JPH0337594B2 - - Google Patents

Description

[Detailed description of the invention]

[Summary] Disclosed is a nematic-cholesteric phase transition type liquid crystal composition for liquid crystal display devices, which is particularly capable of displaying a large capacity. According to the present invention, the optical bistable state of the liquid crystal when a voltage is applied is stabilized by using a liquid crystal composition whose main components are ethane-based and bicyclohexane-based nematic liquid crystal compounds and ester-based chiral nematic liquid crystal compounds. , and by minimizing fluctuations in bistable state maintenance voltage (driving voltage: Vd) due to temperature, a liquid crystal composition that can be stably driven over a wide temperature range is provided. [Industrial Field of Application] The present invention relates to a liquid crystal composition for a liquid crystal display element, and particularly to a nematic-cholesteric phase transition type liquid crystal composition. Liquid crystal display elements (LCDs) are thin, lightweight, and light-receiving, so they are used in calculators, wristwatches, and more recently in televisions and handheld computers, and with the development of office workstations, they are capable of increasingly larger display capacities. LCD
is required. On the other hand, as the application fields of LCDs expand, there is a strong demand for lower-cost LCDs. In this way, in addition to the conventional features of thinness, light weight, and light-receiving type, LCDs have new characteristics such as large capacity display and low cost.Furthermore, LCDs have new features such as large display capacity and low cost. There is a possibility that large-area displays can be implemented at low cost. [Prior Art] The mainstream of conventional liquid crystal display devices (LCDs) is the twisted nematics (TN) method. A liquid crystal display element using this method, for example, as shown in the panel configuration diagram in FIG. , alignment films 6 and 7, one reflective plate 11, and a spacer 8 that holds them.
It is constructed such that display is performed by applying an electric field. In other words, when no electric field is applied, the liquid crystal is continuously twisted by 90 degrees from the upper substrate to the lower substrate due to the alignment force of the alignment film, and the incident light with the same polarization direction is twisted along the twist of the liquid crystal. Since the polarization direction is rotated by 90 degrees, the light passes through the lower polarizing plate, which is perpendicular to the upper polarizing plate, and is reflected by the reflecting plate, resulting in a bright state. When an electric field is applied, all liquid crystal molecules are aligned in the direction of the electric field, so the polarization direction of the incident light cannot be changed by the liquid crystal.
It cannot pass through the polarizing plate below, resulting in a dark state. However, in this TN type liquid crystal display element, since the rise characteristic of the liquid crystal is not steep, when trying to display a large capacity dot matrix, a half-display state up to a non-display point (non-display point), that is, crosstalk occurs. , and large-capacity display cannot be performed. Furthermore, in this type of liquid crystal display element, when attempting to display a large capacity, the viewing angle is significantly limited, resulting in an extremely low contrast. Although efforts such as multi-matrix technology have been made to compensate for these drawbacks, this TN display element overcomes the drawback that the viewing angle range narrows as the number of scanning lines increases, which is fatal to large fixed displays. Can not do it. Under these circumstances, we first developed a large-capacity display with approximately 500 scanning lines in order to overcome the phenomenon of crosstalk in line-sequential scanning, which is inevitable for liquid crystals, and to enable large-capacity display. A storage type liquid crystal display device has been developed. As a result of several subsequent improvements, the phase change type liquid crystal display element that is the subject of the present invention was developed (see, for example, Japanese Patent Application No. 59-00966). Although this LCD has the same structure as a TN LCD except that it does not require a polarizing plate, the driving method, that is, the light control method, is different.
It differs from that of the TN type. This LCD uses the hysteresis loop of the applied voltage vs. light transmittance curve shown in Figure 1, and the driving voltage is set at the center of this loop.
Vd, and the application of this voltage Vd creates two optical states, namely the focal conic state (F:
After writing information, a constant voltage is applied.
The information is displayed by applying Vd as a holding voltage. With this new drive method
LCDs are extremely useful because they can display large amounts of data and can also be used with keyboards. [Problems to be Solved by the Invention] In conventional nematic-cholesteric phase transition liquid crystal materials, the size of the hysteresis loop (Δ) shown in Figure 1 may not be sufficient, or the loop may change due to changes in environmental temperature. Stable operation is difficult because the size and drive voltage Vd change. There are several possible reasons why the size of the hysteresis loop and Vd vary, but the most significant factor is that the helical pitch of the helical structure, which is unique to nematic-cholesteric phase transition liquid crystals, changes with temperature. In conventional liquid crystal materials, the temperature dependence of the helical pitch is very large, so it is difficult to suppress pitch fluctuations due to temperature. In addition, with conventional liquid crystal materials, the temperature range in which a sufficiently large hysteresis loop can be obtained is narrow, and the loop becomes particularly small on the high temperature side, so for applications where an increase in the temperature of the liquid crystal is unavoidable, such as projection type using a backlight. has the disadvantage that it cannot be used. These drawbacks are the problems that the present invention seeks to solve. [Means for Solving the Problems] According to the present invention, the above problems can be solved by using a nematic material in which the composition as a whole exhibits positive dielectric anisotropy.
An ethane-based nematic liquid crystal compound which is a cholesteric phase transition type liquid crystal composition and is represented by the following formula: (In the above formula, R ₁ is a straight chain alkyl group having 2 to 5 carbon atoms,
R ₂ is a straight chain alkyl group having 2 to 5 carbon atoms, an alkoxy group, or fluorine, and X is absent or, when present,

[Formula] or

[Formula], and Y is

【formula】

【formula】

[Formula] or

〔Example〕

Two glass substrates with ITO (indium oxide In ₂ O ₃ ) films were washed with detergent, and then sequentially with isopropyl alcohol, acetone, and pure water. After surface modification of these substrates by irradiation with oxygen plasma, liquid crystal panels were fabricated using spacers made from crushed glass fiber particles with a diameter of 9 μm. As shown in the configuration diagram in FIG. 2, this liquid crystal panel consists of glass substrates 2 and 3, transparent conductive film (ITO) 4
and 5, alignment films 6 and 7, and a spacer 8. The following four types of liquid crystal compositions with different types and mixing amounts of liquid crystal compounds were added to the obtained liquid crystal panel:
A, B, C and D (control; conventional diphenyl type,
(composition mainly composed of cyclohexane) was sealed.

【table】

【table】

【table】

【table】

【table】

【table】

〔Effect of the invention〕

According to the present invention, a bistable state using a nematic-cholesteric phase transition type liquid crystal can be stably and largely maintained, and fluctuations in the bistable state and driving voltage due to temperature are extremely small. An inexpensive large-capacity liquid crystal display element can be obtained.

[Brief explanation of drawings]

FIG. 1 is a diagram of the driving principle of the nematic-cholesteric phase transition type liquid crystal composition according to the present invention, and FIG.
The figure shows a configuration diagram of a panel using a nematic-cholesteric phase transition type liquid crystal composition according to the present invention.
3A and 3B are graphs showing the temperature dependence of Δ and Vd of a liquid crystal panel containing liquid crystal composition A, respectively, and FIGS. 4A and 4B are graphs showing the temperature dependence of Δ and Vd of a liquid crystal panel containing liquid crystal composition B, respectively. Δ and Vd
Graphs showing the temperature dependence of , Figures 5A and 5B
6A and 6B are graphs showing the temperature dependence of Δ and Vd of a liquid crystal panel containing liquid crystal composition C, respectively.
6B and 6B are graphs showing the temperature dependence of Δ and Vd, respectively, of a liquid crystal panel containing liquid crystal composition D, and FIG. 7 is a graph of a panel using a commonly used twisted nematic liquid crystal composition. FIG. In the figure, 1 is a liquid crystal, 2 and 3 are glass substrates,
4 and 5 are transparent conductive films, 6 and 7 are alignment films, and 8 is a spacer.

Claims (1)

[Scope of Claims] 1. A nematic-cholesteric phase transition liquid crystal composition exhibiting positive dielectric anisotropy as a whole, which is an ethane-based nematic liquid crystal compound represented by the following formula: (In the above formula, R ₁ is a straight chain alkyl group having 2 to 5 carbon atoms,
R ₂ is a straight-chain alkyl group having 2 to 5 carbon atoms, an alkoxy group, or fluorine, X is [formula] or [formula] when absent or present, and Y is , [Formula] [Formula] [Formula] or [Formula]); and a bicyclohexane-based nematic liquid crystal compound represented by the following formula: (R ₃ in the formula is a linear alkyl group having 2 to 7 carbon atoms), an ester-based chiral nematic liquid crystal compound represented by the following formula: (In the above formula, R ₄ is an ester group having a non-containing carbon, an alkyl group having a non-containing carbon, or an alkoxy group having a non-containing carbon, and R ₅ is an ester group having a non-containing carbon, a non-containing carbon an alkyl group having a non-satile carbon, an alkoxy group having a non-satile carbon, or a cyano group) is added in an amount sufficient for the obtained liquid crystal composition to exhibit a helical pitch of 0.7 to 1.2 μm. Phase change liquid crystal composition.