JPS62143030A - Ferroelectric liquid crystal display device - Google Patents

Abstract

PURPOSE:To easily obtain a ferroelectric liquid crystal display device having a wide temp. range and good display grade by using a ferroelectric liquid crystal compsn. mixed with a nonchiral liquid crystal for a compsn. mixed with chiral materials which are reversed in twisting directions. CONSTITUTION:The compsn. mixture composed of the liquid crystal having any of SmC, SmI, SmF, and SmG phases which are not chiral is mixed with the compsn. which is prepd. by mixing the left-hand chiral ferroelectric liquid crystal and right-hand chiral ferroelectric liquid crystal, is unravelled in the twisting directions and has a long pitch. The ferroelectric liquid crystal compsn. having the long pitch is thus obtd. Since this liquid crystal compsn. has the long pitch, the compsn. has a high memory characteristic and good orientability. The ferroelectric liquid crystal display device having the high display grade and a wide temp. range is thus easily obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to ferroelectric liquid crystal display devices, which are widely used in optical shutters or matrix display devices.

Conventional technology The conventional technology will be explained below using figures.

Ferroelectric liquid crystal molecules have asymmetric atoms and therefore usually have a twisted structure. This twisted structure is shown in FIG.

In Fig. 3, 31 is the liquid crystal molecule, 32 is the permanent twin seed moment, 23 is the pitch (L) representing the period of twist, 3
4 represents the layer structure, 35 represents the normal direction of the layer, and 36 represents the tilt angle θ.0 When the cell thickness (d) of the ferroelectric liquid crystal panel is thicker than the pitch (d>L), the ferroelectric liquid crystal usually Since the influence of the cell substrate surface does not extend to the center of the cell, it exists in a twisted structure.

However, when the cell thickness is smaller than the pitch (d<L), the twisted structure is unraveled by the force of the substrate surface, and two regions where the molecules are parallel to the substrate surface appear as shown in FIG. In these two regions, the permanent dipole moment of the molecule points in opposite directions, one direction from the edge of the paper toward the front, and the other from the front toward the back of the page. Each of these corresponds to the tilt angle of the molecule with respect to the layer normal.

In FIG. 4, 41 is a liquid crystal molecule, 42 is a permanent dipole moment pointing from the back of the page to the front, 43 is a permanent dipole moment pointing from the front of the page to the back,
44 represents the layer structure, 45 represents the layer normal direction, and 46 represents the inclination angle.

Next, the operating principle of the ferroelectric liquid crystal will be explained using diagrams. FIG. 5 shows the structure of a general ferroelectric liquid crystal panel. In FIG. 5, 51 is a polarizing plate, 52 is a glass substrate, 5
3 represents a transparent electrode, and 54 represents a ferroelectric liquid crystal.

In this way, when a ferroelectric liquid crystal cell is filled with ferroelectric liquid crystal whose pitch is larger than the cell thickness (d>L), a state is created in which the cell has two regions as shown in FIG.

At this time, when an electric field is applied from the back to the front of the paper, all the permanent dipole moments are directed in the direction of the electric field, as shown in Figure 6 (al
The molecules all have a tilt angle of 10, as shown in .

In this state, if the polarization axis of the polarizer (P) of the polarizing plate is made parallel to the long axis of the molecule and the polarization axis of the analyzer (A) is made parallel to the short axis of the molecule (see Figure 6a). ) Polarizer (P)
The linearly polarized light that has passed through passes through the analyzer (
A) and a dark state is obtained.

Furthermore, when an electric field is applied in the opposite direction, the molecules all have a tilt of 10 as shown in Figure 6 ib), and the linearly polarized light that passes through the polarizer becomes bright when it passes through the analyzer due to the birefringence effect. .

As described above, bright and dark states can be obtained depending on the positive and negative electric fields.

(Reference: Fukuda, Takesei, Kondo: High-speed display using ferroelectric liquid crystal, Obtronics, 9th edition, p. 64, 19
1983) In Figure 6, 61 is the direction of the electric field, 62 is the permanent dipole moment of the molecule, 63 is the layer structure, 64 is the tilt angle θ, 65
represent the polarization axes of the polarizer (P) and analyzer (A), respectively. Furthermore, in cells where the cell thickness is smaller than the pitch (d < L), the twisted structure is usually unraveled, so it is said that a memory effect occurs in which the molecules remain in the same state even after the electric field is removed.

In this way, using ferroelectric liquid crystal whose pitch is larger than the cell thickness is considered to be an effective method for creating a word memory effect. Furthermore, since ferroelectric liquid crystals have a layered structure, it has been considered difficult to obtain good alignment compared to conventional nematic liquid crystals. Particularly in compounds having a cholesteric phase in the phase transition series, there is a problem in that it is difficult to obtain good orientation if the pitch of the cholesteric phase is short.

Conventionally, in order to increase the pitch of ferroelectric liquid crystals, as in the case of cholesteric liquid crystals, it is possible to increase the twist by mixing ferroelectric liquid crystals with counterclockwise twist and ferroelectric liquid crystals with clockwise twist. A method of compensation was used.

Problems to be Solved by the Invention In order to actually use a ferroelectric liquid crystal composition in a display device, it is necessary to exhibit ferroelectricity over a wide temperature range. Therefore, it is necessary to mix many types of ferroelectric liquid crystal materials. At this time, in order to obtain a ferroelectric liquid crystal composition with a large pitch, it is necessary to mix many individual ferroelectric liquid crystal materials while taking into consideration the pitch size, left and right orientation, their temperature dependence, etc. Because it is difficult to obtain ferroelectric liquid crystal compositions with large pinches, the twisted structure cannot be unraveled unless a panel with a fairly thin cell thickness is used, and display devices have poor memory performance. There was a problem in that only a display device of poor quality could be obtained.

Means for Solving the Problems In order to solve the above problems, first, chiral substances with opposite twist directions are mixed to create a ferroelectric liquid crystal composition with an untwisted structure and a long pinch. (This composition may have a narrow temperature range and may not be a ferroelectric liquid crystal.) Next, a liquid crystal material that is not inherently chiral, that is, has no twisted structure at all (for example, a liquid crystal material having a smectic C phase). ) to mix. At this time, since the liquid crystal material itself has no twist, it can be mixed without considering left and right pitches. Therefore, it is possible to easily obtain a liquid crystal composition that has a wide temperature range and has no twisted structure.

Finally, by mixing these two liquid crystal compositions, it is possible to obtain a ferroelectric liquid crystal composition that has a wide temperature range and a very long pitch, and displays that do not exhibit a twisted structure even in thick cells. A high-quality ferroelectric liquid crystal display device can be obtained.

Effect of the present invention In contrast to a non-chiral liquid crystal composition which originally does not have any twisted structure, the composition of the present invention is a mixture of chiral substances with clockwise and counterclockwise twisting directions. A dielectric liquid crystal composition is obtained.

For this reason, display panels using such ferroelectric liquid crystal compositions do not develop a wrinkling structure even when the cell thickness is large, providing a memory effect and good alignment, resulting in ferroelectric liquid crystal displays with good display quality. You can get the equipment.

Example As an example of the invention, as a counterclockwise chiral substance (+)
-DOBAMBC, as a counterclockwise chiral substance (-)
- shows the structural formula and phase transition temperature using HOBACPC respectively.

(+)-DOBAMBC 纂CHCOOCH,CHC,H.

(Here * indicates chiral carbon) Here, Iso is an isotropic liquid, SmA is a smectic anthropomorphic SmC tree is a smectic C chiral phase showing ferroelectricity,
Cry indicates a crystalline phase.

(-) -HoBACPC =CHC00CH2CHCH8 ne I s o ←--3 m A◆--1-3m C zero or 1111 The pitch relationship of these mixtures is shown in FIG.

This was measured in a cell with a cell thickness of 200 μm at 4° C. below the TCA (transition point from smectic C chiral phase to smectic A phase).

From FIG. 1, it can be seen that in a mixture in which the weight percent (wt%) of HOBACPC is about 55 wt%, the pinch is almost elongated and the twisted structure disappears.

Next, a liquid crystal composition having a non-chiral smectic C phase was prepared. The wt% of each single substance and the phase transition temperature of the resulting mixture are shown below.

79℃ 56℃ 30℃ Here, N indicates a nematic phase.

Next DOB A M B C (45wt%) and HOB
A mixture of ACPC (55 wt%) and the above ester mixture were mixed at a weight ratio of 20:80,
As a result, a smectic liquid crystal exhibiting ferroelectricity from 0°C to 70'C was obtained.

In this composition, no twisted structure was observed even at a cell thickness of 200 IJm, and good orientation was obtained. Spontaneous polarization is about 2nC
/cd (20°C).

The orientation of the ferroelectric liquid crystal was achieved by applying a polyimide polymer film on a glass substrate and rubbing it.

A ferroelectric liquid crystal composition with such a long pitch can be used with a cell thickness of 3
When encapsulated in a μm cell, a good memory effect was obtained.

The pitch was measured by enclosing a ferroelectric liquid crystal composition in a cell that had been subjected to alignment treatment by rubbing and observing it with a polarizing microscope.

Figure 1 shows the relationship between the voltage and the transmittance of the ferroelectric liquid crystal panel that does not have a twisted structure, created in this way.The cell thickness is 3.5 μm, and the ferroelectric liquid crystal composition is as described above. I used the one from

From FIG. 1, it can be seen that even when the voltage becomes 0 (V), the transmittance does not change and there is a memory effect.

At this time, the height of the voltage pulse is -20 to +20 (V),
The pulse width was 1 sec and the interval between the h pulses was 20 sec.

In FIG. 1, 1) represents a voltage, and 12 represents a transmittance curve.

In a ferroelectric liquid crystal panel, the response time is usually proportional to the cell thickness, so the maximum thickness of the liquid crystal layer is considered to be 10 μm. Therefore, it seems necessary to make the pinch at least 10 μm or more.

Effects of the Invention By mixing each chiral substance with a reverse twist, a long-pitch composition with an untwisted structure is created, and by mixing this composition with a non-chiral liquid crystal composition, a long-pitch ferroelectric material is created. A liquid crystal composition can be obtained. Since this composition has a long pitch, it has high memory properties in devices such as display devices and optical shutters, and also has good orientation, so it is possible to obtain a ferroelectric liquid crystal display device with high display quality.

In addition, because basically only two types of chiral substances are required, there is no need to compensate for the pitches of many types of chiral substances (including temperature dependence), which is required in the past. A display device can be obtained.

[Brief explanation of drawings]

Figure 2 is a characteristic diagram showing the relationship between the voltage and transmittance of the ferroelectric liquid crystal panel in the example, and Figure 2 is the mixing ratio and pitch of the clockwise chiral material and the counterclockwise chiral material in the example. Figure 3 is a schematic diagram showing the twisted structure of a ferroelectric liquid crystal, and Figure 4 shows the state of molecules in a ferroelectric liquid crystal panel when the twisted structure is unwound when the cell thickness is smaller than the pitch. The schematic diagram shown in Figure 5 is a cross-sectional view of a general ferroelectric liquid crystal panel. Figure 6 (al, (bl) is a schematic diagram showing the operation when an electric field is applied to a ferroelectric liquid crystal panel. 1) Voltage curve, 12 Transmittance Curve, 31...Liquid crystal molecule, 32...Permanent dipole moment, 33...Twist period (pitch: L), 34...Layer structure, 35...
・Layer normal direction, 36... Tilt angle (θ), 41・
...Liquid crystal molecules, 42...Direction of electric field (from the front to the back of the paper), 43...Direction of the electric field (from the back of the paper to the front), 44... ...N structure, 45...
...Layer normal direction, 46...Tilt angle (θ), 5
1...Polarizing plate, 52...Glass substrate,
53...Transparent electrode, 54...Ferroelectric liquid crystal, 61...Direction of electric field, 62...
Permanent dipole moment, 63...layer structure, 64
......Tilt angle (θ), 65...Polarizer (
P), the direction of the polarization axis of the analyzer (A>. Agent's name: Patent attorney Toshio Nakao, 1 person Figure 1 V Figure 2 1'10fHoaAcpc Figure 4, 76 σl Figure 5 required

Claims (6)

[Claims] (1) A ferroelectric liquid crystal display device characterized by using a ferroelectric liquid crystal composition formed by mixing a composition of chiral substances having opposite twist directions and a non-chiral substance. (2) Ferroelectricity according to claim (1), wherein the non-chiral substance is a liquid crystal having any one of a smectic C phase, a smectic I phase, a smectic F phase, or a smectic G phase. LCD display device. (3) A ferroelectric liquid crystal display device according to claim (1), wherein the composition of chiral substances with opposite twist directions is a liquid crystal. (4) Claim (1) characterized in that the composition of the chiral substance with the opposite twist direction is a ferroelectric liquid crystal.
The ferroelectric liquid crystal display device described in . (5) The ferroelectric liquid crystal display device according to claim (1), wherein the pitch of the ferroelectric liquid crystal phase of the ferroelectric liquid crystal composition is from 10 μm to infinity. (6) When the ferroelectric liquid crystal composition has a cholesteric phase at a temperature higher than the transition temperature of the ferroelectric liquid crystal phase, the pitch of the cholesteric phase is from 10 μm to infinity. A ferroelectric liquid crystal display device according to item (1).