JPH0553152A - Liquid crystal element - Google Patents

Abstract

PURPOSE:To improve the dependency of a response speed on temp. and to prevent the speed of an image display from lowering even at a low temp. CONSTITUTION:The relation between the dielectric anisotropy DELTAepsilon ((45 deg.C) at 45 deg.C of a chiral smectic liquid crystal and dielectric anisotropy DELTAepsilon (20 deg.C) at 20 deg.C of the liquid crystal element constituted by holding the chiral smectic liquid crystal between two sheets of electrode substrates facing each other is so specified as to satisfy DELTAepsilon(45 deg.C)-DELTAepsilon (20 deg.C)<0.7.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal element used in a liquid crystal display element, a liquid crystal optical shutter or the like, particularly a ferroelectric liquid crystal element. The present invention relates to a liquid crystal element having improved characteristics.

[0002]

2. Description of the Related Art A display device of a type in which transmitted light rays are controlled by using a refractive index anisotropy of ferroelectric liquid crystal molecules in combination with a polarizing element has been proposed by Clark and Lagerwall ( JP-A-56-107216, U.S. Pat. No. 4,367,924.
No. etc.). This ferroelectric liquid crystal generally has a non-helical chiral smectic C phase (SmC ^* ) or H phase (SmH ^* ) in a specific temperature range, and in this state, it has a first phase in response to an applied electric field. Has an optical stable state and a second optical stable state, and has the property of maintaining that state when no electric field is applied, that is, bistability, and has a quick response to changes in the electric field. Therefore, it is expected to be widely used for high-speed and memory type display elements, and in particular, due to its function, application to a large-screen, high-definition display is expected.

In a chiral smectic liquid crystal device provided with such a bistable state, the liquid crystal layer is generally 2 μm.
It is formed with the following extremely thin film thickness and is practically -10 to 5
It is necessary that images can be displayed in a wide temperature range of 0 ° C.

[0004]

However, in general, a chiral smectic liquid crystal has a problem that the viscosity coefficient η becomes larger as the temperature becomes lower, so that the response speed τ when an electric field is applied to the liquid crystal becomes slower. There is. Therefore, on the low temperature side, the speed of image display on a large-screen, high-definition display becomes slow.

Therefore, an object of the present invention is to solve the above problems in a liquid crystal device, improve the temperature dependence of the response speed particularly in a liquid crystal device using a chiral smectic liquid crystal, and not slow down the image display speed. To do so.

[0006]

In order to achieve the above object, according to the present invention, in a liquid crystal element having a chiral smectic liquid crystal layer held between two opposing electrode substrates, the dielectric constant of the chiral smectic liquid crystal at 45 ° C. Anisotropy Δ
ε (45 ° C) and dielectric anisotropy at 20 ° C Δε (20
C) means that the following expression 2 is satisfied.

[0007]

[Equation 2]

[0008]

In general, the response speed τ of a chiral smectic liquid crystal when an electric field is applied is such that the applied voltage is E, the viscosity coefficient is η,
If the spontaneous polarization is P _S , it can be expressed by the following formula 3.

[0009]

[Equation 3] This equation 3 is easily derived from the condition of the balance between the two torques of spontaneous polarization torque and viscous torque (see, for example, Okano, Kobayashi, co-editing, “Liquid Crystal”, Baifukan, p. From the equation (3), in order to improve the temperature characteristic of the response speed τ, the viscosity at low temperature should be reduced, and
It is necessary to increase the spontaneous polarization P _S at low temperature.
Since the spontaneous polarization P _S has the property of increasing at low temperatures, it is essential to reduce the viscosity coefficient η at low temperatures. However, since the viscosity coefficient η usually becomes considerably large at low temperatures, it is not easy to reduce the viscosity coefficient η.

Therefore, the present inventors have examined whether there is any other physical property value that can improve the temperature-dependent characteristic of the response speed other than the viscosity, and if the temperature characteristic of the dielectric anisotropy Δε is improved, It was discovered that it is effective in improving the response speed. This will be described below.

The above equation (3) was derived from the two torque balance conditions of spontaneous polarization torque and viscous torque. The present inventors further calculated and considered the dielectric anisotropy torque, The following equation 4 was obtained easily from the three torque balance conditions.

[0012]

[Equation 4] Here, ε ₀ is the dielectric constant of vacuum, Δε is the magnitude of the dielectric anisotropy, and Θ is the cone angle of the chiral smectic liquid crystal. Conventionally, the spontaneous polarization P _S and the response speed τ are calculated using the equation (3).
Although the viscosity coefficient η was calculated experimentally, the viscosity coefficient η should be calculated using Equation 4 when Δε is also taken into consideration.

When P _S E >> ε ₀ ΔεE ² sin ² Θ holds, the equation (4) is reduced to the equation (3), but it may not hold depending on the values of Δε, E, and Θ. Therefore, the present inventors proceeded with the consideration by using the more accurate equation (4).

As an index of the temperature dependence of the response speed, the ratio τ (T ₁ ) / τ (T ₂ ) of the response speeds when the temperatures at two points of high temperature and low temperature are T ₂ and T ₁ , respectively. When used,
The following equation 5 is obtained from the equation 4.

[0015]

[Equation 5] Therefore, the following consideration was made with a view to improving the temperature characteristic of the response speed τ, paying attention to the dielectric anisotropy Δε.

In order to reduce the value on the left side of Equation 5,
As described above, first, it is necessary to reduce the viscosity coefficient η even at a low temperature, but this is not easy.

Next, since the spontaneous polarization P _S becomes large at low temperatures, it has an advantageous effect on improving the temperature characteristics of the response speed τ. Since the cone angle Θ also increases at low temperature, it is considered that it has the same effect as the spontaneous polarization P _S , but at that time, the sign of the dielectric anisotropy Δε becomes a problem. This is because the magnitude of the dielectric anisotropy Δε generally depends on the temperature, and the sign may change depending on the temperature value. The formulation of this is described in “Liquid crystal” by S. Chandrasekhar, Cambridge.
"University press, page 59". That is, the dielectric anisotropy Δε can be expressed by the following formula 6 using the relative permittivity in the long axis direction of liquid crystal molecules and the relative permittivity in the direction perpendicular to the long axis of liquid crystal molecules. For chiral smectic liquid crystals, the temperature characteristics of the relative permittivity in the long-axis direction and the relative permittivity in the direction perpendicular to the long-axis were measured. The relative permittivity in the direction perpendicular to the axis is
It turned out that there is not much temperature dependence.

[0018]

[Equation 6] As a result, in many chiral smectic liquid crystals, Δε (T ₂ )> 0 on the high temperature side and Δε (T ₁ ) <0 on the low temperature side. In this case, since the numerator is large and the denominator is small on the right side of Equation 5, the entire right side is large, and the temperature characteristic of the response speed τ is deteriorated.
We conclude that the temperature change of the dielectric anisotropy Δε should not be too large. If both Δε (T ₂ ) on the high temperature side and Δε (T ₁ ) on the low temperature side are negative, Δε (T ₁ ) <Δε (T ₂ ) <
Since it becomes 0, the right side of the equation (5) becomes considerably small, which is quite effective for improving the temperature characteristic.

As a result of further study based on the above results, when the temperature T ₂ on the high temperature side is 45 ° C. and the temperature T ₁ on the low temperature side is 20 ° C., the following formula 7 is satisfied: It has been discovered that the temperature characteristics of the response speed τ can be dramatically improved.

[0020]

[Equation 7]

[0021]

EXAMPLES The present invention will be specifically described below with reference to examples. Two 0.7 mm thick glass plates are prepared, and an ITO film is formed on each glass plate to form electrodes for voltage application, and SiO ₂ is vapor-deposited on the electrodes to form an insulating layer. Formed. Furthermore, on this, a silane coupling agent (KBM-602 manufactured by Shin-Etsu Chemical Co., Ltd.) 0.2
% Isopropyl alcohol solution at 2000 rpm.
p. m. Was applied for 15 seconds with the spinner of No. 1 and surface-treated. After that, a heat drying treatment was performed at 120 ° C. for 20 minutes.

Further, a polyimide resin precursor (Toray Industries, Inc. S
P-510) 1.5% dimethylacetamide solution was rotated at 2000 rpm. p. m. Was applied for 15 seconds using the spinner of After this film formation, a heat condensation baking treatment was performed at 300 ° C. for 60 minutes. The film thickness of the coating film at this time was about 250Å.

Next, the fired coating was rubbed with an acetate flocked cloth and then washed with an isopropyl alcohol solution, and silica beads having an average particle diameter of 2 μm were sprinkled on one of the glass plates. A glass plate was attached using an adhesive sealant (Rixon Bond; manufactured by Chisso Corporation) so that the rubbing treatment axes were parallel to each other, and dried by heating at 100 ° C. for 60 minutes to form a cell. When the cell thickness of this cell was measured with a Berek phase plate, it was about 2 μm.

Liquid crystal compositions A to H containing phenylpyrimidine as a main component were injected into this cell in an isotropic liquid state and gradually cooled from the isotropic phase to 25 ° C. at 20 ° C./h to obtain ferroelectricity. A liquid crystal element was created.

Using this ferroelectric liquid crystal device, the optical response (change in transmitted light amount 0 to 90%) under orthogonal Nicols is detected by applying a voltage of peak-to-peak voltage Vpp = 16V, and the response speed τ Was measured.

Next, the dielectric constant was measured as follows. That is, first, an ITO film was formed on a glass plate, and a polyimide alignment film (LP-6 manufactured by Toray Industries, Inc.) was formed thereon.
4) was formed with 50Å and a horizontally aligned cell was formed by rubbing treatment. On the other hand, a vertical alignment cell was formed using a coupling agent (ODS-E manufactured by Chisso Corporation) instead of polyimide.

The above-mentioned ferroelectric liquid crystal was injected into each of these two cells, the relative permittivity in the major axis direction of the liquid crystal molecules was measured using a horizontal alignment cell, and the major axis of the liquid crystal molecules was aligned using a vertical alignment cell. The relative dielectric constant in the direction perpendicular to was measured. This measurement was performed by using an LCR meter 4192A manufactured by YHP Co.
It was performed under the conditions of 5 V sin wave and 20 kHz.

As described above, for the liquid crystals A to E,
The temperature T ₂ on the high temperature side is 45 ° C. and the temperature T ₂ on the low temperature side is respectively
Table 1 shows the results of measuring the response speed τ [μsec] and the dielectric anisotropy Δε with _{1 set} to 20 ° C.

[0029]

[Table 1] From these results, as in the case of the liquid crystal compounds A to F of Examples 1 to 6, the value of Δε (45 ° C.) − Δε (20 ° C.) was 0.
It can be seen that if it is smaller than 7, the value of τ (20 ° C.) / Τ (45 ° C.), which is an index of the temperature characteristic of the response speed τ, becomes small. On the other hand, the liquid crystal compounds G and H of Comparative Examples 1 and 2
When the value of Δε (45 ° C.) − Δε (20 ° C.) is 0.7 or more as in the case of 1, the temperature characteristics are rapidly deteriorated.

From the above, it can be seen that by setting the temperature change of the dielectric anisotropy to a certain value or less, it is possible to obtain a liquid crystal element in which the temperature dependence of the response speed is greatly improved.

[0031]

As described above, according to the present invention, the dielectric anisotropy Δε (45 ° C.) of the chiral smectic liquid crystal at 45 ° C. and the dielectric anisotropy Δε (2 of 20 ° C.
(0 ° C), and the value of Δε (45 ° C) -Δε (20 ° C) is smaller than 0.7, the temperature dependence of the response speed is improved, and the image display speed does not become slow even at low temperatures. You can

Claims (1)

[Claims] 1. A liquid crystal device comprising a chiral smectic liquid crystal layer held between two opposing electrode substrates, wherein the chiral smectic liquid crystal has a dielectric anisotropy Δε (45 ° C.) at 45 ° C. and a dielectric constant at 20 ° C. Anisotropy Δε (2
0 ° C.) is a liquid crystal element characterized by satisfying the following expression 1. [Equation 1]