Comparing Photoactuation of an Azobenzene ... - ACS Publications

Subscriber access provided by the Henry Madden Library | California State University, Fresno

Article

Comparing Photo-Actuation of Azobenzene-Doped Nematic Liquid-Crystal Polymer Through Its Activation Mechanism: trans-cis-trans Reorientation and Photoisomerization Jung-Hoon Yun, Chenzhe Li, Seongseop Kim, and Maenghyo Cho J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12184 • Publication Date (Web): 23 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Comparing Photo-Actuation of Azobenzene-Doped Nematic Liquid-Crystal Polymer Through Its Activation Mechanism: trans-cis-trans Reorientation and Photoisomerization Jung-Hoon Yun†, Chenzhe Li, Seongseop Kim, and Maenghyo Cho* Department of Mechanical Engineering, Seoul National University, Bldg. 301 Room 1524 Seoul, Korea, 08826.

ABSTRACT: We compare the actuation speeds of the azobenzene-doped nematic liquid-crystal polymer (azo-LCP) by examining its activation mechanisms: photoisomerization and trans-cistrans reorientation (TCTR, also known as Weigert effect). Both experiments and modelling are carried out in this study to compare the speed of bending and contraction acting on azo-LCP. We identify that the TCTR-based photo-bending of azo-LCP generated less than half the photo-strain in a unit cell than in the photo-ismerization case, but involves a larger azo-LCP thickness in triggering photo-deformation. Because photo-deformation occurs in a deeper region than in the photoisomerization case, the TCTR-based photo-bending of azo-LCP exhibits a faster bending speed than the photo-isomerization-based photo-bending of azo-LCP, even though less strain is generated in the TCTR case.

ACS Paragon Plus Environment

1

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 33

Introduction The azobenzene-doped nematic liquid-crystal polymer (azo-LCP) is well known for its reversible photo-deformation upon UV light exposure and subsequent return to its original shape under visible light. In contrast to pure azobenzene-actuators,1,2 azo-LCP shows a glassy and rigid nature due to the nematic liquid-crystal inside azo-LCP.3.4 This repeatable and remotecontrollable deformation has motivated researchers to carry out extensive studies on azo-LCP,5-16 from bending17 to twisting.18,19 The main cause of the photo-deformation of azo-LCP is the distraction of the mesogen alignment within the LCP.3 As the alignment of the mesogens transforms from the nematic to the isotropic phase, contraction of the LCP occurs along the major alignment direction of the mesogens. On the other hand, if the mesogens transform from the isotropic to the nematic phase, the shape of the LCP is recovered; this usually occurs under visible-light irradiation. The doped azobenzene monomers inside the LCP act as triggers to increase the distraction of the mesogen alignment in response to light. The azobenzene monomers trigger the alignment distraction of the mesogens in two ways: photoisomerization of azobenzene2, 17, 20-23 and rotation of azobenzene by means of a trans-cistrans reorientation (TCTR) which is also known as the Weigert effect.8, 24-27 In the case of photoisomerization, the trans–cis conversion triggered at around the 365-nm wavelength of light causes distraction of the mesogen alignment. In the case of the TCTR, the rotation of the azobenzene monomer, caused by a certain light wavelength (in the range of 440–500 nm) distracts the mesogen alignment in azo-LCP. A distinctive feature of photo-deformation based on TCTR is that the alignment of the azobenzene monomer can be artificially controlled by varying the light polarization direction so that both expansion and contraction of the LCP can be achieved using a single light source, as


2

Page 3 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


shown in our demonstration (Fig. 1(a)). White’s research group demonstrated such bidirectional photo-bending of azo-LCP based on a TCTR, and characterized the rotation of the azobenzene through the polar absorption of UV light.8,25 White’s group also demonstrated high frequency (greater than 20 Hz) vibration of the azo-LCP film with the TCTR mechanism, thus suggesting the application potential of azo-LCP as a resonator with a single UV-light source.28

Figure 1. (a) Photo bending snapshots for 365 nm and 445 nm light irradiation: E n for light polarization vertical to the alignment, and E ∥ n for light polarization parallel to the alignment. (b) Bending snapshots according to time evolution with light polarization parallel to the major alignment.

The focus of our study is the investigation of the dynamic aspect of the photo-deformation of azo-LCP. In order to investigate the dynamic behavior of the TCTR-based photo-deformation of azo-LCP, we compare the time evolutional change of photo-bending triggered by TCTR and photo-isomerization with the same azo-LCP, as shown in Fig. 1(b). The demonstration shown in Fig. 1(b) illustrates that the bending speed of azo-LCP in the TCTR mechanism is faster than that of the photoisomerization cases. In order to analyze the differences among the bending behaviors shown in Fig. 1(b), dynamic mechanical analysis (DMA) and polarized UV-Vis spectroscopy were used in our study. In addition to our experimental study, we introduce modeling methods to predict the internal behavior of the azo-LCP during photo-deformation. In the case of TCTR-


3


Page 4 of 33

based photo-deformation, we propose a new model to predict both the photo-strain and internal rotation of the azobenzene monomer within azo-LCP. Experimental section Synthesis of azo-LCP film We applied the method suggested by White et al. to prepare films of azo-LCP.6 A “host-guest” system was utilized to fabricate azo-LCP. In the fabrication process, 1,4-bis-[4-(3acryloyloxypropyloxy)benzoyloxy]-2-methylbenzene (RM257, Synthon) was used as the host and 4-4′-bis[6-(acryoloxy)hexyloxy]-azobenzene (A6ZA6, Beam Co.) was used as the guest material for azo-LCP. First, RM257 and A6ZA6 were mixed and melted at 100 °C. The mixture was injected into an Elvamide-coated glass mold by capillary force, and maintained at 72 °C for 30 min. The mixture inside the mold was cured by irradiation with 532-nm light for 30 min, and was then peeled off using an ethanol solution. The properties of the film, such as the dichroic constant and polymerization ratio of the azo-LCP, were measured using a polarized FT-IR spectrum.25 Material characterization As shown in Figure 2(a), the dichroic constant ratio (R) before light irradiation was measured using a Fourier transform infrared (FT-IR) spectrophotometer (Nicolet iS5, Thermo Scientific) with a BaF2 IR polarizer (WP25H-B), according to the procedure by Ikeda et al.29


4

Page 5 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. FT-IR spectrum data for 15 mol% of the azo-LCP. (a) Polarized spectrum data with light polarization parallel to major alignment (R ∥ ) and light polarization vertical to major alignment (R ). (b) Spectrum data before (A ) and after (A) polymerization ratio.

The polymerization ratio (P) was measured as the absorption peak difference before and after the polymerization30 of azobenzene, characterized by the following equation. P 1

(1)

Here, A and A denote absorption coefficient after and before polymerization, respectively, represent the FT-IR absorption peak at 810 cm (Figure 2(b)), which is the peak related to the


5


Page 6 of 33

vibration frequency of the acrylate group. The average dichroic constant and polymerization ratio for azo-LCP are shown in Table 1. Table 1. Dichroic ratio and polymerization ratio of azo-LCP. mol%

PIa [wt%]

Rb

Pc [%]

10

1.0

3.1873

64

15

1.4

2.8745

64

20

2.1

3.2476

63

a

Photo-initiator, bdichroic ratio of the azo-LCP, and cpolymerization ratio.

In the case of the TCTR-based photo-bending experiments, a polarized 445-nm laser (LuxX 445-120-Custom, LuxX, 100:1 polarization ratio) was used as the light source. A half-wave plate was used to vary the light polarization direction. The sample was clamped to a glass substrate and placed on a hot plate for more than 15 min to stabilize the temperature of the testing sample. After saturation, the target temperatures of the samples were verified via an infrared (IR) thermometer (Fluke-561, Fluke). Several snapshots were taken during the photo-bending experiment, and the bending amount was converted into curvature by extracting the position data from the snapshots. In the case of photo-isomerization, we used a 365-nm light-emitting diode (LED) light source (SUV2010-S, UVSMT) with a polarizer (WP25H-B, Thorlabs) to conduct bending experiments, with identical setups to those of our previous experiments.31 Similar to the case of the bending experiments with the TCTR, the sample was clamped to the same glass substrate to conduct the experiment at various temperatures, as shown in Scheme 1. The incident flux of the laser was 150 mW/ for both 365-nm and 445-nm light. In order to minimize the


6

Page 7 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


temperature spike during light irradiation, the light exposure time was limited to only 10-s in both sets of experiments.

Scheme 1. Scheme of photo-bending experiment: (a) light setting, (b) sample clamping.

DMA analysis In addition to the bending experiments, we conducted dynamic mechanical analysis (DMA) tests (Q800, TA Instrument) to measure the average strain during the photo-deformation of the azo-LCP. Polarized UV-Vis spectroscopy was used to measure the change in the dichroic ratio of the azobenzene monomer inside the azo-LCP. The incident flux of the light was set to 50 mW/ for both the 365-nm and 445-nm wavelengths during the DMA and polarized UV-Vis experiment. Simulation study TCTR estimation In our simulation studies, we considered the photo-strain generated by both TCTR and photoisomerization. The TCTR effect occurs via the rotation of the azobenzene, triggered by trans–cis–trans cyclic conversion due to the polarized-light irradiation in the wavelength range of 440-500 nm.28 The rotation angle of the azobenzene monomer is determined by the balance


7


Page 8 of 33

between the rotation force caused by the TCTR and the resisting force generated by the polymeric constraining of the neighboring azobenzene monomers. This balance is expressed by Equation (2).32 0

! "#,%&,',() '

+

+, "') '

(2)

Here, -./( "0, 1̂ , 3, 4) represents the rotation energy profile generated by the TCTR, which is a function of the light intensity I, light polarization direction 67, angular rotation of the azobenzene mesogen θ, and time t. Further, -.9: "3) represents the resisting energy profile originating from the constraint imposed by the neighboring mesogens. Parameter E./( can be predicted using the effective energy profile along the rotation angle of the azobenzene. The effective energy profile can be calculated from Equation (3)32 as follows: E;/à "3)O

(5)

Here, d>/à "3) denotes the average change in distance between the mesogens, which is obtained from molecular dynamic simulations. By rotating a single azobenzene unit in a 50% cross-linked unit cell and acquiring the average distance change in the mesogens neighboring azobenzene, we can obtain dA/à "3){|} ~ ∙ ℓ% /

+exp x-ny z_>/à "3){|} ~ ∙ x_v4 zℓ% {

(9)

Parameters ℓ and ℓ% represent the step lengths before and after the perturbation caused by the rotation of the azobenzene, and nnk/ denotes the mol ratio of azobenzene inside the LCP. Parameter λ"3) denotes the 3 × 3 deformation gradient tensor representing the rotation of the


10

Page 11 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


azobenzene, which is acquired from molecular dynamic simulations.32 Further, ℓ denotes the step length tensor after the alignment distraction of the LCP, and Eny denotes the average interaction energy among the mesogens obtained from the quantum simulations (with the use of the Gaussian 09 package).31 By plugging the distance change through the rotation of the azobenzene into the expression for Eny , we obtained the Boltzmann probability of the alignment distraction for different temperatures as exp x-ny z_>/à "3){ /|} ~ . We then used the Boltzmann probability as a rating factor to calculate ℓ. After comparing the change in the ratio between the major and minor axes in terms of the step length, we derived the actual deformation of the azoLCP along with the amount of rotation in the azobenzene monomer, as expressed in Equation (10).31,32 t%/(/ "⁄ )/

(10)

Here, t%/(/ represents the deformation ratio along the major alignment direction of the azoLCP, and r and r denote the dichroic constants (ratio between major and minor axes in terms of step length) before and after photodeformation, respectively. Data Results Photo-bending result of azo-LCP Figure 3(b) illustrates the photo-bending curvature of azo-LCP along the light irradiation time. The saturation point in the curvature implies that the amount of bending angle of the azo-LPC film becomes 90° . Figure 3(c) illustrates the curvature speed according to the different mole ratios of azobenzene and temperature. The bending curvature shown in Figures 3(b) and 3(c) illustrate that the bending performance of the azo-LCP triggered by TCTR is always greater than that of the azo-LCP triggered by photoisomerization of the azobenzene for different mol ratios and temperature ranges.


11


Page 12 of 33

Figure 3. (a) Photo-bending curvature measurement (spots) and its least square trend (lines). (b) Curvature comparison experiment of 10 mol% azo-LCP for photoisomerization (iso) and trans-cis-trans reorientation (TCTR). (c) Photo bending speed along various temperatures and mole% of azobenzene.

Average strain comparison from DMA Figure 4(a) shows the average photo-strain (measured by DMA) generated via TCTR and photo-isomerization trigger mechanism under two different light wavelengths. Figure 4(b) shows the initial strain at the beginning of the DMA test. In order to minimize the photo-thermal effect


12

Page 13 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


on long-time irradiation, we used low light power (50 mW ∙ cm ) during the DMA test. From Figures 3(a) and 3(b), we observe that the strain efficiency generated by TCTR is 25% lower than that generated by photo-isomerization for every mol% of the azobenzene monomer, which is contrary to the bending trend shown in Figure 3. Figures 4(c) and 4(d) illustrate the polar UV-Vis spectrum for the TCTR and Iso cases, respectively, at 365 nm and 445 nm. In order to determine the average strain in the entire domain of the azo-LCP, low power light (50 mW ∙ cm) was irradiated for 30 min in both the 365 nm and 445 nm cases. The increased dichroic ratio at light polarization vertical to the major alignment of azo-LCP in a 445 nm wavelength of light ( - s in Table 2) indicates that expansion occurs during light irradiation, which indicates that TCTR occurs during light irradiation. The DMA data in Figure 4(a) and the dichroic change ratio in Table 2 indicate that the amount of strain for TCTR-based photo deformation is less than that for the photoisomerization-based photo-deformation. Table 2. Dichroic ratio change according to light irradiation light [nm]

a)

original

∥ b)

445

2.7120

2.6827

2.1018

365

2.0017

2.8419

1.7516

a)

Light polarization vertical to major alignment; b) Light polarization parallel to major alignment


13


Page 14 of 33

Figure 4. (a) Average strain data for TCTR and Iso-based photo deformation, and (b) their initial photo-strain rate. Polar absorption changes during light irradiation for (c) TCTR and (d) Iso case.

Figures 5(a) and 5(b) present the photo-strain rate and photo-bending speed (curvature rate) profile of azo-LCP (20 mol% azobenzene monomer) with time change. As shown in Figure 5(a), the strain rate in the photo-isomerization (Iso) case is greater than that in the TCTR case, or is


14

Page 15 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


comparable to that of the TCTR case (see Electronic Supplementary Information for the remaining data). However, the bending speed of TCTR is much greater than that of the photoisomerization cases, as shown in Figure 5(b). Cases with different mol ratios of azobenzene monomer are illustrated in Electronic Supplementary Information. Figures 5(c) and 5(d) present the light propagation profile along the light irradiating direction and the maximum light propagation depth. As shown in Figure 5(c), light propagation becomes higher than that of the initial state of the azo-LCP due to the photo bleaching effect in both TCTR and the photo-isomerization mechanism. The light propagation depth in the TCTR case is greater than the photo-isomerization case, and their difference increases with time evolution.

Figure 5. Time dependent change on (a) photo-strain rate and (b) photo-bending speed of azo-LCP with 20 mol% of azobenzene monomer. (c) Light propagation (photo bleaching6) according to time evolution (d). Calculated light propagation depth along time evolution. Light intensity is 150 mW ∙ cm , and temperature is 25℃.


15


Page 16 of 33

Figures 6(a) and 6(b) compare the average bending speed (_⁄_4) corresponding to the photobending triggered by TCTR and photo-isomerization. The bending speed (_⁄_4) is estimated by the following Equation (11):23,32,33 o o(

/ o"k,()

/

o(

∙ q ∙ _q

(11)

where represents the thickness of the azo-LCP film, "q, 4) represents the strain profile along the thickness direction, and q represents the coordinate along the thickness direction, starting from the middle point of the film thickness. The bending speed shown in Figures 4(a) and 4(b) also indicates that the TCTR-based photo-bending of azo-LCP exhibits faster bending kinetics than the photo-isomerization-based photo-bending over the entire investigated temperature domain and mol% of azobenzene.

Figure 6. Computational result of average bending speed (_ ⁄_4, [cm ∙ ]) with 150 mW ∙ cm light intensity in (a) photoisomerization and (b) TCTR.

Discussion Strain difference between TCTR and photo-isomerization cases The main reason for the strain difference shown in Figure 4 is clear when comparing the amount of light absorbed for the photo-deformation of the azo-LCP between the two cases.21 For the absorption of the 365-nm light, the triggering photo-isomerization is 7 times greater than that


16

Page 17 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


of the 455-nm-wavelength of light, i.e. more energy is converted into strain with the 365-nmwavelength light than with the 445-nm-wavelength light. Bending speed difference between TCTR and photo-isomerization cases From the results shown in Figures 3 and 6, the bending speed under 445 nm light, triggering TCTR, is always faster than that under 365 nm, triggering photo-isomerization. However, the photo-strain shown in Figures 4 and 5 illustrate that the average strain triggered by photoisomerization is greater than that of the TCTR case, which is contrary to the trend in bending speed shown in Figures 3 and 6. Based on investigations under various mol ratios of azobnezene and temperature, we assume that the reason for the faster bending speed under 445 nm light (shown in Figures 3(b) and 3(c)) is not due to the thermo-mechanical behavior of the azo-LCP. In addition, by adopting the stress-strain relationship proposed by L. Jin et al.,34 (Equation (12)) in the finite element (FE) analysis, the influence of the photo softening of azo-LCP on the photo bending curvature was analyzed. Specific details for the ABAQUS settings and their results are described in Electronic Supplementary Information. σ ≈ 2μ"/ )/ ∙ K % N

(12)

Here, p represents the stress applied to the azo-LCP, and 2μ"/ )/ represents the modulus of the azo-LCP, which is the function of dichroic ratio r. If the softening effect occurs, r is decreased, and the mechanical modulus is decreased. ϵ and ϵ represent the resultant strain and photo-strain induced by light, respectively. The FE analysis demonstrates that the influence of photo softening on the bending speed of azo-LCP is less than 5%, even when we amplified the reduction in the magnitude of modulus during the photo softening effect. As no external force is applied to the azo-LCP in this study, σ


17


Page 18 of 33

in Equation (12) should be 0, and the modulus term (2μ"/ )/ ) should be nonzero. Therefore, the amount of bending caused by ϵ should only be influenced by the photo-strain (ϵ ). Considering the experiments and simulations, the bending under the 445 nm light is faster due to the difference in the light-activated depth for the two cases, as shown in Figure 5(c). As the absorption of 365-nm radiation by azo-LCP is greater than that of 445-nm radiation, 365-nm radiation is less likely to propagate through azo-LCP than 445-nm light, as shown in Figure 5(c). Because of the difference in light propagation, the width of the region in the depth direction generating photo-strain triggered by TCTR becomes greater than that of the domain generated by photo-isomerization along the light irradiation time. Even when the average amount of strain in the 365-nm ("q, 4) in Equation (11)) cases is higher than that in the 445-nm cases, the large domain of the photo-strain (non-zero domain of the "q, 4) in Equation (11)) in the 445-nm cases results in faster bending speed in the TCTR cases. Conclusion In this study, we compared time the evolutional change of the photo-deformation of azo-LCP triggered by TCTR and photo-isomerization. We found that the photo-bending speed of azo-LCP in the TCTR case is always faster than that of photo-isomerization, even when the average strain in TCTR is always smaller than that in the photo-isomerization case. Based on the investigations of the time response of azo-LCP under various temperatures, and various mol ratios of azobenzene, we found that the main reason for the faster bending speed of the TCTR than that of photo-isomerization is due to the light propagation effect of azo-LCP under 445 nm light. This study reveals that both photo-isomerization and the TCTR mechanism have an advantage in the actuation of azo-LCP. The photo-isomerization mechanism triggers faster in-plane contraction speeds than TCTR because of the high contraction efficiency. On the other hand, the


18

Page 19 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TCTR mechanism triggers a faster bending speed than photo-isomerization because of the deeper light penetration.

ASSOCIATED CONTENT Supporting Information. Chemical Structure, UV-Vis spectrum, FT-IR spectrum, DMA data, Computation result of strain/bending speed, ABAQUS setting, Softening effect

AUTHOR INFORMATION Corresponding Author *Tel: +82-2-880-1693; e-mail: [email protected] Present Addresses †LG Display, 10 Magokjungang, 10-ro, Gangseo-gu, Seoul, 07796, Korea

Notes ACKNOWLEDGMENT This work was supported by a grant from the National Research Foundation (NRF) of Korea, funded by the Korea government (MSIP) (No. 2012R1A3A2048841).

REFERENCES


19


Page 20 of 33

1. Ikeda, T.; Nakano, M.; Yu, Y.; Tsutsumi, O.; Kanazawa, A. Anisotropic bending and unbending behavior of azobenzene liquid-crystalline gels by light exposure. Adv. Mater. 2003, 15, 201-205. 2. Yu, Y. L.; Nakano, M.; Ikeda, T. Photomechanics: Directed bending of a polymer film by light. Nature 2003, 425, 145. 3. Harris, K. D.; Cuypers, R.; Scheibe, P.; Oosten, C. L. van; Bastiaansen, C. W. M.; Lub, J.; Broer, D. J. Large amplitude light-induced motion in high elastic modulus polymer actuators J. Mater. Chem. 2005, 15, 5043-5048. 4. Oosten, C. L. van; Harris, K. D.; Bastiaansen, C. W. M.; Broer, D. J. Glassy photomechanical liquid-crystal network actuators for microscale devices. Eur. Phys. J. E 2007, 23, 329-336. 5. Finkelmann, H.; Nishikawa, E.; Pereira, G. G.; Warner, M. A new opto-mechanical effect in solids. Phys. Rev. Lett. 2001, 87, 15501. 6. Ikeda, T. Photomodulation of liquid crystal orientations for photonic applications. J. Mater. Chem. 2003, 13, 2037-2057. 7. White, T. J.; Broer, D. J. Programmable and adaptive mechanics with liquid crystal polymer networks and elastomers. Nature Mat. 2015, 14, 1087-1098. 8. Lee, K. M.; Tabiryan, N. V.; Bunning, T. J.; White, T. J. Photomechanical mechanism and structure-property considerations in the generation of photomechanical work in glassy azobenzene liquid crystal polymer networks. J. Mater. Chem. 2012, 22, 691-698.


20

Page 21 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


9. Koskela, J. E.; Vapaavuori, J.; Ras, R. H. A.; Priimagi, A. Light-driven surface patterning of supramolecular polymers with extremely low concentration of photoactive molecules. ACS Macro Lett. 2014, 3, 1196-1200. 10. 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. 11. Shimamura, A.; Priimagi, A.; Mamiya, J.; Ikeda, T.; Yu, Y. Simultaneous analysis of optical and mechanical properties of cross-linked azobenzene-containing liquidcrystalline polymer films. ACS Appl. Mater. Interfaces 2011, 3, 4190-4196. 12. Basuki, S. W.; Schneider, V.; Strunskus, T.; Elbahri, M.; Faupel, F. Light-controlled conductance switching in azobenzene-containing MWCNT–polymer nanocomposites. ACS Appl. Mater. Interfaces 2015, 7, 11257-11262. 13. Iamsaard, S.; Anger, E.; Aβhoff, S. J.; Depauw, A.; Fletcher, S. P.; Katsonis, N. Fluorinated azobenzenes for shape-persistent liquid crystal polymer networks. Angew. Chem. Int. Ed. 2016, 55, 9908-9912. 14. Kitagawa, D.; Nishi H.; Kobatake, S. Photoinduced twisting of a photochromic diarylethene crystal. Angew. Chem. Int. Ed. 2013, 52, 2013. 15. Hirano, T.; Hashimoto, D.; Kitagawa, K.; Kono, K.; Kobatake S.

Dependence of

photoinduced bending behavior of diarylethene crystals on ultraviolet irradiation power. Cryst. Growth Des. 2017, 17, 4819-4825.


21


Page 22 of 33

16. Kitagawa, D.; Kawasaki, K.; Tanaka R.; Kobatake, S. Mechanical behavior of molecular crystals induced by combination of photochromic reaction and reversible single-crystalto-single-crystal phase transition. Chem. Mater. 2017, 29, 7524-7532. 17. Corbett, D.; Warner, M. Nonlinear photoresponse of disordered elastomers. Phys. Rev. Lett. 2006, 96, 237802. 18. Iamsaard, S.; Villemin, E.; Lancia, F.; Aβhoff, S. J.; Fletcher, S. P.; Katsonis, Preparation of biomimetic photoresponsive polymer springs. Nat. Protoc. 2016, 11, 1788-1797. 19. Aβhoff, S. J.; Lancia, F.; Iamsaard, S.; Matt, B.; Kudernac, T.; Fletcher, S. P.; Katsonis, N. High-pawer actuation from molecular photoswitches in enantiomerically paired soft springs. Angew. Chem. Int. Ed. 2017, 56, 3261-3265. 20. Finkelman, H.; Nishikawa, E.; Pereira, G. G.; Warner, M. A new opto-mechanical effect in solids. Phys. Rev. Lett. 2001, 87, 15501. 21. Hogan, P. M.; Tajbakhsh, A. R.; Terentjev, E. M. UV manipulation of order and macroscopic shape in nematic elastomers. Phys. Rev. E 2002, 65, 41720. 22. Ishikawa, T.; Noro, T.; Shoda, T. Theoretical study on the photoisomerization of azobenzene. J. Chem. Phys. 2001, 115, 7503-7512. 23. Statman, D.; Janossy, I. Study of photoisomerization of azo dyes in liquid crystals. J. Chem. Phys. 2003, 118, 3222-3232. 24. Cheng, L.; Torres, Y.; Lee, K. M.; McClung, A. J.; Baur, J.; White, T. J.; Oates, W. S. Photomechanical bending mechanics of polydomain azobenzene liquid crystal polymer network films. J. Appl. Phys. 2012, 112, 13513.


22

Page 23 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


25. Lee, K. M.; Koerner, H.; Vaia, R. A.; Bunning, T. J.; White, T. J. Light-activated shape memory of glassy, azobenzene liquid crystalline polymer networks. Soft Matter 2011, 7, 4318-4324. 26. Wu. Y; Ikeda, T.; Zhang Q. Three-dimensional manipulation of an azo polymer liquid crystal with unpolarized light. Adv. Mater. 1999, 11, 300-302. 27. 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. 28. 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. 29. Yu, Y.; Nakano, M.; Shishido, A.; Shiono, T.; Ikeda, T. Effect of cross-linking density on photoinduced bending behavior of oriented liquid-crystalline network films containing azobenzene. Chem. Mater. 2004, 16, 1637–1643. 30. Kamal, T.; Park, S. Y. Shape-responsive actuator from a single layer of a liquid-crystal polymer. ACS Appl. Mater. Inter. 2014, 6, 18048-18054. 31. Yun, J.-H.; Li, C.; Chung, H.; Choi, J.; Cho, M. Photo deformation in azobenzene liquidcrystal network: multiscale model prediction and its validation. Polymer 2015, 75, 51-56. 32. Yun, J.-H.; Li, C.; Chung, H.; Choi, J.; Cho, M. Multiscale modeling and its validation of the trans-cis-trans reorientation-based photodeformation in azobenzene-doped liquid crystal polymer. Int. J. Solids Struct. 2017, 128, 36-49.


23


Page 24 of 33

33. Terentjev, E. M.; Warner, M. Liquid crystal elastomers; Oxford University Press: Oxford, U.K., 2006. 34. Jin, L.; Zeng, Z.; Huo, Y. Thermomechanical modeling of the thermo-order–mechanical coupling behaviors in liquid crystal elastomers. J. Mech. Phys. Solids 2010, 58, 19071927.

TOC Graphic


24

Page 25 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


216x80mm (150 x 150 DPI)



(a)Photo bending snapshots for 365 nm and 445 nm light irradiation: E⊥n for light polarization vertical to the alignment, and E∥n for light polarization parallel to the alignment. (b) bending snapshots along with time evolution with light polarization parallel to major alignment. 228x162mm (150 x 150 DPI)


Page 26 of 33

Page 27 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


FT-IR spectrum data for 15 mol% of the azo-LCP. (a) Polarized spectrum data with light polarization parallel to major alignment (R_(E∥N)) and light polarization vertical to major alignment (R_(E⊥N)). (b) Spectrum data before (A_0) and after (A) polymerization ratio. 160x257mm (150 x 150 DPI)



Scheme of photo-bending experiment: (a) light setting, (b) sample clamping. 232x74mm (192 x 192 DPI)


Page 28 of 33

Page 29 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Connecting Beer’s law to the TCTR. Feedback algorithm is used to predict θ and I. 282x142mm (150 x 150 DPI)



(a) Photobending curvature measurement (spots), and its least square trend (lines) (b) curvature comparison experiment of 10 mol% azo-LCP for photoisomerization (iso) and trans-cis-trans reorientation (TCTR) (c) Photo bending speed along various temperature and mole% of azobenzene. 191x356mm (150 x 150 DPI)


Page 30 of 33

Page 31 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(a) Average strain data for TCTR and Iso-based photo deformation, and (b) their initial photo strain rate. Polar absorption change during light irradiation for (c) TCTR and (d) Iso case. 158x289mm (150 x 150 DPI)



Time dependent change on (a) photo-strain rate and (b) photo-bending speed of azo-LCP with 20 mol% of azobenzene monomer. (c) Light propagation (photo bleaching6) along time evolution (d) Calculated light propagation depth along time evolution. Light intensity is 150 mW∙cm^(-2) , and temperature is 25℃. 327x247mm (150 x 150 DPI)


Page 32 of 33

Page 33 of 33 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Computational result of average bending speed (dκ⁄dt, [cm^(-1)∙s^(-1)]) with 150 mW∙cm^(-2) light intensity in (a) photoisomerization and (b) TCTR. 254x122mm (150 x 150 DPI)


Comparing Photoactuation of an Azobenzene ... - ACS Publications

Recommend Documents