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2009, 113, 13882–13885 Published on Web 09/30/2009
Thermally Controllable Reflective Characteristics from Rupture and Self-Assembly of Hydrogen Bonds in Cholesteric Liquid Crystals Wang Hu, Hui Cao, Li Song, Haiyan Zhao, Sijin Li, Zhou Yang, and Huai Yang* Department of Materials Physics and Chemistry, School of Materials Science and Engineering, UniVersity of Science and Technology Beijing, Beijing 100083, China ReceiVed: July 8, 2009; ReVised Manuscript ReceiVed: September 21, 2009
A cholesteric liquid crystal (Ch-LC) composite, made of a series of cholesteryl esters, a nematic LC, and a hydrogen bond (H-bond) chiral dopant (HCD), was prepared and filled into a planar treated cell. When the cell was heated, the selective reflection of the cell exhibited an unusual blue shift. One of the reasonable mechanisms was that the helical twisting power (HTP) value of cholesteryl esters increased with an increasing temperature. The other one was that the H-bonds of HCD were ruptured when the temperature was above 60.0 °C and HCD was split into two kinds of new chiral dopants, which made the HTP value of the chiral dopants change a lot, thus changing the pitch length of the composite greatly. On the basis of this mechanism, a novel thermally controllable reflective color paper could be achieved. Introduction Ch-LC possesses the inherent properties of wavelength- and polarization-selective reflection when the molecules are macroscopically arranged in a periodic helicoidal structure.1 The reflection wavelength of the Ch-LC, λ0, is related to the helix pitch, P, by the Bragg relation (at normal incidence), λ0 ) nP, where n ) (no + ne)/2 is the average of the ordinary (no) and extraordinary (ne) refractive indexes of the locally uniaxial structure. The bandwidth of the selective reflection, ∆λ, is given by ∆λ ) λmax - λmin ) (ne - no)P ) ∆nP, where ∆n ) ne no is the birefringence. The circularly polarized light with the same handedness as the helix is reflected within this reflection band. Outside of the reflection band, both polarization states are transmitted.1-3 Because of the unique property, many efforts have been devoted to the study of selective reflection of Ch-LC. For example, Ch-LC has the ability to shift the selective reflection by changing external factors including electric, magnetic, and acoustic fields, temperature, and light irradiation.4-11 One of the potential applications of Ch-LC with the characteristics is as display devices, such as electrically addressable and electrically erasable display devices, thermally addressable and electrically erasable devices, as well as the electrically addressable and thermally erasable ones.12-16 In these cases, the Ch-LC can be switched either between the planar and the focal conic textures by applying electric fields with different frequencies or between the Ch and the smectic A (SmA) phases with different temperatures. However, the devices based on these mechanisms can only be used as black-white display, which limits their applications in some fields nowadays. In this letter, we study the thermo-optical properties of Ch-LC composites and find that the selective reflection exhibits an unusual blue shift with an increasing temperature. Moreover, we also * To whom correspondence should be addressed. Tel.: +86-10-62333969. Fax: +86-10-623333969. E-mail:
[email protected].
10.1021/jp9064449 CCC: $40.75
Figure 1. Chemical structures of the materials used.
demonstrate that this composite has great potential as one kind of thermally controllable reflective color paper. Materials and Methods In this study, a series of cholesteryl esters (TCI Co. Ltd.), a nematic LC, SLC1717 (Slichem Liquid Crystal Material Co. Ltd.), a HCD, a photopolymerizable LC monomer, C6M, and a photoinitiator, 2,2-dimethoxy-1,2-diphenylethanone, IRG 651 (TCI Co. Ltd.) were used. HCD was obtained by thorough mixing of a chiral pyridine compound and a chiral carboxylic acid in appropriate proportions in a nonprotic solvent, followed by slow evaporation.17-19 C6M was lab-synthesized according to the method suggested by D. J. Broer.20 The chemical structures of the materials used above are listed in Figure 1. The cholesteryl esters used here include cholesteryl nonanoate (CN), cholesteryl oleyl carbonate (COC), cholesteryl oleate (CO), cholesteryl benzoate (CB), and cholesteryl chlorine (CC). Then, the cholesteryl ester mixture (C5) was prepared, whose compositions and weight ratio were CN/COC/CO/CB/CC ) 2009 American Chemical Society
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J. Phys. Chem. B, Vol. 113, No. 42, 2009 13883
TABLE 1: Compositions and Weight Ratios of Samples 1-3 sample
C5/SLC1717/HCD/C6M/IRG 651 (wt %)
1 2 3
-/97.6/2.4/-/37.0/44.0/19.0/-/36.1/43.0/18.6/2.2/0.1
55.0/15.0/11.0/10.0/9.0%. The Ch phase temperature range of the left-handed C5 was from -29.64 to 70.76 °C, which exhibits a Ch phase at room temperature, and the reflection wavelength was 457.0 nm. When the cell containing C5 was heated from 20.0 to 70.0 °C, the reflection wavelength exhibited a blue shift from 457.0 to 400.0 nm because the HTP value of C5 increased constantly with an increasing temperature. The compositions and the weight ratios of the three studied samples are listed in Table 1.
Figure 3. Temperature dependence of the reflection spectra of sample 2. The insets are the photos of the cell containing sample 2 taken at different temperatures and the temperature dependence of the pitch length of sample 2.
Results and Discussion Figure 2a shows the temperature-dependent IR spectra (Perkin-Elmer Spectrum100, U.S.A.) of the HCD, and the stability of the H-bonds of the HCD can be directly confirmed by it. It has been well-known that the appearance of two peaks centered at 2500.9 and 1905.5 cm-1 results from the H-bonds between the carboxylic acid and pyridyl group,17 as shown in Figure 2a. The two H-bonded characteristic peaks change little within 50.0 °C, indicating a good thermal stability of HCD; when above 60.0 °C, they become weak, which indicates that the H-bonds have begun to be ruptured. When the temperature is up to 70.0 °C, the two peaks centered at 2500.9 and 1905.5 cm-1 have disappeared, which demonstrates that all of the H-bonds have been ruptured. Figure 2b shows the temperature dependence of the helical pitch length of sample 1. It can be seen that the helical pitch length changes little under 57.0 and above 67.0 °C, while it has a great decrease between 60.0 and
Figure 2. (a) Temperature-dependent IR spectra of HCD from 2870.0 to 1732.0 cm-1. (b) Temperature dependence of the pitch length of sample 1; the insets are the corresponding POM micrographs taken in the Cano wedges at 57.0 and 67.0 °C.
67.0 °C. We believe that this is because the H-bonds begin to be ruptured when the temperature is above 60.0 °C, and the HCD is split into two kinds of new chiral dopants whose HTP values are much greater than that of the HCD. The insets in Figure 2b are the corresponding micrographs of cholesteric textures of sample 1 in the Cano wedges,21 taken by a polarized optical microscope (POM, Olympus BX51) at 57.0 and 67.0 °C. It is obvious that the distance of two phase stagger lines decreases greatly from 57.0 to 67.0 °C, which proves that the pitch length has a great change in this temperature region. As a result, from sample 1, it can be concluded that the H-bonds of the HCD in the LC can be ruptured above 60.0 °C. In order to study the optical performance in the visible region, some cholesteryl esters were added into sample 1, and the weight ratio of the compositions was adjusted. Then, sample 2 was prepared as shown in Table 1. Figure 3 represents the temperature dependence of the reflection spectra of the cell containing sample 2. The reflection wavelength exhibits an obvious blue shift from 680.0 to 460.0 nm with a constantly increasing temperature from 20.0 to 68.0 °C, and the color which the cell reflects changes from red to orange and finally blue. In particular, within the temperature range of 20.0-60 °C, the reflection wavelength exhibits a blue shift from 680.0 to 648.0 nm. This is because the HTP value of C5 increases with an increasing temperature, which causes the pitch length of sample 2 to decrease somewhat, and the color that the cell reflects changes from red to orange. When the cell is heated from 60.0 to 68.0 °C, the H-bonds of the HCD in sample 2 are ruptured, and the HCD turns into two new chiral dopants. This makes the HTP values of sample 2 increase and the pitch length decrease greatly. Consequently, with an increasing temperature from 60.0 to 68.0 °C, the reflection wavelength of sample 2 shows a blue shift from 648.0 to 460.0 nm, and the reflectance centered at 648.0 nm decreases while that at 460.0 nm increases, as shown in Figure 3. Correspondingly, the color that the cell reflects changes from orange to blue. It is noteworthy that the clear point of sample 2 is 69.2 °C when the cell is heated up to 68.0 °C, which is close to the clear point, and the reflectance of the reflection wavelength centered at 460.0 nm is about 27.5%, not up to 50.0%, as shown in Figure 3. The insets in Figure 3 are the photos of the cell containing sample 2 taken at different temperatures, which show the process of the color change with the increasing temperature, as well as the temperature dependence of the helical pitch length of sample 2, which demonstrates that the pitch length of the sample has a great decrease above 60.0 °C.
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Figure 4. (a) Photos of the prepared cell containing sample 2 without the polymer wall in it after a character T was thermally addressed and (a1-a3) POM micrographs of different regions of the cell. (b) SEM image of the polymer wall network formed in the cell containing sample 3. (c) Scheme of the role of the polymer wall decreasing the heat conduction. (d, e) Photos of the cell containing sample 3 with the polymer wall in it before and after a character T was thermally addressed.
With the mechanism described above, a kind of novel thermally controllable reflective color paper can also be prepared. As soon as a hot pen at 68.0 °C was scanned on one side of the cell containing sample 2, the local region could turn from red to blue because the H-bonds of the HCD were ruptured and the pitch length had a great decrease locally. However, due to that, the heat at the place where the hot pen was scanned would diffuse to the surrounding areas, and a temperature gradient would form from 68.0 °C to room temperature. In this temperature gradient region, the heat was conducting, and some H-bonds of the HCD were ruptured, which caused the molecules of the LC to be disordered, thus leading to a light-scattering state. In this way, a blue character T could be thermally addressed in a red background, but it was unclear, as shown in Figure 4a. Figure 4a1, a2, and a3 represents the POM micrographs of the textures in different regions of sample 2 in the cell. It can be seen that the thermally addressed regions and the ones far away from them are both planar textures, while the regions near the thermally addressed ones exhibit the coexistent textures of focal conic and planar textures, which significantly impacted the display effect. In order to solve this problem, further explore was carried out. On the basis of sample 2, a photopolymerizable LC monomer, C6M, and a photoinitiator, IRG 651, were added, and sample 3 was prepared, as shown in Table 1. Because only minimal amounts of C6M and IRG 651 were added, sample 3 possessed almost the same temperature dependence of the helical pitch length as sample 2. Then, a photomask was put on the cell containing sample 3, and the cell was irradiated with UV light (2.13 mW/cm-2, 365.0 nm) at room temperature for about 15.0 min, and a polymer network was formed from the cross-linking between the molecules of C6M in the regions exposed to the UV light. In this study, the mask used was divided into several squares whose side length was 650.0 µm and the interval of squares was 30.0 µm; only the intervals allowed the transmission of the UV light. Due to that, the polymer network could only form in the regions of the intervals; the patterned polymer network which looked like a “wall” formed in the cell. Figure 4b shows the SEM image
of the polymer wall in the cell. Due to the role of the polymer wall, the heat flow which was conducted from one region to another was hindered greatly, as schematically shown in Figure 4c. After the formation of the polymer wall, as shown in Figure 4d and e, a blue character T could be thermally “written” in a red background after a hot pen was scanned on the surface of the cell with a speed of 0.5 cm/s without the problem as mentioned above because the heat conduction was hindered by the polymer wall; in other words, the resolution in Figure 4e is much higher than that in Figure 4a. Furthermore, it has been demonstrated that the state in Figure 4e had been stable for over 3 days at room temperature without any external fields and then automatically recovered to the initial state as a result of self-assembly of H-bonds, as shown in Figure 4d and e. This indicates that the thermally controllable reflective color paper prepared here exhibits the memory effect. Conclusions In summary, the thermo-optical characteristics of a Ch-LC composite were studied. Because the HTP value of C5 increased and the H-bonds of HCD were ruptured with an increasing temperature, the selective reflection exhibited a blue shift from 680.0 to 648.0 nm continuously and then to 460.0 nm discontinuously, and the color that the cell reflected changed from red to orange and then to blue, correspondingly. When the composite was used as a thermally controllable reflective color paper, the resolution in the thermally addressed region could be improved after the preparation of the polymer wall in the composite. The character (information) addressed in the cell could be memorized over 3 days without obvious change at room temperature and then automatically recovered to the initial state, which indicates that this composite possesses the memory effect. Furthermore, both the character (information) and the background of this kind of novel color paper exhibited both reflective colors by reflecting visible light around it, being unnecessary for back-light and helpful to realize the low power consumption, which must attract attention in the near future.
Letters Acknowledgment. This work is financially supported by the Key Program for Panel Display of 863 Program of China (Grant 2008AA03A318), National Natural Science Foundations (Grant 20674005 and 50973010), National Key Technology Program (Grant 2007BAE31B00), Program of National High Technology 863 program of China (Grant 2006AA03Z108), 2004 Key Fund of Chinese Ministry of Education (Grant 104187), and Doctorate Fund of Chinese Universities (Grant 20050008036). Supporting Information Available: Further details about sample preparation, experimental techniques, physical measurements, and supplemental experimental data. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Lam, L. In Chirality in Liquid Crystals; Kitzerow, H., Bahr, C., Eds.; Springer: New York, 2001; p 159. (2) de Gennes, P. G.; Prost, J. The Physics of Liquid Crystals; Oxford University: New York, 1993. (3) John, W. D. S.; Fritz, W. J.; Lu, Z. J.; Yang, D. K. Phys. ReV. E 1995, 51, 1191. (4) Furumi, S.; Yokoyama, S.; Otomo, A.; Mashiko, S. Appl. Phys. Lett. 2003, 82, 16. (5) Chanishvili, A.; Chilaya, G.; Petriashvili, G.; Barberi, R.; Bartolino, R.; Cipparrone, G.; Mazzulla, A.; Oriol, L. Appl. Phys. Lett. 2003, 83, 5353.
J. Phys. Chem. B, Vol. 113, No. 42, 2009 13885 (6) Morris, S. M.; Ford, A. D.; Pivnenko, M. N.; Coles, H. J. J. Appl. Phys. 2005, 97, 023103. (7) Ma, Y. D.; Wu, B. G. U.S. Patent No. 5 1999, 949, 513. (8) Chien, L. C.; Muller, U.; Nabor, M. F.; Doane, J. W. SID Tech. Digest 1995, 26, 169. (9) Huang, Y. H.; Zhou, Y.; Wua, S. T. Appl. Phys. Lett. 2006, 88, 011107. (10) Xianyu, H.; Lin, T. H.; Wua, S. T. Appl. Phys. Lett. 2006, 89, 091124. (11) Huang, Y. H.; Chen, L. P.; Doyle, C.; Zhou, Y.; Wu, S. T. Appl. Phys. Lett. 2006, 89, 111106. (12) Xu, M.; Yang, D. K. Appl. Phys. Lett. 1997, 70, 720. (13) Hsu, J. S.; Liang, B. J.; Chen, S. H. Appl. Phys. Lett. 2004, 85, 5511. (14) Yang, H.; Kikuchi, H.; Kajiyama, T. Liq. Cryst. 2000, 27, 1695. (15) Yang, H.; Kikuchi, H.; Kajiyama, T. Liq. Cryst. 2002, 29, 1141. (16) Geng, J.; Dong, C.; Zhang, L. P.; Ma, Z.; Shi, L.; Cao, H.; Yang, H. Appl. Phys. Lett. 2006, 89, 081130. (17) Kato, T.; Frechet, J. M. J. J. Am. Chem. Soc. 1989, 111, 8533. (18) Kato, T.; Frechet, J. M. J.; Wilson, P. G.; Saito, T.; Uryu, T.; Fujishima, A.; Jin, C.; Kaneuchi, F. Chem. Mater. 1993, 5, 1094. (19) Osuji, C.; Chao, C. Y.; Bita, I.; Ober, C. K.; Thomas, E. L. AdV. Funct. Mater. 2002, 12, 753. (20) Broer, D. J.; Boven, J.; Mol, G. N. Makromol. Chem. 1989, 190, 2255. (21) Lam, L. In Chirality in Liquid Crystals; Kitzerow, H., Bahr, C., Eds.; Springer: New York, 2001; p 34.
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