KR20230049320A - Liquid crystal display device - Google Patents

Abstract

The present invention provides a liquid crystal display device, which includes: an upper substrate; a lower substrate; and a liquid crystal layer which is formed between the upper substrate and the lower substrate, and in which cholesteric liquid crystal is distributed. When pressure is applied to the liquid crystal layer, a phase of the cholesteric liquid crystal transitions from a focal conic state to a planar phase to display information, and when light is irradiated to the phase-changed liquid crystal layer, the phase of the cholesteric liquid crystal transitions from the planar phase to the focal conic phase, so that the displayed information is locally erased.

Description

Liquid crystal display device {Liquid crystal display device}

The present invention relates to a liquid crystal display device, and more particularly, to a liquid crystal display device capable of locally erasing displayed information using photoisomerization of cholesteric liquid crystals.

Liquid crystals used in conventional liquid crystal displays include twisted nematic liquid crystals, semetic liquid crystals, cholesteric liquid crystals, and the like.

Double cholesteric liquid crystals have been used in reflective display devices. More specifically, the cholesteric liquid crystal can change into three states consisting of two stable states, a planar state and a focal conic state, and a homeotropic state. can

In addition, cholesteric liquid crystals can selectively reflect external light by wavelength, and in particular, during the phase transition of cholesteric liquid crystals, a focal conic phase that scatters external light and a planar image that reflects external light are reflective. have been used for display.

In particular, when a phase change occurs from a planar phase to a focal conic phase or from a focal conic phase to a planar phase in a conventional cholesteric liquid crystal, a voltage is applied to the cholesteric liquid crystal or the applied voltage is adjusted to make a homeotropic phase change. After going through the tropic phase, it has changed to other phases.

Currently, commercially available liquid crystal displays with cholesteric liquid crystal display information by applying pressure locally and then apply an electric field to erase the displayed information. There is a disadvantage that the information is erased at the same time.

In order to solve this problem, a method of patterning an electrode and controlling only a specific area using a thin film transistor has been used, but this has problems in that the manufacturing process is complicated and the manufacturing cost is high, which is not efficient.

1. Reflective liquid crystal display and its control method (Patent Application No. 10-2011-0103726)

The present invention is to solve the above-mentioned problems of the prior art, and an object of the present invention is a liquid crystal display capable of locally erasing displayed information by using photoisomerization of cholesteric liquid crystal by light irradiation without applying an electric field. to provide a device.

The problem to be solved by the present invention is not limited to the above-mentioned problems, and other problems not mentioned will be clearly understood by those skilled in the art from the following description.

A liquid crystal display according to an embodiment of the present invention for achieving the above object includes an upper substrate, a lower substrate, and a liquid crystal layer formed between the upper substrate and the lower substrate and in which cholesteric liquid crystal is distributed, wherein the liquid crystal layer When pressure is applied to the cholesteric liquid crystal, the phase of the cholesteric liquid crystal transitions from a focal conic state to a planar phase, information is displayed, and light is irradiated to one region of the phase-changed liquid crystal layer , the phase of the cholesteric liquid crystal transitions to the focal conic phase on the planar and the displayed information is locally erased.

In one embodiment of the present invention, the upper substrate may be a plastic substrate.

In one embodiment of the present invention, the cholesteric liquid crystal may include an azobenzene group.

In one embodiment of the present invention, the azobenzene group may be included in at least one of a nematic liquid crystal and a chiral dopant constituting the cholesteric liquid crystal.

In one embodiment of the present invention, the azobenzene group may be included in at least one of a photoinitiator, a pigment, and a polymer material added to the liquid crystal layer in addition to the cholesteric liquid crystal.

In an embodiment of the present invention, an alignment layer formed on and below the liquid crystal layer and including an optical alignment material containing an azobenzene group may be further included.

In one embodiment of the present invention, when light is irradiated on the phase-changed liquid crystal layer, the molecular structure of the azobenzene group may change from Trans to Cis.

In one embodiment of the present invention, the wavelength of the irradiated light may be in the ultraviolet region.

In one embodiment of the present invention, the light may be irradiated through an external light emitting body.

The present invention has an effect of enabling local erasure of displayed information by using a phase change of cholesteric liquid crystal by light irradiation without application of an electric field.

Accordingly, since information can be displayed and locally erased without application of an electric field, power consumption can be reduced.

In addition, since patterning of electrodes, thin film transistors, and the like for erasing displayed information are unnecessary, the manufacturing process is shortened and the manufacturing cost is reduced.

This has an effect of providing a liquid crystal display device that can be applied to an electronic book, an electronic blackboard, a personal electronic note pad, etc. by replacing existing paper notebooks, blackboards, and the like.

1 is a diagram showing the structure of a liquid crystal display according to an embodiment of the present invention.
2 is a diagram showing a phase change of a cholesteric liquid crystal layer according to an operating state of a liquid crystal display according to an exemplary embodiment of the present invention.
3 is a view showing photoisomerization of azobenzene according to an operating state of a liquid crystal display according to an embodiment of the present invention.
4 is a diagram showing a phase change and information display state by light irradiation of a liquid crystal display according to an embodiment of the present invention.

Since the present invention can make various changes and have various embodiments, specific embodiments will be illustrated in the drawings and described in detail in the detailed description.

However, this is not intended to limit the present invention to specific embodiments, and should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention. In describing the present invention, if it is determined that a detailed description of related known technologies may obscure the gist of the present invention, the detailed description will be omitted.

Terms used in this application are only used to describe specific embodiments, and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly dictates otherwise.

In the present invention, the term "comprises" or "has" is intended to designate that there is a feature, number, step, operation, component, part, or combination thereof described in the specification, but one or more other features It should be understood that the presence or addition of numbers, steps, operations, components, parts, or combinations thereof is not precluded.

1 is a diagram showing the structure of a liquid crystal display according to an embodiment of the present invention. Referring to FIG. 1 , a liquid crystal display device according to an exemplary embodiment includes an upper substrate, a lower substrate, and a liquid crystal layer formed between the upper substrate and the lower substrate and in which cholesteric liquid crystal is distributed.

As the upper substrate, a substrate used in a general liquid crystal display or a flexible liquid crystal display may be used without limitation, and more specifically, a transparent glass substrate or a plastic substrate may be used, but it is preferable to use a plastic substrate in terms of flexibility and durability. desirable.

For example, cellulose resins such as triacetyl cellulose (TAC) or diacetyl cellulose (DAC), cylic olefin polymers (COPs) such as norbornene derivatives, acrylic resins such as poly (methylmethacrylate) (PMMA), polycarbonate (PC), or Polyolefin such as PP (polypropylene), polyester such as PVA (polyvinyl alcohol), PES (poly ether sulfone), PEEK (polyetheretherketone), PEI (polyetherimide), PEN (polyethylenenaphthalate), PET (polyethyleneterephthalate), PI (polyimide), A sheet or film containing PSF (polysulfone) or a fluororesin may be used, but is not necessarily limited thereto.

The lower substrate serves to absorb scattered light of all wavelengths except light of a specific wavelength reflected when the cholesteric liquid crystal is a planar phase, and thus has a black color. That is, it is displayed in black to indicate a background state before information is displayed in the liquid crystal display or an information erasure state after the displayed information is erased.

The lower substrate may use a substrate coated with black oil paint to absorb light.

The liquid crystal layer includes cholesteric liquid crystal and displays or erases information on the liquid crystal display device by scattering or reflecting incident light through a phase change due to pressure or light irradiation.

Cholesteric liquid crystal is a chiral nematic liquid crystal formed by adding a chiral dopant or a photo-sensitive chiral dopant that induces a periodic helical structure to a nematic liquid crystal. ) means a liquid crystal, and additives such as a photoinitiator may be additionally included in the formation process.

Cholesteric liquid crystals can have a homeotropic phase, a planar phase, or a focal conic phase depending on the molecular arrangement structure. It has the property of maintaining a phase, that is, bistability.

The planar phase is a state in which the spiral axis of cholesteric liquid crystals having a helical structure is arranged in a direction perpendicular to the upper substrate and the lower substrate, and the focal conic phase is a state in which the helical axis of cholesteric liquid crystals having a helical structure is aligned in the upper substrate. A state in which the substrate and the lower substrate are randomly arranged, and the homeotropic phase means a state in which the spiral structure of the cholesteric liquid crystal is unwound and aligned in a direction perpendicular to the upper and lower substrates.

Optically, when the cholesteric liquid crystal is in the homeotropic phase, incident light is transmitted without reflection or scattering and maintains a transparent state.

2 is a diagram showing a phase change of a cholesteric liquid crystal layer according to an operating state of a liquid crystal display according to an exemplary embodiment of the present invention.

Referring to FIG. 2, when the cholesteric liquid crystal is in a planar phase, a specific wavelength of incident light is reflected by bragg reflection, and the reflected wavelength (λ) is the period (p) of the cholesteric liquid crystal and the liquid crystal It is expressed as a product of the average refractive index (n) of (λ = n x p). At this time, since the period of the liquid crystal is determined by the amount of chiral dopant added to the nematic liquid crystal, the wavelength of the reflected light can be determined by selecting an appropriate chiral dopant, and by using this property, visible light (red, blue) , green, etc.), it is possible to display information by reflecting the wavelength of the gamut.

The wavelength of the reflected light is determined by the period (p) of the cholesteric liquid crystal. As the period is longer, a long wavelength range of red series is reflected, and as the period is short, a short wavelength range of blue series is reflected. In addition, by adjusting the period of the cholesteric liquid crystal, it is possible to reflect a green wavelength range, which is a wavelength range near 550 nm, among visible light. can do.

On the other hand, when the cholesteric liquid crystal is in the focal conic phase, incident light is not reflected but scattered, and the scattered light is absorbed by the lower substrate, thereby enabling erasing of information.

As such, in order to display or erase information on a liquid crystal display device using the optical properties of cholesteric liquid crystals, a planar phase and a focal conic phase must be reversibly transferred. In general, when pressure or a high electric field is applied in the initial focal conic phase, the liquid crystal molecules are transferred to the homeotropic phase, and when the electric field is momentarily removed in the homeotropic phase, the liquid crystal molecules are transferred to the planar phase. Transition to the focal conic phase.

At this time, in the method of erasing the information displayed on the liquid crystal display through the phase change of the cholesteric liquid crystal according to the application of the electric field, the electric field is applied while the electrodes cover the entire area of the liquid crystal layer, so that the planar phase is all converted to the focal conic phase. However, there is a problem in that only information desired by the user cannot be erased.

Accordingly, pattern electrodes, thin film transistors, and the like are used as a method for controlling a specific region by applying an electric field only to a portion of the liquid crystal layer, but the manufacturing process is complicated and the manufacturing price is high.

In contrast, according to the present invention, information displayed on a liquid crystal display can be locally erased by allowing the cholesteric liquid crystal to be transferred from the planar phase to the focal conic phase through light irradiation without application of an electric field.

In one embodiment of the present invention, the liquid crystal display device may include a compound having an azobenzene group inside or outside a liquid crystal layer (cholesteric liquid crystal layer) in which cholesteric liquid crystals are distributed. Azobenzene group isomerizes from its stable trans- form to cis- form when exposed to light of wavelengths in the region absorbed by trans- isomers, and isomerizes back to trans- form when exposed to light of wavelengths in the region absorbed by cis- isomers. A reversible photoisomerization occurs. That is, the present invention can locally erase displayed information by inducing a phase transition (focal conic phase on a planar phase) of cholesteric liquid crystal through photoisomerization of an azobenzene group.

As a case where a compound having an azobenzene group exists inside the cholesteric liquid crystal layer, in the cholesteric liquid crystal molecular structure, such as an additive such as a nematic liquid crystal containing azobenzene group, a chiral dopant containing azobenzene group, or a photoinitiator containing azobenzene group. A cholesteric liquid crystal layer may be formed by adding an azobenzene group or a cholesteric liquid crystal, a pigment containing an azobenzene group, a polymer (reactive monomer) containing an azobenzene group, etc. Any configuration in which a compound having an azobenzene group is present inside the liquid crystal layer to induce a phase transition of the cholesteric liquid crystal through photoisomerization is included in the scope of the present invention.

In addition, as a case where a compound having an azobenzene group is present outside the cholesteric liquid crystal layer, an optical alignment layer containing a compound having an azobenzene group may be included to be distinguished from the cholesteric liquid crystal layer. The photo-alignment layer may be formed on the upper and lower portions of the cholesteric liquid crystal layer, and includes a compound having an azobenzene group that causes photoisomerization in response to light, that is, an photo-alignment material, thereby forming a cholesteric layer by irradiation of light. It aligns the liquid crystal molecules and consequently induces a phase transition of the cholesteric liquid crystal.

The compound containing the azobenzene group may be included in an amount of 10 parts by weight or more, preferably 10 to 30 parts by weight, based on 100 parts by weight of the total materials constituting the cholesteric liquid crystal layer or the optical alignment layer.

3 is a view showing photoisomerization of azobenzene according to an operating state of a liquid crystal display according to an embodiment of the present invention.

Referring to FIG. 3, according to the photoisomerization of the azobenzene group, which is easily reversibly converted between cis-trans forms by irradiation of light, the cholesteric liquid crystal changes from the planar phase (information display state) to the focal conic phase (information erased state) to erase information.

In particular, photoisomerization occurs only in a part of the azobenzene group that receives light, resulting in a phase change of the cholesteric liquid crystal. Therefore, through isomerization of the azobenzene group from the trans- form to the cis- form by light irradiation without a pattern electrode. Displayed information can be locally erased by inducing a phase change of only a part of the cholesteric liquid crystal.

As the compound containing the azobenzene group, a nematic liquid crystal containing an azobenzene group in the main chain or side chain (J. Mater. Chem. C, 2014), a chiral dopant (J. Mater. Chem. C, 2014), a pigment (N- Ethyl-N-(2-hydroxyethyl)-4-(4-nitrophenylazo)aniline), polymers (Polydisperse Orange 3 methacrylamide or Polydisperse Red 1 methacrylate), and photo-alignment materials (Langmuir 26, 2010) can be used, but are limited to these it is not going to be

The wavelength of the irradiated light is in the ultraviolet region (absorption by the π-π*), and photoisomerization of the azobenzene group from the trans- form to the cis- form occurs in the above wavelength range, thereby inducing a phase transition of the cholesteric liquid crystal. . Conversely, photoisomerization of the azobenzene group from the cis-form to the trans-form can occur in the visible light region (absorption by the n-π*).

In addition, the irradiation of light may be performed through an external light emitting element capable of erasing information displayed on a liquid crystal display by emitting light of the above wavelength range, for example, an eraser type light emitting element, but is not limited thereto, and azobenzene light It is included in the scope of the present invention as long as light is irradiated through an external light emitting body that emits light in the wavelength range so that tempering can occur.

Hereinafter, a liquid crystal display device according to various embodiments and experimental examples of the present invention will be described in more detail.

Example

1. A cholesteric liquid crystal mixture containing an azobenzene group

Example 1-1

About 5.5% of R1011 is mixed in liquid crystal containing an azobenzene group (J. Photochem. Photobiol. C: Photochem, 2004 or Appl. Chem. Eng, 2015), and trimethylolpropane triacrylate (TMPTA), pentaerythritol tetraacrylate (PETA) and 1,6 - Mix at least one of Hexanediol diacrylate (HDDA) and mix it with cholesteric liquid crystal by about 20% or more to prepare a mixture.

Example 1-2

A chiral dopant (J. Mater. Chem. C, 2014) containing an azobenzene group is blended into the liquid crystal at 1.9%, and any of Trimethylolpropane triacrylate (TMPTA), Pentaerythritol tetraacrylate (PETA) and 1,6-Hexanediol diacrylate (HDDA) A mixture is prepared by mixing one or more and blending about 20% or more of the cholesteric liquid crystal.

Example 1-3

Liquid crystal and R1011 are mixed at about 5.5%, and at least one of Trimethylolpropane triacrylate (TMPTA), Pentaerythritol tetraacrylate (PETA) and 1,6-Hexanediol diacrylate (HDDA) is mixed and mixed at about 20% or more in cholesteric liquid crystal. After that, a mixture was prepared by mixing 0.6% of Disperse Red 1 (Sigma-Aldrich) to the mixture.

Example 1-4

Liquid crystal and R1011 were mixed at about 5.5%, Poly(Disperse Red 1 acrylate), Poly(Disperse Orange 3 methacrylamide), Trimethylolpropane triacrylate (TMPTA), Pentaerythritol tetraacrylate (PETA) and 1,6-Hexanediol diacrylate (HDDA) (Sigma -Aldrich Inc.) is mixed with at least about 20% of cholesteric liquid crystal to prepare a mixture.

2. Liquid crystal display

Example 2-1

Apply one of the cholesteric liquid crystal mixtures together with a ball type spacer so that a certain distance can be maintained between substrates (glass, plastic, or other substrates having a certain level of transparency) including an electrode layer capable of applying a voltage. and curing by irradiating UV at an intensity of about 30 to 50 mW/cm ² for 10 to 30 minutes.

Example 2-2

Liquid crystal and R1011 are mixed at about 5.5%, and at least one of Trimethylolpropane triacrylate (TMPTA), Pentaerythritol tetraacrylate (PETA), and 1,6-Hexanediol diacrylate (HDDA) (Sigma-Aldrich) is mixed to form a cholesteric liquid crystal. A cholesteric liquid crystal mixture is made by blending 20% or more. An optical alignment material (Langmuir 26, 2010) is applied on a substrate (glass, plastic, or other substrate having certain transparency secured) including an electrode layer capable of applying voltage, and then the cholesteric liquid crystal is placed between the two substrates. After applying the mixture with a ball type spacer, it is cured by irradiating UV at an intensity of about 30 to 50 mW/cm ² for 10 to 30 minutes.

Experimental example

4 is a diagram showing a phase change and information display state by light irradiation of a liquid crystal display according to an embodiment of the present invention.

Referring to FIG. 4 , planar image 0 represents an information display state, and a photomask is placed on a cell in planner image 0 and UV is irradiated to convert a partial area into a focal conic image 1 to realize a locally erased information erase state. Next, pressure was applied to the partial area to convert the focal conic phase 1 to the planar phase 1 again, and then the photomask was placed on the cell again and UV was irradiated to convert the partial area to the focal conic phase 2, thereby locally erasing the same as above. The information erasure state was implemented. Through this, it was confirmed that the phase change of the cholesteric liquid crystal by light irradiation, which proceeds reversibly, and the local information erasure accordingly is possible.

Claims (9)

upper substrate;
lower substrate; and
A liquid crystal layer formed between the upper substrate and the lower substrate and in which cholesteric liquid crystal is distributed,
When pressure is applied to the liquid crystal layer, information is displayed by transitioning the cholesteric liquid crystal from a focal conic state to a planar state,
When light is irradiated on one region of the phase-changed liquid crystal layer, a phase of the cholesteric liquid crystal transitions from a planar phase to a focal conic phase, and the displayed information is locally erased. According to claim 1,
The upper substrate is a liquid crystal display device, characterized in that the plastic substrate. According to claim 1,
The liquid crystal layer in which the cholesteric liquid crystal is distributed includes an azobenzene group. According to claim 3,
The liquid crystal display device, characterized in that the azobenzene group is included in at least one of a nematic liquid crystal and a chiral dopant constituting the cholesteric liquid crystal. According to claim 3,
The azobenzene group is a liquid crystal display device, characterized in that included in at least one of a photoinitiator, a pigment and a polymer material added to the liquid crystal layer in addition to the cholesteric liquid crystal. According to claim 1,
The liquid crystal display device further comprises an alignment layer formed on top and bottom of the liquid crystal layer and including an optical alignment material containing an azobenzene group. According to any one of claims 3 to 6,
When light is irradiated on the phase-changed liquid crystal layer, the molecular structure of the azobenzene group is changed from Trans to Cis form. According to claim 1,
The liquid crystal display device, characterized in that the wavelength of the irradiated light is in the ultraviolet region. According to claim 1,
The liquid crystal display device, characterized in that the light is irradiated through an external light emitting body.

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