JPH02296215A - liquid crystal element - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

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 Lager Wall (C1ark) and Lager Wall (C1ark) and Lager Wall (C1ark) and Lager Wall (2013) have developed a type of display element that utilizes the refractive index anisotropy of ferroelectric liquid crystal molecules to control transmitted light in combination with a polarizing element.
wall) (Japanese Patent Application Laid-open No. 107-1983)
No. 216, US Pat. No. 4,367', US Pat. No. 924, etc.). This ferroelectric liquid crystal generally has a non-helical chiral smectic C phase (
SmC*) or H phase (SmH*), in this state, it takes either the first optically stable state or the second optically stable state in response to the applied electric field, and when the electric field is applied It has the property of maintaining its state when it is not present, that is, it has bistability, and it also responds quickly to changes in electric field.It is expected to be widely used as a high-speed and memory-type display element, and is particularly popular due to its function. It is expected to be used as a high-definition screen.

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 [where I. = incident light intensity, I: transmitted light intensity, θ
: 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 225° as possible. .

By the way, as a method for aligning ferroelectric liquid crystals, it is possible to uniaxially align a violent molecular layer organized by a plurality of molecules forming a smectic liquid crystal over a large area along its normal line. It is also desirable that the manufacturing process can 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 the above-mentioned problems.

That is, according to the experiments conducted by the present inventors, the tilt angle (the angle shown in FIG. 3, which will be described later) of a ferroelectric liquid crystal with a non-helical structure obtained by aligning with a polyimide film subjected to a conventional rubbing process is the same as that of a helical one. It was found that the tilt angle is smaller than that of a ferroelectric liquid crystal having a structure (the angle ■ outside the apex angle of the pentagonal pyramid shown in FIG. 2, which will be described later). (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 at most 3 to 5
It was about %. ) According to Clark and Lagauer, the tilt angle of a ferroelectric liquid crystal with a non-helical structure that achieves bistability should be the same as the tilt angle of a ferroelectric liquid crystal with a helical structure. However, in reality, the tilt angle θ in a non-helical structure is smaller than the tilt angle 2 in a 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,
As shown in FIG. 4, the liquid crystal molecules are continuously twisted from the axis 42 of the liquid crystal molecules adjacent to the upper substrate to the axis 43 (direction of twisted arrangement 44) of the liquid crystal molecules adjacent to the lower substrate with respect to the normal to the substrate. They are arranged twisted at an angle δ, and this causes the tilt angle θ in a non-helical structure to be smaller than the tilt angle ■ 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-polarity 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 polarity voltage is removed, the ferroelectric liquid crystal layer A reverse electric field V rav of the other polarity is generated, and this reverse electric field V rav
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''.

[Summary of the Invention] Therefore, it is an object of the present invention to provide a ferroelectric liquid crystal device that solves the above-mentioned problems, and in particular, to produce a large tilt θ in a non-helical structure of chiral smectic liquid crystal, and to produce a high-contrast image. It is an object of the present invention to provide a ferroelectric liquid crystal element that can display images without causing an afterimage.

The present invention is characterized by a liquid crystal element having a pair of substrates having a polyimide film having a structural unit represented by the following general formula on at least one substrate, and a ferroelectric liquid crystal disposed between the pair of substrates.

General formula (wherein R3 and R2 are hydrogen atoms, fluorine atoms, or -
It is CF3. However, at least one of R1 and R2 is a fluorine atom or -CF3. ) [Detailed Description of Embodiments of the Invention] FIG. 1 schematically shows an example of the ferroelectric liquid crystal cell of the present invention, which is a substrate (glass plate) covered with transparent electrodes 12a and 12b such as %Tin Oxide. Thereon, there are insulating films 13a and 13b (SiO2 film, TiO2 film, Ta205 film, etc.) with a thickness of 200 to 1000 Å and orientation control films 14a and 14b with a thickness of 50 to 1000 Å formed of polyimide shown in the general formula above. are stacked on top of each other.

At this time, the alignment control film 14 is rubbed (in the direction of the arrow) so that it is parallel and in the same direction (six directions 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 llb.

According to the experiments conducted by the present inventors, by using the alignment method using a specific polyimide alignment film subjected to rubbing treatment, which is revealed in the above-mentioned example, 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 of a carboxylic anhydride and a diamine.

Examples of the carboxylic anhydride used in the present invention include (1) ()(lifluoromethyl)pyromellitic anhydride (
2) bis(trifluoromethyl)pyromellitic anhydride; (3) fluoropyromellitic anhydride; and the like.

Examples of diamines used in the present invention include m-phenylenediamine, p-phenylenediamine, m-xylenediamine, β-xylenediamine, 4゜4'-diaminodiphenyl ether, 4.4'-diaminodiphenylmethane, 3.3'-dimethyl -4゜4'-diaminodiphenylmethane, 3. 3', 5゜5'-tetramethyl-4,4'-diaminodiphenylmethane, 2.2'
-bis(4-aminophenyl)propane, 4.4'-
Methylene dianiline, benzidine, 4,4'-diaminodiphenylsulfide, 4,4'-diaminodiphenylsulfone, J,5-diaminonaphthalene, 3,3'
-dimethylbenzidine, 3,3'-dimethoxybenzidine, etc. In addition to these diamines, for example,
2,2-bis(4-(4-aminophenoxy)phenyl)hexafluoropropane, 2,2-bis(4-(3-
aminophenoxy)phenyl]hexafluoropropane, 2,2-bis(4-(2-aminophenoxy)phenyl)hexafluoropropane, 2,2-bis[4-(2
-aminophenoxy)-3,5-dimethylphenyl]hexafluoropropane, p-bis(4-amino-2-trifluoromethylphenoxy)benzene, 4,4'-bis(4-amino-2-trifluoromethyl phenoxy)
biphenyl, 4,4'-bis(4-amino-3-trifluoromethylphenoxy)biphenyl, 4,4'-bis(4-amino-2-trifluoromethylphenoxy)diphenylsulfone, 4,4'-bis( 3-Amino-5-trifluoromethylphenoxy)diphenylsulfone, 2
゜2-bis(4-(4-amino-2-trifluoromethylphenoxy)phenyl]hexafluoropropane, 4
．． Fluorinated diamines such as '4'-bis[(4-aminophenoxy)phenyl]hexafluoropropane can be used.

The present invention provides two or more of the above-mentioned carboxylic acid anhydrides, and/or
Alternatively, it is also possible to use two or more types of diamines. Further, in the present invention, a copolymer can be prepared by combining the following carboxylic acid anhydrides in addition to the above-mentioned carboxylic acid anhydrides.

Pyromellitic anhydride, 2.3.6.7-naphthalenetetracarboxylic anhydride, 3.3', 4.4'-diphenyltetracarboxylic anhydride, 1.2.5°6-naphthalenetetracarboxylic acid Anhydride, 2,2'3.3'-diphenyltetracarboxylic anhydride, thiophene-2,3
, 4,5-tetracarboxylic anhydride, 2,2-bis(3
,4-biscarboxyphenyl)propane anhydride, 3,
4-dicarboxyphenylsulfone anhydride, perylene-
3,4,9,10-tetracarboxylic anhydride, bis(3
, 4-dicarboxyphenyl)ether anhydride, 3.
3', 4,4'benzophenonetetracarboxylic anhydride.

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 at 100 to 350°C, preferably 2.
A polyimide film can be formed by heating at a temperature of 00 to 300°C to cause dehydration and ring closure. This polyimide film is
After that, it is rubbed with cloth etc. Further, in the present invention, 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 200 Å or less, preferably 100 Å 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 substances "Lc-1j", "80
A liquid crystal composition containing "B" and r80sI*J in the following ratios is preferably used.

LC-1 "Dan W 8081 * Ittan L- (1) (LC1)so/ (2) (LC-1)80/ (3) (LC-1)70/ (4) (LC1)go/ (5 ) 80SI* (The subscripts in the table represent weight ratios.) (80B) 10 (80B) 20 (80B) 3 (1 (80B) 4゜Figure 2 explains the operation of ferroelectric liquid crystal 21a and 21b are In
A substrate (glass plate) covered with a transparent electrode made of a thin film such as 202.5n02 or ITO, between which a liquid crystal molecular layer 22 is oriented perpendicular to the glass surface.
A liquid crystal of C (chiral smectic C) phase or SmH* (chiral smectic H) phase is sealed. A thick line 23 represents a liquid crystal molecule, and this liquid crystal molecule 23 has a dipole moment (
P l )'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 short axis direction. Therefore, for example, if crossed Nicol polarizers are placed above and below the glass surface, voltage can be applied. It is easily understood that the liquid crystal optical modulator is a liquid crystal optical modulator whose optical characteristics change depending on the polarity.

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 in either the first stable state 33a or the second stable state 33b accordingly.

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. Further, when an electric field E'b in the opposite direction is applied, the liquid crystal molecules are oriented to a second stable state 331b 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.

U 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 molecular axis is twisted greatly from the upper substrate 61a toward the lower substrate 61b.

5(A) is a plan view showing the tilt angle θ of the C-director 81 in the state shown in FIG. 4(B) (referred to as the uniform orientation state), and FIG. 5(B) is a plan view of the C-director 81
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 rev 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 CLC, 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 O and θ charges in a memory state before application of a pulsed electric field, and the direction of spontaneous polarization Ps at this time is from the ■ charge to the e charge. FIG. 6(B) shows that the direction of spontaneous polarization Ps immediately after the pulsed electric field is released is opposite to the direction in FIG. 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 O charges is the same as in Figure 6 (A), a reverse electric field V rev is generated in the liquid crystal in the direction of the defect. . This reverse electric field V r
After a while, ev 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 X1 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 pulse 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 attenuation of rev, a stable alignment state is obtained in which the tilt angle θ slightly increases along the direction of the defect 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 orientation state obtained by the orientation method using the fluorine atom-containing polyimide film described above, the average molecular axes S (A), S under the spray state shown in FIG.
(B) and S (C), and therefore can be arranged on an 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 A 1,000-layer thick ITO film is provided.
Prepare two glass plates with a thickness of m m, and apply N-methylpyrrolidone/polyamic acid represented by the formula below on each glass plate.
A 3.0% by weight solution of n-butyl cellosolve-5/1 was applied for 30 minutes using a spinner with a rotation speed of 300 rpm.

Ta.

A ferroelectric smectic liquid crystal, rC3-1014J (trade name) manufactured by Chisso Corporation, was vacuum injected into this cell under the Tokichi phase, and then slowly cooled from the Tokichi phase to 30°C at a rate of 0.5°C/h. It was possible to orient it by doing this. This r
The phase change in the cell of this example using C3-1014J was as follows.

(degree of polymerization = 700-2000) After the film was formed, it was heated and baked at 250°C for about 1 hour.

The film thickness at this time was 450 people. This coating film was subjected to a unidirectional rubbing treatment using a nylon touch 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. I want to create l5o-Toyoshi phase
ch = cholesteric phase SmA = smectic A phase SmC* = chiral smectic C phase) Subsequent experiments are as follows:
It was carried out at a temperature of 25°C.

The above liquid crystal cell is sandwiched between a pair of 90° crossed nicol polarizers, a 50 μsec 30V pulse is applied, and the 90° crossed nicol is turned to the extinction position (darkest state).
The transmittance at this time was measured using a photomultiplier, and then a 50 V pulse of 50 μsec was applied, and the transmittance (bright state) at this time was measured in the same manner, and the tilt angle θ was At 15° the transmission was 1.0%, so the contrast ratio was 55:1.

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 the image was erased, the afterimage could not be deciphered.
N+0. SN+2 represents the voltage waveform applied to the scanning line, and I represents the voltage waveform applied to the representative information line.

*Examples 2 to 6 Cells were obtained in the same manner as in Example 1, except that the alignment control film and liquid crystal material shown in Table 1 were used.

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

Table 2 shows the results of contrast ratio and optical response delay time.
(The degree of polymerization of polyamic acid in the table is 70.
0 to 2000).

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.

It was performed in μsec.

Table 2 JJJL v> 5X l! T-11's o
5ec2 50 : 1 0.1 3 51 : 1 0.1 4 53 : 1 0.2 5 45 : 1 0.1 6 30 : 1 0.1 Liquid crystal (3) in the same table Comparative Examples 1 to 4 Table 3 A cell was prepared in exactly the same manner as in Example 1 except that the alignment control film and liquid crystal material shown in Table 1 were used (the degree of polymerization of the polyamic acid varnish in the table is 700 to 2000). Table 4 shows the contrast ratio and optical response lag for each cell.

Furthermore, when a display was performed using the same multiplexing drive as in Example 1, the contrast was smaller than that in this example (and an afterimage occurred).

Same as above liquid crystal (3) Table ≦12±shishi 4 upper 5 8:1 7=1 10: 1 8:1 A 5ec 1.5 2.5 1.2 2.2 [Effect 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 in particular, the display contrast during multiplex leg driving is extremely large, and a high-quality display can be obtained. Moreover, there is an effect that no unpleasant afterimage phenomenon occurs.

[Brief explanation of drawings]

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. FIG. 6 is a cross-sectional view showing the charge distribution within the ferroelectric liquid crystal, the direction of spontaneous polarization Ps, and the direction of reverse electric field V rev. Figure 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. 1O is a waveform diagram of the drive voltage used in this example. b2 =135- (Afu('e)) (C) -■---■■訃■God-ichijo-Ashi

Claims (1)

Claims: A liquid crystal element comprising a pair of substrates having a polyimide film having a structural unit represented by the following general formula on at least one substrate, and a ferroelectric liquid crystal disposed between the pair of substrates. General formula▲There are mathematical formulas, chemical formulas, tables, etc.▼ (In the formula, R_1 and R_2 are hydrogen atoms, fluorine atoms, or -CF_3.However, at least one of R_1 and R_2 is a fluorine atom or -CF_3. .)