Liquid crystal optical elements

Abstract

To provide a liquid crystal optical element which has a wide angle of field and an excellent visibility for projecting a high-contrast projection image.SOLUTION: The liquid crystal optical element of the present invention includes two transparent substrates and a display function layer between the transparent substrates, the display function layer including a liquid crystal and a photoisomerization material, and the liquid crystal optical element being switched between an optical transmission state and an optical diffusion state according to the orientation state of the liquid crystal. Since the display function layer includes a coagulant aid, liquid crystal molecules cohere in a screen state and this causes a significant disorder of the orientation of the liquid crystal, whereby a liquid crystal optical element with an excellent visibility can be provided.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a liquid crystal optical element, and more particularly to a liquid crystal optical element that switches between a transparent state that transmits light and a screen state that scatters light. [Background technology]

[0002] Liquid crystal optical elements are known whose optical state can be switched between a transparent state that transmits light and a screen state that scatters light, and visible images are displayed by projecting visible light onto a liquid crystal optical element in the screen state.

[0003] Patent Document 1 discloses an image display using a liquid crystal optical element that is in a screen state when ultraviolet light is irradiated onto a liquid crystal optical element containing liquid crystal and azobenzene, causing the azobenzene molecules to change from trans to cis isomers, and the bent molecular structure of the cis azobenzene disrupts the alignment of the liquid crystal, scattering light. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-185511 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the liquid crystal optical element described in Patent Document 1 has low contrast in the projected image and a narrow viewing angle, so there is room for improvement in visibility.

[0006] The present invention has been made in consideration of the problems associated with the prior art, and its object is to provide a liquid crystal optical element that has a wide viewing angle, is capable of projecting a high-contrast image, and has excellent visibility. [Means for solving the problem]

[0007] As a result of extensive research into achieving the above-mentioned object, the inventors have discovered that adding an aggregation aid makes it easier for liquid crystal molecules to aggregate in the screen state, significantly disrupting the orientation of the liquid crystal, reducing light transmittance and improving diffuse reflectance, thereby achieving the above-mentioned object, and have thus completed the present invention.

[0008] That is, the liquid crystal optical element of the present invention is an optical element that comprises a display function layer sandwiched between two transparent substrates, the display function layer containing liquid crystal and a photoisomerizable material, and that switches between a light-transmitting state and a light-scattering state depending on the orientation state of the liquid crystal. The display function layer further comprises: The liquid crystal is easily aggregated, and the liquid crystal is represented by the structural formula (1). Coagulation aid 3~10% by mass It is characterized by containing [Effects of the Invention]

[0009] According to the present invention, by adding an aggregation aid to the display function layer, liquid crystal molecules aggregate together in the screen state, causing significant disruption to the orientation of the liquid crystal, making it possible to provide a liquid crystal optical element that has low light transmittance, high diffuse reflectance, and excellent visibility. [Brief explanation of the drawings]

[0010] [Figure 1] 1A and 1B are diagrams illustrating a mechanism by which a liquid crystal optical element switches between a light-transmitting state and a light-scattering state. [Figure 2] 10 is a graph showing the relationship between the wavelength of light and the haze value of a liquid crystal optical element of a comparative example in a transparent state. [Figure 3] 1 is a graph showing the relationship between the amount of aggregation additive added and the haze value of a liquid crystal optical element. [Figure 4] 1 is a graph showing the relationship between the amount of aggregation aid added and the diffuse reflectance in Example 1. [Figure 5] 1 is a graph showing the relationship between the amount of aggregation aid added and the diffuse reflectance in Example 2. [Figure 6] 10 shows polarizing microscope images of the screen state of the liquid crystal optical elements of Comparative Example 1 and Examples 1-3. [Figure 7]1 is a graph showing the relationship between the amount of aggregation aid added and light transmittance in Example 1. [Figure 8] 1 is a graph showing the relationship between the amount of aggregation aid added and light transmittance in Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0011] The liquid crystal optical element of the present invention will now be described in detail. The liquid crystal optical element of the present invention comprises a display function layer sandwiched between two transparent substrates, the display function layer being capable of changing between a transparent state that transmits light and a cloudy screen state that scatters light, and the display function layer contains liquid crystal and a photoisomerizable material, and further contains an aggregation aid that facilitates aggregation of the liquid crystal molecules.

[0012] First, the mechanism by which the display function layer changes from a transparent state to a cloudy screen state due to light scattering will be described.

[0013] The liquid crystal molecules are nematic liquid crystal molecules that have a rigid mesogenic skeleton and a flexible long-chain alkyl group, and have optical anisotropy and dielectric anisotropy. Under normal conditions, the rod-shaped liquid crystal molecules associate with each other and align in a substantially uniform direction.

[0014] The photoisomerizable material is a material whose structure changes when exposed to ultraviolet light or an electric charge, causing the liquid crystal molecules to disorganize and become randomly oriented. For example, a compound having an azobenzene structure in which two benzene rings are bonded by an azo group (hereinafter sometimes referred to as azobenzene) can be used.

[0015] When exposed to ultraviolet light, the azobenzene undergoes a structural change from a trans isomer to a cis isomer. The trans isomer of azobenzene has a planar structure, whereas the cis isomer has a curved, three-dimensional structure. In Figure 1, trans azobenzenes are shown as straight lines, and cis azobenzenes are shown as L-shaped.

[0016] When the display function layer is not exposed to ultraviolet light, the azobenzene is in a trans isomer with a planar structure, and the liquid crystal molecules form a nematic phase with directional order, as shown on the left side of Figure 1. The liquid crystal molecules in this nematic phase are aligned in a direction perpendicular to the vertical alignment film by the vertical alignment film, so the display function layer is in a transparent state that allows light to pass through.

[0017] When the display functional layer is irradiated with ultraviolet light, azobenzene isomerizes to a cis form. As azobenzene changes from a planar structure to a curved structure, the alignment of the liquid crystal molecules becomes disordered and they become oriented in random directions, as shown on the right side of Figure 1. As a result, the light incident on the display functional layer is reflected and scattered by the liquid crystal, resulting in a cloudy screen.

[0018] The liquid crystal optical element of the present invention contains an aggregation aid in the display function layer that facilitates aggregation of liquid crystal molecules, and therefore, in the screen state, not only is the alignment of the liquid crystal molecules disturbed by azobenzene, but the liquid crystal molecules are also aggregated by the aggregation aid, resulting in greater disorder of the alignment of the liquid crystal.

[0019] Therefore, light incident on the display functional layer is scattered in all directions approximately isotropically by the liquid crystal, and the amount of incident light that passes through the display functional layer and exits to the opposite side is reduced, thereby improving visibility.

[0020] In other words, when a visible image with a contrast ratio of 90 (brightness ratio of 10:100) is projected, if light scattering is weak and many components pass through the display function layer, for example, if the light transmittance of the display function layer is 80%, the contrast ratio of the projected image will decrease to 18 (brightness ratio of 2:20). In particular, if the light incident from the back side, i.e., the side opposite to the side on which the visible image is projected, is strong, the contrast of the projected image will decrease.

[0021] When light is strongly scattered and only a small amount of light passes through the display function layer, for example, when the light transmittance of the display function layer is 50%, the contrast of the projected image will be 45 (brightness ratio 5:50). In addition, the transmittance of light incident from the back side also decreases, allowing for a high-contrast projected image. Furthermore, because the projected light is scattered in all directions, the viewing angle is widened and visibility is improved.

[0022] The cis azobenzene isomer returns to the trans isomer when left under or exposed to visible light, so that the liquid crystal molecules also return to their original alignment state, and the display function layer can return to its transparent state. The display function layer of the present invention returns to a transparent state when irradiated with visible light, and can be quickly returned to a transparent state by applying an electric charge to the display function layer.

[0023] The aggregation aid may include a compound represented by the following structural formula (1).

[0024] [ka] In the structural formula (1), X and Y represent an electron-withdrawing group or an electron-donating group, and X and Y may be the same or different. a and b each independently represent an integer of 0 to 5.

[0025] The reason why the compound represented by the structural formula (1) easily aggregates liquid crystal molecules in the screen state has not been clarified, but it is presumed that the compound has an affinity for the mesogenic skeleton of the liquid crystal molecules, and therefore attracts and aggregates the liquid crystal molecules when the orientation of the liquid crystal molecules is disturbed by azobenzene.

[0026] The aggregation aid preferably has an electron-donating or electron-withdrawing substituent.

[0027] The liquid crystal contains liquid crystal molecules having dielectric anisotropy in which electron-withdrawing or electron-donating substituents are bonded to the mesogenic skeleton, and the liquid crystal molecules maintain their alignment state due to the dielectric anisotropy.

[0028] Since the charge of the mesogenic skeleton of the liquid crystal molecules is biased in this way, if the charge of the biphenyl structure of the aggregation aid is also biased, the affinity with the liquid crystal molecules will be further increased, and the solubility and dispersibility in the liquid crystal will be improved.

[0029] Therefore, if the aggregation aid has a biased charge, it is possible to increase the amount added without reducing the transparency in the transparent state, and further improve the light scattering properties and visibility.

[0030] Examples of the electron-withdrawing group include a halogeno group (preferably a fluoro group, a chloro group, or a bromo group), a cyano group, a carboxy group (a carboxylic acid group), a carboxylic acid ester group, a carboxylic acid amide group, a sulfo group (a sulfonic acid group), a nitro group, a dicyanoethylene group, and an aldehyde group.

[0031] Examples of the electron-donating group include an alkoxy group, an alkylthio group (preferably an alkylthio group having 1 to 2 carbon atoms (methylthio group, ethylthio group)), an amino group, an amide group, a sulfonamide group, a hydroxyl group, a thiol group, a benzothiazole group, and an indolinyl group.

[0032] Although it depends on the structure of the mesogenic skeleton portion of the liquid crystal molecule, the substituent in the structural formula (1) is preferably a fluoro group.

[0033] The content of the aggregation aid in the display function layer is preferably 1 to 10% by mass. When the content of the aggregation aid is within the above range, the diffuse reflectance of the display function layer is improved, and the light transmittance is reduced, improving the visibility of the projected image.

[0034] The liquid crystal may be a nematic liquid crystal containing liquid crystal molecules with a biphenyl structure and dielectric anisotropy. The nematic liquid crystal may contain multiple types of liquid crystal molecules with different structures, such as E44 and MLC2172 (both manufactured by Merck).

[0035] As the azobenzene, any known azobenzene can be used, but the azobenzene represented by the following structural formula (2) is preferably used because it has a strong power to twist liquid crystal molecules.

[0036] [ka] In the structural formula (2), R represents a linear alkyl group having 5 to 10 carbon atoms.

[0037] The display function layer may contain a non-photoresponsive chiral layer, if necessary. By containing a non-photoresponsive chiral compound, the twisting force induced by azobenzene to the liquid crystal can be canceled in the transparent state, preventing the alignment of the liquid crystal from becoming disordered and increasing transparency.

[0038] The non-photoresponsive chiral compound may be one represented by the following structural formula (3).

[0039] [ka] In the structural formula (3), R represents a linear alkyl group having 5 to 10 carbon atoms.

[0040] Furthermore, the display function layer may have a columnar or wall-shaped resin standing in the thickness direction of the display function layer, if necessary. The presence of such a resin makes it easier for the liquid crystal to be oriented in a standing manner in the thickness direction of the display function layer, thereby improving transparency in the transparent state.

[0041] The columnar or wall-shaped resin can be prepared by polymerizing a photopolymerizable monomer while applying an electric field in the thickness direction of the display function layer.

[0042] The transparent substrate may be made of glass, resin, or the like, and an ITO film or the like may be used as the transparent electrode when applying an electric charge to the display function layer.

[0043] Furthermore, the liquid crystal optical element of the present invention may have an ultraviolet light blocking layer, if necessary. The ultraviolet light-shielding layer is a layer that blocks ultraviolet light incident from the rear side of the display function layer, and is a transparent film that contains an ultraviolet absorber and an ultraviolet light scattering agent.

[0044] The ultraviolet absorber may be a conventionally known ultraviolet absorber that absorbs ultraviolet light having a wavelength of 400 nm or less, does not absorb visible light, and has little coloring, such as benzophenone derivatives, salicylic acid ester derivatives, triazole derivatives, and acrylonitrile derivatives. Furthermore, examples of the ultraviolet light diffuser include titanium oxide and zinc oxide.

[0045] The liquid crystal optical element can be produced by injecting a mixture containing liquid crystal molecules, a photoisomerizable material, and, if necessary, a non-photoresponsive chiral material, a photopolymerizable monomer, and a polymerization initiator between two transparent substrates.

[0046] The liquid crystal optical element of the present invention can be used, for example, in automobile windshields or show windows, and can be switched between a screen state in which a visible light image can be projected and displayed, and a transparent state in which the other side can be seen. [Example]

[0047] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples.

[0048] [Comparative Example] A transparent substrate was obtained by forming a transparent electrode (indium tin oxide: ITO film) on the entire surface of one side of a transparent glass, and a vertical alignment film (polyimide) was formed on the entire surface of the transparent electrode.

[0049] A mixed solution was prepared containing 86.5% by mass of nematic liquid crystal (E44 manufactured by Merck), 4.9% by mass of azobenzene represented by the following structural formula (4), 3.1% by mass of a non-photoresponsive chiral monomer represented by the following structural formula (5), 3.3% by mass of the monomer of structural formula (6) and 1.7% by mass of the monomer of structural formula (7) for a total of 5.0% by mass, and 0.5% by mass of a polymerization initiator (IRGAUCERE819 manufactured by IGM Resins BV).

[0050] [ka]

[0051] [ka]

[0052] [ka]

[0053] [ka]

[0054] Two transparent substrates were placed with the vertical alignment film facing inward, and the above mixture was poured between the two transparent substrates while heating. Using a CCS LED PROCESSOR LSS-26, an electric field was applied between the two transparent substrates, and light with a wavelength of 420 nm and a light intensity of 3 mW / cm was emitted from one of the transparent substrates. 2 The resulting liquid crystal optical element had a resin extending in the thickness direction of the display function layer.

[0055] [Example 1] A liquid crystal optical element was obtained in the same manner as in Comparative Example 1, except that 2-fluorobiphenyl was used as the aggregation aid and the formulation of the mixed liquid was changed as shown in Table 1.

[0056] [Table 1]

[0057] [Example 2] A liquid crystal optical element was obtained in the same manner as in Example 1, except that biphenyl was used as the aggregation aid.

[0058] <Evaluation> (Haze value measurement) The haze value of the liquid crystal optical element in the transparent state was measured using a Murakami color research laboratory HAZEMETER HM-65W (manufactured by Murakami Color Research Laboratory). FIG. 2 is a graph showing the relationship between the wavelength of light and the haze value of the liquid crystal optical element of the comparative example, and FIG. 3 is a graph showing the relationship between the amount of aggregation additive added and the haze value for light with a wavelength of 550 nm.

[0059] The graph in FIG. 3 shows that Example 1, in which 2-fluorobiphenyl was used as the aggregation aid, had a lower haze value in the transparent state and maintained a high transparency, even when the amount of aggregation aid added was greater than that of Example 2, in which biphenyl was used.

[0060] (Measurement of diffuse reflectance) The diffuse reflectance of the liquid crystal optical element in the screen state in the visible light region (550 nm) was measured using a colorimeter CM3600A (manufactured by Konica Minolta). The measurement results are shown in Figures 4 and 5, and polarizing microscope images of Comparative Example 1 and Examples 1-3 in the screen state are shown in Figure 6.

[0061] 4 and 5 show that Examples 1 and 2, which contain an aggregation aid, have higher diffuse reflectance and wider viewing angles than the Comparative Example, which does not contain an aggregation aid. Also, Fig. 6 shows that the inclusion of an aggregation aid causes the liquid crystal to aggregate.

[0062] (Measurement of light transmittance) Using the Murakami Color Research Laboratory HAZEMETER HM-65W (manufactured by Murakami Color Technology Research Institute), the total light transmittance in the visible range of the liquid crystal optical element in the screen state was measured. The measurement results are shown in Figs. 7 and 8.

[0063] From the graphs in Figs. 7 and 8, it can be seen that in Examples 1 and 2 containing the flocculation aid, the light transmittance is lower than that of the Comparative Example without the flocculation aid, indicating that an image with high contrast can be displayed.

Explanation of Reference Signs

[0064] 1 Liquid crystal optical element 2 Display functional layer 21 Liquid crystal molecules 22 Azobenzene molecules 23 Vertical alignment film 3 Transparent substrate

Claims

1. A display function layer is sandwiched between two transparent substrates, the display function layer contains a liquid crystal and a photoisomerizable material, A liquid crystal optical element that switches between a light-transmitting state and a light-scattering state depending on the orientation state of the liquid crystal, A liquid crystal optical element, wherein the display function layer further contains 3 to 10 mass % of an aggregation aid represented by the following structural formula (1) which facilitates aggregation of the liquid crystal: 【Chemistry 1】 In the structural formula (1), X and Y each represent a fluoro group, and a and b each independently represent an integer of 0 to 5.

2. A liquid crystal optical element as described in claim 1, characterized in that the aggregation aid is 2-fluorobiphenyl.

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