JPS6348920B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a liquid crystal composition, and more particularly to a liquid crystal composition used in a liquid crystal display element utilizing dielectric dispersion. Currently, liquid crystal display elements are widely used in digital displays such as calculators and watches. This widely used liquid crystal display element was statically driven in its early days, but recently dynamically driven is the main method. The dynamic drive method currently in use is a method called the voltage averaging method. However, as long as this method is used, four-digit driving is the limit for liquid crystal compositions currently in general use. This has caused problems in application to character displays, televisions, etc. that require multi-digit driving. The two-frequency dynamic drive method was introduced as an idea for this voltage averaging method. (Hereinafter, this method will be abbreviated as the 2F method.) A feature of this method is that the ratio of the voltage when lighting and the voltage when not lighting depends not only on the number of digits but also on the driving voltage and the dielectric constant of the liquid crystal. It will be done. In contrast, conventional voltage averaging methods depend only on the number of digits.
Therefore, theoretically, the number of driving digits in the 2F method can be increased by changing the driving voltage, dielectric constant of the liquid crystal, etc. The conditions required for a liquid crystal composition for such a dual-frequency dynamic drive are: (1) It should exhibit a liquid crystal state over a wide range around room temperature, and (2) It should have a fast response speed when used as a display. 3) The crossover frequency is low; and (4) the dielectric anisotropy △ε _L on the low frequency side and the dielectric anisotropy |△ε _H | on the high frequency side are almost equal. This is suitable for dual-frequency dynamic drive. Although (1) and (2) above generally apply to all liquid crystal compositions, (3) and (4) in particular are required as important characteristics specific to liquid crystal compositions for dual-frequency dynamic drive. and the crossover frequency fc of 3
The level of fc has a large effect on power consumption, and if the crossover frequency fc is high, a higher frequency drive voltage is required, which increases power consumption. In addition, the dielectric anisotropy on the low frequency side or high frequency side in (4) influences the driving voltage on the low frequency measurement or high frequency side, and if the balance cannot be achieved, the respective driving voltages must be changed. Since it is necessary to equalize the power supply voltage level according to each drive voltage, the drive circuit becomes complicated. On the other hand, as a conventional technology, an example of a liquid crystal composition for dynamic drive using a dual frequency method is published by Applied Physics.
Letters, Vol.25, No.4, 15 August1974 P186~
188, and, An example using a 1:1 mixture of According to this, when the voltage is 50 Hz, the dielectric anisotropy on the low frequency side is Δε _L =6.2, and when the voltage is 10 KHz, the dielectric anisotropy on the high frequency side is |Δε _H |=−2.2. In this case, the dielectric anisotropy on the low frequency side and on the high frequency side is not balanced, resulting in the above-mentioned problems. The present invention aims to solve these problems and provide a more balanced liquid crystal composition while maintaining a low crossover frequency. The liquid crystal composition of the present invention can be obtained by appropriately combining the compound groups represented by the following formulas or by combining them with other liquid crystals. (However, R ₁ is a straight chain alkyl group with 5, 7 or 8 carbon atoms, R ₂ is a straight chain alkyl group with 7 or 8 carbon atoms, and R ₃ is a straight chain alkyl group with 2 to 4 carbon atoms. Straight-chain alkyl group, R ₄ is a straight-chain alkyl group with 5 or 6 carbon atoms, R ₅ is a straight-chain alkoxy group with 6 carbon atoms or a straight-chain alkyl group with 5 or 7 carbon atoms. Each of these compounds is shown below.) All of these compounds are essential liquid crystals used in display elements using the 2F method, and exhibit the following properties. The dielectric anisotropy (Δε) of the first group of liquid crystal compounds is positive at low frequencies and negative at high frequencies. For the compounds of Groups 2 and 3, Δε is negative at both low and high frequencies. The compounds of Group 4 are liquid crystalline compounds that have a very large value of Δε at low frequencies. In addition, all compounds other than Group 4 exhibit low melting points. On the other hand, the common characteristics of all these compounds are long molecular length, low crossover frequency, and high frequency |△ε|
The value of is large. The properties of these compounds are shown in Table 1.

【table】

[Table] When the liquid crystal compounds shown in Table 1 were appropriately mixed or mixed with other liquid crystal compounds other than those shown in Table 1, a liquid crystal composition with very good characteristics was obtained. Mixing examples and characteristics of mixed liquid crystals will be described below with reference to Examples. Example 1 When a liquid crystal composition shown in Table 2 was prepared, the liquid crystal composition became grease-like at low temperatures and did not exhibit a significant melting point. However, at around 0°C, it showed a certain degree of fluidity. The clearing point was over 100°C, which caused no practical problems. Figure 1 is a diagram showing the relationship between the magnitude of dielectric anisotropy and frequency at 20°C for this liquid crystal, and it can be seen from this diagram that the crossover frequency is around 2KHz. This is revolutionary considering that the general liquid crystal used in FE type display has a frequency of several hundred KHz or more. Also, △ε _L is relatively large at +4.0, and △ε _H
-3.3, which was well balanced.

[Table] In addition, compositions as shown in Table 3 were prepared, and the dielectric anisotropy and crossover frequency were measured.

[Table] The results were △ε _L =4.0, △ε _H −3.3, and the crossover frequency at this time was 2.1 KHz. This shows that the liquid crystal composition of the present invention has substantially the same dielectric anisotropy at high frequencies and at low frequencies, and also has an extremely low crossover frequency. Example 2 When a liquid crystal composition shown in Table 4 was prepared, the melting point of this liquid crystal composition had the same tendency as in Example 1. Moreover, the clearing point was 105.5°C.

【table】

[Table] When this dielectric anisotropy was measured, △ε _L = +
3.9 and Δε _H −3.3, indicating that the balance of dielectric anisotropy was not disrupted at all. The crossover frequency of this liquid crystal composition was somewhat lower than that of the liquid crystal composition of Example 1. Example 3 4-methoxy-4'-n was added to the liquid crystal composition of Example 2.
A liquid crystal composition containing 20% by weight of -butyl-azoxybenzene was prepared, and the crossover frequency was measured in the same manner as in Example 1.
It was 6KHz. This is thought to be due to the addition of azoxy-based liquid crystal with low viscosity, which increased the crossover frequency. Note that the dielectric anisotropy of this liquid crystal composition is △ε _L =
+3.9, and △ε _H -3.2. Example 4 A liquid crystal composition was prepared by adding 20 weight percent of 4-n-amylcyclohexane-1-carboxylic acid-4'-methoxyphenyl ester to the liquid crystal composition of Example 2, and in the same manner as in Example 1, When the cross frequency was measured, it was 6KHz at 30°C.
There was almost no difference in dielectric anisotropy from the liquid crystal composition of Example 1 in both low frequency and high frequency regions. Example 5 15 weight percent of 4-n-amylcyclohexane-1-carboxylic acid-4'-methoxyphenyl ester and 4-(4-n-
amylphenyl)benzoate-4-n-heptyl-
A liquid crystal composition containing 15% by weight of 2-cyanophenyl ester was prepared. The clearing point of this liquid crystal composition was 99.5°C. When the relationship between the magnitude of dielectric anisotropy and frequency of this liquid crystal composition was investigated at 30°C, the graph shown in FIG. 2 was obtained.
From this, the crossover frequency of this liquid crystal composition is 4.2KHz
It can be seen that it is. Also, △ε on the high frequency side
It can be seen that the value of is smaller than that of Example 1. Example 6 When a liquid crystal cell was filled with the liquid crystal composition of Example 5 and driven, it was possible to drive 64 digits at 25V. Also, when we conducted a lifespan test at room temperature using this cell, we found that
It showed the same increasing trend of current as the liquid crystal used in FE type display elements. From now on, this liquid crystal composition is guaranteed to have the same level of stability as the liquid crystal used in conventional FE type display elements. As described above, according to the liquid crystal composition for dual-frequency dynamic driving of the present invention, (1) the crossover frequency fc exhibits a very low value of 2KHz;
The dielectric anisotropy is also △ε _L = +4.0 on the low frequency side,
Since we were able to provide a well-balanced liquid crystal composition with |△ε _H | on the high frequency side that is not much different from the dielectric anisotropy on the low frequency side, there is no need to use two power supply voltages, and therefore There is no need to complicate the drive circuit, and since the crossover frequency is low, there is no need to apply an extremely high frequency, so power consumption is low, and there is little load on the liquid crystal composition. Therefore, it can have a long life. Even if other liquid crystal compounds are added to further improve other characteristics, the crossover frequency remains approximately 4.
The liquid crystal composition can be kept within the range of ~6KHz, making it a more suitable liquid crystal composition for dual-frequency, multi-digit driving, and enables 25V, 64-digit driving at around room temperature.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between the magnitude of dielectric anisotropy and frequency of the liquid crystal composition of Example 1. FIG. 2 is a graph showing the relationship between the magnitude of dielectric anisotropy and frequency of the liquid crystal composition of Example 5.

Claims (1)

[Scope of Claims] 1. A liquid crystal composition for dynamic drive using a dual frequency method, characterized in that it comprises at least four types of compounds whose general formulas are represented by the following formulas. (However, R ₁ is a straight chain alkyl group with 5, 7 or 8 carbon atoms, R ₂ is a straight chain alkyl group with 7 or 8 carbon atoms, and R ₃ is a straight chain alkyl group with 2 to 4 carbon atoms. Straight-chain alkyl group, R ₄ is a straight-chain alkyl group with 5 or 6 carbon atoms, R ₅ is a straight-chain alkoxy group with 6 carbon atoms or a straight-chain alkyl group with 5 or 7 carbon atoms. (Indicated respectively.)