Electrothermal Switching Characteristics from a Hydrogen-Bonded

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Electrothermal Switching Characteristics from a Hydrogen-Bonded Polymer Network Structure in Cholesteric Liquid Crystals with a Double-Handed Circularly Polarized Light Reflection Band Jinbao Guo,† Fengjin Chen,† Zhijian Qu,† Huai Yang,‡ and Jie Wei*,† † ‡

College of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, P. R. China State Key Laboratory for Advanced Metals and Materials, Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China ABSTRACT: In this study, cholesteric liquid crystal (Ch-LCs) composite with a double-handed circularly polarized light reflection band that contains hydrogen-bonded (H-bonded) polymer network was fabricated by a polymer template method, in which the environmental sensitivity and reversibility of hydrogen bond and memory effect of the polymer template were incorporated into the polymer stabilized cholesteric liquid crystal (PSCLCs) film. The thermal and electrical tunability of the ChLCs composite before and after irradiation were investigated. It has been demonstrated that reflection characteristics of the Ch-LCs composite derived from H-bonded chiral monomer (HBCM) after irradiation exhibited strong temperature sensitivity with temperature change, which we believe is due to that helical twisting power value of both HBCM and cholesteryl additives increased with an increasing temperature. Additionally, the reflective wavelength of the Ch-LCs composite before and after irradiation can be electrically switched to reflect green from the initial state reflecting a red color. The modulating mechanism is due to Helfrich deformation in which the tilt of helices axis in the Ch-LCs composite was induced when a voltage was applied. The technique developed in this study has great applications such as tunable lasers, optical sensors and LCs displays.

average refractive index of the Ch-LCs.22-30 As is well-known, applying voltage on Ch-LC molecules enables the color tuning by extending the helical pitches or inducing tilt of helices;31-36 for example, Hikmet et al.26 used a Ch-LC mixture with polymer network to produce patterned Ch-LC gels reflecting various colors, in which the reflected wavelength had a blue shift with increasing voltage. This behavior can be associated with a Helfrich deformation, indicating uniformly periodical layer deformations in the Ch-LCs. The local tilting of helix leads to the blue shift of the reflection band observed from a normal direction, which can be related to Bragg reflection with a dependence on the angle of incidence in the Ch-LCs. Another simple method of tuning the PBGs of Ch-LCs is to heat the cell with Ch-LCs molecules. Yang et al.28 reported that thermally tuning the PBGs from rupture and self-assembly of hydrogen bonds in Ch-LCs, in which the hydrogen bond (H-bond) of H-bond chiral dopant (HCD) was ruptured when the temperature was >60.0 °C and HCD was split into two new chiral dopants of the initial chiral proton acceptor and chiral proton donor, makes the reflection color of the cell an unusual blue shift. It is a potential application for a novel thermally controllable reflective color paper.

1. INTRODUCTION Cholesteric liquid crystals (Ch-LCs) can be regarded as an important class of 1D photonic crystals (PCs); this is due to the fact that they can spontaneously form periodic helical structures, which lead to selective reflection for circularly polarized light. The reflected wavelength of Ch-LCs is generated by the photonic band gaps (PBGs) from the periodical helical orientation. The pitch p is defined as the distance in which Ch-LC molecules rotate 360° along its helical, and for light propagating along the helical axes, λ0 = p n, where λ0 is the wavelength of the maximum reflection or the middle of selective reflection band and n is the average refractive index n = (ne þ no)/2. The extraordinary and ordinary indices of refraction are denoted by ne and no, respectively. The PBGs width of a conventional Ch-LC is equal to p Δn and is proportional to the anisotropy of refractive indices Δn = ne - no. Within the bandwidth, light with the same handedness as that of the Ch-LCs will be circularly reflected, whereas the component with opposite handedness will be circularly transmitted.1,2 Ch-LCs have been considered for many applications, such as bistable LC displays, optical components, PCs and tunable lasers, and so on.3-21 It has been recognized that the ability to tune the PBGs of ChLCs would enhance their applications to photonics, such as in a tunable dye laser. Many external stimuli, such as pressure, electric field, temperature, and light, can be used to modify the pitch and/or r 2011 American Chemical Society

Received: September 26, 2010 Revised: December 17, 2010 Published: January 13, 2011 861

dx.doi.org/10.1021/jp109193m | J. Phys. Chem. B 2011, 115, 861–868

The Journal of Physical Chemistry B

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In general, the PBG band that could be fabricated from a conventional Ch-LCs with single-handed circularly polarized reflection band could respond rapidly to electrical or thermal addressing and could be wavelength-tuned over a broad range, as mentioned above. This motivated us to investigate electrical or thermal characteristics of a Ch-LCs with a double-handed circularly polarized reflection band. Herein, we employed a polymer template with helical structure and photopolymerizable chiral hydrogen-bonded liquid crystal to achieve this purpose. As is well known, helical structure memory of the polymer network, which is derived from the initial polymer-stabilized cholesteric liquid crystal (PSCLCs), is to template the liquid crystalline order by transferring the respective structure and orientation onto the network during polymerization.37-39 The above mechanism has been utilized to fabricate the Ch-LCs materials with a doublehanded circularly polarized light reflection band in our previous studies.40-43 Additionally, hydrogen bonding as a noncovalent approach has been used for the preparation of a wide variety of self-assembled systems because of its stability, dynamics, directionality, and reversibility.44 In this Article, the memory effect of the polymer template and the stimuli sensitivity of the H-bond of H-bonded chiral monomer (HBCM) were introduced to the ChLCs system. Both electrical and thermal tuning of the PBG band in the Ch-LCs with a double-handed circularly polarized reflection

band was demonstrated, and the mechanism that the reflection color of the Ch-LCs shifts on the thermal and electric fields and that the effects are fully reversible on increasing/decreasing the field strength was investigated.

2. EXPERIMENTAL SECTION 2.1. Materials. In this study, nonreactive nematic LCs, SLC1717 (20 °C, 589 nm, Δn = 0.201; Slichem Liquid Crystal Material); photopolymerizable monomer, C6M; H-bonded chiral monomer (HBCM, AHBA-PyCho), cholesteryl isonicotinate (proton acceptor)/4-(6-acryloyloxyhexyloxy) benzoic acid (proton donor), whose phase transition temperatures are Cr-SmA*, SmA*-TGBA*, TGBA*-Ch, and Ch-I of 96, 138, 149, and 186 °C, respectively; chiral dopant, R811 (Merck) and photoinitiator, 2,2-dimethoxy-2-phenyl-acetophenone (Irgacure 651, TCI) and cholesteryl esters (J&K), were used. Nematic diacrylate monomer, C6M, was synthesized according to the method suggested by Broer et al.45 4-(6-Acryloyloxyhexyloxy) benzoic acid and cholesteryl isonicotinate were prepared as described in previous papers.46,47 The cholesteryl esters of cholesteryl nonanoate (CN), cholesteryl oleyl carbonate (COC), cholesteryl acetate (CA), cholesteryl benzoate (CB), and cholesteryl chloride (CC) were used in this study, and the cholesteryl ester mixture (CEM), which exhibits a Ch phase at room temperature, and the reflection wavelength of 450.0 nm were prepared, in which the composition and weight ratio was CN/COC/CA/CB/CC = 54.0/ 15.0/11.0/10.0/10.0 wt %. The reflective wavelength exhibited a blue shift from 450.0 to 402.0 nm when the cell containing the above CEM was heated from 20.0 to 70.0 °C. Figure 1 shows the chemical structures of C6M, HBCM, R811, cholesteryl esters, and Irgacure 651. 2.2. Experimental Cells. To induce a planar orientation of LC molecules, we coated the inner surfaces of indium-tin-oxidecoated (ITO) glass cells with a 3.0 wt % polyvinyl alcohol (PVA) aqueous solution. The deposited film was dried at 80 °C for 30 min and subsequently rubbed with a textile cloth under a pressure of 0.02 N/cm2 along one direction. PET (polyethylene terephthalate) films of 25 μm thickness were used as the cell spacers. The samples were filled in the cells by capillary action at appropriate temperature. The compositions of the samples are listed in Table 1. 2.3. Preparation of PSCLCs with a Double-Handed Circularly Polarized Light Reflection Band. The PSCLCs with a

double-handed circularly polarized light reflection band were prepared by carrying out the following procedure. At first, the cells containing samples with right-handed helical structure (RHHS) were irradiated with UV light (365 nm, 1.0 mW/cm2) for 30 min for polymerization purposes, as shown in Figure 2a. Following that, the cells were immersed in cyclohexane for ∼48 h and later in tetrahydrofuran for 20 min to remove the nonreactive LCs. After that the cells were kept in a vacuum chamber at 60 °C for ∼3 h. Therefore, the polymer network with an RHHS was obtained, as shown in Figure 2b. Finally, the cells containing

Figure 1. Chemical structures of the materials used.

Table 1. Compositions of Ch-LCs Mixture and Reflection Band of the Cells Obtained cella

A (2/3)

mixture (type)b

weight ratio/wt%

TCh-I/°C

1 (LHHS)

SLC-1717 (95.0)/HBCM (5.0)

92.5

2 (RHHS)

SLC-1717 (67.0)/R811 (18.0)/C6M (15.0)

82.0

3 (LHHS)

SLC-1717 (48.8)/HBCM (20.0)/CEM (30.0)/C6M (1.2)

76.0

a

Cell A (2/3) means that cell A was obtained by infiltrating mixture 3 into the cell containing mixture 2 after polymerization and extraction of nonreactive LCs thereafter. b An amount of 2 wt % (compared with C6M and HBCM) of Irgacure 651 is added for mixtures 2 and 3. 862

dx.doi.org/10.1021/jp109193m |J. Phys. Chem. B 2011, 115, 861–868


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Figure 2. Illustration of the fabrication procedure of the Ch-LCs film.

Figure 3. (a) IR spectra of PyCho, AHBA, and HBCM (AHBA-PyCho). (b) Temperature dependence of the pitch length of mixture 1. (c) Corresponding POM micrographs taken in the Cano wedges at 25.0 and 92.0 °C.

the polymer network were refilled with Ch-LCs mixture with a left-handed helical structure (LHHS) containing HBCM and CEM by vacuum-filling process, as shown in Figure 2c, and followed by ultraviolet (UV) irradiation, as shown in Figure 2d. 2.4. Measurements. We confirmed the formation of H-bonds in complexes by using an FT-IR spectrometer (NICOLET 5700). The samples were observed using a polarizing light microscope

(POM) (Leica, DM2500P) with a heating stage (LTS 420). The optical images were recorded using Linksys 2.43 software. The transmission spectra were obtained by UV/vis spectrophotometer (Hitachi, U-3010) at normal incidence. The transmittance of a blank cell was normalized to 100%. In general, λM and Δλ are defined as the minimum wavelength of the transmitted light and the bandwidth at half-height of the peak, 863



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Figure 4. (a) Transmission spectra of cell with mixture 2 before and after UV irradiation. (b) SEM micrograph of the surface morphology of the cell with mixture 2 after UV irradiation. (c) Transmission spectra of cell with mixture 3 before and after UV irradiation. (d) Transmission spectra of cell A before and after UV irradiation.

respectively. The morphology of the polymer network was observed by scanning electron microscopy (SEM) (Hitachi S-4700). The samples for SEM studies were prepared according to the method described in the previous study.11

of the helical pitch in the Ch-LCs with variation in temperature was possible due to the associated change in the orientational parameter of the molecule or to smectic cybotactic states (smectic fluctuations) as a result of weakened or strengthened H-bonded interaction of the HBCM in the Ch-LCs.48 As a result, according to HTP = 1/P 3 C, in which P is the value of the pitch length and C is the concentration of the HBCM, it can be concluded that the HTP value of the HBCM in the SLC-1717 increased with an increasing temperature. We achieved a polymer template with RHHS by photopolymerizing C6M monomers in PSCLCs, which initially defined the periodicity or pitch (P) of the structure; then, we subsequently removed the nonreactive LCs from the system, as shown in Figure 2b. Then, Ch-LCs with LHHS containing HBCM and CEM were refilled in the above polymer template, followed by second UV irradiation, as shown in Figure 2c,d. As mentioned above, the helical structure memory of the polymer template and the temperature sensitivity/reversibility characteristics of the HBCM determined the optical properties of the Ch-LCs film in this study. Figure 4a shows the transmission spectra of cell with mixture 2 before and after irradiation, and Figure 4b shows SEM photographs of the surface morphology of the cell with mixture 2 after irradiation; the helical structure of the polymer network formed in PSCLCs can clearly be imaged. The reflection wavelength of the cell with mixture 3 after irradiation is ∼800 nm, as shown in Figure 4c, which is identical to that of the cell with mixture 2 after UV irradiation, as shown in Figure 4a. As mentioned above, the mixture 3 contains 1.2 wt % difunctionality C6M and 20 wt % monofunctionality HBCM, in which C6M serves as the formation of cross-link points and HBCM can form a side-chain polymer

3. RESULTS AND DISCUSSION The H-bonded chiral monomer (HBCM, AHBA-PyCho), cholesteryl isonicotinate (proton acceptor)/4-(6-acryloyloxyhexyloxy) benzoic acid (proton donor), was prepared by mixing the precursors at 1:1 proportions in dry tetrahydrofuran, followed by slow evaporation at 65 °C. Before self-assembly of the AHBAPyCho, the AHBA has peaks at 2850-2980 cm-1 associated with CH2 stretching and two peaks at 2560 and 2670 cm-1 associated with OH stretching of H-bonded dimers between the -OH and CdO groups. After the complexation with PyCho, AHBAPyCho exhibits two new peaks at about 2500 and 1901 cm-1, as shown in Figure 3a, the position of which coincides with the position of OH stretching of H-bond between the -OH and pyridyl groups,44 and the disappearance of peaks at 2563 and 2673 cm-1 means that the H-bonds between carboxylic acid and pyridyl groups successfully formed in the AHBA-PyCho complex. Herein, the helical twisting power (HTP) value is measured according to the Cano-rings method.1 Figure 3b shows the temperature dependence of the helical pitch length of mixture 1. It can be seen that the helical pitch length has a great decrease gradually from 25.0 to 92.0 °C. Figure 3c shows the corresponding micrographs of cholesteric textures of mixture 1 in the Cano wedges, taken by a POM at 25.0 and 92.0 °C. It is obvious that the distance of two phase stagger lines decreases greatly from 25.0 to 92.0 °C, which proves that the pitch length has a great change in this temperature region. The mechanism for the change 864



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Figure 5. (a) Temperature dependence of the transmission spectra of cell A after irradiation. (b) Reflection wavelength as a function of temperature measured from the left-handed polarized window of cell A after irradiation. (c) Images of cell A obtained under a polarizer at 25, 50, 70, and 75 °C, respectively.

as a part of the network. Therefore, a lightly cross-linked network (low cross-linked density) was formed in the system after second irradiation. As for cell A, the reflection intensity of cell A before irradiation can approach ∼100%; this is due to the fact that equal pitch lengths of the polymer template and mixture 3 combined with the opposite helicity sense cause the reflection bands of the polymer template and mixture 3 to overlap. That is, both righthanded circularly polarized light (R-CPL) and left-handed circularly polarized light (L-CPL) were reflected within the same reflection band by cell A. The film characteristic that reflects both R-CPL and L-CPL is attributed to the polymer template that has a characteristics of memorizing the helical structure of the initial PSCLCs, as mentioned above.40-43 It should be pointed out that the reflection intensity of cell A after UV irradiation became weak compared with that of cell A before irradiation, as shown in Figure 4d. This is because that increased network concentration after second irradiation restricted the formation of helical structure of the bulk Ch-LCs. Figure 5a shows the temperature dependence of the transmission spectra of cell A after irradiation; it can be seen that the reflection intensity of cell A is to approach ∼100% at 25 °C, which means that both red-colored L-CPL and R-CPL have been reflected within the same reflection band by cell A at 25 °C. With an increase in the temperature, it can be observed that the reflection band began to change gradually with the temperature ranging from 25 to 75 °C. When the temperature was increased to 75 °C, two reflection bands in the spectra of cell A appeared owing to the difference in the pitch lengths of the polymer

template and the bulk LCs. This reveals that the single-layer ChLC film can reflect green-colored L-CPL and red-colored R-CPL at this temperature. Figure 5b presents the reflection wavelength as a function of temperature measured from the left-handed polarized window of cell A after irradiation. As described in our previous study,41-43 the polarizer used here has two windows for both left-handed polarized light and right-handed polarized light, which is prepared by stacking multilayer polyvinyl alcohol (PVA) films on triacetyl cellulose (TAC) substrate, followed by a dyeing and uniaxial tension process. It is obvious that the reflection band of cell A observed by the left-handed polarized window has a blue shift from 780 to 540 nm with an increase in the temperature, which indicates that the pitch length of mixture 3 increases with the temperature increasing. Herein, the color tuning for cell A is mainly due to the temperature dependence of the HTP value of both HBCM and CEM, as suggested above. The images of cell A obtained at these four temperatures under a polarizer are shown in Figure 5c. It can be seen that cell A exhibits different reflection modes within left-handed polarizer and right-handed polarizer windows at 25, 50, 70, and 75 °C, respectively, which is in accordance with the above spectrum characteristics. Additionally, it is worth mentioning that the above process is a fully reversible process on increasing/decreasing the temperature, and the color tuning can be achieved in a lower temperature range, which is basic foundation for its practical application. Because of simplicity, compatibility, and applicability, electric field driving is a feasible choice for modulating the reflection 865



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Figure 6. (a) Voltage dependence of the transmission spectra of cell A before irradiation at a frequency of 100 Hz and a temperature of 25 °C. (b) Reflection central wavelength and reflection intensity as a function of applied voltage (rms) measured from the left-handed polarized window of cell A before irradiation. (c) POM of cell A before irradiation at different voltage strengths.

Figure 7. (a) Voltage dependence of the transmission spectra of cell A after irradiation at a frequency of 100 Hz and a temperature of 25 °C. (b) Frequency dependence of the transmission spectra of cell A after irradiation at an electric field of 50 V at 25 °C. (c) POM of cell A after irradiation at different voltage strengths. 866


The Journal of Physical Chemistry B characteristics of Ch-LCs.31-36 However, the most common method of applying a longitudinal field across the Ch-LC generally leads to a phase transition either to a focal-conic state (highly scattering) or a nematic state, in which the reflection band is destroyed. It has been suggested that polymer-stabilized LCs have been used to weaken this phase transition.36 In this study, we investigated the electric tuning characteristics of cell A with a double-handed circularly polarized reflection band. The spectrum characteristics and optical texture were investigated in detail when an ac electric field was applied to cell A before irradiation and after irradiation. Figure 6a demonstrates the reflection spectrum of cell A before irradiation with different electric field (E-field) intensity. It is easy from these spectra to observe directly the overall reflection band and especially to observe a shift in the wavelength for maximum reflected intensity. For example, at 0 V, it is shown that the reflection intensity is largest in the red region of the visible spectrum (peak at 715 nm) and is to approach ∼100%, corresponding to red-colored L-CPL and R-CPL. At the E-field strength of 75 V, the reflection peak shifts to ∼695 nm, corresponding to red-colored R-CPL, and another reflection band appeared, corresponding to green-colored L-CPL. As the E-field strength continued to increase, the peak reflection wavelength of green-colored L-CPL because obvious until the E-field strength of 125 V. It should be noted that the HBCM and CEM used in this study were cholesteryl derivates, which have large volume fraction, therefore resulting in the driving voltage becoming larger than that of the conventional Ch-LCs system. The starting intensity of the reflection wavelength decreased with an increase in the applied voltage, as shown in Figure 6a; this decrease is associated with an increase in light scattering of the system. In particular, Figure 6b shows the reflectance central wavelength and reflectance intensity of electrically switched color as a function of applied voltage of cell A observed by a left-handed polarizer; it can be seen that the spectrum blue shift and reflectance intensity decreased with the voltage increase, which is in agreement with the above spectrum characteristics. The morphology of the optical texture with different E-field strength is shown in Figure 6c when viewed under POM; as the E-field strength continued to increase, focal conic domains have increased. The observed phenomenon originates from the E-fieldinduced Helfrich deformation.49,50 Because of the undulated nature of the Helfrich deformation, light scattering of the cell increases with the increased applied voltage, which also contributes to the spectrum blue shift and decreased reflectance. Figure 7a shows the reflection properties of cell A after irradiation with different E-field intensity. It is obvious that the reflection band with E-field intensity also has an obvious change as well as cell A before irradiation. That is, the overall intensity of the peak reflected band decreased with an increase in the applied E-field strength, and another reflection band began to appear, as shown in Figure 7a. Additionally, the tuning of the reflection band was found to be independent of frequency for the range 100 Hz to 5 kHz, as shown in Figure 7b. This frequency independence may be a result of the stabilization effect in the presence of the polymer network. For comparison, the textures observed for cell A after irradiation are shown in Figure 7c. The optical micrographs correspond to the same E-field strengths as that presented in Figure 6c. It is evident that the textures and the morphology with E-field strength are very different from that observed for cell A before irradiation. That is, the light scattering of cell A after irradiation are not obvious and even disappeared with the increased applied voltage, which contributes to the

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anchoring effect of the polymer network, which suppressed the formation of focal conic textures in the system. As mentioned above, the polymer network formed in the system after second UV irradiation, which is derived from 1.2 wt % C6M and 20 wt % HBCM, provides an anchoring effect on the Ch-LCs molecules.

4. CONCLUSIONS In summary, thermal and electrical tunability of the Ch-LCs composite with a double-handed circularly polarized reflection band before and after irradiation were investigated. It has been suggested that the L-CPL reflection band of the Ch-LCs composite derived from HBCM (AHBA-PyCho) after irradiation exhibited a blue shift from 780 to 540 nm over the temperature range of 25-75 °C, and the reflected color of cell A observed by left-handed polarized window changed from red to green, correspondingly, whereas the R-CPL reflection band of Ch-LCs composite changed little. This is due to the fact that the HTP value of both HBCM and CEM with LHHS increased with an increase in the temperature. Additionally, the L-CPL reflection band of the Ch-LCs composite before and after irradiation also exhibited a blue shift with E-field strength increase, reflecting from the initial red to green color, correspondingly. Meanwhile, the R-CPL reflection band of the Ch-LCs composite had little change; this behavior is a result of the tilting of the cholesteric helix, followed by the spectrum blue shift in response to the applied voltage. However, the variation trend of the reflected color with an E-field was very slow, and the driving voltage was very large; this is because that high network concentration in the system restricted the reorganization of molecules to some extent. This novel Ch-LCs composite can be considered to be a promising material for photonic and optical applications. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This research was supported by the National Natural Science foundation (grant no. 50903004). ’ REFERENCES (1) Chilaya, G. In Chirality in Liquid Crystals; Kitzerow, H., Eds.; Springer Series Partially Ordered Systems; Springer: New York, 2001. (2) de Gennes, P. G.; Prost, J. The Physics of Liquid Crystals; Clarendon Press: Oxford, U.K., 1993. (3) Shibaev, P. V.; Kopp, V. I.; Genack, A. Z. J. Phys. Chem. B. 2003, 29, 6961. (4) Morris, S. M.; Hands, P. J. W.; Findeisen-Tandel, S.; Cole, R. H.; Wilkinson, T. D.; Coles, H. J. Opt. Express. 2008, 16, 18827. (5) Ozaki, M.; Matsuhisa, Y.; Yoshida, H.; Ozaki, R.; Fujii, A. Phys. Status Solidi 2007, (a)204, 3777. (6) Manabe, T.; Sonoyama, K.; Takanishi, Y.; Ishikawa, K.; Takezoe, H. J. Mater. Chem. 2008, 18, 3040. (7) Goto, H. Adv. Funct. Mater. 2009, 19, 1335. (8) Broer, D. J.; Lub, J.; Mol, G. N. Nature 1995, 378, 467. (9) Broer, D. J.; Mol, G. N. Adv. Mater. 1999, 11, 573. (10) Hikmet, R. A. M.; Kemperman, H. Nature 1998, 392, 476. (11) Mitov, M.; Nouvet, E.; Dessaud, N. Eur. Phys. J. E. 2004, 15, 413. (12) Relaix, S.; Bourgerette, C.; Mitov, M. Appl. Phys. Lett. 2006, 89, 251907. (13) Fan, B.; Vartak, S.; Eakin, J. N.; Faris, S. M. Appl. Phys. Lett. 2008, 92, 061101. 867



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Electrothermal Switching Characteristics from a Hydrogen-Bonded

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