阅读说明：本技术 一种全反射波段可调控的液晶薄膜材料的制备方法 (Preparation method of liquid crystal thin film material with adjustable total reflection wave band ) 是由 曹晖 史玮婷 张学涛 韩锐 李慧 杨洲 王冬 何万里 于 2021-07-06 设计创作，主要内容包括：一种全反射波段可调控的液晶薄膜材料的制备方法,属于光学薄膜材料技术领域。将小分子向列相液晶、紫外自由基可聚合单体、手性化合物、紫外吸收染料和自由基光引发剂混合材料混合均匀,经紫外光辐照聚合后形成具有宽波反射特性的聚合物稳定胆甾相液晶薄膜,其体系内形成了螺距梯度分布的聚合物网络,将未反应的液晶及其他化合物洗出后得到聚合物网络；将与第一部分手性化合物旋向相反的光/热响应的手性化合物和小分子向列相液晶混合均匀,填充到聚合物网络中,即可得到具有光/热响应的全反射波段可调控的液晶薄膜材料。本发明制作工艺简单,方法灵活,材料来源丰富,价格低廉,应用领域广。(A preparation method of a liquid crystal film material with adjustable total reflection wave band belongs to the technical field of optical film materials. Uniformly mixing small molecular nematic liquid crystal, an ultraviolet free radical polymerizable monomer, a chiral compound, an ultraviolet absorption dye and a free radical photoinitiator mixed material, irradiating and polymerizing by ultraviolet light to form a polymer stable cholesteric liquid crystal film with wide wave reflection characteristic, forming a polymer network with pitch gradient distribution in the system, and washing out unreacted liquid crystal and other compounds to obtain the polymer network; and uniformly mixing the chiral compound with the optical/thermal response opposite to the rotation direction of the first part of chiral compound and the micromolecular nematic liquid crystal, and filling the mixture into a polymer network to obtain the liquid crystal film material with the optical/thermal response and adjustable total reflection wave band. The invention has simple manufacturing process, flexible method, rich material source, low price and wide application field.)

1. A preparation method of a liquid crystal film material with adjustable total reflection wave band is characterized by comprising the following specific preparation processes:

step 1: mixing small-molecular nematic liquid crystal, an ultraviolet free radical polymerizable monomer, a chiral compound, an ultraviolet absorption dye and a free radical photoinitiator according to a certain proportion, adding a solvent, performing ultrasonic treatment, drying the solvent to form a uniform system, and preparing a polymerizable liquid crystal composite system, wherein the polymerizable liquid crystal composite system is protected from light in the operation process;

step 2: pouring the polymerizable liquid crystal composite system mixed in the step 1 into a parallel orientation liquid crystal box, keeping the polymerizable liquid crystal composite system in an oven for dust prevention for a certain time to stabilize the system, wherein the temperature of the oven is 10-15 ℃ lower than the clearing point of the system;

and step 3: exposing the liquid crystal box obtained in the step 2 under an ultraviolet radiation lamp to enable polymerizable monomers to perform polymerization reaction to form a polymer network with a pitch and network density gradient distribution;

and 4, step 4: immersing the liquid crystal box subjected to ultraviolet irradiation polymerization in the step 3 in an organic solvent for 2-15 days, and removing liquid crystal small molecules, chiral compounds, ultraviolet absorption dyes, free radical photoinitiators and unreacted polymerizable monomers from the liquid crystal box by using the solvent to leave a polymer network;

and 5: placing the polymer network obtained in the step 4 in a fume hood, and preserving for 4-12 h in a dustproof manner to remove the organic solvent, so as to obtain the polymer network with the broadband reflection memory characteristic;

step 6: mixing small molecular nematic liquid crystal and a light/thermal response chiral compound according to a certain proportion, adding a solvent, performing ultrasonic treatment, drying the solvent to form a uniform system, and obtaining a response liquid crystal composite system, wherein light is avoided in the operation process;

and 7: and (3) after the responsive liquid crystal composite system mixed in the step (6) is poured into the polymer network obtained in the step (5), heat preservation and plane texture optimization treatment are carried out by using an oven under the condition of keeping out of the sun, so that the responsive liquid crystal composite system is fully filled and is in a stable plane texture state, and the chiral compound with good responsiveness of light/heat response is ensured. The temperature of the oven is 10-15 ℃ lower than the clearing point, and the oven is subjected to stabilizing treatment for 2-5 h;

and 8: irradiating the composite system obtained after the stabilization in the step 7 by using ultraviolet light or performing variable temperature treatment to obtain a liquid crystal film material with adjustable and controllable required total reflection wave band;

in the system, the position of the reflection band of the newly filled cholesteric liquid crystal in the step 7 is overlapped with the position of the reflection band of the liquid crystal system used in the step 1 before the reoccurring organic solvent is washed out, but the two rotation directions are opposite, so that total reflection is formed.

2. The method for preparing a single-layer liquid crystal thin film material with adjustable total reflection waveband of claim 1, wherein in the step 1, the weight percentage of the small-molecular nematic liquid crystal is 65-90%, the weight percentage of the ultraviolet free radical polymerizable monomer is 8-25%, the weight percentage of the chiral compound is 0.6-10%, the weight percentage of the ultraviolet absorption dye is 0.1-0.4%, and the weight percentage of the free radical photoinitiator is 0.1-1%.

3. The method for preparing a single-layer liquid crystal thin film material with adjustable total reflection waveband of claim 1, wherein in the step 2, the thickness of a liquid crystal box is 10-150 μm; in the step 3, the ultraviolet irradiation conditions are as follows: the wavelength of ultraviolet light is 365nm, the polymerization temperature is 0-70 ℃, the ultraviolet light irradiation time is 2-150 min, and the ultraviolet light irradiation intensity is 0.1-50 mW/cm²。

4. The method for preparing a single-layer liquid crystal film material with adjustable total reflection waveband of claim 1, wherein the ultraviolet absorption dye comprises one or more of benzophenone, benzotriazole and benzotriazole ultraviolet absorption dyes; the chiral compound comprises one or more of isosorbide, binaphthol, binaphthyl amine, azobenzene binaphthalene compounds and derivatives thereof; the polymerizable monomer is one or more of acrylate and methacrylate; the organic solvent for washing out comprises one or more of cyclohexane, acetone, toluene and tetrahydrofuran.

5. The method for preparing a single-layer liquid crystal film material with adjustable total reflection waveband of claim 1, wherein the position of the reflection band of the newly filled cholesteric liquid crystal in the step 7 in the system is overlapped with the position of the reflection band of the liquid crystal system used in the step 1 before the reoccurring organic solvent is washed out, but the two rotation directions are opposite, so that the total reflection is formed.

6. The method of claim 1, wherein the liquid crystal film material is capable of forming a tunable total reflection band or a plurality of wide bands.

Technical Field

The invention belongs to the field of functional material application, and provides a method for preparing a single-layer cholesteric liquid crystal film with adjustable total reflection wave band of light/heat response. The material can be widely applied to the preparation of energy-saving building glass, laser protection and near-infrared light shielding films.

Background

The liquid crystal is a special form existing in nature, has optical anisotropy of crystals and flow property of liquid, and is a low-dimensional ordered fluid. The liquid crystal is an optically anisotropic substance sensitive to an external field because the centroid of the liquid crystal molecules is disordered (the crystals are ordered) and the director thereof is ordered (the ordinary liquid disorder). Due to this special structure, liquid crystals can modulate light and are widely used in the display field. The cholesteric liquid crystal material with the reflection wave band in the visible light region can be applied to an optical brightness enhancement film of a liquid crystal display, and people know that most of liquid crystals cannot emit light and need a backlight source system to provide light sources. The light utilization rate and brightness of the liquid crystal display screen can be obviously improved by using the light brightness enhancement film, and the light brightness enhancement film can be realized by stabilizing the cholesteric liquid crystal film by using the polymer; the cholesteric liquid crystal with the reflection wave band in the visible light region can also be applied to temperature indication, tumor examination, anti-counterfeiting trademarks, reflective liquid crystal display, color filters, reflective circular polarizers and the like. The cholesteric liquid crystal with the reflection wave band in the near infrared light region can be applied to energy-saving and environment-friendly building glass or paint. The cholesteric liquid crystal with the reflection wave band in the mid-infrared light region has potential application prospect in shielding and hiding aspects in military affairs.

Cholesteric liquid crystals, also known as chiral nematic liquid crystals (N x-LCs), can be formed by adding a chiral compound to a nematic liquid crystal. Due to the unique helical structure, the liquid crystal material can selectively reflect incident light with different wavelengths, wherein the wavelength lambda of the incident light is equal to nP, n is the average refractive index of the liquid crystal material, and P is the helical pitch of cholesteric liquid crystal. The wavelength range of the cholesteric liquid crystal with single pitch selectively reflecting the incident light is between lambda_max＝n_eP and lambda_min＝n_oP between (n)_eAnd n_oExtraordinary and ordinary refractive indices, respectively); reflection bandwidth Δ λ ═ λ_max-λ_min＝(n_e-n_o)P＝ΔnP(Δn＝n_e-n_oIs the birefringence index). The reflection bandwidth refers to the width of the reflection band, in the reflection bandwidth, the circularly polarized light with the same rotating direction as the cholesteric liquid crystal spiral structure is reflected, the circularly polarized light with the rotating direction opposite to the cholesteric liquid crystal spiral structure or outside the reflection bandwidth is transmitted, the reflectivity is only about 50%, but in practical application, a material with better overall transmittance and the light with a certain wave band is required to be totally reflected.

Generally, racemization occurs when chiral molecules with opposite handedness exist in a single liquid crystal film, and when multiple layers of films are stacked, the whole transmittance is obviously influenced by the refractive index mutation among the films. Therefore, an important research objective in the research field of the totally-reflective polymer stabilized cholesteric liquid crystal is to obtain a film material with a flexible and controllable reflection band, high overall transmittance and a simple manufacturing process.

Disclosure of Invention

The invention aims to provide a simple and efficient method for preparing a total reflection liquid crystal film material with optical/thermal response, which has the characteristics of simple and easily obtained raw materials, simple process, low cost and the like.

The invention utilizes the memory effect of the polymer network to prepare the single-layer liquid crystal film with adjustable total reflection wave band of light/heat response, and the specific preparation process comprises the following steps:

step 1: mixing small molecular nematic liquid crystal, ultraviolet free radical polymerizable monomer, chiral compound, ultraviolet absorbing dye and free radical photoinitiator in certain proportion, adding solvent, ultrasonically drying the solvent to form a uniform system, and keeping the mixture away from light during operation.

Step 2: and (2) pouring the polymerizable liquid crystal composite system mixed in the step (1) into a parallel orientation liquid crystal box, keeping the polymerizable liquid crystal composite system in an oven for a certain time to stabilize the system, wherein the temperature of the oven is 10-15 ℃ lower than the clearing point of the system.

And step 3: exposing the liquid crystal box obtained in the step 2 under an ultraviolet radiation lamp to enable polymerizable monomers to perform polymerization reaction to form a polymer network with a pitch and network density gradient distribution;

and 4, step 4: immersing the liquid crystal box subjected to ultraviolet irradiation polymerization in the step 3 in an organic solvent for 2-15 days, and removing liquid crystal small molecules, chiral compounds, ultraviolet absorption dyes, free radical photoinitiators and unreacted polymerizable monomers from the liquid crystal box by using the solvent to leave a polymer network;

and 5: placing the polymer network obtained in the step 4 in a fume hood, and preserving for 4-12 h in a dustproof manner to remove the organic solvent, so as to obtain the polymer network with the broadband reflection memory characteristic;

step 6: mixing small molecular nematic liquid crystal and a light/thermal response chiral compound according to a certain proportion, adding a solvent, performing ultrasonic treatment, drying the solvent to form a uniform system, and obtaining a response liquid crystal composite system, wherein light is avoided in the operation process;

and 7: and (3) after the responsive liquid crystal composite system mixed in the step (6) is poured into the polymer network obtained in the step (5), heat preservation and plane texture optimization treatment are carried out by using an oven under the condition of keeping out of the sun, so that the responsive liquid crystal composite system is fully filled and is in a stable plane texture state, and the chiral compound with good responsiveness of light/heat response is ensured. The temperature of the oven is 10-15 ℃ lower than the clearing point, and the stabilizing treatment is carried out for 2-5 h.

And 8: and (4) irradiating the composite system obtained after the stabilization in the step (7) by using ultraviolet light or carrying out variable temperature treatment to obtain the liquid crystal film material with the adjustable and controllable required total reflection wave band.

In the system, the position of the reflection band of the newly filled cholesteric liquid crystal in the step 7 is overlapped with the position of the reflection band of the liquid crystal system used in the step 1 before the reoccurring organic solvent is washed out, but the two rotation directions are opposite, so that total reflection is formed.

Further, the handedness of the chiral compound in the polymerizable liquid crystal composite system is opposite to the handedness of the chiral compound in the responsive liquid crystal composite system.

Further, the polymerizable liquid crystal composite system comprises 65-90 wt% of small molecular nematic liquid crystal, 8-25 wt% of ultraviolet free radical polymerizable monomer, 0.6-10 wt% of chiral compound, 0.1-0.4 wt% of ultraviolet absorption dye and 0.1-1 wt% of free radical photoinitiator.

The weight percentage of the ultraviolet absorbing dye is a major factor affecting the reflection bandwidth of the polymerizable liquid crystal composite system, and thus affecting the reproducible reflection bandwidth of the polymer network. The larger the concentration of the ultraviolet absorption dye is, the wider the reflection bandwidth is, but the excessive concentration can cause that the ultraviolet absorption dye can not be completely dissolved in a composite system when a sample is mixed, and the phase separation is easy to occur in the polymerization process. When the weight percentage is 0.1-0.4%, the optimal reflection bandwidth can be realized, and the ultraviolet absorption dye is completely dissolved during mixing.

The weight percentage of the chiral compound will affect the position of the reflection band and different amounts of photoinitiator will result in different polymerization rates, which can achieve the best polymer network density.

Further, in the step 2, the thickness of the liquid crystal box is 10-150 μm.

Further, in the step 3, the conditions of the ultraviolet irradiation are as follows: the wavelength of ultraviolet light is 365nm, the polymerization temperature is 0-70 ℃, the ultraviolet light irradiation time is 2-150 min, and the ultraviolet light irradiation intensity is 0.1-50 mW/cm²。

Further, the ultraviolet absorption dye comprises one or more of benzophenone, benzotriazole and benzotriazole ultraviolet absorption dyes; the chiral compound comprises one or more of isosorbide, binaphthol, binaphthyl amine, azobenzene binaphthalene compounds and derivatives thereof; the polymerizable monomer is one or more of acrylate and methacrylate; the organic solvent for washing out comprises one or more of cyclohexane, acetone, toluene and tetrahydrofuran.

According to a second aspect of the present invention there is provided a single layer cholesteric polymer film, characterised in that said single layer cholesteric polymer film is prepared using a method according to any one of the preceding aspects, and said liquid crystal film material is capable of forming a tuneable total reflection band or bands.

The invention has the advantages that: the material system used by the film material is simple to mix, the experimental materials are easy to obtain, the sources are rich, the process is simple, and the large-scale production is easy to realize. The film material can freely adjust the reflection wave band, and the position of the reflection band is adjusted mainly by changing the type or the content of the chiral compound in the step 1 and the step 6; the central reflection wavelength of total reflection can also be freely adjusted, and ultraviolet irradiation or temperature change treatment in the step 8 is mainly carried out. The prepared liquid crystal film is a single-layer film, and adverse effects of a multi-layer film structure on the transmittance are avoided.

Drawings

FIG. 1 is a transmitted light spectrum of a sample in the preparation process of example 1 according to the present invention;

FIG. 2 is a photograph showing polarization of a sample before and after filling a liquid crystal mixture in example 1;

FIG. 3 is a transmitted light spectrum of a sample in the preparation process of example 2;

FIG. 4 is a transmitted spectrum of a sample prepared in example 3 according to the present invention.

The specific implementation mode is as follows:

the technical solution of the present invention is further explained below with reference to specific embodiments.

Example 1:

preparation of a liquid crystal box: and (3) placing the cleaned Indium Tin Oxide (ITO) glass in a drying oven at 60 ℃ for 3h for drying. 3g of polyvinyl alcohol (PVA) white solid particles are added into 97mL of deionized water, stirred by magnetic force, slowly heated to 95 ℃ until the PVA is completely dissolved, and then a PVA solution with the concentration of 3% is obtained. Coating the ITO glass with a glue homogenizing machine at an initial speed of 600r/min and 10s and at a high speed of 2000r/min and 30s, and then carrying out heat treatment on the glass substrate coated with the PVA high polymer film at the temperature of 90 ℃ for 120 min. And (3) directionally rubbing the surface coated with the PVA orientation layer by using flannelette for 3 times, and avoiding excessive force as much as possible. The two glass substrates were combined, rubbing directions were antiparallel, and a PET film having a thickness of 60 μm was used as a spacer to prepare a liquid crystal cell.

Uniformly mixing nematic liquid crystal SLC-1717, ultraviolet polymerizable monomer C6M, chiral compound R5011, photoinitiator IRG651 and ultraviolet absorbing dye UV-327 according to the mass ratio of 83.6%, 15%, 0.8%, 0.3% and 0.3% respectively to obtain cholesteric liquid crystal, heating to dissolve, vibrating and ultrasonically treating, and filling into a planar oriented liquid crystal box at room temperature.

The chemical structural formula of the ultraviolet light polymerizable monomer C6M is as follows:

the chiral compound R5011 has the chemical structural formula:

the ultraviolet absorption dye UV-327 has a chemical structural formula:

photoinitiator IRG651 has the chemical structural formula:

after the system was stabilized, UV light (1.0) was applied to the cellmW/cm²365nm) for 30min, so that the polymerizable monomer is cured and crosslinked to form a gradient polymer network.

And soaking the polymerized sample in cyclohexane to immerse the sample in a liquid crystal box, storing for 5 days, taking out, and placing in a fume hood for dustproof storage for 7 hours to obtain the liquid crystal box containing the polymer network. The cyclohexane can be slowly volatilized at room temperature, the influence on the polymer network structure is avoided being too large, and the dry polymer network is obtained.

Uniformly mixing nematic liquid crystal SLC1717 and ultraviolet light response chiral compound 2D according to the mass ratio of 98% to 2%, pouring into polymer network at room temperature, keeping away from light in a drying oven at 60 deg.C for 3h, and then using ultraviolet light (3.0 mW/cm)²365nm) and the transmission spectrum of the sample was measured every 15s, resulting in fig. 1. The samples maintained good planar texture throughout the process, and FIG. 2 is a photograph of a polarizing microscope before and after filling with the liquid crystal mixture.

The ultraviolet light response chiral compound 2D has the chemical structural formula:

the ultraviolet light intensity can be controlled by adjusting the power of the ultraviolet lamp and the distance between the lamp and a sample, and the ultraviolet irradiation is measured by using an ultraviolet irradiation meter. The clearing point of the mixed system, namely the temperature of the mixed system when the mixed system is converted from anisotropy to isotropy, can be accurately measured by Differential Scanning Calorimetry (DSC). The transmitted spectrum of the sample can be measured at room temperature using a uv-vis-nir spectrophotometer (JascoV-570). The texture of the sample can be observed using a polarizing microscope (Olympus BX 51).

Example 2:

the preparation method comprises the following steps of uniformly mixing nematic liquid crystal SLC-1717, an ultraviolet polymerizable monomer C6M, a chiral compound R5011, a photoinitiator IRG651 and an ultraviolet absorbing dye UV-327 according to the mass ratio of 83.2%, 15%, 1.2%, 0.3% and 0.3%, heating for dissolving, vibrating for ultrasonic treatment, and filling into a planar oriented liquid crystal box at room temperature.

After the system was stabilized, UV light (1.0 mW/cm) was applied to the cell²365nm) for 30min, so that the polymerizable monomer is cured and crosslinked to form a gradient polymer network.

And soaking the polymerized sample in cyclohexane to immerse the sample in a liquid crystal box, storing for 5 days, taking out, and placing in a fume hood for dustproof storage for 7 hours to obtain the liquid crystal box containing the polymer network. The cyclohexane can be slowly volatilized at room temperature, the influence on the polymer network structure is avoided being too large, and the dry polymer network is obtained.

Uniformly mixing nematic liquid crystal SLC1717 and ultraviolet light response chiral compound 2D according to the mass ratio of 98% to 3%, pouring into a polymer network at room temperature, keeping away from light in a drying oven at 60 ℃ for 3h, and then using ultraviolet light (3.0 mW/cm)²365nm) was irradiated and the transmission spectrum of the sample was measured every 20s, resulting in fig. 3.

Example 3:

uniformly mixing the nematic liquid crystal SLC1717 and the thermal response chiral compound CD according to the mass ratio of 98% to 2%, pouring the mixture into the polymer network in the embodiment 1 at room temperature, keeping the mixture away from light in a drying box at 60 ℃ for 3 hours, then placing the system on a hot table, heating at the speed of 1 ℃/min, and measuring the transmission spectrum of the sample once at 5 ℃ to obtain a graph shown in FIG. 4.

The chemical structural formula of the thermal response chiral compound CD is as follows:

while the present invention has been described with reference to the embodiments shown in the drawings, the present invention is not limited to the embodiments, which are illustrative and not restrictive, and it will be apparent to those skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the invention as defined in the appended claims.