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We report five ultra-low viscosity nematic liquid crystal mixtures with birefringence around 0.1, dielectric anisotropy in the range of 3 to 6, and clearing temperature about 80°C. A big advantage of these low viscosity mixtures is low activation energy, which significantly suppresses the rising rate of viscosity at low temperatures. Using our mixture M3 as an example, the response time of a 3-μm cell at −20°C is only 30 ms. Widespread application of these materials for display devices demanding a fast response time, especially at low temperatures, is foreseeable.
© 2015 Optical Society of America
1. Introduction
Fast response time is one of the most critical requirements for most liquid crystal display (LCD) devices [1] because it helps reduce motion picture image blur and crosstalk, enhance optical efficiency, and suppress color mixing for field-sequential displays [2, 3]. Mobile displays, wearable displays, and car navigation systems are often used in outdoor and they have to endure harsh weather conditions, like low temperatures (−20°C). In such a cold ambient temperature, LC response time is usually as sluggish as several hundreds of milliseconds. As a result, the displayed image quality is severely degraded [4].
To shorten response time, a straightforward approach is to decrease the LC cell gap (d). However, for an LCD a certain dΔn value is required in order to obtain high transmittance; here Δn is the LC birefringence. For example, the commonly used fringe field switching (FFS) LCD requires dΔn≈320-340 nm in order to achieve high transmittance [5, 6]. Although thin cell gap helps greatly to achieve fast response time [7], this approach imposes two problems: it reduces manufacturing yield and it demands a higher Δn LC, which has stronger wavelength dispersion [8]. To obtain white color, the transmittance of red, green, and blue sub-pixels should be balanced. From experimental studies, the preferred Δn for FFS is around 0.10 ± 0.01. Under such circumstance, the cell gap is about 3 μm, which is still manageable for high-yield manufacturing. With abovementioned constraints, the simplest way to reduce response time is to employ a low viscosity LC.
In this paper, we formulated five ultra-low viscosity LC mixtures with Δn≈0.1, dielectric anisotropy Δε≈3 to 6, and clearing temperature about 80°C. A big advantage of these low viscosity LC mixtures is their small activation energy, which significantly suppresses the rising rate of viscosity at low temperatures. Using our mixture M3 as an example, the response time of a 3-μm FFS cell at −20°C is about 30ms. These materials will find widespread applications for display devices that demand a fast response time.
2. Mixture formulation
Our low viscosity LC mixtures contain three major ingredients: 1) high Δn and large Δε compounds, 2) ultra-low viscosity diluters, and 3) wide nematic range compounds. Table 1 lists the chemical structures and compositions of our five mixtures. Compounds 1 and 2 have high Δn and large Δε (>25) [9, 10], but their viscosity is also high. To lower the viscosity, we added more than 40% non-polar diluters (#3) [11]. To widen nematic range and achieve high clearing point, we added some terphenyl compounds (#4). To obtain different Δε values, we formulated five LC mixtures by varying the compound concentrations as Table 1 shows.
Table 1. Chemical structures and compositions of LC mixtures; R and R’ represent alkyl chains.
3. Material characterization
In experiment, we measured the dielectric anisotropy, birefringence, visco-elastic constant, and activation energy of these five mixtures. To avoid crowdedness of data presentation, here we only show the measured results of M3, M4, and M5 in the following Sections. Table 2 summarizes the key results of these five mixtures.
Table 2. Measured properties of the five LC mixtures at T = 23°C, λ = 633nm, and f = 1 kHz.
3.1 Dielectric anisotropy
Dielectric anisotropy affects the operation voltage, peak transmittance [12], and response time (through viscosity) of the FFS LCD. To reduce the power consumption of a mobile display, it is desirable to keep the on-state voltage below 5V. This requirement demands a fairly large Δε. On the other hand, to obtain low viscosity we should keep Δε as small as possible. Thus, there exist contradicting requirements for Δε between low operation voltage and fast response time. A compromised Δε value is in the range of 3 to 6.
In experiment, we used the capacitance method to measure the dielectric constants (ε// and ε⊥) of our five LC mixtures at room temperature (23°C). Detailed procedures have been reported in Ref [13], and the measured results are listed in Table 2. From Table 2, the ε// and ε⊥ of M3 is 6.26 and 2.76, respectively, i.e., Δε = 3.50, which is much lower than that used in conventional p-FFS LCD (Δε = 8~10) [14]. With such a low Δε, the operation voltage, which is inversely proportional to the square root of Δε, would undoubtedly increase [12]. Fortunately, the transmittance of p-FFS increases as Δε gradually decreases. As a result, we can still get high transmittance at a relatively low voltage (5V) using a low Δε LC material [15]. For M4 and M5, the Δε value is 4.60 and 6.18, respectively. Among these three mixtures studied, M3 contains the largest amount of diluters, thus its viscosity is the lowest but its dielectric anisotropy is also the smallest.
3.2 Temperature dependent birefringence
Birefringence of an LC is mainly governed by the conjugation length and order parameter [16]. To measure Δn, we filled the LC mixture into a homogeneous cell made of indium tin oxide (ITO) glass substrates. The inner surface of the ITO-glass was over-coated with a thin polyimide alignment layer. The pretilt angle was about 2°. The cell was sandwiched between two crossed linear polarizers. By measuring the voltage dependent transmittance through LabView system, we can obtain Δn easily. Detailed method has been described in [17]. From Table 2, the measured birefringence at room temperature is Δn = 0.100 for M3, 0.097 for M4, and 0.099 for M5. These values are very close to our ideal one, which is 0.1.
Next, we measured the temperature dependent birefringence. We placed the LC cell on a Linkam heating stage controlled by the temperature program (Linkam TMS94). Results are shown in Fig. 1, where dots stand for measured data and solid lines for the fittings using Haller’s semi-empirical equation [18]:
where Δn0 is the extrapolated birefringence at T = 0, S is the order parameter, T is the temperature, Tc is the clearing point, and β is a material parameter. Through fittings, we found Δn0 = 0.138 and β = 0.174 for M3, Δn0 = 0.133 and β = 0.177 for M4, and Δn0 = 0.135 and β = 0.165 for M5, respectively. Using these fitting parameters, we can calculate the order parameter (S), which will be used later.
Fig. 1 Temperature dependent birefringence of M3, M4, and M5 at λ = 633nm. Dots are experimental data and solid lines are fitting curves with Eq. (1).
3.3 Elastic constant and viscosity
In an LCD, the response time is proportional to the visco-elastic coefficient (γ1/Kii), where Kii is the corresponding elastic constant depending on the LC alignment. For examples, for vertical alignment, Kii = K33 is the bend elastic constant, and for in-plane switching (IPS) cell [19] Kii = K22 is the twist elastic constant. However for FFS, the electric field has transversal and longitudinal components so that both K22 and K11 are involved, although twist dominates [20]. Several approaches have been proposed to measure γ1 and K11. Here, we used the time dependent transmittance method described in Ref [13].
For a homogeneous cell, the threshold voltage is related to K11 and Δε as [21]:
where K11 is the splay elastic constant and ε0 is the permittivity of vacuum. From the measured threshold voltage and dielectric anisotropy, we can extract K11 from Eq. (2). As listed in Table 2, all the five mixture we prepared have a very similar K11 value (~12pN) because they basically consist of same compounds except at different compositions.Next, we used the same setup as described in Sec. 3.2 to measure γ1/K11. Detailed method has been described in [13]. Since K11 has already been obtained from Eq. (2), we can extract γ1 from the measured γ1/K11. The measured γ1 is 45.1 mPas, 50.4 mPas, and 52.3 mPas for M3, M4, and M5, respectively. These γ1 values seem to correlate with Δε linearly, as will be examined in more detail later.
3.4 Activation energy
As the temperature decreases, rotational viscosity increases exponentially as [22, 23]:
where E is the activation energy and kB is the Boltzmann constant. From Eq. (3), activation energy determines the rising rate of rotational viscosity in the low temperature region. Key parameters affecting E include molecular structure and conformation, and intermolecular interactions [13]. As Table 1 shows, the low Δε LC mixture contains more non-polar diluters. As a result, its activation energy is relatively small, which in turn only causes a mild increase as the temperature decreases. To extract E, we measured the temperature dependent visco-elastic coefficient of these mixtures using the same method discussed above. In theory, temperature dependent γ1/K11 (homogenous cell) can be described as follows [22]:Figure 2 depicts the measured data (dots) and fitted curves (solid lines). The measured data fit well with Eq. (5). Through fittings, we found E = 205 meV for M3, 228 meV for M4, and 260 meV for M5. For comparison, the reported activation energy of MLC-6686 (Δε = 10) is 353.9 meV and MLC-6608 (Δε = −4.2) is 496.0 meV [15, 24]. Our low viscosity LC mixtures exhibit much lower activation energy. In experiment, we tested a 3.5-μm FFS cell with electrode width l = 3μm, electrode gap g = 4μm using M3. Peak transmittance (90.4%) was achieved at 7.1 Vrms under λ = 514nm. The measured response time [rise, decay] is [10.3ms, 10.7ms] at room temperature. As the temperature decreases to −20°C, the decay time increases to 42ms. If we use a thinner cell gap (e.g. d = 3μm), the expected decay time, which is proportional to d2, is ~30ms. This result is >10X faster than that of the MVA cell reported in Ref [4], at the same temperature. More details about the electro-optic properties using ultra-low viscosity and low dielectric anisotropy materials have been reported in Ref [15].
Fig. 2 Temperature dependent γ1/K11 of M3, M4, and M5. Dots are experimental data and solid lines are fittings with Eq. (5).
4. Discussion
Table 2 summarizes the measured physical properties of the five mixtures we prepared. Their Δn is around 0.1 and clearing point ≈80°C, which is desirable for FFS LCD applications. As Δε decreases from 6.2 to 3.1, γ1 decreases from 53 mPas to 41 mPas. The correlation seems to be linear between these two parameters. To further investigate this empirical relation, more mixtures using the compounds listed in Table 1 are prepared for comparison. Figure 3 depicts the results, from which a linear relation between Δε and γ1 is indeed observed. The extrapolated γ1 is about 30 mPas for the employed non-polar diluters whose Δε≈0. For some LCDs, such as desktop computers and TVs, they can afford to have a higher operation voltage, say 7.5V. Thus, we can use a lower Δε LC mixture and achieve a faster response time.
5. Conclusion
We have formulated five ultra-low viscosity LC mixtures with positive Δε and characterized their physical properties. In addition to low viscosity, their Δn is around 0.1 and Tc~80°C, which is ideal for FFS LCDs. Another big advantage is their small activation energy, which significantly suppresses the rising rate of viscosity at low temperatures. Widespread applications of these ultra-low viscosity LC mixtures are expected.
Acknowledgment
The authors are indebted to AFOSR for partial financial supports under contract No. FA9550-14-1-0279.
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