KR20180114450A - Blue phase liquid crystal composition and photo control device comprising the same - Google Patents

Abstract

According to an embodiment of the present invention, a blue phase liquid crystal composition comprises: a bendable liquid crystal compound comprising a bent core at the center and a rod-like wing on both sides of the bent core; a rod-like liquid crystal mixture having a nematic phase; a chiral dopant twisting the rod-like liquid crystal to form a blue phase; a monomer for polymer formation; and a photoinitiator to induce polymerization of the monomer.

Description

TECHNICAL FIELD [0001] The present invention relates to a blue phase liquid crystal composition and a light control device including the blue phase liquid crystal composition.

The present invention relates to a blue phase liquid crystal composition and a light control device including the same, and more particularly, to a blue phase liquid crystal composition and a light control device applicable to a smart window.

In recent years, the complexity of the ordered structure of a chiral liquid crystal called a blue phase (blue phase) has a completely different optical property from that of a nematic phase.

Particularly, for the display, the blue phase exhibits a faster electrical response than the nematic phase, which is why the attention is paid to the fact that the substrate surface treatment of the liquid crystal panel is simpler than the display using the conventional nematic phase.

However, there are a few problems with Blue Phase, which cause many problems in practical applications. In particular, the problem that must be solved urgently for the application of blue phase is the thermophysically stable mesophase blue phase at a narrow temperature range between isotropic (Iso) phase and chiral nematic (N *) phase It can not be expressed within a usable temperature range.

The blue phase has a double-twist cylinder (DTC) structure, which is due to the self-assembly of a double-twisted cylinder (DTC) in a three-dimensional space.

The Blue Phase is classified into three categories according to the temperature rising order: Blue Phase I (BP I), Blue Phase II (BP II), and Blue Phase III (BP III). In the case of BP I, which is a bilaterally symmetrical structure of a tightly packed double-twisted cylinder (DTC), it is a body-centered cubic structure and BP II is a simple cubic structure. BP III is composed of a double-twisted cylinder (DTC) having an arbitrary direction, that is, an amorphous structure.

 The blue phase is optically active, non-birefringent and exhibits Bragg reflection for circularly polarized light in the visible light range. The unique electro-optical characteristics of the Blue Phase are attractive from the standpoint of their potential application, but the fact that they exist in a narrow temperature range is a major obstacle for future applications.

There is a method of producing a polymer liquid crystal film having a blue phase structure by polymerizing a photoreactive liquid crystal monomer having a blue phase structure by raising the temperature range of Blue Phase (BP) I.

However, in this case, all the constituent molecules were fixed, resulting in no liquid crystal power.

Furthermore, Blue Phase (BP) II has an extremely narrow temperature range (0.1 ° C), so a solution that broadens the temperature range must be presented to utilize Blue Phase II.

Korean Patent No. 10-1531103 (June 17, 2015) Korean Patent No. 10-1632611 (Jun. 26, 2016)

It is an object of the present invention to provide a blue phase (BP) composition having a wide temperature range between an isotropic (Iso) phase and a chiral nematic (N *) phase.

A blue phase liquid crystal composition according to an embodiment of the present invention includes a bent liquid crystal compound including rod-shaped wings on both sides of a bent core and a bent core, a rod-shaped liquid crystal mixture having a nematic phase, A chiral dopant which twists to form a blue phase, a monomer for forming a polymer, and a photoinitiator for inducing the polymerization of the monomer.

Here, the bent type liquid crystal compound may be at least one of the compounds represented by the following formulas (1) to (3) having a band structure.

[Chemical Formula 1]

(2)

(3)

Here, the center substitution angle of the bent type liquid crystal compound may be 60 ° to 120 °.

Here, the chiral dopant may be at least one of the compounds represented by Chemical Formulas 4 to 5 below.

[Chemical Formula 4]

[Chemical Formula 5]

Here, the monomer may include a liquid crystalline monomer having two functional groups and a non-liquid crystalline monomer having three functional groups as shown in Formula (7) as shown in Formula (6).

[Chemical Formula 6]

(7)

Here, the photoinitiator may be a photoreactive material, which forms radicals that increase the reactivity of monomers when irradiated with ultraviolet light having a wavelength of 340 nm to 380 nm.

[Chemical Formula 8]

Here, the blend ratio of the rod-shaped liquid crystal to the curved liquid crystal may be from 16 wt% to 30 wt% with respect to the rod-shaped liquid crystal.

Here, the chiral dopant is mixed at a ratio of 29 wt% to 35.5 wt% with respect to a mixture of the rod-shaped liquid crystal and the bend-like liquid crystal to form a host liquid crystal.

Here, the monomer may be mixed at a ratio of 4 wt% to 6 wt% with respect to the host liquid crystal.

Here, the photoinitiator may be mixed with 0.5 wt% of the material mixed with the host liquid crystal and the monomer in the mixed material.

Here, the blue phase expressed by the chiral dopant may be Blue Phase II (BP II) having a simple cubic structure.

According to the blue phase liquid crystal composition according to the embodiment of the present invention and the light control device including the same, a thermophilically stable mesophase blue phase is provided in a wide temperature range between an isotropic phase and a chiral nematic phase.

1 is a view showing the structure of a blue phase according to an embodiment of the present invention.
2 is a view showing a double twisted cylinder structure according to an embodiment of the present invention.
3 is a view showing a double twist cylinder including a rod-shaped liquid crystal mixture according to an embodiment of the present invention.
4 is a view showing a structure of a blue phase in which a polymer is formed according to an embodiment of the present invention.
FIG. 5 is a graph showing changes in temperature range of Blue Phase I in which polymers are bonded. FIG.
6 is a state diagram when an electric field is applied to the blue phase liquid crystal composition according to the embodiment of the present invention.
7 is a graph of reflectance according to temperature and wavelength.
Fig. 8 is a graph showing a wavelength with a maximum reflectance according to temperature. Fig.
9 is a Kossel diagram image of a blue phase according to an embodiment of the present invention.
10 is a graph of reflectance of a light control device using Blue Phase II according to an embodiment of the present invention.
11 is a graph of reaction time measurement of a light control device using Blue Phase II according to an embodiment of the present invention.

It is to be understood that the specific structural or functional description of embodiments of the present invention disclosed herein is for illustrative purposes only and is not intended to limit the scope of the inventive concept But may be embodied in many different forms and is not limited to the embodiments set forth herein.

The embodiments according to the concept of the present invention can make various changes and can take various forms, so that the embodiments are illustrated in the drawings and described in detail herein. It is not intended to be exhaustive or to limit the invention to the particular forms disclosed, but on the contrary, is intended to cover all modifications, equivalents, or alternatives falling within the spirit and scope of the invention.

Hereinafter, a blue phase liquid crystal composition according to the present invention and a light control device including the blue phase liquid crystal composition will be described.

1 is a view showing the structure of a blue phase according to an embodiment of the present invention.

2 is a view showing a double twisted cylinder structure according to an embodiment of the present invention.

The blue phase on the left side of FIG. 1 is Blue Phase I having a body-centered cubic structure, and the blue phase on the right side of FIG. 1 is Blue Phase II having a simple cubic structure.

A blue phase composition according to an embodiment of the present invention includes a curved liquid crystal compound including rod-like wings on both sides of a bent core and a curved core, a rod-shaped liquid crystal mixture having a nematic phase, A chiral dopant for expressing a blue phase, a monomer for forming a polymer, and a photoinitiator for inducing polymerization of the monomer.

The curved liquid crystal compound may be at least one of compounds represented by the following formulas (1) to (3) having a band-form structure.

[Chemical Formula 1]

(2)

(3)

The bent type liquid crystal compound has a structure including a bar-shaped wing on both sides of a central bent core and a bent core, and the central substitution angle may be 60 ° to 120 °.

As shown in FIG. 1, blue phases I and II formed of a double twist cylinder (DTC) have a defect line formed between double twisted cylinders constituting one cubic unit having a cubic shape, The bent liquid crystal compound is located on the vacancy line of the blue phase I, II liquid crystal.

A rod-shaped liquid crystal mixture having a nematic phase is a mixture constituting a double twisted cylinder constituting a blue phase as shown in Fig.

3 is a view showing a double twist cylinder including a rod-shaped liquid crystal mixture according to an embodiment of the present invention.

If the liquid crystal phase in which the uniaxial twist structure is extended three-dimensionally as shown in Fig. 3 is a chiral nematic phase, the rod-shaped liquid crystal mixture having the nematic phase shown in Fig. 2 is twisted two- Expanded is Blue Phase.

Also, a column having a certain radius to maintain a twin-axis twisted structure is called a double twisted cylinder. In FIG. 1, the blue phase I and II are distinguished from each other according to the structure of the double phase twisted cylinder.

As shown in FIGS. 2 and 3, the double-twisted cylinder forming the blue phase has a rod-shaped liquid crystal mixture having a nematic phase formed in a double spiral structure along two axes perpendicular to each other in the cylinder, It is extended to three dimensions.

If double twisted cylinders are arranged like Blue Phase I and II, there is inevitably an empty space, and the line connecting this empty space is the defect line mentioned above. With the double twisted cylinder and the vacancy line coexisting, the blue phase becomes thermodynamically stable, and it forms the hundreds of nanoseconds in the spontaneous visible light region, which is the fast response speed which is the advantage of the blue phase.

The chiral dopant causes the blue phase to be expressed by twisting the rod-like liquid crystal in the rod-like liquid crystal mixture as described above.

The chiral dopant may be at least one of the compounds represented by Formulas (4) to (5), but is not necessarily limited thereto.

[Chemical Formula 4]

[Chemical Formula 5]

Formula 4 is a structural formula of R811, which is a kind of chiral dopant, and Formula 5 is a structural formula of S811.

4 is a view showing a structure of a blue phase in which a polymer is formed according to an embodiment of the present invention.

The monomer according to an embodiment of the present invention includes a liquid crystalline monomer having two functional groups and a non-liquid crystalline monomer having three functional groups as shown in Chemical Formula 7,

[Chemical Formula 6]

(7)

Monomers are low-molecular substances that constitute polymer compounds and the like, and are low-molecular substances that become a unit when a polymer compound is produced by a chemical reaction. When a monomer is added to the blue phase, a polymer compound is generated by the induction of a photoinitiator, and a phase line is formed between the blue phase and the polymer compound, and the polymer is driven to the defect line. In the state where the polymer is firmly fixed by the cross linking between the polymers, the polymers bind to each other in the form of grabbing the bellows strongly. The vacancy line is stabilized through such a bond, and the temperature range of the blue phase is increased by the polymers bonded to the vacancy line.

Photoinitiators induce polymerisation of monomers. The photoinitiator is a compound having a structure as shown in Formula 8, and forms a radical which increases the reactivity of monomers when irradiated with ultraviolet light having a wavelength of 340 nm to 380 nm with a photoreactive substance.

[Chemical Formula 8]

FIG. 5 is a graph showing changes in temperature range of Blue Phase I in which polymers are bonded. FIG.

As shown in FIG. 5, it can be seen that the temperature range between the isotropic phase and the chiral nematic phase is significantly widened in the blue phase including the stabilized polymer.

6 is a state diagram when an electric field is applied to the blue phase liquid crystal composition according to the embodiment of the present invention.

When electrical anisotropy, which is one of physical properties of the liquid crystal, has a positive value, when an electric field is applied as shown in FIG. 6, the blue phase liquid crystal molecules rotate in parallel with the applied direction. This causes the double helical structure of the blue phase liquid crystal to disappear and no longer have a periodic structure. As a result, it is impossible to reflect light of a specific wavelength. In this case, light of all wavelengths passes through the blue phase liquid crystal. Then, when the electric field is removed again, the liquid crystal molecules form a photonic crystal structure immediately upon returning to the initial position. The same applies to the case where the electric anisotropy of liquid crystal molecules has a negative value. The characteristics of the blue phase liquid crystal can be applied to a smart window.

The components of the blue phase liquid crystal composition according to the embodiments of the present invention have been described above. Hereinafter, practical examples of the blue phase liquid crystal composition according to the present invention will be described.

<Experimental Example>

In order to prepare a blue phase liquid crystal composition according to an embodiment of the present invention, rod-shaped liquid crystal weight curves are mixed at a ratio of 16 wt% to 30 wt% in a rod-shaped liquid crystal having a nematic phase for expressing blue phase. More preferably, the curved liquid crystal is mixed at a ratio of 20 wt%.

Next, a mixture of rod-shaped liquid crystals and curved liquid crystals is mixed with a chiral dopant at a ratio of 29 wt% to 35.5 wt%. By adjusting the amount of chiral dopant, it is possible to reflect wavelengths in all regions from the red region to the blue region. In the vicinity of the red region, 29 wt% of chiral dopant, 35.5 wt% it is possible to reflect light in the vicinity of the blue region. A liquid crystal mixture to which a chiral dopant is added is hereinafter referred to as a host liquid crystal.

Bar-shaped liquid crystals and curved liquid crystals are mutually non-reactive substances. Therefore, the two liquid crystal materials are uniformly mixed with each other and do not affect the inherent properties of the respective materials. In addition, the photo crystal period of the blue phase can be determined by changing the chiral dopant ratio, and only the wavelength of the selected light can be reflected through the film.

Chiral dopants are also unreactive with rod-shaped liquid crystals and bendable liquid crystals, and twist the rod-shaped liquid crystals without affecting the properties of each other.

Next, the above-mentioned host liquid crystal is mixed with the monomer at a ratio of 4 wt% to 6 wt%. Preferably at a ratio of 5 wt%. RM257, a liquid crystal monomer having two functional groups as a monomer, and TMPTA having three functional groups are mixed in a host liquid crystal.

The ratio of the mixed material to the photoinitiator is suitably 0.5 wt%. DMPAP is used as a photoinitiator. The photoinitiator absorbs light of about 340nm ~ 380nm to generate two radicals. The radicals react with two monomers, not the host liquid crystal, to form a polymer.

The polymer was stabilized and reflectance was measured in the blue phase liquid crystal composition prepared as described above.

7 is a graph of reflectance according to temperature and wavelength.

Fig. 8 is a graph showing a wavelength with a maximum reflectance according to temperature. Fig.

It can be seen that a sharp reflectivity can be obtained at each temperature as shown in FIG. Further, as shown in FIG. 8, it can be confirmed that the wavelength of the spectrum is hardly changed according to the change of the temperature.

9 is a Kossel diagram image of a blue phase according to an embodiment of the present invention.

In order to confirm whether the blue phase according to the embodiment of the present invention is Blue Phase I (BP I) or Blue Phase II (BP II), a Kossel diagram image was measured, and a typical Blue Phase II (BP (100) plane of the elliptic ellipsoid (II).

7 to 9, it can be seen that the section where there is almost no change in the wavelength according to the temperature change of Blue Phase II is from about 10 ° C to 60 ° C. In spite of the Blue Phase II, the blue phase II according to the embodiment of the present invention Ⅱ shows a broad temperature range of 50 캜 due to the bonding of the polymer material to the defect line.

10 is a graph of reflectance of a light control device using Blue Phase II according to an embodiment of the present invention.

In order to confirm the usefulness of Blue Phase II as a light control device as shown in FIG. 10, a reflection type device is fabricated, a blue phase II according to an embodiment of the present invention stabilized by polymer is injected into a cell constituting the device, And the reflectivity was measured. As a result of the measurement, it was confirmed that the device has almost no hysteresis. In addition, the phenomenon that the reflectivity decreases gradually can be analyzed as the initial alignment of the twisted twisted cylinder causes the liquid crystal molecules to rotate in the electric field direction by the electric field, and the double twisted cylinder structure gradually disappears and the reflectivity decreases.

11 is a graph of reaction time measurement of a light control device using Blue Phase II according to an embodiment of the present invention.

As shown in FIG. 11, it can be seen that the reaction time of the optical control element is very short as 0.6 ms (decay time) and 0.9 ms (rise time), and even when the blue phase is applied to the device, It can be confirmed that it is expressed without.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art. Accordingly, the true scope of the present invention should be determined by the technical idea of the appended claims.

Claims (13)

A bendable liquid crystal compound comprising a rod of rod on both sides of a central bent core and a curved core;
Rod-shaped liquid crystal mixture having a nematic phase;
A chiral dopant for twisting the rod-shaped liquid crystal to form a blue phase;
Monomers for polymer formation; And
And a photoinitiator for inducing polymerization of the monomer. The method according to claim 1,
Wherein the bendable liquid crystal compound is at least one of compounds represented by the following formulas (1) to (3) having a band-form structure.
[Chemical Formula 1]

(2)

(3)

3. The method of claim 2,
Wherein the central substitution angle of the bent type liquid crystal compound is 60 ° to 120 °. The method according to claim 1,
Wherein the chiral dopant is at least one of compounds represented by the following Chemical Formulas (4) to (5).
[Chemical Formula 4]

[Chemical Formula 5]

The method according to claim 1,
Wherein the monomer comprises a liquid crystalline monomer having two functional groups and a non-liquid crystalline monomer having three functional groups as shown in Formula (6), as shown in Formula (6).
[Chemical Formula 6]

(7)

The method according to claim 1,
Wherein the photoinitiator is a photoreactive substance and forms a radical which increases the reactivity of the monomers when irradiated with ultraviolet light having a wavelength of 340 nm to 380 nm.
[Chemical Formula 8]

The method according to claim 1,
Wherein the mixture of the rod-shaped liquid crystal and the bendable liquid crystal has a blending ratio of the curved liquid crystal to the rod-shaped liquid crystal of from 16 wt% to 30 wt%. 8. The method of claim 7,
Wherein the chiral dopant is mixed in a ratio of 29 wt% to 35.5 wt% with respect to a mixture of the rod-shaped liquid crystal and the curved liquid crystal to form a host liquid crystal. 9. The method of claim 8,
Wherein the monomer is mixed at a ratio of 4 wt% to 6 wt% with respect to the host liquid crystal. 10. The method of claim 9,
Wherein the photoinitiator is mixed with a material obtained by mixing the host liquid crystal and the monomer in an amount of 0.5 wt% based on the mixed material. The method according to claim 1,
Wherein the blue phase expressed by the chiral dopant is Blue Phase II (BP II) having a simple cubic structure. 12. A light control element comprising the blue phase liquid crystal composition of any one of claims 1 to 11. A smart window comprising the light control element of claim 12.

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