JP2003327966A - Liquid crystal material for optical modulation element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a liquid crystal material which can develop a blue phase over such a sufficiently wide temperature range as being supplied for practical uses as optical modulation elements. <P>SOLUTION: This liquid crystal material for optical modulation elements comprises the blue phase of a composite liquid crystal composition (polymer network/low molecular liquid crystal) which comprises a low molecule liquid crystal capable of developing the blue phase between a cholesteric phase and an isotropic phase, and a polymer network formed in the low molecule liquid crystal by polymerizing an amorphous monomer (for example, an alkyl group side chain-having acrylate-based monomer) together with a crosslinking agent. The liquid crystal material for optical modulation elements is produced by dispersing the monomer and the crosslinking agent in the low molecule liquid crystal and then polymerizing the dispersion at a temperature holding the blue phase. Thus, such the liquid crystal material that the developed temperature of the blue phase is ranged in a temperature width of ≥60°C nipping room temperature can be obtained. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention belongs to the technical field of light modulation using liquid crystals, and more particularly to a novel liquid crystal material suitable for use as an optical modulator and a method for producing the same.

[0002]

2. Description of the Related Art An element (optical modulation element) for controlling the intensity and polarization state of light at high speed and with high precision is indispensable in optical information processing technology. Up to now, an inorganic single crystal exhibiting an electro-optical effect has been mainly used as an optical modulator. However, since an inorganic single crystal is generally expensive and has a low degree of freedom in shape, an inexpensive and small-sized optical modulation element using liquid crystal has recently been attracting attention. Liquid crystals exhibit large birefringence and various electro-optical effects, but the problem is that the response speed is low and a half of the transmitted light is lost because a polarizing plate is required.

The blue phase (hereinafter sometimes abbreviated as BP) is a few degrees Celsius between the cholesteric phase and the isotropic phase.
It is one of the liquid crystal phases that may appear in the temperature range (generally 1 to 3 ° C) (temperature range), and is a body-centered cubic lattice (BPI) or a simple cubic lattice (BPI) with a lattice constant of the order of several 100 nm.
I) It is known to be caused by the formation of a three-dimensional periodic structure such as a structure. BP exhibits Bragg reflection and optical rotatory power with respect to visible light, and is a unique electro-optical characteristic that can change the diffraction angle and polarization state of incident light with a microsecond-order response time by an external field such as an electric field or magnetic field. Indicates. Therefore, a liquid crystal exhibiting BP can have a response speed far superior to that of a conventional liquid crystal modulator and a multifunctional optical modulation. However, BP is only 1 between cholesteric and isotropic phases.
Since it appears only in the temperature range of up to 3 ° C., there is a problem that precise temperature control of the element is required, and an optical modulation element made of BP liquid crystal has not yet been put to practical use.

[0004]

An object of the present invention is to develop a new liquid crystal material capable of exhibiting a blue phase over a sufficiently wide temperature range that can be put to practical use as an optical modulator.

[0005]

Means for Solving the Problems As a result of extensive studies, the present inventors have found that by forming a polymer network derived from a monomer having a specific structure in a low-molecular liquid crystal exhibiting a blue phase, the temperature at which the blue phase appears. We succeeded in greatly expanding the range (temperature range) and derived the present invention.

Thus, according to the present invention, a low molecular weight liquid crystal capable of exhibiting a blue phase between a cholesteric phase and an isotropic phase, and a polymer network formed in the low molecular weight liquid crystal and having a non-liquid crystalline monomer are used. A liquid crystal material for an optical modulator, which is composed of a blue phase of a (polymer network / low-molecular liquid crystal) composite liquid crystal composition composed of a polymer network formed by polymerizing with a crosslinking agent. It is provided. According to the invention, there is further provided a method for producing the above liquid crystal for optical modulation element, which comprises a step of dispersing a monomer and a cross-linking agent in a low-molecular liquid crystal, and polymerizing at a temperature at which a blue phase is maintained. A method is provided comprising:

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION The liquid crystal material of the present invention suitable for use as an optical modulator is different from the (polymer network / low molecular weight liquid crystal) composite liquid crystal composition known so far. It is composed of BP (blue phase) expressed by a new type (polymer network / low-molecular liquid crystal) composite liquid crystal composition based on a technical idea. That is, PDLC (Polymer Dispersed Liquid) which is a liquid crystal mode used as a display element (display) or the like.
As one of the (Crystal) modes, a composite liquid crystal composition in which a minute amount of a polymer network structure is formed in a low-molecular liquid crystal (polymer network / low-molecular liquid crystal) is well known. The (polymer network / low-molecular liquid crystal) structure is different from the conventional one. In the conventionally known (polymer network / low molecular weight liquid crystal) composite liquid crystal composition, as a monomer molecule which is a raw material of a polymer forming a network, it is liquid crystalline itself and is compatible with the low molecular weight liquid crystal. The alignment of the liquid crystal phase is stabilized by forming a polymer network using the material.

On the other hand, the present invention is based on the fact that when a polymer molecule is formed by polymerizing a monomer molecule having low compatibility with a low molecular weight liquid crystal, a temperature range in which BP is expressed is expanded. Is. This is understood to be due to the following reason: BP is in a state coexisting with line defects in the molecular arrangement, and the energy for generating defects is large, so that BP is in a high energy state, that is, just under the isotropic phase. BP is expressed only in a wide temperature range. When a monomer having low compatibility with a low molecular weight liquid crystal is used as the monomer, the monomer molecules are concentrated in the BP line defects of the low molecular weight liquid crystal, and a polymer network corresponding to the structure of the BP line defect is formed by polymerization. , The amount of energy for generating line defects of BP is reduced, line defects of BP are generated even in a lower temperature region, and the temperature range of BP development may be expanded.

Thus, the monomer used to form the polymer network in the liquid crystal material for an optical modulator of the present invention is a non-liquid crystalline monomer. Here, the non-liquid crystal monomer used in the present invention is a monomer that can be polymerized by photopolymerization or thermal polymerization, and has a well-known rod-shaped molecular structure (for example, biphenyl group or biphenyl).・ A monomer that does not have a molecular structure such as an alkyl group, cyano group, or fluorine attached to the terminal such as a cyclohexyl group)
For example, a monomer having a polymerizable group such as an acryloyl group, a methacryloyl group, a vinyl group, an epoxy group, a fumarate group, or a cinnamoyl group in its molecular structure can be used, but the monomer is not limited thereto.

A preferred example of the non-liquid crystalline monomer used for forming a polymer network in the liquid crystal material for an optical modulator of the present invention is an acrylate-based monomer having an acryloyl group or a methacryloyl group in its molecular structure, Preferred is an acrylate-based monomer having a branched structure having an alkyl group as a side chain. The alkyl group is generally an alkyl group having 1 to 4 carbon atoms, and a monomer having at least one side chain composed of such an alkyl group per monomer unit is used. It has been found that the effect of expanding the temperature range for expressing BP is small when the polymer network is formed from monomers that are not branched even if they have a non-liquid crystalline molecular structure. Cyclohexyl acrylate is a preferred example of the acrylate-based monomer,
Preferable examples of the acrylate-based monomer having an alkyl group as a side chain include 2-ethylhexyl acrylate and 1,3,3-trimethylhexyl acrylate.

The above-mentioned monomer is subjected to polymerization together with the crosslinking agent to form a polymer network. The cross-linking agent may be either a liquid crystal or non-liquid crystal compound, and one having a reactive site capable of forming a network structure by binding the monomer molecules corresponding to the monomer used may be used. Good. For example, when an acrylate-based monomer is used as the monomer according to the preferred embodiment of the present invention, a liquid crystalline diacrylate monomer can also be used as the crosslinking agent. However,
If no crosslinking agent is used or the concentration of the crosslinking agent is too low, B
Since P (blue phase) is not expressed or the expression temperature range (temperature range) is narrowed, it is necessary to use a sufficient amount of the crosslinking agent. Further, the concentration of the polymer network is also important, and in order to widen the temperature range of BP expression, it is necessary to use a sufficient amount of the monomer and the cross-linking agent so that the polymer network having high continuity is formed. Yes (see Examples below).

The low molecular weight liquid crystal constituting the (polymer network / low molecular weight liquid crystal) composite liquid crystal composition in the liquid crystal material of the present invention which is suitable for use as an optical modulator is a cholesteric phase (chiral nematic phase) or the like. A blue phase can be developed between the phases. Such a low-molecular liquid crystal includes a molecular structure such as biphenyl, terphenyl, biphenyl / cyclohexyl, and has chirality (chirality) by itself due to the presence of an asymmetric atom.
Alternatively, it is a substance that can develop a cholesteric phase (chiral nematic phase) by adding a chiral substance (chiral dopant), and the helical pitch length in the cholesteric phase (chiral nematic phase) is about 500 nm or less. It is selected from the following. Such a liquid crystal develops a cholesteric phase (chiral nematic phase) at a low temperature and an isotropic phase at a higher temperature, and in a slight temperature region between the cholesteric phase (chiral nematic phase) and the isotropic phase. It is known to develop a blue hue. These low-molecular liquid crystals are
Generally, it is preferable to use a mixture of a plurality of types.

The liquid crystal material for an optical modulator of the present invention is composed of a blue phase of a composite liquid crystal composition (polymer network / low molecular liquid crystal) composed of the above low molecular liquid crystal and polymer network. The blue phase of this (polymer network / low-molecular liquid crystal) composite liquid crystal composition is obtained by dispersing a monomer and a cross-linking agent in the low-molecular liquid crystal and performing a polymerization reaction at a temperature at which the blue phase is maintained. To be

The retention of the blue phase can be confirmed by observation with a polarization microscope and measurement of the reflection spectrum. That is, when the blue hue appears,
The blue and yellow-green platelets (platelet-like tissue) characteristic of the blue phase are observed by a polarization microscope, and a peak is observed in the reflection spectrum at a wavelength of about 550 nm corresponding to the yellow-green platelets.

The polymerization can be carried out by either thermal polymerization or photopolymerization, but in the case of thermal polymerization, there is a limit to the range in which the temperature at which the blue phase is retained and the polymerization temperature (heating temperature) overlap, and Since the morphology of the polymer network may change by heating, photopolymerization using ultraviolet light is preferable. Further, in the polymerization, it is preferable to disperse the polymerization initiator in addition to the monomer and the crosslinking agent in the low molecular weight liquid crystal in order to accelerate the polymerization rate. As the photopolymerization initiator, various initiators such as acetophenone-based, benzophenone-based, benzoin ether-based, and thioxanthone-based initiators can be used, and specific examples thereof include 2,2-dimethoxy-2-phenylacetophenone. . Furthermore, as described above, it may be necessary to add a chiral dopant for expressing a chiral nematic phase in the low-molecular liquid crystal.

Thus, according to the present invention, in order to prepare a liquid crystal material comprising a blue phase of the (polymer network / low molecular weight liquid crystal) composite liquid crystal composition, a monomer and a crosslinking agent are added to the low molecular weight liquid crystal as described above. Further, if necessary, a mixed solution in which a polymerization initiator or a chiral dopant is dispersed is injected into an appropriate cell and subjected to a polymerization reaction as follows: First, a sample (mixed solution) before polymerization is cooled or It is confirmed that BP (blue phase) is developed by raising the temperature by observing with a polarization microscope and / or reflection spectrum measurement as described above. Next, when the sample was heated or cooled from the temperature at which BP expression was confirmed and the yellow-green brightness of the platelets was weakened (by observation with a polarizing microscope and / or reflection spectrum measurement), ultraviolet light was emitted. Irradiate, and once the intensity of yellow-green becomes strong, stop the irradiation of ultraviolet light once. Then, further lower or raise the temperature of the sample,
UV light is radiated at a temperature at which the yellow-green brightness of No. 2 becomes weaker, and when the yellow-green brightness of the platelets becomes stronger, the UV light irradiation is temporarily stopped. By repeating this operation, the temperature at which BP is expressed (the temperature at which the yellow-green brightness of the platelets becomes stronger) almost coincides with the BP expression temperature of the system of the low-molecular liquid crystal alone, and then UV light is further maintained for a certain time (for example, 1 hour). The polymerization is completed by irradiating with light. Although the above operation is based on photopolymerization, in the case of thermal polymerization, similarly, by maintaining the system at a temperature at which BP expression is confirmed by a polarization microscope observation and / or reflection spectrum measurement and the polymerization reaction proceeds. Polymerization can be carried out.

The liquid crystal material of the present invention composed of the blue phase (BP) of the composite type liquid crystal composition (polymer network / low molecular liquid crystal) obtained by the above polymerization reaction has an extremely wide temperature range (temperature range). ) Presents a stable blue hue. For example, a liquid crystal material including a polymer network formed of an acrylate-based monomer having an alkyl group side chain, which is a preferred example of the present invention, has a BP over a temperature range of 60 ° C or more across room temperature (15 to 25 ° C). Some can be expressed. The BP expression of the obtained liquid crystal material can also be confirmed by observation with a polarization microscope and reflection spectrum measurement as described above.

[0018]

EXAMPLES The features of the present invention will be more specifically described below.
Examples are shown for the sake of clarity, but the present invention is not limited to these examples.
Is not limited by.Example 1: Preparation of liquid crystal material Non-liquid crystalline 2-ethylhexyl as photopolymerizable monomer
Leacrylate (2EHA) (Aldrich), hex
Luacrylate (HA) (manufactured by Aldrich), and 1,
3,3-Trimethylhexyl acrylate (TMHA)
(Manufactured by Aldrich), and liquid crystalline 6- (4'-shear
Nobiphenyl-4-yloxy) hexyl acrylate
(6CBA) was used. Liquid crystalline diacryl as a cross-linking agent
Monomer (RM257) (Merck), photopolymerization initiator
2,2-dimethoxy-2-phenylacetopheno
(Manufactured by Aldrich) was used. As a low-molecular liquid crystal,
Elementary nematic mixed liquid crystal JC-1041XX (7) (Chisso
Company) and cyanobiphenyl nematic liquid crystal 4-system
Ano-4'-pentylbiphenyl (5CB) (Aldrich
Chiral dopa
ZLI-4572 (9) (manufactured by Merck) is used as the component.
It was Photopolymerizable monomer used, crosslinking agent and photopolymerization initiation
The chemical structural formula of the agent is shown in Fig. 1, and the used low-molecular liquid crystal and
Figure 2 shows the chemical structural formulas of chiral dopants and chiral dopants, respectively.
Shows. In addition, the chemical structural formulas shown in FIGS.
In the conventional expression, carbon atoms and hydrogen
The child is omitted.

A mixed solution of the above constituents prepared in a predetermined composition was poured in a non-oriented, sandwich type cell having a cell thickness of 14 μm in an isotropic phase state. The cell into which each sample was injected was observed with a polarizing microscope under crossed Nicols, and the irradiation intensity obtained from the metal halide lamp was 1.5 mW · c while confirming that the BP was retained according to the method described above.
Photopolymerization was performed by irradiating with m ⁻² ultraviolet light for 1 hour or more.

Further, by using a high sensitivity multi-channel photodetector (C4564-010G, Hamamatsu Photonics KK),
The reflection spectrum of each sample before and after the polymerization was measured in the temperature range between the isotropic phase and the chiral nematic liquid crystal phase. A xenon lamp was used as a light source. Table 1
Shows the composition of each sample when 2EHA is used,
Further, Table 2 shows the phase transition temperature after photopolymerization and the BP expression temperature range of the sample shown in Table 1 obtained by observation with a polarization microscope.

[0021]

[Table 1]

[0022]

[Table 2]

As shown in Table 2, in the absence of the polymer network (Sample 1), the BP expression temperature range (temperature range) was 1.1 K (1.1 ° C), but the concentration of the polymer network was Is a 4 mol% complex system (Sample 2)
In, the expression temperature range of BP was expanded to 6.8K (6.8 ° C). Furthermore, the concentration of the polymer network is 7 mol
In the composite system (samples 3 to 5) of more than%, the expression temperature range of BP was greatly expanded to 60 K (60 ° C.) or more. This is because the polymer network corresponding to the line defect of BP was formed by using 2EHA, which is a non-liquid crystalline monomer having an alkyl group side chain, and thus the line defect having a coexisting relationship with the molecular arrangement structure of BP was formed. It is considered that BP is also stabilized as a result of being stabilized.

Similar to 2EHA, when TMHA having a non-liquid crystalline and having an alkyl group side chain was used, a remarkable effect of expanding the BP expression temperature range was recognized, and the BP expression temperature range was 60 ° C. with almost the same composition as Sample 3. It has been greatly expanded. However, HA that is non-liquid crystalline but does not have an alkyl group side chain
When BP was used, the expression temperature range of BP was 8.2 ° C. even when the composition was almost the same as that of sample 3, which was smaller than that when 2EHA was used. On the other hand, the liquid crystalline 6
When CBA was used, the BP expression temperature range was almost constant at about 2 ° C. without depending on the composition, and expansion of the BP expression temperature range was not substantially observed.

FIG. 3 shows a polarization microscope observation image of the sample 3 as an example of the polarization microscope observation.
When the temperature was lowered from the isotropic phase ((A) of FIG. 3), blue and yellow-green platelets associated with the expression of BP were observed (B of FIG. 3). Similar images were observed for other samples over a wide temperature range.

FIG. 4 shows the temperature dependence of the reflection spectrum of Sample 3 as an example. Unlike the low molecular weight liquid crystal alone system, peaks due to the expression of BP were observed in a wide temperature range ((A) and (B) of FIG. 4). Moreover, the expression of a peak corresponding to the pitch length of the chiral nematic liquid crystal phase was not observed ((C) of FIG. 4). Therefore, it was revealed that the molecular arrangement structure of BP was stabilized over a wide temperature range.

[0027]Example 2: Evaluation of electro-optical characteristics of liquid crystal material The applicability of the liquid crystal material according to the present invention as an optical modulator
At the room temperature using an electro-optical characteristic evaluation device for evaluation.
Sine wave, 1kHz AC electric field is applied under (20 ℃),
Dependence of electro-optical response speed and transmitted light intensity on applied voltage
The sex was measured. The incident light used is He-Ne laser light.
(Λ = 632.8nm), and the transmitted light is detected by a check grid.
It was performed with the polarizer rotated by 45 degrees.

FIG. 5 shows Example 1 as an example of the measurement results.
3 shows the time dependence of the intensity of transmitted light when the above-mentioned AC electric field (80 V) is applied to the liquid crystal material produced from Sample 3 of FIG. It is understood that the transmitted light intensity is increased and the optical rotatory power is reduced in response to the application of an electric field, and the polarization state of incident light changes depending on the liquid crystal material. It is considered that this is because the low molecular weight liquid crystal molecules are arranged along the direction of the applied electric field to break the helical structure of BP. Further, both rising and falling are about 1-2 ms, which is a high-speed electro-optical response as compared with the nematic liquid crystal. Figure 6
Shows the dependence of the transmitted light intensity on the square of the applied voltage. Since the transmitted light intensity and the square of the applied voltage have a proportional relationship, it can be said that this is an electro-optical response behavior associated with the dielectric anisotropy.

[0029]

According to the present invention, the expression temperature range of the blue phase (BP) can be greatly expanded while maintaining the peculiar electro-optical characteristics of the blue phase (BP), and the BP is put into practical use as an optical modulator. The problem of narrowing the expression temperature, which was a hindrance to, is overcome. INDUSTRIAL APPLICABILITY The liquid crystal material of the present invention can be used in a wide temperature range as a novel optical modulator utilizing the diffraction and optical rotation properties of BP and can contribute to new development in the field of micro-optoelectronics.

[Brief description of drawings]

FIG. 1 shows chemical structural formulas of a photopolymerizable monomer, a crosslinking agent and a photopolymerization initiator used in Examples of the present invention.

FIG. 2 shows chemical structural formulas of a low molecular weight liquid crystal and a chiral dopant used in Examples of the present invention.

FIG. 3 shows an example of a polarization micrograph of a liquid crystal material sample according to the present invention.

FIG. 4 shows an example of temperature dependence of a reflection spectrum of a liquid crystal material sample according to the present invention.

FIG. 5 shows the electro-optical characteristics of the liquid crystal material according to the present invention, which is one of the time dependence of the transmitted light intensity when an AC electric field is applied.
Here is an example:

FIG. 6 shows an example of the relationship between transmitted light intensity and applied voltage as electro-optical characteristics of the liquid crystal material according to the present invention.

Claims (5)

[Claims] 1. A low-molecular liquid crystal capable of exhibiting a blue phase between a cholesteric phase and an isotropic phase, and a polymer network formed in the low-molecular liquid crystal and having a non-liquid crystal monomer are polymerized together with a crosslinking agent. A liquid crystal material for an optical modulation device, which is composed of a blue phase of a composite liquid crystal composition (polymer network / low-molecular liquid crystal) composed of a polymer network formed by the above. 2. The liquid crystal material for an optical modulation element according to claim 1, wherein the non-liquid crystal monomer is an acrylate-based monomer having an alkyl group as a side chain. 3. (Polymer network / low-molecular liquid crystal) composite liquid crystal composition has a blue phase development temperature range of 60 below room temperature.
The liquid crystal material for an optical modulation element according to claim 2, which has a temperature of not less than ° C. 4. A method for producing a liquid crystal material for an optical modulation element according to claim 1, wherein a monomer and a cross-linking agent are dispersed in a low molecular weight liquid crystal, and a blue phase is retained. A method comprising the step of polymerizing at a temperature. 5. The method for producing a liquid crystal material for an optical modulation device according to claim 4, wherein the polymerization is performed by photopolymerization.