsive materials in order to provide these materials with potential applications in desired areas. Recently, multi-component nanocomposite systems have been exploited because they can integrate several functions into one system.1 Generally, stimuli-responsive materials can change their shapes or dimensions in response to external stimuli, such as magnetic field,2,3 electric field,4,5 temperature,6−8 solvent,9 moisture,10 pressure,11 pH,12 light

13−16

and so on. Among them, light is particularly attractive

because it is a clean energy that can be remotely, instantly and precisely controlled in one


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uncontacted way. Thus, photoresponsive materials have sparked enormous interests due to their promisingly photomechanical applications, converting light energy directly into mechanical work, and making it possible to fabricate various photo-driven devices such as flexible micro robots,17 switches,18,19 artificial muscles,20 motors,21 optical pendulum generator,22 and lightpowered tumbler.23 Recently, light-responsive polymers, including hydrogels, shape-memory polymers and liquid-crystalline elastomers (LCEs) have attracted considerable attentions in scientific and engineering fields due to their distinctive advantages.24−28 Especially, LCEs have been widely studied because they combine the self-organization performance of liquid crystals (LCs) with the mechanical properties of polymers. Recently, photochromic molecules such as azobenzenes have been incorporated into polymer networks to achieve light-responsive LCEs.13−19,29,30 Then photoinduced molecular cooperative motion of LC molecules is utilized to magnify the structural change of the azobenzene molecules and the alignment change of the mesogens caused by actinic light. Thus, significant macroscopic photo-deformations of the whole materials can be achieved correspondingly. Generally, photoresponsive materials have been modified and improved when they are incorporated with different nanomaterials such as carbon nanotube (CNT),

27,28,31

graphene and

graphene oxide (GO).18,19,32 For instance, Yu and co-workers fabricated light-responsive LCEs capable of photoinduced bending upon exposure to near infrared (NIR) light at 980 nm by introducing upconversion nanophosphors into matrices.33,34 Li et al. also developed LC materials responding to NIR light by using this upconversion technique.35−40 In addition, CNT and GO often show high-efficient photothermal conversion and thermal conductivity, which may act as nanoscale heat source to increase the temperature of nanocomposites.41−44 We previously


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reported one polymer-dispersed liquid crystal (PDLC)/GO hybrid film,32 showing similar photomechanical behaviors to LCEs upon exposure to visible (VIS) light. In addition, PDLC/GO nanocomposites exhibited excellent properties of easy fabrication, low cost, good film-forming and mechanical features. However, materials that can be responsive to UV-VIS-NIR light, especially nature sunlight are scarcely reported. In this study, we design a novel kind of UVVIS-NIR light-responsive PDLC/GO nanocomposite materials composed of GO, mixing LCs and polyvinyl alcohol (PVA). As shown in Scheme 1, the low-mass compound 4-cyano-4’pentyloxyazobenzene (5CAZ) containing an azobenzene moiety shows photoisomerization upon UV-light irradiation (Supporting Information, Figure S4). Correspondingly, the photochemical nematic (N) to isotropic (I) phase transition of the guest/host LC mixtures (5CB and 5CAZ) can be obtained by the photoinduced molecular cooperative motion.45 In the region of VIS or NIR light, GO functions as the photo-absorbent and nanoscale heat source to raise the temperature of PDLC/GO nanocomposites. The LC mixtures can undergo thermally induced N-to-I phase transition when the temperature of PDLC/GO composites is higher than their clearing point (TNI). Here, PVA acts as the photo-inert polymer host matrix, providing the nanocomposite with good film forming and mechanical properties.32 Thus, photoinduced deformation of PDLC/GO nanocomposites was obtained upon irradiation of NIR-VIS-UV light. Moreover, their quick response to sunlight was also observed.

（Scheme 1） 2. RESULTS AND DISCUSSION Characterization of Guest/Host LCs. The structures and properties of the host and guest molecules for LC mixtures are shown in Figure 1a. The host 5CB is a typical nematic LC with a melting point of 24 °C and the N-to-I phase transition temperature of 35 °C, exhibiting lower


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viscosities than macromolecular LC materials. The guest 5CAZ was synthesized and purified according to literature.46 The synthetic details, 1HNMR spectrum, different scanning calorimetry (DSC) thermogram, LC texture and light-responsive property are presented in the Supporting Information. As shown in Figure 1b, 5CB and 5CAZ were separately dissolved in tetrahydrofuran (THF), and the two solutions mixed together at one desired ratio. Then the solvent THF was removed completely to obtain guest/host LC mixtures. Accompanying the increase in the molar concentration of 5CAZ, the color of the LC mixtures became deeper and deeper (Figure 1c), and thermal properties of the LC mixtures were also changed accordingly, as shown in Table S1. (Figure 1) Figure 1d shows the phase diagram of the guest /host LC mixtures of 5CAZ and 5CB. TNI increases gradually with the rise of molar fraction of 5CAZ. The photochemical phase transition behavior of the LC mixtures was in-situ observed at room temperature upon irradiation of UV light at 365 nm with one polarizing optical microscope (POM). Before irradiation, all the samples exhibited typical schlieren texture of nematic LC phase. Once UV light was turned on, dark spots appeared quickly in the LC mixtures, and they developed until all the area became dark. The formation of the dark spots indicates that the photochemical N-to-I phase transition occurred in local regions. Eventually, the N-to-I phase transition took place throughout the whole area of the sample. The processes of photoinduced phase transition of the LC mixtures with 5CAZ molar ratios of 0.5%, 1%, 3%, 5% were in-situ recorded and given in Movies S1-1, S1-2, S1-3, S1-4, respectively. Here, the N-to-I phase transition behaviors were not obtained upon irradiation of VIS or NIR light. In all samples of LC mixtures, the rate of photochemical phase transition is fastest when the molar fraction of 5CAZ is 5%. Therefore, this concentration


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(sample 5 in Figure 1c) was selected for fabrication of the PDLC/GO nanocomposites containing 5CAZ. Fabrication of PDLC/GO Nanocomposites. The films were prepared by directly casting an aqueous PVA solution (10 wt%) containing homo-dispersed 1 wt% GO and 5 wt% LC mixture (doped with 5CAZ or without 5CAZ) for photomechanical studies. It has been reported that little volume change can be detected when the phase transition of materials occurs from non-aligned LC to the isotropic phase. Therefore, homogenous alignment of mesogens in the nanocomposite film is one of the necessary preconditions for their light-actuating behaviors.24,32 The mesogenic alignment of was acquired through a simple method of mechanical stretching, as shown in Figure 2a. As reported previously,24 the molecular ordering in the nanocomposite film can be manipulated by control of their elongation ratios. Before mechanical treatment, the mesogens in LC domains were randomly orientated since no LC alignment was induced in the fabrication process. After the mechanical treatment, structural anisotropy of LC domains was clearly observed as a result of shape deformation from spheres to ellipsoids (Figure 2b & 2c, Figure S6). As shown in Figure 2d & 2e, the lowest transmittance can be observed when the stretching direction was parallel to one of the polarization direction of the polarizer or the analyzer, while the highest transmittance appeared when the angle between the stretching direction and the polarization direction of either polarizer was 45°. The periodic change of the transmittance indicates that the mesogens in LC microdomains were homogenously aligned along the stretching direction, similarly to our previous results about PDLC-like or PDLC/GO systems.24,32 （Figure 2）） As shown in Figure 2f & 2g, bilayer structures were clearly observed in scanning electron microscopy (SEM) images of the cross-sectional films. In the fabrication process of the


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nanocomposite, the LC domains were homo-dispersed in aqueous PVA solutions, which were then cast on glass slides. Upon slow evaporation of the solvent, the LC domains gradually floated to upper part of the mixture solution due to its lower density of the LC mixture compared to water, leading to the formation of airside of the film.24 Thus a bilayer-like structure was spontaneously obtained by this simple way of solvent evaporation. As shown in Figure 2f, the airside of the film is LC-rich layer, and the non-airside is LC-poor layer. Moreover, the crosssectional SEM images also indicate the occurrence of shape deformation of LC microdomains in the films from spheres to ellipsoids through mechanical stretching. NIR-VIS Light Response of the PDLC/GO Nanocomposites. The photo-actuating behaviors of the stretched freestanding films of PDLC/GO containing 5CAZ were first investigated upon irradiation of NIR or VIS light at room temperature. Compared to our previous work,32 the present PDLC/GO film has controllable TNI by changing the mole fraction of 5CAZ. As shown in Figure 3a, upon exposure to NIR light (808 nm, 66 mw/cm2), the film bent toward the light source along the stretching direction when its airside faced to the light source (Movie S2-1). If the non-airside of the film was irradiated, it bent backward the light source (Movie S2-2 and Figure 3b). This is similar to the photomechanical behaviors of PDLC/GO without 5CAZ, which have been realized in our previous work.32 It has been explained with a so-called bimetal mechanism,21, 24−26 and volume contraction of LCEs usually occurs upon the phase transition from homogeneously-aligned LC to isotropic phase.24−26 In the present nanocomposite film with a bilayer-like structure, the LC-rich layer showed a larger contraction in volume compare to the LC-poor layer upon NIR irradiation. It is the structural feature of the films that enables them to show completely different bending direction. The photoinduced motion was accomplished with a few seconds, and longer-time photoirradiation did not cause any more significant deformation to


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the film. Upon irradiation of VIS light (450 nm, 54 mw/cm2), the films showed very similar photomechanical behaviors (Movie S3-1, S3-2), but a slower response comparing to NIR light actuation. (Figure 3) For comparison, three sample films of pure PVA, PVA/GO without LC and PVA/LC without GO were respectively fabricated upon stretching treatment. Our previous work had proved that films of PVA/GO without LC showed no response to VIS light.32 All the sample films had the similar phenomenon that they were not responsive to NIR light, indicating that the co-existence of the LC domains and GO is necessary for the nanocomposite films to obtain the photoinduced deformation. Many groups have reported that graphene and GO are highly efficient photothermal conversion and thermal conductivity nanomaterials.41−44,47,48 Figure 4a shows the temperature variation of the nanocomposite films upon photoirradiation of NIR or VIS light. It increased from 25 °C to 60 °C with 1s of NIR light irradiation, and then quickly decreased to 25 °C within 1s after the light is turned off. Similarly, the temperature of the nanocomposite films increased to 48 °C with 1.5s upon VIS light irradiation. As a result, the mechanism of the photo-actuating behaviors should be related with the photothermal effect of GO in films. Here, GO may act as the only photo-absorbent and nanoscale heat source since both the LC mixtures and the polymer substrate of PVA are inert to NIR or VIS light. Upon photoirradiation, the film temperature rapidly increased to the clearing point of the LC mixtures. Volume contraction occurred along the mechanically stretching direction in the upper layer of films (airside) caused by N-to-I phase transition while the lower layer of films (non-airside) exhibited fewer changes due to the lack of LC domains. Thus, the films bent toward the light source along the stretching direction when its airside faced to light source, as shown in Figure 4b.


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Thanks to the highly thermal conductivity of GO, N-to-I phase transition of the LC domains was also caused even though its non-airside faced to light source, leading to a reverse bending direction, as shown in Figure 3b. （Figure 4） Similarly to our previously reported results,24 the mesogenic alignment of LC domains in films can be controlled by the elongation ratio upon mechanical stretching, producing a dramatic influence on the photomechanical motion. As shown in Figure S6, the long axis of ellipsoidal LC microdomains was two times of the diameter of the spherical ones when the film was drawn with a 100% elongation ratio. Here, a parameter of bending degree (F) was defined to quantitatively evaluate the macroscopic deformation, as expressed in Equation (1). F=

L0 − L L0

(1)

Where L0 and L are linear length of the films before and after photoirradiation. As shown in Figure 5a, no apparent deformation was observed for the nanocomposite films at lower elongation ratio (< 20%) upon photoirradiation. When the elongation ratio was lower than 120%, faster photomechanical response was observed, and the F value almost linearly increased with the elongation ratio. Above 120%, the change in elongation ratio decreased appreciably with respect to F. Because almost all the mesogens in the LC domains were aligned along the stretching direction at this case, and increasing the elongation ratio only induced a larger change in the shape of the films, whereas the alignment of mesogens was not obviously improved.24 (Figure 5) UV Light Response of PDLC/GO Nanocomposites containing 5CAZ. It is well known that photochemically induced N-to-I phase transition of guest/host LC mixtures can be obtained by photoisomerization of the guest azobenzene dye.13−16,30 To render the nanocomposite film with


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UV-light response, 5CAZ was introduced into the LC domains (5 mol% in 5CB). Then a stretched film of PDLC/GO containing 5CAZ was exposed to UV light at 365 nm (50 mW/cm2) under ambient conditions at 25 °C. As shown in Movie S4, the films bent toward the light source along the stretching direction (Figure 6a) when its airside faced to the light source, similarly to that actuated by NIR or VIS light. (Figure 6) To get ride of the function of GO in the nanocomposite upon UV irradiation, one stretched sample film containing 5CAZ was prepared without GO. Although such a film of PVA/LC without GO exhibited no response to NIR or VIS irradiation due to the lack of photothermal effect of GO, it was actuated by exposure to UV light due to the existence of 5CAZ, as shown in Movie S5-1. In addition, the temperature variation of sample films of PDLC/GO and PVA/LC without GO were also measured upon UV-light irradiation. Both of the films showed increased temperature from 25 °C to 30 °C within 1s (Figure S5). The highest temperature was still lower than TNI of the LC mixtures. Generally, GO is photo-inert to UV light and photothermal effect was very weak at this case. However, the existence of GO contributes to the significant enhancement of mechanical properties of the nanocomposite films, as shown in Figure S7.15,23 These indicate that the UV light-responsive behavior of the PDLC/GO nanocomposite was obtained by the photochemically induced phase transition of LC microdomains in films, as shown in Figure 6b. When UV light was irradiated to the no-airside of the PDLC/GO nanocomposite film, it bent away from the light source (Movie S5-2), similarly the photomechanical behaviors under NIR or VIS light. However, its mechanism of photoinduced bending was not photothermally induced phase transition of LC mixtures, but photochemically caused volume change in the LC


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microdomains. Because the polymer substrate PVA is transparent, UV light can easily penetrate the film due to the separated phase of LC microdomains in bulk films. Obviously, almost all of the LC domains can undergo photochemical phase transition. But the volume change of the LCrich layer (airside) was larger that the LC-poor layer (non-airside), leading to the bending of the film away from the light source following the principal of bimetal mechanism of LCEs.13−19,21 Similarly, the photomechanical behaviors of the nanocomposite films containing 5CAZ were quantitatively studied by changing their elongation ratios upon UV irradiation. Compared with the bending degree of NIR or VIS light-driven films, similar performance was achieved when UV light was used. As shown Figure 7a, the F value increased with increasing the elongation ratio. However, the bending degree is a little smaller that obtained from the NIR light actuating films. Besides, the nanocomposite films demonstrated slower response to UV light comparing to NIR light (Movie S2-1 and Movie S4). These can be ascribed to the different mechanism of the phase transition of the LC microdomains when different actinic light source was used. Irradiation of NIR or VIS light brings about photothermal effect of GO in the nanocomposites and thermally induced phase transition of LC domains can be quickly obtained because of the high thermal conductivity of GO. On the other hand, UV light-caused phased transition of LC domains is based on the photoinduced molecular cooperative motion of the LC mixtures. This photochemical process includes two steps, as shown in Figure 7b. One is the photoisomerization of azobenzene molecules from rod-like trans-forms to their bent cis-forms. The other is that the bent cis-azobenzenes destroy the LC ordering, leading to isothermally changing into isotropic phase. That means the photochemical process is accompanied with molecular relaxation, which is greatly influenced by the viscosity of LC mixtures. As a result, lower speed of phase transition was obtained from photochemical process than that from the


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photothermally caused case with a lower viscosity at a higher temperature. Table S2 summarizes all the sample films with detailed composition for different photomechanical response upon NIRVIS-UV irradiation. (Figure 7) Finally, the photomechanical behaviors of the fabricated nanocomposite films of PDLC/GO containing 5CAZ were investigated under nature sunlight at room temperature. As shown in Movie S6, the film quickly bent toward to sunlight as soon as it was taken out from a dark box. It is well known that sunlight covers light of almost full-wave bands from UV to VIS and NIR ranges, and both photochemical (UV) and photothermal (NIR and VIS) effect should be involved in the photomechanical behavior. To show the photochemical process under sunlight, one film of PDLC containing 5CAZ without GO was also fabricated. As our expected, such films still showed sunlight-induced actuating behaviors, as shown in in Movie S7. This direct nature sunlight-driven motion makes the fabricated PDLC/GO nanocomposite films advantageous over other photoresponsive films triggered by light with narrow-band wavelength, which is undoubtedly beneficial for designing full-plastic soft actuators.

3. CONCLUSIONS In summary, NIR-VIS-UV light-responsive nanocomposite films of PDLC/GO containing one azobenzene dye 5CAZ were successfully fabricated with GO, LC mixtures and PVA. Bilayerlike structures spontaneously formed in the fabrication process, enabling the film to exhibit photomechanical motion with a bimetal way. Fast photoinduced deformation of the nanocomposite film was realized due to photothermal effect of GO upon NIR or VIS light irradiation. Benefiting from the doping of 5CAZ in the LC microdomains, photochemical phase


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transition was obtained isothermally, endowing the nanocomposite film with UV-light response. Direct exposure to nature sunlight, the film demonstrated quick photomechanical behaviors because both photochemical and photothermal effect were involved. The nanocomposite films showed a much-improved mechanical strength and light-responsive properties, promising their numerous applications of soft actuators and optomechanical systems directly driven by sunlight.

4. EXPERIMENTAL SECTION Chemical. Graphite was obtained from China Graphite Co., Ltd. Polyvinyl alcohol (PVA) was received from Sinopharm Chemical Reagent Co., Ltd. 4-aminobenzonitrile, sodium nitrite (NaNO2), hydrochloric acid (HCl), sodium acetate (CH3COONa), ammonia water (NH3·H2O), N,N-dimethylformide (DMF), potassium carbonate (K2CO3) were purchased from Beijing Chemical Works. Synthesis of the azobenzene dye 5CAZ. 5CAZ was prepared as previously reported.46 The synthetic details are given in the supporting information. Fabrication of PDLC/GO Nanocomposites. GO was prepared via the modified method. The nanocomposite films were fabricated as following steps. First, one GO solution was prepared through sonicating GO (30.3 mg) in the 54 mL deionized water for two hours which was then added with 3.0 g PVA and 150.2 mg LC mixture 5CAZ/5CB (the LC mixture in PVA was 5 wt%). The obtained solution was rotated for 48 hours at 95 °C to ensure that the LC mixture was homogeneously dispersed. Then the resultant solution was cast on clean glass slide and dry for one day at room temperature. Eventually, the nanocomposite film having dimensions of 70 mm × 20 mm × 0.8 mm was achieved by peeling off the glass substrate.


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The samples for photomechanical studies were obtained by cutting the nanocomposite film into dimensions of 20 mm × 5 mm × 0.8mm. Then, the samples were stretched to different elongation ratios by using a tensile test machine at 80 °C, just higher than the glass transition temperature of PVA. Finally, the stretched films holding at the elongated status on the grippers were slowly cooled down to room temperature to make sure the alignment of mesogens and complete relaxation of the residual stress. Characterizations. 1HNMR spectra were recorded on a Bruker 500MHz spectrometer to confirm the molecular structure of the synthesized 5CAZ. All spectra were run in chloroform-d (CDCl3) solution. The mesomorphic properties of 5CAZ and LC mixtures were studied by differential scanning calorimetry (Perkin-Elmer DSC8000). The heating and cooling rates were 5 °C/min. The birefringent properties of the PDLC/GO nanocomposite films were performed using optical polarizing microscopy (ZEISS Axio Scope A1). Scanning electron microscopy (SEM) images were obtained on a Hitach S-4800. Three driving light sources are implemented by different instruments (UV-light: HTLD-4ⅡShenzhen Height-LED Opto-electronic Tech Co., Ltd. VIS-light: CEL-M500 mercury lamp. NIR-light: MDL-H Changchun New Industries Optoelectronics Technology Co., Ltd). Figure S9 shows one schematic illustration of the experimental setup for photomechanical response of the nanocomposite film. ASSOCIATED CONTENT Supporting Information Available: Synthesis and characterization of the azobenzene compound 5CAZ, thermal properties of guest/host LCs, the temperature variation of pure PDLC and PDLC/GO nanocomposite upon UV irradiation, supporting movies S1-(1-4) showing the photochemical phase transition behaviors of 5CAZ/5CB mixtures, and supporting movies S2-S7


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showing photomechanical response of PDLC/GO nanocomposite upon irradiation of NIR-VISUV light or nature sunlight. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author E-mail: [email protected], [email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The National Natural Science Foundation of China and Science and Technology Opening Cooperation Project of Henan Province. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge the National Natural Science Foundation of China (Grant No. 51322301, 51573005) and Science and Technology Opening Cooperation Project of Henan Province (Grant No. 142106000050).


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Scheme and Figure Captions

Scheme 1. Schematic illustration of NIR-VIS-UV light-responsive actuator films of PDLC/GO nanocomposite containing 5CAZ. Both photothermal effect of GO upon NIR or VIS light irradiation and photochemical phase transition of azobenzene LC mixtures upon UV light were used in this system. Figure 1. Photochemical phase transition of guest/host LCs with one azobenzene compund (5CAZ) as guest dye in host nematic LC (5CB). (a) Structure and thermal properties of the LC mixtures. K, N and I correspond to the crystal, nematic LC, and isotropic phases, respectively. (b) Fabrication process of the Guest/Host LCs. (c) Photographs of LC mixture samples (1-5) with different molar ratios of the azobenzene dye, 5CAZ/5CB (molar ratio) = 0, 0.5%, 1%, 3%, 5%. (d) The phase diagram of the guest /host LC mixtures of 5CAZ and 5CB. Figure 2. Structural characterization of the fabricated nanocomposite films upon mechanical stretching. (a) Schematic illustration of the mechanical stretching process. (b) and (c) are optical microscopic images of the nanocomposite films with 5wt% 5CAZ/5CB and 1wt% GO before and after mechanical stretching with an elongation ratio of 100%. (d) and (e) are POM images of mechanically stretched films with a stretching direction parallel to one of the polarization direction and angle between the stretch direction and polarization direction of either polarizer was 45°. A, analyzer; P, polarizer. (f) and (g) are SEM images of cross-section of the nanocomposite film before and after stretching. Figure 3. Photomechanical behaviors of the stretched PDLC/GO nanocomposite films upon irradiation of NIR or VIS light. (a) and (b) are the schematic illustration and experimental results when the films were irradiated from the airside or the other side. Figure 4. Photomechanical response of the nanocomposite films due to the photothermal effect of GO and the thermally induced N-to-I phase transition of LC domains upon NIR or VIS irradiation. (a) Temperature change of the films as a function of on and off cycles with NIR or VIS light. (b) Scheme illustration of the possible mechanism of light-responsive behaviors. Figure 5. Quantitative study on the light-responsive behaviors of the nanocomposite films. (a) Definition of bending degree of photomechanical films. (b) Relationship between the mechanical stretching and the light-responsive behaviors of the film. Figure 6. Photoresponsive behaviors of the PDLC/GO nanocomposite films containing 5CAZ upon UV-light irradiation. (a) and (b) are experimental results and the possible mechanism of UV-light responsive behaviors. Figure 7. (a) Relationship between the elongation ratio and bending degree. (b) Schematic illustration of photochemically induced N-to-I phase transition of LC mixtures containing 5CAZ.


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Scheme 1


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Figure 1


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Figure 2


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Figure 3


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(a)

(b) Tlow

Thigh NIR or VIS light

LC domain

Figure 4


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Figure 5

(a)

(b) Tlow

Tlow UV light

LC domain

Figure 6


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Figure 7


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TOC

NIR-Vis

Thigh

UV

Tlow

NIR/Vis/UV

Tlow


31

NIRâVisâUV Light-Responsive Actuator Films of Polymer-Dispersed

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