Temperature-responsive and fluorescence-variable liquid crystal film, and preparation method and application thereof

Abstract

The application belongs to the field of liquid crystal polymer material preparation, and discloses a temperature-responsive and fluorescence-variable liquid crystal film as well as a preparation method and application thereof. The temperature-responsive and fluorescence-variable liquid crystal film material is prepared by adding a fluorescence-variable cholesteric liquid crystal solution into a 10%-20% polyvinyl alcohol solution. The color pattern change of the liquid crystal film driven by temperature, ultraviolet light and blue light is realized for the first time. The preparation method has the advantages of low cost, environmental protection and simplicity. The prepared liquid crystal film material has the functions of temperature response and fluorescence variability, and has good fatigue resistance. The liquid crystal film material can be applied to the fields of research and development of a new generation of temperature sensors, light-controlled intelligent materials, information storage and anti-counterfeiting encryption.

Description

Technical Field

[0001] This invention belongs to the field of liquid crystal polymer material preparation, specifically relating to a temperature-responsive and fluorescence-variable liquid crystal thin film, its preparation method, and its application. Background Technology

[0002] Temperature-responsive and fluorescence-variable liquid crystal films are materials that can sense environmental and human skin temperatures and can change color and pattern under light-driven conditions. Light energy is a clean energy source with advantages such as ease of availability, non-contact remote control, and ease of control in time and space, making it an ideal driving method.

[0003] Polyvinyl alcohol (PVA) is a non-toxic, harmless, and easily film-forming polymer with good compatibility with cholesteric liquid crystals. The resulting liquid crystal films are portable and retain the inherent properties of cholesteric liquid crystals, showing great promise for applications such as photothermal conversion. However, as the area to be covered by the liquid crystal film increases, problems such as uneven film color easily arise. Therefore, the key issue is how to prepare liquid crystal films with large area, uniform color, reusability without affecting liquid crystal performance, and low cost. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of the prior art and provide a temperature-responsive and fluorescence-variable liquid crystal thin film, its preparation method, and its application. Specifically, the following technical solution is adopted: According to a first aspect of the present invention, a method for preparing a temperature-responsive and fluorescence-variable liquid crystal thin film is provided, comprising the following steps: Cholesteric liquid crystal monomers, chiral molecules, and fluorescent molecules were mixed uniformly to obtain a liquid crystal composite solution. Then, the liquid crystal composite solution was added to a 10%-20% polyvinyl alcohol solution, mixed uniformly, and dried to obtain a liquid crystal film with variable temperature response and fluorescence. All liquid crystal monomers used were synthesized in this study, and their structural formulas are at least one of formulas (1) to (18), as detailed below: ; R1-R 18 Selected independently or n is any positive integer from 1 to 12; R 19 -R 23 Selected independently , or n is any positive integer from 1 to 12; X is selected from F, Cl, Br or I.

[0005] For example, in molecular formula (1), R1 and R3 are n-hexyl, and R2 is methoxy; in molecular formula (2), R5 and R6 are n-hexyl, and R5 is methoxy; in molecular formula (3), R7 is n-hexyl, and R8 is methyl or methoxy. Mixing these three compounds in a mass fraction of 20%:55%:20% is the optimal choice for this invention. The carboxylic acid used in this method can be compatible with ester groups and chiral molecules, thereby promoting the compatibility between ester groups and chiral molecules. Ester groups can increase the temperature range of liquid crystals, enabling cholesteric liquid crystals to achieve red, green, and blue color changes over a wider temperature range. Liquid crystals containing only carboxylic acid will result in a short temperature range for cholesteric liquid crystals and will generate smectic phases, leading to large differences in color changes during heating and cooling, uneven color changes, and are not conducive to subsequent experiments; while liquid crystals containing only ester groups have poor compatibility with chiral molecules, and the color changes during heating are not obvious.

[0006] This invention prepares a temperature-responsive and fluorescence-variable liquid crystal film by adding a fluorescence-variable cholesteric liquid crystal solution to a 10%-20% polyvinyl alcohol solution. The main process involves encapsulating the resulting mixture with the polyvinyl alcohol solution to obtain a fluorescent cholesteric liquid crystal. This film is then spread evenly on a petri dish or black PET plastic sheet and placed at room temperature. The polyvinyl alcohol solution evaporates, forming a transparent film, while the cholesteric liquid crystal remains within the polyvinyl alcohol, creating a colored cholesteric liquid crystal film that retains the characteristics of a cholesteric liquid crystal.

[0007] This invention marks the first realization of color and pattern changes in a liquid crystal film driven by temperature, ultraviolet light, and blue light. The color and pattern changes of this liquid crystal film material can be adjusted by controlling temperature, ultraviolet light, and the mask pattern, exhibiting fatigue resistance and repeatability. The preparation method involves simply pouring the mixture into a petri dish, spreading it evenly, and allowing it to air dry. This method is inexpensive, environmentally friendly, and simple, and can be applied to the development of next-generation temperature sensors, light-controlled smart materials, information storage, and anti-counterfeiting encryption.

[0008] The polyvinyl alcohol used in this invention is a high-molecular-weight polymer with advantages such as good biocompatibility, non-toxicity, and easy degradation. Compared with the chemical doping method in traditional research, this invention adopts a non-invasive modification method to prepare responsive liquid crystal films. This method allows for repeated processing and utilization of raw materials through the removal and recoating of the coating, making it a very inexpensive and environmentally friendly process.

[0009] Preferably, the polyvinyl alcohol (PVA) mass fraction is 18%. The preparation method of 18% PVA includes the following steps: mixing PVA and water at a mass ratio of 82:18 until homogeneous, and stirring at 100 °C for 2 h. The PVA solution at this concentration, when mixed with cholesteric liquid crystal, yields the best liquid crystal film performance. Too low a PVA concentration results in the cholesteric liquid crystal not adhering well to the film, leading to insignificant color changes. Too high a concentration results in high adhesiveness of the liquid crystal film, making it difficult to spread the liquid crystal mixture evenly on the evaporating dish.

[0010] Preferably, the chiral molecule is at least one of formulas (19) to (29), and the specific structural formula is as follows: ; R 24 The structure is shown in equation (30); R 25 Selected from equation (31) or equation (32); R 26 -R 29 Selected independently n is any positive integer from 1 to 12; .

[0011] The chiral molecules used are easy to synthesize and can achieve RGB color changes. More preferably, the chiral molecular formula (19) is a commonly used chiral molecule that is easy to synthesize. Adding only 3%-5% by mass can transform the liquid crystal into a cholesteric liquid crystal. Under heating conditions, the red, green and blue (RGB) color changes can be achieved, which is highly efficient.

[0012] Preferably, the fluorescent molecule is at least one of formulas (33) to (37), with the following specific structural formulas: ; X is selected from S or CH2=CH2. More preferably, formula (33) is inexpensive and easy to synthesize, and only 1%-2% is needed to achieve a good fluorescent pattern effect.

[0013] Preferably, in the liquid crystal composite solution, the mass fraction ratio of liquid crystal monomers, chiral molecules, and fluorescent molecules is (67%-96%):(3%-30%):(1%-3%). At this ratio, the prepared cholesteric liquid crystal exhibits both color and temperature changes; when chiral molecules are in excess, the color change disappears; while when fluorescent molecules are in excess, although the fluorescence effect is good, the compatibility between the substances is poor.

[0014] Preferably, the mass fraction ratio of the liquid crystal composite solution to polyvinyl alcohol is 1:10. When the cholesteric liquid crystal is in excess, it will cause liquid crystal clusters, resulting in uneven color of the obtained liquid crystal film; while when the polyvinyl alcohol is in excess, the obtained liquid crystal film will be lighter in color, and the color and pattern changes will not be obvious.

[0015] According to a second aspect of the present invention, a temperature-responsive and fluorescence-variable liquid crystal film is also provided, prepared by the above-described method. This liquid crystal material can undergo red, green, and blue color changes within a certain temperature range; it can switch patterns and colors under irradiation with light of wavelengths of 365 nm and 450 nm. At room temperature, the liquid crystal film displays red; when touched, the film color changes from red to green, and as the temperature reaches 30 °C, the film color returns to red; when the contact temperature reaches 40 °C, the liquid crystal film loses its cholesteric liquid crystal properties and becomes colorless and transparent. When a liquid crystal cell containing cholesteric liquid crystal is irradiated with 450 nm blue light, the hydrazone fluorescent molecules undergo isomerization through irradiation, changing from a fluorescent configuration to a non-fluorescent configuration. After covering the liquid crystal cell with a mask and irradiating it with a 365 nm ultraviolet lamp for 5 minutes, and then observing the pattern under 365 nm or 450 nm blue light, it can be seen that the prepared cholesteric liquid crystal material has fluorescence-variable functionality and good fatigue resistance.

[0016] According to a third aspect of the present invention, the above-described liquid crystal thin film is also provided for use in the fabrication of temperature sensors, light-controlled smart materials, information storage, anti-counterfeiting encryption, and optical driving devices. In the application to the fabrication of optical driving devices, it is characterized in that the film can be fabricated under constant near-infrared light and light conditions with wavelengths of 365 nm and 450 nm.

[0017] The beneficial effects of this invention are as follows: This invention prepares a temperature-responsive and fluorescence-variable liquid crystal thin film material by adding a fluorescence-variable cholesteric liquid crystal solution to a 10%-20% polyvinyl alcohol solution. For the first time, color pattern changes in a liquid crystal thin film driven by temperature, ultraviolet light, and blue light have been achieved. This preparation method has advantages such as low cost, environmental friendliness, and simplicity. The prepared liquid crystal thin film material has temperature-responsive and fluorescence-variable functions and good fatigue resistance properties, and can be applied in the research and development of next-generation temperature sensors, light-controlled smart materials, information storage, and anti-counterfeiting encryption. Attached Figure Description

[0018] Figure 1 The image shows a test graph of the temperature change of the liquid crystal film during human skin contact. Figure 2 The image shows the ultraviolet absorption test of the liquid crystal film during human skin contact. Figure 3 The image shows the color change of the liquid crystal thin film material prepared in Example 1 during contact with human skin; Figure 4 The image shows the pattern changes of the liquid crystal thin film material prepared in Example 1 under irradiation at 365 nm and 450 nm. Detailed Implementation

[0019] The following will provide a clear and complete description of the concept and technical effects of the present invention in conjunction with the embodiments and accompanying drawings, so as to fully understand the purpose, solution and effects of the present invention. It should be noted that, unless otherwise specified, the embodiments and features in the embodiments of this application can be combined with each other.

[0020] Example 1 A temperature-responsive and fluorescence-variable liquid crystal thin film, the preparation method of which includes the following steps: First, C7T, E7, and chiral molecules were mixed at a mass ratio of 8:85:7. Then, 1.0 wt% of fluorescent molecules were melted and mixed thoroughly. The resulting liquid crystal system was then mixed with 18% PVA at a mass ratio of 1:10 and injected into an 18% PVA solution. The mixture was stirred at room temperature for 2 hours and left overnight to eliminate air bubbles. It was then poured into a petri dish, evenly spread, and allowed to air dry to obtain a temperature-responsive and fluorescence-variable liquid crystal film.

[0021] The chiral molecular structure used in this embodiment is shown below: ; The fluorescent molecule structure used in this embodiment is shown below: .

[0022] The structural formula of the C7T liquid crystal monomer (4-hexyloxybenzoic acid-4'-methoxyphenyl ester) used in this embodiment is shown below: .

[0023] The synthetic method involves dissolving p-hexyloxybenzoic acid (11.11 g, 50 mmol) and 4-dimethylaminopyridine (0.367 g, 3 mmol) in 60 mL of dichloromethane. After stirring and dissolving, another 60 mL of dichloromethane containing p-methoxyphenol (3.72 g, 30 mmol) and N,N'-dicyclohexylcarbodiimide (6.18 g, 30 mmol) is added. The reaction is carried out at room temperature for 24 h. The reaction product is filtered, the solvent is removed by vacuum distillation, purified by silica gel column chromatography (petroleum ether: ethyl acetate = 10:1), and then recrystallized from ethanol to obtain a pure sample of 4-hexyloxybenzoic acid-4'-methoxyphenyl ester, denoted as C7T.

[0024] The structural formula of the chiral molecule (C4O) used in this embodiment is shown below: .

[0025] The synthetic method involves dissolving S-naphthol (5.72 g, 20 mmol) and 4-dimethylaminopyridine (0.48 g, 4 mmol) in 60 mL of dichloromethane. After stirring and dissolving, another 60 mL of dichloromethane containing p-butoxybenzoic acid (7.76 g, 40 mmol) and N,N'-dicyclohexylcarbodiimide (8.24 g, 40 mmol) is added. The reaction is carried out at room temperature for 24 h. The reaction product is filtered, the solvent is removed by vacuum distillation, purified by silica gel column chromatography (petroleum ether: ethyl acetate = 10:1), and then recrystallized from ethanol to obtain a chiral molecule, denoted as C4O.

[0026] The structural formula of the fluorescent molecule (CNH) used in this embodiment is shown below: .

[0027] The synthetic method involves the following steps: Isosorbide (4.38 g, 30 mmol), benzoylformic acid (10.50 g, 70 mmol), oxaloyl chloride (8.88 g, 70 mmol), and 1 drop of N,N'-dimethylformamide were dissolved in 60 mL of dichloromethane and stirred until fully dissolved. The mixture was then heated to 35 °C and reacted for 48 h. The reaction product was filtered, and the solvent was removed by vacuum distillation. The product was purified by silica gel column chromatography (petroleum ether: ethyl acetate = 10:1) and recrystallized from ethanol to obtain the product of the first step, denoted as CNH1. CNH1 (1.46 g, 10 mmol), p-carboxyphenylhydrazine (2 g, 26 mmol), and 5 drops of acetic acid were dissolved in 20 mL of ethanol and reacted at 80 °C under reflux for 4 h. The reaction product was filtered and dried to obtain the product of the second step, denoted as CNH2. CNH₂ (6.78 g, 10 mmol) and 4-dimethylaminopyridine (0.38 g, 2 mmol) were dissolved in 60 mL of dichloromethane. After stirring and dissolving, 60 mL of dichloromethane containing 4-hexyloxyphenol (1.88 g, 20 mmol) and N,N'-dicyclohexylcarbodiimide (4.12 g, 20 mmol) was added. The reaction was carried out at room temperature for 24 h. The reaction product was filtered, the solvent was removed by vacuum distillation, purified by silica gel column chromatography (petroleum ether: ethyl acetate = 10:1), and then recrystallized from ethanol to obtain the fluorescent molecule, denoted as CNH.

[0028] Example 2 A temperature-responsive and fluorescence-variable liquid crystal film (compared to Example 1, only the polyvinyl alcohol mass fraction was adjusted to 10%) is prepared by the following steps: First, C7T, E7, and chiral molecules were mixed in a mass ratio of 8:85:7. Then, 1.0 wt% of fluorescent molecules were melted and mixed thoroughly. The resulting liquid crystal system was then mixed with 10% PVA at a mass ratio of 1:10 and injected into a 10% PVA solution. The mixture was stirred at room temperature for 2 hours and left overnight to eliminate air bubbles. It was then poured into a petri dish, evenly spread, and allowed to air dry to obtain a temperature-responsive and fluorescence-variable liquid crystal film.

[0029] The chiral molecular structure used in this embodiment is shown below: ; The fluorescent molecule structure used in this embodiment is shown below: .

[0030] The structural formula of the C7T liquid crystal monomer (4-hexyloxybenzoic acid-4'-methoxyphenyl ester) used in this embodiment is shown below: .

[0031] The steps of its synthesis method are the same as those in Example 1.

[0032] The structural formula of the chiral molecule (C4O) used in this embodiment is shown below: .

[0033] The steps of its synthesis method are the same as those in Example 1.

[0034] The structural formula of the fluorescent molecule (CNH) used in this embodiment is shown below: .

[0035] The steps of its synthesis method are the same as those in Example 1.

[0036] Example 3 A temperature-responsive and fluorescence-variable liquid crystal film (compared to Example 1, only the polyvinyl alcohol mass fraction was adjusted to 15%) is prepared by the following steps: First, C7T, E7, and chiral molecules were mixed at a mass ratio of 8:85:7. Then, 1.0 wt% of fluorescent molecules were melted and mixed thoroughly. The resulting liquid crystal system was then mixed with 15% PVA at a mass ratio of 1:10 and injected into a 15% PVA solution. The mixture was stirred at room temperature for 2 hours and left overnight to eliminate air bubbles. It was then poured into a petri dish, evenly spread, and allowed to air dry to obtain a temperature-responsive and fluorescence-variable liquid crystal film.

[0037] The chiral molecular structure used in this embodiment is shown below: ; The fluorescent molecule structure used in this embodiment is shown below: .

[0038] The structural formula of the C7T liquid crystal monomer (4-hexyloxybenzoic acid-4'-methoxyphenyl ester) used in this embodiment is shown below: .

[0039] The steps of its synthesis method are the same as those in Example 1.

[0040] The structural formula of the chiral molecule (C4O) used in this embodiment is shown below: .

[0041] The steps of its synthesis method are the same as those in Example 1.

[0042] The structural formula of the fluorescent molecule (CNH) used in this embodiment is shown below: .

[0043] The steps of its synthesis method are the same as those in Example 1.

[0044] Example 4 The liquid crystal thin film material obtained in Example 1 was tested, and the testing method included the following steps: (1) When the cholesteric phase liquid crystal film comes into contact with human skin, such as Figure 1 As shown, the results indicate that as the temperature decreases, the wavelength of the thin film redshifts from 470 nm to 700 nm, and the film's appearance changes from blue to red; Figure 2 The reflection-wavelength diagram of the thin film is shown, indicating that the film exhibits red, green, and blue colors and has a transmittance of 50%. Figure 3 As shown in the actual color change diagram, since the cholesteric liquid crystal film in this embodiment is sensitive to temperature and temperature can penetrate the liquid crystal, when the film touches human skin, as the temperature rises, both sides of the film can simultaneously undergo temperature-responsive red, green and blue color changes.

[0045] (2) Irradiate the cholesteric phase liquid crystal film with 450 nm blue light. The hydrazone fluorescent molecules undergo isomerization through light irradiation, changing from a fluorescent configuration to a non-fluorescent configuration. Cover the film with a mask, and then irradiate the film with a 365 nm ultraviolet lamp for 15 min. After irradiation, place the film under 450 nm blue light to observe the pattern. Figure 4 The pattern change process is shown in the diagram. The pattern was erased under 365 nm and 450 nm lamps. The cholesteric phase liquid crystal film was irradiated with a 365 nm ultraviolet lamp. The hydrazone fluorescent molecules underwent isomerization through the light, changing from a non-fluorescent configuration to a fluorescent configuration. A mask was placed on the film, and then the film was irradiated with a 450 nm blue lamp for 15 minutes. After irradiation, the liquid crystal cell was placed under a 365 nm ultraviolet lamp to observe the pattern.

[0046] In summary, based on UV testing and visual observation, the liquid crystal films obtained with 10% and 15% polyvinyl alcohol exhibit less noticeable RGB color changes and weaker UV absorption due to the lower mass fraction of cholesteric liquid crystal loaded in polyvinyl alcohol. In contrast, the liquid crystal film obtained with 18% polyvinyl alcohol shows significant RGB color changes and absorbs UV light at all wavelengths (red, green, and blue). Therefore, the liquid crystal film obtained with 18% polyvinyl alcohol demonstrates the best performance.

[0047] Although the description of the invention has been quite detailed and particularly of several described embodiments, it is not intended to limit it to any of these details or embodiments or any particular embodiment, but should be considered as providing a broad possible interpretation of the claims by referring to the appended claims and taking into account the prior art, thereby effectively covering the intended scope of the invention. Furthermore, the invention has been described above with respect to embodiments foreseeable by the inventors in order to provide a useful description, and non-substantial modifications to the invention that have not yet been foreseen may still represent equivalent modifications.

Claims

1. A method for preparing a temperature-responsive and fluorescence-variable liquid crystal thin film, characterized in that, Includes the following steps: Cholesteric liquid crystal monomers, chiral molecules, and fluorescent molecules are mixed uniformly to obtain a cholesteric liquid crystal composite solution; then the cholesteric liquid crystal composite solution is added to a 10%-20% polyvinyl alcohol solution, mixed uniformly, and dried to obtain a liquid crystal film with variable temperature response and fluorescence. The cholesteric liquid crystal monomer is a C7T liquid crystal monomer, and its structural formula is shown below: ； The chiral molecular structure is shown below: ； The specific structural formula of the fluorescent molecule is as follows: ； In the cholesteric liquid crystal composite solution, the mass ratio of the cholesteric liquid crystal monomer, the chiral molecule, and the fluorescent molecule is (67%-96%):(3%-30%):(1%-3%).

2. The preparation method according to claim 1, characterized in that, The polyvinyl alcohol solution has a mass fraction of 18%.

3. The preparation method according to claim 2, characterized in that, The preparation method of the polyvinyl alcohol solution includes the following steps: mixing polyvinyl alcohol and water at a mass ratio of 82:18 until homogeneous, and stirring at 100°C for 2 hours.

4. The preparation method according to claim 1, characterized in that, The mass ratio of the cholesteric phase liquid crystal composite solution to the polyvinyl alcohol solution is 1:

10.

5. A temperature-responsive and fluorescence-variable liquid crystal thin film, characterized in that, It is prepared by the preparation method according to any one of claims 1-4.

6. The application of the liquid crystal film according to claim 5 in the preparation of temperature sensors, light-controlled smart materials, information storage, anti-counterfeiting encryption and optical driving devices.

7. The application of claim 6 in the preparation of temperature sensors, light-controlled smart materials, information storage, anti-counterfeiting encryption, and optical drive devices, characterized in that, The preparation was carried out under constant near-infrared light and light conditions with wavelengths of 365 nm and 450 nm.

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