JPH03287231A - Liquid crystal element - Google Patents

Abstract

PURPOSE:To attain a display of a high contrast without generating after-images by disposing a ferroelectric liquid crystal between a pair of substrates provided with specific polyimide films on at least one substrate. CONSTITUTION:This liquid crystal element has a pair of the substrates having the polyimide films obtd. by polymerizing the unit expressed by formula I on at least one substrate and the ferroelectric liquid crystal disposed between a pair of these substrates. In the formula I, the part B contains either of the structure expressed by formula II or formula III. In the formulas II, III, R denotes hydrogen, methyl group, ethyl group, hydroxyl group, methoxy group, ethoxy group, and acetyl oxy group; the part A denotes a quadrivalnt residual org. group. the high-grade display of the extremely high contrast in a bright state and a dark state and the extremely large display contrast particularly at the time of multiplexing driving is obtd. in this way and the generation of the after images unpleasing to the eyes is obviated.

Description

[Detailed Description of the Invention] [Field of Industrial Application] The present invention relates to liquid crystal elements used in liquid crystal display elements, liquid crystal light shutters, etc., particularly ferroelectric liquid crystal elements, and more specifically relates to a method for improving the alignment state of liquid crystal molecules. This invention relates to a liquid crystal element with improved display characteristics.

[Prior art]

Clark (C1ark) and Lagerwall (L a g
er w a l l ) proposed by (
JP-A-56-107216, U.S. Patent Section 4,36
7.924 specification, etc.). This ferroelectric liquid crystal generally has a non-helical chiral smectic C phase (S m C *) or H phase (S m H *) in a specific temperature range.
and in this state, takes either the first optically stable state or the second optically stable state in response to an applied electric field, and maintains that state when no electric field is applied. In other words, it is bistable and responds rapidly to changes in electric field, and is expected to be widely used as a high-speed and memory-type display element. Applications are expected.

In order for an optical modulation element using this bistable liquid crystal to exhibit predetermined driving characteristics, the liquid crystal disposed between a pair of parallel substrates must be
It is necessary that the molecules be in such a state that conversion between two stable states can occur effectively.

In addition, in the case of a liquid crystal element that utilizes the birefringence of liquid crystal, the transmittance under crossed Nicols is as follows
: tilt angle, Δn: refractive index anisotropy, d, film thickness of liquid crystal layer, λ - wavelength of incident light. ] The tilt θ in the non-helical structure described above appears as an angle between the average molecular axis direction of the twisted liquid crystal molecules in the first and second alignment states. According to the above formula, the maximum transmittance occurs when the tilt θ is 22.5°, and it is necessary that the tilt angle θ in a non-helical structure to achieve bistability be as close to 22.5° as possible. It is.

By the way, as a method for aligning ferroelectric liquid crystals, it is possible to uniaxially align a molecular layer organized by a plurality of molecules forming a smectic liquid crystal over a large area along its normal line, and the manufacturing process is simple. It is also desirable that the process be realized by a simple rubbing process.

As an alignment method for ferroelectric liquid crystals, particularly chiral smectic liquid crystals with a non-helical structure, for example, US Pat. No. 4,561,726 is known.

However, when the alignment methods that have been used so far, especially the alignment method using a rubbed polyimide film, are applied to the non-helical ferroelectric liquid crystal that exhibits bistability as described by Clark and Lagauer, had problems similar to that of subordinates.

That is, according to the experiments conducted by the present inventors, the tilt angle of a ferroelectric liquid crystal with a non-helical structure obtained by aligning with a conventional rubbed polyimide film is the same as that of a ferroelectric liquid crystal with a helical structure. It was found that the angle was smaller than the angle. (In particular, the tilt angle θ of a ferroelectric liquid crystal with a non-helical structure obtained by aligning with a conventional rubbed polyimide film is generally about 3° to 8°, and the transmittance at that time is 3 to 5% at most. In this way, according to Clark and Lagauer, the tilt angle of a ferroelectric liquid crystal with a non-helical structure that achieves bistability is the same as that of a ferroelectric liquid crystal with a helical structure. It should have an angle, but actually the tilt angle θ in a non-helical structure
is smaller than the tilt angle ■ in the helical structure. Moreover, it has been found that the reason why the tilt angle θ in the non-helical structure is smaller than the tilt angle 2 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, liquid crystal molecules are continuous from the axis of liquid crystal molecules adjacent to the upper substrate to the axis of liquid crystal molecules adjacent to the lower substrate (direction of twisted alignment) with respect to the normal to the substrate. They are arranged in a twisted manner with a twist angle δ, and this is the reason why the tilt angle θ in a non-helical structure is smaller than the tilt angle 2 in a helical structure.

Furthermore, the alignment state of the chiral smectic liquid crystal produced by the conventional rubbed polyimide alignment film is changed to the first optically stable state (e.g. When a one-polar voltage is applied for switching from a white display state) to a second optically stable state (e.g., a black display state), after the application of this negative polarity voltage is removed, the ferroelectric liquid crystal layer A reverse electric field V rev of the other polarity is generated, and this reverse electric field V
rev was causing an afterimage on the display. The above-mentioned reverse electric field generation phenomenon is described, for example, in Akio Yoshida, October 1988, "Liquid Crystal Symposium Proceedings JP, 142-143, r.
``Switching characteristics of ssFLc''.

[Purpose of the invention]

Therefore, an object of the present invention is to provide a ferroelectric liquid crystal device which solves the above-mentioned problems, and which produces a large tilt θ in the non-helical structure of a chiral smectic liquid crystal, displays a high contrast image, and An object of the present invention is to provide a ferroelectric liquid crystal element that can achieve a display that does not cause afterimages.

[Means and effects for achieving the object] Therefore, the present invention provides a pair of substrates having a polyimide coating obtained by polymerizing units represented by the following formula [I] on at least one of the substrates, and a polyimide film formed between the pair of substrates. A liquid crystal element having ferroelectric liquid crystals arranged therein is provided.

(The ■ part in the formula contains any of the structures shown in the following formula (a) or (b).) F3 F3 (However, R is hydrogen, a methyl group, an ethyl group, a hydroxyl group, methoxy group, ethoxy group, acetyloxy group) represents a tetravalent organic residue. ) The present invention also provides a pair of substrates having a polyimide coating obtained by copolymerizing units represented by the following formulas [I[] and [I[[] on at least one substrate, and a pair of substrates disposed between the pair of substrates. The present invention provides a liquid crystal element having a ferroelectric liquid crystal.

(However, R represents hydrogen, methyl group, ethyl group, hydroxyl group, methoxy group, ethoxy group, acetyloxy group, and moiety represents a tetravalent organic residue.) Figure 1 shows the ferroelectric liquid crystal of the present invention. This is a schematic drawing of an example of a cell.

11a and llb are In 203 and ITO (In
It is a substrate (glass plate) coated with transparent electrodes 12a and 12b such as diumTin Oxide), and insulating films 13a and 13b (S1
02 film, TiO2 film, Ta205 film, etc.) and the above-mentioned polyimide, the orientation control film 14 has a thickness of 50 to 1000 films.
a and 14b are each laminated.

At this time, the alignment control films 14 are rubbed (in the direction of the arrow) so that they are parallel and in the same direction (the incoming direction in FIG. 1).
a and 14b are arranged. A ferroelectric smectic liquid crystal 15 is arranged between the substrates 11a and llb, and the distance between the substrates 11a and llb is sufficient to suppress the formation of a helical alignment structure of the ferroelectric smectic liquid crystal 15. The distance is set to be small (for example, 0.1 μm to 3 μm), and the ferroelectric smectic liquid crystal 15 is in a bistable alignment state.

The sufficiently small distance described above is maintained by bead spacers 16 (silica beads, alumina beads) placed between the substrates 11a and Ilb.

According to experiments conducted by the present inventors, by using an alignment method using a specific polyimide alignment film subjected to rubbing treatment, which will be explained in the examples below, a large optical contrast between the bright state and the dark state is exhibited, and in particular, U.S. Patent No. 4,65
5,561, etc., produces a large contrast for non-selected pixels during multiplex puff drive, and also does not cause optical response lag during switching (when multiplex puff drive), which causes afterimages during display. An oriented state has been achieved.

The polyimide film used in the present invention is obtained by ring-closing a polyamic acid synthesized by a condensation reaction between a carboxylic anhydride and a diamine.

Specifically, a polyimide film can be formed by applying and baking a polyamic acid synthesized by a reaction between at least one diamine such as F3 F3 and tetracarboxylic anhydride.

Examples of the tetracarboxylic anhydride include pyromellitic dianhydride, benzophenonetetracarboxylic dianhydride, biphenyltetracarboxylic dianhydride, naphthalenetetracarboxylic dianhydride, and 2,2-bis(dicarboxyphenyl)propanihydride. Anhydride, bis(dicarboxyphenyl)
Sulfone dianhydride, bis(dicarboxyphenyl)ether dianhydride, etc. can be used.

When preparing the polyamic acid used in the present invention, the compound represented by formula (a) or (b) is used in a ratio of 0.1 to 10 parts by weight, preferably 1 part by weight, per 1 part by weight of the tetracarboxylic anhydride. used in Further, the compounds shown in the formulas (a) and (b) can be used alone or in combination as a copolymer. Even in the case of a copolymer having the compounds of formulas (a) and (b), the mixing ratio of each and the tetracarboxylic anhydride is as described above. In addition,
The copolymer includes both block copolymerization and random copolymerization.

The polyimide of the present invention may have an average molecular weight of 1 to 10,000, preferably 30,000 to 70,000, and more preferably 50,000.

When providing the polyimide film used in the present invention on a substrate,
Polyamic acid, which is a precursor of polyimide, is dissolved in a solvent such as dimethylformamide, dimethylacetamide, dimethylsulfoxide, or N-methylpyrrolidone.
．． After applying the solution as a 01 to 40% (by weight) solution onto a substrate by a spinner coating method, a spray coating method, a roll coating method, etc., the solution is heated to 100 to 350°C, preferably 20°C.
A polyimide film can be formed by heating at a temperature of 0 to 300°C to cause dehydration and ring closure. This polyimide film is then rubbed with a cloth or the like. Further, the polyimide film used in the present invention has a thickness of about 30A to 1μ, preferably 20A to 1μ.
The film thickness is set to 0A to 200OA. In this case, the use of the insulating films 13a and 13b shown in FIG. 1 can be omitted. Further, 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 200A or less, preferably 100A or less.

The liquid crystal material used in the present invention has a
A liquid crystal that generates a chiral smectic C phase through a cholesteric phase and a smectic A phase is preferred. In particular, it is preferable that the pitch in the cholesteric phase is 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). As specific liquid crystals, the following liquid crystal materials rLC-IJ, r80
A liquid crystal composition containing BJ and r80sI''J in the following ratios is preferably used.

LC-1 ”DanW 80SI * (1) (LC-1)9o/(80B)+.

(2) (LC-1)8o / (80B)20(
3) (LC-1)70 / (80B)30(
4) (LC-1)so / (80B)40(
5) 80SI* (The subscripts in the table each represent a weight ratio.) FIG. 2 schematically depicts an example of a cell to explain the operation of a ferroelectric liquid crystal. 21a and 21b are In2
A substrate (glass plate) coated with a transparent electrode made of a thin film of O2, SnO2 or ITO, between which a liquid crystal molecular layer 22 is oriented perpendicular to the glass surface.
*(chiral smectic C) phase or SmH* (chiral smectic H) phase liquid crystal is sealed. A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment (P
Sat) has 24. When a voltage equal to or higher than 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 the liquid crystal molecules 23 are aligned in the direction such that the dipole moment (P) 24 is all directed in the direction of the electric field. can be changed. The liquid crystal molecules 23 have an elongated shape and exhibit refractive index anisotropy in the long axis direction and the short axis direction. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, the voltage application polarity can be changed. It is easily understood that the liquid crystal optical modulation element has optical characteristics that change depending on the amount of 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 have a sufficiently thin thickness (for example, 0.1 μm to 3 μm). As the liquid crystal layer becomes thinner, the helical structure of the liquid crystal molecules unwinds and becomes a non-helical structure even when no electric field is applied, as shown in Figure 3, and its dipole moment P or P' is directed upward ( 34a) or downward (34b). When an electric field Ea or Eb of different polarity above a certain threshold value is applied to such a cell by the voltage applying means 31a and 31b as shown in FIG. 3, the dipole moment corresponds to the electric field vector of the electric field Ea or Eb. The liquid crystal molecules are oriented either upward 34a1 or downward 34b, and accordingly the liquid crystal molecules are oriented in either the stable state 33a or the second stable state 33b in FIG.

The effects obtained by this ferroelectric liquid crystal cell are, firstly, that the response speed is extremely fast, and secondly, that the orientation of the liquid crystal molecules has bistability.

To further explain the second point, for example, with reference to FIG. 3, when the electric field Ea is applied, the liquid crystal molecules are oriented in a first stable state 33a, and this state remains stable even when the electric field is turned off. When an electric field Eb in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 33b and change their orientation, but they remain in this state even after the electric field is turned off. Further, as long as the applied electric field Ea does not exceed a certain threshold value, each orientation state is maintained.

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

61a and 61b shown in FIG. 4(A) represent an upper substrate and a lower substrate, respectively. 60 is a molecular layer organized by liquid crystal molecules 62, and the liquid crystal molecules 62 are attached to the bottom surface 64 of the cone 63.
They are arranged by changing their positions along the (circle).

FIG. 4(B) is a diagram showing a C-director.

Ul in FIG. 4(B) is the C director 81 in one stable orientation state, and U2 is the C-director 81 in the other stable orientation state. The C-director 81 is shown in FIG. 4(A).
This is a projection of the molecular long axis onto a virtual plane perpendicular to the normal to the molecular layer 60 shown in FIG.

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. 4(C), the tilt angle θ is small because the twist of the molecular axis is large from the upper substrate 61a toward the lower substrate 61b.

FIG. 5(A) is a plan view showing the tilt angle θ when the C-director 81 is in the state shown in FIG. 4(B) (referred to as the uniform orientation state).
is a plan view showing the tilt angle θ in the state of FIG. 4(C) (referred to as the spray alignment state). In the figure, 50 indicates the axis of the rubbing treatment applied to the specific polyimide film of the present invention described above, 51a indicates the average molecular axis in the orientation state U, and 51
b is the average molecular axis in orientation state U2, 52a is orientation state S
1, and 52b indicates the average molecular axis in orientation state S2.

The average molecular axes 51a and 51b can be converted by applying voltages of opposite polarity that exceed a threshold voltage. The same thing occurs between the average molecular axes 52a and 52b.

Next, the usefulness of the uniform orientation state for optical response delay (afterimage) due to the reverse electric field v rav will be explained.

When the capacitance C1 of the insulating layer (alignment control film) of the liquid crystal cell, the capacitance of the liquid crystal layer is CLo, and the spontaneous polarization of the liquid crystal is Ps, V reV, which causes an afterimage, is expressed by the following formula.

FIG. 6 is a cross-sectional view schematically showing the charge distribution, the direction of Ps, and the direction of the reverse electric field within the liquid crystal cell.

FIG. 6(A) shows the distribution state of the ■ and O charges under the memory state before the application of a pulsed electric field, and the direction of the spontaneous polarization Ps at this time is from the ■ charge to the e charge. Figure 6(B) shows that the direction of the spontaneous polarization Ps immediately after the pulsed electric field is released is opposite to the direction in Figure 6(A) (therefore, the liquid crystal molecules change from one stable orientation state to the other stable orientation state). However, since the distribution state of the ■ and e charges is the same as in Fig. 6(A), a reverse electric field V rev % is generated in the liquid crystal in the direction of the arrow. There is. This reverse electric field V
After a while, rev disappears as shown in FIG. 6(C), and the distribution state of ■ and ○ charges changes.

FIG. 7 shows the change in the optical response of the spray alignment state caused by a conventional polyimide alignment film, expressed as a change in the tilt angle θ. According to FIG. 7, when a pulsed electric field is applied, an overflow occurs along the direction of the mark Xl from the average molecular axis S (A) under the spray orientation state to the average molecular axis U2 under the uniform orientation state near the maximum tilt angle ■. Immediately after the shot is released and the pulsed electric field is released, the action of the reverse electric field V rev shown in FIG. The angle θ decreases and the reverse electric field V shown in FIG. 6(C)
Due to the effect of attenuation of rev, a stable alignment state is obtained in which the tilt angle θ slightly increases along the direction of arrow X3 up to the average molecular axis S (C) under the spray alignment state. The optical response at this time is clarified in FIG.

According to the present invention, in the alignment state obtained by the alignment method using the fluorine atom-containing polyimide film described above, the average molecular axis S (A) under the spray state shown in FIG.
S (B) and S (C) are not generated, and therefore it is possible to align the molecules on the average molecular axis that produces a tilt angle θ close to the maximum tilt angle ■. The optical response of the present invention at this time is shown in FIG. According to FIG. 9, it can be seen that there is no delay in optical response caused by afterimages and that high contrast is produced under the memory state.

Hereinafter, the present invention will be explained according to examples.

Example 1 Two glass plates with a thickness of 1.1 mm each provided with an IrO film with a thickness of 1,000 mm were prepared, and on each glass plate, N-methylpyrrolidone/n-butyl cellosolve-571, a polyamic acid represented by the following formula, was placed on each glass plate. A 3.0 wt% solution of
After film formation at 0 rpm, heating and baking treatment was performed at 250° C. for about 1 hour (molecular weight: 50,000). The film thickness at this time was 450 people.

This coating film was subjected to a unidirectional rubbing treatment using a nylon flocked cloth.

After that, alumina beads with an average particle size of approximately 1.5 μm are scattered on one glass plate, and the two glass plates are stacked so that their rubbing axes are parallel to each other and in the same treatment direction. was created.

A ferroelectric smectic liquid crystal, rcs-'1014J (trade name) manufactured by Chisso Corporation, was vacuum injected into this cell under the Tokichi phase, and then gradually heated to 30°C at a rate of 0.5°C/h from the Tokichi phase. Orientation could be achieved by cooling. The phase change in the cell of this example using this rC3-1014J was as follows.

(Iso-Toyoshi 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 was applied, and then a 90°
' Set the crossed nicols to the extinction position (darkest state),
The transmittance at this time was measured using a photomultiplier, then a 50 μsec pulse of 30 V was applied, and the transmittance (bright state) at this time was measured in the same manner, and the tilt angle θ was 15°. , the transmittance in the darkest state is 1%,
The transmittance in the bright state was 35%, so the contrast ratio was 35:1.

The delay in optical response, which causes afterimages, was 0.2 seconds or less.

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 after displaying an image by inputting a predetermined character, the entire screen became white. When I erased the image, the afterimage could not be read.
S Nil I S N+2 C represents the voltage waveform applied to the scanning line, and ■ represents the voltage waveform applied to a typical information line.

(I-3N) is a composite waveform applied to the intersection of the information line I and the scanning line SN. Also, in this example, ■.

It was carried out at 5■~8V1ΔT-20μsec~70μsec.

Examples 2 to 5 Cells were 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 in each example was 50,000) and liquid crystal material were used.

The same test as in Example 1 was conducted on each of them.

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

Furthermore, when display was performed using the same multiplexing drive as in Example 1, results similar to those in Example were obtained regarding contrast and afterimage.

Table 2 44 cases of Kazumi-Yu no 1j and 4 1kJfXi response” (D#
h 5ec231・10.2 325 : 1 0.3 429 : 1 0.2 530 : 1 0.1 Liquid crystal (3) above in the same table Comparative Examples 1 to 4 Using the alignment control film and liquid crystal material shown in Table 3 A cell was prepared in the same manner as in Example 1, except that the cell was prepared in the same manner as in Example 1.

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

Furthermore, when a display was performed using the multiplex puff drive similar to that in Example 1, the contrast was smaller than that in this example, and moreover, an afterimage occurred.

Same as above liquid crystal (3) Table ≦ Technique 2'' Back ≧ 8=1 7:1 10: 1 8:1 ＼Italy 4, Missing 5ec 1.5 2.5 1.2 2.2 [Effects of the Invention] As clarified in the above Examples and Comparative Examples, according to the present invention, the contrast between the bright state and the dark state is high, and especially the display contrast during multiplex puffer drive is very large and high quality is achieved. It is possible to obtain a display with no unpleasant afterimage phenomenon.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a liquid crystal element of the present invention. FIG. 2 is a perspective view showing the orientation state of a chiral smectic liquid crystal having a helical structure, and FIG. 3 is a perspective view showing the orientation state of a chiral smectic liquid crystal having a non-helical molecular arrangement. FIG. 4(A) is a cross-sectional view showing the alignment state of chiral smectic liquid crystal oriented by the alignment method of the present invention, FIG. 4(B) is a C-director diagram in the uniform alignment state, and FIG. ) is a C-director diagram in a spray orientation state. FIG. 5(A) is a plan view showing the tilt angle θ in the uniform orientation state, and FIG. 5(B) is a plan view showing the tilt angle θ in the spray orientation state. Figure 6 (A), (E) and (C
) is a cross-sectional view showing the charge distribution in the ferroelectric liquid crystal, the direction of spontaneous polarization Ps, and the direction of reverse electric field V rev. FIG. 7 is a plan view showing changes in the tilt angle θ during and after application of an electric field. FIG. 8 shows the optical response characteristics in the conventional example, and FIG. 9 shows the optical response characteristics in the example of the present invention. FIG. 10 is a waveform diagram of the drive voltage used in this example. 207- 210-

Claims (1)

[Claims] (1) A liquid crystal element having a pair of substrates having a polyimide coating obtained by polymerizing units represented by the following formula [I] on at least one substrate, and a ferroelectric liquid crystal disposed between the pair of substrates. ▲There are mathematical formulas, chemical formulas, tables, etc.▼ [I] (The [B] part in the formula contains one of the structures shown in the following formula (a) or (b). (a) ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (b) ▲There are mathematical formulas, chemical formulas, tables, etc.▼ (However, R is hydrogen, methyl group, ethyl group, hydroxyl group, methoxy group, ethoxy group, acetyloxy group) A]
The part indicates a tetravalent organic residue. )(2) A pair of substrates having a polyimide coating obtained by copolymerizing units represented by the following formulas [II] and [III] on at least one of the substrates, and a ferroelectric liquid crystal disposed between the pair of substrates. A liquid crystal element with ▲There are mathematical formulas, chemical formulas, tables, etc.▼ [II] (However, R is hydrogen, methyl group, ethyl group, hydroxyl group, methoxy group, ethoxy group, acetyloxy group, or [A]
The part indicates a tetravalent organic residue. )