JPH05216034A - Ferroelectric liquid crystal element - Google Patents

Abstract

PURPOSE:To improve the durability of a device by preventing the transfer of a liquid crystal and lessening the increase in the cell thickness at the ends of a cell. CONSTITUTION:This ferroelectric liquid crystal element has the ferroelectric liquid crystal 15 and a pair of substrates 11a, 11b which face each other by holding this ferroelectric liquid crystal 15 in-between, are formed with electrodes 12a, 12b for impressing a voltage to the ferroelectric liquid crystal on the respective opposite surfaces and are subjected to a uniaxial orientation treatment of the same direction in nearly parallel with each other for orienting the ferroelectric liquid crystal 15. The relation between the cone angle THETA of the ferroelectric liquid crystal 15 and the inclination angle delta of a chiral smectic layer has the orientation state expressed by delta/THETA>=0.80. In addition, the orientation state thereof exhibits at least two stable states and thetaa which is half the angle formed by these optical axes and the tilt angle theta of the ferroelectric liquid crystal have a relation theta/2<thetaa<theta.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a liquid crystal element having a cell structure suitable for a ferroelectric liquid crystal.

[0002]

2. Description of the Related Art A display device of the type in which transmitted light rays are controlled by a combination with a polarizing device utilizing the refractive index anisotropy of ferroelectric liquid crystal molecules has been proposed by Clark and Lagerwall. (JP-A-56-107216, U.S. Pat. No. 4,367,924).
No. etc.). This ferroelectric liquid crystal generally has a chiral smectic C phase (SmC ^* ) or H phase (SmH ^* ) in a specific temperature range, and in this state, it has a first optical stability in response to an applied electric field. State or the second optically stable state, and has the property of maintaining that state when no electric field is applied, that is, bistability, and has a quick response to changes in the electric field, and high speed. In addition, it is expected to be widely used as a memory type display element.

[0003]

However, in such a conventional ferroelectric liquid crystal cell, when it is continuously driven for a long time, the cell thickness at the cell edge gradually increases, and the cell appears to be colored yellow. The problem of coming is recognized.

An object of the present invention is to reduce an increase in cell thickness at a cell edge in a high contrast liquid crystal device using a ferroelectric liquid crystal.

[0005]

In order to achieve the above object, a ferroelectric liquid crystal device of the present invention is provided with a ferroelectric liquid crystal and a ferroelectric liquid crystal which are opposed to each other while holding the ferroelectric liquid crystal therebetween. Electrodes for applying a voltage to the ferroelectric liquid crystal are formed respectively, and a pair of substrates, which are substantially parallel to each other for orienting the ferroelectric liquid crystal and are uniaxially oriented in the same direction, are provided. The relationship between the cone angle Θ of the dielectric liquid crystal and the tilt angle δ of the chiral smectic layer is δ / Θ> 0.80,
Preferably, it has an orientation state represented by δ / Θ> 0.90, and the orientation state exhibits at least two stable states, and θ _a which is ½ of the angle formed by the optical axes thereof and the ferroelectric And the tilt angle θ of the liquid crystal has a relationship of θ / 2 <θ _a <θ.

Here, the pretilt angle of the ferroelectric liquid crystal is 1
It is preferably 0 ° or more. Further, it is preferable to provide an organic polymer alignment control film for controlling the alignment of the ferroelectric liquid crystal. Further, in the above-mentioned orientation state, when the temperature at which SmA is transformed into Sm ^* C is T ₁ ° C, the temperature is (T ₁ -5) ° C,
₁ -10) to ° C. or (T ₁ -20) ° C., more preferably up to (T ₁ -30) ° C., more preferably (T ₁
It exists in the whole temperature range up to -40 ° C.

[0007]

According to the research conducted by the present inventors, in a liquid crystal cell having such a ferroelectric liquid crystal, the liquid crystal itself moves in a specific direction between the liquid crystal cells by driving,
It has been observed that the pressure at the cell edges has increased, resulting in an increase in cell thickness. The cause of the force that the liquid crystal molecules move in the liquid crystal cell is unknown, but it is presumed that it is probably an electrodynamic effect caused by the fluctuation of the dipole moment of the liquid crystal molecules in the alternating electric field due to the driving pulse. .. Further, according to an experiment by the present inventors, FIG.
As shown in (A), the moving direction 22 of the liquid crystal is determined by the rubbing direction 20 and the average molecular axis directions 21, 21 'of the liquid crystal molecules. Since the moving direction of the liquid crystal molecules depends on the rubbing direction as described above, it is presumed that the phenomenon depends on the pretilt state at the substrate interface.
The average molecular axis directions 21 and 21 'indicate average molecular positions in the bistable state of the ferroelectric liquid crystal molecules. Here, for example, when an appropriate AC electric field that does not cause switching of the liquid crystal is applied in a state where the average molecular axis direction is 21,
Liquid crystal molecules move in the direction of arrow 22. However, here, a case where a liquid crystal material having a negative spontaneous polarization direction is used is described. Furthermore, this liquid crystal movement phenomenon depends on the alignment state of the cell as described below.

The orientation including the chevron structure of the smectic layer can be explained by two types of orientation models, C1 and C2. In FIG. 3, 31 is a smectic layer, 3
Reference numeral 2 represents a C1-oriented region, and 33 represents a C2-oriented region.
Smectic liquid crystals generally have a layered structure, but when the SA phase transitions to the SC phase or SC * phase, the layer spacing shrinks, so that the layers are bent at the center of the upper and lower substrates 14a and 14b (chevron structure) as shown in FIG. Take As shown in the figure, there can be two bending directions C1 and C2, but as is well known, the liquid crystal molecules at the substrate interface form an angle (pretilt) with the substrate by rubbing, and that direction is the rubbing direction. The direction is such that the liquid crystal molecules lift their heads toward A (the tip becomes floating). Due to this pretilt, the C1 orientation and the C2 orientation are not elastically energy-equivalent, and a transition occurs at a certain temperature as described above. Further, mechanical strain may cause dislocation. When the layered structure of FIG. 3 is viewed in a plan view, the boundary 34 when the C1 orientation shifts to the C2 orientation in the rubbing direction A is a zigzag lightning bolt-like lightning defect, and the boundary 35 when the C2 orientation shifts to C1. Has a wide, gentle curve and is called a hairpin defect.

The ferroelectric liquid crystal is provided with a pair of substrates that are substantially parallel to each other and are uniaxially aligned in the same direction. The ferroelectric liquid crystal has a pretilt angle of α and a tilt angle (cone). (1/2 of the angle) is θ, and the tilt angle of the Sm * C layer is δ, and when the orientation state represented by the formula 1 is provided, there are four states having a chevron structure in the C1 orientation state. To do.

[0010]

[Equation 1] These four C1 orientation states are different from the conventional C1 orientation states, and among them, two of the four C1 orientation states form a bistable state (uniform state). Here, assuming that the apparent tilt angle when there is no electric field is θ _a , of the four states in the C1 orientation state, the state showing the relationship of the equation 2 is called the uniform state.

[0011]

[Equation 2] In the uniform state, it is considered that the director is not twisted between the upper and lower substrates in view of its optical properties. FIG. 4A is a schematic view showing the arrangement of directors at respective positions between the substrates in each state of C1 orientation. 5 in the figure
1 to 54 show the state in which the director is projected on the bottom surface of the cone in each state and viewed from the bottom surface direction.
1 and 52 are splayed states, and 53 and 54 are director arrangements considered to be uniform states. As can be seen from the figure, in the two states 53 and 54 of the uniform, the positions of the liquid crystal molecules on either the upper or lower substrate interface are replaced with the positions in the splay state. FIG. 4B shows the C2 orientation, and there are two states 55 and 56 due to internal switching without interface switching. The uniform state of the C1 orientation produces _a larger tilt angle θ _a than the conventionally used bistable state of the C2 orientation, and has a large luminance and a high contrast.

In the actual liquid crystal cell, if the liquid crystal molecule position is in the state shown by the arrow 21 in the whole cell as shown in FIG. The liquid crystal moves from right to left. As a result, as shown in FIG. 2 (B), the cell thickness of the region 23 becomes thicker with time, and coloring occurs. When the liquid crystal molecules are in the state shown by the arrow 21 ', the movement direction under the AC electric field is opposite, but in any case, the movement of the liquid crystal in the direction perpendicular to the rubbing direction 20, that is, in the smectic layer. Occurs. Such liquid crystal movement is
It is obvious that the durability of the liquid crystal element in continuous driving for a long time is adversely affected.

However, in the present invention, the relationship between the cone angle Θ of the ferroelectric liquid crystal and the tilt angle δ of the chiral smectic layer is δ / Θ ≧ 0.80, preferably δ / Θ> 0.90.
Therefore, as shown in the following examples, the movement of the liquid crystal is prevented and the durability in continuous driving for a long time is improved.

[0014]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a ferroelectric liquid crystal (FL) according to an embodiment of the present invention.
It is sectional drawing which shows the liquid crystal cell of a C) element typically. As shown in the figure, this liquid crystal cell has an upper substrate 1 arranged in parallel.
1a and a lower substrate 11b, and transparent electrodes 12a and 12b formed on the opposing surfaces of these substrates and having a thickness of, for example, about 400 to 3000 Å. Between the upper substrate 11a and the lower substrate 11b, a ferroelectric liquid crystal 1 having a non-helical structure is provided.
5 are arranged. On the transparent electrodes 12a and 12b,
Insulating films 13a and 13b having a thickness of 100 to 3000 Å are formed. As this insulating film, coating,
Baking type inorganic oxide, sputtered film, etc. can be used. Further, a multilayer film having a two-layer structure or more may be used. Alignment control films 14a and 14b are formed on the insulating films 13a and 13b.
It is formed with a film thickness of 0 to 1000Å. As a material for the alignment control film, a polymer is usually used, but in the device of the present invention, an alignment control film that gives a high pretilt angle such as a fluorine-containing polyimide is preferable.

As the ferroelectric liquid crystal 15, one having a chiral smectic phase state can be used, and specifically, a chiral smectic C phase (SmC ^* ) and an H phase (SmH).
^* ), I-phase (SmI ^* ) and G-phase (SmG ^* ) liquid crystals can be used. As a particularly preferable ferroelectric liquid crystal, a liquid crystal exhibiting a cholesteric phase on the higher temperature side can be used, and for example, a pyrimidine-based mixed liquid crystal exhibiting the phase transition temperature shown in Chemical formula 1 can be used.

[0016]

[Chemical 1] The relationship between the cone angle Θ of the ferroelectric liquid crystal 15 and the tilt angle δ of the chiral smectic layer has an orientation state represented by δ / Θ ≧ 0.80, preferably δ / Θ> 0.90, and The orientation state exhibits at least two stable states, and θ _a , which is ½ of the angle formed by the optical axes thereof, and the tilt angle θ of the ferroelectric liquid crystal have a relation of θ / 2 <θ _a <θ. The tilt angle θ, the apparent tilt angle θ _a , the layer tilt angle δ, and the pretilt angle α of the ferroelectric liquid crystal can be measured as follows.

Measurement of Tilt Angle θ While applying DC of 10 to 30 V between the upper and lower substrates of the FLC element, the FLC element arranged between them is horizontally rotated with the polarizing plate under the orthogonal crossed Nicols to make the first extinction. Position (the position where the transmittance is the lowest) is searched for, and then the second extinction position is searched for while applying DC of the opposite polarity to the above. 1 / the angle from the first extinction position to the second extinction position at this time
Let 2 be the tilt angle θ.

Measurement of apparent tilt angle θ _a A single pulse having a threshold value of the liquid crystal was applied, and thereafter, the FLC element disposed between them was rotated horizontally with the polarizing plate under no electric field and under orthogonal crossed Nicols. Then, the first extinction position is searched for, then a pulse having a polarity opposite to that of the above-mentioned single pulse is applied, and then the second extinction position is searched under no electric field. The apparent tilt angle θ _a is ½ of the angle from the first extinction position to the second extinction position at this time.

Measurement of layer inclination angle δ The layer inclination angle δ is measured by an X-ray analysis method using an X-ray analyzer RAD-IIB (45 KV, 30 mA).

Measurement of pretilt angle α “Jpn. J. Appl. Phys. Vol19 (1980) NO. 10, Short No.
tes 2013 ”and obtain according to the method (crystal rotation method). That is, substrates rubbed in parallel and in opposite directions are bonded to each other to form a cell having a cell thickness of 20 μm, liquid crystal having an SmA phase is enclosed in the cell in the range of 0 to 60 ° C., and this liquid crystal cell is used as upper and lower substrates. While rotating on a plane that is vertical and includes the alignment treatment axis,
Helium-neon laser light with a polarization plane that makes an angle of ° is irradiated from a direction perpendicular to the rotation axis, and the intensity of the transmitted light is increased by a photodiode through a polarizing plate that has a transmission axis parallel to the incident polarization plane on the opposite side. taking measurement. The pretilt angle α can be calculated by the formula 1 below, where φ _X is the angle formed by the center line of the hyperbola group of transmitted light intensity caused by interference and the line perpendicular to the liquid crystal cell.

[0021]

[Equation 1] Examples 1 to 5 and Comparative Examples 1 to 2 Two glass plates having a thickness of 1.1 mm provided with an ITO film having a thickness of 1000 Å were prepared, and a coating type insulating layer (Ti-Si = 1) was provided thereon. : 1; manufactured by Tokyo Ohka Co., Ltd., and applied 3
By firing at 00 ° C., an insulating film having a thickness of 1200 Å was formed.

Next, a fluorine-containing polyamic acid was printed on this insulating film as a 4% solution of NMP and n-butyl cellosolve, and then baked at 270 ° C. to 200Å.
Of the film thickness.

Next, these two substrates were rubbed with a nylon cloth in parallel and in the same direction, and then they were rubbed with 1.2.
The liquid crystal cell as described above was prepared by facing each other using silica beads of μm.

Next, the liquid crystal cells thus produced were respectively subjected to Examples 1 to 5 and Comparative Examples 1 to 3 in Table 1.
The orientation of the entire liquid crystal cell is shown in FIG. 2 by injecting a ferroelectric liquid crystal composition having the cone angle Θ, the inclination angle δ of the chiral smectic layer, the ratio δ / Θ and the dielectric polarization P _S shown in FIG.
A rectangular wave having a pulse width Δt = 25 μs, a voltage amplitude V _PP = 40 V, and a ½ duty is applied for about 7 hours and aligned in the average molecular axis direction 21 shown in FIG.
In addition to measuring the cell thickness in the region 23 shown in (1), the liquid crystal cell was arranged between the crossed Nicols polarizing plates, and the coloring was visually observed.

[0025]

[Table 1] FIG. 5 is a graph showing the change in cell thickness with respect to δ / Θ obtained from the measurement results. Further, according to visual observation, in the cases of Examples 2 and 3 where δ / Θ> 0.90, no coloring was observed and no region where the cell thickness was increased was recognized at all. Further, in the case of Examples 1, 4 and 5 where δ / Θ ≧ 0.80, a slightly yellow area was observed, but it was hardly noticeable.

On the other hand, in the case of Comparative Examples 1 to 3 in which δ / Θ <0.80, the area changed to yellow due to the increased cell thickness is clearly recognized, and the image quality is deteriorated. Was confirmed.

[0027]

As described above, according to the present invention, the relationship between the cone angle Θ of the ferroelectric liquid crystal and the tilt angle δ of the chiral smectic layer is δ / Θ ≧ 0.80, preferably δ / Θ.
Since the alignment state represented by> 0.90 is provided, the movement of the liquid crystal can be prevented and the durability of the device can be improved.

[Brief description of drawings]

FIG. 1 shows a ferroelectric liquid crystal (FL) according to an embodiment of the present invention.
It is sectional drawing which shows the liquid crystal cell of a C) element typically.

FIG. 2 is an explanatory diagram showing how liquid crystal moves.

FIG. 3 is an explanatory diagram showing a layer structure of C1 orientation and C2 orientation.

FIG. 4 is a schematic diagram showing an arrangement of directors at respective positions between substrates in respective states of C1 orientation and C2 orientation.

FIG. 5 is a graph showing changes in cell thickness with respect to δ / Θ in Examples and Comparative Examples.

[Explanation of symbols]

11a, 14a: upper substrate, 11b, 14b: lower substrate, 1
2a, 12b: transparent electrode, 15: ferroelectric liquid crystal, 12
a, 12b: transparent electrode, 13a, 13b: insulating film, 14
a, 14b: orientation control film, 17a, 17b: polarizing plate, 2
0: rubbing direction, 22: liquid crystal moving direction, 23:31:
Smectic layer, 32: C1 oriented region, 33: C2 oriented region, 51, 52: splayed director arrangement, 53, 54: uniformed director arrangement

Claims (8)

[Claims] 1. A ferroelectric liquid crystal and a ferroelectric liquid crystal which face each other with the ferroelectric liquid crystal held therebetween, and electrodes for applying a voltage to the ferroelectric liquid crystal are formed on the opposing surfaces, respectively. A pair of substrates that are substantially parallel to each other and are subjected to uniaxial alignment treatment in the same direction for aligning the liquid crystal, and the relationship between the cone angle Θ of the ferroelectric liquid crystal and the tilt angle δ of the chiral smectic layer, θ, which has an orientation state represented by δ / Θ ≧ 0.80, exhibits at least two stable states, and is 1/2 of an angle formed by the optical axes thereof.
_A ferroelectric liquid crystal device, wherein _a and the tilt angle θ of the ferroelectric liquid crystal have a relationship of θ / 2 <θ _a <θ. 2. The relationship between the cone angle Θ and the tilt angle δ is δ / Θ> 0.90.
The ferroelectric liquid crystal device described. 3. The pretilt angle of the ferroelectric liquid crystal is 10
3. The ferroelectric liquid crystal element according to claim 1, wherein the ferroelectric liquid crystal element has an angle of at least °. 4. The ferroelectric liquid crystal device according to claim 2, further comprising an alignment control film of an organic polymer for controlling the alignment of the ferroelectric liquid crystal. Wherein said orientation state is a temperature at which the transition to the Sm ^* C When T ₁ ℃ (T ₁ -5) from ° C. from SmA (T ₁
It exists in the whole temperature range up to -10) ° C.
3. The ferroelectric liquid crystal device according to any one of 3 above. Wherein said orientation state is a temperature at which the transition to the Sm ^* C When T ₁ ℃ (T ₁ -5) from ° C. from SmA (T ₁
It exists in the whole temperature range up to -20) ° C.
3. The ferroelectric liquid crystal device according to any one of 3 above. 7. When the temperature at which the above-mentioned orientation state transitions from SmA to Sm ^* C is T ₁ ° C., (T ₁ −5) ° C. to (T ₁
It exists in the whole temperature range up to -30) ° C.
3. The ferroelectric liquid crystal device according to any one of 3 above. Wherein said orientation state is a temperature at which the transition to the Sm ^* C When T ₁ ℃ (T ₁ -5) from ° C. from SmA (T ₁
It exists in the whole temperature range up to -40) ° C.
3. The ferroelectric liquid crystal device according to any one of 3 above.