CN116300161B - Method for regulating color of liquid crystal coating by utilizing ultraviolet light, product and application thereof - Google Patents

Abstract

The present invention discloses a method for regulating the color of a liquid crystal coating by using ultraviolet light. The cholesteric liquid crystal coating is cross-linked to a certain extent by ultraviolet light at room temperature. The color of the coating depends not only on the degree of cross-linking, but also on the subsequent heating temperature. Therefore, after different degrees of cross-linking are achieved by ultraviolet light irradiation, the final display color of the film can be controlled by the specific temperature change of the subsequent heating. When heated, parts with different cross-linking conditions will show different colors at the same temperature. Therefore, a suitable heating temperature can be selected as needed to obtain the desired color coating, so that the film can be applied to thermosensitive elements, information display and anti-counterfeiting encryption.

Description

A method and product for regulating the color of liquid crystal coating using ultraviolet light and its application

Technical Field

The present invention relates to the field of liquid crystal application technology development, and in particular to a method and product for regulating the color of a liquid crystal coating by utilizing ultraviolet light, and applications thereof.

Background Art

Liquid crystal is a common material in daily life. As a phase between liquid and solid, liquid crystal has many unique properties. Among them, cholesteric liquid crystal has a certain symmetry in its structure. The molecular orientation in each layer is orderly. The orientation between each layer will return to the initial orientation after undergoing a 360° arrangement change along the long axis spiral direction. This periodic interlayer spacing is called the pitch of cholesteric liquid crystal. Cholesteric liquid crystal can reflect light of different wavelengths according to the length of the pitch, thereby showing different colors. For the change of the pitch, on the one hand, it can be regulated by designing specific liquid crystal molecules. They are responsive to specific environments. When external factors such as temperature or pressure change, their pitch will also change accordingly; on the other hand, when the end of the liquid crystal molecule contains some cross-linkable molecular structures, such as the carbon-carbon double bond of the acrylate group, it can be cross-linked by chemical bonds in the presence of a photoinitiator through ultraviolet light irradiation, thereby forming a polymer network and fixing the pitch of the liquid crystal molecule.

Patent CN109143712A discloses a cholesteric liquid crystal composite film and its preparation method and application. The cholesteric liquid crystal system in the cholesteric liquid crystal composite film has temperature response characteristics, and the reflection wavelength can be flexibly adjusted by controlling the temperature. However, the disadvantage of this technology is that the heating process of adjusting the wavelength and the ultraviolet irradiation process of cross-linking the liquid crystal are inseparable, resulting in high requirements for its preparation scene, and it cannot be applied to precision instruments to make smaller patterns, and multiple ultraviolet irradiation processes will consume more energy.

Therefore, it is urgent to find a new technical method to solve the above problems and overcome the above defects.

Summary of the invention

The technical solution of the present invention discloses a method for regulating the pitch of cholesteric liquid crystal by ultraviolet light, thereby controlling the color change of the cholesteric liquid crystal coating with temperature change. Since the cholesteric liquid crystal has a diacrylate group, when a photoinitiator is present, an appropriate amount of ultraviolet light under suitable conditions will cause the acrylate group to have different degrees of cross-linking, and different degrees of cross-linking will limit the degree of movement of the reflection wavelength of the system when the temperature changes.

The object of the present invention is to provide a method for regulating the color of a liquid crystal coating using ultraviolet light, which comprises the following steps:

S1, heating and stirring a liquid crystal monomer, a chiral dopant, a chain extender and a solvent to react to obtain a liquid crystal oligomer;

S2, mixing the liquid crystal oligomer with a surfactant, a chiral dopant, a photoinitiator and a solvent to obtain a liquid crystal mixture;

S3, placing the liquid crystal compound on a glass sheet to form a liquid crystal coating, and then applying different ultraviolet light doses to different areas on the glass sheet at room temperature, so that different parts of the liquid crystal coating show different degrees of cross-linking and curing, thereby obtaining a liquid crystal film;

S4, subjecting the liquid crystal film to continuous ultraviolet irradiation at elevated temperature, so that the liquid crystal film is completely cross-linked to obtain a product.

The technical solution of the present invention utilizes ultraviolet light to indirectly regulate the pitch of the cholesteric liquid crystal coating; wherein the raw materials of the cholesteric liquid crystal coating include liquid crystal oligomers, chiral dopants, surfactants and photoinitiators; the ultraviolet light can be a type of surface light source or a point light source; and a mask plate or other opaque object of any shape can be used to control the dose of ultraviolet light irradiated to different areas of the subsequent liquid crystal coating.

Preferably, the raw materials of the cholesteric liquid crystal coating include 75-85 mass parts of liquid crystal oligomer, 5-15 mass parts of chiral dopant, 1-5 mass parts of surfactant, 1-2 mass parts of photoinitiator, and the balance is solvent.

Preferably, the photoinitiator includes but is not limited to at least one of Irg184, Irg819 or Irg651.

Preferably, the solvent is selected from at least one of tetrahydrofuran, dichloromethane or cyclopentanone.

Furthermore, the liquid crystal monomer contains at least two acrylate groups.

Furthermore, in step S3, the degree of cross-linking is 1-100%.

Furthermore, in step S3, the range of the ultraviolet light dosage is 20-400 mJ/cm ² .

Furthermore, in step S3, the wavelength range of the ultraviolet light is 300-400 nm.

Furthermore, in step S3, the normal temperature ranges from 25°C to 30°C.

Furthermore, in step S4, the heating temperature is 35-70°C.

Furthermore, in step S4, different regions of the product present gradual or sudden color differences.

Another object of the present invention is to provide a liquid crystal coating obtained by the above method of regulating the color of a liquid crystal coating using ultraviolet light.

Another object of the present invention is to provide the above-mentioned liquid crystal coating thermosensitive element, and its application in information display and anti-counterfeiting encryption.

Preferably, the cholesteric liquid crystal coating is coated on the surface of the object by using a blade coating technique.

Preferably, the emission wavelength band of the ultraviolet light can be adjusted to a matching wavelength band according to different photoinitiators.

Preferably, when used in an ultraviolet lithography device containing a digital micro-mirror device (DMD), the mask should be a non-physical digital mask.

The present invention has the following beneficial effects:

First, the technical solution uses ultraviolet light at room temperature to crosslink the cholesteric liquid crystal coating to a certain extent. The color display of the coating depends not only on the degree of crosslinking, but also on the subsequent heating temperature. Therefore, after achieving different degrees of crosslinking through ultraviolet light irradiation, the final display color of the film can be controlled by the specific temperature change of the subsequent heating. When heated, parts with different crosslinking conditions will show different colors at the same temperature. Therefore, the appropriate heating temperature can be selected as needed to obtain the desired color coating, so that the film can be applied to thermosensitive elements, information display and anti-counterfeiting encryption.

Preferably, this method is used on a UV lithography device containing a DMD to prepare tiny patterns at room temperature, which not only proves that the resolution of the system can reach the millimeter level, but also expands the application range of the cholesteric liquid crystal system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(A) is a schematic diagram of the operation flow of Example 1 of the present invention.

Figure numerals: 1 - ultraviolet light source; 2 - mask; 3 - cholesteric liquid crystal.

FIG1(B) shows the reflection wavelengths of different areas of the coating at corresponding temperatures after the operation process of FIG1(A), wherein the dotted line is the wavelength and temperature trend line, as well as the actual picture.

FIG. 2(A) is an operation flow chart of Embodiment 2 of the present invention.

Figure numerals: 1 - ultraviolet light source; 2 - computer; 3 - digital micromirror array; 4 - reflector; 5 - focusing objective lens.

FIG. 2(B) is a picture of the actual object prepared after the operation process of FIG. 2(A).

FIG3(A) is an operation flow chart of Embodiment 3 of the present invention.

Figure numerals: 1 - ultraviolet light source; 2 - mask; 3 - schematic diagram of the cross-linking degree of cholesteric liquid crystal.

FIG. 3(B) shows the reflection wavelengths of different regions after the operation process of FIG. 3(A) and a real-object picture.

DETAILED DESCRIPTION

The following will be combined with the embodiments to clearly and completely describe the concept of the present invention and the technical effects produced, so as to fully understand the purpose, characteristics and effects of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, other embodiments obtained by those skilled in the art without creative work are all within the scope of protection of the present invention.

Unless otherwise specified, the reagents and preparation methods involved in the embodiments and comparative examples of the present invention should be regarded as commercially available reagents and conventional preparation methods.

The method for determining the degree of crosslinking in the embodiments of the present invention is carried out using surface attenuated total reflection infrared spectroscopy.

Example 1

The preparation method of the cholesteric liquid crystal coating comprises the following steps:

S1. Spin-coat a polyvinyl alcohol aqueous solution with a mass fraction of 5 on a clean transparent glass sheet, and then anneal at 90°C for 10 minutes to evaporate excess water. After the glass sheet cools to room temperature, rub it on a velvet cloth in the same direction to make it have parallel oriented grooves so that the cholesteric oligomer liquid crystal coated later can obtain a better orientation effect;

S2, take 82 parts by mass of liquid crystal monomer RM82 containing diacrylate groups, 12 parts by mass of chiral dopant LC756 and 4 parts by mass of chain extender n-butylamine, dissolve them in sufficient amount of dichloromethane. Stir evenly and react at 100°C for 18 hours to obtain the desired liquid crystal oligomer;

S3. In order to make the liquid crystal oligomer prepared in S2 suitable for coating on the glass sheet prepared in S1, it is necessary to additionally add 4 parts by mass of surfactant 2-(N-ethylperfluorooctanesulfonamide)-ethyl methacrylate, 1 part by mass of chiral dopant LC756, 1 part by mass of photoinitiator Irg184 and 24 parts by mass of solvent tetrahydrofuran to prepare a liquid crystal mixture.

S4, on the glass sheet with the parallel alignment layer prepared in S1, at the parameters of temperature of 60°C, speed of 15mm/s, and thickness of 12μm, the liquid crystal mixture prepared in S3 is scraped, and then placed in an environment of 60°C to volatilize the solvent, and annealing is continued for 12h, at which time the film is 432nm blue; then naturally cooled to room temperature, at which time the film is 600nm red. Finally, a temperature-responsive cholesteric liquid crystal film is obtained;

The process of using UV light to control the color of the coating is shown in Figure 1 (A). At room temperature of 25°C and a nitrogen environment, with the help of the movement of the mask, the UV dose received by each area is divided into an uncrosslinked area, a partially crosslinked area (crosslinking degree is 65%) and a completely crosslinked area. The UV doses received by the three areas are 0, 10.5 mW/ ^cm2 × 20s and 10.5 mW/ ^cm2 × 30s, respectively.

In the opaque part under the mask, which is not irradiated by ultraviolet light, the blue shift of the reflection wavelength becomes larger and larger as the temperature gradually increases; when the partially cross-linked area is irradiated by ultraviolet light with a dose of 10.5mW/ ^cm2 ×20s, the diacrylate groups are cross-linked while the cholesteric liquid crystal system still retains temperature responsiveness, but when the temperature rises, the blue shift of its reflection wavelength is lower; when the completely cross-linked area is irradiated by ultraviolet light with a dose of 10.5mW/ ^cm2 ×30s, the diacrylate groups are completely cross-linked and lose temperature responsiveness.

As shown in Figure 1(B), at the same temperature, the reflection bands of the uncrosslinked area, partially crosslinked area and fully crosslinked area are significantly different, thus producing different colors. Using this mechanism, at room temperature of 25°C, the same film is divided into uncrosslinked area, partially crosslinked area and fully crosslinked area, where the UV doses received by the three areas are 0, 10.5mW/ ^cm2 ×20s and 10.5mW/ ^cm2 ×30s respectively. When the temperature is raised to 60°C, a dose of 10.5mW/ ^cm2 ×50s is used to completely crosslink the system, so that the pitch of the liquid crystal molecules is completely fixed, and finally the desired color film is obtained. The specific results are shown in Table 1.

Table 1 Reflection wavelengths obtained at different temperatures for uncrosslinked, partially crosslinked, and fully crosslinked regions

Example 2

The preparation method of the cholesteric liquid crystal composite film in this embodiment is substantially the same as the preparation method in Embodiment 1, and the difference between the two is that in step S3, the photoinitiator Irg184 in the liquid crystal mixture is replaced by Irg819.

The process of using ultraviolet light to control the color of the coating is shown in Figure 2 (A)-(B). Using Photoshop software on a computer, a digital mask is prepared on a layer of 1920*1080 pixels, where the white pixel part represents the part irradiated by ultraviolet light, and the black pixel part represents the part without ultraviolet light irradiation. Different digital masks can be designed to prepare different patterns as needed. This embodiment designs a heart-shaped pattern with a length of 3mm and a width of 2mm. The film is placed in a DMD digital mask lithography machine, and after leveling, the previously designed digital mask is imported, and the appropriate exposure intensity and time are set; in order to obtain multiple patterns, the moving distance of the light source is continued to be set. The exposure process will run automatically according to the setting program, and finally a film with four heart-shaped patterns is obtained. When the temperature rises, the heart-shaped pattern and the background show different colors.

Example 3

The preparation method of the cholesteric liquid crystal composite film of this embodiment is basically the same as the preparation method of embodiment 1, and the difference between the two is that in step S4, after annealing at 60° C. for 12 hours, the film is irradiated with ultraviolet light while it is still blue.

The process of using ultraviolet light to control the color of the coating is shown in Figure 3 (A). Because the cross-linking caused by ultraviolet light irradiation will limit the movement of the reflection band, the color displayed by each part is different at the end of the red shift process at room temperature. The degree of red shift of each part depends on the size of the ultraviolet light dose. The larger the ultraviolet light dose, the smaller the red shift. The color presented by each area and the actual picture are shown in Figure 3 (B). The benefit of this method is that a film that is colorful at room temperature can be obtained without the need for a heating step.

It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above and that the present invention can be implemented in other specific forms without departing from the spirit or essential features of the present invention. Therefore, the embodiments should be considered exemplary and non-restrictive in all respects, and the scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes falling within the meaning and scope of the equivalent elements of the claims be included in the present invention.

In addition, it should be understood that although the present specification is described according to implementation modes, not every implementation mode contains only one independent technical solution. This narrative method of the specification is only for the sake of clarity. Those skilled in the art should regard the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other implementation modes that can be understood by those skilled in the art.

Claims (7)

1. The method for regulating and controlling the color of the liquid crystal coating by using the ultraviolet light is characterized by comprising the following steps of:

s1, heating and stirring a liquid crystal monomer, a chiral dopant, a chain extender and a solvent for reaction to obtain a liquid crystal oligomer;

s2, mixing the liquid crystal oligomer with a surfactant, a chiral dopant, a photoinitiator and a solvent to obtain a liquid crystal mixture;

S3, placing the liquid crystal mixture on a glass sheet to form a liquid crystal coating, and then applying different ultraviolet light doses to different areas on the glass sheet at normal temperature to enable different parts of the liquid crystal coating to show different degrees of crosslinking and curing to obtain a liquid crystal film;

S4, continuously irradiating the liquid crystal film with ultraviolet light under the condition of heating so that the liquid crystal film is completely crosslinked to obtain a product;

The liquid crystal monomer contains at least two acrylate groups;

The ultraviolet light dosage is in the range of 20-400 mJ/cm ²;

in step S3, the wavelength range of the ultraviolet light is 300-400 nm.

2. The method for controlling color of liquid crystal coating according to claim 1, wherein the degree of crosslinking is 1-100% in step S3.

3. The method for controlling color of liquid crystal coating according to claim 1, wherein in step S3, the normal temperature range is 25-30 ℃.

4. The method according to claim 1, wherein in step S4, the temperature of the heating is 35-70 ℃.

5. The method for controlling color of liquid crystal coating according to claim 1, wherein in step S4, different regions of the product exhibit gradual or abrupt color differences.

6. A liquid crystal coating obtainable by a method of modulating the colour of a liquid crystal coating using ultraviolet light as claimed in any one of claims 1 to 5.

7. The use of a liquid crystal coating according to claim 6 in heat sensitive elements, information presentation and security encryption.