All-cellulose-based photonic crystal thin film and preparation method thereof

Abstract

The application discloses a kind of all-cellulose-based photonic crystal films, with regenerated cellulose film (stretching / without stretching) as base material, cellulose nanocrystal is coated on its surface, and after self-assembly, all-cellulose-based photonic crystal film is formed, so that it has excellent flexibility and can be bent and folded, and also effectively maintains the structural color of cellulose nanocrystal;The prepared all-cellulose-based photonic crystal film can also adjust its left-handed and / or right-handed.Innovatively, a flexible photonic crystal film composed entirely of cellulose is prepared, and the all-cellulose-based flexible film has advantages such as ecological friendliness and sustainability, and meets the application of biocompatibility and biodegradability.

Description

Technical Field

[0001] This invention belongs to the field of nanomaterials technology, and specifically relates to an all-cellulose-based photonic crystal thin film and its preparation method. Background Technology

[0002] Cellulose, as a renewable resource, is abundant in nature. Its highly crystalline component—cellulose nanocrystals (CNCs)—can be produced through chemical or physical grinding methods. The unique left-handed chiral nematic phase of cellulose nanocrystals enables them to self-assemble through evaporation-induced self-assembly into iridescent films that reflect LCP light, finding wide applications in optical anti-counterfeiting, novel sensors, and information storage and encryption. However, because the left-handed chiral nematic phase of CNCs is thermodynamically stable, preparing CNC composite films capable of simultaneously reflecting both LCP and RCP light presents a challenge. Furthermore, the brittleness of CNC films limits their versatility, particularly in applications requiring mechanical flexibility. Existing research has successfully enhanced the flexibility of CNC films by incorporating polyvinyl alcohol or polyethylene glycol, including the introduction of zwitterionic surfactants, glycerol, and glucose or ionic liquids into CNC suspensions. While these materials retain the structural color of the film, they all, to some extent, affect the self-assembly of CNCs and render them unsuitable for biocompatibility and biodegradability.

[0003] Current methods for preparing CNC (carbon nanotube) films cannot achieve uniform size control, resulting in a wide size distribution that is unfavorable for photonic crystal fabrication. Furthermore, existing gradient centrifugation methods for purifying and classifying CNC are inefficient and cannot be scaled up. CNC photonic crystal films have poor mechanical properties and are difficult to produce on a large scale. Existing literature reports on coating CNC onto plastic films to construct composite photonic crystal films, which compromises renewability, sustainability, and environmental friendliness. Summary of the Invention

[0004] To address the brittleness of CNC films and improve their flexibility, this invention utilizes regenerated cellulose film as a substrate to prepare a fully cellulose-based photonic crystal film. Furthermore, the regenerated cellulose film (stretched and unstretched) allows for the control of the left and right rotation of the fully cellulose-based photonic crystal film. The resulting composite film exhibits a bright, vibrant color and is smooth, easily foldable without any breakage.

[0005] This invention provides a fully cellulose-based photonic crystal film. Using a regenerated cellulose film as the substrate material, a CNC dispersion is coated onto its surface. After self-assembly, the CNC layer forms a composite photonic crystal film. On one hand, the substrate enhances the tensile strength and elongation of the cellulose composite film, giving it excellent flexibility during bending and folding while maintaining a high Young's modulus, and effectively preserving the structural color of the CNC film. On the other hand, two types of regenerated cellulose substrate films (stretched and unstretched) allow for the control of the left-hand and right-hand rotation of the fully cellulose-based photonic crystal film (the regenerated cellulose substrate is the reverse side of the composite film, and the CNC layer is the front side). The composite film with the stretched regenerated cellulose film as the substrate has a left-handed front side and a right-handed back side; the composite film with the unstretched regenerated cellulose film as the substrate has a left-handed front side and a right-handed back side. Furthermore, this innovatively prepares a flexible photonic crystal film composed entirely of cellulose. The fully cellulose-based flexible film has advantages such as eco-friendliness and sustainability, meeting the requirements for biocompatibility and biodegradability applications.

[0006] The present invention provides an all-fiber-based photonic crystal film, the all-fiber-based photonic crystal film comprising a regenerated cellulose membrane substrate and cellulose nanocrystals coated on the surface of the substrate and self-assembled to form a composite photonic crystal film. Preferably, the all-fiber-based photonic crystal film is tunable to be left-handed and / or right-handed.

[0007] Specifically, the cellulose nanocrystals are purified and size-graded using ultrafiltration, wherein the ultrafiltration is an ultrafiltration membrane or ultrafiltration fiber. Preferably, the pore size of the ultrafiltration membrane is 5-600 nm, more preferably, the pore size is 10-500 nm. Preferably, the cellulose nanocrystals are prepared by acid hydrolysis.

[0008] Specifically, the regenerated cellulose membrane is a stretched regenerated cellulose membrane and / or an unstretched cellulose membrane. Preferably, the regenerated cellulose membrane is stretched longitudinally with a stretch ratio of 5-15% to obtain a stretched regenerated cellulose membrane. More preferably, the stretch ratio is 10%.

[0009] Specifically, the regenerated cellulose membrane uses one or more of the following cellulose dissolution systems to dissolve the cellulose-containing material: N-methylmorpholine-N-oxide (NMMO) system, ionic liquid system, sodium hydroxide / urea (NaOH / urea) system, and carbon disulfide / sodium hydroxide (CS2 / NaOH) system.

[0010] Preferably, an ionic liquid is used to dissolve the cellulose-containing material, and preferably, the ionic liquid is a 1-allyl-3-methylimidazolium chloride solution.

[0011] The present invention also provides a method for preparing the above-mentioned all-cellulose-based photonic crystal thin film, comprising the following steps:

[0012] S1. Preparation of cellulose nanocrystals: cellulose-containing materials are mixed with 60%~70% sulfuric acid at a material-to-liquid ratio of 1g:(5~8)mL. The mixture is reacted and diluted to obtain a neutralized solution. The pH is adjusted to 5~6.5. The neutralized solution is then subjected to ultrafiltration. Water is added during the ultrafiltration process until the conductivity of the filtrate is 4~10us / cm. The filtrate and filtrate retained by the filter membrane are recovered. The filtrate is concentrated to obtain cellulose nanocrystals.

[0013] S2. Preparation of regenerated cellulose membrane: The cellulose-containing material and ionic liquid are mixed and impregnated at a material-to-liquid ratio of 1g:(19-30)g. The mixture is heated and dissolved. The resulting cellulose ionic liquid is defoamed, coated into a film, and then washed with water, regenerated, and dried to obtain the regenerated cellulose membrane.

[0014] S3. Preparation of all-cellulose-based photonic crystal film: The regenerated cellulose film obtained in S2 is fixed, and an appropriate amount of cellulose nanocrystals from step S1 are taken and evenly spread on the regenerated cellulose film substrate of S1. After drying, an all-cellulose-based photonic crystal film is obtained.

[0015] Specifically, the ultrafiltration in S1 is an ultrafiltration membrane or ultrafiltration fiber. Preferably, the pore size of the ultrafiltration membrane is 5-600 nm, and more preferably, the pore size is 10-500 nm.

[0016] Specifically, in S1, the reaction is carried out at 40-60°C for 1-3 hours, the dilution factor in S1 is 5-15 times, the pH is adjusted using sodium hydroxide with a mass fraction of 4%-10%, and the concentration is brought to a solid content of more than 2%, preferably more than 3%.

[0017] Specifically, the regenerated cellulose membrane in S2 is a stretched regenerated cellulose membrane and / or an unstretched regenerated cellulose membrane. Preferably, the regenerated cellulose membrane is stretched longitudinally with a stretching ratio of 5-15% to obtain a stretched regenerated cellulose membrane. More preferably, the stretching ratio is 10%.

[0018] Specifically, the ionic liquid in S2 is a 1-allyl-3-methylimidazolium chloride solution, the heating and dissolving conditions are 60-100°C for 2-4 hours, and the defoaming conditions in S2 are vacuuming at 50-80°C for 4-8 hours, preferably vacuuming at 60-80°C for 5-7 hours.

[0019] Specifically, the fixation of the regenerated cellulose membrane in S3 is selected from methods such as glass plate clamping, vacuum adsorption fixation, or temporary adhesive fixation.

[0020] Specifically, the uniform spreading is achieved by methods such as flow coating, scraping, spin coating, or spraying.

[0021] Specifically, in S3, the ratio of the substrate area to the cellulose nanocrystals during spreading is (2-6) cm². 2 1ml.

[0022] Specifically, the cellulose-containing material is selected from one or more of cotton, refined cotton, chemical pulp, dissolving pulp, and microcrystalline cellulose.

[0023] Beneficial effects

[0024] (1) The present invention uses sulfuric acid hydrolysis to prepare cellulose nanocrystals. The cellulose nanocrystal film prepared has stable structural color and can produce bright and dazzling colors in a drying environment of 25 to 105℃. In particular, it can form structural color at temperatures above 80℃. High temperature has little effect on the self-assembly of CNC.

[0025] (2) In the preparation of cellulose nanocrystals, the present invention uses ultrafiltration to achieve rapid purification and size classification of CNCs by using filter membranes with different pore sizes, resulting in CNCs with more uniform particle size distribution, which is more conducive to their self-assembly into photonic crystal films with uniform color. Compared with traditional centrifugation and dialysis, ultrafiltration is simpler and more efficient in purification and classification, enabling large-scale production.

[0026] (3) In this invention, cellulose solvents such as ionic liquids are used to dissolve cellulose and prepare regenerated cellulose films. CNC dispersion (with the addition of certain additives as needed) is coated on the surface of the regenerated cellulose film to prepare a full cellulose-based photonic crystal film with excellent mechanical properties.

[0027] (4) Left- or right-hand rotation modulation of all-cellulose-based photonic crystal films: Stretching the regenerated cellulose substrate allows the composite film to simultaneously reflect LCP and RCP light. The stretched regenerated cellulose substrate serves as the reverse side of the composite film, and the CNC layer serves as the front side. The front side is left-handed, and the reverse side is right-handed. For composite films with an unstretched regenerated cellulose film as the substrate, both the front and back sides are left-handed. Depending on individual needs, a suitable regenerated cellulose film (stretched or unstretched) can be selected as the substrate to prepare all-cellulose-based photonic crystal films with left- or right-hand rotation characteristics. This method achieves left- or right-hand rotation modulation simply and efficiently, and can be mass-produced. Left- or right-hand rotation modulation allows for precise control of the polarization state of the CNC photonic crystal film, optimizing its reflection and transmission characteristics. This is of great significance for optical devices requiring specific polarization states and for designing efficient optical filters, mirrors, and other optical components. The left- or right-hand rotation modulation characteristics of regenerated cellulose make the composite film a potential application in anti-counterfeiting and encryption fields. By designing optical patterns or codes with specific polarization states, highly secure anti-counterfeiting labels and encrypted information storage can be achieved. All-cellulose photonic crystal films capable of left- and right-handed rotational control can be applied to optoelectronic displays, biomedicine, anti-counterfeiting and encryption, and other fields, improving product performance and added value, and promoting the transformation, upgrading and sustainable development of related industries. Stretching the regenerated cellulose membrane substrate not only achieves positive and negative circular polarization reflection, but also improves the mechanical properties of the composite film and enhances its flexibility. Attached Figure Description

[0028] Figure 1 This is the spectrum of the dried cellulose nanocrystal film prepared in step (1) of Example 1 of the present invention, where the solid line - the left vertical axis is the UV-Vis transmission spectrum and the dashed line - the right vertical axis is the CD spectrum.

[0029] Figure 2 This is a schematic diagram of the all-cellulose-based photonic crystal thin film obtained in Example 1 of the present invention;

[0030] Figure 3 It is a cellulose-based photonic crystal film that has been bent at a certain angle;

[0031] Figure 4 The results are from a polarizing microscope (POM) of the all-cellulose-based photonic crystal thin film prepared in Example 1;

[0032] Figure 5 These are photographs of the dried cellulose nanocrystal film prepared in step (1) of Comparative Examples 1-2 of this invention;

[0033] Figure 6 These are photographs (A, B, and C) of the all-cellulose-based photonic crystal thin films (front and back sides) obtained in Embodiments 1, 2, and Comparative Example 1 of the present invention, taken through LCP and RCP films.

[0034] Figure 7 The CD spectra of the all-cellulose-based photonic crystal thin film (front and back) obtained in Example 1 are shown.

[0035] Figure 8 The CD spectra of the all-cellulose-based photonic crystal thin film (front and back) obtained in Comparative Example 1 are shown. Detailed Implementation

[0036] To better understand the technical content of this invention, specific embodiments are provided below to further illustrate the invention.

[0037] Unless otherwise specified, the experimental methods used in the embodiments of this invention are all conventional methods.

[0038] Unless otherwise specified, all materials and reagents used in the embodiments of this invention are commercially available.

[0039] To investigate the effects of different preparation conditions of cellulose nanocrystals, filter membranes with different pore sizes during ultrafiltration, and different preparation conditions of regenerated cellulose films on all-cellulose-based photonic crystal films, this invention conducted Examples 1, 1-1, 1-2, 2, 3, and 4, the details of which are as follows:

[0040] Example 1: Preparation of all-cellulose-based photonic crystal thin films

[0041] The specific conditions and steps for preparing all-cellulose-based photonic crystal thin films are as follows:

[0042] (1) Preparation of cellulose nanocrystals: 10g of paper strips were dispersed in 70ml of preheated 64% sulfuric acid solution. The mixture was stirred at 200rpm in a 50℃ water bath for 1.5h to obtain a mixed solution. The resulting mixed solution was diluted 10 times with deionized water, and 8% sodium hydroxide solution was added to adjust its pH to pH=5.5 to obtain a neutralized solution. Then, ultrafiltration was performed using a 10nm filter membrane, during which pure water was continuously added for desalination until the conductivity of the filtrate was 6us / cm. The filtrate retained by the filter membrane was then recovered, and the filtrate was concentrated by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals. The UV-Vis transmittance spectrum and circular dichroism spectrum of the dried cellulose nanocrystal film are shown below. Figure 1 As shown, the solid line and the left vertical axis (transmittance) represent the UV-Vis transmission spectrum of the dried cellulose nanocrystal film, while the dashed line and the right vertical axis (circular dichroism) represent the CD spectrum. A distinct photonic bandgap (the lowest value of transmittance) and a strong left-handed signal (the peak of the circular dichroism curve) characteristic of the chiral nematic structure can be observed near 440 nm.

[0043] (2) Preparation of stretched regenerated cellulose film: 1g of pulp powder was immersed in 25g of 1-allyl-3-methylimidazolium chloride solution. The mixture was heated and dissolved for 2h in an oil bath at 80℃ and mechanical stirring speed of 150rpm. The resulting cellulose ionic liquid solution was placed in a vacuum drying oven at 70℃ for 6h to remove bubbles. While still hot, the above cellulose ionic liquid solution was coated onto a glass plate using a glass rod to form a film. The film was then steadily placed in water for regeneration, with the coagulation bath being changed continuously. The film was dried in an oven at 105℃ for 10min to obtain a regenerated cellulose film. The regenerated cellulose film was stretched longitudinally (stretch ratio 10%).

[0044] (3) Preparation of all-cellulose-based photonic crystal thin film: The regenerated cellulose film is coated with CNC solution under a stretched state. After removing air bubbles from the cellulose nanocrystals in step (1), an appropriate amount is dropped onto the substrate (substrate area cm²). 2 The sample volume ratio (ml = 4:1) was used to ensure uniform spreading (flow method). Then, it was placed in a constant temperature and humidity drying oven at 30℃ and 50% humidity. After drying, a fully cellulose-based photonic crystal film was obtained, such as... Figure 2 As shown, both the front and back sides exhibit bright, vibrant colors and are smooth. The all-cellulose-based photonic crystal film, after being bent at a certain angle, is as follows... Figure 3 As shown, the composite film can be easily folded without any breakage, demonstrating a significant advancement in the fabrication conditions of the composite circularly polarized thin film of this invention for realizing flexible cellulose-based photonic materials. A schematic diagram of its polarizing microscope (POM) is shown below. Figure 4 As shown, the prepared all-cellulose-based photonic crystal film exhibits a fingerprint structure with distinct chiral nematic characteristics. The transmission results of the prepared all-cellulose-based photonic crystal film through LCP and RCP films are as follows... Figure 6 As shown in Figure A, the prepared composite membrane exhibits left-handed rotation on the front and right-handed rotation on the back. The corresponding CD spectra of the front and back sides of the composite membrane are as follows. Figure 7 .

[0045] Comparative Example 1-1: Effect of Unstretched Regenerated Cellulose Film on All-Cellulose Photonic Crystal Film

[0046] (1) Preparation of cellulose nanocrystals: 10g of paper strips were dispersed in 70ml of preheated 64% sulfuric acid solution. The mixture was stirred at 200rpm in a 50℃ water bath for 1.5h to obtain a mixture. The mixture was diluted 10 times with deionized water, and 8% sodium hydroxide solution was added to adjust the pH to 5.5 to obtain a neutralized solution. Ultrafiltration was then performed using a 10nm filter membrane, during which pure water was continuously added for desalination until the conductivity of the filtrate was 6µs / cm. The filtrate retained by the filter membrane was then recovered, and the filtrate was concentrated by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals.

[0047] (2) Preparation of unstretched regenerated cellulose film: 1g of pulp powder was impregnated in 25g of 1-allyl-3-methylimidazolium chloride solution. The mixture was heated and dissolved for 2h in an oil bath at 80℃ and mechanical stirring speed of 150rpm. The resulting cellulose ionic liquid solution was placed in a vacuum drying oven at 60℃ for 6h to remove bubbles. While still hot, the above cellulose ionic liquid solution was coated onto a glass plate using a glass rod to form a film. The film was then steadily placed in water for regeneration, with the coagulation bath being changed continuously. The film was dried in an oven at 105℃ for 10min to obtain the regenerated cellulose film.

[0048] (3) Preparation of all-cellulose-based photonic crystal thin film: First, place the regenerated cellulose film from step (2) on a petri dish (130 mm in diameter, fixed around the edges). After removing air bubbles from the cellulose nanocrystals from step (1), take an appropriate amount and drop it onto the substrate (substrate area cm²). 2 The sample volume ratio (ml = 4:1) was used to ensure uniform spreading (flow method). The sample was then placed in a constant temperature and humidity drying oven at 30℃ and 50% humidity, and dried to obtain a fully cellulose-based photonic crystal thin film.

[0049] Compared to the all-cellulose-based photonic crystal film prepared in Example 1, in Comparative Example 1-1, the regenerated cellulose film was not stretched during preparation. Although other processes were the same as in Example 1, the composite film with the reverse right-handed rotation effect as in Example 1 could not be formed. The all-cellulose-based photonic crystal film prepared in Comparative Example 1-1 showed the following results when transmitted through LCP and PCR membranes: Figure 6 As shown in Figure C, both its front and back sides are left-handed. The CD spectra of the composite film on both sides are as follows. Figure 8 .

[0050] Comparative Examples 1-2: The Influence of CNC Obtained with Different Filter Membrane Pore Sizes on All-Cellulose Photonic Crystal Thin Films

[0051] (1) Preparation of cellulose nanocrystals: 10g of paper strips were dispersed in 60ml of preheated 64% sulfuric acid solution. The mixture was stirred at 200rpm with a magnetic stirrer and reacted in a 45℃ water bath for 1.5h to obtain a mixed solution. The resulting mixed solution was diluted 10 times with deionized water, and 8% sodium hydroxide solution was added to adjust its pH to pH=5.5 to obtain a neutralized solution. Then, ultrafiltration was performed using 100nm and 500nm filter membranes. Pure water was continuously added to desalinate the solution until the conductivity of the filtrate was 6us / cm. The filtrate retained by the filter membrane was then recovered, and the filtrate was concentrated by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals. The dried film photograph is shown below. Figure 5 As shown.

[0052] (2) Preparation of stretched regenerated cellulose film: 1g of pulp powder was immersed in 25g of 1-allyl-3-methylimidazolium chloride solution. The mixture was heated and dissolved for 2.5h in an oil bath at 80℃ and mechanical stirring at 150rpm. The resulting cellulose ionic liquid solution was placed in a vacuum drying oven at 70℃ for 6h to remove bubbles. While still hot, the above cellulose ionic liquid solution was coated onto a glass plate using a glass rod to form a film. The film was then steadily placed in water for regeneration, with the coagulation bath being changed continuously. The regenerated cellulose film was stretched longitudinally (stretch ratio 10%) and dried in a rapid Kassel paper forming machine (100℃) to obtain the regenerated cellulose film.

[0053] (3) Preparation of all-cellulose-based photonic crystal thin film: First, the regenerated cellulose film from step (2) is fixed using a glass plate clamping method. After removing air bubbles from the cellulose nanocrystal dispersion from step (1), an appropriate amount is taken and placed on the substrate (substrate area cm²). 2 The sample volume (ml = 4:1) was scraped and spread evenly. Then it was placed in a constant temperature and humidity drying oven at 50℃ and 50% humidity, and then dried to obtain a composite circularly polarized film.

[0054] Compared with Example 1, by changing the preparation conditions of cellulose nanocrystals and using ultrafiltration with filter membranes of different pore sizes, CNCs of different lengths were obtained. The different lengths of the CNCs resulted in different colors of the membranes obtained after self-assembly.

[0055] Example 2: Effects of different regenerated cellulose membrane preparation conditions on all-cellulose-based photonic crystal thin films

[0056] (1) Preparation of cellulose nanocrystals: 10g of paper strips were dispersed in 60ml of preheated 64% sulfuric acid solution. The mixture was stirred at 200rpm with a magnetic stirrer and reacted in a 45℃ water bath for 2h to obtain a mixed solution. The resulting mixed solution was diluted 10 times with deionized water, and 8% sodium hydroxide solution was added to adjust its pH to pH=5.7 to obtain a neutralized solution. Then, ultrafiltration was performed using a 100nm filter membrane, during which pure water was continuously added for desalination until the conductivity of the filtrate was 6us / cm. The filtrate retained by the filter membrane was then recovered, and the filtrate was concentrated by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals.

[0057] (2) Preparation of stretched regenerated cellulose film (carbon disulfide / sodium hydroxide (CS2 / NaOH) system): Pulp powder was soaked in 18-20% sodium hydroxide solution at 20-25℃ for 1-2 hours to generate alkali cellulose. The alkali cellulose was placed in a petri dish and exposed to air in a 25℃ constant temperature incubator for 24-48 hours to allow it to oxidize and degrade. The aged alkali cellulose was transferred to a glass bottle with a sealed cap. CS2 was added at 30% of the cellulose mass, and the mixture was sealed and shaken at room temperature for 2 hours. After the reaction was completed, the product became orange-yellow cellulose xanthate. The xanthate was slowly added to 4% NaOH solution and magnetically stirred until completely dissolved to form a viscous adhesive. The adhesive was allowed to stand for 12-24 hours to mature until the fluidity was stable. The adhesive was filtered under reduced pressure using a filter membrane to remove insoluble matter. The filtrate was transferred to a centrifuge tube and centrifuged at low speed (2000 rpm, 5 minutes) to remove air bubbles. The adhesive was poured onto a clean glass plate and scraped into a uniform liquid film with a spatula. Immerse the cellulose in a coagulation bath (10% H2SO4 + 15% Na2SO4, 40℃) for 1-2 minutes to regenerate it into a solid film. Peel the film off with tweezers and rinse it in deionized water. Dry the film in an oven at 105℃ for 10 minutes to obtain a regenerated cellulose membrane. Stretch the regenerated cellulose membrane longitudinally (stretch ratio 17%).

[0058] (3) Preparation of all-cellulose-based photonic crystal thin film: First, the regenerated cellulose film from step (2) is placed in a petri dish for fixation. After removing air bubbles from the cellulose nanocrystals from step (1), an appropriate amount is dropped onto the substrate (substrate area cm²). 2 The sample volume (ml = 4:1) was spread evenly. Then it was placed in an oven at 45℃ and dried to obtain a composite circularly polarized film.

[0059] In Example 2, by changing the preparation method of the substrate regenerated cellulose membrane, the same effect as in Example 1 can be achieved. The results of the prepared all-cellulose-based photonic crystal thin film transmitted through LCP and PCR membranes are as follows: Figure 6 As shown in Figure B, the prepared composite membrane exhibits left-handed rotation on the front and right-handed rotation on the back.

[0060] Combining Examples 1 and 2, composite circularly polarized films were prepared using the same fabrication process but with different conditions for preparing cellulose nanocrystals, different pore sizes of CNC membranes obtained during ultrafiltration, and different conditions for preparing stretched regenerated cellulose films. These composite circularly polarized films are simple to prepare, exhibit enhanced strength and flexibility, retain their structural color, and possess a certain degree of stability. Furthermore, the all-cellulose-based photonic crystal film simultaneously reflects both LCP and RCP light.

[0061] Example 3: Effect of Regenerated Cellulose Film Preparation Conditions on All-Cellulose-Based Photonic Crystal Films

[0062] (1) Preparation of cellulose nanocrystals: 10g of paper strips were dispersed in 70ml of preheated 64% sulfuric acid solution. The mixture was stirred at 200rpm in a 50℃ water bath for 1.5h to obtain a mixture. The mixture was diluted 10 times with deionized water, and 8% sodium hydroxide solution was added to adjust the pH to 5.5 to obtain a neutralized solution. Ultrafiltration was then performed using a 10nm filter membrane, during which pure water was continuously added for desalination until the conductivity of the filtrate was 6µs / cm. The filtrate retained by the filter membrane was then recovered, and the filtrate was concentrated by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals.

[0063] (2) Preparation of stretched regenerated cellulose membrane (using N-methylmorpholine-N-oxide (NMMO) system): At 50℃, slowly add pulp powder (8-15wt%) to 50% NMMO aqueous solution and stir for 1-2 hours to form a uniform suspension; slowly evaporate the water using a vacuum drying oven (85-100℃) until the NMMO concentration reaches 77-83% (forming a uniform, transparent, viscous solution). Continue stirring for 1-2 hours to ensure complete dissolution (the solution should be amber-colored and transparent, without particles); place the solution in an ultrasonic instrument to remove air bubbles (30-60 minutes). Pour the cellulose-NMMO solution onto a clean glass plate and use a scraper to control the thickness to form a film. Immediately immerse in a coagulation bath (pure water) to regenerate the cellulose. Dry the film in a 60℃ oven to obtain the regenerated cellulose membrane. Stretch the regenerated cellulose membrane longitudinally (stretch ratio 10%).

[0064] (3) Preparation of all-cellulose-based photonic crystal thin film: First, fix the regenerated cellulose membrane from step (2). After removing air bubbles from the cellulose nanocrystal dispersion from step (1), take an appropriate amount and place it on the substrate (substrate area cm²). 2 The sample volume (ml = 4:1) was scraped and spread evenly. Then it was placed in a constant temperature and humidity drying oven at 50℃ and 50% humidity, and then dried to obtain a full cellulose-based photonic crystal film.

[0065] Example 4: Effect of Regenerated Cellulose Membrane Preparation Conditions on All-Cellulose-Based Photonic Crystal Thin Films

[0066] (1) Preparation of cellulose nanocrystals: 10g of paper strips were dispersed in 70ml of preheated 64% sulfuric acid solution. The mixture was stirred at 200rpm in a 50℃ water bath for 1.5h to obtain a mixture. The mixture was diluted 10 times with deionized water, and the pH was adjusted to 5.5 with 8% sodium hydroxide solution to obtain a neutralized solution. Ultrafiltration was then performed using a 10nm filter membrane, during which pure water was continuously added for desalination until the conductivity of the filtrate was 6µs / cm. The filtrate retained by the filter membrane was then recovered, and the filtrate was concentrated by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals.

[0067] (2) Preparation of stretched regenerated cellulose membrane (using sodium hydroxide / urea (NaOH / urea) system): Pulp powder was vacuum dried at 60℃ for 12 hours to remove moisture. A solution was prepared according to the mass ratio of NaOH:urea:water = 7:12:81. The mixed solution was pre-cooled at -12℃ for 2 hours to form a low-temperature transparent solvent. At a low temperature (-12℃), dried cellulose (4-8wt%) was added to the pre-cooled solvent. The mixture was stirred vigorously (800-1200rpm) for 1-2 hours until a transparent viscous solution was formed. The temperature was kept low (-12℃) throughout the process to avoid gelation caused by temperature rise. After centrifuging the mixture to remove air bubbles, it was poured onto a clean glass plate and coated with a film using a scraper to control the thickness. The film was immediately immersed in a coagulation bath (5% sodium sulfate (Na2SO4) aqueous solution) to regenerate the cellulose. The film was then transferred to deionized water for soaking to completely remove residual solvent. The film was laid flat and dried naturally at room temperature to obtain the regenerated cellulose membrane. The regenerated cellulose membrane was stretched longitudinally (stretch ratio 10%).

[0068] (3) Preparation of all-cellulose-based photonic crystal thin film: First, fix the regenerated cellulose membrane from step (2). After removing air bubbles from the cellulose nanocrystal dispersion from step (1), take an appropriate amount and place it on the substrate (substrate area cm²). 2 The sample volume (ml = 4:1) was scraped and spread evenly. Then it was placed in a constant temperature and humidity drying oven at 50℃ and 50% humidity, and then dried to obtain a full cellulose-based photonic crystal film.

[0069] Examples 1, 2, 3, and 4, using the same preparation process but different preparation conditions for the regenerated cellulose membrane, employed four solvent systems—N-methylmorpholine-N-oxide (NMMO), ionic liquid (1-allyl-3-methylimidazolium chloride solution), sodium hydroxide / urea (NaOH / urea), and carbon disulfide / sodium hydroxide (CS2 / NaOH)—to dissolve cellulose. All four examples yielded all-cellulose-based photonic crystal films with the same effect as in Example 1. The prepared composite films exhibited left-handed rotation on the front and right-handed rotation on the back. This all-cellulose-based photonic crystal film is simple to prepare, exhibits enhanced strength and flexibility, retains its structural color, and possesses a certain degree of stability. Furthermore, the all-cellulose-based photonic crystal film simultaneously reflects both LCP and RCP light. The use of an ionic liquid system for preparing the regenerated cellulose membrane is more environmentally friendly and simplifies the operation.

[0070] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a fully cellulose-based photonic crystal thin film, characterized in that, The all-cellulose-based photonic crystal film includes a regenerated cellulose film substrate and cellulose nanocrystals coated on the surface of the substrate and self-assembled to form a composite photonic crystal film. The all-cellulose-based photonic crystal film can be tuned to be left-handed or right-handed. All-cellulose-based photonic crystal thin films were prepared using the following method: S1. Preparation of cellulose nanocrystals: cellulose-containing materials are mixed with 60%–70% sulfuric acid at a material-to-liquid ratio of 1g:(5–8)mL. The mixture is reacted and diluted to obtain a neutralized solution. The pH is then adjusted to 5–6.

5. The neutralized solution is then subjected to ultrafiltration. Water is added during the ultrafiltration process until the conductivity of the filtrate is 4–10 μS / cm. The filtrate retained by the filter membrane is recovered and concentrated to obtain cellulose nanocrystals. S2. Preparation of regenerated cellulose membrane: The cellulose-containing material and ionic liquid are mixed and impregnated at a material-to-liquid ratio of 1g:(19-30)g. The mixture is heated and dissolved. The resulting cellulose ionic liquid is defoamed, coated into a film, and then washed with water, regenerated, and dried to obtain the regenerated cellulose membrane. The regenerated cellulose membrane is a stretched regenerated cellulose membrane, and the stretch ratio of the regenerated cellulose membrane along the longitudinal direction is 5-15%; S3. Preparation of all-cellulose-based photonic crystal thin film: Fix the regenerated cellulose film obtained in S2, and take an appropriate amount of the cellulose nanocrystals from step S1 after removing air bubbles, according to the ratio of substrate area to cellulose nanocrystals of (2-6) cm². 2 1 mL was evenly spread on the regenerated cellulose membrane substrate of S1 and dried to obtain a full cellulose-based photonic crystal film; The cellulose nanocrystals are purified and fractionated using ultrafiltration, wherein the ultrafiltration is performed using an ultrafiltration membrane or ultrafiltration fiber, and the pore size of the ultrafiltration membrane is 5-600 nm.

2. The method for preparing a full cellulose-based photonic crystal thin film according to claim 1, characterized in that, The pore size of the ultrafiltration membrane is 10-500 nm.

3. The method for preparing a fully cellulose-based photonic crystal thin film according to claim 1, characterized in that, The regenerated cellulose membrane is stretched longitudinally with a stretch ratio of 10%.

4. The method for preparing a full cellulose-based photonic crystal thin film according to claim 1, characterized in that, Regenerated cellulose membranes can also be made by dissolving cellulose-containing materials using one or more of the following cellulose dissolving systems: N-methylmorpholine-N-oxide system, sodium hydroxide / urea system, and carbon disulfide / sodium hydroxide system.

5. The method for preparing a full cellulose-based photonic crystal thin film according to claim 1, characterized in that, The ionic liquid is a solution of 1-allyl-3-methylimidazolium chloride.

6. The method for preparing a full cellulose-based photonic crystal thin film according to claim 1, characterized in that, The reaction conditions in S1 are as follows: reaction at 40-60℃ for 1-3 hours, dilution ratio of 5-15 times, pH adjustment using sodium hydroxide with a mass fraction of 4%-10%, and concentration to a solid content of more than 2%.

7. The method for preparing a fully cellulose-based photonic crystal thin film according to claim 6, characterized in that, Concentrate to a solid content of 3% or more.

8. The method for preparing a full cellulose-based photonic crystal thin film according to claim 1, characterized in that, The heating and dissolving conditions are 60–100°C and the dissolving time is 2–4 hours. The defoaming conditions in S2 are vacuuming at 50–80°C.

9. The method for preparing a full cellulose-based photonic crystal thin film according to claim 8, characterized in that, Vacuum degassing is performed at 60–80°C.

10. The method for preparing an all-cellulose-based photonic crystal thin film according to claim 1, characterized in that, The regenerated cellulose membrane fixation in S3 is selected from glass plate clamping, vacuum adsorption fixation, and temporary adhesive fixation methods, while the uniform spreading is carried out by the following methods: flow coating, scraping coating, spin coating, and spraying coating.

11. The method for preparing a full cellulose-based photonic crystal thin film according to claim 1, characterized in that, The cellulose-containing material is selected from one or more of cotton, refined cotton, chemical pulp, dissolving pulp, and microcrystalline cellulose.

12. The all-cellulose-based photonic crystal thin film prepared by the method of any one of claims 1-11.

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