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This paper investigates the effects of ZnO nanoparticles (NPs) on the switching voltages of polymer dispersed liquid crystal (PDLC) films. The threshold and driving electric fields of PDLC film doped with 2.44 wt% ZnO NPs were 0.13 and 0.31 V/μm, respectively, with a contrast ratio of 26. The results of field emission scanning electron microscopy show that the size of the droplets in doped PDLC films increases with the doping concentration. The development of ZnO-doped PDLC films with low driving voltages greatly broadens the applicability of these devices.
© 2016 Optical Society of America
1. Introduction
Liquid crystals (LCs) are widely applied in liquid crystal displays (LCDs) as well as optical elements, such as lenses [1,2], light shutters [3,4], and switches [5]. Polymer dispersed liquid crystal (PDLC) films have attracted particular interest due to their light scattering properties and lack of polarizers [6]. The light scattering of PDLC films is affected by surface scattering and volume scattering [7]. Surface scattering is produced by interfacial roughness or a mismatch in the refraction index between the LCs and the polymer matrix. Volume scattering is caused by the inhomogeneous distribution of LC droplets. Conventional PDLC film consists of submicron–sized nematic LC droplets, which are randomly distributed throughout a polymer matrix [8]. When no electric field is applied to the film, the effective index of LC droplets is around (2no + ne)/3, where no and ne are the ordinary and extraordinary index of LCs, respectively. The scattering of incident light due to the index mismatch between LC droplets and the surrounding polymer matrix (np), such that it appears opaque. The application of a sufficiently strong electric field to the PDLC film causes it to become transparent, as long as the index of the electrically-aligned LC molecules (no) matches the index of polymer np. PDLC films are widely used in switchable windows [9], displays [9,10], holographic-PDLC [11,12], and phase modulation [13]. However, many properties, such as the contrast ratio and viewing angle, still require improvement. In particular, the switching voltages of PDLC are excessively high (>20V) due to strong surface anchoring effects at the polymer matrix walls [6,14]. To expand the applicability of PDLC, researchers have succeeded in lowering the switching voltage via doping with nanoparticles (NPs) [15–19] or clay [20]. Doping with organic/inorganic materials appears to be a particularly effective means of achieving these goals, because the physical properties of LC droplets and the anchoring force at the LC-polymer interface are largely affected by the characteristics of the materials. The size of the droplets also influences scattering performance and driving voltage [21]. In this study, we fabricated ZnO Quantum Dots (QDs) in an ethanol solution and doped them into PDLC films. Our results demonstrate that the driving voltage of PDLC film can be lowered without sacrificing contrast ratio. We also investigate in detail the electro-optical properties and morphology of PDLC.
2. Experiment
ZnO QDs were fabricated using the sol-gel process [22,23]. Zinc acetate dihydrate [Zn(CH3COO)2⋅2H2O], lithium hydroxide monohydrate (LiOH⋅H2O), and ethanol (EtOH) were used to synthesize ZnO NPs. A total of 2.4 g Zn(CH3COO)2⋅2H2O was added to 100 ml of ethanol using a sonicator and stirrer at 70 °C for 5 min. This was followed by the addition of 0.8 g LiOH⋅H2O using the sonicator and stirrer for 1 hr, whereupon the sonicator was switched off but stirring was maintained for another 1 hr., before cooling the solution down to room temperature. After several cycles of centrifugation and cleaning with ethanol, the precipitate was collected and re-dispersed in ethanol. The final weight percentage of ZnO NPs in the ethanol solution was approximately 22 wt% and the size of the ZnO QDs was less than 10 nm, as observed using transmission electron microscopy (TEM), as shown in Fig. 1. The QD colloidal solution presented with a light orange color under UV illumination and the index of refraction of the ZnO was approximately 2.0.
The materials used in the fabrication of conventional PDLC include Norland optical adhesive (NOA 65 with index of refraction 1.524 of cured polymer) and nematic LC E7. The ordinary and extraordinary refractive indexes of E7 (at 589 nm, 20 °C) are no (~1.5216) and ne (~1.7462), respectively. Ethanol solutions with ZnO NPs were added to the E7/NOA65 (~1/1) mixture at ZnO concentrations of 0.28, 0.79, 1.34, and 2.44 wt%. To clarify the electro-optical effects of ZnO NPs and ethanol, we prepared PDLCs with 2.63, 6.56, and 10.37 wt% of ethanol in a mixture of E7/NOA65. All of the ingredients were blended using a sonicator and then used to fill empty cells via capillary action in a chamber at 60 °C. The gaps in the empty cells were approximately 15 μm, with planar alignment in antiparallel directions. The prepared cells were cooled to room temperature over a period of 1 hr prior to UV curing. Polymerization-induced phase separation (PIPS) led to the formation of PDLC films. The morphology of the resulting polymer matrix was studied using field emission scanning electron microscopy (FE-SEM) following the removal of LCs by methanol.
The optical system in Fig. 2 was used to measure the electro-optic characteristics of PDLC films. Non-scattered light and light scattering of the PDLC cell within an angle of 3.8° was detected. Voltage-transmittance (V-T) curves were recorded with increasing voltages of a square-wave at 1 kHz. The threshold voltage (Vth) and driving voltage (Vd) were acquired when the transmittance respectively reached 10% and 90% of their maximum values Tmax, i.e., at T10 and T90. Contrast ratio (CR) is defined as follows:
where Tmax and Tmin are the maximum and minimum transmittance of the V-T curves, respectively.The switching characteristics of the PDLC film are dominated by the properties of the LCs as well as the size and shape of the droplets, and the anchoring properties at droplet boundaries [24]. Under the assumption of rigid anchoring, Vth are related to some material parameters as follows [4,21]:
where R is the characteristic radius of the droplets, d0 is the thickness of PDLC, and εp and εlc are the real part of the dielectric constant of the polymer matrix and LC material, respectively. K is the Frank elastic constant for nematic LCs under one constant approximation, l is the aspect ratio of the droplets (≡ a/b with a and b respectively representing the major radius and minor radius of the droplets), ε0 ( = 8.854 × 10−12 F m−1) is the permittivity under vacuum, and Δε is the dielectric anisotropy of the LCs.Many researches [24,25] also derived some formulas to include other influences on Vth. They all concluded that an increase in the size of the LC droplets, which is strongly influenced by manufacturing processes [26,27], leads to a decrease in the threshold voltage.
3. Results and discussion
Figure 3 presents the normalized V-T curves of PDLC films with added ethanol. The curves shift to the left and somewhat upward with an increase in the concentration of ethanol. This leads to decreases in Vd and Vth and an increase in the transmittance of zero applied voltage, which decreases the contrast ratio.
Fig. 3 Voltage-transmittance curves of ethanol doped PDLC showing that an increase in the concentration of ethanol leads to a reduction in Vth and Vd.
Table 1 displays the Vd, Vth, and CR of PDLC cells with various concentrations of ethanol. Adding ethanol (nethanol = 1.36) to LCs may alter the elastic constant as well as decrease the effective electric anisotropy, viscosity, and anchoring force of the LC at the boundaries [28]. We believe that the ethanol in the PDLC may exist in the LC droplets, as long as it does not evaporate in the manufacturing process. The switching voltage of the PDLC film doped with 6.56 wt% of ethanol was only ~11 V (0.75 V/μm). However, when no voltage is applied to the film, the effective index of refraction of the LC droplets decreased due to the lower index of ethanol. As the mismatch in refractive index between the LC droplets and the surrounding polymer lowered, the light scattering decreased. Thus, an increase in the concentration of ethanol leads to an increase in transmittance at zero applied voltage and a decrease in CR. Figures 4(a)-4(c) present FESEM images of these ethanol-doped PDLC cells in which the size of the droplets does not increase noticeably. We believed that the reductions in Vth and Vd are due to changes in the physical properties of the LCs and a reduction in the anchoring force at the boundary of droplets.
Table 1. Driving voltages, threshold voltages, and contrast ratio of PDLC cells with various concentrations of ethanol
Fig. 4 FESEM images of (a) pure PDLC, (b) 3.40 wt% ethanol, (c) 6.56 wt% ethanol doped PDLC films which are under magnification of 7500x. And (d) doped with 10.37 wt% ethanol magnified 2500x.
A further increase in the concentration of ethanol to 10.37 wt% led to a dramatic increase in the overall transmittance noticeable to the naked eye. The differences in size and non-uniformity in the dispersion of droplets are clearly shown in Fig. 4(d) under magnification of 2500x. Although the switching voltages can be decreased by doping small amount of ethanol, the decrease in the CR was harmful to their applications.
Figure 5 presents normalized V-T curves of PDLC films doped with various concentrations of ZnO NPs. An increase in doping concentration results in curves rising earlier and steepening more as well as lowering Vth and Vd values. All the transmittances of zero applied voltage are kept as low as that of the pure PDLC film, so the contrast ratios of the doped PDLC films are not reduced. Despite the fact that the doped PDLC films contain ethanol, the reductions in Vth and Vd are more pronounced than in ethanol-doped PDLC films. Table 2 shows the Vth, Vd, and CR of PDLC films doped with various concentrations of ZnO NPs. The voltage differences between Vth and Vd decrease as the concentration increases, which was also observed in the ethanol-doped PDLC films.
Table 2. Driving voltages, threshold voltages, and contrast ratio of PDLC cells with doped with various concentrations of ZnO NPs
Figure 6 presents FESEM images of PDLC films doped with various concentrations of ZnO NPs. The size of the droplets increased noticeably with an increase in concentration. As shown in Eq. (2), the size of droplets is inversely proportional to Vth. Doping with ZnO NPs increases the sizes of LC droplets and leads to a decrease in the Vth. In addition, the presence of ethanol lowered the anchoring force at the droplet boundaries as well as alters the physical properties of the LCs. These factors greatly reduce the voltage required to switch the PDLC film from opaque to transparent. For the PDLC film doped with 2.44 wt% of ZnO NPs, Vth is 1.9 V (0.13 V/μm) and Vd is 4.6 V (0.31 V/μm), which is the lowest switching voltage ever as far as we know.
Fig. 6 FESEM images PDLC doped with ZnO NPs: (a) 0.28 wt%, (b) 1.34 wt%, and (c) 2.44 wt%. All the diagrams are under magnification of 7500x.
In conventional PDLC film, an increase in the size of droplets decreases the scattering of light. Consequently, when the transmittance of zero applied voltage increases, CR decreases. In this study, the transmittance of zero applied voltage did not increase with the increasing size of LC droplets. Voloschenko et al. revealed that NPs tend to accumulate in regions with the strongest director distortions during phase separation [29], and Yaroshchuk et al. showed that NPs exist within a polymer matrix [19]. However, due to the presence of ethanol, we believe that ZnO NPs may locate near the boundaries between LCs and the polymer matrix. Due to the high refractive index of ZnO, the light scattering and CR can be maintained even with large LC droplets. Nevertheless, the detail mechanisms of these ZnO NPs/ethanol doped PDLC films are not fully understood and we are still working on it.
4. Conclusions
This study investigated the switching voltages of PDLC films doped with ZnO NPs/ethanol solution. Doping ethanol alone leads to decrease in Vth, Vd and CR without changing the LC droplet sizes. However, the droplet sizes of the ZnO NPs/ethanol doped PDLC films increase as doping concentration increases. We believed that this is the main reason for the reduction in switching voltage. The inclusion of ethanol may also have some positive impacts on the reduction of switching voltages, such as lowering the anchoring force on the boundaries. The lowest Vth (1.9 V) and Vd (4.6 V) were obtained at a ZnO NP doping concentration of 2.44 wt%, which led to a CR of 26. The preserved CR and low driving voltages can save energy and broadening the applicability of scattering LC mode.
Acknowledgments
The corresponding authors J.-S. Hsu are grateful to Professor Ching-Ling Hsu for providing materials and laboratory equipment for the synthesis of ZnO NPs. This work was financially sponsored by Chung Yuan Christian University, through Grant No. 107011012.
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