JPH01225691A - Ferroelectric liquid crystal composition and ferroelectric liquid crystal display device - Google Patents

Abstract

PURPOSE:To readily obtain excellent orientation and, at the same time, to obtain high contrast display in a liq. crystal display device using a rubbing method of an org. oriented film, by employing a particular ferroelectric liq. crystal compsn. which has a phase sequence exhibiting from the high-temp. side an isotropic liq. a smectic A a chiral smectic C. CONSTITUTION:This ferroelectric liq. crystal compsn. exhibits from the high- temp. side all phase sequences of an isotropic liq., a smectic A phase and chiral smectic C phase, and has the characteristic of undergoing a pre-transition phenomenon (P) during the transition from the isotropic liq. to the smectic A phase. This compsn. pref. has a tilt angle of at least 20 deg. and a spontaneous polarizability of at least 20nC/cm<2>.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to bistable liquid crystals, and more particularly to ferroelectric liquid crystal compositions and ferroelectric liquid crystal display devices.

Conventional technology In recent years, liquid crystal displays have been used not only in watches, calculators, etc.
It has become widely used in imaging equipment, and LCD color televisions are also starting to appear on the market. Currently, the majority of color display liquid crystal panels are those using nematic liquid crystals. However, the characteristics of the nematic liquid crystal are far from ideal and include many problems. Ferroelectric liquid crystals have characteristics not found in nematic liquid crystals, such as fast response speed and memory properties, and are being studied in a variety of fields for potential applications in display devices. (For example, Optronics, 1983, Magnetic 9) Ferroelectric liquid crystal will be explained below with reference to the drawings. FIG. 4 is a schematic diagram of ferroelectric liquid crystal molecules. A ferroelectric liquid crystal is usually a liquid crystal having a layered structure called a smectic liquid crystal, and the liquid crystal molecules have a structure tilted by θ with respect to the normal direction of the layers. Furthermore, ferroelectric liquid crystal molecules are usually composed of optically active liquid crystal molecules that are not racemic. In FIG. 5, 7 is a liquid crystal molecule, 8 is spontaneous polarization, 9 is a C director, 1) 0 is a cone, 1) is a layer structure, 12 is a layer normal direction, and 13θ is a tilt angle. As shown in Figure 4, ferroelectric liquid crystal molecules have spontaneous polarization, and in the chiral smectic C phase, they can move freely outside the cone 8 in Figure 4. . The direction of the long axis of the molecules deviates slightly from layer to layer, resulting in a twisted structure as a whole. Next, we will discuss the display principle of ferroelectric liquid crystal. FIG. 5 is a diagram showing the operating principle of a ferroelectric liquid crystal. Figure 5 (al is the state where no voltage is applied, Figure 5 (b) is when a voltage is applied from the back of the paper to the front,
FIG. 5(C) is a diagram showing the principle of operation when voltage is applied in the opposite direction. 14 is a liquid crystal molecule whose long axis of the molecule is tilted by +θ degrees with respect to the layer normal, 15 is a liquid crystal molecule that is elongated by -θ degrees, 16 is a dipole moment pointing toward the front of the paper, and I7 is a liquid crystal molecule facing toward the back of the paper. The dipole moment 18 is the direction of the two polarizing plates. When a ferroelectric liquid crystal is sandwiched between glass substrates with transparent electrodes and the thickness of the panel is made less than the helical pitch, the helix unravels as shown in Figure 5+al, and the molecules are +6 to the layer.
It is divided into a tilted area and a 1θ degree tilted area.
As shown in the figure (bl), the entire cell becomes a uniform monodomain tilted by +θ degrees. Also, when a reverse voltage is applied, the fifth
As shown in Figure fc), the entire cell becomes a uniform monodomain tilted by 1θ degree. Therefore, by using birefringence or dichroism due to the electro-optic effect, brightness and darkness can be represented by two uniform states tilted by ±θ degrees. However, in general, there has been no report to date of such ideal uniform switching when using the rubbing method of an organic alignment film, and as shown in Figure 6(a), the molecules are It is tilted by +θ degree, and at the bottom surface of the substrate, it is tilted by 1θ degree, and in the middle, the molecule is stable in a twisted state in which it is twisted by +6 degrees to 1θ degree from the top surface of the substrate to the bottom surface. In FIG. 6, 19 is an upper substrate, 20 is a lower substrate, 21 is a +θ tilted liquid crystal molecule, 22 is a 10 tilted liquid crystal molecule, 23 is a cone, and 24 is a C director. 6th
Figure (bl) shows the display by the C director. The molecules have a pretilt of a certain angle with respect to the substrate, and the amount of transmitted light differs depending on whether the molecule is twisted to the right or left. (8) to ( In d), a possible twist state when pre-tilt is taken into account is shown by a C director. In Fig. 7, 25 is an upper substrate, 26 is a lower substrate, 27 is a cone, and 28 is a C director. According to previous reports, when the alignment by rubbing of an organic alignment film is used, the alignment between (al, (b)) or (C1, fd
Display is performed by switching between twists. M Murakami, Nis Myake, T Masumi, T & Fukami, Proceedings of the 6th International Display Research Conference, 344 (198)
6) (M, Murakami, S.
Mi yake, ToMasumt, T, And
o, A, Fukami; Proceedings o
fthe 5th InternationalD
isplay Re5each Confe-r
en ce, 344 (1986)) When applying this ferroelectric liquid crystal to a display device, the following items are required of the liquid crystal material.

(1) Exhibits a ferroelectric liquid crystal phase (for example, chiral smectic C phase) over a wide temperature range including room temperature.

(2) The response speed τ of the ferroelectric liquid crystal to the electric field is τ=
η/(Ps-E) where η; viscosity Ps; spontaneous polarization E; given by applied electric field. Therefore, in order to achieve a high-speed response on the order of several microseconds, it is necessary to have a large spontaneous polarization and a low viscosity.

(3) As mentioned above, the optical response of a ferroelectric liquid crystal is first realized by a stable two-state [(bistable 5tate). (According to::1erk et al., in order to realize this state, the cell gap d must be
It is necessary to unwind the spiral by reducing the number to below p.

N, Knee, Clark, Nis, T, Raghaval; Apple, Fizz, Left 0, Average 899 (1980) (
N, A, C1erk, S, T, Lagerw-a 1)
;Ap l 1. Phys, Let t, , 36
899 (1980)'') Therefore, in order to utilize a cell with a thick cell gap that is easy to fabricate, it is necessary to lengthen the helical pitch of the ferroelectric liquid crystal.

(4) The alignment state of the ferroelectric liquid crystal varies depending on the phase series of the liquid crystal material. The phase series includes the following:

■I s o-*N*SmA-*SmC*■
I s o→SmA-3mC*■ r s
o-+N*-*SmC*■ Iso-esmc* However, Iso: Isotropic liquid N*: Chiral nematic, liquid phase S m A i Smectic A phase SmC*; Chiral smectic C phase Conventionally, these four phase series Among them, the one with the phase series ■ and the N* phase with a long helical pitch gives the best orientation. , alignment film cracking occurs due to large deformation.

For this reason, most conventional ferroelectric liquid crystal compositions exhibit a phase series of (1).

Few conventional ferroelectric liquid crystals have reached a practical level not only in terms of temperature range but also in terms of response speed.
At present, a practically usable liquid crystal composition that exhibits good uniform-to-uniform transition and high contrast with respect to dark value characteristics has not yet been obtained. An example of a conventional ferroelectric liquid crystal material is shown below.

Transition temperature Iso -I N* -e SmA -*
SmC*75℃ 64℃ 57℃ Melting point 1) 'C Spontaneous polarization 1) nC/cm2 Response speed 300 μsec (44VI) When applied) Tilt angle 26° In a liquid crystal display device using such a liquid crystal composition, the response speed is However, in panels using the late-rubbing orientation of an organic alignment film, no transition between uniforms is observed as described above, but instead a transition between twists is observed, which is unsatisfactory in terms of display quality. Ta.

Problems to be Solved by the Invention However, since there are almost no conventional ferroelectric liquid crystal materials with other phase series that exhibit good alignment, the phase series is Iso-+N*-+SmA-3mC. Most of them were *. For this reason, there has been a problem in that display devices aligned by rubbing an organic alignment film do not show any transition between uniforms, making it impossible to obtain good display quality.

Means for Solving the Problems In order to solve the above problems, the ferroelectric liquid crystal composition and ferroelectric liquid crystal display device of the present invention have a phase series of ■s o-4Sm.
A-"SmC* is a ferroelectric liquid crystal composition with a pre-transition phenomenon during the transition from an isotropic liquid to an SmA phase. By using this liquid crystal composition, good alignment is easily achieved. In a liquid crystal display device using a rubbing method of an organic alignment film, a transition between tej and niform can be obtained, and a display with high contrast can be obtained.

Function Generally speaking, liquid crystal compositions exhibiting a phase sequence of Iso-=SmA→SmC* do not have an N* phase, so during the transition from an isotropic liquid to an SmA phase, the molecular long axis is regulated and the molecular long axis changes from a completely random state. The deformation is too rapid because the layer direction regulation occurs at the same time. For this reason, a good orientation state cannot be obtained. However, in the liquid crystal composition of the present invention, a phenomenon called pretransition occurs during the transition from the isotropic liquid to the SmA layer.
Cybotic ticks or rasters occur in the random state of an isotropic liquid. Therefore, since the transition to the SmA layer is from this cytotactic cluster, there is no rapid deformation as described above, and the transition is gradual, resulting in good orientation. In addition, since there is no N* phase, the orientation state after the transition to the SmC* phase has almost no discrepancy called zigzag disclination, which is specific to the rubbing method of organic alignment films, and the spontaneous polarization is 20n C/cm. Since the tilt angle is large at 2 or more and the tilt angle is also large at 20° or more, good uniform-to-uniform switching is observed instead of twist-to-twist transition. Further, by using this liquid crystal composition, a liquid crystal display device having a good alignment state and excellent display quality can be easily obtained.

Example An embodiment of the present invention will be described below with reference to the drawings.

First, FIG. 3 shows the structure of a liquid crystal cell used to measure the response of the ferroelectric liquid crystal material in this example. here,
1 is a polarizing plate, 2 is a glass substrate, 3 is a transparent electrode, 4 is an organic film obtained by rubbing an organic alignment film, 5 is a liquid crystal, and 6 is a sealant for bringing the opposing substrates into close contact.

A ferroelectric liquid crystal material was sealed in a cell with such a structure, and its response characteristics, spontaneous polarization, and dark value characteristics were measured. Spontaneous polarization was measured using the triangular wave method.

In addition, regarding the phase transition temperature, the text
Measurements were performed by ure observation and differential scanning calorimetry (DSC).

Example 1 Ps, tilt angle, transition temperature, response speed, dark value characteristics, etc. of the ferroelectric liquid crystal composition used were measured. The results are shown below.

Transition temperature Iso −+ SmA −SmC *65℃
53°C Spontaneous polarization 40nC/cm2 Tilt angle 24°Response speed 30μsec In addition, Fig. 1 ta+ shows the DSC calorific value curve during the transition of this ferroelectric liquid crystal composition from an isotropic liquid to an SmA phase.
FIG. 1 (Is conventionally known as bl.

Fig. 3 shows a calorific value curve of a ferroelectric liquid crystal composition exhibiting a phase series of -3mA-+SmC*. Here, the liquid crystal composition in FIG. 1(a) is a triple critical point liquid crystal composition of an isotropic liquid, an N* phase, and an SmA phase.
The b+ liquid crystal composition is a liquid crystal composition that does not have a triple critical point. As is clear from the figure, in the liquid crystal composition of FIG. 1 fat, a shift in the baseline, that is, a gradual heat generation phenomenon is observed just before the peak of the transition from the isotropic liquid to the SmA phase. However, in the conventional liquid crystal composition shown in FIG. 1 (bl), no deviation of the baseline is observed. From this, it is clear that the liquid crystal composition shown in FIG. Figure 1(b)
The liquid crystal composition is a liquid crystal composition that does not have a pre-transition phenomenon. When these liquid crystal compositions were placed in a cell rubbed with a 2.5 μm organic alignment film and the alignment state was observed, the liquid crystal composition shown in Figure 1+al showed a very good alignment state;
In the liquid crystal composition shown in FIG. 1 (bl), the liquid crystal molecules lost their uniaxiality and a good alignment state could not be obtained.

FIG. 2 shows the dark value characteristics of these ferroelectric liquid crystal compositions. In FIG. 2, (01 indicates the applied waveform used when measuring the dark value characteristics, Vr indicates the reset pulse, ■t indicates the actual dark value characteristic evaluation pulse, and Vn indicates the non-selection pulse.
In Figure Tal, (b), ◯ indicates the maximum brightness immediately after application of the evaluation voltage. × indicates the brightness of the memory state after 1000 line scans after application of the evaluation voltage. Figure 2 fal is the first
The dark value characteristics of the liquid crystal composition shown in FIG.

On the other hand, although FIG. 2 (bl shows the dark value characteristics of the liquid crystal composition in FIG. 1 (bl), it shows good dark value characteristics like (al),
Indicates the dark value characteristic between twists. Moreover, a liquid crystal display device made using such a liquid crystal composition showed good display quality. In addition, a liquid crystal composition exhibiting such a pre-transition phenomenon has a spontaneous polarization of 20nC/cm2.
As described above, when the tilt angle was 20° or more, good alignment was easily obtained as in this example, and good alignment between uniforms was obtained.

Effects of the Invention As described above, the present invention provides an isotropic liquid with a phase series of -8mA-
It is a ferroelectric liquid crystal composition which is 4S m C* and exhibits a pre-transition phenomenon during the transition from isotropic liquid to SmA, and can easily obtain good alignment in a display device subjected to an organic film rubbing method. A ferroelectric liquid crystal display device using this ferroelectric liquid crystal composition exhibits uniform switching and exhibits high contrast and excellent display quality.

[Brief explanation of the drawing]

Figure 1 shows Sm from the isotropic liquid in Example 1 of the present invention.
FIG. 2 is a characteristic diagram of the dark value characteristic in Example 1 of the present invention, FIG. 3 is a configuration diagram of a ferroelectric liquid crystal cell, and FIG. Schematic diagram of dielectric liquid crystal, 5th
The figure is a schematic diagram showing the operating principle of a ferroelectric liquid crystal, FIG. 6 is a schematic diagram of a twisted state, and FIG. 7 is an explanatory diagram of switching between twists. 1... Polarizing plate, 2... Upper and lower glass substrates, 3... Transparent electrode, 4... Organic alignment film subjected to alignment treatment, 5... ...ferroelectric liquid crystal phase,
6... Seal material for adhering opposing substrates, 7... Ferroelectric liquid crystal molecules, 8... Spontaneous polarization, 9... C director -1) 0...
... Cone, 1) ... Layer, 12 ... Layer normal, 13 ... Tilt angle θ of the molecule with respect to the layer normal
, 14... Liquid crystal molecules whose long axis of the molecule is tilted by 10 degrees with respect to the layer normal, 15... Liquid crystal molecule whose long axis of the molecule is tilted by -θ with respect to the layer normal , 16...Dipole moment facing toward the back of the page, 17...Dipole moment pointing toward the back of the page, 18...
...Direction of the two polarizing plates, 19...Top substrate,
2゜・・・Lower substrate, 21・・・+θ tilted liquid crystal molecules, 22・・・−θ tilted liquid crystal molecules, −
23...Cone, 24...C director-125...Top board, 26...Bottom board, 27...Cone, 28...・・・・・・C
Director-0” Name of agent Patent attorney Toshio Nakao Figure 1 (a)
(b) Figure 2 Voltage Vt Figure 3 Voltage Vt Figure 3 Figure 4 I Figure 5

Claims (7)

[Claims] (1) It shows all phase series from the high temperature side: isotropic liquid, smectic A phase, and chiral smectic C phase, and is characterized by being accompanied by a pre-transition phenomenon at the time of transition from isotropic liquid to smectic A phase. Dielectric liquid crystal composition. (2) The ferroelectric liquid crystal composition according to claim (1), wherein the ferroelectric liquid crystal composition has a tilt angle of 20° or more. (3) Spontaneous polarization of ferroelectric liquid crystal composition is 20 nC/cm
2. The ferroelectric liquid crystal composition according to claim 1, wherein the ferroelectric liquid crystal composition is 2 or more. (4) A ferroelectric liquid crystal composition exhibiting all phase series from the high temperature side: isotropic liquid, smectic A phase, and chiral smectic C phase, and accompanied by a pre-transition phenomenon during the transition from the isotropic liquid to the smectic A phase. A ferroelectric liquid crystal display device characterized by using. (5) A ferroelectric liquid crystal composition having a tilt angle of 20° or more is used.
) A ferroelectric liquid crystal display device according to item 2. (6) A ferroelectric liquid crystal display device according to claim (4), characterized in that a ferroelectric liquid crystal composition having a spontaneous polarization of 20 nC/cm2 or more is used. (7) The ferroelectric liquid crystal according to any one of claims (4), (5), and (6), wherein the alignment treatment of the liquid crystal display device is performed by rubbing an organic alignment film. Display device.