Preparation method of mechanochromic photonic crystal composite film and product and application of mechanochromic photonic crystal composite film

Abstract

The invention discloses a preparation method of a mechanochromic photonic crystal composite film as well as a product and application of the mechanochromic photonic crystal composite film. The TPU / CLCE composite film is prepared by compounding a mechanically force-induced color-changing cholesteric liquid crystal elastomer (CLCE) film onto a black polyurethane (TPU) film purchased on the market through a water transfer printing technology. The composite film prepared by a water transfer printing method has good tensile property, the shape and the color can be completely recovered, and the composite film has strong metal reflection color. The composite film meets the basic performance requirements of the color-changing car cover, can be further expanded to be applied to the fields of intelligent wearable equipment, consumer electronics and the like, and provides unique visual effects and functional characteristics for products. Good market application prospects are realized.

Description

Technical Field

[0001] The present invention relates to a preparation method of a stretchable photonic crystal TPU composite film for automobile exterior modification, specifically a preparation method of a force-induced color change photonic crystal composite film, its product and application. Background Art

[0002] Photonic crystal films selectively reflect light through their internal periodic structures, thereby achieving the presentation of specific colors. This structural color has high stability and selectivity, and can effectively avoid the problems of fading and color difference of traditional pigments. Among them, cholesteric liquid crystal elastomers, as an important photonic crystal material, have developed continuously and have the advantages of simple preparation and rich colors, providing new possibilities for fields such as sensors and consumer electronics.

[0003] With the continuous increase in the global automobile ownership, consumers' demands for the personalization, aesthetics and functionality of automobiles are growing. As an emerging automobile exterior modification material, color-changing car wraps have received extensive attention. Traditional color-changing car wraps mainly achieve color changes through methods such as dyeing or printing, but these methods have problems such as poor color stability, easy fading, and environmental pollution. In addition, traditional car wrap materials also have certain limitations in terms of protective performance, weather resistance and service life. In the preparation of photonic crystal composite films, thermoplastic polyurethane, as a commonly used substrate, has excellent mechanical properties, weather resistance and processing properties, and can provide good support and protection for photonic crystal films. However, it is not easy to composite photonic crystals with thermoplastic polyurethane. Traditional methods such as solution coating and vacuum evaporation have problems such as complex processes, high costs and environmental pollution during the preparation process, and the composite films still need to be improved in terms of tensile properties, color uniformity and stability.

[0004] Therefore, the development of a stretchable photonic crystal TPU composite film with excellent performance and its preparation method is of great significance for promoting the development of color-changing car wrap technology. Summary of the Invention

[0005] Object of the Invention: The technical problem to be solved by the present invention is to provide a preparation method, its product and application of a force-induced color change photonic crystal composite film with force-induced color change performance, mechanical stability and excellent mechanical properties.

[0006] Technical Solution: To solve the above technical problems, the present invention provides a preparation method of a force-induced color change photonic crystal composite film, including the following steps: (1) Dissolve the liquid crystal monomer 4-(3-acryloyloxypropoxy)benzoic acid 2-methyl-1,4-phenylene ester, the chiral agent (3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-((acryloyloxy)butoxy)carbonyl)oxy)benzoyl)oxy)benzoate), the crosslinking agent pentaerythritol tetra-3-mercaptopropionate, and the chain extender 3,6-dioxa-1,8-octanedithiol in an organic solvent, and obtain a mixed reaction solution after fully mixing evenly; (2) Add the photoinitiator 651 to the mixed reaction solution described in step (1), mix evenly, add the catalyst dipropylamine, and obtain a pigment solution after ultrasonic treatment; (3) Coat the pigment solution in (2) on a polyvinyl alcohol film treated with air plasma, let it stand to complete the self-assembly of the CLCE layer, and obtain a PVA / CLCE composite film after light curing; (4) Immerse the PVA / CLCE composite film in water. After the PVA layer is fully dissolved, place a black TPU film in the area where the CLCE floats. After the TPU film and the CLCE film are fully composite, take it out and perform a drying treatment to finally obtain a photonic crystal composite film with mechanochromic properties.

[0007] Among them, the molar ratio of the liquid crystal monomer 4-(3-acryloyloxypropoxy)benzoic acid 2-methyl-1,4-phenylene ester, the chiral agent (3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-((acryloyloxy)butoxy)carbonyl)oxy)benzoyl)oxy)benzoate), the crosslinking agent pentaerythritol tetra-3-mercaptopropionate, and the chain extender 3,6-dioxa-1,8-octanedithiol in step (1) is 39:1~1.25:2.7:30.8.

[0008] Among them, the organic solvent described in step (1) is a volatile organic solvent, including acetone, ethyl acetate, dichloromethane or toluene, which is beneficial to fabricating the film by the deswelling method.

[0009] Among them, ultrasonic treatment is performed using an ultrasonic crusher before mixing in step (2); the ultrasonic power of the treatment is 100~1000 W, and the ultrasonic time is 5 min to make the solution mix evenly.

[0010] Among them, the mass fraction of the photoinitiator 651 in the total mass of the liquid crystal monomer, chiral agent, crosslinking agent and chain extender in step (2) is 4~6 wt.%, and the mass fraction of the catalyst dipropylamine in the total mass of the liquid crystal monomer, chiral agent, crosslinking agent and chain extender is 0.5~3%.

[0011] Among them, the thickness of the PVA film in step (3) is 23~26 μm.

[0012] Among them, the time of the air plasma treatment described in step (3) is 1 to 6 min.

[0013] Among them, the wavelength of the ultraviolet LED lamp described in step (3) is 365 nm, and the light power density is about 5 to 200 mW cm -2 , and the irradiation time is 30 s to 1 min.

[0014] Among them, the volume of water described in step (4) is 1 to 5 L, and the dissolution time of the PVA layer is 30 to 90 min.

[0015] The present invention also provides a force-induced color-changing photonic crystal composite film prepared by the preparation method.

[0016] The present invention also provides the application of the force-induced color-changing photonic crystal composite film in the preparation of a color-changing car wrap.

[0017] Among them, after the water transfer CLCE layer is dried into a film, different colors such as red, green, and blue can be displayed according to different chiral agent components.

[0018] In the preparation of the stretchable photonic crystal TPU composite film of the present invention, PVA is selected as the key material in the water transfer process, mainly based on the following considerations: First, the TPU material is prone to swelling in organic solvents, which will increase the difficulty of film composite and affect the composite effect. However, PVA has good solubility in water and will not cause swelling of TPU, thus ensuring the integrity and performance stability of the TPU film. Second, after being treated by plasma, PVA can be coated more uniformly, which helps to form a flat and uniform CLCE film, providing a good basis for the subsequent composite process. In the water transfer process, PVA is compounded with CLCE to form a film, and then the composite film is immersed in water, and the PVA component gradually dissolves, enabling the CLCE film to be in direct contact and composite with the TPU film. The dissolution process of PVA provides convenient conditions for the transfer of the CLCE film, ensuring the uniform distribution and stable existence of the CLCE film on the water surface. After the dissolution of PVA, the remaining PVA forms a thin adhesion layer between the CLCE film and the TPU film. This remaining PVA forms hydrogen bonds with the polar groups on the surfaces of the TPU film and the CLCE film through its hydroxyl groups, thereby enhancing the adhesion force between the two. The flexibility and stretchability of the TPU film match the liquid crystal structure of the CLCE film, enabling the composite film to maintain the stability and uniformity of color change during the stretching process. After atmospheric drying treatment, the residual moisture of PVA completely volatilizes, and finally a photonic crystal composite film with force-induced color-changing characteristics is obtained.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following remarkable advantages: (1) Avoiding TPU swelling: The present invention selects polyvinyl alcohol (PVA) as the key material in the water transfer printing process. PVA has good solubility in water and will not cause the swelling of TPU, thus ensuring the integrity and performance stability of the TPU film. This is in contrast to the characteristic that TPU is easily swollen in organic solvents, effectively avoiding the problems of film performance degradation and poor composite effect caused by swelling; (2) Simple and efficient: The present invention adopts the water transfer printing technology, which is simple to operate, easy to control, and environmentally friendly. Compared with traditional methods such as solution coating and vacuum evaporation, the water transfer printing technology avoids problems such as complex processes, high costs, and environmental pollution, greatly improving the preparation efficiency and the feasibility of practical applications; (3) PVA in water can assist in enhancing the adhesion effect of CLCE / TPU composites: During the water transfer printing process, the residual PVA after dissolution forms a thin adhesion layer between the CLCE film and the TPU film. This residual PVA forms hydrogen bonds with the polar groups on the surfaces of the TPU film and the CLCE film through its hydroxyl groups, thereby enhancing the adhesion force between the two. In addition, the residual PVA can also play a certain toughening role in the composite film, improving the flexibility and crack resistance of the film, and further enhancing the stability and durability of the composite film. Description of the Drawings

[0020] Figure 1 It is a diagram of the raw material molecular formula of the composite film in Example 1; Figure 2 It is a process flow chart of the water transfer printing method for the force-induced color change composite film of the present invention; Figure 3 Figures a, b, and c in it are SEM pictures of the CLCE layer of the force-induced color change composite films prepared in Examples 1, 2, and 3 at a magnification of 15000x; Figure 4 They are the transmission spectra (a) and optical photos (b) of the red, green, and blue three kinds of photonic crystal composite films; Figure 5 It is a stress-strain curve diagram of the red photonic crystal composite film, TPU, and PVA prepared in Example 1; Figure 6 They are the optical photo and enlarged surface diagram of the green photonic crystal composite film prepared in Example 2 after the cross-cut test with 3M681 test tape; Figure 7 a is the adhesion force test method of the photonic crystal composite film in Example 1, and b is the adhesion strength diagram of the CLCE film and the TPU film measured by the tensile test method; Figure 8 They are the (a) reflection spectra and (b) optical photos of the red photonic crystal composite film in Example 1 at different stretching rates; Figure 9 The effect diagrams of water transfer printing with (a) PVP film; (b) PSS film; (c) PVA film as the substrate respectively. Figure 10 The effect diagrams of the water transfer printing process with different plasma treatment times: (a) untreated; (b) 2.5 min; (c) 5 min. Figure 11 The effect diagrams of the water transfer printing process at different water temperatures: (a) 60 °C; (b) 40 °C; (c) 20 °C. Figure 12 The effect diagrams of the water transfer printing process after ultraviolet curing for 30 s (a) or 1 min (b). Detailed implementation mode

[0021] The technical solution of the present invention will be further described below with reference to the accompanying drawings.

[0022] The main raw materials and their actual sources in the embodiments of the present invention are shown in Table 1.

[0023] Table 1 Main raw materials and reagent sources for the experiments of the present invention Experiment main raw materials reagents

[0024] Example 1 Preparation of red force - induced color - changing photonic crystal composite film The specific steps are as Figure 2 shown: (1) Weigh monomer RM257 (367.9 mg, 0.624 mmol), LC756 (16 mg, 0.016 mmol), PETMP (21.3 mg, 0.04355 mmol), EDDET (90.9 mg, 0.494 mmol) respectively in four 5 - mL transparent culture bottles. After mixing the above solutes with acetone (2.0 mL), put them into the 5 - mL transparent culture bottles and seal them, and mix well.

[0025] (2) Add photo - initiator 651 (2.6 mg) and catalyst DPA (2.5 μL) to the uniformly dispersed mixture, and use a high - power ultrasonic cleaner to ultrasonically dissolve it at an ultrasonic power of 100 W for 5 minutes to obtain a pigment solution.

[0026] (3) Treat the commercially available PVA film with an air plasma using a plasma processor for 5 minutes. Then, immediately coat the pigment solution onto the PVA film by blade coating, and let it stand for 4 hours. After the CLCE layer has completed self-assembly, cure it by irradiating with a UV LED lamp at a light power density of 10 mW / cm² for 1 minute to obtain a PVA / CLCE composite film.

[0027] (4) Immerse the PVA / CLCE composite film in water (water temperature is 40 °C). After the PVA component has fully dissolved, place the black TPU film in the area where the CLCE is floating. The TPU film is in full contact with the CLCE film to achieve composite. Subsequently, take out the composite film from the water and perform a drying treatment under normal pressure environment to finally obtain a red photonic crystal composite film with mechanochromic properties.

[0028] Figure 1 It is the raw material molecular formula diagram of the CLCE layer of the photonic crystal composite film in Example 1. The CLCE layer is prepared by organic polymerization through a thiol-ene click reaction using RM257 as the liquid crystal monomer, LC756 as the chiral agent, PETMP as the cross-linking agent, EDDET as the chain extender, photoinitiator 651 as the initiator, DPA as the catalyst, and acetone as the organic solvent.

[0029] Preparation of the green mechanochromic photonic crystal composite film in Example 2 The specific steps are as follows: (1) Weigh the monomer RM257 (367.9 mg, 0.624 mmol), LC756 (18 mg, 0.018 mmol), PETMP (21.3 mg, 0.04355 mmol), and EDDET (90.9 mg, 0.494 mmol) respectively in four 5 mL transparent culture bottles. After mixing the above solutes with acetone (2.0 mL), put them into the 5 mL transparent culture bottles and seal them, and mix well.

[0030] (2) Add photoinitiator 651 (2.6 mg) and catalyst DPA (2.5 μL) to the uniformly dispersed mixture and use a high-power ultrasonic cleaner to ultrasonically dissolve it for 5 minutes at an ultrasonic power of 100 W to obtain a pigment solution.

[0031] (3) Treat the commercially available PVA film with an air plasma using a plasma processor for 5 minutes. Then, immediately coat the pigment solution onto the PVA film by blade coating, and let it stand for 4 hours. After the CLCE layer has completed self-assembly, cure it by irradiating with a UV LED lamp at a light power density of 10 mW / cm² for 1 minute to obtain a PVA / CLCE composite film.

[0032] (4) Immerse the PVA / CLCE composite film in water (water temperature is 40 °C). After the PVA component is fully dissolved, place the black TPU film in the area where the CLCE is floating. The TPU film is in full contact with the CLCE film to achieve lamination. Subsequently, take out the laminated film from the water and conduct drying treatment under normal pressure environment to finally obtain a green photonic crystal composite film with mechanochromic properties.

[0033] Example 3 Preparation of Blue Mechanochromic Photonic Crystal Composite Film The specific steps are as follows: (1) Weigh the monomers RM257 (367.9 mg, 0.624 mmol), LC756 (20 mg, 0.02 mmol), PETMP (21.3 mg, 0.04355 mmol), and EDDET (90.9 mg, 0.494 mmol) respectively in four 5 mL transparent culture bottles. After mixing the above solutes with acetone (2.0 mL), put them into the 5 mL transparent culture bottles and seal them, and mix well.

[0034] (2) Add photoinitiator 651 (2.6 mg) and catalyst DPA (2.5 μL) to the uniformly dispersed mixture and use a high-power ultrasonic cleaner to ultrasonically dissolve it at an ultrasonic power of 100 W for 5 minutes to obtain a pigment solution.

[0035] (3) Treat the commercially available PVA film with an air plasma processor for 5 minutes, and then immediately coat the pigment solution on the PVA film by scraping. Let it stand for 4 hours. After the CLCE layer completes self-assembly, cure it with a UV LED lamp at a light power density of 10 mW / cm² for 1 minute to obtain a PVA / CLCE composite film.

[0036] (4) Immerse the PVA / CLCE composite film in water (water temperature is 40 °C). After the PVA component is fully dissolved, place the black TPU film in the area where the CLCE is floating. Keep this state for a period of time to ensure that the TPU film is in full contact with the CLCE film and achieve lamination. Subsequently, take out the laminated film from the water and conduct drying treatment under normal pressure environment to finally obtain a blue photonic crystal composite film with mechanochromic properties.

[0037] Figure 3a, b, and c are respectively the schematic cross-sectional scanning electron micrographs of the force-induced color change layers of the red, green, and blue photonic crystal composite films. It can be seen from the figure that the pitch of the red force-induced color change layer is 438 nm, the pitch of the green force-induced color change layer is 410 nm, and the pitch of the blue force-induced color change layer is 368 nm. It can be seen that as the addition amount of the chiral agent LC756 increases continuously, the pitch also decreases continuously, and the macroscopic manifestation is the continuous blue shift of the color of the reflected light.

[0038] Figure 4 are the transmission spectra and optical photographs of the red, green, and blue photonic crystal composite films. It can be seen from the figure that the center of the transmission wavelength. According to Bragg's law, the wavelength of the transmitted structural color (λ) can be given by the following formula: λ = 2npcosθ ， where n is the refractive index of the liquid crystal, p is the helical pitch of the matrix, and θ is the incident angle. The calculated results can be deduced to be consistent with the pitch shown in the scanning electron microscope in Figure 3 Among them, Figure 4 3.2%, 3.6%, and 4% in

[0039] Example 4 tests the stress-strain ability, adhesion cross-cut test, and force-induced color change performance of the red photonic crystal composite film prepared in Example 1 To study the mechanical properties of the photonic crystal composite film prepared by the water transfer printing method, a tensile testing machine was used to conduct stress-strain tests on PVA, TPU, and the red photonic crystal composite film of Example 1. Figure 5 is the stress-strain curve graph of the red photonic crystal composite film of Example 1 and PVA, TPU. As Figure 5 shown, the tensile fracture strength of the PVA film is 48.36 MPa, the Young's modulus is 701.4 MPa, and the elongation at break is 312%. The tensile fracture strength of the TPU / CLCE composite film prepared by the water transfer printing method is 57.8 MPa, the Young's modulus is 8.65 MPa, and the elongation at break is 1420%. It can be seen that the tensile fracture strength and Young's modulus of the TPU / CLCE composite film prepared by the water transfer printing method have been improved to a certain extent compared with the pure TPU film, which is attributed to the additional strengthening effect of the residual PVA on the strength of the composite film.

[0040] To explore the reliability of the photonic crystal composite film as a force-induced color change car wrap, an adhesion tester was used to conduct a cross-cut test on the film. Figure 6 shows the optical photograph and the magnified surface image after pulling up the 3M Scotch test tape for the green photonic crystal composite film of Example 2. It shows that the edge of the cut area is flat and smooth, without signs of peeling or delamination, meeting the adhesion level of ISO-0.

[0041] In addition, in order to explore the effects of the water transfer printing method and plasma treatment on the composite adhesion of the thin film, the present invention designed three groups of experiments: directly preparing the CLCE / TPU composite by the anisotropic deswelling method in a tetrafluoroethylene tank, without air plasma treatment of the TPU, and with air plasma treatment of the TPU film for 5 minutes. Among them, the CLCE / TPU composite film was prepared by the anisotropic deswelling method: in four 5 mL transparent culture flasks, monomers RM257 (367.9 mg, 0.624 mmol), LC756 (18 mg, 0.018 mmol), PETMP (21.3 mg, 0.04355 mmol), and EDDET (90.9 mg, 0.494 mmol) were weighed respectively. After mixing the above solutes with acetone (2.0 mL), they were put into a 5 mL transparent culture flask and sealed, and then thoroughly mixed. Then, a high-power ultrasonic cleaner was used to ultrasonically dissolve them for 5 minutes at an ultrasonic power of 100 W to obtain a pigment solution. Subsequently, the above-prepared uniform solution was poured into a polytetrafluoroethylene mold (specification: 4 cm in length × 3 cm in width × 1 cm in thickness), vacuum-dried to remove the bubbles in the solution, and the mold was horizontally placed at room temperature for 2 h to complete the pre-crosslinking reaction of the CLCE film. 2 mL of n-hexane was poured into the mold, and the film was taken out with a spatula. Finally, a UV LED lamp was used to irradiate for 1 minute at a light power density of 10 mW / cm² for curing to carry out the secondary crosslinking reaction to obtain a single-layer pure CLCE film. The preparation was completed by directly laminating the TPU with the prepared CLCE film. The specific steps of not performing air plasma treatment on the TPU were the preparation process in Example 1 above, excluding the step of treating the commercially available PVA film with an air plasma treatment machine for 5 minutes. The specific steps of treating the TPU film with air plasma for 5 minutes were the same as those in Example 1 above.

[0042] Using the water transfer printing technique, these three groups of samples were prepared into composite films, and the composite area was set to 4×4 mm, and the composite method was as Figure 7 shown in a. A tensile machine was used to test the adhesion between the films, and the test results were as Figure 7As shown in Figure b. The experimental results show that directly compounding CLCE with the TPU film does not exhibit an obvious adhesion effect, so it is not included in the figure. The adhesion force of the sample prepared by water transfer printing without air plasma treatment is 0.43 N, while that of the sample prepared by water transfer printing with air plasma treatment is 0.7 N. Directly preparing the CLCE / TPU composite by the anisotropic deswelling method in a tetrafluoroethylene bath fails to show an adhesion effect, indicating that the adhesion force between CLCE and TPU is relatively weak without surface treatment. The water transfer printing technique can significantly improve the adhesion force between CLCE and TPU, and the adhesion force between CLCE and TPU can be further enhanced by air plasma treatment of the TPU surface. This is because air plasma treatment enhances the surface roughness of TPU or activates the surface, removes surface contaminants and oxides while increasing the number of surface active groups, thereby improving the adhesion performance of TPU.

[0043] To study the force-induced color change performance of the composite film prepared by the water transfer printing method, the reflection spectra of the red photonic crystal composite film in Example 1 at different stretching rates are as Figure 8 shown. When not stretched initially, the center of the reflection wavelength of the sample is 606 nm. As the stretching rate gradually increases to 100% at intervals of 20%, the center of the reflection wavelength of the sample becomes 561 nm, 532 nm, 514 nm, 485 nm, and 472 nm in sequence, showing a continuous blue shift of the reflection wavelength.

[0044] Example 5 Exploration of Process Parameters for Preparing TPU / CLCE Composite Film by Water Transfer Printing Method 1. Exploration of the Best Water-Soluble Film Substrate To explore the influence of water-soluble film substrates on the water transfer printing process, the present invention not only uses commercially available PVA films, but also additionally prepares PVP films and PSS films, and processes them using the same water transfer printing method (the method of Example 1). Among them, the preparation method of the PVP film is as follows: Weigh 1 g of polyvinylpyrrolidone (PVP) and place it in a 5 mL sample bottle. Add 2 mL of deionized water and stir vigorously to dissolve it to complete the preparation of the PVP solution. Cut out a glass plate (specification: 5 cm in length × 5 cm in width × 0.5 cm in thickness). After washing the surface, place it on the surface of the spin coater tray after air plasma treatment for 5 min. Use a pipette to slowly drop about 0.5 ml of the PVP solution onto the center of the glass plate. First, spread the solution at a low speed of 100 rpm for 10 s, and then spin coat it at a high speed of 1000 rpm for 50 s. After the spin coating is completed, quickly transfer it to a hot stage at 90 °C and heat it to quickly evaporate the water to obtain the PVP film. The preparation method of the PSS film is the same as that of the PVP film, except that the above spin coating solution is replaced with an aqueous solution containing 30 wt% sodium polystyrene sulfonate (PSS), and the rest of the process is the same as above. The results are as Figure 9 shown. It can be clearly observed that at the same placement time (4 h), different films have a significant impact on the self-assembly effect of the CLCE layer during the water transfer printing process. Among them, when the PVA film is used as the substrate, the self-assembly effect is the most ideal, and a distinct red color can be clearly observed.

[0045] 2. Plasma treatment time To explore the influence of plasma treatment time on the water transfer printing process, the present invention set up three groups of different treatment times: untreated, treated for 2.5 min, and treated for 5 min for experiments. Other conditions are the same as in Example 1. The results are as Figure 10 shown. The experimental results show that when not treated with air plasma, obvious shrinkage holes appear in some areas of the coating, the self-assembly effect is poor, and the display of structural color is also relatively weak. Under the condition of a treatment time of 2.5 min, although the shrinkage hole phenomenon is alleviated, the structural color of the CLCE layer is still not obvious. In contrast, when the treatment time is 5 min, the best coating effect and self-assembly performance are shown. As the treatment time increases, the modification effect of the plasma on the coating surface gradually enhances, providing a more ideal interface condition for the self-assembly of the CLCE layer, thus significantly improving the coating effect.

[0046] 3. Water temperature To explore the influence of water temperature on the water transfer printing process, the present invention set up three water temperature conditions: 20 °C, 40 °C, and 60 °C for experiments. Other conditions are the same as in Example 1. The results are as Figure 11As shown. The experimental results show that when the water temperature is 60 °C, due to the too high temperature, the PVA film dissolves violently, resulting in the film being twisted and entangled, which is not conducive to the continuation of the water transfer printing process. Under the condition of 40 °C, the dissolution effect of the PVA film is relatively ideal. The moderate temperature makes the dissolution rate of PVA relatively fast, and at the same time does not affect the optical properties of the CLCE film. The next transfer can be carried out after 10 min, effectively shortening the soaking time of water transfer printing. In contrast, the dissolution rate of the PVA film is slower at 20 °C, and the next transfer can only be carried out after 30 min. In addition, if the soaking time is too short, the residual amount of PVA is relatively large, which will cause a significant increase in the overall Young's modulus of the CLCE film and a sharp decrease in the tensile fracture rate, which is extremely unfavorable for the production of the force-induced color change layer.

[0047] 4. UV curing time Due to the high flexibility of the CLCE film, it is easily affected by the water flow during the dissolution of PVA during the water transfer printing process, resulting in the spreading of the film, and then causing a blue shift in the reflected color of the film, as Figure 11 shown. In order to effectively solve this problem, the present invention adopts different UV curing times at a water temperature of 40 °C. According to the steps of Example 1, the UV curing time is set to 30 s or 1 min respectively, and the experimental results are as Figure 12 shown. It can be found that when cured for 1 min, there is no obvious spreading phenomenon on the CLCE film, and its optical properties are well maintained. When cured for 30 s, the CLCE film shows uneven color due to spreading.

Claims

1. A preparation method of a force-chromic photonic crystal composite film, characterized in that, It includes the following steps: (1) Dissolve the liquid crystal monomer 4-(3-acryloyloxypropoxy)benzoic acid 2-methyl-1,4-phenylester, the chiral agent (3R,3aR,6S,6aR)-hexahydrofuro[3,2-b]furan-3,6-diyl bis(4-((acryloyloxy)butoxy)carbonyl)oxy)benzoate), the crosslinking agent pentaerythritol tetra-3-mercaptopropionate and the chain extender 3,6-dioxa-1,8-octanedithiol in an organic solvent. After fully mixing evenly, a mixed reaction solution is obtained; (2) Add the photoinitiator 651 to the mixed reaction solution described in step (1), mix evenly, add the catalyst dipropylamine, and obtain a pigment solution after ultrasonic treatment; (3) Coat the pigment solution in (2) on a polyvinyl alcohol film treated by air plasma, let it stand still to complete the self-assembly of the CLCE layer, and obtain a PVA / CLCE composite film after light curing; (4) Immerse the PVA / CLCE composite film in water. After the PVA layer is fully dissolved, place a black TPU film in the area where the CLCE floats. After the TPU film and the CLCE film are fully compounded, take it out and perform a drying treatment to finally obtain a photonic crystal composite film with mechanochromic properties.

2. The preparation method of the force-responsive color-changing photonic crystal composite film according to claim 1, wherein, The molar ratio of the liquid crystal monomer, chiral agent, crosslinking agent and chain extender described in step (1) is 39:1 to 1.25:2.7:30.

8.

3. The preparation method of the force-induced color-changing photonic crystal composite film according to claim 1, characterized in that, The organic solvent described in step (1) is acetone, ethyl acetate, dichloromethane or toluene.

4. The preparation method of the force-responsive color-changing photonic crystal composite film according to claim 1, characterized in that, In step (2), ultrasonic treatment is performed using an ultrasonic crusher before mixing; the ultrasonic power of the treatment is 100 to 1000 W.

5. The preparation method of the force-responsive color-changing photonic crystal composite film according to claim 1, characterized in that, The mass fraction of the photoinitiator 651 described in step (2) is 4 to 6 wt.%, and the mass fraction of the catalyst dipropylamine is 0.5 to 3%.

6. The preparation method of the force-responsive color-changing photonic crystal composite film according to claim 1, wherein The thickness of the PVA film described in step (3) is 23 to 26 μm.

7. The preparation method of the force-responsive color-changing photonic crystal composite film according to claim 1, characterized in that, The time of the air plasma treatment described in step (3) is 1 to 6 min.

8. The method for preparing the force-responsive color-changing photonic crystal composite film according to claim 1, wherein, The optical power density of the ultraviolet LED lamp described in step (3) is about 5~200 mW cm -2 , and the irradiation time is 30 s~1 min.

9. A mechanochromic photonic crystal composite film prepared by the preparation method according to any one of claims 1 to 8.

10. Application of the mechanochromic photonic crystal composite film according to claim 9 in the preparation of a color-changing car wrap.

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