A structural color multi-responsive hydrogel and its preparation method and application

Abstract

The present invention discloses a method for preparing a structural color multi-responsive hydrogel, which first self-assembles monodisperse polymer microspheres into a three-dimensional photonic crystal, then fills hydrophilic monomers acrylamide and carboxymethyl chitosan into the gaps between the internal spheres of the photonic crystal, initiates polymerization through light, and controls the light intensity of different areas through mask technology to prepare a structural color composite hydrogel with specific graphic information. The present invention also discloses a structural color multi-responsive hydrogel prepared by the above method and the application of the structural color multi-responsive hydrogel in anti-counterfeiting encryption. The structural color hydrogel in the present invention has multiple effects such as solvent response, gas response, mechanical force response, color angle dependence, etc. It is difficult to imitate and has good application prospects in the anti-counterfeiting of tobacco and alcohol products, the anti-counterfeiting of high-end luxury goods, and the anti-counterfeiting of electronic technology products.

Description

Technical Field

[0001] The present invention belongs to the technical field of optical anti-counterfeiting materials, and specifically relates to a structural color multi-responsive hydrogel and a preparation method and application thereof. Background Art

[0002] Responsive photonic crystal materials are a type of optical functional material that is a combination of photonic crystals and stimulus-responsive smart materials.

[0003] At present, most responsive photonic crystal materials have a single response mode and the technology is relatively mature. The existing responsive photonic crystal materials can be divided into: ion response type, temperature response type, liquid / gas response type, force response type and light response type according to the response characteristics. Their technical content is low, the structural combination is easy to decipher, and the anti-counterfeiting effect is poor, which greatly limits the application and development of responsive photonic crystal materials in anti-counterfeiting functions.

[0004] Therefore, in the field of optical anti-counterfeiting, the preparation of anti-counterfeiting labels with multiple responses will be one of the future trends. It can effectively improve the complexity of anti-counterfeiting, greatly increase the difficulty of counterfeiting and cracking, and break through the bottleneck of low applicability, low added value and low performance of traditional anti-counterfeiting materials, opening up a new way for anti-counterfeiting technology. For anti-counterfeiting materials, multiple responses are achieved to increase the cost of cracking counterfeiting. Summary of the invention

[0005] The purpose of the present invention is to provide a method for preparing a structural color multi-responsive hydrogel. The method fills hydrophilic monomers acrylamide and carboxymethyl chitosan into the gaps between balls inside a photonic crystal, and controls the light intensity of different areas through photoinitiated polymerization and mask technology to prepare a structural color composite hydrogel with specific graphic information. Multiple responses can be achieved on anti-counterfeiting materials, thereby cracking the high cost of counterfeiting.

[0006] The present invention also aims to provide a structural color multi-responsive hydrogel prepared by the above method, wherein the structural color multi-responsive hydrogel has multi-responsive properties such as solvent response, angle response, gas response and mechanical force response.

[0007] The last object of the present invention is to provide the application of the above-mentioned structural color multi-responsive hydrogel in encryption and anti-counterfeiting.

[0008] The first object of the present invention can be achieved by the following technical solution: A method for preparing a structural color multi-responsive hydrogel, comprising the following steps:

[0009] (S1) dispersing polymer monodisperse nanospheres in water to obtain a polymer monodisperse nanosphere dispersion liquid, vertically inserting a hydrophilic glass sheet into the polymer monodisperse nanosphere dispersion liquid, evaporating water in a constant temperature and humidity environment, and self-assembling the polymer monodisperse nanospheres on the surface of the hydrophilic glass sheet by a vertical sedimentation method to obtain a polymer photonic crystal;

[0010] (S2) dissolving carboxymethyl chitosan in water and dispersing it ultrasonically, and when it becomes transparent and uniform, adding acrylamide and a cross-linking agent in sequence, and dispersing them ultrasonically until uniform, to obtain a multi-responsive polymer monomer solution, adding a photoinitiator aqueous solution to the multi-responsive polymer monomer solution, and mixing them to obtain a polymer precursor mixed solution;

[0011] (S3) adding the polymer precursor mixed solution in step (S2) to the polymer photonic crystal in step (S1), and keeping it still in a dark place until the polymer precursor mixed solution completely fills the gaps between the balls inside the photonic crystal;

[0012] (S4) placing a photomask with a pattern on the photonic crystal filled with the polymer precursor mixed solution obtained in step (S3), and irradiating it under ultraviolet light to initiate a polymerization reaction;

[0013] (S5) After the polymerization reaction in step (S4) is completed, the polymer photonic crystal is embedded in the polymer precursor mixed solution, removed from the hydrophilic glass, and cleaned to obtain a structural color multi-responsive hydrogel.

[0014] In the preparation method of the structural color multi-responsive hydrogel:

[0015] Preferably, the polymer monodisperse nanospheres in step (S1) are polystyrene monodisperse nanospheres or polymethyl methacrylate monodisperse nanospheres, and the particle size thereof is 160 to 350 nm.

[0016] The polymer monodisperse nanospheres in the present application are not removed and are always embedded in the hydrogel, e.g. Figure 1 As shown in the electron microscope image.

[0017] Preferably, the mass percentage of the polymer monodisperse nanospheres in the polymer monodisperse nanosphere dispersion in step (S1) is 0.15-0.25wt%.

[0018] Preferably, the constant temperature and humidity environment in step (S1) has a temperature of 50-65° C. and a relative humidity of 55-70%.

[0019] Preferably, the cross-linking agent in step (S2) is N-N'-methylenebisacrylamide, divinylbenzene, diisocyanate or 3,3'-dithiobis(sulfosuccinimidyl propionate).

[0020] Preferably, in step (S2), the mass ratios of methyl chitosan, acrylamide, cross-linking agent and water are 0.1-0.5:1.5-2.5, 0.03-0.07:3.5-5.

[0021] In step (S2) of the present invention, carboxymethyl chitosan is used as a component, and carboxymethyl chitosan is a key component that causes the hydrogel to swell to different degrees in different solvents, thereby showing different colors.

[0022] Preferably, the cross-linking agent in step (S2) is N-N'-methylenebisacrylamide, divinylbenzene, diisocyanate or 3,3'-dithiobis(sulfosuccinimidyl propionate).

[0023] Preferably, the mass percentage of the photoinitiator in the photoinitiator aqueous solution in step (S2) is 10-40%, and the amount added is 1%-6% of the total mass of the multi-responsive polymer monomer solution.

[0024] Preferably, the photoinitiator in step (S2) is azobisisobutylamidine hydrochloride, azobisisobutylimidazoline hydrochloride, azobiscyanovaleric acid, azobisisopropylimidazoline or azobis(2-methylpropylimidazoline) dihydrochloride.

[0025] Preferably, in step (S4), a photomask with graphics is arranged above the photonic crystal filled with the polymer precursor mixed solution obtained in step (S3) to make the illumination levels of different areas different, wherein the photomask is a transparent polymer film or a quartz glass sheet, and the transparent polymer film is a polypropylene, polyethylene or polyethylene terephthalate transparent sheet, and the graphic information is set on the photomask by inkjet printing or an oil-based marker.

[0026] Preferably, in step (S4), the polymerization reaction is initiated by irradiating the sample with 32-40W ultraviolet light for 100-300 minutes.

[0027] Preferably, in step (S5), the thickness of the structural color multi-responsive hydrogel is 1 to 4 mm.

[0028] Preferably, the water used in steps (S1) to (S2) is ultrapure water, and ultrapure water is used for immersion and rinsing during cleaning and impurity removal in step (S5) to remove residual reactants and by-products.

[0029] The above second object of the present invention can be achieved by the following technical solution: a structural color multi-responsive hydrogel is prepared by any of the methods described above.

[0030] The last object of the present invention can be achieved by the following technical solution: application of the above-mentioned structural color multi-responsive hydrogel in encryption and anti-counterfeiting.

[0031] Furthermore, the method for preparing the structural color multi-responsive hydrogel provided by the present invention comprises the following steps:

[0032] (I) A method for preparing a polymer three-dimensional photonic crystal, comprising the following steps:

[0033] Three-dimensional photonic crystals are obtained by self-assembly using the vertical sedimentation method:

[0034] After being cleaned, the commercially available glass slide is immersed in concentrated sulfuric acid / hydrogen peroxide solution = 7:3 V / V for 12 to 24 hours for hydrophilic treatment.

[0035] The monodisperse polymer microsphere emulsion (microsphere particle size is 160-350 nm) is diluted with water to a mass fraction of 0.15-0.25 wt %, and then injected into a vial.

[0036] The hydrophilized glass slide is then vertically inserted into the diluted polymer microsphere emulsion and fixed, and placed in a constant temperature and humidity chamber, maintaining a relative humidity of 55-70% and a temperature of 50-65°C for 3-5 days. After the water evaporates, the polymer microspheres are assembled on the glass slide to obtain a three-dimensional photonic crystal.

[0037] (ii) A method for preparing a structural color multi-responsive hydrogel, comprising the following steps:

[0038] Place 0.1-0.5 g of carboxymethyl chitosan in a centrifuge tube, add 3.5-5 mL of ultrapure water and disperse by ultrasonication. When it becomes transparent and uniform, add 1.5-2.5 g of acrylamide and 0.03-0.07 g of a photocrosslinking agent (such as N-N'-methylenebisacrylamide, divinylbenzene, diisocyanate, 3,3'-dithiobis(sulfosuccinimidyl propionate), etc.) in sequence and disperse by ultrasonication until it is uniform.

[0039] Subsequently, 0.1 to 0.2 g of a water-soluble photoinitiator (such as azobisisobutylamidine hydrochloride, azobisisobutylimidazoline hydrochloride, azobiscyanovaleric acid, azobisisopropylimidazoline, azobis(2-methylpropylimidazole) dihydrochloride, etc.) is dissolved in 0.5 to 1.0 mL of ultrapure water, and the dissolved initiator solution is added to the above mixed solution, and the mixture is mixed evenly to obtain a polymer precursor.

[0040] Place the three-dimensional photonic crystal in a polytetrafluoroethylene mold, then pour the polymer precursor into the mold, and let it stand for 2 to 5 minutes away from light until the mixed solution completely fills the gaps between the balls inside the photonic crystal.

[0041] Place the photomask with the pattern on top of the mold so that the graphic information area is partially shielded from light and the background area is exposed. Put it into a 36W ultraviolet lamp for photopolymerization for 100 to 300 minutes. The photonic crystal assembled from polymer microspheres has been embedded in the hydrogel. At this time, the obtained structural color hydrogel can be peeled off the hydrophilic glass sheet and soaked and rinsed with a large amount of clean water (ultrapure water) to remove residual reactants and by-products, and finally obtain a multi-responsive structural color hydrogel.

[0042] The thickness of the obtained hydrogel is determined by the liquid level of the polymer precursor injected into the mold. The thickness of the hydrogel finally obtained is preferably 1 to 4 mm.

[0043] The principle of the present invention is:

[0044] (1) During the photoinitiated polymerization process, the portion with higher light intensity has higher polymerization degree and cross-linking degree, and has lower swelling degree in solvent (such as water, ethanol, etc.); while the portion with weaker light intensity has lower polymerization degree and cross-linking degree, and has higher swelling degree in solvent. Because the swelling degree of different regions is different, the period of photonic crystal embedded in the hydrogel is also different, and the wavelength of its diffracted light is also different, which ultimately forms a color difference between the background part and the pattern part, so different graphic information is displayed.

[0045] (2) Since the swelling degree of hydrogels in different solvents is also different, the colors of the structural color multi-responsive hydrogel pattern and the background are also different in different solvents.

[0046] (3) During the stretching or squeezing process, due to the deformation of the structural color multi-responsive hydrogel, the period of the microspheres inside it will also change accordingly, causing the overall color of the hydrogel to change.

[0047] (4) According to the Bragg diffraction law, the wavelength of the diffracted light of the photonic crystal material is related to the angle of the incident light and the angle of the outgoing light. Therefore, when the observation angle changes, the structural color multi-responsive hydrogel sample will also show different colors.

[0048] Therefore, in order to overcome the defects and technical deficiencies of the existing single-responsive photonic crystal material technology, the method of the present invention provides a structural color multi-responsive hydrogel. The more responsive characteristics a structural color hydrogel has, the more information can be obtained. By comparing and referencing different signals, the reliability of its response signal can be enhanced, and the possibility of being interfered by other factors can be reduced, thereby improving its anti-counterfeiting level. The preparation method of the structural color multi-responsive hydrogel provided by the present invention obtains a structural color hydrogel with solvent response, angle response, gas response and mechanical force response.

[0049] The resultant multi-color responsive hydrogel in this application has the following characteristics:

[0050] ① Concealment: In the dry state, due to the small difference in refractive index between the dry gel matrix and the polymer microspheres, the diffraction effect is small, and the structural color pattern is difficult to observe with the naked eye; after being soaked in the solvent and the gel swells, the refractive index difference between the swollen polymer and the polymer microspheres becomes larger, the diffraction effect becomes stronger, and a structural color pattern visible to the naked eye is presented.

[0051] ②Solvent responsiveness: Since the swelling degree of gel polymer in different solvents is different, the period of microspheres in the gel after swelling is different, the wavelength of diffracted light is different, and the pattern presents different colors. Due to the solvent responsiveness of structural color hydrogel, it is more difficult to forge, and it also gives it the ability to indicate different solutions (such as different alcohols, different alcoholic beverages, etc.).

[0052] ③ Mechanochromic ability: Stretching or squeezing the hydrogel to cause it to deform to varying degrees can change the wavelength of the diffracted light it presents, change the color of its pattern, enhance the anti-counterfeiting effect, and at the same time give it the ability of mechanical sensing.

[0053] ④ Color angle dependence: According to Bragg's diffraction law, the wavelength of the diffracted light of the structural color hydrogel is related to the angle of the incident light and the angle of the outgoing light. Therefore, as the observation angle changes, the structural color multi-responsive hydrogel sample will also present different colors, increasing its anti-counterfeiting effect.

[0054] Compared with the prior art, the present invention has the following advantages:

[0055] (1) The present invention first self-assembles monodisperse polymer microspheres into a three-dimensional photonic crystal, then fills the hydrophilic monomers acrylamide and carboxymethyl chitosan into the gaps between the microspheres inside the photonic crystal, initiates polymerization through light, and controls the light intensity of different areas through mask technology to prepare a structural color composite hydrogel with specific graphic information;

[0056] (2) The present invention uses mask technology to make the polymers in different regions of the hydrogel have different cross-linking degrees, so that they obtain different swelling properties under solvent infiltration, and finally display different colors. In addition, since the polymers swell to different degrees in different solvents, they obtain solvent responsiveness, that is, the colors of graphic information are different under different solvent infiltration. In addition, during the stretching deformation process, the lattice constant of the photonic crystal inside the hydrogel changes, which also causes the color of the anti-counterfeiting pattern to redshift / blueshift, making it have a mechanochromic effect.

[0057] (3) Therefore, the structural color hydrogel in the present invention has multiple effects such as solvent response, gas response, mechanical force response, and color angle dependence, which is difficult to imitate. It has good application prospects in the anti-counterfeiting of tobacco and alcohol products, the anti-counterfeiting of high-end luxury goods, and the anti-counterfeiting of electronic technology products. BRIEF DESCRIPTION OF THE DRAWINGS

[0058] Figure 1 is a scanning electron microscope image of the structural color multi-responsive hydrogel in Example 1;

[0059] Figure 2 The structural color multi-responsive hydrogel in Example 1 is in a dry state;

[0060] Figure 3 These are photos of the structural color multi-responsive hydrogel in Example 1 in water immersion and ethanol immersion states, the upper picture: the response effect of the hydrogel in water, the lower picture: the response effect of the hydrogel in ethanol;

[0061] Figure 4 These are photos of the structural color multi-responsive hydrogel in Example 1 at different observation angles, the upper picture: the response effect of the hydrogel at 90°, the lower picture: the response effect of the hydrogel at 30°;

[0062] Figure 5 is the color change of the structural color multi-responsive hydrogel during stretching in Example 1;

[0063] Figure 6 This is a graph showing the changes in the dry structural color multi-responsive hydrogel in Example 1 when it is fumigated in water vapor. DETAILED DESCRIPTION

[0064] The raw materials used in the following examples are all commercially available products or prepared according to conventional methods or references unless otherwise specified.

[0065] Example 1

[0066] The method for preparing the structural color multi-responsive hydrogel provided in this embodiment comprises the following steps:

[0067] (I) Preparation of polystyrene three-dimensional photonic crystals:

[0068] (S1) Three-dimensional photonic crystals are self-assembled using the vertical sedimentation method:

[0069] After being cleaned, the commercially available glass slide was immersed in concentrated sulfuric acid / hydrogen peroxide solution = 7:3 V / V for 24 hours for hydrophilic treatment.

[0070] The monodisperse polystyrene microsphere emulsion (microsphere particle size of 180 nm) was diluted with water to a mass fraction of 0.2 wt %, and then injected into a vial.

[0071] The hydrophilized glass slide was then vertically inserted into the diluted polystyrene microsphere emulsion and fixed, and placed in a constant temperature and humidity chamber, maintaining a relative humidity of 55% and a temperature of 55°C for 5 days. After the water evaporated, the microspheres were assembled on the glass slide to obtain a three-dimensional photonic crystal (also called a polymer photonic crystal).

[0072] (II) Preparation of structural color multi-responsive hydrogels:

[0073] (S2) 0.12 g of carboxymethyl chitosan was placed in a centrifuge tube, and 4 mL of ultrapure water was added for ultrasonic dispersion. When the chitosan was transparent and uniform, 1.6 g of acrylamide and 0.04 g of a photocrosslinker N-N'-methylenebisacrylamide were added in sequence, and ultrasonic dispersion was performed until the chitosan was uniform.

[0074] Subsequently, 0.12 g of a water-soluble photoinitiator azobisisobutylamidine hydrochloride was dissolved in 0.5 mL of ultrapure water, and the dissolved initiator solution was added to the above mixed solution, and mixed evenly to obtain a polymer precursor (ie, a polymer precursor mixed solution).

[0075] (S3) placing the three-dimensional photonic crystal in a polytetrafluoroethylene mold, and then pouring the polymer precursor into the mold, leaving it to stand in the dark for 5 minutes until the mixed solution completely fills the gaps between the balls inside the photonic crystal.

[0076] (S4) placing a photomask with a pattern on top of the mold, partially shielding the image information area and exposing the background area, and placing it in a 36W ultraviolet lamp for 240 minutes of light polymerization to carry out a polymerization reaction; (The mask plate is a transparent PET plastic sheet with a thickness of 0.2 mm, and different patterns are drawn on the PET sheet with a commercially available oil pen. In this embodiment, three different mask plates are used: ① Arabic numeral "2" and a hollow "√" in a solid circle (corresponding to Figure 2 , 3 );②The letters "Z", "K" and diamond (corresponding to Figure 4 );③Blank mask (corresponding to Figure 5 );④Arabic numeral "2" (corresponding to Figure 6 ))

[0077] (S5) After the polymerization reaction, the photonic crystals assembled from the polymer microspheres have been embedded in the hydrogel. At this time, the obtained structural color hydrogel can be peeled off from the hydrophilic glass sheet and soaked and rinsed with a large amount of clean water to remove the residual reactants and by-products, and finally a multi-responsive structural color hydrogel can be obtained.

[0078] The thickness of the obtained hydrogel is determined by the liquid level of the polymer precursor injected into the mold, and the thickness of the final hydrogel is 2 mm.

[0079] The scanning electron microscope image of the structural color multi-responsive hydrogel in this embodiment is as follows: Figure 1 As shown, from Figure 1 It can be seen that the polystyrene microspheres with a particle size of 170 nm have been embedded in the polymer matrix.

[0080] In this embodiment, the structural color multi-responsive hydrogel is in a dry state. Figure 2 As shown, from Figure 2It can be seen that at this time the structural color hydrogel is translucent and slightly yellow, the pattern information is hidden and cannot be recognized by the naked eye.

[0081] Therefore, the structural multi-corresponding hydrogel in the present invention has concealment. In the dry state, due to the small difference in refractive index between the dry gel matrix and the polymer microspheres, the diffraction effect is small, and the structural color pattern is difficult to observe with the naked eye; after being soaked in the solvent and the gel swells, the refractive index difference between the swollen polymer and the polymer microspheres becomes larger, the diffraction effect becomes stronger, and a structural color pattern visible to the naked eye is presented.

[0082] The photos of the structural color multi-responsive hydrogel in this example in water and ethanol infiltration states are shown in the figure below. Figure 3 As shown, the upper figure shows the response effect of the hydrogel in water, and the lower figure shows the response effect of the hydrogel in ethanol. Figure 3 It can be seen that in water, the pattern part appears orange-red, while the background part appears green, with a sharp contrast, and the graphic information can be read; in ethanol, the pattern part appears green, while the background part appears blue, with a sharp contrast, and the graphic information can be read. In different solvents, the color of the displayed pattern is obviously different.

[0083] Therefore, the structural color multi-response hydrogel in the present invention has solvent responsiveness. Since the swelling degree of the gel polymer in different solvents is different, the period of the microspheres in the gel after swelling is different, the wavelength of the diffracted light is different, and the color of the pattern is different. Due to the solvent responsiveness of the structural color hydrogel, the difficulty of counterfeiting is increased, and it also gives it the ability to indicate different solutions (such as different alcohols, different alcoholic beverages, etc.).

[0084] The photos of the structural color multi-responsive hydrogel at different observation angles are shown in the figure below. Figure 4 As shown, the upper figure: the response effect of the hydrogel at 90°, the lower figure: the response effect of the hydrogel at 30°, Figure 4 It can be seen that when observed vertically at 90°, the pattern part appears green and the background part appears orange-red, while when observed at 30°, the pattern part appears blue and the background part appears green. The display color of the multi-responsive structural color hydrogel changes significantly when observed from different angles.

[0085] Therefore, the structural color multi-responsive hydrogel in the present invention has color angle dependence. According to the Bragg diffraction law, the wavelength of the diffracted light of the structural color hydrogel is related to the incident light angle and the outgoing light angle. Therefore, when the observation angle is changed, the structural color multi-responsive hydrogel sample will also show different colors, increasing its anti-counterfeiting effect.

[0086] The color changes of the structural color multi-responsive hydrogel in this embodiment at different stretching degrees are as follows: Figure 5 As shown in Figure 5It can be seen that as the hydrogel is gradually stretched, its color gradually shifts to blue, from orange to green, and finally to blue.

[0087] Therefore, the structural color multi-responsive hydrogel in the present invention has mechanochromic ability. Stretching or squeezing the hydrogel to cause it to deform to varying degrees can change the wavelength of the diffracted light it presents, change the color of its pattern, enhance the anti-counterfeiting effect, and at the same time give it the ability of mechanical sensing.

[0088] The dried structural color multi-responsive hydrogel of this embodiment changes after being fumigated with water vapor as follows: Figure 6 As shown in , as the fumigation time increases, the hidden pattern (Arabic numeral "2") in the dried hydrogel sample gradually emerges, the background is light red, and the text pattern is gray-green.

[0089] Therefore, the structural color multi-responsive hydrogel in the present invention has gas responsiveness. When the dried hydrogel is fumigated with water vapor, the sample gradually expands as the hydrogel absorbs water. Due to the difference in the degree of crosslinking between the background area and the pattern area, the degree of water absorption and swelling is also different, resulting in different periods of microspheres in the gel after water absorption and swelling, different wavelengths of diffracted light, and different colors of the pattern. This further enriches the responsiveness of the hydrogel material.

[0090] Example 2

[0091] The method for preparing the structural color multi-responsive hydrogel provided in this embodiment comprises the following steps:

[0092] (I) Preparation of polystyrene three-dimensional photonic crystals:

[0093] (S1) Three-dimensional photonic crystals are self-assembled using the vertical sedimentation method:

[0094] After being cleaned, the commercially available glass slide was immersed in concentrated sulfuric acid / hydrogen peroxide solution = 7:3 V / V for 24 hours for hydrophilic treatment.

[0095] The monodisperse polystyrene microsphere emulsion (microsphere particle size of 350 nm) was diluted with water to a mass fraction of 0.15 wt %, and injected into a vial.

[0096] The hydrophilized glass slide was then vertically inserted into the diluted polystyrene microsphere emulsion and fixed, and placed in a constant temperature and humidity chamber, maintaining a relative humidity of 60% and a temperature of 60°C for 4 days. After the water evaporated, the microspheres were assembled on the glass slide to obtain a three-dimensional photonic crystal.

[0097] (II) Preparation of structural color multi-responsive hydrogels:

[0098] (S2) 0.16 g of carboxymethyl chitosan was placed in a centrifuge tube, and 5 mL of ultrapure water was added for ultrasonic dispersion. When the chitosan was transparent and uniform, 1.5 g of acrylamide and 0.04 g of a photocrosslinker 3,3'-dithiobis(sulfosuccinimidyl propionate) were added in sequence, and ultrasonic dispersion was performed until the chitosan was uniform.

[0099] Subsequently, 0.15 g of azobisisobutylimidazoline hydrochloride, a water-soluble photoinitiator, was dissolved in 1 mL of ultrapure water, and the dissolved initiator solution was added to the above mixed solution, and the mixture was mixed evenly to obtain a polymer precursor.

[0100] (S3) placing the three-dimensional photonic crystal in a polytetrafluoroethylene mold, and then pouring the polymer precursor into the mold, leaving it to stand in the dark for 5 minutes until the mixed solution completely fills the gaps between the balls inside the photonic crystal.

[0101] (S4) placing a photomask with a pattern (for example, the pattern may be the word "Zhongkai", and no drawings are provided here) on top of the mold, so that the image information area is partially shielded from light, while the background area is exposed, and placing it in a 36W ultraviolet lamp for photopolymerization for 180 minutes to perform a polymerization reaction;

[0102] (S5) After the polymerization reaction, the photonic crystals assembled from the polymer microspheres have been embedded in the hydrogel. At this time, the obtained structural color hydrogel can be peeled off from the hydrophilic glass sheet and soaked and rinsed with a large amount of clean water to remove the residual reactants and by-products, and finally a multi-responsive structural color hydrogel can be obtained.

[0103] The thickness of the obtained hydrogel is determined by the liquid level of the polymer precursor injected into the mold, and the thickness of the final hydrogel is 1.5 mm.

[0104] Example 3

[0105] The method for preparing the structural color multi-responsive hydrogel provided in this embodiment comprises the following steps:

[0106] (I) Preparation of polystyrene three-dimensional photonic crystals:

[0107] (S1) Three-dimensional photonic crystals are self-assembled using the vertical sedimentation method:

[0108] After being cleaned, the commercially available glass slide was immersed in concentrated sulfuric acid / hydrogen peroxide solution = 7:3 V / V for 24 hours for hydrophilic treatment.

[0109] The monodisperse polystyrene microsphere emulsion (microsphere particle size of 160 nm) was diluted with water to a mass fraction of 0.25 wt %, and injected into a vial.

[0110] The hydrophilized glass slide was then vertically inserted into the diluted polystyrene microsphere emulsion and fixed, and placed in a constant temperature and humidity chamber, maintaining a relative humidity of 70% and a temperature of 65°C for 3 days. After the water evaporated, the microspheres were assembled on the glass slide to obtain a three-dimensional photonic crystal.

[0111] (II) Preparation of structural color multi-responsive hydrogels:

[0112] (S2) 0.3 g of carboxymethyl chitosan was placed in a centrifuge tube, and 3.5 mL of ultrapure water was added for ultrasonic dispersion. When the chitosan was transparent and uniform, 2 g of acrylamide and 0.04 g of photocrosslinker N-N'-methylenebisacrylamide were added in sequence, and ultrasonic dispersion was performed until the chitosan was uniform. 0.06 g of photocrosslinker diisocyanate was added, and ultrasonic dispersion was performed until the chitosan was uniform.

[0113] Subsequently, 0.1 g of a water-soluble photoinitiator azobis(2-methylpropylimidamide) dihydrochloride was dissolved in 1.0 mL of ultrapure water, and the dissolved initiator solution was added to the above mixed solution, and the mixture was mixed evenly to obtain a polymer precursor.

[0114] (S3) placing the three-dimensional photonic crystal in a polytetrafluoroethylene mold, and then pouring the polymer precursor into the mold, leaving it to stand in the dark for 3 minutes until the mixed solution completely fills the gaps between the balls inside the photonic crystal.

[0115] (S4) placing a photomask with a pattern (for example, the pattern may be the word "Packaging", and no drawings are provided here) on top of the mold, so that the graphic information area is partially shielded from light and the background area is exposed, and placing it in a 36W ultraviolet lamp for photopolymerization for 140 minutes to perform a polymerization reaction;

[0116] (S5) After the polymerization reaction, the photonic crystals assembled from the polymer microspheres have been embedded in the hydrogel. At this time, the obtained structural color hydrogel can be peeled off from the hydrophilic glass sheet and soaked and rinsed with a large amount of clean water to remove the residual reactants and by-products, and finally a multi-responsive structural color hydrogel can be obtained.

[0117] The thickness of the obtained hydrogel is determined by the liquid level of the polymer precursor injected into the mold, and the thickness of the final hydrogel is 4 mm.

[0118] Example 4

[0119] The method for preparing the structural color multi-responsive hydrogel provided in this embodiment comprises the following steps:

[0120] (I) Preparation of polymethyl methacrylate three-dimensional photonic crystals:

[0121] (S1) Three-dimensional photonic crystals are self-assembled using the vertical sedimentation method:

[0122] After being cleaned, the commercially available glass slide was immersed in concentrated sulfuric acid / hydrogen peroxide solution = 7:3 V / V for 24 hours for hydrophilic treatment.

[0123] The monodisperse polymethyl methacrylate microsphere emulsion (microsphere particle size is 220 nm) is diluted with water to a mass fraction of 0.25 wt %, and injected into a vial.

[0124] The hydrophilized glass slide was then vertically inserted into the diluted polystyrene microsphere emulsion and fixed, and placed in a constant temperature and humidity chamber, maintaining a relative humidity of 60% and a temperature of 60°C for 4 days. After the water evaporated, the microspheres were assembled on the glass slide to obtain a three-dimensional photonic crystal.

[0125] (II) Preparation of structural color multi-responsive hydrogels:

[0126] (S2) 0.16 g of carboxymethyl chitosan was placed in a centrifuge tube, and 5 mL of ultrapure water was added for ultrasonic dispersion. When the chitosan was transparent and uniform, 1.5 g of acrylamide and 0.04 g of a photocrosslinker 3,3'-dithiobis(sulfosuccinimidyl propionate) were added in sequence, and ultrasonic dispersion was performed until the chitosan was uniform.

[0127] Subsequently, 0.15 g of azobisisobutylimidazoline hydrochloride, a water-soluble photoinitiator, was dissolved in 1 mL of ultrapure water, and the dissolved initiator solution was added to the above mixed solution, and the mixture was mixed evenly to obtain a polymer precursor.

[0128] (S3) placing the three-dimensional photonic crystal in a polytetrafluoroethylene mold, and then pouring the polymer precursor into the mold, leaving it to stand in the dark for 5 minutes until the mixed solution completely fills the gaps between the balls inside the photonic crystal.

[0129] (S4) placing a photomask with images and texts (for example, the images and texts may be "ZK Zhongkai", and no drawings are provided here) on top of the mold, so that the image and text information area is partially shielded from light, while the background area is exposed, and placing it in a 36W ultraviolet lamp for photopolymerization for 160 minutes to perform a polymerization reaction;

[0130] (S5) After the polymerization reaction, the photonic crystals assembled from the polymer microspheres have been embedded in the hydrogel. At this time, the obtained structural color hydrogel can be peeled off from the hydrophilic glass sheet and soaked and rinsed with a large amount of clean water to remove the residual reactants and by-products, and finally a multi-responsive structural color hydrogel can be obtained.

[0131] The thickness of the obtained hydrogel is determined by the liquid level of the polymer precursor injected into the mold, and the thickness of the final hydrogel is 2 mm.

[0132] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, based on the principle of the present invention, optimization, improvement, combination, substitution, etc. of the present invention should be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing a structural color multi-responsive hydrogel, characterized in that The following steps are involved: (S1) dispersing polymer monodisperse nanospheres in water to obtain a polymer monodisperse nanosphere dispersion liquid, vertically inserting a hydrophilic glass sheet into the polymer monodisperse nanosphere dispersion liquid, evaporating water in a constant temperature and humidity environment, and self-assembling the polymer monodisperse nanospheres on the surface of the hydrophilic glass sheet by a vertical sedimentation method to obtain a polymer photonic crystal; (S2) dissolving carboxymethyl chitosan in water and dispersing it ultrasonically, and when it becomes transparent and uniform, adding acrylamide and a cross-linking agent in sequence, and dispersing them ultrasonically until uniform, to obtain a multi-responsive polymer monomer solution, adding a photoinitiator aqueous solution to the multi-responsive polymer monomer solution, and mixing them to obtain a polymer precursor mixed solution; (S3) adding the polymer precursor mixed solution in step (S2) to the polymer photonic crystal in step (S1), and keeping it still in a dark place until the polymer precursor mixed solution completely fills the gaps between the balls inside the photonic crystal; (S4) placing a photomask with a pattern on the photonic crystal filled with the polymer precursor mixed solution obtained in step (S3), and irradiating it under ultraviolet light to initiate a polymerization reaction; (S5) After the polymerization reaction in step (S4) is completed, the polymer photonic crystal is embedded in the polymer precursor mixed solution, removed from the hydrophilic glass, and cleaned to remove impurities, thereby obtaining a structural color multi-responsive hydrogel; The polymer monodisperse nanospheres in step (S1) are polystyrene monodisperse nanospheres or polymethyl methacrylate monodisperse nanospheres, and the particle size thereof is 160 to 350 nm; In step (S2), the mass ratios of carboxymethyl chitosan, acrylamide, crosslinking agent and water are 0.1-0.5:1.5-2.5 and 0.03-0.07:3.5-5.

2. The method for preparing the structural color multi-responsive hydrogel according to claim 1, characterized in that: The mass percentage of polymer monodisperse nanospheres in the polymer monodisperse nanosphere dispersion in step (S1) is 0.15-0.25wt%; the constant temperature and humidity environment in step (S1) is 50-65°C and 55-70% relative humidity.

3. The method for preparing the structural color multi-responsive hydrogel according to claim 1, characterized in that: The cross-linking agent in step (S2) is N-N'-methylenebisacrylamide, divinylbenzene, diisocyanate or 3,3'-dithiobis(sulfosuccinimidyl propionate).

4. The method for preparing the structural color multi-responsive hydrogel according to claim 1, characterized in that: The mass percentage of the photoinitiator in the photoinitiator aqueous solution in step (S2) is 10-40%, and the amount added is 1%-6% of the total mass of the multi-responsive polymer monomer solution.

5. The method for preparing the structural color multi-responsive hydrogel according to claim 1, characterized in that: The photoinitiator in step (S2) is azobisisobutylamidine hydrochloride, azobisisobutylimidazoline hydrochloride, azobiscyanovaleric acid or azobisisopropylimidazoline.

6. The method for preparing the structural color multi-responsive hydrogel according to claim 1, characterized in that: In step (S4), a photomask with images and texts is arranged above the photonic crystal filled with the polymer precursor mixed solution obtained in step (S3), so that the illumination levels of different areas are different, wherein the photomask is a transparent polymer film or a quartz glass sheet, and the transparent polymer film is a transparent sheet of polypropylene, polyethylene or polyethylene terephthalate, and the image and text information is set on the photomask by inkjet printing or an oil-based marker pen; in step (S4), the illumination is carried out for 100 to 300 minutes under 32 to 40W ultraviolet light conditions to initiate a polymerization reaction.

7. The method for preparing the structural color multi-responsive hydrogel according to claim 1, characterized in that: The thickness of the structural color multi-responsive hydrogel in step (S5) is 1 to 4 mm.

8. The method for preparing the structural color multi-responsive hydrogel according to claim 1, characterized in that: The water used in steps (S1) to (S2) is ultrapure water, and ultrapure water is used for immersion and rinsing during cleaning and impurity removal in step (S5) to remove residual reactants and by-products.

9. A structural color multi-responsive hydrogel, characterized by: The method is prepared by any one of claims 1 to 8.

10. Application of the structural color multi-responsive hydrogel described in claim 9 in encryption and anti-counterfeiting.

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