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Body Temperature Controlled Optical and Thermal Information Storage Light Scattering Display with Fluorescence Effect and High Mechanical Strength Si Chen, Xiaoqian Tong, Huiwen He, Meng Ma, Yanqin Shi, and Xu Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b03092 • Publication Date (Web): 08 Mar 2017 Downloaded from http://pubs.acs.org on March 12, 2017

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ACS Applied Materials & Interfaces

Body Temperature Controlled Optical and Thermal Information Storage Light Scattering Display with Fluorescence Effect and High Mechanical Strength Si Chen, a§ Xiaoqian Tong, a§ Huiwen He, a Meng Ma , a Yanqin Shi a and Xu Wang a* College of Materials Science and Engineering, Zhejiang University of Technology, NO. 18 Chaowang Road, Hangzhou 310000, China.

ABSTRACT：：A kind of body temperature controlled optical and thermal information storage light scattering display based on super strong liquid crystalline physical gel with special “loofahlike gel network” was successfully prepared. Such liquid crystal (LC) gel was obtained by mixing a dendritic gelator (POSS-G1-BOC), an azobenzene compound (2Azo2), a phosphor tethered liquid crystalline host (5CB) which could show its best contrast ratio at around human body temperature under UV light because of the phosphor’s fluorescence effect. The gel also has quite strong mechanical strength, which could be used in wearable device field especially under sunlight, even under the forcing conditions as hush as being centrifuged for 10 min at the speed of 2000 r/min in centrifuge. The whole production process of such display is quite simple and could get displays at any size through non-contact writing. We believe it will have wide applications in the future.

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INTRODUCTION In recent years, LC physical gels have drawn immense interest because they combine the properties of gels’ thermal reversibility and liquid crystals’ anisotropy. LC physical gels are novel kinds of soft materials acquired by the constraint of networks formed by gelators’ selfassembly in LC host molecules, dividing the LC host into countless tiny domains. Most of the driving forces of the gelators’ self-assembly depend on non-covalent interactions, mainly including intermolecular hydrogen bond, π-π stacking, van der Waals force, electrostatic interaction, hydrophobic effect, etc.1-4 The micro phase separation structure obtained from the domains of the gels might induct or improve the photoelectric, photochemical, and electrical performance of liquid crystals5, providing the applications in light scattering displays, grating, semiconductor materials or other fields.6 Until now, researches about LC physical gels and their performances have made some progresses. Gelators can self-assemble in LC compounds to acquire supramolecular LC gels. Relational researches about their critical concentration, sol-gel transition temperature (Tsol-gel) and other gel behaviors have been deeply discussed. A series of responsive properties including electric, magnetic and light responsiveness also vividly perform in practical applications. Meanwhile, the mechanism and regularity behind those properties are discussed as well.7 Among them, LC physical gels with fluorescence emission are a fascinating area for research. For example, Stéphane Diring et. al demonstrated luminescent ethynyl-pyrene gels could be used for the fabrication of OLED-like devices.8 Yue Zhao et. al reported a fluorescent self-assembled liquid-crystal gel (LCG) which could have its fluorescence intensity modulated through electricfield-controlled LC orientation and it might open the door to a new exploitation for applications of quantum dots (QD) such as emission-based liquid-crystal display (LCD).9 Takashi Kato et. al 2


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prepared gels that could produce intense red light when they were irradiated with UV-light and could be used in on-off application of electric fields.10 Hongbo Lu et. al proposed an optical switch based on a fluorescent molecule doped CLC which had three stable – focal conic, planar and fingerprint states.11 They also synthesized an AIE-active luminescent liquid crystal, (Z)-CNAPHP, which could be converted into E-isomers via 440 nm photoexcitation. Based on the CNAPHP, a new optical switching effect doped liquid crystals had been demonstrated and it emitted partially linearly polarized light.12 Broadly speaking, LC physical gels with fluorescence characteristics had its unique good integrated performances and their applications had become more widespread. However, we still find some problems from the reported literatures which have not been solved and fundamentally limit the LC physical gels’ practical application. One problem is that at present the strength of LC physical gels is often not high enough to show a practical use or needed to add into the system at high concentration to maintain the steady of gels.13 For instance, W. C. K. Poon et. al discovered a class of soft solids, with shear moduli up to 104 pascals, containing high concentrations of colloidal particles (volume fraction ϕ ≥ 20%) directly dispersed into a nematic liquid crystal.14 Xie, X. L. et. al prepared three LC gels which could exhibit good self-supporting ability with storage moduli higher than 104 Pa when the gelator content is increased to 1.5 wt%.15 And matrix LC molecules must have good compatibility with gelators to guarantee the stability of the LC physical gel. This finally greatly restricted the selection and practical application of different types of matrix LC compounds. Another problem is that the heat-stability of the reported LC physical gels is normally not high enough. Takashi Kato et. al reported a kind of gelation of room-temperature nematic liquid crystals which was through self-aggregation of low molecular weight molecules with the action of hydrogen

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bonding and their Tsol-gel was only from 50 oC to 110 oC changing as the different ratio of materials.16 The range of the temperature might limit the applications at higher temperature. Hirotsugu Kikuchi et. al prepared a kind of liquid crystal gel with low Tsol-gel at 42 oC.17 Takashi Kato et. al prepared LC physical gels showing bright, intense red luminescence light with low Tiso-Ic (isotropic-nematic transition temperature of LCs) around 33 oC and Tsol-gel around 50 oC.10 At the same time, usually, Tsol-gel is lower than Tiso-Ic.15,18,19 That is to say, before reaching phase transition temperature of the LC host, the system showed up the phenomenon of changing from gel to sol firstly.15 So Tsol-gel becomes a key factor of the application temperature of the LC physical gel, which is quite hard to adjust to a special narrow target scope, not to mention just happen to be human body temperature. To solve these problems, we prepared novel kinds of fluorescent LC physical gels by mixing dendritic gelators (POSS-G1-BOC), azobenzene compound (2Azo2), phosphors and matrix LC host (5CB) together, which have excellent ultraviolet and visible light response performance, achieving the properties of light and heat responsibilities. In order to expand the application of the physical gels, we tried to apply them to LC displays. The fluorescent LC display has quite clear image under UV light owning to the addition of phosphors. Meanwhile, after studying the system’s gelation behavior, dynamic rheological measurements, surface morphology, UV absorption, emission spectrum, centrifugal experiments and temperature tunable experiments, we found that the display was a kind of body temperature controlled optical and thermal information storage materials with fluorescence effect and high mechanical strength. EXPERIMENTAL SECTION

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Materials: The nematic liquid crystals 5CB (as shown in Scheme 1(a)) were purchased from Yantai Valiant Fine Chemicals Co., Ltd. 4,4'-Diethoxyazobenzene (97%, 2Azo2) (as shown in Scheme 1(b)) were supplied by Alfa Aesar (China) Chemical Co., Ltd. Oily phosphors (pink) were purchased from Jinan XiangFa chemical technology co., LTD. The gelators POSS-G1-BOC (as shown in Scheme 1(c)) were synthesized in our lab and had been reported in the literature.20

(a)

C5H11

(b)

C2H5O

CN

N

N

O O C HN

(c) O CO NH

O O C HN

O C NHO OC

OC O N H

H O N C O

HN O C NH

OHN C NH C O O

OC2H5

O C O C NHO HN

Si O Si O O SiO SiOO O O OSi Si O O Si Si O HN OC

O O C HN

O C NHO

C NHO H O NC HNC OO

H O N CO

HN O C NHO C NH O CO O HN C O O

HN O CO

Scheme 1. Chemical structure of (a) the host nematic liquid crystals 5CB, (b) 4,4’Diethoxyazobenzene (2Azo2), (c) POSS-G1-BOC. Characterization: Tube inversion method: The samples mixed with different proportions of POSS-G1-BOC, 2Azo2, phosphors and 5CB were dissolved in a glass test tube at 150 oC in advance and then

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cooled to room temperature in air. Finally the test tube was heated slowly at the speed of 1 o

C/min in an oil bath and was then inverted to observe and record Tsol-gel. The test was repeated

three times to get its mean value. Meanwhile the performance of the samples at different temperatures in natural light and under UV light was also observed. Dynamic rheological measurements: Dynamic rheological measurements were carried out using a controlled-stress rheometer (MCR302, Anton Paar, Austria). A parallel plate geometry of 25 mm diameter with a gap of 1 mm was employed for the experiments. The test was performed as follows: firstly, the freshly prepared samples were coated onto the parallel-plate fast and made them steady for a period of time before test. Dynamic strain sweep tests were performed under a frequency ω = 10 rad s-1 with the increasing amplitude of oscillation from 0.01% up to 200% apparent strain shear at 25 oC. Dynamic time sweep tests was firstly given a strain (γ = 0.1%) for 100s and then given a large strain (γ = 100%) for 200 s, then kept at a frequency ω = 6.28 rad s-1 for its recovery. FESEM measurements: The microstructure was probed by a Hitachi S-4700 field emission scanning electron microscope (FESEM, Hitachi, Japan). Samples for FESEM measurements were obtained by soaking in n-hexane and extracting for 48 h, subsequently removing the residual solvent by pulling a vacuum on them at room temperature. TEM measurements: The material soaked in n-hexane at room temperature for 48 h and the sol was drop coated on amorphous carbon-coated Cu grids and dried in room temperature. The morphology of TEM was then observed by using a JEM-1230 microscope at an accelerating voltage of 80 kV.

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UV absorption measurements: Lambda 750S UV VIS spectrophotometer (Perkin Elmer) was used to test the samples in liquid crystal cell (7 µm). Firstly, the UV absorption spectra of the sample was studied through being irradiated by UV every 5 s until reaching steady state. On this basis the sample was then irradiated by Vis every 2 s to get back LC gels into the initial state. 365 nm UV light source (CREE B-530, Send nits) and 455 nm Vis light source (SM-7031, Yueqing Shishi Electric Appliance Co., Led (Zhejiang, China)) were used in the experiments. Fluorescent measurements: Fluorescent measurements were taken on a Hitachi F-2700 fluorescence spectrophotometer (Hitachi, Japan) for studying the fluorescent properties of the samples. The EX WL was 365.0 nm and scan speed was 240 nm/min. The EX Slit was 10.0 nm and EM Slit was 20.0 nm. The PMT Voltage was 400 V. The fluorescent LC display was tested before and after the irradiated of UV for 3 h. Fluorescence lifetime was detected by a HORIBA FL-1057 lifetime and steady state spectrometer (HORIBA, USA). Centrifugal experiment: To clearly prove the mechanical strength of the system, centrifugal experiment was applied to the display made by ourselves. The experiments were carried out using a high speed centrifuge (HC-3018, ZhongJia ZONKIA) at the speed of 2000 r/min for 10 min. Temperature tunable experiments: The transformation of the displays as the tunable temperature could be clearly shown by heating stage (MR Hei-Standard, Heidolph Instruments D-91126 Schwabach). Preparation of LC physical gels: The fluorescent LC physical sol was prepared by dissolving certain amount of POSS-G1-BOC, phosphors and 2Azo2 in 5CB at 150 oC in stoppered tubes using an oil bath under constant agitation. Then the sol was cooled to the room temperature to 7



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form LC physical gel. The LC physical gel without phosphors was prepared by the same way.21 The samples were named with percentages (firstly) and components (secondly). Preparation of displays: The fluorescent LC light scattering display was prepared by dropcasting the homogeneous sol which was firstly heated at 150 oC into a homemade container composed with two slides and 3M tape as spacer and then cooled to room temperature in air. After that, the displays should be sealed with glue at both ends to avoid the influence of ambient atmosphere. The LC light scattering display was prepared by the same way.22 And the making process of the display only consisting of phosphors, 2Azo2 and 5CB was also the same. The displayed image on the device written by ultraviolet was successfully obtained by the use of a “ZJUT” photomask, as shown in Figure S1: the thin display cell was covered with the dark “ZJUT” photomask and exposed to 35.4 mW/cm2 UV irradiation of 365 nm wavelength for 3 h at room temperature. RESULTS AND DISCUSSION Gelation behaviours and properties: Special phase transition behavior of the LC gel (0.5wt%POSS-G1-BOC/5wt%2Azo2/5wt%Phosphor/5CB) through repeating heating-cooling processes shown in Figure S2 indicating a high thermo-stable LC gel with fluorescence-emission ability had been obtained. From Figure S2 (b1) we find that the Tsol-gel of LC gel is about 120 oC and the fluorescence flaming out from phosphors (Figure S2 (b2)) is more clear than Figure S2 (a2). This may because the strong light transmission in Figure S2 (a2) relatively weaks the fluorescence-emission ability of the uniformly scattered pink phosphors under UV light. At this time, a three-dimensional gel network filled with isotropic liquid crystals forms in the system and the phosphors still well dispersed, which is called isotropic transparent LC gel (Figure S2 8


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(b1)) below 120 oC. Figure S2 (a) showed sol state at 150 oC owning to the destruction of hydrogen bonding force in the gel network with the increasing temperature. Obviously adding of 5 wt% pink phosphors decreases the transparency comparing to the system without phosphors which reported in the literature by us.21 This may because the color of phosphors is darker than 2Azo2 and the mixture of pink and yellow makes the final color much darker than the system without phosphors. As we all know, the isotropic-anisotropic transition temperature of 5CB is about 35 oC. With the mixture being continuously cooled below Tiso-LC, the state of liquid crystals transformed from isotropic to anisotropic phase and the director of every domain in liquid crystals devided by the gel network was inconformity. This results in the scattering state of the system showing opaque condition at 25 oC (Figure S2 (c2)) and it has stronger fluorescent intensity than its isotropic state seen in Figure S2 (a2). We could clearly find out that the whole process is reversible. In addition, Tiso-LC and Tsol-gel with different content of POSS-G1-BOC were tested through tube

inversion

method.

Sol-gel

and

phase

transition

temperature

of

the

0.3/0.5/0.7/0.9/1.1wt%POSS-G1-BOC/5wt%2Azo2/5wt%Phosphor/5CB LC gel can be seen in Figure 1. Even though the gelator’s concentration was as low as 0.3 wt%, Tsol-gel was still as high as 113 oC. With the concentration of gelator up to 1.1 wt%, Tsol-gel reached to 138 oC. In the meantime, Tiso-LC with different content of gelator was all around 37 oC, decided by the natural phase transition properties of host 5CB. Comparing to the result of 0.3/0.5/0.7/0.9/1.1wt%POSSG1-BOC/5wt%2Azo2/5CB LC gel reported in the literature21, the Tiso-LC and Tsol-gel were about the same. And the phase transition temperature of 5wt%Phosphor/5CB was also around 37 oC. It indicates that the addition of gelators, photo responsive agent and phosphors has no significant influences on the LC phase transition of host 5CB (35 oC) and does not influence its thermo-

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stability. These experimental results proved that the LC physical gel has a very significant thermal stability which can retain the gel state even at high temperature.

Figure 1. Sol-gel and phase transition temperature of the 0.3/0.5/0.7/0.9/1.1wt%POSS-G1BOC/5wt%2Azo2/5wt% Phosphor/5CB LC gel. To further study the mechanical properties of LC physical gel, dynamic rheological measurements were carried out as shown in strain scanning of the system with different content of gelators (Figure 2 and Figure S3). Figure 2(a) shows an intersection at the strain of 20% represented a solid-like structure kept in this strain range. And when the adding of gelators were up to 0.5 wt% and 0.7 wt%, G’ and G” increased by almost one order of magnitude, at the same time the strain still stayed within 20 %. The deformation resistant capacity of the LC gel system was increased with the increasing of gelators’ content. It illustrates that adding gelators in host 5CB can relatively resist great outer force. When the level of gelators was up to 0.9 wt% and 1.1 wt%, the intersections slightly reduced to 13 % simultaneously and the storage modulus reached a maximum. So to some extent the LC gel is a kind of material which has the ability of

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G' 0.5wt% POSS-G1-BOC/5wt% 2Azo2/5wt% Phosphor/5CB G" 0.5wt% POSS-G1-BOC/5wt% 2Azo2/5wt% Phosphor/5CB 0.1

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Figure 2. Strain dependence of G’ and G” for gels, measured at 25 oC with a strain from 0.01 % to 200 % and frequency ω = 10 rad s-1; (a-e) 0.3/0.5/0.7/0.9/1.1wt%POSS-G1BOC/5wt%2Azo2/5wt%Phosphor/5CB LC gel; (f) 5wt%2Azo2/5wt%Phosphor/5CB. maintaining its own independent shape even though there is an external small force. Comparing to the system without phosphors21, G’ and G” of the two systems are analogous, so it can be deduced that phosphors do not affect the strength of LC physical gels. The recoverability of LC

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gel is exhibited in Figure S3, which also provides the comparison of the LC gel with different content of gelators. We can see that the initial storage modulus continued to rise as the increase of gelator contents up to 1.1 wt% and at the same time the extent of variation of G’ got larger and larger in the presence of 0.1% strain, showing the LC gel has a fairly good recoverability. And the rheological properties of 5wt%2Azo2/5wt%Phosphor/5CB without gelators were shown in Figure 2(f) and Figure S3 (f). The comparison of the two systems of whether adding gelators or not clearly indicates the gelators’ function of forming quite well network and having strong mechanical properties. To clearly observe the network structure of the system, SEM images of 0.5wt%POSS-G1BOC/5wt%2Azo2/5wt%Phosphor/5CB LC xerogel are shown in Figure 3 a-d. The special loofah-like gel networks greatly help the other materials reduce the movement in the system to provide the mechanical strength. Loofah-like gel networks are supported by a sectional type hexagonal columnar assembly mode rather than physical entanglement of the length-limited fibers.20 From the image of SEM we can clearly see the furcated fibers with a width of 100-300 nm which seem to have no endpoint. And we find that the fibers in Figure 3c and Figure 3d appear obvious spiral status which needs to be further studied. Most notably, compared with the SEM image (Figure 3e) of the loofah-like xerogel network morphology obtained in 0.5 wt% MMA, the xerogel network of our system did not collapse and its size and morphology of fibers almost kept unchanged. However, compared to the collapsing network has reported in the literature20 (Figure 3f), the networks of our system successfully kept the original appearance and didn’t change as guest molecules changed. The situation appeared in Figure 3f might because the elution and dry processes destroy the integrity of network.

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Figure 3. SEM images of the loofah-like xerogel network morphology obtained in (a-d) 0.5wt%POSS-G1-BOC/5wt%2Azo2/5wt%Phosphor/5CB; (e) MMA (0.5 wt%)20; (f) 0.5wt% POSS-G1-BOC/5wt%2Azo2/5CB which has been removed by rinsing with hexane.22 To further prove the gel network structure of POSS-G1-BOC in the system, TEM test is also taken, which is shown in Figure S4 (a-c) similarly reveals that loofah-like gel network exists in the LC gel and has not been disrupted with the adding of phosphors. More clearly can be seen the furcated fibers seem to have no endpoint and grows together inherently to form a continuous and integral network without physical crosslinking of individual fibers. The phenomenon of one thick mother-fiber branching off into two relatively thin daughter-fibers which is identical with the structure has been reported in the literature20 called Y-limb. The existence of the network improves the response capability of the system owing to its ability of dividing the LC molecules

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to each small domain. So the special network exactly exists in the system and provides a super strong ability of resisting exterior force.

Figure 4. Proposed mechanism for the formation of LC physical gel: (a) isotropic liquid, 150 oC; (b) isotropic gel, 120 oC; (c) nematic LC gel, 25 oC; (d) a cross-sectional view of the loofah-like hexagonal columnar network; (e) single columns are formed via the plane-to-plane stacking of free gelator molecules and superimpose together to form hexagonal columnar bundle structure.

The phase-separated structures of the gels are dependent on the comparation of Tsol-gel and Tiso-LC. There are two types of phase transition behaviors of the gelators and liquid crystals.23 According to the high thermal stability of our gelators, we set up the construction principle shown in Figure 4. In our system, Tsol-gel is much higher than Tiso-LC, the state of liquid crystals and gelators are different with the decreasing temperature (Figure 4 (a) → (b) → (c)). Randomly scattered gelators form loofah-like network across the isotropic liquid crystals which

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is called isotropic gel (Figure 4 (a) → (b)). Single columns aggregates together to heap up in a hexagonal columnar bundle structure20 (Figure 4 (d) with randomly arranged liquid crystals around it. And its structure arranges via the plane-to-plane stacking of free gelator molecules which is shown in Figure 4 (e). Subsequently at lower temperatures it starts a process of isotropic-anisotropic transition of liquid crystals, called nematic LC gel (Figure 4 (b) → (c)). All the above transition processes are reversible. Photoresponse performance: Azobenzene-containing LC mixtures commonly possess photoinduced isomerization, well stability, reversibility and other characters.24 The irradiation of 365 nm and 455 nm light sources are used to study the photoresponsive performances of the system. The gel with a large absorption peak around 365 nm at the beginning met a stable minimum level of the absorption peak until the irradiation time was up to 20 s (see in Figure S5). And it was faster than the system without phosphors which needed 35 s to reach a stable state reported in the literature.21 On this basis the sample almost totally returned to the initial state after being irradiated by Vis light for 6 s (see in Figure S6) while the system without phosphors only needed 2 s. It may because the 5 wt% phosphors absorb the UV light and accelerate the speed of the system to reach the stable state. And at the same time, the recovery rate also slows down because of the function of phosphors. This phenomenon of LC gel retains the performance of azobenzene which can easily realize ultraviolet-visible responsive property. Under visible light, the gel network divides LC molecular up into countless small domains and its order directions of each phase are disunity. It results in the scattering state of LC gel. With the action of 365 nm and 455 nm light, the trans-cis photoisomerization of azobenzene occurs and breaks LC’s ordered arrangement. As a result, LC gel transforms from anisotropic state (opaque) to

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isotropic state (transparent). In a word, with the action of specific wavelengths of light, the fluorescent LC gel can successfully realize the transition from opaque to transparent. Fluorescence property: Fluorescence effect25 means the phenomenon of fluorescent materials irradiated by UV (shorter wavelength) emitting fluorescence in the long wave band. To get to know the fluorescence property of the fluorescent LC displays, emission spectrum was shown in Figure S7. It was evident that there was a huge peak up to 765 around 607 nm which was prior to be irradiated in the condition of light scattering. While the system changed to be transparent after being irradiated by 365 nm UV light as an excitation source for 3 h, the intensity of the system was increasing twice as much as before. By the huge difference of fluorescence emission intensity, the displayed contents of fluorescence display could be clearly seen through the UVtransmitting partition. Luminescence decay curves of fluorescent LC gel in Figure S8 show that the fluorescence lifetime of gel is about 0.3 ms. The fluorescence irradiated by phosphors oscillated greatly in the gel network. The contrast between the two can further expand the scope of display application due to the obvious fluorescence effect. For example, the warm color tone of the display (see in Figure S9) may have special application places such as rooms in cold area, traffic lights and so on. Fluorescent LC light scattering display: The obvious fluorescence effect vividly reflects in Figure 5. Comparing the two kinds of displays, fluorescent LC light scattering display (seen in Figure 5 (d)) could perfectly display the contents written by non-contact control of 365nm UV lights owing to the strong fluorescence emitting by phosphors. While the display without phosphors reported before22 (seen in Figure 5 (b)) hardly showed anything under 365nm UV lamp. So the fluorescent LC light scattering display can be used in the UV environments such as

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ultraviolet light disinfection rooms in hospitals, ultraviolet accelerated weatherometers and so forth.

Figure 5. LC light scattering display (0.5wt%POSS-G1-BOC/5wt%2Azo2/5CB), under 365nm UV lamp: (a) prior to irradiation; (b) after irradiation of UV; fluorescent LC light scattering display (0.5wt%POSS-G1-BOC/5wt%2Azo2/5wt% Phosphor/5CB), under 365nm UV lamp: (c) prior to irradiation; (d) after irradiation of UV. Display properties in practical situation: In order to study the properties of the display in practical situation, many researches have been done. Firstly, the impacts of display time on display effect is studied. From Figure S10 we can see a gradual change of the fluorescent LC gel display images at room temperature for nearly 9 h. We still have a good view of the images at 4 h and totally lose sight 9 h. While changing images of the LC gel display shown in Figure S11 indicate the display time only could be maintained for 3 h and it’s hard to observe the information after 1 h. The fluorescent LC gel display has much longer display time comparing to the LC gel display at room temperature. It may because the existence of even dispersed 17



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phosphors reduces the space of free motion among the molecules and hinders the process of azobenzene’s trans-cis photoisomerization. Furthermore, the impacts of ambient temperature on display effect is also studied. From the experiments of repeated heating and cooling treatment to the display shown in Figure 6 and Figure S12, we fortunately found it’s a recyclable process with almost no clear change of images as the temperature changing between 25 oC and 37 oC and had the best degree of clarity at 37 oC. With tests we could clearly find out the phase transformation of 5wt%Phosphor/5CB was around 37 oC and it’s quite similar to 5CB host which was around 35 o

C, indicating the best display temperature is determined by the host. Comprehensive analyzing

the influences of time and temperature, we observed the gradually changeable process of displays for every half an hour with constant temperature around 37 oC. From Figure S13 and Figure S14 we discovered that the information written into the displays before could perfectly maintain for almost 3 h at 37 oC and its contrast was much higher than the display at room temperature. The reduce of display time from 9 h ( 25 oC ) to 3 h ( 37 oC ) mainly because the vigorous movement of molecules promoting azobenzene’s trans-cis photoisomerization. And the clearing point ( around 35 oC ) of 5CB helps realizing the best display effect at 37 oC. In addition, to know the highest usage temperature of the displays, the comparison of fluorescent LC displays whether adding gelators or not was shown in Figure S15 (grabbing images from video 1 in supporting information) with the constant rising of the heating stage temperature. In the system of fluorescent LC displays without gelators, the image (Figure S15 a1) could slightly be seen at 37 oC. As the temperature slowly rised to 74 oC, the image totally vanished. After cooling to 25 o

C, the display continued being heated to 37 oC. The whole process could hardly have a distinct

image. On the contrary, the display with the addition of gelators had a high-definition image seen in Figure S15 a2 at 37 oC. As the temperature slowly rised to 74 oC, the images vanished as well

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in Figure S15 c2. Then similarly nothing could be observed in Figure S15 d2 when cooled to 25 o

C. Finally we heat it again to 37 oC, it could be simply observed the clear “ZJUT” on the display

in Figure S15 e2 while the Figure S15 e1 was not. These two illustrate that the gelator has shape memory ability at specified temperature range and could keep the original message and definition. Broadly speaking, the displays containing gelators have wide usage temperature range and long usage time.

1cm

Figure

6.

Images

of

fluorescent

LC

gel

displays

(0.5wt%POSS-G1-

BOC/5wt%2Azo2/5wt%Phosphor/5CB) during repeated heating and cooling. (a) 25 oC; (b) 37 o

C; (c) 25 oC; (d) 37 oC; (e) 25 oC.

Special high mechanical properties of LC displays: In the pratical use, the display might have to face hash situations that need good mechanical properties. In order to simplify the complicated actual condition, we use centrifuge experiment to simulate rigorous mechnical environment to confirm the results of dynamic rheological characterization, as shown in Figure 7 and Figure S16. Comparing the two images, it’s evident to find that the images had hardly changed after centrifuging for 10 mins at the speed of 2000 r/min because of function of the strong gel network.

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And the stretchable light scattering display based physical gels without phosphors reported by our group22 exhibits quite good stretchability at room temperature and can clearly show pictorial information at strains as large as 145%. They are two similar systems with the same gelator. The centrifugation experiments explain the adding of phosphors does not have remarkable influence on the mechanical properties of the display. In general, the displays have quite strong mechanical properties and the prepared display could overcome the harsh environments, being widely used in road transportation, railway transportation, air transportation and other fields.

Figure 7. Fluorescent LC displays (0.5wt%POSS-G1-BOC/5wt%2Azo2/5wt%Phosphor/5CB): (a) the initial opaque state at 25 oC; (b) after being heated at 50 oC; (c) after irradiation at 365 nm, 25 oC; (d) after centrifuging for 10 min at the speed of 2000 r/min, 25 oC; (e) in a 50 mL plastic centrifuge tube. (The thickness of the LC physical gel was 0.14 mm and the distance between the display and the paper was 5.5 mm.) Body temperature controlled display properties: To further expand the application of the displays, we tried to use our hands to change the contrast according to the clearest display temperature at 37 oC. Figure 8 (grabbing images from video 2) and Figure S17 (grabbing images from video 3) clearly indicated that the display had body temperature controlled ( 37 oC ) display 20


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performance and could obtain optimal display effects though being placed on one’s palms. And it could realize circular display by the way of being covered with hands which would have intensively potential applications in the fields of artificial skin.

Figure 8. Fluorescent LC displays (0.5wt%POSS-G1-BOC/5wt%2Azo2/5wt%Phosphor/5CB): (a) the initial state, 25 oC; (b) covered with hands for 30 s; (c) after covering with hands; (d) returned to the initial state after 2 min, 25 oC; (e) put on the hand for 40 s; (f) after putting on the hand; (g) returned to the initial state after 90 s, 25 oC.( The thickness of the LC physical gel was 0.14 mm and the distance between the display and the paper was 5 cm.) Sunlight controlled display: To make the display more lifelike, we placed the displays under the sunlight for an hour. Then transfered the displays to the shade areas and the information written before would appear for a while with the contrast of opacity and transparence. Just after

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nearly less than 1 min, the informations also exhibited well which were shown in Figure 9 (a3) and Figure 9 (b3). So it’s more convenient to get the displays with the irradiation of sunlight not just limited to the certain wavelength of 365 nm. The displays can be used as long as there is sunlight at around human body temperature.

1cm

Figure 9. Displays being exposed under sunlight for 90 min (14:00-15:30, 45 oC ): (a1) fluorescent LC displays (0.5wt% POSS-G1-BOC/5wt%2Azo2/5wt%Phosphor/5CB), 45 oC; (b1) LC displays (0.5wt%POSS-G1-BOC/5wt%2Azo2/5CB), 45 oC; Comparison images after being shined: fluorescent LC displays (a2) 37 oC; (a3) 25 oC; LC displays (b2) 37 oC; (b3) 25 oC. SUMMARY AND CONCLUSIONS In summary, an optical and thermal information storage material were reported here and it has a great deal of benefits such as body temperature response, fluorescence effect, wide range of usage temperature, high mechanical strength, shape memory ability and so on. The display gets out off polarizing films usually containing in a conventional liquid crystal displays (LCD). Based on the results of test-tube-inversion, the temperature controlled experiment, rheological measurement, centrifugation experiment, SEM and TEM, it’s clearly shown that the strong loofah-like 3D networks could promote the fabrication of devices with a fine thermal stability

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and relatively strong gel strength. The UV absorption spectra indicated that the fluorescent LC gels responded faster than the system of LC gels without phosphors. Emission spectrum of fluorescent LC gel vividly showed the property of fluorescence effect. Owning to the special properties, the display could use not only in harsh conditions and UV atmosphere, but also with different kinds such as body skin and under sunlight. The whole production process of display is quite simple and could get displays at any size though non-contact writing. We believe it will have wide applications in the future. ASSOCIATED CONTENT Supporting Information Preparation of displays (Figure S1), phase transition behavior and schematic illustration of LC gel (Figure S2), dynamic rheological characterization (Figure S3), gel network morphology (Figure S4), photoresponse performance (Figure S5 and Figure S6), fluorescence property (Figure S7, Figure S8 and Figure S9), fluorescent LC gel displays properties in practical situation (Figure S10, Figure S13 and Figure S15), LC gel display properties in practical situation (Figure S11, Figure S12, Figure S14, Figure S16 and Figure S17) (PDF) Comparison of fluorescent LC displays with 0.5wt%POSS-G1-BOC or not (video 1), body temperature controlled fluorescent LC gel display (video 2) and body temperature controlled LC gel display (video 3). AUTHOR INFORMATION Corresponding Author: Xu Wang * E-mail:[email protected]; Fax: +86-571-88320855

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§

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Si Chen and Xiaoqian Tong contributed equally.

ACKNOWLEDGMENT This research work is supported by National Natural Science Foundation of China (Grant No: 51173167 and 21004052), ZheJiang Provincial Natural Science Foundation of China (Grant No: LY14E030003 and LY14E030004) and Collaborative Innovation Center of Membrane Separation and Water Treatment of Zhejiang Province (Grant No: 2050999093). The authors would like to thank Professor ZaiFa Pan in the College of Chemical Engineering in Zhejiang Univeristy of Technology for the guidance of Luminescence decay curves. REFERENCES (1) Palui, G.; Banerjee, A. Fluorescent Gel from a Self-Assembling New Chromophoric Moiety Containing Azobenzene Based Tetraamide. J. Phys. Chem. B. 2008, 112, 10107-10115. (2) Kuang, G. C.; Ji, Y.; Jia, X. R.; Chen, E. Q.; Gao, M.; Yeh, J. M.; Wei, Y. Supramolecular Self-Assembly of Dimeric Dendrons with Different Aliphatic Spacers. Chem. Mater. 2009, 21, 456-462. (3) Qu, S. N.; Zhao, L. J.; Yu, Z. X.; Xiu, Z. Y.; Zhao, C. X.; Zhang, P.; Long, B. H.; Li, M. Nanoparticles, Helical Fibers, and Nanoribbons of an Achiral Twin-Tapered Bi-1,3,4-oxadiazole Derivative with Strong Fluorescence. Langmuir. 2009, 25, 1713-1717. (4) Hayashi, Y.; Ichimura, K. Fluorescence Studies on Intermolecular Interactions of Cyanobiphenyl Groups Chemisorbed on a Quartz Surface with Liquid Crystals. Langmuir. 1996, 12, 831-835.

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Body Temperature Controlled Optical and Thermal ... - ACS Publications

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