We report the photo-isomerization and electro-isomerization effects in azobenzenes-doped polymer-dispersed liquid crystals during the switching of the liquid crystal (LC) device between transparent (cis-isomers dominant) and scattering states (trans-isomers dominant). The isothermal phase transition, which is a result of the illumination of UV light and the application of DC voltage, was the main mechanism to switch the LC device between transparency, scattering, and gray scales. This study discusses in detail the variations in the population of cis-isomers as functions of the period and the amplitude of the applied DC voltage.
© 2014 Optical Society of America
In recent decades, polymer-dispersed liquid crystals (PDLCs) have been extensively developed because of their high scattering, electrically and optically switchable properties, simple fabrication, and so on [1–5]. PDLCs possess high potential to fabricate many liquid crystal (LC) devices, such as light switches , light shutters [7, 8], smart cards/windows [9, 10], applications of photonic crystals [11, 12], and others. Generally, the structures of PDLCs are the micron-size LC droplets distributed in continuous polymer networks fabricated by phase separation processes [13, 14]. To obtain high performance light shutters using PDLCs, the refraction index of the selected polymers (np) should conform to the ordinary refractive index (no) of the used LCs. Considering the scattering state (voltage off), the boundaries between the polymer networks and LC droplets will produce light scattering because of the refractive index mismatch. On the other hand, when the directors of the LC droplets (∆ε > 0) are aligned along the applied electric field, the transparent state can be generated because of the matching refractive index of polymer and LCs (no = np). However, in general, the polymer walls also produce strong surface anchoring effect, which increases the driving voltage. The operating voltage of a traditional PDLC scattering mode light shutter is extremely high [1, 13, 14]. Therefore, the high driving voltage, the low contrast ratio, the absence of bistability, and so on comprise the main disadvantages of PDLC devices. In particular, with regard to the large size of PDLC device, the concern of high energy consumption should be significantly reduced by multiple stabilizations or the reductions of the driving voltage [15, 16]. Moreover, both long-term stabilization and low operation energy properties for the applications of PDLCs can be achieved in this study.
In addition, the well-known azobenzene liquid crystals (azo-LCs), which have key functions in this study, are used to enhance the performances of PDLCs. Briefly, azo-LCs, presenting the combined properties of azobenzene dyes, and nematic LCs, have two isomers of stably rod-like trans-isomers and unstably bent cis-isomers [17–22]. Both can be transferred between each other by light illumination with different wavelengths (photo-isomerization) [17–20]. Moreover, the unstable cis-isomers can spontaneously transfer back to stable trans-isomers through a process called dark relaxation. The energy level of cis-isomers is higher than that of trans-isomers. The external applied energy, including thermal treatment, light illumination, specifically electric field, and others can be used to stimulate the isomerization process. The comparison of trans- and the cis-azo-LCs showed that the former are much similar to the common nematic LCs because of their physical properties, including dielectric anisotropy, optical anisotropy, and elastic properties . Moreover, according to the absorption spectra, photo-isomerization of the used azo-LCs in this study from trans- to cis-isomers can be initiated by UV light illumination . Then, the order parameter can be reduced because of the formation of bent cis-isomers in situ as well as the orientation of the entire LCs [18–20]. Therefore, the isothermal phase transition of LCs in the whole LC cell will occur with the increase of the population of the cis-azo-LCs . By contrast, the unstable cis-azo-LCs can be recovered back to the stable trans-azo-LCs by illumination with green light. The transformation rate from cis- to trans-isomers is much higher than that of dark relaxation .
Moreover, it was also proposed that the transformation from cis- to trans-isomers of azobenzenes can be sped up by electric field application [27–29]. This effect is called electro-isomerization. Liu et al. first reported the electrochemical effect of the azobenzene materials and showed the three status systems to demonstrate the electrochemical effect in the DC voltage system . Tong et al. studied the fast electro-isomerization from cis- to trans-isomers of azobenzene materials and showed the chemical reaction equations during the DC application. The chain reaction of the electron and the unstable cis-isomers indicate the fast isomer transition by application of low DC voltages. Briefly, some parts of ions existing in the materials of azobenzene-doped LCs can diffuse forward and aggregate onto the indium-tin-oxide (ITO) surfaces when an electric field (DC field) is applied onto the cell. After that, the aggregate ions can generate an internal field, which is higher than the applied field, at the azobenzene solution and the electrode interface, indicating that the operating voltage for switching the cis- to trans-isomers can be reduced . However, so far, no direct evidence has been proposed to explain that the interaction of the cis-isomers and the generated internal field results in such rapid isomerization processes from cis- to trans-isomers. Moreover, the polymer networks in such polymer-dispersed liquid crystals doped with azobenzenes provide the high scattering performance in this system.
This study used the azo-LCs to develop optically and electrically switchable scattering mode light shutters in PDLCs. Initially, the LC light shutter depicts scattering (opaque) mode when the azo-LCs are stable at trans-isomers. With UV light illumination, the transformation from trans- to cis-isomers results in isothermal phase transition so that the switchability from scattering (opaque) to transparent states can be obtained optically. Notably, the transmittance can be maintained for at least 10 hours (long-term stabilization) depending on the life time of cis-isomers [24–26]. In contrast, our previous study demonstrated that the green light illumination onto the transparent LC cell will switch the transparent state back to the scattering state because of phase transition from isotropic to LC states, but the required green light intensity is extremely high . Given this reason, the electro-isomerization (applying DC voltage) in this study from cis- to trans-isomers was adopted to switch the PDLC device from transparent to scattering states [27–29]. The contrast ratio is measured to be approximately 120. The stable (>10 hours) gray scales can be achieved by applying pulsed voltages. Thus, the optical and electrical switching methods of PDLC devices, having long-term stability (>10 hours), and high contrast ratio (~120), are reported. Additionally, the population of trans- and cis-azo-LCs, which directly affect the clearing temperature (TC), absorption spectra, and the dielectric anisotropy of the mixtures, will be discussed in this study.
The nematic LCs used in the present study was MDA-00-3461 (ne = 1.7718, no = 1.5140, TC = 92 °C) and purchased from Merck. The monomer [1,6-hexanediol diacrylate (HDDA), nHDDA = 1.456] and azo-LC (1205; Δn ≈0.21, nematic phase from 8 °C to 59 °C) were purchased from Alfa-Aesar and BEAM Corp., respectively. Two non-rubbed ITO-coated glass substrates were combined to fabricate an empty cell, whose cell gap was 12 μm. The nematic LCs (MDA-00-3461), azo-LCs (1205), monomer (HDDA), and thermal initiator with a weight ratio of 58:22:18:2 were homogeneously filled into the empty cell. Notably, the azo-LCs account for 27.5% of the total LCs (nematic LCs and azo-LCs). The edges of the LC cell were sealed with epoxy. Finally, the LC cell was baked in an oven at 90 °C for 90 minutes for completing polymerization. After cooling, the fresh scattering PDLC cell was fabricated.
The absorption spectra of trans-azo-LCs (1205) show a strong maximum absorption near 350 nm ; thus, UV derived from Ar+ laser (λ = 365 nm) was selected to achieve the photo-isomerization from trans- to cis-isomers in this study. Consequently, excitation by UV light resulted in the increase of cis-isomer population and the reduction of order parameter. Notably, the isothermal phase transition from nematic state to isotropic state can be initiated as the concentration of the cis-azo-LCs in the entire LC cell attains higher than the critical concentration, approximately 23%. Experimentally, the life time of cis-azo-LCs was confirmed to be longer than 10 hours [24–26]. Moreover, as previously described, the cis- to trans-isomerization process can be sped up by illumination with blue-green light because of the red-shifted absorption spectrum of cis-azo-LCs . The electric field (DC voltage) was also demonstrated to speed up not only the isomerization from cis- to trans-isomers but also the isothermal phase transition from isotropic to LC states, resulting from the variation of the concentration of trans- and cis-isomers.
3. Results and discussion
Figure 1 shows the dynamic transmittance variations of a fresh scattering LC cell illuminated with different UV intensities. The probed beam was a He–Ne laser (λ = 632.8 nm), whose wavelength was not in the absorption band of the azo-LCs (1205). Initially, the fresh scattering PDLC cells presented extremely low transmittance. The higher intensities and longer duration of UV laser (Ar+ laser, λ = 365 nm) irradiation onto the cell resulted in the increase of transmittance and reduction of order parameter because of phase transition and photo-isomerization. Notably, the high transmittance, resulting from isothermal phase transition by UV laser illumination, should be stable for at least 10 hours (data not shown). In general, the transition from transparent state to scattering state can be sped up by green light illumination or thermal treatment . However, electro-isomerization effect was used in this study to accelerate the phase transition from isotropic state (cis-isomer, transparent state) to nematic state (trans-isomer, scattering state). Experimentally, both of the continuous DC voltages with different amplitudes and the periodically pulsed voltages were adopted to accelerate the switching from transparent to scattering states.
With regard to the switch from transparent (cis-isomer dominant) to scattering states (trans-isomer dominant), electro-isomerization was used to accelerate the isomerization processes. Additionally, the life time of the cis-isomers (dark relaxation) can also be influenced by the polymer networks [31, 32]. White et al. reported that the polymer networks can obviously reduce the life time of photo-induced isotropic state (cis-isomers). The local interactions of azobenzene materials and the polymer networks speed up the restoration of cis-isomers. Regarding our experiments, shown in this paper, the polymer network also provides the capability to reduce the life time of the isotropic state (cis-isomers). Experimentally, the life time can be reduced from 12 to 10 hours. However, the life time reduced by the polymer stabilization in this system is not quick enough, comparing with the response of electro-isomerization effect. Restated, the mechanism of electro-isomerization dominates the properties of the switching. Theoretically, the electro-isomerization from cis- to trans-isomers can be initiated by applying a DC voltage but not by an AC voltage [27–29]. Figure 2 shows the variations of dynamic transmittance of the initially transparent LC cell, prepared by irradiating with UV laser (46 mW/cm2) for 25 s [as the transparent spot shown in the inset (I) of Fig. 2], during the electro-isomerization by applying a DC voltage (20 V). The highly transmissive LC cell can be switched to low transmissive LC cell within a few tens of seconds. Compared with the dark relaxation process, the required time to transfer the isomers from cis- to trans-states using such an electro-isomerization technique was much shorter than the dark relaxation (10 hours). As shown in Fig. 2, the transfer time to stable state by electro-isomerization was 40 s. Notably, the residual transmittance resulted from the electrical orientation of LC molecules so that a little leakage of light would remain. However, when the applied DC voltage was switched off, the transmittance of the LC device suddenly decreased to extremely low value (dark-state) because of the mismatch of refractive indices of LCs and polymers [inset (II) of Fig. 2]..
Figure 3 shows the required duration for completing electro-isomerization from cis- to trans-states by applying different amplitudes of continuous DC voltage. Clearly, the higher the amplitude of the applied DC voltage is, the shorter the duration is required. Therefore, the decreasing ratio of the cis-isomer population in the mixing LCs would correspond to the rising DC voltage amplitude. The cell thickness also determined the operation voltage of the electro-isomerization (including the threshold field that could start the isomerization effect) . The threshold field in this experiment was approximately 8 V. In this system, the optimized amplitude of the applied DC voltage was approximately 15 V, which depended on the mixing ratio of nematic LCs and azo-LCs. Moreover, the required time (switching time) would not be shortened obviously when the applied DC voltage was higher than 15 V.
Considering the populations of trans- and cis-isomers, which can be tuned electrically, in the PDLCs doped with azobenzene LC cell, the variations in the population (%) of cis-isomers as a function of the period of DC voltage (15 V) application were calculated according to their absorption spectra, as shown in Fig. 4(a).Additionally, the stable populations (%) of cis-isomers, defined as the populations of cis-isomers when the electrically tuned transmittance reaches stability, versus the amplitudes of the applied DC voltage is also calculated [Fig. 4(b)]. Theoretically, the populations of trans- and cis-isomers depend on the optical absorbance . The selected wavelength in the spectrum for calculating the populations of cis-isomers was green light (λ = 532 nm) so that the population can be obtained by mathematically conversing the optically measured absorption spectra. Notably, regarding the materials in cells, as shown in Experiments Section, the employed azo-LCs (1205) is the only component absorbing the optical energy at the wavelength of 532 nm. The difference between the energy absorbed by trans- and cis-isomers from their absorption spectra can be used to obtain the population of cis-isomers directly. In this paper, the 100% (maximum population) of cis-isomer population is defined as the conversion of the absorption spectrum of the used materials after the LC cell is illuminated with UV laser (46 mW/cm2 for 25 s), resulting in photo-isomerization. The termination of red-shifting in the absorption spectrum was used to confirm the saturation of cis-isomers generated by the illumination of UV laser. As previously mentioned, the high transparent state can be obtained at this state (cis-isomer dominant). Moreover, the 0% (minimum population) of cis-isomer population is defined as the conversion of the absorption spectrum of the used materials after the dark relaxation is completed. The trans-isomers should dominate the azobenzene materials at this state. Notably, the population of cis-isomers indicates the percentage of the cis-azo-LCs in the azo-LC (1205; azobenzene), including trans- and cis-isomers, rather than the cis-isomers in the total LCs (azo-LCs 1205 and MDA-00-3461).
Figure 4(a) shows the reduction of cis-isomer population during the application of DC voltage (15 V). Clearly, the reduction of the population of cis-isomers stopped at approximately 35% within 75 s. According to the calculation, the cis-isomer concentration accounted for approximately 10% of the total LCs in the cell [35% multiplied by 27.5% (concentration of azo-LCs in the total LCs) was approximately 10%]. Experimentally, the critical concentration of the cis-azo-LCs in the total LCs to initiate isothermal phase transition at room temperature (25 °C) was measured to be approximately 23%. The calculated cis-isomer concentration (10%) was low enough to obtain isothermal phase transition from isotropic to nematic states. As previously described, the phase transition from isotropic (cis-isomer dominant, transparency) to nematic (trans-isomer dominant, scattering) states can be achieved by applying a DC voltage (15 V). However, the isomers cannot be completely transferred from cis-isomers back to trans-isomers by applying 15 V of a DC voltage via electro-isomerization effect, but the transmittance can be switched between transparency and scattering. This finding is reasonable because the scattering state results from the mismatch of refractive indices of LCs and polymers. Therefore, if the isotropic (nematic) state, induced by high enough percentage of cis-isomers (trans-isomers), can be achieved, the transparent (scattering) state can be obtained, as shown in Figs. 1 and 2. Figure 4(b) shows the stable population (%) of cis-isomers versus the amplitudes of the applied DC voltage. Obviously, the cis-isomers are difficult to be transferred back to trans-isomers via electro-isomerization effect when the applied DC voltage was lower than 8 V, which is consistent with the results shown in Fig. 3. Moreover, according to the reduction of the stable population of cis-isomers, the electro-isomerization effect can be clearly enhanced by increasing the amplitude of the applied DC voltage. Again, the saturated population of cis-isomers transferred by 15 V of a DC voltage was approximately 35% of azo-LCs. This result indicates the incomplete transfer of azo-isomers to trans-isomers by DC voltage application via electro-isomerization effect .
The aforementioned experimental results indicated that the electro-isomerization effect can be used to accelerate the isomerization from cis- to trans-isomers. However, several points should be considered in this system as described below. First, during the processes of electro-isomerization effect, the transformation of cis- to trans-isomers, as well as phase transition from isotropic to nematic states, are spontaneously occurred. It indicates that the transmittance cannot be tuned to minimum without switching off the applied DC voltage because of the electrical orientation of LCs, as shown in Fig. 2. Moreover, the DC voltage application into LCs will produce the surface charge accumulation or the so-called ion-charge effect . The disadvantage of holding voltage will reduce the performance of the LC devices. Accordingly, periodically pulsed voltage is much better for driving the LC devices that clearly reduce the surface charge accumulation. Besides, the LC devices applied with pulsed driving voltage can be switched between bright state, dark state, and gray scales by applying different numbers of pulsed voltages. Figure 5 shows the variations in transmittance with the applications of pulsed voltages. The initially high transmittance (isotropic state) was obtained by UV laser illumination that was defined as 100%. Experimentally, each application of pulsed voltages with amplitude of 40 V and pulsed width of 2 s could be used to achieve the corresponding stable transmittances (gray scales) of 81%, 69%, 56%, 43%, 32%, and 20%. The contrast ratio between the two switched states, transparent and scattering states, was measured to be approximately 120. Importantly, no significant thermal effect, which can induce isomerization from cis- to trans-isomers, was observed after the LC cell was illuminated (photo-isomerization) and applied with DC voltages (electro-isomerization) in these experiments.
In conclusion, an optically (photo-isomerization) and electrically (electro-isomerization) switchable PDLC light modulator was demonstrated with long-term stabilization. The isothermal phase transition by UV illumination was used to transfer scattering PDLC cell to the transparent state. The illumination of green light and the application of DC voltage can speed up the isothermal phase transition from isotropic (bright state) to nematic state (dark state). The variations in cis-isomer population in the LC cell as functions of the period and the amplitude of the applied DC voltage were discussed. The gray scales were also demonstrated by the application of pulsed DC voltages. To our knowledge, this study is the first to report that DC voltage application was adopted to control the phase transition processes in azobenzene-doped PDLCs, as well as to demonstrate a PDLC light modulator. Given the long life time of cis-azo-LCs, the LC device exhibited long-term stability at transparent state and gray scales and permanently stable at scattering state. The contrast ratio was approximately 120.
The authors would like to thank the National Science Council (NSC) of Taiwan for financially supporting this research under Grant Nos. NSC 101-2112-M-006-011-MY3 and NSC 102-2112-M-008-016. Additionally, this work is partially supported by Advanced Optoelectronic Technology Center. Correspondences about this paper can be addressed to Prof. Ko-Ting Cheng at email@example.com or Prof. Andy Ying-Guey Fuh at firstname.lastname@example.org.
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