JPH0727140B2 - Liquid crystal element - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electro-optical element using a liquid crystal, and more particularly to a liquid crystal element using a ferroelectric liquid crystal as the liquid crystal.

[Conventional technology]

Currently, electro-optical elements using liquid crystals, that is, display elements and shutter arrays for printer heads are under active development, and display elements have been widely put into practical use. However, the response speed of the conventional liquid crystal element is limited, and the response speed of the twisted nematic type widely used as a display element is about 30 milliseconds at room temperature, and it is extremely difficult to shorten this response speed by one digit. It is common sense that Although the display element can withstand use at this level of response speed, it still has insufficient operation at low temperatures, and it is necessary to reduce the response speed by about two digits in order to use it in a shutter array for a printer head. is there. Under such circumstances, a chiral smectic liquid crystal of a type called a ferroelectric liquid crystal has recently attracted attention. This is because the response speed of this type of ferroelectric liquid crystal is reduced by about 2 to 4 digits compared to the twisted nematic type liquid crystal and has high speed response.

It is said that Noel A. Clark and Sven T. Lag-erwall were the first to confirm this high-speed response exhibited by the ferroelectric liquid crystal. The contents are Vol. 36, No. 11 (198) of Applied Physics Letters.
It is described in their papers published on pages 899 to 901 (published 0 years). That is, in the chiral smectic liquid crystal exhibiting ferroelectricity, liquid crystal molecules (directors) 22 having a spontaneous polarization 21 have a layered structure and at the same time have a helical orientation as shown in FIG. As it is, the spontaneous polarization 21 is distributed evenly around the spiral axis 23 and cancels each other. However, as shown in FIG. When sandwiched by 32 and the gap, that is, the thickness of the liquid crystal is reduced to at least the pitch length of the helical structure, the liquid crystal molecule 22 has two orientation states in which the spontaneous polarization 21 is perpendicular to the substrates 31 and 32. That is, the C director, which is the unit vector indicating the local average orientation direction of the long axis of the liquid crystal molecule, and the C director, which indicates the direction of the orthogonal projection onto the layer surface of the director, is forcibly oriented in one of two states in which they are oriented in parallel. To be In FIG. 4, region A is a state in which the spontaneous polarization 21 faces the lower substrate 31, and region B is a state in which the spontaneous polarization 21 faces the upper substrate 32. FIG. 5 is a view seen from the upper surface of the substrate, and shows that the liquid crystal molecules in the regions A and B have two different alignment states 41 and 42. FIG. 6 is a diagram showing the two orientation states of the area A and the area B by the orientation state of the C director 51 viewed from the direction of arrow a in FIG. As shown in FIG. 5, these two regions A and B are sandwiched by two polarizing plates whose polarization directions are orthogonal to each other, and the polarization direction 43 of one polarizing plate is aligned with the alignment direction.
When observed by being matched with 41 liquid crystal molecules, the region A looks dark and the region B looks bright. In this way, when a chiral smectic liquid crystal having a molecular long-axis helical orientation and a layered structure, that is, a ferroelectric liquid crystal is sandwiched between two substrates with electrodes having a very small gap, the C directors of the liquid crystal molecules are parallel to each other. It comes into two oriented states that are optically distinct from each other. Moreover, the spontaneous polarization of the ferroelectric liquid crystal directly responds to the external electric field and is oriented in the direction of the electric field. Therefore,
When an electric field which is parallel to the layer and whose direction is reversed is applied, the direction of spontaneous polarization is reversed according to the reversal of the electric field. That is, the fifth
Areas A and B in the figure are electrically switched.
Moreover, since this electrical switching phenomenon is due to the spontaneous polarization and the direct response of the external electric field, the response speed is extremely high. According to the above-mentioned paper, the response speed on the order of microseconds has been confirmed. In addition, the two states of region A and region B in FIG. 5 are energetically stable even when no external electric field is applied, and therefore the two states are electrically switchable and at the same time the external electric field is applied. It remains stable even after removing the. That is, it has a memory property. As described above, since the ferroelectric liquid crystal device described in the above-mentioned paper has high speed and memory property, research and development have been advanced, and development examples of large-capacity display devices have been reported.

[Problems to be solved by the invention]

However, in the above-mentioned ferroelectric liquid crystal element, it is necessary to eliminate the helical alignment in order to produce its characteristics, and in reality, this is realized by making the liquid crystal extremely thin. Specifically, the liquid crystal thickness of the above-mentioned ferroelectric liquid crystal element (so-called panel cap) is about 2 microns, which is extremely small compared with the conventional twisted nematic liquid crystal element having a liquid crystal thickness of about 6 to 8 microns. thin. In order to realize such a thin liquid crystal thickness, it is necessary to make special efforts in the panel manufacturing technology, the yield is low, it takes a long time to inject the liquid crystal substance into the panel, and the glass substrate is required. On the other hand, highly accurate smoothness is required, and it is necessary to use an expensive glass substrate. Thus, although the above-mentioned ferroelectric liquid crystal element is excellent in performance, the manufacturing cost becomes high,
It had a drawback that the device was expensive.

The object of the present invention is to overcome the above-mentioned drawbacks and to provide a liquid crystal of excellent performance having the same high speed and memory property as the above-mentioned ferroelectric liquid crystal element even with a liquid crystal thickness as thick as the conventional twisted nematic liquid crystal element. It is to provide an element.

[Means for solving problems]

The present invention relates to a liquid crystal device in which a chiral smectic liquid crystal having a molecular long-axis helical orientation and a layered structure is sandwiched between two substrates each having an electrode for applying an electric field which is substantially parallel to the layers and whose direction is reversed. In the first substrate, the C directors provided on the respective inner surfaces of the two substrates in contact with the liquid crystal and showing the direction of the orthogonal projection of the long axis of the liquid crystal molecules to the layer surface are oriented substantially in parallel.
And second orientation means for causing the C director to assume either a second orientation state in which the C director is rotationally oriented in the layer, and the first and second orientation means formed by reversing the direction of the electric field. Means for distinguishing the light passing through the liquid crystal regions in the respective alignment states in the alignment state and the second alignment state, and the first and second alignment means have liquid crystal molecules in the vicinity of the alignment means. Aligning substantially parallel to the aligning means and the substrate, and the first aligning means has a strong binding force such that liquid crystal molecules in the vicinity of the aligning means hardly change in alignment by application of the electric field, The second alignment means has such a weak binding force that liquid crystal molecules in the vicinity of the alignment means easily change their alignment by the application of the electric field, and the distance between the first and second alignment means is large. Is the second orientation state A liquid crystal element characterized in that long enough to present a constant.

[Action / Principle]

The operation and principle of the liquid crystal device of the present invention will be described with reference to FIGS. 1 and 2.

FIG. 1 is a schematic view showing an alignment state of liquid crystal molecules as seen from the top surface of the device, and 11, 12, 13 represent liquid crystal molecule long axes (directors). Reference numeral 11 represents a state at the interface with the lower substrate (substrate 17 in FIG. 2), and 12 represents a state at the interface with the upper substrate (substrate 16 in FIG. 2). Therefore, in the device, the direction from 11 to 12 ( The angle between them is θ) and a twist occurs. Reference numeral 13 indicates that they are oriented substantially in the same direction at the interface with the upper and lower substrates and in the element. FIG. 2 is a schematic diagram showing an alignment state of liquid crystal molecules as seen from the direction of arrow a in FIG. 1, where 14 is a C director and 15 is a direction of spontaneous polarization. In FIG. 2, the layer surface of the layer structure formed by the chiral smectic liquid crystal is substantially parallel to the paper surface. Second
As can be clearly seen from the figure, in the region B, the C director 14 is in a first alignment state in which the C director 14 is oriented substantially in parallel, and in the region A, the C director 14 is rotationally aligned in the layer in a second alignment state. It is in a state. In FIG. 2, 16
Reference numerals 17 and 17 represent an upper substrate and a lower substrate, respectively, on which electrodes are formed on their inner surfaces, and it is possible to apply and switch electric fields E _A and E _B which are substantially parallel to the layers and whose directions are inverted. Area A is an electric field
Represents a liquid crystal molecule orientation state after removal of it after applying the case and the electric field E _A is applied to E _A, after region B is removed it after applying the case and the electric field E _B is applied an electric field E _B Represents the alignment state of liquid crystal molecules. The inner surfaces of the substrates 16 and 17 in contact with the liquid crystal have been subjected to an alignment treatment (alignment means) so that the liquid crystal molecule alignment state of FIG. The orientation means has a strong binding force so that the orientation of the liquid crystal molecules near the substrate 17 does not change not only when the electric field E _B is applied but also when the electric field E _{A in the} direction opposite to the spontaneous polarization is applied. On the other hand, board 16
The second alignment means provided on the inner surface of the device has such a weak binding force that the liquid crystal molecules in the vicinity of the substrate 16 easily change their alignment due to the switching of the electric fields E _A and E _B. Such two kinds of alignment means are formed by forming a polymer film on the inner surface of the substrate and, if necessary, using a method known in the field of liquid crystals called rubbing on the surface of the polymer film. According to an experiment by the inventor, when a polymer film having a molecular structure having a strong permanent dipole is used, the direction of spontaneous polarization is strongly bound by a strong polar interaction with liquid crystal molecules.
When a polymer film with a molecular structure that has a relatively long methylene group and has a very small or no permanent dipole is formed, the interaction with the liquid crystal molecules has a non-polar dispersion force. As a main component, the second alignment means is formed so that the direction of the spontaneous polarization is not restricted and the direction of the long axis of the liquid crystal molecule is restricted substantially parallel to the alignment means. The polymer film material used for the first orientation means found by the inventor is a ferroelectric polymer material such as polyvinylidene fluoride or a polyimide material, and the polymer film material used for the second orientation means. Examples include polyvinyl alcohol materials and alkylsilane materials. In addition, the rubbing treatment described above is added to the first orientation means, or an oblique method using silicon monoxide, alumina, yttrium oxide, or the like as a base before forming the polymer film described above is used. Experiments by the inventor have found that means such as forming a vapor deposition film is more effective. Further, the liquid crystal element of the present invention is provided with a means for distinguishing the transmitted light between the region B in the first alignment state and the region A in the second alignment state. That is, linear polarizing plates are attached to the outsides of the second substrates 16 and 17, and their polarization axes are set in the directions shown by 18 and 19 in FIG. 1, for example. In the liquid crystal element having such a structure,
Considering the light incident from the lower surface side of the element, that is, from the lower side of the substrate 17 in FIG. 2, the light incident on the liquid crystal region inside the element by the linear polarizing plate provided on the substrate 17 is shown by 19 in FIG. It is a linearly polarized light in which the electric field component oscillates in the vertical direction. In the region A, the liquid crystal molecule orientation is twisted in the direction from 11 to 12, and the twisting period is sufficiently smaller than the wavelength of light.
Since the vibration direction of the electric field component of the linearly polarized light is twisted according to the alignment state of the liquid crystal molecule long axis as the light advances, the vibration direction of the linearly polarized electric field component is directed in the direction of 12 when reaching the substrate 16. Become. The direction of 12 is the direction of the absorption axis of the linearly polarizing plate provided on the substrate 16 (perpendicular to 18, ie,
The light is transmitted because it is out of alignment. On the other hand, in the region B, since the major axes of the liquid crystal molecules are all oriented in the direction of 13, the twist of the linearly polarized light as in the region A does not occur, and the incident linearly polarized light remains in the polarized state to the substrate 16 side. Reach However, its polarization direction 19 is
Since it coincides with the direction of the absorption axis of the linear polarizing plate provided in 16, light does not pass through the upper surface of the device. Thus, region A allows light to pass while region B blocks light. And
The regions A and B can be electrically switched by switching the direction of the applied electric field to E _A and E _B. Moreover, the response speed is very high. This is because the high-speed molecular rotation motion called the Goldstone mode rotating on the cone shown in FIG. 3 which is peculiar to the ferroelectric liquid crystal is utilized also in the liquid crystal device of the present invention. It should be noted that FIG. 1 shows the case where the twist angle of the liquid crystal molecule long axis (θ described above) is 45 °. Axes 11 and 13
The light quantity ratio of the transmitted light between the area A and the area B can be maximized by making the values match with. Further, in FIG. 1, the case where two linear polarizing plates are used as a means for distinguishing the transmitted light of two liquid crystal regions has been described. By mixing the dye in the ferroelectric liquid crystal, it is possible to distinguish the transmitted light of two liquid crystal regions with one linear polarizing plate. The second alignment state of the region A shown in FIGS. 1 and 2 is derived from the continuum elasticity theory, and it is derived that the second alignment state exists energetically stably when the liquid crystal thickness is relatively large. The distance between the first and second orienting means in FIG. 2, ie approximately two substrates
The gaps 16 and 17 need to be long enough so that the second alignment state exists stably. In the experiments by the inventor, this value was typically 5-8 microns.

〔Example〕

Hereinafter, examples performed for confirming the effect of the liquid crystal element of the present invention will be described.

On the surface of two glass substrates (16, 17) equipped with transparent electrodes of indium oxide, one is formed with a 1000 Å thick film of polyimide as a first orientation means by a spinner and rubbed on the surface. Then, on the other one, a 750 Å thick film of polyvinyl alcohol was formed by a spinner as a second orientation means. The two substrates were faced to each other with the orienting means inside, and a glass fiber having a diameter of 6 μm was interposed as a spacer in the gap, and the periphery was fixed with an epoxy adhesive leaving one defective portion. From this defect portion, chiral smectic liquid crystal (ferroelectric liquid crystal) CS-1014 (manufactured by Chisso Corporation) having a helical alignment of the long axis of the molecule and a layer structure was injected. When observed with a polarization microscope in this state, a slightly colored and bright state was observed, which indicates that the C director was in the second orientation state in which the C director was rotationally oriented. Also, through the transparent electrodes provided on the two substrates
When a DC voltage of 10 V was applied, a dark-field state was obtained under a polarizing microscope in the crossed Nicols state (where the polarizing plates were orthogonal to each other). That is, it was confirmed that this state was the first orientation state in which the C directors were oriented substantially in parallel. When the polarity of the applied voltage is reversed in this state,
It switched to the slightly colored bright state described above. Further, it was confirmed that although the transmitted light intensity slightly changes when the applied voltage is cut off in any of the states, the above-described two distinguishable states are maintained, that is, the memory property is maintained. In this state, two polarizing plates were attached to the outside of each substrate as a means for distinguishing the transmitted light intensities in the two alignment states. The polarization axes thereof were matched with the polarization axis directions of the two polarizing plates (polarizers) in the above-mentioned polarization microscope. When a DC voltage of ± 10 V was switched and applied to the liquid crystal element completed in this way, a change in brightness was observed visually. When the change in transmitted light intensity was measured by irradiation with light, the contrast ratio of light and dark was 10: 1, and the switching time was 250 microseconds. These values have almost the same contrast ratio and a switching time of about 1/100 as compared with the conventional twisted nematic element. For comparison, the same ferroelectric liquid crystal CS-1014 was used to fabricate a conventional liquid crystal device with a liquid crystal thickness of 2 microns, which was described in the above-mentioned document. The ratio was 10: 1. Since the liquid crystal element of the embodiment according to the present invention has a liquid crystal thickness of 6 μm and is similar to the twisted nematic element, it goes without saying that it can be manufactured with high yield and low cost.

〔The invention's effect〕

As described above, according to the present invention, a liquid crystal element capable of performing high-speed switching with a response speed reduced by about two digits as compared with a twisted nematic element and having a memory property is the same as the twisted nematic element. It can be supplied at a price.

[Brief description of drawings]

1 and 2 are diagrams showing the alignment state of liquid crystal molecules in the liquid crystal element of the present invention, FIG. 3 is a view showing the helical alignment state of ferroelectric liquid crystal molecules, and FIG. 4 is a ferroelectric liquid crystal of conventional structure. FIG. 5 is a view showing an orientation state in which the spiral is eliminated in the device, FIG. 5 is a top view of FIG. 4, and FIG. 6 is a sectional view. 11,12,13 …… Director, 14 …… C director 15 …… Spontaneous polarization, 16,17 …… Substrate 18,19 …… Polarizing axis of polarizing plate

Claims (1)

[Claims] 1. A chiral smectic liquid crystal having a helical alignment of a long axis of a molecule and a layer structure is sandwiched between two substrates each having an electrode for applying an electric field which is substantially parallel to the layer and whose direction is reversed. In a liquid crystal element, a first alignment state or a C director in which C directors, which are provided on respective inner surfaces of the two substrates in contact with liquid crystal, and which indicate a direction of orthogonal projection of a liquid crystal molecule major axis to a layer surface, are aligned substantially in parallel. And first and second orientation means for causing one of the second orientation states rotationally oriented in the layer, and the first and second orientation states formed by reversing the direction of the electric field. Means for distinguishing the transmitted light of the liquid crystal regions in the respective alignment states from the alignment state, and the first and second alignment means have liquid crystal molecules in the vicinity of the alignment means and the alignment means and the substrate. Aligned approximately parallel to Said first alignment means has a strong binding force, such as the liquid crystal molecules in the vicinity does not cause a substantial change in the orientation of the orientation means by application of said electric field, said second
The aligning means has a weak binding force so that even the liquid crystal molecules in the vicinity of the aligning means easily change the alignment by the application of the electric field, and the distance between the first and second aligning means is equal to the first aligning means. A liquid crystal device characterized in that the liquid crystal device is made long so that the alignment state of 2 exists stably.