KR101253094B1 - Method for preparing liquid crystalline blue phase I with excellent thermal stability - Google Patents

Abstract

The present invention relates to a method for producing a blue phase I liquid crystal having excellent thermal stability, and more particularly, by controlling the mixing ratio of the vent-core material added to the host nematic liquid crystal, the elastic constant K ₃₃ is lower than K _11. By controlling so that the blue phase I liquid crystal having excellent thermal stability can be prepared.

Description

Method for preparing liquid crystalline blue phase I with excellent thermal stability

The present invention relates to a method for manufacturing a blue phase I liquid crystal having excellent thermal stability by adjusting the elastic constant value of the host nematic liquid crystal.

Recently, the possibility of its application has been attracting attention because of its complex optical structure, which is three-dimensionally formed by a chiral liquid crystal called a blue phase (blue phase). Lee, S.-T. et al . J. Mater . Chem ., 2010, 20, pp. 5813). In particular, the display is attracting attention because the blue phase shows an electric field response faster than the nematic phase, and the substrate surface treatment of the liquid crystal panel is simpler than the display using the normal nematic phase. In addition, understanding the basic properties of the blue phase is indispensable for its application, and researches aiming at it are being actively promoted. However, some problems with the blue phase cause many problems in actual applications. Among these problems, the blue phase needs to be solved urgently for display applications. The mesophase is thermodynamically stable in a narrow temperature range (about 1 ° C) between an isotropic (iso) phase and a chiral nematic (N *) phase. Much research is being conducted on whether the blue phase can be expressed within the usable temperature range (PP Crooker, in Chirality in Liquid Crystals (Eds: H.-S. Kitzerow, C. Bahr), Springer, New York 200l, pp. 186-222; H. Kikuchi, Struct . Bonding , 2008, 128, pp. 99.). This is because their great potential for future applications is useful for developing devices such as fast optical modulators or adjustable photonic crystals.

The blue phase has a double-twist cylinder (DTC) structure, and their unusual structure is believed to be due to the way the DTC self-assembles in three-dimensional space. Blue phases are classified into three categories, namely BPI, BPII, and BPIII, in order of temperature rise. The left-right symmetry of the filled DTC is a body-centred cubic structure for BPI, and a simple cubic structure for BPII. BPIII is any direction, that is thought to consist of DTC that has an amorphous structure (DC Wright and ND Mermin, Rev Mod Phys, 1989, 61, pp 385;.... H.-S. Kitzerow and PP Crooker, Phys . Rev. Lett., 1991, 67, pp. 2151). The blue phases are optically active and non-birefringent, and they show Bragg reflections for circularly polarized light in the visible range. Their unique electro-optic features are fascinating in terms of their potential applications, but the fact that they exist in a narrow temperature range is a major obstacle to future applications. In particular, raising the temperature range of the BPI is indispensable. Because BPII is not observed in strong chiral systems, BPIII is rather vague in its properties.

Several successful attempts have been made to overcome this problem. Kitzerow et al. Polymerized a photoreactive liquid crystal monomer having a blue phase structure to prepare a polymer liquid crystal film having a blue phase structure ( Liq . Cryst ., 1993, 14, pp. 911). However, in this case, all the constituent molecules were fixed and consequently there was no liquid crystal dynamics. In 2002, Kikuchi et al. Developed a polymer-stabilized blue phase by doping a small amount of polymer (7-8 wt%) to increase the temperature range of BPI by several decades ( Nat . Mater ., 2002, 1 , pp. 64). BPI stabilized by the polymer showed a high speed electro-optic response. In 2005, Coles et al. Reported the observation of BPI with a very wide temperature range in bimesogenic molecular systems with large bending electromagnetisms ( Nature , 2005, 436, pp. 997). In this case, the reflection color of the blue phase grating can be reversibly adjusted by applying an electric field. More recently, Choi et al. Described the temperature range of BPI in a simple mixture of bent-core materials with nematic phases and a few percent chiral additives with high helical twisting power of the helix. Investigate. They experimentally confirmed the presence of BPI with a wide temperature range (within 15 ° C) in a vent-core material system that was previously expected in theoretical studies (GP Alexander, JM Yeomans, Phys . Rev. E , 2006, 74, pp. 061706; F. Castles, SM et al ., Phys . Lett . , 2010, 104, pp. 157801). In addition, a group of Chinese researchers has reported a BPI with a wide temperature range, ie, a temperature range of 29 ° C., in a mixture of chiral and bent-core liquid crystals (Z. Zheng, D et. al . New J. Phys ., 2010, 12, pp. 113018). The latter can be explained by the reduction of the elastic energy values of the bleeding (K ₁₁ ) and bending (K ₃₃ ) deformations of warp defect lines due to the vent-core material (M. Nakata et al . Phys . Rev. E , 2003, 68, pp. 041710; H. Chen et al . Appl . Phys . Lett ., 2010, 97, pp. 181919).

Several research groups have contributed to the theoretical and experimental findings of the broadening of the BPI's temperature range, but to direct experimental evidence of the correlation between the actual temperature of the blue phase and the total temperature range of the physical phase, such as the elastic constant of the host nematic liquid crystal. There is little research.

It is an object of the present invention to provide a method for producing a blue phase I having excellent thermal stability by investigating the correlation between the elastic constant of the host nematic liquid crystal and the thermal stability of the blue phase I liquid crystal.

In order to achieve the above object, the present invention provides a blue phase I liquid crystal from the host nematic liquid crystal,

It provides a method for producing a blue phase I liquid crystal having excellent thermal stability comprising the step of controlling the elastic constant of the host nematic liquid crystal as shown in Equation 1 below:

[Equation 1]

Where

K ₃₃ is the bend Frank elasticity constant,

K ₁₁ represents the splay Frank elasticity constant.

The present invention can induce thermal stability of the blue phase I liquid crystal prepared therefrom by adjusting the elastic constant of the host nematic liquid crystal.

1 shows the chemical structure of a vent-core material used in one embodiment of the present invention.
FIG. 2 shows the elastic constants, K ₁₁ , for the four host nematic liquid crystals with the concentration of vent-core material added at the same reduced temperature ( T _NI -20 ° C.). And K ₃₃ .
Figure 3 shows the entire temperature range of the blue phase I liquid crystal derived from four types of host nematic liquid crystal in which the vent-core material is added for each concentration.
FIG. 4 is a typical polarization optical micrograph of a host nematic liquid crystal (BC30) doped with 25 parts by weight of a chiral dopant, upon cooling (1 ° C./min), showing a colored plate of blue phase I liquid crystal ((a)-(e ), After the preservation at 30 ° C. for 24 hours, it indicates that the blue phase I liquid crystal is maintained as it is (f).

EMBODIMENT OF THE INVENTION Hereinafter, the structure of this invention is demonstrated concretely.

The present invention provides a blue phase I liquid crystal from a host nematic liquid crystal.

It relates to a method for producing a blue phase I liquid crystal having excellent thermal stability comprising the step of controlling the elastic constant of the host nematic liquid crystal as shown in Equation 1 below:

[Equation 1]

Where

K ₃₃ is the bend Frank elasticity constant,

K ₁₁ represents the splay Frank elasticity constant.

The method for producing a blue phase I liquid crystal having excellent thermal stability according to the present invention is that when the blue phase I liquid crystal is produced from a host nematic liquid crystal, the elastic constant of the host nematic liquid crystal, K ₃₃ is the value of K ₁₁ , as shown in Equation 1 above. The smaller control feature extends the operating temperature range of the Blue Phase I liquid crystal.

The blue phase I liquid crystal may be prepared by cooling the host nematic liquid crystal in an isotropic state, but is not particularly limited thereto.

The host nematic liquid crystal may be a rod-shaped, band-shaped, T-shaped liquid crystal, or a mixture of these liquid crystals. There is no particular limitation to this.

The elastic constants can be controlled by the vent-core material added to the host nematic liquid crystal.

Preferably, 27 to 50 parts by weight of the vent-core material may be added to 100 parts by weight of the host nematic liquid crystal. When the content of the vent-core material is out of the range, K ₃₃ exceeds the value of K ₁₁ , and thus the operating temperature range of the blue phase I liquid crystal may be narrowed to within 5 ° C.

The vent-core material may use at least one of compounds represented by the following Chemical Formulas 1 to 3 having a band or T-shaped structure, but is not particularly limited thereto.

[Formula 1]

[Formula 2]

(3)

In addition, in the manufacturing method of the present invention, in order to control the elastic constant of the host nematic liquid crystal, 5 to 50 parts by weight of the chiral dopant may be further added to 100 parts by weight of the host nematic liquid crystal containing the vent-core material. . In the above range, the elastic constant value satisfies Equation 1 above.

The chiral dopant may be used without limitation as long as it is a material capable of inducing a host nematic liquid crystal to chiral nematic regardless of the type of helical twisting power (HTP). For example, LC756 of BASF Corporation, S-811 of Merck Corporation, etc. can be used.

Blue Phase I liquid crystal according to the production method of the present invention is characterized in that it is stable in the temperature range of -20 ℃ to 80 ℃.

Doping the vent-core material into the nematic liquid crystal significantly reduces K ₃₃ and reduces the free energy value for defects, leading to stabilization of the double-twist cylinder (DTC) structure of the blue phase I liquid crystal. do. These factors broaden the operating temperature range of Blue Phase I liquid crystals and can be applied to devices such as fast optical modulators and adjustable photonic crystals as well as liquid crystal display devices.

Hereinafter, the present invention will be described in more detail with reference to Examples of the present invention, but the scope of the present invention is not limited by the following Examples.

Example 1 Preparation of Host Nematic Liquid Crystal Added with Vent-Core Material

Four host mixtures of the vent-core material and known nematic liquid crystals (NLC) shown in FIG. And 30 parts by weight (BC30) was prepared. NLC (BYLC53XX, BaYi Space Co.) used in this example is a eutectic mixture with positive dielectric anisotropy (Δε) and shows the following phase sequence during the cooling process. Namely, isotropic (Iso)-(101 ° C.)-Nematic (N)-(-40 ° C.)-Crystal (Cryst). The vent-core material had N phases and exhibited the following phase sequence during the cooling process. Namely, Iso- (99 ° C) -N- (82 ° C) -B4- (76 ° C) -Cryst.

The constituent fractions and phase-transition temperatures of the four host NLCs synthesized are summarized in Table 1.

Experimental Example 1 Investigation of Elastic Constants of Host Nematic Liquid Crystal

Elastic constants of host nematic liquid crystals with concentration of vent-core material at the same reduced temperature ( T _NI -20 ° C, T _NI : N-Iso phase transition temperature), K ₁₁ And K ₃₃ were measured. Elastic Constant K ₁₁ of Mixed Host NLC And K ₃₃ were measured by performing capacitance measurements. For capacitance measurement, sandwich cells with polyimide coated glass substrates having rubbing directions opposite to each other were prepared. The cell gap between the two substrates was kept at 10 μm using a ball spacer.

As shown in FIG. 2, K ₁₁ The values of and K ₃₃ showed the following tendency as the concentration of the vent-core material increased: K ₁₁ slightly increased, while K ₃₃ decreased dramatically. As the concentration of vent-core material increased, the decrease in K ₃₃ of the host NLC was due to the bending distortion and the curved shape of the vent-core material as a strong basis. The reduction in K ₃₃ of the NLC system with the addition of vent-core material has already been reported by Dodge et al. For BC17 K ₃₃ was much higher than K ₁₁ , while for BC30 K ₃₃ was much lower than K ₁₁ . Interestingly, K ₁₁ And values of K ₃₃ span the periphery of BC25. That is, in areas where the concentration of vent-core material is smaller than that of BC25, K ₃₃ is higher than K ₁₁ , whereas in areas where the concentration of vent-core material is higher than BC25, the value of K ₃₃ is higher than K ₁₁ . Lower.

Example 2 Preparation and Thermal Stability of Blue Phase I Liquid Crystal from Host Nematic Liquid Crystal Added Chiral Dopant

The total temperature range of BPI derived from four types of host NLC was measured for each concentration of the vent-core material. To introduce chirality into the mixed host NLC, 25 parts by weight of chiral dopants (S811, Merck) were added to 100 parts by weight of the total host NLC to which the vent-core material of Example 1 was added. These mixtures were injected into a 5 μm thick cell on the two substrates without a rubbing treatment such as polyimide coating and observed with a polarized optical microscope (POM). The temperature of the sample was precisely controlled by using a hot stage at a cooling rate of 1 ° C / min.

Figure 3 shows the overall temperature range of BPI derived from four types of host NLC according to the concentration of the vent-core material. For BC17 and BC20, where K ₃₃ of the host NLC was higher than K ₁₁ , the temperature range of BPI was ˜2 ° C., which is the same as conventional NLC with chiral additives. For BC25, where K ₃₃ of host NLC was lower than K ₁₁ , the temperature range of BPI was observed at ˜5 ° C. Finally, for BC30, where the K ₃₃ value of the host NLC is much lower than K ₁₁ , the temperature range of BPI was observed at ˜30 ° C., which is much wider than that of conventional NLC mixed with chiral additives. That is, when K ₃₃ is higher than K ₁₁ , BPI was only seen in a narrow temperature range. In general, if it is ₃₃ K was higher, K ₃₃ is lower than ₁₁ K in the case of the conventional stick-shaped form of the NLC, the temperature range of BPI is widened. Finally, when K ₃₃ is much lower than K ₁₁ , the temperature range of BPI was observed to be much wider than that of conventional NLC.

FIG. 4 shows a typical polarization optical micrograph of BC30 doped with 25 parts by weight of chiral dopant, gradually cooling the isotropic phase, showing a colored plate representative of the typical optical structure of BPI (FIG. 4 (a). )-(e)), its color is reflected by the blue phase (BP) grating.

In order to confirm the thermodynamic stability of the BPI of Example 1, the cell was stored at 30 ° C. for 24 hours, and as shown in FIGS. 4E and 4F, the BPI was maintained as it is. Thus, the extended temperature range of BPI was not attributed to super-cooling, and the obtained BPI was confirmed to be a thermodynamically stable phase.

In conclusion, the present invention analyzed the correlation between the elastic constants of host nematic liquid crystals and the thermal stability of their BPI, and the addition of vent-core materials to conventional nematic liquid crystals resulted in a much wider BPI temperature range than conventional systems. It was found that to get. This result indicates that doping the vent-core material to a conventional nematic liquid crystal leads to a significant reduction in K ₃₃ , a reduction in free energy value for defects and stabilization of the DTC. These factors increase the temperature range of the BPI. From the experimental results, it is demonstrated that the thermal stability of the BPI depends strongly on the value of the elastic constant of the host nematic liquid crystal. The stability is greater when K ₃₃ is smaller than K ₁₁ .

Claims (6)

In preparing a blue phase I liquid crystal from a host nematic liquid crystal,
The host nematic liquid crystal and at least one of a bent-core material represented by the following Chemical Formulas 1 to 3 having a band or T-shaped structure are mixed, and the elastic constant of the host nematic liquid crystal is Method for producing a blue phase I liquid crystal having excellent thermal stability including the step of controlling as in Equation 1:
[Formula 1]

(2)

(3)

[Equation 1]

In this formula,
K ₃₃ is the bend Frank elasticity constant,
K ₁₁ represents the splay Frank elasticity constant.
The method of claim 1,
A host nematic liquid crystal is a rod-shaped, band-shaped, T-shaped, or a mixture of these liquid crystals of the blue phase I liquid crystal having excellent thermal stability.
The method of claim 1,
The vent-core material is a method for producing a blue phase I liquid crystal excellent in thermal stability is mixed in 27 to 50 parts by weight with respect to 100 parts by weight of the host nematic liquid crystal.
delete The method of claim 1,
A method for producing a blue phase I liquid crystal having excellent thermal stability by further adding 5 to 50 parts by weight of a chiral dopant with respect to 100 parts by weight of a host nematic liquid crystal containing a vent-core material.
The method of claim 1,
Blue phase I liquid crystal is a method for producing a blue phase I liquid crystal excellent in stable thermal stability in the temperature range of -20 ℃ to 80 ℃.

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