JPH0473847B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a ferroelectric liquid crystal electro-optical device using chiral smectic liquid crystal. [Prior Art] Liquid crystals are used in a variety of displays.
Liquid crystal panels have excellent characteristics such as being small, thin, and consuming less power. Therefore, it is often used to display clocks and calculators.
The most commonly used liquid crystal for these displays is a thermotropic liquid crystal. It assumes various liquid crystal phases within a predetermined temperature range. This liquid crystal phase is roughly divided into a nematic phase (hereinafter referred to as N) without a layered structure and a smectic phase (hereinafter referred to as Sm) having a layered structure. Sm is further uniaxial Smektic A phase (hereinafter
SmA) and the biaxial smectic C phase (hereinafter SmC
are categorized. The thickness of the layer approximately corresponds to the length of one liquid crystal molecule. Figure 2 schematically shows the molecular arrangements of N, SmA, and SmC. Figure 2a shows N, Figure 2b shows SmA, and Figure 2c shows SmC. Furthermore, if the liquid crystal molecule has an asymmetric carbon and is not racemic, it will take on a helical structure. In the case of N, the long axes of liquid crystal molecules are located within the thin layer and are aligned in one direction.
This results in a chiral nematic, in which the direction of the molecules within each layer is slightly twisted in each layer. FIG. 3 is a diagram schematically showing the molecular arrangement of chiral nematics. In the case of Sm, the molecules are arranged in a spiral with the normal direction of the layer as the helical axis, resulting in a chiral smectic C phase (hereinafter referred to as SmC ^* ). Let me explain a little more about SmC ^* . The long axis direction (hereinafter referred to as molecular axis) of the liquid crystal molecules within one layer is inclined at an angle θ with respect to the normal direction of the layer, and this angle is constant in all layers. 4b shows the relationship between the molecular axis and the normal direction. On the other hand, when looking at the molecular arrangement of SmC ^* from the normal direction of the layers, the azimuth rotates by a constant value (a in Figure 4 shows the case where it changes by 45 degrees) for each layer, and the molecules The arrangement results in a helical structure. In general, SmC ^* not only has a helical structure but also has an electric dipole in the direction perpendicular to the molecular axis and exhibits ferroelectricity. Ferroelectric liquid crystal was developed in 1975 by Meyer (J.de.Phys.
36, 69, 1975) and their existence was demonstrated. The liquid crystal synthesized at that time was commonly known as DOBAMSB (2-
Methylbutyl P-[(P-n-decyloxybenzylidene)amino]) It is still actively used in research on ferroelectric liquid crystals. SmC ^* has a helical structure as mentioned above, but the period of the helix is thinner than that of the liquid crystal.
When implanted into a cell with a gap of about 1 μm, the helical structure disappears. The molecular arrangement structure after the helical structure disappears is shown in FIG. 5 together with its geometric relationship with the cell substrate. The liquid crystal molecules become parallel to the cell substrate. That is, the liquid crystal molecules are arranged so that the molecular axes are parallel to the substrate and are tilted by θ from the normal direction of the layers. Here, the normal direction of the layer is parallel to the substrate. The layers are therefore formed perpendicular to the substrate. When tilted by θ from the normal direction of the layer, domains that are tilted clockwise by θ and domains that are tilted counterclockwise from the normal coexist. SmC ^* liquid crystal molecules generally have an electric dipole perpendicular to the molecular axis. Assuming that in one domain the electron dipoles are aligned upward with respect to the cell substrate,
In the other domain, the electric dipoles are aligned downward. When an electric field is applied between the cell substrates, the liquid crystal molecules in the entire cell will move +θ or −θ from the normal direction of the layers.
(+ and - are determined by the direction of the electric dipole) Align at an inclined position. Hereinafter, these will be referred to as the +θ position and the −θ position. When an electric field is applied in the opposite way to the above, the liquid crystal molecules will move +θ
Move from position to -θ position or move from -θ position to +θ position. This phase structure is SmC because the molecules of the entire cell are arranged in the +θ or -θ position.
It is. By thinning the cell gap, the helical structure disappeared and an SmC phase was formed. However, when this SmC moves from the ±θ position to the opposite position as a trace of its helical structure, it moves along the circular vertebrae shown in FIG. 4b. These two states are very stable, and the speed at which the two states can be alternately switched by an electric field is normal.
While TN is on the order of milliseconds, it is on the order of several microseconds. In other words, a cell consisting of two substrates holding SmC ^* and attached with electrodes has memory properties and can change into two states at extremely high speed. Clark and Lagerwall (Appl, Phys.
lett. 36, 899, 1980) et al. They also claimed that chiral smectic liquid crystals have another property. In other words, it is a "desirable threshold characteristic."
In this case, the threshold characteristic is different from the threshold characteristic for the effective voltage in a TN liquid crystal, and is determined only by the applied voltage value. However, neither our experiments nor any other research institutes in the world have produced data indicating the existence of desirable threshold characteristics. Therefore, we selectively turn on the voltage ±Vap with the desired pulse-to-pulse duration. Select non-lit,
Lights up with an AC pulse with equal positive and negative amplitudes smaller than the Vap. We have developed a driving method that memorizes the non-lighting state. FIGS. 6a and 6b show an example of driving waveforms according to the above driving method. A and b in FIG. 6 are drive waveforms in which the potentials from the scanning electrodes are +Vap and -Vap, respectively, when selected. When a chiral smectic liquid crystal is driven with this driving waveform, the quality of display is greatly influenced by the alignment state of the liquid crystal. The reason for this is thought to be that the molecular orientation due to uniaxial alignment treatment is stable, and the state in which the molecules become parallel to the substrate when a voltage is applied has a large strain. FIG. 7 shows the molecular arrangement when the surfaces of two substrates were subjected to uniaxial alignment treatment. The layer is considered to be tilted in the thickness direction of the substrate, and if we focus on a single molecule, if we consider the virtual cone in Figure 4, we can assume that it is at the apex of the cone shown in Figure 8. It will be done. It is considered to be parallel to the substrate and located at the apex of a virtual cone. For a virtual cone that is parallel to the substrate, it is necessary for the molecules to be stably positioned at one of two molecular positions on the central plane of the cone from the initial orientation state. Conventionally, to obtain this state, the molecules were oriented laterally using spacers, etc., as advocated by Professor Fukuda of Tokyo Tech.
A method of taking a position parallel to the substrate and on the central plane of a virtual cone was known. However, this method requires arranging a large number of spacers having a lateral orientation force in the cell, making it difficult to mass-produce the spacer on an industrial scale. [Means for solving the problem] The following method is an orientation direction that can provide molecules with two stable positions parallel to the substrate and is compatible with the driving method that we have developed. That is, one substrate is subjected to an alignment process with uniaxial alignment, and the other substrate is subjected to a random horizontal alignment process without directionality. [Operation] It is thought that liquid crystal molecules are literally randomly attached horizontally to a film that has been subjected to random horizontal alignment treatment without directionality. On the other hand, the uniaxial alignment treatment of the other substrate fixes the molecules horizontally in one direction. The orientation state of molecules within the cell due to these orientations can be considered as follows. FIG. 1 is a schematic diagram showing the orientation state of molecules within a cell. The figure is a view seen from the uniaxial orientation direction. Liquid crystal molecules hardly move on a uniaxially aligned film. It is thought that the molecules attached to the uniaxially aligned film do not move at all or move only a few molecular layers. On the other hand, molecules subject to random horizontal alignment are also thought to be in a few molecular layers and do not move much. The uniaxially oriented region is called a uniaxially oriented region, and the randomly horizontally oriented region is called a random horizontally oriented region. The former defines the alignment direction of molecules so that the chiral smectic liquid crystal forms uniform domains with aligned directions. On the other hand, the latter serves to make the interface with the domain that has grown under the influence of the uniaxial orientation region a free interface. Free means that there is little interaction between molecules above and below the interface. Therefore, the liquid crystal molecules grown from the uniaxial alignment film can remain in the most stable position without being forced. Its position is horizontal to the board and is located on the line of intersection between the virtual cone and the center plane. This position is the ±θ position. Example 1 As an example of uniaxial alignment treatment, a polyimide thin film is formed by printing or dipping, and then rubbing treatment is performed in one direction. On the other hand, as an example of the random horizontal alignment process, there is the following process.
That is, SiO ₂ sputtering → organic silane vertical alignment compound, 0.01 to 5.0% by weight aqueous solution immersion → washing → drying, then heat treatment at around 300°C for about 30 minutes. The suitability of the driving method is highly dependent on the final heat treatment temperature. 250°C to 350°C is most compatible. Examples of organic silane vertically aligned compounds include compounds such as (CH ₃ O) ₃ −(CH ₂ ) ₃ −N ⁺ (CH ₃ ) ₂ −(CH ₂ ) ₁₇ CH ₃ Cl ⁻ (DMOAP). Example 2 The uniaxial alignment treatment is the same as in Example 1. An organic silane compound is used for the random horizontal alignment treatment. The alignment treatment using an organic silane compound is as follows. The process is as follows: SiO ₂ spatter → immersion in an aqueous solution containing 0.01 to 5.0% by weight of an organic silane compound → washing → drying. For organic silane compounds, _CH3 -NH-( _CH2 ) ₃ -Si( _OCH3 ) ₃ , _NH2- ( _CH2 ) _2- NH-( _CH2 ) _3- Si( _OCH3 ), NH ₂ −(CH ₂ ) ₃ −Si(OC ₂ H ₅ ) ₃ Si(OC ₂ H ₅ ) ₄ , CH ₃ −Si(OCH ₃ ) ₃ , CH ₃ −(CH ₂ ) ₄ −Si(OC ₂ H ₅ ) ₃ ,

【formula】

_CH3− ( _CH2 ) ₇ −Si( _OCH3 ) ₃ , etc. [Effect] Performing the conventional uniaxial alignment process on one of the two substrates and random horizontal alignment process on the other is a simple and mass-producible alignment process. By using such a simple method, most of the molecules are located parallel to the substrate and on the line of intersection between the virtual cone and the central plane. The molecules at this location are stable and the open memory is semi-permanent. It has good compatibility with the driving method described above, and a good ferroelectric electro-optical device can be obtained, and a large-capacity LCD display can be realized.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of the arrangement of molecules when the orientation method according to the present invention is used. FIG. 2 is a schematic diagram of the molecular arrangement of the nematic phase, smectic A phase, and smectic C phase. FIG. 3 is a schematic diagram of the molecular arrangement of the chiral nematic phase. FIG. 4 is a schematic diagram of the molecular arrangement of chiral smectic C phase. FIG. 5 is a schematic diagram of the molecular arrangement within the cell after the helical structure has disappeared. FIG. 6 is an example of a driving waveform for driving a chiral smectic. FIG. 7 is a schematic diagram showing the molecular arrangement when both of the two substrates are subjected to uniaxial alignment treatment. FIG. 8 is an enlarged view of the cone. DESCRIPTION OF SYMBOLS 1...Substrate, 2...Liquid crystal molecules, 3...Uniaxially aligned film, 4...Random horizontally aligned film, 5...
electric dipole.

Claims (1)

[Scope of Claims] 1 A ferroelectric liquid crystal is sandwiched between substrates at least one of which is transparent and has an electrode provided on the surface, and an electric field is applied while limiting the distance between the substrates to a helical pitch of the ferroelectric liquid crystal or less. In a ferroelectric liquid crystal electro-optical device driven by A ferroelectric liquid crystal electro-optical device characterized by: 2. The ferroelectric liquid crystal electro-optical device according to claim 1, wherein the alignment film having uniaxial alignment characteristics is an alignment film subjected to a rubbing treatment. 3. The ferroelectric electro-optical device according to claim 2, wherein the alignment film having uniaxial alignment characteristics is made of a polyimide thin film. 4. The ferroelectric liquid crystal according to claim 1, wherein the alignment film having random horizontal alignment characteristics is an alignment film obtained by firing an alignment film having vertical alignment at a temperature equal to or higher than the temperature at which the vertical alignment disappears. Electro-optical device. 5. The ferroelectric liquid crystal electro-optical device according to claim 1, wherein the alignment film having random horizontal alignment characteristics is made of an organic silane compound thin film.