Cellulose photonic gel with multiple explicit-hidden optical functions and preparation method thereof

Abstract

The invention belongs to the technical field of optical materials, and discloses cellulose photonic gel with multiple explicit-hidden optical functions and a preparation method of the cellulose photonic gel. The method comprises the following steps: (1) uniformly mixing CNC suspension liquid with polyacrylamide, and constructing a blue structural color film by a self-assembly method; 2) dissolving PVA in water to obtain a PVA solution; uniformly mixing a PVA solution, a photoinitiator and a cross-linking agent to obtain a mixed solution; 3) removing bubbles in the mixed solution, immersing the blue-structured color film into the mixed solution, carrying out photocuring, and removing impurities to obtain cellulose photon gel; and 4) stimulating the cellulose photonic gel by adopting ethanol solutions with different concentrations to realize the multiple explicit-hidden optical functions of the cellulose photonic gel. The method is simple, wide in raw material source, green and environment-friendly. The prepared cellulose photon gel controls the display / hiding of structural color information under multiple stimulation sources, displays / hides different information, and has excellent optical information anti-counterfeiting encryption capability.

Description

Technical Field

[0001] The present invention belongs to the technical field of functional optical materials, and in particular relates to a cellulose photonic gel with multiple visible and invisible optical functions and a preparation method thereof. Background Art

[0002] With the rapid development of society, traditional static optical materials can no longer meet the needs of dynamic information interaction in complex environments. Materials with reversible optical response can achieve controllable hiding and visualization of optical signals through external stimuli (such as humidity, temperature, and electric field), which is of great significance for the construction of anti-counterfeiting labels, environmentally responsive packaging, and rewritable display devices. However, existing material systems generally have defects such as a single response mode, poor cyclic stability, or insufficient environmental compatibility. Moreover, most of them rely on petroleum-based polymers or rare metal materials, which are difficult to meet the needs of green manufacturing. There is an urgent need to develop environmentally friendly intelligent optical materials.

[0003] Cellulose nanocrystals (CNCs), due to their unique self-assembled photonic crystal structure, can produce iridescent structural colors through nanoscale spiral arrangements, providing ideas for the development of environmentally friendly invisible materials. The optical properties of CNC films are responsive to external stimuli such as humidity and stress, but their invisible function is usually limited to a single response mode, and there are problems such as low color contrast and insufficient response sensitivity. Current fields such as smart packaging and multi-level anti-counterfeiting require materials to present differentiated optical signals under different triggering conditions, which poses multiple dynamic control challenges to the structural design of CNCs. How to break through the limitations of the existing single response mechanism through molecular modification, multi-dimensional assembly, and other means to construct a multiple invisible system with resolution has become a key bottleneck restricting the practical application of cellulose-based smart materials. There is an urgent need to carry out research on the coordination of cross-scale structural design and response mechanism. Summary of the Invention

[0004] To overcome the shortcomings of the existing technology, the present invention provides a cellulose photonic gel with multiple optical functions, including visible and invisible colors, and a method for preparing the same. This invention introduces a CNC liquid crystal structure into a polyvinyl alcohol (PVA) hydrogel system, inducing phase separation of the PVA hydrogel using an ethanol medium to promote the visible and invisible color changes of the CNC structure.

[0005] The purpose of the present invention is achieved through the following technical solutions:

[0006] A method for preparing a cellulose photonic gel having multiple optical functions of visibility and concealment comprises the following steps:

[0007] 1) The CNC suspension was mixed evenly with polyacrylamide, and then a blue structural color film was constructed by self-assembly.

[0008] 2) dissolving PVA in water to obtain a PVA solution; and mixing the PVA solution, a photoinitiator, and a cross-linking agent to obtain a mixed solution;

[0009] 3) removing bubbles from the mixed solution in step 2), immersing the blue structural color film in step 1) into the mixed solution, and then photocuring to remove impurities to obtain a cellulose photonic gel;

[0010] 4) Cellulose photonic gel was stimulated and treated with ethanol solutions of different concentrations to obtain cellulose photonic gel with multiple visible and invisible optical functions.

[0011] The self-assembly conditions in step 1) are: temperature of 20-28° C., 50-70% RH.

[0012] The self-assembly time refers to standing still for 2 to 7 days.

[0013] In step 1), the polyacrylamide is used in the form of an aqueous solution with a concentration of 0.3-1 wt%.

[0014] Step 1) The concentration of the CNC suspension, i.e., the cellulose nanocrystal suspension, is 4-5 wt %. The length of the nanocrystalline cellulose in the nanocrystalline cellulose suspension is 200-500 nm.

[0015] The nanocrystalline fiber suspension is also a cellulose nanocrystal suspension, which can be obtained by dispersing cellulose nanocrystals in water, or by using a conventional sulfuric acid hydrolysis method.

[0016] The volume ratio of the CNC suspension to polyacrylamide is (25-40):1-5, preferably (7-12):1.

[0017] The molecular weight of the PVA in step 2) is 89,000-124,000 (number average molecular weight), and the alcoholysis degree is ≥99%; the mass ratio of PVA to water in the PVA solution is 5-15:100, preferably 5-11:100.

[0018] When the PVA is dissolved, DMSO is added to water, and the mass ratio of DMSO to water is (0.5-1.5):100. At this time, the mass ratio of PVA to (water+DMSO) in the PVA solution is 5-15:100, preferably 5-11:100.

[0019] In step 2), the photoinitiator is preferably Irgacure 2959. The mass ratio of the photoinitiator to PVA is 0.5 to 2:100.

[0020] The crosslinking agent is preferably glycidyl methacrylate, and the mass ratio of the crosslinking agent to PVA is 1 to 3:100.

[0021] In step 3), the step of removing bubbles from the mixed solution is to place the mixed solution in a nitrogen environment for bubbling treatment for 10-15 minutes.

[0022] The blue structural color film is immersed in the mixed solution for 3 to 10 minutes.

[0023] The conditions for light curing are ultraviolet light irradiation, and the irradiation time is 5 to 15 minutes.

[0024] The UV lamp has a wavelength of 365nm and a power of 20-30mW / cm 2 .

[0025] The impurity removal refers to removing unreacted monomers and initiator residues; specifically, it refers to removing unreacted monomers and initiator residues using water.

[0026] The impurities are removed by soaking the gel in water for 2 to 4 times, with each soaking time not exceeding 2 minutes. After each soaking, the gel needs to be placed in an environment with a RH of 50-60% for 10-15 minutes.

[0027] Specifically, the first soaking time in the soaking is 1.5 to 2.5 minutes, and the subsequent soaking time is ≤ 1 minute.

[0028] The different ethanol concentrations described in step 4) refer to adjacent concentrations differing by 20-40%, and the different concentrations refer to three different ethanol solutions. The different ethanol concentrations are 25-35% by volume, 55-65% by volume, and 90-100% by volume. The gel is immersed in the above ethanol concentrations for 20-40 minutes.

[0029] The present invention provides a cellulose photonic gel prepared by the above preparation method.

[0030] The cellulose photon gel of the present invention is used in the field of optical information encryption and anti-counterfeiting.

[0031] The cellulose photonic gel of this invention exhibits distinct structural colors when stimulated by different ethanol concentrations. For example, after immersion in a 30% ethanol solution, the sample's transparency increases due to increased water absorption, but no transmission peak is observed in the visible light region. After immersion in a 60% ethanol solution, the sample exhibits a distinct transmission peak in the red region and reflects a red structural color on the surface. After immersion in a 98% ethanol solution, the sample undergoes a phase transition, becoming opaque white. Therefore, the same sample can exhibit distinct optical topography characteristics depending on the ethanol concentration, demonstrating its excellent multiple-layered optical performance.

[0032] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0033] (1) The method of the present invention is simple to operate and the process is controllable;

[0034] (2) The mixed raw materials selected in the present invention are widely available, environmentally friendly, and biodegradable, and thus have certain economical and practical properties for the preparation, utilization, and recycling of photonic gels;

[0035] (3) The cellulose photonic gel obtained by the present invention has the ability of sensitive and reversible response and has the potential for application in practical scenarios;

[0036] (4) The cellulose photonic gel obtained by the present invention can control the display / hiding of structural color information under multiple stimulus sources, and display / hide different information under different stimulus sources. It has controllable and adjustable characteristics and has excellent optical information anti-counterfeiting encryption capabilities. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is the morphology of the sample prepared in Example 1 after being immersed in 30% ethanol;

[0038] Figure 2 This is the morphology of the sample prepared in Example 1 after being immersed in 60% ethanol;

[0039] Figure 3 This is the morphology of the sample prepared in Example 1 after being immersed in 98% ethanol;

[0040] Figure 4 Transmission spectra of the sample prepared in Example 1 after being immersed in 30%, 60% and 98% ethanol;

[0041] Figure 5 This is a fracture morphology diagram of the internal structural color film in the sample prepared in comparative example 1. DETAILED DESCRIPTION

[0042] The specific implementation of the present invention will be further described below in conjunction with specific examples, but the implementation and protection of the present invention are not limited thereto. It should be noted that if there are any processes not particularly described in detail below, they can be implemented or understood by those skilled in the art with reference to the prior art. If the manufacturer of the reagents or instruments used is not indicated, they are deemed to be conventional products that can be purchased commercially.

[0043] Example 1

[0044] A method for preparing a cellulose photonic gel having multiple optical functions of visibility and concealment comprises the following steps:

[0045] 1) A 4 wt% CNC suspension (cellulose nanocrystals with a length of 300-500 nm) was mixed with 0.5 wt% polyacrylamide (solvent: water) in a volume ratio of 35:3 to obtain a CNC / polyacrylic acid mixture. A blue structural color film was constructed under self-assembly conditions at 25°C and 60% relative humidity (standing for 8 days).

[0046] 2) Dissolve 5 g of PVA powder (molecular weight 100,000) in 100 mL of water (containing 1 wt% DMSO) at 88°C and magnetically stir until completely transparent. Terminate the reaction and cool the PVA solution to room temperature. Add 0.5% Irgacure 2959 (based on the weight of the PVA) and 1% glycidyl methacrylate (based on the weight of the PVA) to the solution and stir at 25°C to obtain a mixed solution.

[0047] 3) The mixed solution in step 2) was placed in a nitrogen environment for bubbling treatment for 10 minutes, and then the mixed solution was injected into a mold with a thickness of 2 mm (the thickness of the mixed solution was 2 mm). At the same time, the blue structural color film in step 1) was immersed in the mixed solution for 3 minutes, and then the blue structural color film was immersed in the mixed solution at a wavelength of 365 nm and a power of 20 mW / cm 2 The mixed solution containing the structural color film was irradiated under ultraviolet light for 5 minutes to solidify it;

[0048] 4) Soaking the gel from step 3) in deionized water three times for 2 minutes, 1 minute, and 1 minute, respectively. After each soaking, the gel was placed in an environment with a relative humidity of 50-60% for 10 minutes to remove unreacted monomers and initiator residues and retain the initial morphology. The above samples were stimulated in ethanol solutions with volume concentrations of 30%, 60%, and 98% for 30 minutes, respectively, to construct a cellulose photonic gel (structural color: red) with multiple optical functions.

[0049] Example 2

[0050] A method for preparing a cellulose photonic gel having multiple optical functions of visibility and concealment comprises the following steps:

[0051] 1) A 4 wt% CNC suspension and 0.5 wt% polyacrylamide were mixed in a volume ratio of 35:3 to obtain a CNC / polyacrylic acid mixture. A blue structural color film was constructed under self-assembly conditions at 25°C and 60% relative humidity (standing for 8 days).

[0052] 2) 8 g of PVA powder was dissolved in 100 mL of deionized water (containing 1 wt% DMSO) at 85°C and magnetically stirred until completely transparent. The reaction was terminated and the PVA solution was cooled to room temperature. 1% Irgacure 2959 (based on the weight of the PVA) and 1% glycidyl methacrylate (based on the weight of the PVA) were added to the solution and mixed vigorously at 25°C.

[0053] 3) The mixed solution in step 2) was placed in a nitrogen environment for bubbling treatment for 12 minutes, and then the mixed solution was injected into a mold with a thickness of 2 mm. At the same time, the blue structural color film in step 1) was immersed in the mixed solution for 5 minutes. Then, the blue structural color film was immersed in the mixed solution at a wavelength of 365 nm and a power of 20 mW / cm 2 The mixed solution containing the structural color film was irradiated under ultraviolet light for 8 minutes to solidify it;

[0054] 4) Soaking the gel from step 3) in deionized water three times for 2 minutes, 1 minute, and 1 minute, respectively. After each soaking, the gel was placed in an environment with a relative humidity of 50-60% for 10 minutes to remove unreacted monomers and initiator residues and retain the initial morphology. The above samples were stimulated in ethanol solutions with volume concentrations of 30%, 60%, and 98% for 30 minutes, respectively, to construct a cellulose photonic gel with multiple optical functions of visibility and concealment.

[0055] Example 3

[0056] A method for preparing a cellulose photonic gel having multiple optical functions of visibility and concealment comprises the following steps:

[0057] 1) A 5 wt% CNC suspension and 0.5 wt% polyacrylamide were mixed in a volume ratio of 35:5 to obtain a CNC / polyacrylic acid mixture, and a blue structural color film was constructed under self-assembly conditions at 25°C and 60% relative humidity.

[0058] 2) Dissolve 10 g of PVA powder in 100 mL of deionized water (containing 1 wt% DMSO) at 90°C and magnetically stir until completely transparent. Terminate the reaction and cool the PVA solution to room temperature. Add 1.5% Irgacure 2959 (based on the weight of the PVA) and 2% glycidyl methacrylate (based on the weight of the PVA) to the solution and stir vigorously at 25°C to mix thoroughly.

[0059] 3) The mixed solution in step 2) was placed in a nitrogen environment for bubbling treatment for 15 minutes, and then the mixed solution was injected into a mold with a thickness of 2 mm. At the same time, the blue structural color film in step 1) was immersed in the mixed solution for 7 minutes. Then, the blue structural color film was immersed in the mixed solution at a wavelength of 365 nm and a power of 30 mW / cm2 The mixed solution containing the structural color film was irradiated under ultraviolet light for 10 minutes to solidify it;

[0060] 4) Soaking the gel from step 3) in deionized water three times for 2 minutes, 1 minute, and 1 minute, respectively. After each soaking, the gel was placed in an environment with a relative humidity of 50-60% for 10 minutes to remove unreacted monomers and initiator residues and retain the initial morphology. The above samples were stimulated in ethanol solutions with volume concentrations of 30%, 60%, and 98% for 30 minutes, respectively, to construct a cellulose photonic gel with multiple optical functions of visibility and concealment.

[0061] Comparative Example 1

[0062] Compared with Example 1, the blue structural color film does not contain polyacrylamide molecules, and the remaining steps are the same.

[0063] Comparative Example 2

[0064] Compared with Example 1, the initial color of the CNC structural color film was changed from blue to red (the self-assembly temperature was 35-40° C.), and the remaining steps were the same.

[0065] Figure 1 、 2 3 are the morphologies of the gel prepared in Example 1 after being immersed in 30%, 60% and 98% ethanol, respectively. Figure 4 The transmission spectra of the gel prepared in Example 1 after being immersed in 30%, 60% and 98% ethanol are shown in FIG. Figure 5 This is a fracture morphology diagram of the internal structural color film in the gel prepared in Comparative Example 1. Figure 1-4 As can be seen, after immersing the gel in 30% ethanol, the sample's transparency increased due to increased water absorption, but no transmission peak was observed in the visible light region. After immersion in 60% ethanol, the sample exhibited a distinct transmission peak in the red region and reflected a red structural color on the sample surface. After immersion in 98% ethanol, the sample underwent a phase transition, becoming an opaque white sample. Therefore, the same sample exhibited different optical topography characteristics based on different ethanol concentrations, demonstrating its excellent multi-visibility optical capabilities.

[0066] The performance of the samples prepared in Example 2 and Example 3 is similar to that in Example 1.

[0067] In Comparative Example 1, since the structural fixative polyacrylamide was not added to the structural color film, the structural color film was not well integrated into the gel when preparing the gel sample, resulting in broken structural color areas on the gel surface, which had an adverse effect on the final optical performance display; in Comparative Example 2, the color of the initial structural color film was red. During the process of preparing the gel, the structural color film absorbed water and swelled, resulting in the final gel being unable to display structural color under any conditions. Therefore, the expected photonic gel could not be constructed under this condition.

[0068] The polyacrylamide selected in the present invention is used in the form of an aqueous solution with a concentration of 0.3-1 wt% and a molecular weight of 5-12 million. The polyacrylamide fixes the CNC cholesteric liquid crystal structure and prevents the CNC cholesteric liquid crystal structure from being dispersed and damaged during the subsequent dipping process.

[0069] The nanocrystalline cellulose suspension of the present invention is obtained by treating hardwood pulp with concentrated sulfuric acid. The nanocrystalline cellulose or suspension can also be purchased.

[0070] The nanocrystalline cellulose suspension is specifically obtained by the following method: a) pre-treating hardwood pulp in a weak alkaline environment, and then drying and dehydrating; mixing the pre-treated hardwood pulp with concentrated sulfuric acid, heating under stirring, adding water to terminate the reaction, mixing and standing, centrifuging, collecting the upper suspension to obtain a suspension; b) centrifuging the suspension, taking the upper suspension, and diluting to obtain a nanocrystalline cellulose suspension.

[0071] The pH of the weak alkaline environment for pretreatment of the hardwood pulp in step a) is 8.5-9.5, provided by NaOH solution and KOH solution, and the pretreatment time is 10-14 hours.

[0072] The concentration of the concentrated sulfuric acid solution in step a) is 62-66 wt %, and the mass volume ratio of the pretreated hardwood pulp to the concentrated sulfuric acid is 1 g: (6-8) mL.

[0073] The stirring speed in step a) is 1000-1200 rpm, the heating temperature is 40-55° C., and the heating time is 20-40 min.

[0074] In step a), the volume ratio of concentrated sulfuric acid to water for terminating the reaction is 1:(9-12); and the standing time is 20 to 24 hours.

[0075] The number of centrifugation in step a) is 5 to 6 times, the centrifugal speed is 10,000 to 12,000 rpm, the centrifugal time is 8 to 10 minutes, and the centrifugal temperature is 10 to 15°C.

[0076] The number of centrifugation in step b) is 3 to 4 times, the centrifugal speed is 10,000 to 11,000 rpm, the centrifugal time is 15 to 20 minutes, and the centrifugal temperature is 10 to 15°C.

[0077] In the present invention, after the bubbling treatment, the mixed solution is injected into a mold with a thickness of ≤3 mm. The blue structural color film is immersed in sunlight or natural light for 3-10 minutes and then irradiated with ultraviolet light.

[0078] The structural color of the cellulose photonic gel of the present invention is controlled within the yellow-red range.

[0079] The above embodiments are only preferred implementation modes of the present invention and are only used to explain the present invention rather than to limit the present invention. Any changes, substitutions, modifications, etc. made by those skilled in the art without departing from the spirit of the present invention should fall within the scope of protection of the present invention.

Claims

1. A method for preparing a cellulose photonic gel with multiple optical functions, characterized by: The steps include: 1) mixing a CNC suspension and polyacrylamide uniformly, and then constructing a blue structural color film by self-assembly; the self-assembly conditions in step 1) are: temperature 20-28° C., 50-70% RH; 2) dissolving PVA in water to obtain a PVA solution; and mixing the PVA solution, a photoinitiator, and a cross-linking agent to obtain a mixed solution; 3) removing bubbles from the mixed solution in step 2), immersing the blue structural color film in step 1) in the mixed solution, and then photocuring to remove impurities to obtain a cellulose photonic gel; the blue structural color film is immersed in the mixed solution for 3 to 10 minutes; 4) Cellulose photonic gel was stimulated and treated with ethanol solutions of different concentrations to obtain cellulose photonic gel with multiple visible and invisible optical functions.

2. The method for preparing the cellulose photonic gel with multiple optical functions according to claim 1, characterized in that: The polyacrylamide in step 1) is used in the form of an aqueous solution, and the concentration of the polyacrylamide aqueous solution is 0.3-1 wt%; the concentration of the CNC suspension, i.e., the cellulose nanocrystal suspension, in step 1) is 4-5 wt%; The volume ratio of the CNC suspension to the polyacrylamide aqueous solution is (25-40):1-5.

3. The method for preparing the cellulose photonic gel with multiple optical functions according to claim 2, characterized in that: The volume ratio of the CNC suspension to the polyacrylamide aqueous solution is (7-12):

1.

4. The method for preparing the cellulose photonic gel with multiple optical functions according to claim 1, characterized in that: The molecular weight of the PVA in step 2) is 89,000-124,000, and the degree of alcoholysis is ≥99%; the mass ratio of PVA to water in the PVA solution is 5-15:100; The self-assembly time in step 1) is 2 to 7 days of standing; the length of the nanocrystalline cellulose in the nanocrystalline cellulose suspension is 200 to 500 nm.

5. The method for preparing the cellulose photonic gel with multiple optical functions according to claim 1, characterized in that: When the PVA in step 2) is dissolved, DMSO is added to water, and the mass ratio of DMSO to water is (0.5-1.5):

100. At this time, the mass ratio of PVA to (water+DMSO) in the PVA solution is 5-15:

100.

6. The method for preparing the cellulose photonic gel with multiple optical functions according to claim 1, characterized in that: Step 2) The photoinitiator is Irgacure 2959; The mass ratio of the photoinitiator to PVA is 0.5 to 2:100; The cross-linking agent is glycidyl methacrylate; The mass ratio of the cross-linking agent to PVA is 1 to 3:

100.

7. The method for preparing the cellulose photonic gel with multiple optical functions according to claim 1, characterized in that: The step of removing bubbles from the mixed solution in step 3) is to place the mixed solution in a nitrogen environment for bubbling treatment; the light curing condition is ultraviolet light, and the irradiation time is 5 to 15 minutes; The impurity removal refers to the removal of unreacted monomers and initiator residues; specifically, it refers to the removal of unreacted monomers and initiator residues using water; The impurities are removed by soaking the gel in water for 2 to 4 times, with each soaking time not exceeding 2 minutes, and placing the gel in an environment with a RH of 50-60% for 10-15 minutes after each soaking. The different concentrations of ethanol in step 4) refer to adjacent concentrations differing by 20-40%, and the different concentrations refer to three ethanol solutions of different concentrations.

8. The method for preparing the cellulose photonic gel with multiple optical functions according to claim 7, characterized in that: The bubbling treatment time in step 3) is 10-15 minutes; The UV lamp has a wavelength of 365nm and a power of 20-30mW / cm 2 ; In the specific step of removing impurities, the first soaking time is 1.5 to 2.5 minutes, and the subsequent soaking time is ≤ 1 minute; The different concentrations of ethanol refer to ethanol solutions with a volume concentration of 25-35%, ethanol solutions with a volume concentration of 55-65%, and ethanol solutions with a volume concentration of 90-100%. The gel is immersed in the ethanol solutions with different concentrations for 20-40 minutes.

9. A cellulose photonic gel having multiple optical functions of visibility and concealment obtained by the preparation method according to any one of claims 1 to 8.

10. The use of the cellulose photonic gel with multiple optical functions according to claim 9, characterized in that: The cellulose photonic gel is used in the field of optical information encryption and anti-counterfeiting.

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