Holocellulose-based photonic crystal film and preparation method thereof

Abstract

The invention discloses a holocellulose-based photonic crystal film, which is characterized in that a regenerated cellulose film (stretched / unstretched) is used as a substrate material, cellulose nanocrystals are coated on the surface of the regenerated cellulose film, and the holocellulose-based photonic crystal film is formed after self-assembly, so that the holocellulose-based photonic crystal film has excellent flexibility and can be bent and folded; meanwhile, the structural color of the cellulose nanocrystal is effectively maintained; the prepared holocellulose-based photonic crystal film can also be regulated and controlled to rotate leftwards and / or rightwards. The flexible photonic crystal film completely composed of cellulose is innovatively prepared, and the holocellulose-based flexible film has the advantages of eco-friendliness, sustainability and the like, and meets the application of biocompatibility and biodegradability.

Description

Technical Field

[0001] The present invention belongs to the technical field of nanomaterials, and in particular relates to a full-cellulose-based photonic crystal film and a preparation method thereof. Background Art

[0002] Cellulose is a renewable resource abundant in nature. Its highly crystalline components, cellulose nanocrystals (CNCs), can be produced by chemical or physical grinding. The unique left-handed chiral nematic phase of CNCs allows them to self-assemble into iridescent films that reflect LCP light through evaporation, resulting in widespread applications in optical anti-counterfeiting, novel sensors, and information storage and encryption. Because the left-handed chiral nematic phase of CNCs is thermodynamically stable, the preparation of CNC composite films that simultaneously reflect both LCP and RCP light presents challenges. However, the brittleness of CNC films limits their versatility, particularly in applications requiring mechanical flexibility. Previous studies have successfully enhanced the flexibility of CNC films by incorporating polyvinyl alcohol or polyethylene glycol, including zwitterionic surfactants, glycerol, and glucose, or ionic liquids into CNC suspensions. While these materials maintain the structural color of the films, they all hinder the self-assembly of CNCs to some extent, rendering them unsuitable for biocompatibility and biodegradability.

[0003] Existing CNC preparation methods cannot achieve uniform size control, resulting in a wide size distribution, which is not conducive to the preparation of photonic crystals. Furthermore, existing gradient centrifugation methods for purifying and fractionating CNCs 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 reports of CNC coating on plastic films to construct composite photonic crystal films have lost their renewability, sustainability, and environmental friendliness. Summary of the Invention

[0004] In order to solve the brittle defects of CNC films and improve their flexibility, the present invention uses regenerated cellulose film as a substrate to prepare a full-cellulose-based photonic crystal film. At the same time, the regenerated cellulose film (stretched and unstretched) can realize the regulation of the left-right rotation of the full-cellulose-based photonic crystal film. The prepared composite film shows bright colors and is flat, and can be easily folded without any breakage.

[0005] This invention provides an all-cellulose-based photonic crystal film. Using a regenerated cellulose film as a substrate, a CNC dispersion is coated on its surface, resulting in the CNC self-assembly forming a composite photonic crystal film. The substrate enhances the tensile strength and elongation of the cellulose composite film, enabling excellent flexibility even during bending and folding while maintaining a high Young's modulus and effectively preserving the structural color of the CNC film. Furthermore, two regenerated cellulose substrates (stretched and unstretched) enable the control of the left-handed and right-handed nature of the all-cellulose-based photonic crystal film (the regenerated cellulose substrate serves as the back surface of the composite film, and the CNC layer serves as the front surface). The composite film with the stretched regenerated cellulose film as the substrate exhibits left-handedness on the front surface and right-handedness on the back surface; the composite film with the unstretched regenerated cellulose film as the substrate exhibits left-handedness on both the front and back surfaces. Furthermore, this innovative method allows the preparation of a flexible photonic crystal film composed entirely of cellulose. The all-cellulose-based flexible film offers advantages such as eco-friendliness and sustainability, making it suitable for biocompatible and biodegradable applications.

[0006] The present invention provides an all-fiber-based photonic crystal film, which 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. Preferably, the all-cellulose-based photonic crystal film can control left-handedness and / or right-handedness.

[0007] Specifically, the cellulose nanocrystals are purified and size-classified by ultrafiltration, the ultrafiltration is an ultrafiltration membrane or ultrafiltration filament, preferably, the pore size of the ultrafiltration membrane is 5-600nm, preferably, the pore size is 10-500nm, preferably, the cellulose nanocrystals are prepared by acid hydrolysis.

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

[0009] Specifically, the regenerated cellulose membrane adopts one or more of the following cellulose dissolving systems to dissolve the cellulose-containing material: N-methylmorpholine-N-oxide (NMMO) system, ionic liquid system, sodium hydroxide / urea (NaOH / urea) system, carbon disulfide / sodium hydroxide (CS2 / NaOH) system, Preferably, an ionic liquid is used to dissolve the cellulose-containing material. Preferably, the ionic liquid is a 1-allyl-3-methylimidazolium chloride solution.

[0010] The present invention also provides a method for preparing the above-mentioned all-cellulose-based photonic crystal film, comprising the following steps: S1. Preparation of cellulose nanocrystals: A cellulose-containing material is mixed with 60% to 70% sulfuric acid at a material-liquid ratio of 1 g: (5 to 8) mL, reacted, and the resulting mixed solution is diluted and the pH is adjusted to 5 to 6.5 to obtain a neutralized solution. The neutralized solution is ultrafiltered, and water is added during the ultrafiltration process until the conductivity of the filtrate is 4 to 10 μS / cm. The filtrate and filtrate retained by the filter membrane are recovered, and the filtrate is concentrated to obtain cellulose nanocrystals. S2. Preparation of regenerated cellulose membrane: a cellulose-containing material and an ionic liquid were mixed and impregnated at a material-liquid ratio of 1 g:(19-30) g, the mixed liquid was heated to dissolve, the obtained cellulose ionic liquid was defoamed, and a film was formed by blade coating, which was then washed, regenerated, and dried to obtain a regenerated cellulose membrane; S3. Preparation of a fully cellulose-based photonic crystal film: Fix the regenerated cellulose film obtained in S2, take an appropriate amount of cellulose nanocrystals obtained in step S1, evenly spread it on the regenerated cellulose film substrate of S1, and dry it to obtain a fully cellulose-based photonic crystal film.

[0011] Specifically, the ultrafiltration in S1 is an ultrafiltration membrane or an ultrafiltration filament. Preferably, the pore size of the ultrafiltration membrane is 5 to 600 nm, and preferably, the pore size is 10 to 500 nm.

[0012] Specifically, the reaction conditions in S1 are 40-60° C. for 1-3 hours, the dilution ratio in S1 is 5-15 times, and the pH is adjusted using sodium hydroxide with a mass fraction of 4%-10%. The solution is concentrated to a solid content of more than 2%, preferably, a solid content of more than 3%.

[0013] Specifically, the regenerated cellulose film in S2 is a stretched regenerated cellulose film and / or an unstretched regenerated cellulose film. Preferably, the regenerated cellulose film is stretched longitudinally with a stretching ratio of 5-15% to obtain a stretched regenerated cellulose film. Preferably, the stretching ratio is 10%.

[0014] Specifically, the ionic liquid in S2 is 1-allyl-3-methylimidazolium chloride solution, the heating dissolution conditions are 60-100°C, and the dissolution is 2-4 hours. 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.

[0015] Specifically, the fixing of the regenerated cellulose film in S3 is selected from a glass plate clamping method, a vacuum adsorption fixing method or a temporary adhesive fixing method.

[0016] Specifically, the uniform spreading is carried out by a flow coating method, a blade coating method, a spin coating method or a spray coating method.

[0017] Specifically, the ratio of the substrate area to the cellulose nanocrystals in the spread in S3 is (2-6) cm 2 :1ml.

[0018] Specifically, the cellulose-containing material is selected from one or more of cotton, refined cotton, chemical pulp, dissolving pulp, and microcrystalline cellulose.

[0019] Beneficial effects

[0020] (1) The present invention adopts sulfuric acid hydrolysis method to prepare cellulose nanocrystals. The structural color of the prepared cellulose nanocrystal film is stable and can produce bright and colorful colors in a dry environment of 25 to 105°C. In particular, it can also form structural colors above 80°C. High temperature has little effect on the self-assembly of CNC.

[0021] (2) The present invention adopts an ultrafiltration method in the preparation process of cellulose nanocrystals, using filter membranes of different pore sizes to achieve rapid purification and size classification of CNC, thereby obtaining CNC with a more uniform particle size distribution, which is more conducive to its self-assembly into a photonic crystal film with uniform color. Compared with traditional centrifugation and dialysis, the purification and classification process of ultrafiltration is simpler and more efficient, and large-scale production can be achieved.

[0022] (3) The present invention uses cellulose solvents such as ionic liquids to dissolve cellulose to prepare a regenerated cellulose film, and a CNC dispersion (certain additives can be added 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.

[0023] (4) All-cellulose-based photonic crystal films achieve left-handed or right-handed control. Stretching the regenerated cellulose base film enables the composite film to simultaneously reflect LCP and RCP light. The stretched regenerated cellulose base serves as the back of the composite film, and the CNC layer serves as the front of the composite film. The front is left-handed and the back is right-handed. The composite film with the unstretched regenerated cellulose film as the base has both left-handed and right-handed properties. According to one's own needs, a suitable regenerated cellulose film (stretched or unstretched) can be selected as the base to prepare all-cellulose-based photonic crystal films with left-handed or right-handed properties. This method is simple and efficient to achieve left-handed or right-handed control and can be mass-produced. Left-handed or right-handed control can precisely control the polarization state of the CNC photonic crystal film and optimize its reflection and transmission properties, which is of great significance for optical devices that require specific polarization states and the design of efficient optical filters, reflectors and other optical components. The left-handed or right-handed control properties of regenerated cellulose make the composite film have potential applications in the fields of anti-counterfeiting and encryption. By designing optical patterns or codes with specific polarization states, high-security anti-counterfeiting labels and encrypted information storage can be achieved. All-cellulose photonic crystal films that achieve left-handed and right-handed control can be applied in optoelectronic displays, biomedicine, anti-counterfeiting, and encryption, improving product performance and added value, and driving the transformation, upgrading, and sustainable development of related industries. Stretching the regenerated cellulose film substrate not only achieves both positive and negative circularly polarized reflection, but also enhances the mechanical properties and flexibility of the composite film. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 : is a spectrum of the cellulose nanocrystal film after drying prepared in step (1) of Example 1 of the present invention, wherein the solid line - left ordinate is the UV-Vis transmittance spectrum, and the dotted line - right ordinate is the CD spectrum; Figure 2 This is a schematic diagram of the all-cellulose-based photonic crystal film obtained in Example 1 of the present invention; Figure 3 It is a fully cellulose-based photonic crystal film that is bent at a certain angle; Figure 4 This is the polarizing microscope (POM) result of the all-cellulose-based photonic crystal film prepared in Example 1; Figure 5 This is a photograph of the film of cellulose nanocrystals after drying prepared in step (1) of Comparative Example 1-2 of the present invention; Figure 6 These are photos of the all-cellulose-based photonic crystal films (front and back) obtained in Example 1, Example 2, and Comparative Example 1 of the present invention, taken through LCP and RCP films (Figures A, B, and C, respectively); Figure 7 is the CD spectrum of the all-cellulose-based photonic crystal film (front and back) obtained in Example 1; Figure 8 1 is the CD spectrum of the all-cellulose-based photonic crystal film (front and back) obtained in Comparative Example 1. DETAILED DESCRIPTION

[0025] In order to better understand the technical content of the present invention, specific examples are provided below to further illustrate the present invention.

[0026] Unless otherwise specified, the test methods used in the examples of the present invention are all conventional methods.

[0027] Unless otherwise specified, the materials, reagents, etc. used in the examples of the present invention can be obtained from commercial sources.

[0028] In order to study 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, the present invention carried out Example 1, Comparative Example 1-1, Comparative Example 1-2, Example 2, Example 3, and Example 4. The specific contents are as follows: Example 1: Preparation of all-cellulose-based photonic crystal films The specific conditions and steps for preparing the all-cellulose-based photonic crystal film are as follows: (1) Preparation of cellulose nanocrystals: Take 10g of paper strips and disperse them in 70ml of preheated 64% sulfuric acid solution, stir with a magnetic stirrer at 200rpm, and react in a 50℃ water bath for 1.5h to obtain a mixed solution. Dilute the obtained mixed solution 10 times with deionized water, add 8% sodium hydroxide solution to adjust its pH to pH=5.5, and obtain a neutralized solution. Then use a 10nm filter membrane for ultrafiltration, and continuously add pure water for desalination until the conductivity of the filtrate is 6us / cm. Then recover the filtrate retained by the filter membrane, and concentrate the filtrate by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals. The UV-Vis transmittance spectrum and circular dichroism spectrum of the film after drying of cellulose nanocrystals are shown in Figure 2. Figure 1 As shown, the solid line and the left ordinate (transmittance) are the UV-Vis transmittance spectrum of the cellulose nanocrystal film after drying, and the dotted line and the right ordinate (circular dichroism) are the CD spectrum. An obvious photon band gap (the lowest value of transmittance) and a strong left-handed signal (peak of the circular dichroism curve) with the characteristics of the chiral nematic structure can be observed around 440nm.

[0029] (2) Preparation of stretched regenerated cellulose film: 1 g of pulp powder was immersed in 25 g of 1-allyl-3-methylimidazolium chloride solution. The mixture was heated and dissolved in an oil bath at 80°C with a mechanical stirring speed of 150 rpm for 2 h. The resulting cellulose ionic liquid solution was placed in a vacuum drying oven at 70°C for degassing for 6 h. While still hot, the 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 continuously replaced. The film was then dried in an oven at 105°C for 10 min to obtain a regenerated cellulose film. The regenerated cellulose film was stretched longitudinally (stretching ratio 10%).

[0030] (3) Preparation of all-cellulose-based photonic crystal film: The regenerated cellulose film is coated with CNC solution in a stretched state. After removing the bubbles from the cellulose nanocrystals in step (1), an appropriate amount is dropped on the substrate (substrate area cm 2 : sample volume ml = 4:1), so that it can be evenly spread (casting method). Then it is placed in a constant temperature and humidity drying oven, set at 30℃ and 50% humidity, and then dried to obtain a full cellulose-based photonic crystal film, such as Figure 2 As shown in the figure, both the front and back sides show bright colors and are flat. Figure 3 As shown in Figure 2, the composite film can be easily folded without any breakage, which proves that the preparation conditions of the composite circular polarizing film of the present invention are a significant advancement in realizing flexible cellulose-based photonic materials. Figure 4As shown in Figure 2, the prepared all-cellulose-based photonic crystal film has a fingerprint structure with obvious chiral nematic structure characteristics. The test results of the prepared all-cellulose-based photonic crystal film through LCP and RCP films are shown in Figure 2. Figure 6 As shown in Figure A, the front side of the prepared composite film is left-handed and the back side is right-handed. The CD spectra of the front and back sides of the composite film are shown in Figure Figure 7 .

[0031] Comparative Example 1-1: Effect of Unstretched Regenerated Cellulose Film on All-cellulose-based Photonic Crystal Film (1) Preparation of cellulose nanocrystals: 10 g of paper strips were dispersed in 70 ml of preheated 64% sulfuric acid solution, stirred at 200 rpm with a magnetic stirrer, and reacted in a 50°C water bath for 1.5 h to obtain a mixed solution. The obtained mixed solution was diluted 10 times with deionized water, and an 8% sodium hydroxide solution was added to adjust its pH to pH = 5.5 to obtain a neutralized solution. Ultrafiltration was then performed using a 10 nm filter membrane, and pure water was continuously added for desalination until the filtrate conductivity reached 6 μs / cm. The filtrate retained by the filter membrane was then recovered and concentrated by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals.

[0032] (2) Preparation of unstretched regenerated cellulose film: 1 g of pulp powder was immersed in 25 g of 1-allyl-3-methylimidazolium chloride solution. The mixture was heated and dissolved in an oil bath at 80°C with a mechanical stirring speed of 150 rpm for 2 h. The resulting cellulose ionic liquid solution was placed in a vacuum drying oven at 60°C for degassing for 6 h. While still hot, the 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 continuously replaced. The film was then dried in an oven at 105°C for 10 min to obtain a regenerated cellulose film.

[0033] (3) Preparation of all-cellulose-based photonic crystal film: First, place the regenerated cellulose film in step (2) on a surface dish (diameter 130 mm) and fix it around the edges. After removing the bubbles from the cellulose nanocrystals in step (1), take an appropriate amount and drop it on the substrate (substrate area cm 2 The sample volume (ml) was adjusted to 4:1 (spreading method) to allow for uniform spreading. The film was then placed in a constant temperature and humidity drying oven at 30°C and 50% humidity, and dried to obtain a fully cellulose-based photonic crystal film.

[0034] Compared with the preparation of 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 the other processes were the same as in Example 1, the composite film with the reverse right-handed effect in Example 1 could not be formed. The test results of the all-cellulose-based photonic crystal film prepared in Comparative Example 1-1 through the LCP and PCR films are as follows: Figure 6As shown in C, both the front and back sides are left-handed. The CD spectra of the composite film corresponding to the front and back sides are as follows Figure 8 .

[0035] Comparative Example 1-2: Effect of CNC obtained with different filter membrane pore sizes on all-cellulose-based photonic crystal films (1) Preparation of cellulose nanocrystals: Take 10g of paper strips and disperse them in 60ml of preheated 64% sulfuric acid solution. Use a magnetic stirrer at 200rpm and react in a 45℃ water bath for 1.5h to obtain a mixed solution. Dilute the obtained mixed solution 10 times with deionized water, add 8% sodium hydroxide solution to adjust its pH to pH=5.5, and obtain a neutralized solution. Then use 100nm and 500nm filter membranes for ultrafiltration. During this period, pure water is continuously added for desalination until the conductivity of the filtrate is 6us / cm. Then recover the filtrate retained by the filter membrane and concentrate the filtrate by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals. The photo of the film after drying is as follows: Figure 5 shown.

[0036] (2) Preparation of stretched regenerated cellulose film: 1 g of pulp powder was immersed in 25 g of 1-allyl-3-methylimidazolium chloride solution. The mixture was heated and dissolved in an oil bath at 80°C with a mechanical stirring speed of 150 rpm for 2.5 h. The resulting cellulose ionic liquid solution was placed in a vacuum drying oven at 70°C for degassing for 6 h. While still hot, the 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. During this period, the coagulation bath was continuously replaced. The regenerated cellulose film was stretched longitudinally (stretching ratio 10%) and dried in a rapid Kasai paper sheet former (100°C) to obtain a regenerated cellulose film.

[0037] (3) Preparation of all-cellulose-based photonic crystal film: First, fix the regenerated cellulose film in step (2) by using the glass plate clamping method. After removing the bubbles from the cellulose nanocrystal dispersion in step (1), take an appropriate amount of it on the substrate (substrate area cm 2 The film was then placed in a constant temperature and humidity drying oven at 50°C and 50% humidity to obtain a composite circularly polarizing film.

[0038] Comparative Example 1, the preparation conditions of cellulose nanocrystals were changed, and ultrafiltration was performed using filter membranes with different pore sizes to obtain CNCs of different lengths. The different lengths of CNCs will result in different colors of the membranes obtained after self-assembly.

[0039] Example 2: Effects of different regenerated cellulose film preparation conditions on all-cellulose-based photonic crystal films (1) Preparation of cellulose nanocrystals: 10 g of paper strips were dispersed in 60 ml of preheated 64% sulfuric acid solution, stirred at 200 rpm with a magnetic stirrer, and reacted in a 45°C water bath for 2 h to obtain a mixed solution. The obtained mixed solution was diluted 10 times with deionized water, and an 8% sodium hydroxide solution was added to adjust its pH to pH = 5.7 to obtain a neutralized solution. The solution was then ultrafiltered using a 100 nm filter membrane, and pure water was continuously added for desalination until the filtrate conductivity reached 6 μs / cm. The filtrate retained by the filter membrane was then recovered and concentrated by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals.

[0040] (2) Preparation of stretched regenerated cellulose film (carbon disulfide / sodium hydroxide (CS2 / NaOH) system): Soak pulp powder in 18-20% sodium hydroxide solution at 20-25°C for 1-2 hours to generate alkali cellulose. Place the alkali cellulose in a culture dish and expose it to air in a 25°C constant temperature box for 24-48 hours to oxidize and degrade it. Transfer the aged alkali cellulose to a glass bottle with a sealed lid. Add CS2 at 30% of the cellulose mass, seal it, and oscillate at room temperature for 2 hours. After the reaction is completed, the product becomes orange-yellow cellulose xanthate. Slowly add the xanthate to 4% NaOH solution and stir it magnetically until it is completely dissolved to form a viscous viscose liquid. Let the viscose liquid stand for 12-24 hours to mature until the fluidity is stable. Filter the viscose liquid with a filter membrane under reduced pressure to remove insoluble matter. Transfer the filtrate to a centrifuge tube and centrifuge it at low speed (2000 rpm, 5 minutes) to remove bubbles. Pour the viscose liquid onto a clean glass plate and scrape it into a uniform liquid film with a spatula. Immerse the cellulose in a coagulation bath (10% H₂SO₄ + 15% Na₂SO₄, 40°C) for 1-2 minutes to regenerate the cellulose into a solid film. Remove the film with tweezers and rinse in deionized water. Dry the film in an oven at 105°C for 10 minutes to obtain a regenerated cellulose film. Stretch the regenerated cellulose film longitudinally (stretch ratio 17%).

[0041] (3) Preparation of all-cellulose-based photonic crystal film: First, place the regenerated cellulose film in step (2) on a surface dish and fix it. After removing the bubbles from the cellulose nanocrystals in step (1), take an appropriate amount and drop it on the substrate (substrate area cm 2 The film was then placed in an oven at 45°C and dried to obtain a composite circularly polarizing film.

[0042] In Example 2, the preparation method of the base regenerated cellulose film was changed, and the same effect as in Example 1 was achieved. The test results of the prepared all-cellulose-based photonic crystal film through the LCP and PCR films were as follows: Figure 6 As shown in B, the front side of the prepared composite film is left-handed and the back side is right-handed.

[0043] Combining Examples 1 and 2, composite circularly polarizing films were prepared using the same preparation process, but with different cellulose nanocrystal preparation conditions, different pore size filter membranes used during ultrafiltration to obtain CNCs of varying sizes, and different conditions for stretching the regenerated cellulose film. These composite circularly polarizing films are simple to prepare, exhibit enhanced strength and flexibility, maintain structural color, and exhibit a certain degree of stability. Furthermore, the all-cellulose-based photonic crystal film achieves simultaneous reflection of both LCP and RCP light.

[0044] Example 3: Effect of Regenerated Cellulose Film Preparation Conditions on All-cellulose-based Photonic Crystal Films (1) Preparation of cellulose nanocrystals: 10 g of paper strips were dispersed in 70 ml of preheated 64% sulfuric acid solution, stirred at 200 rpm with a magnetic stirrer, and reacted in a 50°C water bath for 1.5 h to obtain a mixed solution. The obtained mixed solution was diluted 10 times with deionized water, and an 8% sodium hydroxide solution was added to adjust its pH to pH = 5.5 to obtain a neutralized solution. Ultrafiltration was then performed using a 10 nm filter membrane, and pure water was continuously added for desalination until the filtrate conductivity reached 6 μs / cm. The filtrate retained by the filter membrane was then recovered and concentrated by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals.

[0045] (2) Preparation of stretched regenerated cellulose membrane (prepared using N-methylmorpholine-N-oxide (NMMO) system): Slowly add pulp powder (8-15 wt%) to a 50% NMMO aqueous solution at 50°C and stir for 1-2 h to form a uniform suspension; slowly evaporate the water in a vacuum drying oven (85-100°C) until the NMMO concentration reaches 77-83% (forming a uniform, transparent, viscous solution). Continue stirring for 1-2 h to ensure complete dissolution (the solution should be amber, transparent, and free of particles); place the solution in an ultrasonicator to remove bubbles (30-60 min). 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 an oven at 60°C to obtain a regenerated cellulose membrane. Stretch the regenerated cellulose membrane longitudinally (stretching ratio 10%).

[0046] (3) Preparation of all-cellulose-based photonic crystal film: First, fix the regenerated cellulose film in step (2), remove the bubbles in the cellulose nanocrystal dispersion in step (1), and take an appropriate amount on the substrate (substrate area cm 2 The film was then placed in a constant temperature and humidity drying oven at 50°C and 50% humidity to obtain a fully cellulose-based photonic crystal film.

[0047] Example 4: Effect of Regenerated Cellulose Film Preparation Conditions on All-cellulose-based Photonic Crystal Films (1) Preparation of cellulose nanocrystals: Take 10g of paper strips and disperse them in 70ml of preheated 64% sulfuric acid solution. Use a magnetic stirrer at 200rpm and react in a 50℃ water bath for 1.5h to obtain a mixed solution. Dilute the obtained mixed solution 10 times with deionized water, add 8% sodium hydroxide solution to adjust its pH to pH=5.5, and obtain a neutralized solution. Then use a 10nm filter membrane for ultrafiltration, and continuously add pure water for desalination until the conductivity of the filtrate is 6us / cm. Then recover the filtrate retained by the filter membrane and concentrate the filtrate by rotary evaporation to a solid content of 3% to obtain cellulose nanocrystals. (2) Preparation of stretched regenerated cellulose membrane (prepared using sodium hydroxide / urea (NaOH / urea) system): The pulp powder was vacuum dried at 60°C 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°C for 2 hours to form a low-temperature transparent solvent. Dry cellulose (4-8wt%) was added to the pre-cooled solvent under a low-temperature environment (-12°C). Vigorously stirred (800-1200rpm) for 1-2 hours until a transparent viscous solution was formed. The temperature was kept low (-12°C) throughout the process to avoid gelation caused by temperature increase. The mixed solution was centrifuged to remove bubbles and poured onto a clean glass plate. The film was coated with a scraper to control the thickness. The cellulose was immediately immersed in a coagulation bath (5% sodium sulfate (Na2SO4) aqueous solution) to regenerate the cellulose. The membrane was transferred to deionized water for soaking to completely remove the residual solvent. The regenerated cellulose membrane was naturally dried at room temperature. The regenerated cellulose membrane was stretched in the longitudinal direction (stretching ratio 10%).

[0048] (3) Preparation of all-cellulose-based photonic crystal film: First, fix the regenerated cellulose film in step (2), remove the bubbles in the cellulose nanocrystal dispersion in step (1), and take an appropriate amount on the substrate (substrate area cm 2 The film was then placed in a constant temperature and humidity drying oven at 50°C and 50% humidity to obtain a fully cellulose-based photonic crystal film.

[0049] Examples 1, 2, 3, and 4 used the same preparation process but different regenerated cellulose membrane preparation conditions. 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), were used to dissolve cellulose in the regenerated cellulose membranes. All of these produced fully cellulose-based photonic crystal films with the same effects as in Example 1. The prepared composite films exhibited left-handed rotation on the front and right-handed rotation on the back. The fully cellulose-based photonic crystal films were simple to prepare, exhibited enhanced strength and flexibility, maintained structural color, and exhibited a certain degree of stability. Furthermore, the fully cellulose-based photonic crystal films simultaneously reflected both LCP and RCP light. The use of an ionic liquid system to prepare regenerated cellulose membranes is more environmentally friendly and has simple operation steps.

[0050] 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 in the scope of protection of the present invention.

Claims

1. A fully cellulose-based photonic crystal film, characterized in that: The invention comprises 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. Preferably, the all-cellulose-based photonic crystal film can be controlled to be left-handed and / or right-handed.

2. The all-cellulose-based photonic crystal film according to claim 1, characterized in that: The cellulose nanocrystals are purified and graded by ultrafiltration, the ultrafiltration is an ultrafiltration membrane or an ultrafiltration filament, preferably, the pore size of the ultrafiltration membrane is 5-600nm, preferably, the pore size is 10-500nm, preferably, the cellulose nanocrystals are prepared by acid hydrolysis.

3. The all-cellulose-based photonic crystal film according to claim 1, characterized in that: The regenerated cellulose film is a stretched regenerated cellulose film and / or an unstretched cellulose film. Preferably, the regenerated cellulose film is stretched in the longitudinal direction with a stretching ratio of 5-15% to obtain a stretched regenerated cellulose film. Preferably, the stretching ratio is 10%.

4. The all-cellulose-based photonic crystal film according to claim 3, characterized in that: The regenerated cellulose membrane uses one or more of the following cellulose dissolving systems to dissolve the cellulose-containing material: N-methylmorpholine-N-oxide (NMMO) system, ionic liquid system, sodium hydroxide / urea (NaOH / urea) system, carbon disulfide / sodium hydroxide (CS2 / NaOH) system, Preferably, an ionic liquid is used to dissolve the cellulose-containing material. Preferably, the ionic liquid is a 1-allyl-3-methylimidazolium chloride solution.

5. A method for preparing the all-cellulose-based photonic crystal film according to any one of claims 1 to 4, characterized in that: The steps include: S1. Preparation of cellulose nanocrystals: A cellulose-containing material is mixed with 60% to 70% sulfuric acid at a material-liquid ratio of 1 g: (5 to 8) mL, reacted, and the resulting mixed solution is diluted and the pH is adjusted to 5 to 6.5 to obtain a neutralized solution. The neutralized solution is ultrafiltered, and water is added during the ultrafiltration process until the filtrate has a conductivity of 4 to 10 μS / cm. The filtrate retained by the filter membrane is recovered and concentrated to obtain cellulose nanocrystals. S2. Preparation of regenerated cellulose membrane: a cellulose-containing material and an ionic liquid were mixed and impregnated at a material-liquid ratio of 1 g:(19-30) g, the mixed liquid was heated to dissolve, the obtained cellulose ionic liquid was defoamed, and a film was formed by blade coating, which was then washed, regenerated, and dried to obtain a regenerated cellulose membrane; S3. Preparation of all-cellulose-based photonic crystal film: fix the regenerated cellulose film obtained in S2, remove bubbles from the cellulose nanocrystals in step S1, take an appropriate amount, evenly spread it on the regenerated cellulose film substrate in S1, and dry it to obtain the all-cellulose-based photonic crystal film.

6. The method according to claim 5, characterized in that The ultrafiltration in S1 is an ultrafiltration membrane or an ultrafiltration filament. Preferably, the pore size of the ultrafiltration membrane is 5 to 600 nm, preferably, the pore size is 10 to 500 nm; the reaction conditions in S1 are 40 to 60° C. for 1 to 3 hours, a dilution ratio of 5 to 15 times, and pH adjustment using sodium hydroxide with a mass fraction of 4% to 10%. The concentration is to a solid content of more than 2%, preferably, a solid content of more than 3%.

7. The method according to claim 5, characterized in that The regenerated cellulose film in S2 is a stretched regenerated cellulose film and / or an unstretched regenerated cellulose film. Preferably, the regenerated cellulose film is stretched in the longitudinal direction with a stretching ratio of 5-10% to obtain a stretched regenerated cellulose film. Preferably, the stretching ratio is 15%.

8. The method according to claim 7, characterized in that The ionic liquid in S2 is 1-allyl-3-methylimidazolium chloride solution, the heating dissolution condition is 60-100°C, the dissolution time is 2-4h, and the defoaming condition in S2 is vacuuming at 50-80°C, preferably, vacuum defoaming at 60-80°C.

9. The method according to claim 4, characterized in that The regenerated cellulose film in S3 is fixed by a method selected from glass plate clamping, vacuum adsorption fixation, and temporary adhesive fixation, and the uniform spreading is carried out by a flow-through method, a blade coating method, a spin coating method, or a spray coating method. Preferably, the ratio of the base area to the cellulose nanocrystals in the spreading in S3 is (2-6) cm 2 :1ml.

10. The method according to any one of claims 5 to 9, characterized in that: The cellulose-containing material is selected from one or more of cotton, refined cotton, chemical pulp, dissolving pulp, and microcrystalline cellulose.

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