Liquid crystal compositions and liquid crystal display devices

Details Liquid crystal compositions and liquid crystal display devices Liquid crystal compositions and liquid crystal display devices

2000-08-21

2002-01-01

Wu, Shean C. (Department: 1756)

Stock material or miscellaneous articles

Liquid crystal optical display having layer of specified...

C252S299630, C252S299660

Reexamination Certificate

active

06335064

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a nematic liquid crystal composition. More particularly, the invention relates to a liquid crystal composition suitable for an active-matrix (AM) mode and to a liquid crystal display device using the liquid crystal composition.
BACKGROUND OF THE INVENTION
The liquid crystal display device of the active-matrix mode (AM-LCD) is the center of attention as the favorite of LCD, since it permits a high-definition display. It has been applied for a display screen of laptop computers, digital still camera, digital video camera and the like.
The AM-LCD liquid crystal compositions have required the following characteristics (1) to (5).
(1) The liquid crystal composition should exhibit a nematic phase in as wide a temperature range as possible in order to extend an environmental temperature at which the liquid crystal display device is used. (The upper-limit temperature of the nematic phase is increased as high as possible and the lower-limit temperature of the nematic phase is reduced as low as possible.)
(2) A viscosity of the liquid crystal composition should be reduced as low as possible in order to decrease a response time of the liquid crystal display device.
(3) An optical anisotropy (&Dgr;n) of the liquid crystal composition can take a suitable value depending on a cell thickness (d) of the liquid crystal display device in order to increase a contrast of the liquid crystal display device.
(4) A specific resistance of the liquid crystal composition should be increased and a voltage holding ratio of a cell in which the liquid crystal composition is contained should be increased in order to increase a contrast of the liquid crystal display device. In particular, a voltage holding ratio at high temperatures should be increased. The measurement of the voltage holding ratio at high temperatures is equivalent to an accelerated test to make sure of the durability of the liquid crystal composition.
(5) A threshold voltage of the liquid crystal composition should be reduced in order to provide a smaller-sized battery serving as a power for driving the liquid crystal display device.
In view of such background, Japanese Patent Kokai 8-73857 discloses the liquid crystal compositions having high voltage holding ratios, low threshold voltage and moderately large optical anisotropy. Japanese Patent Kokai 9-31460 also discloses the liquid crystal compositions particularly having low threshold voltage, good compatibility at low temperatures and a wide temperature range of a nematic phase.
As liquid crystal compounds for low-voltage driving in various modes including AM and STN modes, WO 96/11897 discloses new liquid crystalline compounds having large dielectric anisotropy and very low viscosity as well as liquid crystal compositions containing said compounds.
The liquid crystalline compounds having large dielectric anisotropy are required to use in order to reduce the threshold voltage of the liquid crystal composition. In general, the use of the liquid crystalline compounds having large dielectric anisotropy for the preparation of the liquid crystal composition increases the viscosity of the liquid crystal composition. A response time is proportional to the square of a cell gap, as proposed by E. Jakeman et al., Phys. Lett., 39A (1972) 69. Accordingly, the liquid crystal display device using the liquid crystal composition having reduced threshold voltage becomes slow at the response rate. To decrease a response time, it is recommendable to reduce a thickness (d) of a cell for the liquid crystal display device. However, it is necessary to set a value expressed by a product (&Dgr;n·d) of a cell thickness (d) and an optical anisotropy (&Dgr;n) of the liquid crystal composition at a predetermined value of about 0.5 in order to obtain high contrast under the first-minimum condition for TN mode, as shown in the above item (3). Reducing a cell thickness necessitates an increase in an optical anisotropy of the liquid crystal composition.
The compositions disclosed in Japanese Patent Kokai 8-73857 have the problems that the threshold voltage is low, the optical anisotropy is moderately large, but the upper-limit temperature of a nematic phase is too low and the voltage holding ratio at high temperatures is low, or they have the problems that the optical anisotropy is moderately large, the upper-limit temperature of a nematic phase is high, but the threshold voltage is too high and the voltage holding ratio at high temperatures is low, as shown in comparative examples which will be given later.
The compositions disclosed in Japanese Patent Kokai 9-31460 have the problems that the optical anisotropy is small, the threshold voltage is high and the upper-limit temperature of a nematic phase is low, as shown in comparative examples which will be given later. The liquid crystal compositions for AM-LCD disclosed in WO 96/11897, those in which no compound having a cyano group at the end group is contained have the problems that the threshold voltage is not sufficiently low and the optical anisotropy is small, as shown in comparative examples which will be given later. (The compounds having a cyano group cannot be used as the liquid crystal composition for AM-LCD, because of the voltage holding ratio being low.)
Thus a liquid crystal composition for AM-LCD has not been known which satisfies the requirements of keeping the voltage holding ratio at high temperatures high, having sufficiently low threshold voltage and large optical, as shown in comparative examples which will be given lateranisotropy, while satisfying the characteristics (1) to (4) as mentioned above.
SUMMARY OF THE INVENTION
An object of the invention is to provide a liquid crystal composition especially having high voltage holding ratio at high temperatures, sufficiently low threshold voltage and large optical anisotropy, while satisfying general characteristics required for AM-LCD.
Through our intensive studies on compositions using various liquid crystalline compounds in an effort to solve the above-mentioned problems, we have found that the above object can be achieved by using the liquid crystal composition of the present invention in a display device for AM-LCD.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a liquid crystal composition which comprises:
as a first component at least one of the compounds of the following formulas (1-1) and (1-2); and
as a second component at least one of the compounds of the following formulas (2-1) to (2-3)
 in which R
1
, R
2
, R
3
, R
4
and R
5
each independently represent an alkyl group of 1-10 carbons; A
1
, A
2
, A
3
, A
4
, A
5
and A
6
each independently represent —CH
2
CH
2
— or a single bond, provided that at least one of A
3
and A
4
is a single bond; X
1
and X
3
each independently represent F, —OCF
3
or —OCF
2
H; and X
2
and X
4
each independently represent H or F.
In one embodiment of the liquid crystal composition, the first component comprises 20-70% by weight and the second component comprises 30-80% by weight, based on the total weight of the liquid crystal composition.
The liquid crystal composition of the invention may further comprise as a third component at least one of the compounds of the following formula (3)
wherein R
6
and R
7
each independently represent an alkyl group of 1-10 carbons. In one embodiment of said liquid crystal composition, the first component comprises 20-70% by weight, the second component comprises 30-80% by weight and the third component comprises not more than 20% by weight, preferably 5 to 15% by weight, based on the total weight of the liquid crystal composition.
Further, the present invention provides a liquid crystal display device using each of the above-described liquid crystal compositions according to the present invention.
PREFERRED EMBODIMENTS OF THE INVENTION
Preferred examples of the compounds of formula (1-1) used in the liquid crystal composition of the invention can include those of the following formulas (1-1-1) to (1-1-12):
in which R represents an alkyl grou

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