KR100285540B1 - Polymer dispersed liquid crystal - Google Patents

Abstract

PURPOSE: Provided is a polymer dispersed liquid crystal which has a short response time and enhanced voltage holding ratio, and which can be used in dynamic image display. CONSTITUTION: The polymer has a structure which polymeric compound and liquid crystal compound are dispersed between two sheets of substrate coated with transparent electrode. The polymeric compound consists of smectic or nematic polymer liquid crystal having mesogenic group such as the formulas(1)-(15). In the formulas, each m and n are an integer of 1-20. Acrylate monomers having one or at least two single functional group or a plurality of functional groups may be added to the liquid crystal compound. The liquid crystal component is halogen based liquid crystal.

Description

Polymer dispersed liquid crystal composite

The present invention relates to a polymer dispersed liquid crystal composite of a new type, and more particularly, a nematic or smectic liquid crystal polymer having a mesogen group as a polymer component and a halogen-substituted liquid crystal as a liquid crystal component. The present invention relates to a novel polymer dispersed liquid crystal composite which can be used for display of moving picture display with a fast response speed and improved voltage retention.

Twisted Nematic (TN) or Super Twisted Nematic (STN), which are currently the mainstream of liquid crystal displays, have low light utilization efficiency due to the use of polarizing plates, resulting in poor contrast. The alignment treatment around the device becomes difficult, and the viewing angle is narrow to around 20 °. Therefore, many efforts have been attempted to use a display by using a transmission scattering mode of light without using polarization. That is, recently, a polymer dispersed liquid crystal (PDLC) or a polymer network liquid crystal (PNLC) of light scattering mode in which a liquid crystal is dispersed in a polymer has appeared.

In the PDLC, when the electric field is not applied, the refractive indices of the liquid crystal and the polymer do not coincide, so that incident light is scattered and the cell appears to be opaque. When the electric field is applied, the liquid crystal is arranged in the electric field direction and the cell appears to be transparent. Since the polarizing plate is not used according to the principle of using light scattering and transmission in this way, the light utilization efficiency is higher than that of the conventional liquid crystal display device, and the luminance is high, and the viewing angle is excellent, so that dimming display is possible. However, the PDLC has a higher driving voltage than the conventional liquid crystal display device, so there is no problem in using a switchable window that can be used only by switching within a range of 100 volts. It is required to have a low driving voltage, a fast response speed, and a high voltage holding ratio (VHR).

Conventional liquid crystal displays using TN or STN use a polarizing plate, so the light utilization efficiency is low and the contrast is bad. Therefore, many efforts have been attempted to use the display using the impulse scattering mode of light without using polarization. The first published as such PDLC technology is a method in which a liquid crystal is dispersed in a transparent polymer resin (US Pat. No. 4,435,047). This is evenly dispersed liquid crystal in gelatin, gum arabic or polyvinyl alcohol aqueous solution, and then uniformly coated on the glass plate or polyester film coated with conductive material ITO to 5-20㎛ thickness, and then evaporated water Disclosed is a method of adhering a glass plate or a polyester film coated with another IT0.

Another more advanced manufacturing method of a polymer dispersed liquid crystal composite (PDLC) is a phase-separated manufacturing method using a solubility difference between a polymer and a liquid crystal monomer (US Pat. Nos. 4,688,900 and 4,685,771). This is prepared by dissolving the liquid crystal monomer in the monomer or oligomer of the transparent polymer resin and then reducing the solubility of the liquid crystal monomer as the polymerization reaction proceeds by UV or heat to precipitate the liquid crystal monomer in the form of a droplet.

When the refractive index n _p of the transparent polymer resin, the normal light refractive index n _o of the liquid crystal, and the ideal light refractive index are n _e , when no electric field is applied, the liquid crystal molecules become disordered and the refractive indexes of n _e and n _p are different so that incident light scatters. Therefore, the film appears opaque, and when the electric field is applied, the liquid crystals are arranged in the electric field direction, so that n _o and n _p coincide, and incident light is transmitted, so that the film appears transparent.

Unlike PDLC, polymers are continuous and liquid crystals form drooplets. PNLC is a three-dimensional network structure in which liquid crystals are continuous and polymers are cross-linked. (Japanese Patent No. 01-198725), In general, the driving voltage is lower than that of PDLC.

However, in order to be used as a display, it is also important to be driven within a range of driving ICs capable of driving a polymer dispersed liquid crystal composite film, but a moving picture can be realized only with a fast response speed and a high voltage holding ratio.

An object of the present invention is to provide a novel polymer dispersed liquid crystal composite which can be used for display of moving image display with low driving voltage, fast response speed and high voltage holding ratio.

That is, the present invention is a polymer dispersed liquid crystal composite in which a polymer compound and a liquid crystal compound are dispersed between two substrates coated with a transparent electrode, wherein the polymer component is a mesogen group represented by the following general formulas (1) to (15): It is to provide a polymer dispersed liquid crystal composite, characterized in that consisting of smectic or nematic liquid crystal polymer.

In general, the polymer dispersed liquid crystal composite is uniformly dispersed in a gelatin, gum arabic, or polyvinyl alcohol aqueous solution, and then uniformly coated on the glass plate or polyester film coated with conductive material IT0 with a thickness of 5-20 μm. Then, by evaporating water to bond another ITO coated glass plate or polyester film, and uniformly mixing the oligomer and monomer and the liquid crystal, the solubility of the liquid crystal decreases according to the polymerization of the oligomer and monomer to phase separation. It can be divided into the method of making.

In the phase-separation method, a liquid crystal monomer, a liquid crystal polymer, and an acrylate or methacrylate monomer capable of ultraviolet polymerization can be uniformly mixed with a photoinitiator between two glass plates or polyester films coated with a transparent electrode, followed by heat or ultraviolet rays. As the solubility of the liquid crystal decreases according to the polymerization of the oligomer and the monomer, (PIPS: Polymerization Induced Phase Separation), the liquid crystal of the oligomer and monomer such as chloroform dissolve all of these components in the co-solvent to form a homogeneous mixed solution. It is coated on a glass plate or polyester film coated with conductive material ITO using a bar coater with uniform thickness to remove the solvent and then phase separated by solvent (SIPS: Solvent Induced Phase Separation) After the liquid crystal is mixed uniformly, When the phase separation method for cooling the resin: and the like (TIPS Thermal Induced Phase Separation).

In the present invention, when the compatibility between the liquid crystal polymer and the acrylate or methacrylate monomer is good, it may be prepared by the PIPS method, but when the compatibility is poor or weak, it may be prepared by the SIPS method using a common solvent. At this time, if the evaporation rate of the solvent is too large, it appears in the form of polymer, and if it is too small, the size of the liquid crystal droplet becomes too large and the response speed becomes slow.

The liquid crystal polymer used in the present invention is preferably a nematic or smectic phase with a mesogenic group, and more preferably a smectic phase. The liquid crystal polymer of the smectic phase forms a layered structure to give the alignment force of the liquid crystal monomer, thereby minimizing the influence of the elastic modulus of the liquid crystal monomer to reduce the response speed. In addition, the voltage holding ratio can be improved by using a halogen-based liquid crystal, which is advantageous in improving the voltage holding ratio but is difficult to use due to a slow response speed.

The liquid crystal polymer used in the present invention preferably has a molecular weight of 5000 or less so that it can be easily dissolved in a solvent, and a low viscosity so as to further improve the response speed. The liquid crystal polymer used in the present invention has a structure such as the following general formulas (1) to (15).

In the above formula, m and n are each an integer of 1 or more and 20 or less.

Monomers for ultraviolet polymerization usable in the present invention include styrene and derivatives thereof; Methyl, ethyl, propyl, butyl, pentyl, hexyl, amyl, 2-ethylhexyl, octyl, nonyl, dodecyl, isodecyl, lauryl, hexadecyl, cyclohexyl, benzyl, methoxyethyl, ethoxyethyl, butoxy Ethyl, phenoxyethyl, allyl, metaallyl, glycidyl, 2-hydroxyethyl, 2-hydroxypropyl, 3-chloro-2-hydroxypropyl, dimethylaminohexyl, diethylaminohexyl, isobornyl and the like Acrylate or methacrylate having the same single functional group; Ethylene glycol, diethylene glycol, triethylene glycol, polyethylene glycol, propylene glycol, tripropylene glycol, polypropylene glycol, butylene glycol, tetramethylene glycol, hexamethylene glycol, neopentyl glycol, butanediol, hexanediol, trimethylolpropane And polyfunctional acrylates or methacrylates such as pentaerythritol and dipentaerythritol, and derivatives thereof.

The composition of the polymer compound used in the present invention may include an ultraviolet polymerization initiator and other additives in addition to the liquid crystal polymer and monomer described above. Preferred initiators for ultraviolet polymerization include 2-hydroxy-2-methyl-1-phenylpropane-1-silver (Merck, Darocure 1173), 1-hydroxycyclohexylphenyl ketone (Ciba-Geigy, Irgacure184), 1- (4-isopropylphenyl) -2-hydroxy-2-methylpropan-1-one (Merck, Darocure 1116), benzyldiethyl metal (Ciba-geigy, Irgacure651) and the like can be used.

As the liquid crystal monomer used in the present invention, it is preferable to use a halogen-based liquid crystal substituted with halogen such as fluorine at the end of the molecule having excellent suppression retention ratio so as to realize moving images. In general, cyan-based liquid crystals, which are widely used in the manufacture of polymer liquid crystal composites, are preferred for use in dimming glass because they have high contrast, good driving voltage, and fast response speed, but they are suitable for various display devices or displays requiring moving images. This low can not be used.

Halogen-based liquid crystals have other electro-optical properties, particularly voltage retention, but have a slow response time, and thus, there are limitations in the use of a polymer dispersed liquid crystal composite to realize a moving image. However, in the present invention, even when the halogen-based liquid crystal is used, the liquid crystal monomer has an orientation control force due to the side of the nematic or smectic liquid crystal polymer having a mesogenic group, so that the fall time at the time of removing the electric field is faster, resulting in excellent voltage retention and response speed. Fast polymer dispersed liquid crystal composite can be prepared.

When the liquid crystal polymer used in the present invention is prepared using a common solvent, it is preferable to use about 10-50% by weight with respect to the total composition, and more preferably 10-3O% by weight.

In the present invention, when mixed with an ultraviolet polymerizable monomer, such as acrylate or methacrylate, and prepared by the PIPS method of ultraviolet polymerization under a photoinitiator, the ratio of the liquid crystal polymer and the acrylate or methacrylate monomer is about 1: 1. However, when the monomer is insufficient and the liquid crystal polymer does not melt well, the acrylate or methacrylate monomer may be added up to about 1: 5. As the content of the liquid crystal polymer increases, the driving voltage increases, and as the content of the liquid crystal polymer decreases, the effect of reducing the response speed is reduced. In this case, the total addition amount of the liquid crystal polymer and the acrylate or methacrylate monomer is preferably used in an amount of about 10-50% by weight based on the total composition, as in the case of using a common solvent mixed with liquid crystal, and 10-30% by weight. The best photoinitiator is 1 to 5% by weight based on the acrylate or methacrylate monomer.

The substrate coated with the transparent electrode used in the present invention may be made of indium tin oxide (ITO) glass or a transparent plastic film coated with ITO, or use of SnO ₂ glass or transparent plastic film. In this case, it is important that the transmittance of the transparent conductive substrate is 80% or more, in particular 85% or more, and the area resistance of the conductive layer may be 30Ω / 口 or less. Alumina, rod-shaped glass fibers, glass beads, polymer beads, micro pearls, etc. may be used as spacers used to maintain a constant cell thickness of the polymer dispersed liquid crystal composite layer, and the thickness of the manufactured cell is 1 μm to 50 μm. Μm levels are good.

When manufactured by PIPS method, the UV light used for phase separation can be used from several mW to hundreds of mW, and the irradiation time of UV light varies depending on the composition, but the curing time is completely completed in 5 seconds to 30 minutes. It's time. In addition, if the intensity of the ultraviolet rays irradiated is too strong, the polymerization reaction to the polymer proceeds so quickly that all of the liquid crystal monomers dissolved in the monomer and oligomer may not be precipitated. Thus, the morphology of the polymer dispersed liquid crystal composite appears in the form of polymer balls. Hysteresis increases and driving voltage increases. In addition, if the intensity of ultraviolet light is too weak, even if a sufficient amount of light for polymerization is irradiated, the size of the deposited liquid crystal droop is too large, the driving voltage is low, but the response speed is a problem. Therefore, the intensity of ultraviolet light is preferably irradiated from 1mW to 50mW intensity.

The liquid crystal of the present invention is actually closer to the form in which the liquid crystal is present in the continuous phase than in the form of the droplet. Since the distance between the liquid crystal polymer layers is very short in order to have the alignment force because the liquid crystal is present in the form of a droplet between the mesogenic groups of the branched liquid crystal polymer, the size of the droplet becomes very small even if it is actually formed as a drawlet. Cause an increase. Therefore, it is similar to the PNLC structure in which the liquid crystal forms a continuous phase between the polymers, but the mesogenic group has liquid crystallinity and the structure of the whole liquid crystal monomer is slightly homeotropic due to the mesogenic group. Is the point.

Unlike the general TN or STN liquid crystals, the polymer dispersed liquid crystal composite of the present invention does not use a polarizing plate, so the light utilization efficiency is good and the luminance is high. In addition, since the polymer dispersed liquid crystal composite of the present invention uses a liquid crystal polymer having a mesogenic group, its response speed is high due to the orientation control force on the liquid crystal monomer, and the halogen-based liquid crystal can be used, so that the suppression retention rate is improved to display a moving image. This has a possible advantage.

Hereinafter, the present invention will be described in detail with reference to Examples, but it should be noted that the present invention is not limited by the following Examples.

Example 1

The ratio of 1: 3 using a polybiphenyl methacrylate-based compound (molecular weight 2000, R is OCH ₃ , n is 2) as the liquid crystal polymer and ethylhexyl acrylate as the acrylate monomer The composition was. As an initiator, 2% by weight of Darocure 1173 (Darocure 1173, Merck) was used based on the acrylate monomer. The mixture was uniformly mixed with a handy mixer in a water bath at 60 ° C. to form a single phase, and TL-204 (Merck Co., Ltd.), a fluorine-based liquid crystal, was prepared by forming a total polymer component / fluorine-based liquid crystal monomer at a weight ratio of 20/80. Mix evenly with the Handy Mixer. The mixture was injected between two ITO-coated glass plates having a transmittance of 86% and an area resistance of 30 Ω / 口 by using a spacer. When the injection is completed, the UV light having a wavelength of 365 nm is cured by irradiating for 3 minutes at an intensity of 10 mW / cm 2.

Example 2

As the liquid crystal polymer, a polybiphenyl methacrylate-based compound (molecular weight: 2000, R is OCH ₃ , n is 2) as shown in Formula (1), and ethylhexyl acrylate and trifunctional acrylate monomer are used as acrylate monomers. Phosphorous trimethylolpropane triacrylate was formulated in a weight ratio of 1:10. The liquid crystal polymer and the acrylate monomer were prepared in the same manner as in Example 1 except for the composition in a weight ratio of 1: 3 to prepare a polymer dispersed liquid crystal composite, and the physical properties thereof are shown in Table 1 below.

Example 3

As the liquid crystal polymer, a cholesteric substituted polyamide ester-based compound (molecular weight: 2500, n is 2) as shown in Formula (9) was used, and butyl acrylate and butyl methacrylate were used as the acrylate monomer. The liquid crystal polymer and the acrylate monomer were prepared in a weight ratio of 1: 5, and the butyl acrylate and butyl methacrylate were prepared in the same manner as in Example 1 except that the butyl acrylate and butyl methacrylate were formed in a weight ratio of 2: 1. To prepare and evaluate the physical properties are shown in Table 1 below.

Example 4

After dissolving the cholesteric substituted polyphenylester-based compound (molecular weight: 4000, n is 6) as a liquid crystal polymer in chloroform, TL-204 was dissolved in this solution as a liquid crystal monomer. Is added so that the weight ratio is 20:80 to dissolve the liquid crystal. This mixed solution is coated to a thickness of 12 μm using a bar coater. If the amount of solvent is too high, the wetting phenomenon due to the difference in the surface tension with the glass plate or the polyester film during coating occurs severely. If the amount of the solvent is too small, thin film coating and bubble removal are difficult, so the amount of solvent that dissolves the polymer is dissolved. After adjusting the viscosity of the mixture to about 100 ~ 500cps.

After the coated substrate is completely removed from the solvent remaining in the oven at 40 ℃, another ITO coated substrate is pressed to prepare a PDLC film and the physical properties are shown in Table 1 below.

Example 5

A polymer dispersed liquid crystal composite was prepared in the same manner as in Example 4, except that a polybiphenyl methacrylate-based (molecular weight: 5000, R is CH ₃ , n is 6) compound as the liquid crystal polymer was used. And the physical properties were evaluated and shown in Table 1 below.

Comparative Example 1

An aromatic urethane acrylate was used as the oligomer of the polymer component and ethylhexyl acrylate was used as the monomer to form a weight ratio of 1/7 oligomer / monomer. As an initiator, Darocure 1173 (Darocure 1173, Merck) is used in an amount of 2% by weight based on the acrylate oligomer and monomer, and TL-204 (Merck), a fluorine-based liquid crystal, is used as the liquid crystal. The composition was set to 20:80.

The mixture was injected between two ITO-coated glass plates having a transmittance of 86% and an area resistance of 30 Ω / 口 by using a spacer. Upon completion of the injection, the UV light having a wavelength of 365 nm was irradiated and cured for 3 minutes at an intensity of 10 mW / cm 2 to prepare a polymer dispersed liquid crystal composite, and the physical properties thereof are shown in Table 1 below.

Comparative Example 2

BL-001 (Merck Co., Ltd.) was used as the cyan liquid crystal in the composition of the liquid crystal monomer. A polymer dispersed liquid crystal composite was prepared in the same manner as in Comparative Example 1 except that the ratio of the fluorine liquid crystal and the cyan liquid crystal was 70:30. To prepare and evaluate the physical properties are shown in Table 1 below.

Comparative Example 3

A polybiphenyl methacrylate-based compound (molecular weight: 2000, R is OCH ₃ , n is 2) as the liquid crystal polymer, and ethylhexyl acrylate is used as the acrylate monomer. The composition was proportioned. As an initiator, 2% by weight of Darocure 1173 (Darocure 1173, Merck) was used based on the acrylate monomer. The mixture was uniformly mixed with a handy mixer in a water bath at 60 ° C. to form a single phase, and TL-204 (Merck), a fluorine-based liquid crystal, and BL-001, a cyan liquid crystal, were prepared at a weight ratio of 70:30. Mix uniformly with a handy mixer. Subsequent processes were carried out as in Comparative Example 1 to prepare a polymer dispersed liquid crystal composite, and evaluated for physical properties thereof.

TABLE 1

Claims (3)

In a polymer dispersed liquid crystal composite in which a polymer compound and a liquid crystal compound are dispersed between two substrates coated with a transparent electrode, the polymer component is a smectic or nema with mesogenic groups such as the following general formulas (1) to (15). Polymer dispersed liquid crystal composite comprising a tick liquid crystal polymer. In the above formula, m and n are each an integer of 1 or more and 20 or less. The polymer dispersed liquid crystal composite according to claim 1, wherein an acrylate monomer having one or two or more single functional groups or polyfunctional groups is added to the liquid crystal polymer. The polymer dispersed liquid crystal composite according to any one of claims 1 to 2, wherein the liquid crystal component is a halogen-based liquid crystal.