JPH0699683B2 - Nematic liquid crystal composition - Google Patents

Description

TECHNICAL FIELD The present invention relates to a nematic liquid crystal composition, and particularly to a nematic liquid crystal composition suitable for dynamic driving.

[Prior art]

The nematic liquid crystal composition used for dynamic driving needs the following characteristics.

The rise in the vicinity of the threshold voltage of the voltage-transmittance curve is steep (hereinafter abbreviated as steepness in the text).

The response speed of the transmittance change is fast with respect to the voltage change.

Can be driven near room temperature.

It is chemically stable and has excellent moisture resistance and light resistance.

The drive voltage (or threshold voltage) can be freely selected.

, Are related to display quality, and are related to reliability. The importance of these is as follows.

When the simple matrix display is dynamically driven, the relative voltage difference applied to the selected electrode and the non-selected electrode becomes smaller as the number of scan electrodes increases. Therefore, in order to obtain a sufficient contrast, the difference between the lighting voltage (that is, the voltage at which the transmittance (or the shielding rate) is saturated) of the liquid crystal and the non-lighting voltage (threshold voltage) must be relatively small. That is, in order to obtain a sufficient contrast in a simple matrix display, increase the number of scanning electrodes, and increase the number of pixels, the voltage-transmittance curve has a lighting voltage (saturation voltage) and a non-lighting voltage ( It is necessary that the curve between (threshold voltage) is steep.

The response speed does not matter so much when displaying a still image. However, when it is necessary to switch images frequently such as in computer terminals and word processors, high speed response is required. Needless to say, a faster response is required when displaying a moving image such as a television image.

・ For indoor use, it is the same as before. However, the demands for some applications such as outdoor use, such as those for automobiles, are becoming stricter.
It is desirable to be able to drive above 80 degrees and below 30 degrees Celsius on the low temperature side. Therefore, the liquid crystal may be required to exhibit a nematic phase in this temperature range.

The liquid crystal display body consumes less power than other display bodies. When the liquid crystal can be driven at a low voltage, not only the power consumed for moving the liquid crystal itself is reduced, but also the driving circuit is advantageous because it can use a low power consumption and an inexpensive integrated circuit. However, if the purpose is to use it in a place where sufficient electric power can be obtained and the image quality is prioritized over the production cost, the drive voltage condition is relaxed.

The characteristic conditions imposed on the liquid crystal composition are as shown in the above items. However, none of the liquid crystal compositions that have hitherto been commercially available satisfy all of these conditions. For example, the general formula (Where R and R'represent a straight-chain alkyl group, straight-chain alkoxy group or cyano group each having an arbitrary number of carbon atoms) is a liquid crystal phase in the temperature range (in the text below, (Abbreviated as temperature range) is relatively wide around the room temperature, and the liquid crystal composition using the same has relatively good electro-optical characteristics. However, this compound is chemically unstable and is easily decomposed by water, and it cannot be said that the compound has sufficient reliability.

Also, the general formula The represented azoxy compound also has a liquid crystal temperature range near room temperature, and a liquid crystal composition using the same has excellent electro-optical characteristics. However, it absorbs ultraviolet rays and visible light of a short wavelength to cause photodegradation, resulting in a decrease in electrical resistance and generation of bubbles. In order to prevent deterioration, it is necessary to protect the liquid crystal from external light with a yellow filter, but this has the disadvantages that the display looks yellowish and the cost of the filter and the time and effort to attach it increase. Therefore, the use of the azoxy compound is limited.

Furthermore, the general formula Compounds represented by The compound represented by has a wide liquid crystal temperature range. A liquid crystal composition using this has excellent electro-optical characteristics such as steepness and viewing angle dependency. However, the response speed is slow, so it is not suitable for displaying television images.

As described above, it is apparent that the liquid crystal composition according to the conventional technique has various drawbacks for stable and high-quality image display.

〔Purpose〕

An object of the present invention is to eliminate such drawbacks of the prior art, and to provide a nematic liquid crystal composition having steepness suitable for obtaining high contrast in dynamic driving, fast response speed, and high reliability. is there.

〔Overview〕

The nematic liquid crystal composition of the present invention is characterized by containing a compound represented by the following general formula A, a compound represented by the following general formula B, and a compound represented by the following general formula C.

(However, R and R'represent a straight-chain alkyl group having 1 to 10 carbon atoms.) The nematic liquid crystal composition according to the present invention has the general formulas A and B.
In addition to the above compounds represented by C and C, a compound having a molecular skeleton composed of a cyclohexane ring, a benzene ring, a naphthalene ring, etc., to which an alkyl group and an alkoxy group, etc. are bonded, characterized by a low viscosity, a cholesteric property, It may contain an optically active substance, dye or pigment. In the nematic liquid crystal composition according to the present invention, the compound represented by the general formula A is 45.0 to 84.0% by weight, the compound represented by the general formula B is 4.2 to 36.6% by weight, and the compound represented by the general formula C is 4.0 to 3.0% by weight. A composition range is desirable.

〔Example〕

 Hereinafter, the present invention will be described in detail based on examples.

The characteristics of the liquid crystal composition were measured as follows.

FIG. 1 shows a measuring system for electro-optical characteristics. The measuring cell 3 is provided with a transparent electrode such as tin oxide on one surface of a glass substrate by an operation such as vapor deposition, and the surface is covered with an organic thin film for orientation treatment, and a nylon film frame also serving as a spacer is placed between them. Two glass substrates are fixed so as to face each other so that the liquid crystal layer has a constant thickness when the liquid crystal composition is enclosed between them. One polarizing plate is provided on each side of the cell. The plate is attached by adjusting the direction of the polarization axis so that the light is transmitted when no voltage is applied and the light is blocked when a voltage is applied. The thickness of the liquid crystal layer will be referred to as the cell thickness hereinafter. The light beam emitted from the white light source 1 passes through the lens system 2 and enters the cell 3 from the vertical direction,
The intensity of the transmitted light is measured by a detector provided at the rear. At this time, an alternating rectangular voltage having a frequency of 1 kilohertz having an arbitrary effective voltage is applied to the cell 3 by the drive circuit 5. FIG. 2 is a voltage-transmittance curve obtained by measuring a liquid crystal cell using the measurement system shown in FIG. In Fig. 2, the transmittance is expressed as 100% and 0% at the brightest and the darkest, respectively, within the normal applied voltage range. To the threshold voltage Vth when the transmittance changes by 10%, and the effective voltage when the transmittance changes from 100% to 90% from the saturation voltage Vsat. And determine each. At this time, the rise (that is, steepness) near the threshold voltage of the voltage-transmittance curve is defined as the β value in the following equation.

The rms voltage (denoted as Von) when turned on (when selected in the matrix cell) is equal to Vsat, and the rms voltage (denoted as Voff) when not lit (when not selected) is equal to Tth When an electric signal is applied, the transmittance becomes 90% and 10%, respectively, and the lighting and non-lighting of pixels are recognized. Furthermore, if Von is slightly larger than Vsat and Voff is slightly smaller than Vth, the respective transmittances are 90% or more and 10% or less. At this time, Von / Voff> Vsat / Tth = β. On the contrary, Von
Is smaller than Vsat and Voff is larger than Vth, the respective transmittances are 90% or less and 10% or more, resulting in poor visibility. That is, when the signal voltage Von / Voff <Vsat / Tth = β is applied, the visibility is deteriorated. In this way, if the β value is smaller than the effective voltage ratio Von / Voff of the electric signal, a pixel display with good visibility can be obtained. To obtain the same image display, the smaller the β value, the smaller the Von / Voff ratio. In a simple matrix display, the larger the number of scanning lines, the smaller Von / Voff, and therefore the β value must be small (close to 1). The above β value is the index of the multiplex characteristic because it shows the minimum value that allows Von / Voff.

The response speed to changes in applied voltage is as follows. When the applied effective value voltage is instantaneously switched from Vth to Vsat, the time required for the transmittance to change by 90% of the difference between the transmittances for each effective voltage in the steady state (that is, the transmittance is 90% The time required to change from 18% to 18%) is expressed as Ton in milliseconds, and similarly, when the effective value voltage is instantaneously switched from Vsat to Vth, the difference in transmittance between each effective voltage in the steady state. The time required for the transmittance to change by 90% (time required for the transmittance to change from 10% to 82%) is expressed as Toff in milliseconds. T (millisecond unit), which is the sum of Ton and Toff, is used as an index of the response speed.

Generally, the time required for the transmittance to change from 0 to 90% after instantaneously switching the applied voltage from 0 to an arbitrary voltage υ (V) is ton, and the applied voltage is instantaneously changed from υ to 0. It is known that the time required to change the transmittance from 100% to 10% after switching to is represented by the following formula (reference: M. Schadt, Japan Society for the Promotion of Science, Information Science). Organic Materials 142nd Committee A Subcommittee (Liquid Crystal Group) 11th Workshop Material, 1978).

(Here, η is bulk viscosity, ε ₀ is vacuum permittivity, Δε is anisotropy of relative permittivity, E is electric field, K ₁₁ + (K ₃₃ −2K ₂₂ ) / 4 elastic constant term, d is cell Each represents a thickness, and η, Δε and K are unique to the liquid crystal composition.

Therefore, both ton and toff grow proportionally to d ² ₁₂ .

The response speed T defined in this example also has a close relationship with the cell thickness, and although it is qualitative, T is short if the cell thickness is thin,
It was found that the thicker the cell, the longer the tendency. Those skilled in the art can easily understand these relationships. Therefore, when a liquid crystal display is manufactured using the same liquid crystal composition, the response speed can be increased as the cell thickness is reduced.

On the other hand, it was found that the steepness β is the smallest (best) when the product Δn · d of the cell thickness d (μ) and the refractive index anisotropy Δn is around 0.8 (reference: Yamazaki). Yoshio, Yutaka Takeshita,
Mitsuo Nagata, Yukio Miyaji, Proceedings of the 3rd Inte
rnational Display Research Conference “JAPAN D
ISPLAY '83 ", p. 320, 1983, SID.) Therefore, when importance is placed on contrast, it is best to make a liquid crystal display so that the cell thickness d is about 0.8. It is considered most appropriate to compare the steepness of objects with this cell thickness.

Since the response time is also related to the cell thickness as described above, it is necessary to measure with an appropriate thickness in order to compare the response times of liquid crystal compositions.

In view of the above, in the present embodiment, Δn · d is 0.8 when the steepness, the response speed, and the threshold voltage are measured from the standpoint of emphasizing the steepness.
The measurement was performed using a cell having a cell thickness of

Since a twist cell was used in this example, a small amount of a cholesteric optically active agent was added to the nematic liquid crystal composition in order to enhance the stability of alignment, and after mixing, it was enclosed in the cell.

Hereinafter, in the table, high temperature liquid crystallinity and low temperature liquid crystallinity are assumed to be room temperature of 20 degrees Celsius, respectively, and with this as a standard, a temperature 30 degrees higher and 30 degrees lower (that is, 50 degrees Celsius and minus 10 degrees Celsius). Degree) indicates whether or not the liquid crystal composition exhibits a stable nematic phase. If the nematic phase is stable, ○ mark, Celsius
It is indicated by I for an isotropic liquid at 50 degrees, Sr for a smectic phase at -10 degrees Celsius, and an X for precipitation. First
Table is a general formula as a conventional example A phenyl cyclohexane ester compound represented by (R and R'represent a straight-chain alkyl group having an arbitrary carbon number, and the compound is hereinafter abbreviated as PCE (negative)) and a general formula 3 shows the composition and characteristics of a liquid crystal composition comprising an aromatic ester compound represented by (hereinafter abbreviated as ARE (positive) in the text). The voltage is changed from 3.04V to 1.38V by changing the content of ARE (positive). The response time T is approximately 340 milliseconds over the entire threshold voltage, which is much slower than the target value of 200 milliseconds in the present invention. The β value is also constant at 1.29, but today more excellent electro-optical properties are required for liquid crystal compositions and display devices using the same.

PCE for the purpose of improving electro-optical characteristics compared to the above conventional example
A compound represented by the general formula A in place of (negative) is represented by the general formula Liquid crystal compositions using the phenyl cyclohexane compound represented by (hereinafter abbreviated as PCH (negative) in the text) are shown in Table 2 as Comparative Examples 1 to 3. Response time is 18
It was on the order of 0 milliseconds, which was a remarkable improvement.

On the other hand, the β value is as good as 1.27 in the composition near the threshold voltage of 2.4V. However, when the composition is such that the threshold voltage is 1.6 V or less, the β value is 1.29 or more and there is a tendency that the β value is not improved at all and is rather deteriorated. It can be seen from the characteristics of Comparative Examples 1 to 3 that the β value in the composition greatly depends not only on the characteristics of the compound itself used but also on the type and amount of the other compound to be combined. Therefore, the inventors of the present invention arrived at the present invention as a result of examining various compositions using various liquid crystal compounds.

Example -1,2,3,4] R-H- (CH ₂₎ a compound represented by the general formula A ₂ - ○
Examples 1 to 4 shown in Table 3 are nematic liquid crystal compositions of the present invention containing 60% by weight and 45% by weight of -OR '.
Is.

The β value is extremely good and the response speed is fast. The liquid crystal temperature range is also sufficient.

A general formula which is a compound other than the present invention in place of the compound represented by the general formula C A composition using a composition I (comprising the composition shown in Table 4 (a)) consisting of a compound represented by the formula (1) and PCH (negative) which is a compound other than the present invention in place of the compound represented by the general formula A. The product is shown as Comparative Example 4. The β value of Comparative Example 4 is 1.28.
It is slightly inferior, and the response time is 243 ms, which is quite slow.

As shown in Examples 1 to 4 above, the nematic liquid crystal composition according to the present invention is significantly superior to the liquid crystal compositions according to the prior art in both steepness and responsiveness. Further, by appropriately changing the content of the compound represented by the general formula B, that is, the compound having a positive dielectric anisotropy, the level of the threshold voltage can be adjusted without changing the liquid crystal temperature range.

[Advantages of the Invention] As described above, according to the present invention, the sharpness, that is, the dynamic characteristic is remarkably improved, and the response speed is significantly increased. When used as a twisted nematic display element, an excellent image display which has never been obtained is obtained. Has the effect of giving.

Further, the nematic liquid crystal composition of the present invention is stable and highly reliable because it does not use compounds that are chemically or photophysically weak such as Schiff compounds and azoxy compounds.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing a measuring device used in Examples. FIG. 2 is a diagram showing a graph of relative transmittance-effective voltage generally obtained by using the measuring apparatus of FIG. 1 ... Light source 2 ... Ray 3 ... Lens and filter system 4 ... Cell 5 ... Light receiving part (photoelectric multiplication tube)

Claims (1)

[Claims] 1. A nematic liquid crystal composition comprising a compound represented by the following general formula A, a compound represented by the following general formula B, and a compound represented by the following general formula C. (However, R and R'represent a linear alkyl group having 1 to 10 carbon atoms.)