JPH01167825A - Liquid crystal display element - Google Patents

Abstract

PURPOSE:To maintain the same change of d/p of a cell with temp. by using an optically active agent for right rotation among plural pairs of enantiomers which are the same in molecular structure and are different only in the optical characteristics as one of the chiral agents in liquid crystal compsns. to be packed in a display cell and compensation cell and an optically active agent for right rotation as the other. CONSTITUTION:A pair of the compds. which are the same in molecular structure as each other, contain just one asymmetric carbon to make all of 4 kinds of substituents different and are in the enantiomeric relation with each other as shown in the formulas are used as the chiral agents. Such compds. are identical in the molecular structure and are, therefore, equal in thermal behavior to each other. The directions of the chiral degree are opposite and the absolute values are the same. The optical rotatory power of the resulted compdsn. are reverse and the helical pitch (p) and the dependency of the temp. thereof are the same if the clockwise rotatory and counter-clockwise rotatory compds. are compounded at an equal ratio each with the liquid crystals to be packed in the display cell 1 and the compensation cell 2. The same d/p (d is an inter-cell spacing) of both cells is obtd. over a wide temp. range and the black and white display is enabled at all times if the compd. which increases the change of the helical pitch and the compd. which decreases the same as temp. rises are used in combination.

Description

[Detailed Description of the Invention] "Industrial Application Field" This invention relates to super twisted nematic type (S
TN type) liquid crystal display element.

"Prior Art" Recently, a two-layer cell structure as shown in FIG. 1 has been proposed as a type of STN liquid crystal display element. In FIG. 1, reference numeral 1 is a display cell, and 2 is a compensation cell.

Display cell 1 includes a nematic liquid crystal (N
P liquid crystal) is filled with a liquid crystal composition in which, for example, an optically dextrorotatory chiral agent is mixed, and the -force compensation cell 2 is filled with:
Similarly, a liquid crystal composition in which an optically levorotatory chiral agent is mixed with Np liquid crystal as a mother liquid crystal is filled.

This liquid crystal display element has T for display in its display cell l.
An L pole is formed and voltage is applied,
No display voltage is applied to the compensation cell 2 at all. Further, in this liquid crystal display element, the cell spacing of display cell 1 and compensation cell 2 is d8, d2, display cell!, respectively. If the helical pitch of each liquid crystal composition of compensation cell 2 is pl, p, then (L/p+ = dt/
pt.

With such a configuration, the liquid crystal display element can display black/white in the S'rN type since the distortion of the surface light is eliminated by the compensation cell 2.
Therefore, it is possible to solve the problem of unclear display due to yellow or blue, which was a drawback of the conventional STN type.

``Problems to be Solved by the Invention'' However, in the liquid crystal display element described in ``-'', the amount of variation (1/G) of each cell caused by temperature changes is always constant between the display cell and the compensation cell. Must be constant.

In other words, if the amount of variation in d/p due to temperature change in the display cell and the amount of variation in d/p due to temperature change in the compensation cell do not match, even if the temperature is lower than room temperature or the temperature is higher than room temperature. This is because even if the compensation cell is used, the distortion of the display light cannot be sufficiently resolved, making clear black and white display impossible.

Here, the cell interval d of each cell is a constant that does not change with temperature changes, but l/p is a temperature-dependent variable determined by the state of the liquid crystal and the like. Therefore, it is necessary to equalize the amount of variation in I/p due to temperature change in both cells.

By the way, one of the liquid crystal compositions filled in the above two cells is a mixture of an Np liquid crystal and a dextrorotatory chiral agent, and the other is a mixture of an N.p liquid crystal and a levorotatory chiral agent. Usually, a dextrorotatory chiral agent and a levorotary chiral agent have different molecular structures, and therefore their physical properties such as inherent pitch, dislocation temperature, and viscosity are different, and furthermore, the formulation m is also different, so display cells l and The I/p of each liquid crystal composition filled in the compensation cell 2 does not have the same temperature dependence. Here, for example, one cell contains an Np liquid crystal for STN (manufactured by Merck & Co., trade name ZLI-2293) 99 ,
Ovt'% and chiral agent (manufactured by Merck & Co., Ltd.: product name S-8
11) 1. A liquid crystal composition formed by blending Ovt% with 98.8wt% of the above 5TNJTINp liquid crystal and a chiral agent (manufactured by BDH Company: trade name CB15) I,
When liquid crystal compositions containing 2 wt% are filled and used, the d/p of each of these liquid crystal compositions is
As shown in the figure, there is a large difference in the low temperature range or high temperature range.

Therefore, such a liquid crystal display element has a problem in that it cannot provide clear black and white display in a low temperature range or a high temperature range other than room temperature.

In addition, in order for the liquid crystal composition to undergo a cholesteric-nematic phase transition without generating domains in either the display cell 1 or the compensation cell 2, when the pretilt angle is small and the twist angle is φ, (II
/ p + (dt/ It is necessary to configure the cell so that P satisfies the following relationship.

(φ−π/2)/2π≦d/p≦(φ+π/2)/2π
... (1) However, the characteristic pitch of the chiral agent that determines the d/p of a liquid crystal composition generally varies greatly depending on the temperature. In a liquid crystal display element filled with such a chiral agent, there is a fear that domains may be generated that deteriorate display contrast and drive characteristics.

``Means for Solving the Problems'' The liquid crystal display element of the present invention has a dextrorotating helical pitch in which the helical pitch increases as the temperature rises. A mixture of a chiral agent and a dextrorotatory mixed chiral agent whose helical pitch decreases as the temperature rises is used to fill both of the above cells as the other liquid crystal composition. , the mother liquid crystal is composed of a plurality of levorotatory chiral agents each having an enantiomeric relationship with each of the dextrorotatory chiral agents, and these levorotatory chiral agents are mixed in the same blending ratio as the dextrorotatory mixed chiral agent. This uses a mixture to which a levorotatory mixed chiral agent is added.

Therefore, in the liquid crystal display element of the present invention, one of the chiral agents in the liquid crystal composition filled in the display cell and the compensation cell is selected from among a plurality of pairs of enantiomers that have the same molecular structure but differ only in optical properties. Since a rotatory optically active compound and a levorotatory optically active compound were used at the same mixing ratio, the temperature dependence of I/p in each cell could be made equal. .

In addition, each of the chiral agents in the liquid crystal composition filled in both cells was mixed by combining chiral agents whose helical pitch increases with increasing temperature and chiral agents whose helical pitch decreases, so each mixed chiral agent In both cases, the change in helical pitch due to temperature is small, and therefore the d/p in both cells is as described above in a wide temperature range (
1) satisfies the formula.

In the liquid crystal display element shown in FIG. 1, the liquid crystal composition filled in display cell 1 and compensation cell 2 includes a dextrorotatory mixed chiral agent on one cell side and a levorotary mixed chiral agent on the other cell side. A chiral agent is added. A dextrorotatory mixed chiral agent is a mixture of a plurality of types (two or more types) of dextrorotatory chiral agents in an appropriate composition.

These dextrorotatory chiral agents are liquid crystalline compounds each containing one asymmetric carbon and are optically dextrorotatory.

Furthermore, at least one of these dextrorotatory chiral agents increases its helical pitch as the temperature rises, and at least one of the other agents decreases.

With this combination, the helical pitch of the dextrorotatory mixed chiral agent will not change as a whole due to temperature because the chiral agent that increases and the chiral agent that decreases as the temperature increases cancel each other out. Become. The levorotatory mixed chiral agent is composed of multiple types of levorotatory chiral agents that have enantiomeric relationships with each of the dextrorotatory chiral agents in the dextrorotatory mixed chiral agent, and these levorotatory chiral agents are It is mixed in the same blending ratio as the mixed chiral agent. Therefore, even in this levorotatory mixed chiral agent, since some helical pitches become larger and others become smaller as the temperature rises, the overall effect of the levorotatory mixed chiral agent is similar to the above-mentioned dextrorotatory mixed chiral agent. Changes in helical pitch due to temperature are small.

Here, enantiomers are those with the same molecular structure,
It refers to a pair of compounds that contain only one asymmetric carbon that makes all four types of substituents different, and are mirror images of each other, such as a pair of optically active compounds such as. These compounds are preferably used as chiral agents in the present invention.

Since such enantiomers have the same molecular structure, their thermal behavior is exactly the same, and their chiral degrees are also opposite in direction but have the same absolute value. Therefore, we used these as chiral agents, and used a dextrorotatory one for one and a levorotatory one for the other, and in both cells, compounds with the same molecular structure were mixed in the same -m ratio, and the same compounding ratio was used. A mixed chiral agent is prepared as shown in FIG.
If both are blended, the resulting composition will have opposite optical rotation on the right and left sides, but the helical pitch (p) will be the same, and the temperature dependence of the helical pitch will also be the same.

Furthermore, these dextrorotatory mixed chiral agents and levorotatory mixed chiral agents each include those whose helical pitch changes larger and smaller as the temperature rises, so both of them have a change in helical pitch as a whole. changes with temperature are small, so that d
/p satisfies the above equation (+) over a wide temperature range.

Therefore, in such a liquid crystal display element, since the temperature dependence of d/p of each cell is the same, the d/p of display cell l and compensation cell 2 match at any temperature, and this This enables black and white display at all times from low to high temperatures. In addition, since d/p satisfies equation (1) above over a wide temperature range, no domains occur in the normal operating temperature range, and therefore display contrast and drive characteristics due to these domains are reduced. The decline can be prevented.

"Examples" The present invention will now be described in more detail with reference to Examples.

(Example 1) 1.8wt% of a dextrorotatory mixed chiral agent obtained by blending a dextrorotatory chiral agent at a ratio of 1:1, and NP for STN as a mother liquid crystal.
Liquid crystal (manufactured by Merck; product name ZLI-2293) 98
, 2vt% mixture was filled into the display cell 1 shown in FIG. Furthermore, a levorotatory mixed chiral agent 1.
8wt% and the same Np for STN as above as the mother liquid crystal.
Compensation cell 2 was filled with a mixture of 98.2 wt % liquid crystal.

In this way, 2406 twisted cells were fabricated, and 2FJ
This is a liquid crystal display element with a cell structure. Here, the chiral agent shown in (2) above, the chiral agent shown in (4), (3)
The chiral agent shown in (5) and the chiral agent shown in (5) are in an enantiomeric relationship with each other. In addition, the chiral agents shown in (3) and (5) above each have a large change in helical pitch due to temperature rise, and the chiral agents shown in (2) and (4) above each have a large change in helical pitch due to temperature rise. Changes due to temperature rise are reduced.

In the produced cells, each d/p was set to 0.5 at room temperature.

When the change in d/p with temperature in both these cells was investigated, almost no change was observed in the temperature range of 0 to 60°C.

C from the compensation cell side to the liquid crystal display element obtained in this way.
White light is incident from the C light source specified by the IE (International Commission on Illumination), and the hue of the display obtained at each point of 0°C, 25°C, and 60°C is examined, and the results are compared to the CIE shown in Figure 2.
Color system (1) - Shown in chromaticity diagram. For comparison, the display hue of the conventional liquid crystal display element mentioned above was also investigated and is shown in FIG.

From the results shown in FIG. 2, it was confirmed that the liquid crystal display element of the present invention is capable of providing clearer black and white display than conventional products.

Also, when I checked the display contrast, I found that it was 1/200
When driving at a duty ratio of 16/1, a high contrast value of 16/1 was obtained.

Furthermore, in l/200 duty drive, -20℃
The occurrence of domains at each temperature of , 25°C, and 80°C was investigated by microscopic observation, and the results are shown in the following table in comparison with conventional products.

table However, no O; domain was observed.

Δ: Slight occurrence of domains is observed.

×; Occurrence of domains is observed considerably.

shall be.

From the results shown in the table, it was confirmed that the products of the present invention did not generate domains over a wide temperature range.

(Example 2) In Example 1, the dextrorotatory mixed chiral agent used as the mixed chiral agent for display cell 1 was used for compensation cell 2, and the levorotatory mixed chiral agent was used as the mixed chiral agent for compensation cell 2. These were used for Cell I, and each was added to the mother liquid crystal to form a liquid crystal composition, and both cells were produced to form liquid crystal display elements.

The compounding ratio of the mother liquid crystal and the mixed chiral agent in the liquid crystal composition was the same as in Example 1, and the cell dimensions etc. were also the same as in Example 1.
was made the same as

Even with such a liquid crystal display element, results similar to those of Example 1 were obtained in terms of display hue, contrast, and domain generation.

(Other Examples) In Examples 1 and 2, dextrorotatory and levorotatory mixed chiral agents of combinations A to C below were used as chiral agents for display cells and compensation cells.

A.

B.

Even in liquid crystal display devices made from mixed chiral agents with combinations of C1 or above, all of the display hue, contrast,
The same results as in Example 1 were obtained in terms of domain generation.

"Effects of the Invention" As explained above, the liquid crystal display element of the present invention has a dextral helical pitch that increases as the temperature rises in the mother liquid crystal as one of the liquid crystal compositions filled in display cells or compensation cells. The other of the liquid crystal compositions filled in both of the cells is prepared by using a mixture containing a dextrorotatory mixed chiral agent, which is a mixture of a dextrorotatory chiral agent and a dextrorotatory chiral agent whose helical pitch decreases as the temperature increases. , the mother liquid crystal is composed of a plurality of levorotatory chiral agents having enantiomeric relationships with each of the dextrorotatory chiral agents, and these levorotatory chiral agents are in the same compounding ratio as the dextrorotatory mixed chiral agent. A mixture containing a levorotatory mixed chiral agent is used.

Therefore, in the liquid crystal display element of the present invention, by using optically active compounds having enantiomeric relationships as chiral agents in both cells, the d/p of each cell is
Since the temperature dependence of is the same, the d/p of the display cell and the compensation cell are the same at any temperature, and as a result, black and white display is always possible from a low temperature state to a high temperature state.

In addition, by using a mixed chiral agent in which the helical pitch increases as the temperature rises and in which the helical pitch decreases as the temperature rises, the d/p of each cell can be maintained over a wide temperature range. Since the above-mentioned formula (1) is satisfied, no domains occur in the normal usage range, and therefore, it is possible to prevent the display contrast and drive characteristics from deteriorating due to the domains.

Furthermore, since the blending ratio of the mother liquid crystal and each chiral agent in the liquid crystal composition to be filled is the same in both the display cell and the compensation cell, blending of the liquid crystal composition can be simplified.

[Brief explanation of the drawing]

FIG. 1 is a diagram related to the liquid crystal display element of the present invention,
A schematic configuration diagram of a liquid crystal display element. Fig. 2 is an (XY)-chromaticity diagram of the CIE color system showing hue changes due to temperature changes when the power is turned off for products of the present invention and conventional products. Fig. 3 is a diagram of the conventional product. 1 is a graph showing changes in d/p due to temperature changes of a liquid crystal composition used in a liquid crystal display element. ■...Display cell, 2...Nli compensation cell.

Claims (1)

[Claims] In a liquid crystal display element having an STN type two-layer cell structure consisting of a display cell and a compensation cell, one of the liquid crystal compositions filled in the display cell or the compensation cell is in the mother liquid crystal, and the helical pitch is This is a mixture containing a dextrorotatory chiral agent whose helical pitch increases as the temperature rises, and a dextrorotatory chiral agent whose helical pitch decreases as the temperature rises, with the addition of a dextrorotatory mixed chiral agent. The other liquid crystal composition is comprised of a plurality of levorotatory chiral agents in enantiomeric relationships with each of the dextrorotatory chiral agents in the mother liquid crystal, and these levorotatory chiral agents are in contact with the dextrorotatory mixed chiral agent. What is claimed is: 1. A liquid crystal display element characterized in that it is a mixture containing a levorotatory mixed chiral agent mixed in the same proportion as the agent.