Ferroelectric liquid crystal mixture

Details Ferroelectric liquid crystal mixture Ferroelectric liquid crystal mixture

1998-06-29

2001-01-09

Kelly, C. H. (Department: 1721)

Compositions

Liquid crystal compositions

Containing nonsteryl liquid crystalline compound of...

C252S299620, C252S299630, C252S299650, C252S299660, C252S299670, C349S182000

Reexamination Certificate

active

06171519

ABSTRACT:

The present invention relates to a novel ferroelectric liquid crystal mixture. More particularly, it relates to a ferroelectric liquid crystal mixture, which shows a high switching speed when driven at a low voltage, and a liquid crystal display device with the use of this liquid crystal mixture.
In these days, liquid crystal mixtures have been widely employed in practice as display devices of clocks, electronic calculators, small-sized television sets, etc. Among all, the most commonly employed products are those with the use of the nematic phase of liquid crystals and their display systems are called the twisted nematic (TN) type or the super twisted nematic (STN) type. However, the TN type is unsuitable for a display of a large information capacity, since its contrast is deteriorated with an increase in line number. Owing to the TFT driving system wherein each pixel is provided with a thin film transistor (TFT), the display characteristics of the TN type become more and more like those of a cathode ray tube (CRT) and thus the information capacity can be enlarged. However, its production process is highly complicated and achieves only a low yield, which results in an extremely high production cost. Moreover, it is scarcely applicable to a large screen.
Although the STN system, which is composed of a simple matrix, has improved display characteristics compared with the TN type, it is still insufficient when compared with the TFT-TN system. However, the STN system requires only a low production cost. Since Clark and Lagerwall found Surface Stabilized Ferroelectric Liquid Crystals (SSFLC) in 1980 [N. A. Clark and S. T. Lagerwall, Appl. Phys. Lett., 36, 899 (1980)], these liquid crystals have attracted attentions as display materials in the coming generation and a number of studies have been carried out thereon. The reasons therefor are as follows. (1) These ferroelectric liquid crystals have a high response speed. (2) They have memory properties which enable a display of a large information capacity and they can be produced at a relatively low cost, since no active device (thin film transistor, etc.) is needed. (3) They have a broad viewing angle. Thus, these liquid crystals are expected to be usable in a display device having a large screen size and a large display capacity.
To use a ferroelectric liquid crystal display device in practice, it is an important factor to achieve a highly defined contrast. It is very difficult to establish a highly defined contrast at the desired level by using ferroelectric liquid crystals. The reasons therefor reside in, for example, the zigzag defect in the smectic C phase, a decrease in the effective cone angle due to the chevron geometry, the insufficient memory properties, etc. There have been proposed various methods for achieving a highly defined contrast. Examples of these method include the use of an oblique vapor-deposition film as an alignment layer, the C1 uniform method by using an alignment layer having a high pretilt, the utilization of a quasi-bookshelf geometry through an AC electric field processing or by using a naphthalene-based compound, and the use of a material having a negative dielectric anisotropy. Among the above-mentioned methods, the one with the use of a material having a negative dielectric anisotropy (&Dgr;&egr;) depends on a phenomenon that, when an electric field of a high frequency is applied perpendicularly to the electrode substrate, liquid crystal molecules having a negative &Dgr;&egr; are aligned in parallel with the electrode substrate. This phenomenon is called the AC stabilization effect.
Surguy [P. W. H. Surguy, et al., Ferroelectrics, 122, 63 (1991)] further proposed “a method with the use of a liquid crystal material having negative dielectric anisotropy”. This method, which is largely accepted as a method for successfully achieving a highly defined contrast, is described in P. W. Ross, Proc. SID, 217, (1992).
A material with a negative dielectric anisotropy has a so called
T
−V characteristic, i.e., the pulse width (
T
) required for switching shows the minimum value (
T
−Vmin) with an increase in the voltage (V). Surguy et al. reported a driving system with the use of this characteristic. In this driving system, switching is effected under the voltage |Vs−Vd| but not under |Vs+Vd| or |Vd|.
The driving voltage in this system is determined by (
T
−Vmin) characteristics for the materials. According to Surguy et al., the value Vmin is defined as follows.
Vmin=Emin*d=Ps*d/3*&egr;
0
*&Dgr;&egr;*sin
2
&thgr;.
In the above formula, Emin stands for the minimum strength of the electric field; d stands for the cell gap, Ps stands for the spontaneous polarization; &Dgr;&egr; stands for the dielectric anisotropy; and &thgr; stands for the tilt angle of the liquid crystal material.
By taking the biaxial anisotropy (&dgr;&egr;) into consideration, furthermore, Towler et al. obtained the values Vmin and Tmin as defined below.
&LeftBracketingBar;
V
⁢
 
⁢
min
&RightBracketingBar;
=
Ps
·
D
ε
0
⁢
3
⁢
(
sin
2
⁢
θ
-
∂
ε
)
τmin
∝
η
⁡
(
Δε
⁢
 
⁢
sin
2
⁢
θ
-
∂
ε
)
Ps
2
(&eegr;: viscosity)
[M. J. Towel et al., Liquid Crystal, 11 (1992)].
However, the ferroelectric liquid crystal material disclosed by Ross et al. still shows only a slow response speed and [Vs+Vd] exceeds 55 V, which makes it less usable in practice. Accordingly, it has been required to develop a liquid crystal material which has a sufficiently large absolute value of the negative dielectric anisotropy, a large spontaneous polarization and a low viscosity. Ferroelectric liquid crystal mixtures appropriate for driving systems with the use of the AC stabilization effect or driving systems with the use of the
T
−Vmin characteristics are disclosed, e.g. in JP A 168792/1989, 306493/1989 and 4290/1992, JP B 29990/1995 and JP A 503444/1990.
However, since the development of ferroelectric liquid-crystal mixture in particular can in no way be regarded as complete, the manufactures of displays are still interested in a very wide variety of mixtures. Another reason for this is that only the interaction of the liquid-crystalline mixtures with the individual components of the display device or of the cells (for example the alignment layer) allows conclusions to be drawn on the quality of the liquid-crystalline mixtures too.
The object of the present invention was therefore to provide liquid crystalline mixtures which are suitable for improving the property profile of liquid crystal displays, particulary ferroelectric liquid crystal displays and especially ferroelectric liquid crystal (FLC) displays operated in the inverse mode.
A further object of the present invention is to provide a ferroelectric liquid crystal mixture, which has a negative value of &Dgr;&egr; and is capable of achieving a high response speed and a low voltage driving, and a liquid crystal display device with the use of this liquid crystal mixture.
The present invention provides a ferroelectric liquid crystal mixture comprising at least two compounds selected from at least two different of the following groups of compounds:
A. (1,3,4)-thiadiazole derivatives of the formula (I),
wherein the symbols and indices have the following meanings:
R
1
and R
2
independently of one another, are
(a) a hydrogen atom,
(b) a straight-chain or branched-chain alkyl group, with or without an asymmetric carbon atom, having from 2 to 16 carbon atoms, in which one or two, preferably non-adjacent, —CH
2
— groups may be replaced by —O—, —S—, —CO—, —CO—O—, —O—CO—, —CH═CH—, —C≡C—, —Si(CH
3
)
2
—, 1,4-cyclohexylene, 1,4-phenylene, cyclopropane-1,2-diyl or —O—CO—O—, with the proviso that —O— and/or —S— atoms must not be directly bonded to one another; one or more hydrogen atoms of the alkyl group may be substituted by F, Cl, CN and/or CF
3
; or
(c) any one of

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