JP2675893B2 - Liquid crystal element - Google Patents

Description

TECHNICAL FIELD The present invention relates to a liquid crystal element used in a liquid crystal display element, a liquid crystal-optical shutter or the like, particularly a ferroelectric liquid crystal element, and more specifically, to improve the alignment state of liquid crystal molecules. Thus, the present invention relates to a liquid crystal element having improved display characteristics.

(Prior art)

A display element of the type in which transmitted light rays are controlled by combining with a polarizing element by 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, etc.). This ferroelectric liquid crystal generally has a non-helical chiral smectic C phase (SmC ^* ) or H phase (SmH ^* ) in a specific temperature range, and in this state, it has a first phase in response to an applied electric field. One of the second optical stable state and the optical stable state of
And the property of maintaining the state when no electric field is applied,
In other words, it has bistability and quick response to changes in the electric field, and is expected to be widely used as a high-speed and storage-type display element. Is expected.

In order for the optical modulation element using the liquid crystal having the bistability to exhibit a predetermined driving characteristic, the liquid crystal arranged between the pair of parallel substrates has the two stable states regardless of the electric field application state. It is necessary to be in a state of molecular alignment such that conversion between states can occur effectively later.

In the case of a liquid crystal element utilizing birefringence of liquid crystal, the transmittance under crossed Nicols is [Wherein, I ₀ : incident light intensity, I: transmitted light intensity, θ: tilt angle, Δn: refractive index anisotropy, d: thickness of liquid crystal layer, and λ: wavelength of incident light. The tilt θ in the non-helical structure described above appears as an angle in the average molecular axis direction of the twisted liquid crystal molecules in the first and second alignment states. According to the above equation, it is necessary that the tilt angle θ in the non-helical structure that realizes the bistability becomes the maximum transmittance when the tilt θ is at an angle of 22.5 ° and is as close as possible to 22.5 °.

By the way, as a method of orienting a ferroelectric liquid crystal, a molecular layer composed of a plurality of molecules forming a smectic liquid crystal can be uniaxially oriented along its normal line over a large area, and a manufacturing process It is desirable that the process can be realized by a simple rubbing process.

For example, US Pat. No. 4,561,726 is known as an alignment direction for a ferroelectric liquid crystal, especially for a chiral smectic liquid crystal having a non-helical structure.

However, when the alignment direction used up to now, especially the alignment method using a rubbing-treated polyimide film, is applied to the ferroelectric liquid crystal of the non-helical structure exhibiting the bistability, which was announced by Clark and Lagauall. Had the following problems.

That is, according to the experiments by the present inventors, the tilt angle in the ferroelectric liquid crystal having the non-helical structure obtained by aligning the film with the conventional rubbing-treated polyimide film is the tilt angle in the ferroelectric liquid crystal having the spiral structure. It turned out to be smaller than the corners. (In particular, the tilt angle θ of a non-helical ferroelectric liquid crystal obtained by aligning with a conventional rubbing-treated polyimide film is generally about 3 ° to 8 °,
The transmittance at that time was about 3 to 5% at most. Thus, according to Clark and Ragall, the tilt angle of a non-helical ferroelectric liquid crystal that realizes bistability should be the same as the tilt angle of a ferroelectric liquid crystal having a helical structure. However, in practice, the tilt angle θ in the non-helical structure is smaller than the tilt angle in the helical structure. Moreover, it has been found that the cause of the tilt angle θ in the non-helical structure being smaller than the tilt angle in the helical structure is due to the twisted arrangement of the liquid crystal molecules in the non-helical structure. In other words, in a ferroelectric liquid crystal with a non-helical structure, the liquid crystal molecules are continuous with respect to the normal line of the substrate in the axis of the liquid crystal molecules adjacent to the lower substrate (direction of twist alignment) than the axis of the liquid crystal molecules adjacent to the upper substrate. Are arranged with a twist angle δ, which causes the tilt angle θ in the non-helical structure to be smaller than the tilt angle in the spiral structure.

In addition, the alignment state of the chiral smectic liquid crystal generated by the conventional rubbing-treated polyimide alignment film is the first optically stable state (for example, due to the presence of the polyimide alignment film as an insulator between the electrode and the liquid crystal layer). , A white display state) to a second optically stable state (for example, a black display state), when a one-polarity voltage is applied to the ferroelectric liquid crystal layer after the application of the one-polarity voltage is released. On the other hand, a reverse electric field V _rev of the other polarity was generated, and this reverse electric field V _rev caused an afterimage at the time of display. The above-mentioned reverse electric field generation phenomenon is clarified in, for example, "Switching Characteristics of SSFLC", written by Akio Yoshida, October 1987, "Transactions of the Liquid Crystal Symposium", pp. 142-143.

[Object of the invention]

Therefore, an object of the present invention is to provide a ferroelectric liquid crystal device that solves the above-mentioned problems, and particularly to generate a large tilt θ in the non-helical structure of a chiral smectic liquid crystal and display a high-contrast image. Another object of the present invention is to provide a ferroelectric liquid crystal element capable of achieving a display without causing an afterimage.

[Means and actions for achieving the object]

Therefore, the present invention provides a pair of substrates having a polyimide film represented by the following general formula (I) on at least one of the substrates and a liquid crystal element having a ferroelectric liquid crystal disposed between the pair of substrates.

(The above R ¹ , R ² , R ³ and R ⁴ each represent H or CF _3. However, at least one is a CF ₃ group. Further, part represents a tetravalent organic residue.) Is a schematic drawing of an example of the ferroelectric liquid crystal cell of the present invention.

11a and 11b are In ₂ O ₃ and ITO (Indium Tin Oxide) respectively
(Glass plate) covered with transparent electrodes 12a and 12b, etc., and insulating films 13a and 13b (SiO 2
₂ film, TiO ₂ film, Ta ₂ O ₅ etc.) and the above polyimide
Alignment control films 14a and 14b each having a thickness of 50 Å to 1000 Å are laminated.

At this time, they are parallel and in the same direction (direction A in FIG. 1).
Alignment control 14a with rubbing treatment (arrow direction)
And 14b are arranged. A ferroelectric smectic liquid crystal 15 is disposed between the substrates 11a and 11b, and the distance between the substrates 11a and 11b is sufficient to suppress the formation of a helical array structure of the ferroelectric smectic liquid crystal 15. When the distance is set to a small distance (for example, 0.1 μm to 3 μm), the ferroelectric smectic liquid crystal 15 has a bistable alignment state. The above-described sufficiently small distance is held by bead spacers 16 (silica beads, alumina beads) arranged between the substrates 11a and 11b.

According to the experiments conducted by the present inventors, by using the alignment direction of the specific polyimide alignment film subjected to the rubbing treatment which will be clarified in the examples described below, a large optical contrast in the bright state and the dark state is exhibited, and particularly, , U.S. Pat.
No. 561, etc., achieves high contrast for non-selected pixels during multiplexing drive, and achieves an alignment state that does not cause optical response delay during switching (during multiplexing drive) that causes afterimages during display Was done.

The polyimide film used in the present invention is (R ¹ , R ² , R ³ and R ⁴ are as described above) and the polyamic acid synthesized by subjecting a diamine and a tetracarboxylic acid anhydride to a condensation reaction can be ring-closed by heating.

Examples of the tetracarboxylic acid anhydride include pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, biphenyltetracarboxylic dianhydride, naphthalenetetracarboxylic dianhydride, 2,2-bis (dicarboxyphenic acid). (Enyl) propane dianhydride, bis (dicarboxyphenyl)
Sulfone dianhydride and bis (dicarboxyphenyl) ether dianhydride can be used.

The diamine used in preparing the polyamic acid used in the present invention is used in an amount of 0.1 to 10 parts by weight, preferably 1 part by weight, relative to 1 part by weight of the carboxylic acid anhydride.

The polyimide of the present invention may have an average molecular weight of 1 to 100,000, preferably 30,000 to 70,000, and more preferably 50,000.

When providing the polyimide film used in the present invention on the substrate, the polyamic acid precursor of the polyimide is dissolved in a solvent such as dimethylformamide, dimethylacetamide, dimethylsulfoxide, N-methylpyrrolidone 0.01 to 40. (% By weight) After applying the solution on a substrate by a spinner coating method, a spray coating method, a roll coating method, or the like, the solution is heated at a temperature of 100 to 350 ° C., preferably 200 to 300 ° C. to be dehydrated and closed. To form a polyimide film. This polyimide film is then rubbed with a cloth or the like. The thickness of the polyimide film used in the present invention is set to about 30 to 1 μm, and preferably 200 to 2000 μm. At this time, the insulating films 13a and 13a shown in FIG.
The use of 3b can be omitted. In the present invention, when a polyimide film is provided on the insulating films 13a and 13b, the thickness of the polyimide film can be set to 200 mm or less, preferably 100 mm or less.

As the liquid crystal substance used in the present invention, a liquid crystal that generates a chiral smectic C phase through an isotropic phase, a cholesteric phase, and a smectic A phase in a temperature decreasing process is preferable. Especially,
The pitch in the cholesteric phase is preferably 0.8 μm or more (the pitch in the cholesteric phase is measured at the center point in the temperature range of the cholesteric phase). Specific liquid crystals include the liquid crystal substances “LC-1” and “80
A liquid crystal composition containing "B" and "80SI ^* " in the following ratio is preferably used.

Liquid crystal (1) (LC-1) ₉₀ / (80B) ₁₀ (2) (LC-1) ₈₀ / (80B) ₂₀ (3) (LC-1) ₇₀ / (80B) ₃₀ (4) (LC-1) ₆₀ / (80B) ₄₀ (5) 80SI ^* (Subscripts in the table represent weight ratios, respectively.) FIG. 2 schematically shows an example of a cell for explaining the operation of a ferroelectric liquid crystal. It is drawn in. 21a and 21b are In ₂ O ₂ , S
A substrate (glass plate) covered with a transparent electrode composed of a thin film of nO ₂ or ITO, in which the SmC ^* (chiral smectic C) phase in which the liquid crystal molecular layer 22 is oriented perpendicular to the glass surface Or SmH ^* (Chiral smectic H)
Phase liquid crystals are enclosed. A bold line 23 represents a liquid crystal molecule, and the liquid crystal molecule 23 has a dipole moment (P⊥) 24 in a direction perpendicular to the molecule. When a voltage above a certain threshold is applied between the electrodes on the substrates 21a and 21b, the helical structure of the liquid crystal molecules 23 is unraveled, and all the dipole moments (P⊥) 24 are oriented in the electric field direction.
23 can change the orientation direction. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the major axis direction and the minor axis direction. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, the voltage application polarity It can be easily understood that the liquid crystal optical modulation element whose optical characteristics change depending on the liquid crystal.

The surface stable ferroelectric liquid crystal cell in a bistable alignment state used in the liquid crystal element of the present invention can be made sufficiently thin (for example, 0.1 μm to 3 μm). As shown in FIG. 3, as the liquid crystal layer becomes thinner, the helical structure of the liquid crystal molecules is unwound and becomes a non-helical structure even when no electric field is applied, and the dipole moment P or P 'thereof is upward ( 34a) or downward (34b). When an electric field Ea or Eb having a polarity equal to or higher than a certain threshold is applied to such a cell by the voltage applying means 31a and 31b, the dipole moment corresponds to the electric field vector of the electric field Ea or Eb. And the liquid crystal molecules are oriented in either the stable state 33a or the second stable state 33b of FIG. 1 accordingly.

The first effect obtained by the ferroelectric liquid crystal cell is that the response speed is extremely fast, and the second effect is that the orientation of the liquid crystal molecules has bistability. The second point will be further described with reference to, for example, FIG. 3. When an electric field Ea is applied, the liquid crystal molecules are aligned in the first stable state 33a.
This state is stable even when the electric field is turned off. When a reverse electric field Eb is applied, the liquid crystal molecules are abandoned in the second stable state 33b to change the direction of the molecules, but they remain in this state even when the electric field is cut off. As long as the applied electric field Ea does not exceed a certain threshold value, the respective alignment states are also maintained.

FIG. 4A is a cross-sectional view schematically illustrating an alignment state of liquid crystal molecules generated in an alignment direction according to the present invention, and FIG. 4 is a diagram illustrating a C-director thereof.

Reference numerals 61a and 61b shown in FIG. 4A represent an upper substrate and a lower substrate, respectively. Numeral 60 denotes a molecular layer organized by liquid crystal molecules 62, and the liquid crystal molecules 62 are arranged by changing the position along the bottom surface 64 (circle) of the cone 63. 66a and 66b are directions of rotation with respect to the adjacent substrates 61a and 61b of the bent structure of the molecular layer 60 which has a bent structure, respectively.
66a and 66b are floating rotation directions of the liquid crystal molecules 62 adjacent to the adjacent substrates 61a and 61b, respectively.

FIG. 4B is a diagram showing a C-director. Figure 4 in U ₁ is C- director 81 in one stable orientation state of (B), U ₂ is C- directors 81 in the other stable orientation state
It is. The C-director 81 is a projection of the molecular long axis onto a virtual plane perpendicular to the normal of the molecular layer 60 shown in FIG. 4 (A).

On the other hand, the orientation state produced by the conventional rubbed polyimide film is shown by the C-director diagram in FIG. 4 (C). In the orientation state shown in FIG. 4C, the tilt of the molecular axis is large from the upper substrate 61a to the lower substrate 61b, so that the tilt angle θ is small.

FIG. 5A is a plan view showing the tilt angle θ of the C-director 81 in the state of FIG. 4B (referred to as a uniform alignment state), and FIG. 5B is a plan view of the C-director. 81
Is a plan view showing a tilt angle θ in the state of FIG. 4C (referred to as a splay alignment state). In the figure, 50 denotes the rubbing axis was subjected to a particular polyimide film of the present invention described above, the average molecular axis in 51a the orientation state U _1, 51b denotes an average molecular axis in the orientation state U _2, 52a are aligned state the average molecular axis in the S _1, 52 b denotes an average molecular axis in the orientation state S _2. The average molecular axes 51a and 51b can be converted by the ignition of opposite polarity voltages that exceed each other's threshold voltage. The same occurs between the average molecular axes 52a and 52b.

Next, the usefulness of the uniform alignment state for the optical response (afterimage) due to the reverse electric field V _rev will be described.

Capacitance C _i of the insulating layer of the liquid crystal cell (alignment layer), the capacitance of the liquid crystal layer to the C _LC and _the spontaneous polarization of the liquid crystal and Ps, V _rev causing after-image is expressed by the following equation.

FIG. 6 is a cross-sectional view schematically showing the distribution of charges in a liquid crystal cell, the direction of Ps, and the direction of a reverse electric field. FIG. 6 (A) shows the distribution state of the electric charge under the memory state before the application of the pulse electric field, and the direction of the spontaneous polarization Ps at this time is from the electric charge to the electric charge. FIG. 6 (B)
Is that the direction of the spontaneous polarization Ps immediately after the release of the pulse electric field is opposite to the direction in FIG. 6A (therefore, the liquid crystal molecules are inverted from one stable alignment state to the other stable alignment state). ), And the distribution state of the charge is the sixth
Since it is the same as in the case of FIG. (A), the reverse electric field V _rev ,
Occurs in the direction of the arrow. After a while, the reverse electric field V _rev disappears as shown in FIG. 6C, and the distribution state of the charge changes.

FIG. 7 shows a change in optical response in a splay alignment state caused by a conventional polyimide alignment film, instead of a change in tilt angle θ. According to Figure 7, when the pulse electric field is applied, the average molecular axis S under splay alignment state (A) to an average molecular axis U ₂ under Yunifuomu alignment state in the vicinity of the maximum tilt angle along the direction of Shirushishirube X ₁ over-sheet Ute, immediately after pulsed electric field cancellation is working action of the reverse electric field V _rev as shown in FIG. 6 (B), Yashirube X ₂ average molecular axis under splay alignment state along the direction S (B ) tilt angle θ is reduced to, and by the action of attenuation of the reverse electric field V _rev as shown in FIG. 6 (C), to an average molecular axis S under splay alignment state along the direction of Yashirube X ₃ (C) A stable alignment state in which the tilt angle θ is slightly increased can be obtained. The optical response at this time is clarified in FIG.

According to the present invention, in the alignment state obtained by the alignment direction using the above-mentioned fluorine atom-containing polyimide film, the seventh state is obtained.
Average molecular axis S (A), S under the spray condition shown in the figure
(B) and S (C) do not occur, and thus can be arranged on the average molecular axis that produces a tilt angle θ close to the maximum tilt angle. FIG. 9 shows the optical response of the present invention at this time. According to FIG. 9, it can be seen that the optical response caused by the afterimage is not delayed, and that high contrast is caused in the memory state.

 Hereinafter, the present invention will be described with reference to examples.

Example 1 Two 1.1 mm-thick glass plates provided with a 1000Å-thick ITO film were prepared, and N-methylpyrotidone / n-butyl cellosolve of amic acid represented by the following formula: 5 /
3.0 wt% solution of 1 at 3000 rpm After film formation, heat treatment was performed at 250 ° C. for about 1 hour (molecular weight 50,000). The film thickness at this time was 450 °. The coated film was subjected to a unidirectional rubbing treatment with a nylon blanket.

Then, sprinkle alumina beads with an average particle size of about 1.5 μm on one glass plate, and then stack the two glass plates so that the rubbing treatment axes are parallel to each other and are in the same treatment direction. It was made.

The cell name "CS-1014", which is a ferroelectric smectic liquid crystal manufactured by Chitso Corp., is vacuum-injected into the cell in the isotropic phase, and then gradually increased from the isotropic phase to 30 ° C at 0.5 ° C / h. It was possible to orient by cooling. The phase change in the cell of this example using "CS-1014" was as follows.

(Iso = isotropic phase Ch = cholesteric phase SmA = smectic A phase SmC ^* = chiral smectic C phase) After sandwiching the above liquid crystal cell between a pair of 90 ° crossed Nicol polarizers, a 30 V pulse of 50 μsec is applied. After applying
Set the 90 ° crossed Nicols to the extinction position (darkest state), measure the transmittance at this time with a photomultiplier, and then apply a -30 V pulse of 50 μsec, and check the transmittance (bright state) at this time. When measured by the same method, the tilt angle θ was 15
And the transmittance in the darkest state was 1%, the transparency in the bright state was 30%, and the contrast ratio was 30: 1.

The delay in the optical response that caused the afterimage was less than 0.2 seconds.

When this liquid crystal cell was displayed by multiplexing drive using the drive waveform shown in Fig. 10, a high-contrast, high-quality display was obtained, and the entire screen was in a white state after the image was displayed by inputting a predetermined character. After erasing, the afterimage reproduction was unreadable. In addition, S _N , S _{N + 1} , S in FIG.
_{N + 2} represents a voltage waveform applied to the scanning line, and I represents a voltage waveform applied to a typical information line.
(I-S _N ) is a composite waveform applied to the intersection of the information line I and the scanning line _SN . Further, in this embodiment, V ₀ = 5V to 8V, Δ
T = 20 μsec to 70 μsec.

Examples 2 to 5 A cell was obtained in the same manner as in Example 1 except that the alignment control film shown in Table 1 (the molecular weight of the polyimide film was 50,000 in each example) and the liquid crystal material were used.

 The same test as in Example 1 was performed for each.

Table 2 shows the results of the contrast ratio and the blistering time of the optical response.

Further, when a display was performed by the same multiplexing driving as in Example 1, the same results as in Example were obtained with respect to contrast and afterimage.

Table 2 Examples Contrast ratio Dependence of optical response (sec) 2 25: 1 0.2 3 31: 1 0.3 4 29: 1 0.1 5 24: 1 0.1 Comparative Examples 1 to 4 The alignment control films and liquid crystal materials shown in Table 3 were used. A cell was prepared in exactly the same manner as in Example 1 except that it was used.

Table 4 shows the contrast ratio and the optical response of each cell.

Further, when the display was performed by the multiplexing drive similar to that of the first embodiment, the contrast was smaller than that of the first embodiment, and an afterimage occurred.

Table 4 Comparative example Contrast ratio Optical response blemish (sec) 1 8: 1 1.5 2 7: 1 2.5 3 10: 1 1.2 4 8: 1 2.2 [Effect of the invention] As shown in the above examples and comparative examples. In addition, according to the present invention, the contrast in the bright state and the dark state is high, the display contrast is very large especially in the case of multiplexing driving, and a high-quality display can be obtained, and there is an effect that a noticeable afterimage phenomenon does not occur.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a sectional view of a liquid crystal element of the present invention. FIG. 2 is a perspective view showing an alignment state of a chiral smectic liquid crystal having a helical structure, and FIG. 3 is a perspective view showing an alignment state of a chiral smectic liquid crystal having a non-helical structure molecular arrangement. . FIG. 4 (A) is a cross-sectional view showing an alignment state of a chiral smectic liquid crystal aligned by the alignment method of the present invention.
FIG. 4 (B) shows C- in the uniform orientation state.
FIG. 4 (C) is a C-director diagram in a splay alignment state. FIG. 5A is a plan view showing the tilt angle θ in the uniform alignment state.
FIG. 2B is a plan view showing the tilt angle θ in the splay alignment state. 6 (A), (B) and (C) show the charge distribution in the ferroelectric liquid crystal, the direction of spontaneous polarization Ps and the reverse electric field V
It is sectional drawing which shows the direction of _rev . FIG. 7 is a plan view showing a change in the tilt angle θ during and after application of an electric field. FIG. 8 shows an optical response characteristic in a conventional example, and FIG. 9 shows an optical response characteristic in an example of the present invention. FIG. 10 is a waveform diagram of the drive voltage used in this embodiment.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 62-13436 (JP, A) JP 62-28713 (JP, A) JP 62-146984 (JP, A)

Claims (8)

(57) [Claims] 1. A substrate having at least one of the following general formulas (I):
A liquid crystal device having a pair of substrates having a polyimide film obtained by polymerizing the unit represented by and a chiral smectic liquid crystal disposed between the pair of substrates. (R ¹ , R ² , R ³ and R ⁴ each represent H or CF _3. However, at least one is a CF ₃ group. Further, part represents a tetravalent organic residue.) 2. A layer structure in which a pair of substrates having a polyimide coating film obtained by polymerizing a unit represented by the following general formula (I) on at least one substrate and a plurality of layers organized by a plurality of liquid crystal molecules are formed. And an arrangement structure in which the formation of an inherent helical arrangement structure is suppressed, and the layered structure has a bent structure, and the rotation direction of the bent structure with respect to the adjacent substrate rises up of the liquid crystal molecules adjacent to the adjacent substrate. A liquid crystal device having a chiral smectic liquid crystal formed in the same direction as the rotation direction. (R ¹ , R ² , R ³ and R ⁴ each represent H or CF _3. However, at least one is a CF ₃ group. Further, part represents a tetravalent organic residue.) 3. The liquid crystal device according to claim 2, wherein the polyimide film is unidirectionally rubbed. 4. A pair of substrates having a polyimide coating film obtained by polymerizing a unit represented by the following general formula (I) on both substrates, and a layer structure having a plurality of layers organized by a plurality of liquid crystal molecules. The layered structure has a bent structure, and the rotation direction of the bent structure with respect to the adjacent substrate is a floating rotation of liquid crystal molecules adjacent to the adjacent substrate. A liquid crystal element having a chiral smectic liquid crystal formed in the same direction as the direction. (R ¹ , R ² , R ³ and R ⁴ each represent H or CF _3. However, at least one is a CF ₃ group. Further, part represents a tetravalent organic residue.) 5. The liquid crystal device according to claim 4, wherein the polyimide coating is unidirectionally rubbed. 6. A liquid crystal device according to claim 4, wherein the polyimide coatings provided on the pair of substrates are subjected to a unidirectional lapping treatment in parallel with each other. 7. The chiral smectic liquid crystal has a temperature range in which a smectic A phase is generated on a higher temperature side than a temperature range of the chiral smectic C phase, and is cooled to the chiral smectic C phase via the temperature range of the smectic A phase. The liquid crystal element according to claim 4, which is a liquid crystal formed by: 8. The chiral smectic liquid crystal has a temperature range in which a smectic A phase and a cholesteric phase are generated on the higher temperature side than the temperature range of the chiral smectic C phase, and the chiral is passed through the temperature range of the cholesteric phase and the smectic A phase. The liquid crystal device according to claim 4, which is a liquid crystal that is cooled to a smectic C phase.