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This paper reports the electro- and photo-isomerization-induced isothermal phase separation of liquid crystals (LCs) and poly(N-vinylcarbazole) (PVK). The proposed phase separation process determines reformed PVK films on substrates to obtain switchable LC light valves. UV illumination induces simultaneous isothermal phase transition of the mixture and dissolution of PVK into the LCs. Phase separation of PVK and LCs occurs by the reversed phase transition via rapid electro-isomerization and slow dark-relaxation. During rapid phase separation, micron-sized LC domains (branch-like PVK structures) are generated to develop stable light scattering; during slow dark-relaxation, a uniform PVK film is obtained, thereby providing stable transparency.
© 2014 Optical Society of America
1. Introduction
In recent decades, liquid crystals (LCs) have been used in numerous optical devices, such as lenses, displays, shutters, and wave plates [1–3]. The mechanism of LC light scattering shutters, such as polymer-dispersed LCs (PDLCs), focal conic textures of cholesteric LCs, and polymer-stabilized cholesteric textures, has been given considerable attention [4–7]. Applications of LCs light scattering shutters, including smart cards, smart windows, light shutters, light modulators, have also been extensively developed [8–10]. However, conventional LC scattering devices exhibit several disadvantages that reduce device mobility and hinder their popularization; these disadvantages include high operation voltage, low contrast ratio, and the absence of transmittance stability [8–10]. Especially, reducing the energy consumption of such devices is thus one of the most important issues concerning the natural environment.
We previously proposed mechanisms to develop scattering-mode light shutters with high scattering efficiency based on poly(N-vinylcarbazole) (PVK) films [11–13]. In the study, we developed PVK film-based scattering-mode light shutters and proposed a particular thermally induced phase separation (TIPS) of LCs and PVK [11]. The particular TIPS, which was achieved by direct thermal heating, natural cooling, and light-induced thermal effect of PVK films-coated LC cells, can be adopted to generate scattering-mode LC light shutters with high scattering efficiency; these shutters are polarizer-free and feature a high contrast ratio, low operating field, and fast responses [11, 12].
The absence of stability limits the applications of scattering-mode LC light shutters. Thus, optically controllable scattering LC light shutters with multi-stability capabilities based on azobenzene and PVK films have been developed [13]. In these shutters, the varying degrees of roughness of PVK surfaces (between extremely uniform surfaces and multi-domain roughness) can be repeatedly generated and finely controlled [13].
PVK is a common material employed in studies of photoconductive [14] and photorefractive properties [15]. In our recent papers, mechanically rubbed PVK layers were shown to provide homogeneous alignment with their easy axes perpendicular to the rubbing direction. Thermal effects on surface alignment, including thermally-switched LC alignments and particular TIPS, have also been reported [11–13, 16, 17]. According to the mechanism of particular TIPS [11], which involves dissolution and phase separation of PVK and LCs, the coated PVK can be dissolved into LCs at temperatures higher than the clearing temperature of the employed LCs. Phase separation of LCs and PVK occurs during phase transition caused by cooling. We have proposed three approaches to generate reformed PVK structures: thermally induced phase transition [11], optically induced thermal effect [12], and photo-isomerization-induced isothermal phase transition [13].
Azobenzene LC possesses the properties of nematic LCs and azobenzene dyes [18–20]. The rod-like trans-isomers exhibit several characteristics similar to those of nematic LCs, such as dielectric anisotropy and birefringence. Based on the absorption spectra, light illumination at a proper range of wavelengths is the general approach for initiating transformation between trans- and cis-isomers; this process is called photo-isomerization [21, 22]. The bent-like cis-isomers transferred by UV illumination (from trans-isomers) can disturb LC alignment and induce isothermal phase transition [23]. In our previous study [24], the phase transition rate and populations of trans- and cis-isomers, which reflect the order parameter of the materials, were controlled by the intensity or duration of light irradiation. Without irradiation, unstable cis-isomers spontaneously transform back into stable trans-isomers; this process is called dark-relaxation [13, 21].
Several studies have proposed that reverse isomerization of azobenzenes from cis- to trans-isomers can be accelerated by application of an external electric field, called the electro-isomerization (electrochemical) effect [24–27]. Liu et al. first reported electro-isomerization of azobenzenes and demonstrated the different status of the reactions in a DC voltage system [25]. Tong et al. subsequently proposed the chemical reactions of azobenzenes during DC voltage application and demonstrated the rapid electro-isomerization from cis- to trans-isomers [26]. The generated internal field was in the order of 103 V/μm, significantly higher than the initial applied field; this result indicates that voltages applied for reverse isomerization from cis- to trans-isomers can be reduced [26]. Compared with the rapid photo-isomerization from cis- to trans-isomer transition by high intensity illumination of green light (~300 mW/cm2), the low operating voltage of electro-isomerization is much better than that by such high laser intensity during photo-isomerization processes [13].
In this study, a combined mechanism involving isothermal electrically and optically induced phase separation of LC, azobenzenes, and PVK is proposed to reform PVK surfaces and develop scattering-mode LC light modulators. The resultant LC light modulators exhibit several advantages, such as low power consumption, polarizer-free (no requirement of any polarizer for this modulators) [4–7, 11–13, 22, 24], multi-stabilization capacity, and highly efficient scattering. Moreover, due to quick phase separation of the solved PVK and LCs, as well as the advantages of electro-isomerization effect, the response time of the proposed LC light modulators is significantly shorter than that in the previous works [24, 26].
2. Experiments
The materials used in this experiment were azo-LCs (nematic phase from 8 °C to 59 °C, BEAM Co.), MDA-00-3461 (nematic LCs, TC = 92 °C, Merck), and PVK (polymer, Sigma-Aldrich). Azo-LCs and MDA-00-3461 were homogeneously mixed at a 25:75 weight ratio. The powdered PVK was dissolved in chlorobenzene at a 98.36:1.64 weight ratio; the solution was then spin-coated onto indium tin oxide-coated glass substrates. PVK-coated substrates were pre-baked at 80 °C for 20 min and post-baked at 120 °C for 120 min. Two non-rubbed PVK substrates were assembled with 6 µm cell gap. Finally, the LC mixtures were filled into the cell, and the edges of the cell were sealed with epoxy. The fresh LC cell fabricated by two PVK-coated substrates provided a transparent mode because of the uniform PVK surface [13].
Stable trans-azo-LCs, with maximum absorption near 350 nm, can be transformed to cis-azo-LCs by UV illumination. The absorption spectrum of the UV-illuminated azo-LCs (cis-isomers dominant), is red-shifted toward the blue-green region. According to our previous study [13] and on the absorbance spectrum of the employed azo-LCs [28], the intensity of visible light required to achieve reverse isomerization from cis- to trans-isomers is fairly high because of the low absorbance of visible light by the azo-LCs. Hence, in the present study, a DC electric field was adopted in the cis-isomer-dominant LC cell to accelerate reverse isomerization from cis- to trans-isomers and phase transition from isotropic to LC phases. In this manner, the solubility of PVK and LCs can be reduced, resulting in their phase separation. The roughness of the reformed PVK films depends on the rates of phase separation. Rapid (or slow) phase separation results in roughly (or uniformly) reformed PVK surfaces [13]. The LC cell with roughly reformed PVK surfaces shows small-sized LC domains, which highly scatter incident light and provide an efficient dark state. By contrast, the UV-illuminated (cis-isomer-dominant) cell treated without any electric/thermal treatment after blocking the UV light shows that cis-isomers with long lifetimes (~10 h) [13,24] spontaneously transform into trans-isomers (dark-relaxation). Such spontaneous and slow reverse isomerization processes result in a highly transparent state. Gray scales resulting from the various degrees of roughness of the reformed PVK surfaces can be demonstrated by applying different amplitudes of the DC electric field to control the rates of phase separation.
3. Results and discussion
Figure 1 shows a schematic diagram of the mechanism of an electrically and optically controllable scattering-mode light modulator; the fresh LC cell, which is filled with a homogeneous mixture of nematic and azobenzene LCs, presents a transparent state [Fig. 1(a)]. Figure 1(b) shows the transparent LC cell with completely isotropic materials obtained after UV irradiation-induced isothermal phase transition. Here, part of the coated PVK films on the substrate can be dissolved into isotropic LC mixtures. After blocking the UV light, phase separation can be divided into two parts, namely, slow dark-relaxation and rapid electro-isomerization, which are shown in Figs. 1(c) and 1(d), respectively. During slow dark-relaxation, the LC cell with isotropic materials, formed by UV illumination, possesses high population of cis-isomers that spontaneously transform back into trans-isomers. The lifetime of cis-azo-LCs was experimentally measured to be ~10 h. Long-term dark-relaxation corresponds to slow phase transition and phase separation processes, thereby producing a transparent state with two uniformly reformed PVK surfaces [Fig. 1(c)]. Application of a DC electric field can be used to accelerate the reverse isomerization effect at constant temperature. Consequently, rapid phase separation occurs to reform rough PVK surfaces, resulting in a high scattering state because of the generated small-sized LC domains [Fig. 1(d)]. It should be noted that the scattering mode LC light modulators, resulting from LC domains, do not need any polarizer to present their performances [4–7, 11–13, 22, 24]. Stably transparent [Fig. 1(c)] and scattering [Fig. 1(d)] states can be alternately switched via the transparent state shown in Fig. 1(b).
Fig. 1 Mechanism of the electrically and optically controllable light modulator of (a) initial fresh transparent cell; (b) transparent LC cell with isotropic LCs obtained by UV illumination; (c) stable transparent cell resulted from dark-relaxation induced slow phase separation; and (d) stable scattering cell caused by electro-isomerization induced rapid phase separation.
Figure 2(a) shows the dynamics of the transmission variations of LC modulators from transparent to scattering states during electro-isomerization by applying various DC voltages for 8 s (from time = 0 s to time = 8 s). The corresponding mechanism is shown in Fig. 1(d). DC voltage application for 8 s optimally initialized electro-isomerization and prevented the LCs being continuously aligning along the electric field direction during the dynamic measurements. An unpolarized He–Ne laser (λ = 632.8 nm) was employed as a probe beam during measurement because the wavelength of this laser is not in the absorption band of the azo-LCs. The distance between the photo-detector and the LC cell was 20 cm. Each of the initial high transmittance LC cells (cis-isomers dominant) [Fig. 2(b)] was prepared by UV illumination (~41 mW/cm2, λ = 365 nm) for 50 s before dynamic measurement. After switching off the 8s-DC voltage, the high transmittance [transparency, Fig. 2(b)] spontaneously decreased to a low and stable transmittance [scattering, Fig. 2(c)] within a few seconds because of the electrochemical chain reaction and internal electric field [24]. Figure 2(c) shows the extremely high scattering state produced by applying a DC voltage of 16 V for the first 8 s; the measured contrast ratio was ~340. Figure 2(d) shows the switching time of a LC light modulator from transparent to scattering states by applying 8s-DC voltages with various amplitudes; a higher amplitude of the applied DC voltage results in more rapid electro-isomerization chain reactions (shorter switching time). Electro-isomerization caused the existing ions to diffuse onto the substrates, thereby generating an internal field that induces the reverse isomerization effect (from cis- to trans-isomers). Refer to Fig. 2(d), the optimized amplitude of the operating DC voltage was ~16 V, which was dependent on several factors, such as cell gap and nematics and azobenzenes used. In this system, the shortest switching time achieved by applying a DC voltage higher than 20 V was ~11 s, which is much shorter than that in the previous works [24, 26]. The experiment also demonstrated that applied DC voltages higher than 8 V can be used to switch the LC light modulator from transparent to extremely dark states.
Fig. 2 (a) Dynamics of transmission variations of LC modulator from transparent (cis-azo-LCs dominant) to scattering states by applying DC voltages of (I) 16, (II) 13, (III) 10, (IV) 9, and (V) 8 V for the 8 s. Insets: (b) high transmittance state and (c) high scattering state corresponding to curve (I); (d) Switching time of LC light modulator from transparent to stable scattering states as a function of the amplitude of the applied 8 s DC voltage.
If the amplitude of the applied DC voltage is lower than 8 V, the initiated electrochemical effect is too weak to induce rapid phase separation of LCs and PVK required achieving a high scattering state. However, phase separation caused by applying a DC voltage lower than 8 V generates slightly rough PVK surfaces; thus, gray scales can be obtained. Gray scales can be produced by applying various DC voltages (<7.5 V) for 8 s (from time = 0 s to time = 8 s), as shown in Fig. 3(a). Applying lower DC voltages produces higher stable transmittance. Figures 3(b)–3(e) show the images of the stable transmittance (gray scales) generated by applying DC voltages of 7.5, 7, 6.5, and 4.2 V, respectively, for 8 s.
Fig. 3 Stable gray scales obtained by applying DC voltage with various amplitudes. (a) Dynamics of transmission variations of LC light modulator from transparent (cis-azo-LCs dominant) to gray scales by applying DC voltages of (I) 7.5, (II) 7, (III) 6.5, (IV) 5, and (V) 4.2 V for 8 s. Images of the stable gray scales switched by applying DC voltages of (b) 7.5, (c) 7, (d) 6.5, and (e) 4.5 V.
The scattering LC light modulator can be optically switched back to the transparent state and various gray scales. Figure 4 shows the dynamics of transmittance variations of the LC light modulator from scattering to transparency by irradiation under various intensities of UV laser (Ar+ laser, λ = 365 nm). The initial scattering state was prepared by applying a 16 V DC voltage for 8 s, as depicted in curve (I) of Fig. 2(a). UV illumination can induce isothermal phase transition of LCs and re-dissolve the rough PVK structures on the substrates. The switching time from scattering dark state to transparent states decreases as the increase of the UV intensity, indicating that the switching time can be shortened further by shinning with high-intensity UV light. Finally, dark-relaxation, which results in slow phase separation processes, generates a uniform PVK surface on the substrates and maintains high transparent states. The corresponding mechanism is shown in Fig. 1(c). Gray scales can also be obtained by the approach shown in Fig. 3.
Fig. 4 Dynamics of transmission variations of LC light modulator from scattering to transparent states under UV irradiation at various intensities of (I) 48, (II) 41, (III) 35, (IV) 29, and (V) 22 mW/cm2.
Figure 5 shows the transmission variations of the LC light modulator at transparent and gray scales, produced by isothermal electrically and optically induced phase separation of nematics, azobenzene LCs, and PVK. Curve (I) depicts the transmission variation of a highly transparent LC cell prepared by UV illumination (~41 mW/cm2, λ = 365 nm for 50 s) and kept indoors at room temperature after UV was blocked (dark-relaxation). The highly transparent state remains stable during dark-relaxation process. Curves (II) to (IV) of Fig. 5 also show the variations in the transmission of the LC light modulators (gray scales) prepared with applied DC voltages of 4.2, 6.5, and 7.5 V, respectively, for 8 s, onto the cis-isomer-dominant cell. Electrically controllable multi-stable properties were observed. All scattering states, transparent state, and gray scales can be repeatedly switched to other states by UV irradiation (photo-isomerization) and electric field application (electro-isomerization). Additionally, the switching cycles of the LC light modulator was experimentally repeated about 30 times; the performances of the modulator exhibit no significant aging effect after the experiments were done. Accordingly, we infer that the device has high potential for practical applications.
Fig. 5 Variations in transmission of LC light modulator at different states. (I) Highly transparent LC cell, and gray scales with transmissions of (II) 87%, (II) 40%, and (IV) 6.7%.
Figure 6 shows top-view scanning electron microscopy (SEM) images of the PVK surfaces. The untreated PVK layer is uniformly coated [Fig. 6(a)]. Figure 6(b) depicts the top-view SEM image of the reformed PVK surface of the LC cell produced by UV illumination (~41 mW/cm2, λ = 365 nm for 50 s) and DC voltage application (16 V for 8 s). This result is consistent with that shown in Fig. 2(c). Rapid phase separation and reformation processes resulted in the high-scattering PVK structure. Figure 6(c) shows the top-view SEM image of the LC cell substrate disassembled when the irradiated UV light (~41 mW/cm2, λ = 365 nm, for 50 s) on a scattering-mode LC cell was blocked. This means that the azobenzenes and LCs presented an isotropic state and that the partially rough PVK film on the substrates was dissolved into the LC mixture. Here, the LC cell was a transparent cell with isotropic LCs because of UV-induced isothermal phase transition. Compared with the structures shown in Fig. 6(b), the substrate [Fig. 6(c)] contained fewer PVK molecules and the surface was relatively more uniform; the relevant mechanism is shown in Fig. 1(b). Under the stably bright state (transparent state), Fig. 6(d) displays the extremely uniform structure of a PVK surface obtained via UV-induced photo-isomerization (~41 mW/cm2, λ = 365 nm for 50 s) and completion of slow phase separation (dark-relaxation, 20 h) for an initially scattering-mode LC cell. The structures of the PVK surface in Fig. 6(d) and the initially coated PVK film [Fig. 6(a)] were similar, which indicates that the uniform PVK surface can be recovered to obtain stable transparent states. For the multi-stable gray scales produced by electro-isomerization, Figs. 6(e)–6(g) show the PVK structures generated by applying various DC voltages of 4.2, 5, and 6.5 V, respectively, for 8 s on the cis-isomer-dominant cell. The corresponding transmittances in Figs. 6(e)–6(g) are 87%, 72%, and 39%, respectively. The PVK structures became increasingly rough with increasing applied DC voltage because of the higher rates of phase separation.
Fig. 6 Top-view SEM images of (a) the initially coated PVK surface, (b) reformed rough PVK surface produced by UV illumination and DC voltage application (16 V for 8 s), (c) partially dissolved PVK surface (transparent state), (d) uniform PVK surface obtained by UV illumination and dark-relaxation, and (e)-(g) slightly rough PVK surfaces (gray scales) produced by applying DC voltages of 4.2, 5, and 6.5 V for 8 s, respectively.
4. Conclusion
In conclusion, an optically and electrically controllable LC light modulator based on LCs, PVK, and azobenzene materials was demonstrated. Phase separation induced by photo-isomerization, electro-isomerization, and dark-relaxation is the main mechanism for switching in the LC light modulator. Various transmissions (gray scales) obtained by different phase separation rates remained stable even without application of any external energy; all stable gray scales can also be reversibly switched. The PVK surface structures were examined using SEM, and the measured contrast ratio of the developed LC light modulator was ~340.
Acknowledgments
The authors would like to thank the Ministry of Science and Technology (MOST) of Taiwan for financially supporting this research under Grant Nos. NSC 101-2112-M-006-011-MY3 and NSC 102-2112-M-008-016. This work is partially supported by Advanced Optoelectronic Technology Center.
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