Monodisperse lignin colloidal spheres, preparation method thereof, and application thereof as structural color material

Abstract

The present invention discloses a monodisperse lignin colloidal sphere, a preparation method, and an application as a structural color material. The method comprises the following steps: first, lignin is steamed under alkaline conditions, and then ultrafiltration is performed to remove low-molecular-weight and high-molecular-weight lignin to obtain a uniform medium-molecular-weight lignin, which is then reacted with a glycidyl ether compound to further regulate polar groups such as hydroxyl groups, and then short-chain alcohols are used to separate alcohol-insoluble lignin. The alcohol-insoluble lignin is dissolved in an organic solvent-water mixed solvent, water is added dropwise to cause phase separation, and finally, centrifugal sedimentation is performed to obtain monodisperse lignin colloidal spheres. The preparation process of the method of the present invention is green and environmentally friendly, the organic solvents used can be recycled and reused, and the product yield reaches more than 70%. The obtained monodisperse lignin colloidal spheres can be used as a structural color material.

Description

Technical Field

[0001] The invention belongs to the field of derivatives of natural polymer compounds, and particularly relates to monodisperse lignin colloid spheres, a preparation method thereof, and application thereof as structural color materials. Background Art

[0002] Lignin is a high-molecular polymer second only to cellulose in plant abundance and the only aromatic biomass resource in nature. In industries like pulp and papermaking and biorefining, lignin is often discharged as a byproduct or burned as a low-value fuel, significantly wasting resources and placing significant pressure on ecological conservation. Therefore, achieving its effective and high-value utilization is key to promoting the development of forest biomass and improving the economic benefits of related industries.

[0003] Lignin colloidal spheres possess UV and aging resistance, as well as low cost, good biocompatibility, and natural biodegradability. They demonstrate significant potential for application in biomedicine, food, and daily chemical products, and have garnered widespread attention and research. The size distribution of the colloids significantly influences their performance, with significant differences in physicochemical properties between spheres of different sizes. Uniformly sized monodisperse colloids can be assembled into ordered structures, attracting significant attention due to their potential applications in sensors, coatings, and cosmetics. For example, arranging monodisperse colloids into long-range or short-range ordered arrays with periodically varying refractive indices can induce Bragg reflection of specific wavelengths of light, resulting in unique optical properties and the potential for the preparation of long-lasting, stable functional structural color materials. Currently, the raw materials for the preparation of structural color materials are mostly synthetic polymers such as polystyrene and polymethyl methacrylate and inorganic substances such as silicon oxide and titanium dioxide (Goerlitzer ESA, Klupp Taylor RN, Vogel N. Bioinspired Photonic Pigments from Colloidal Self-Assembly [J]. Advanced Materials, 2018, 30 (28): 1706654.). Most of these raw materials have poor biocompatibility, and the structural color formed becomes white as the thickness of the stacking increases. Lignin not only has good biocompatibility, but also has a rich content of functional groups. Its own chromophores can effectively improve the saturation of the structural color. It is a raw material for the preparation of bio-based structural color materials with great application potential.

[0004] The polydispersity index (PDI) of particle size distribution is a key factor affecting the saturation of structural color. The lower the PDI, the higher the saturation, and monodisperse colloidal spheres (PDI < 0.10) are a necessary condition for preparing structural color materials. However, due to the heterogeneity of the molecular weight and polar functional groups of industrial lignin, the lignin colloidal spheres prepared by the prior art have problems such as a wide particle size distribution and heterogeneous polarity. The preparation of monodisperse lignin colloidal spheres remains a challenge. For example, Chinese patent application CN110452396A discloses a method for preparing lignin micro-nanospheres, which is prepared by dissolving lignin in an organic solvent, adding water to obtain a colloidal solution, and further evaporating the organic solvent to obtain lignin micro-nanospheres. This method is simple to operate and environmentally friendly, but the resulting colloidal spheres have a wide size distribution due to the lack of separation, purification or modification of lignin, and do not have the application characteristics of structural color materials. In order to regulate the polarity of lignin, Chinese patent CN103242555B acetylated alkali lignin and then added water to change the microphase environment to prepare acetylated lignin colloid spheres, which were successfully applied to the field of pesticide embedding. In this method, the alkali lignin was only acetylated and modified without separation and purification. The molecular weight distribution was uneven, and the size distribution of the prepared lignin colloid spheres was wide, making it difficult to prepare structural color materials.

[0005] In response to the problem that the wide molecular weight distribution of lignin limits its application field, some scholars have adopted a solvent step-by-step purification and separation method to separate lignin components with uniform molecular weight, which are further self-assembled into lignin colloidal spheres. For example, Chuanling Si et al. used Kraft lignin as raw material, used different concentrations of ethanol / water solution for step-by-step purification and separation, and obtained three lignin components with different molecular weights, which were further dissolved in ethanol and added water for self-assembly to prepare lignin colloidal spheres (Cao QW, Wu Q., Si CL, et al. Size-Controlled Lignin Nanoparticles for Tuning the Mechanical Properties of Poly (Vinyl Alcohol) [J]. Industrial Crops and Products, 2021, 172: 114012.). Since the polar functional groups of lignin were not regulated, the polydispersity coefficient value was between 0.156 and 0.865, which still could not meet the preparation conditions of structural color. Jae Hoon Lee et al. used organic solvents such as ethyl acetate, 2-butanone, methanol, acetone and 95% dioxane to sequentially purify and separate commercial Soda lignin, obtaining 6 lignin components with significantly different molecular weights, which were then dissolved in tetrahydrofuran and self-assembled with water to obtain lignin colloid spheres with a PDI value of 0.30 to 1.0 (Lee JH, Park SY, Choi JW, et al. Investigation of Molecular Size Effect on The Formation of Lignin Nanoparticles by Nanoprecipitation [J]. Applied Sciences, 2020, 10 (14): 4910.). Although the above-mentioned method of step-by-step purification and separation of lignin raw materials to regulate molecular weight has improved the size uniformity of the colloid spheres to a certain extent, its PDI value is still large (greater than 0.10), which does not meet the standard of monodisperse colloid spheres and cannot be used to prepare structural color materials.

[0006] Jingyu Wang et al. found that the intermolecular interactions of lignin with moderate polarity are more uniform, which is the key to preparing monodisperse colloidal spheres. They used anhydrous ethanol and acetone to perform two-step purification and separation of enzymatic lignin to obtain a lignin component with moderate polarity. They also used water-added self-assembly to prepare monodisperse lignin colloidal spheres (PDI of 0.04-0.13) for the first time, and successfully prepared structural color materials (Wang JY, Chen WH, Yang DJ, et al., Monodispersed Lignin Colloidal Spheres with Tailorable Sizes for Bio-Photonic Materials[J]. Small 2022, 18: e2200671.). However, this method requires the use of highly volatile organic solvents such as acetone, which makes it difficult to scale up production. At the same time, the molecular weight of lignin is not regulated, and the prepared lignin colloidal spheres have poor structural stability, especially the two-step purification and separation, which results in low yield (less than 10%) and low color saturation.

[0007] In summary, due to the complex components, wide molecular weight distribution and uneven polarity of industrial lignin, the lignin colloidal spheres prepared by existing technologies or processes still have key problems such as wide size distribution, poor surface property uniformity and low yield, which makes it difficult to scale up production and application. Summary of the Invention

[0008] In order to address the shortcomings and deficiencies of the prior art, the primary purpose of the present invention is to provide a method for preparing monodisperse lignin colloidal spheres.

[0009] The preparation of lignin colloidal spheres mainly includes two processes: the depolymerization of lignin aggregates and the reaggregation of molecules after depolymerization. First, the lignin raw material needs to be fully dissolved and dispersed in a certain pure solvent or mixed solvent system to reduce the intermolecular attraction of lignin molecules, so that the originally randomly aggregated lignin molecules are depolymerized and dissolved. Then, by changing the external environment (such as adding a poor solvent or reducing a good solvent), the intermolecular attraction of lignin molecules is increased, thereby causing the lignin molecules to gradually aggregate and form colloidal spheres with a stable and regular supramolecular structure. In the above process, the change in the intermolecular force of lignin is the essential reason for driving the formation of its supramolecular structure, and the magnitude of each intermolecular force is closely related to the strength of its molecular polarity. Therefore, the polarity difference between lignin molecules is the main reason for the wide size distribution and poor uniformity of lignin colloidal spheres.

[0010] The monodisperse lignin colloidal spheres described in the present invention are prepared by homogenizing and regulating polar functional groups and molecular weight. Lignin is first digested under alkaline conditions to obtain an alkaline aqueous solution of lignin. Ultrafiltration is then performed to separate and purify low- and high-molecular-weight lignin, resulting in a uniform medium-molecular-weight lignin. This lignin is then reacted with a glycidyl ether compound to further regulate polar groups such as hydroxyl groups. Short-chain alcohols are then used to purify and separate the resulting components, yielding components with uniform molecular weight and polarity. These components are then dissolved in an organic solvent-water mixture, and water is added dropwise to induce phase separation. Finally, centrifugation is performed to obtain the monodisperse lignin colloidal spheres. The entire preparation process is environmentally friendly, the organic solvents used are recyclable, and the product yield exceeds 70%.

[0011] Another object of the present invention is to provide monodisperse lignin colloidal spheres prepared by the above method. The resulting monodisperse lignin colloidal spheres have a PDI value of 0.01 to 0.10, and the particle size can be adjusted to 170 to 260 nm by varying the lignin concentration. As a structural color material, the lignin colloidal spheres can exhibit blue, green, or red colors.

[0012] Another object of the present invention is to provide the use of the aforementioned monodisperse lignin colloidal spheres as a structural color material. Compared to traditional raw materials (such as polystyrene and silica), lignin not only has excellent biocompatibility but also contains a rich content of functional groups. Its inherent chromophores can effectively enhance the saturation of the structural color, preventing it from fading to white with increasing stacking thickness.

[0013] The purpose of the present invention is achieved through the following technical solutions:

[0014] A method for preparing monodisperse lignin colloidal spheres comprises the following steps:

[0015] (1) Cooking lignin under alkaline conditions, then separating and removing low molecular weight and high molecular weight lignin to obtain uniform medium molecular weight lignin;

[0016] (2) reacting medium molecular weight lignin with glycidyl ether compounds to obtain etherified modified lignin;

[0017] (3) dispersing and dissolving the etherified modified lignin in a short-chain alcohol, separating, and obtaining the alcohol-insoluble lignin;

[0018] (4) Dissolve the alcohol-insoluble lignin in an organic solvent-water mixed solution, add water dropwise, and centrifuge to obtain monodisperse lignin colloidal spheres.

[0019] Preferably, the alkaline condition in step (1) refers to an aqueous solution with a pH of 9 to 11; the mass concentration of the lignin in the aqueous solution with a pH of 9 to 11 is 10 to 30%; more preferably, the aqueous solution with a pH of 9 to 11 is a NaOH aqueous solution with a pH of 9 to 11.

[0020] Preferably, the lignin in step (1) is at least one of enzymatic lignin in a biorefining industrial process, organic solvent lignin in a biorefining industrial process, wood pulp alkali lignin from alkaline pulping black liquor, bamboo pulp alkali lignin from alkaline pulping black liquor, and wheat straw alkali lignin from alkaline pulping black liquor.

[0021] Preferably, the cooking temperature in step (1) is 110-150° C. and the cooking time is 1-3 hours.

[0022] Preferably, the method for separating and removing low molecular weight and high molecular weight lignin in step (1) is: first separating with an ultrafiltration membrane with a molecular weight cutoff of 10,000, then separating the obtained filtrate with an ultrafiltration membrane with a molecular weight cutoff of 2,000, and finally concentrating and drying; the drying conditions are drying at 50-80°C for 12-24 hours.

[0023] Preferably, the weight average molecular weight of the uniform medium molecular weight lignin in step (1) is in the range of 2000 to 10000.

[0024] Preferably, the mass ratio of the medium molecular weight lignin to the glycidyl ether compound in step (2) is 100:5-10.

[0025] Preferably, the glycidyl ether compound in step (2) is at least one of allyl glycidyl ether, n-butyl glycidyl ether and octyl glycidyl ether.

[0026] Preferably, the reaction temperature in step (2) is 60-80° C. and the reaction time is 1-3 h.

[0027] Preferably, the reaction medium in step (2) is an aqueous solution with a pH of 9 to 11, and the mass concentration of the medium molecular weight lignin in the aqueous solution with a pH of 9 to 11 is 10 to 30%; more preferably, the aqueous solution with a pH of 9 to 11 is a NaOH aqueous solution with a pH of 9 to 11.

[0028] Preferably, after the reaction in step (2) is completed, a drying treatment is performed, and the drying conditions are drying at 50-80° C. for 12-24 hours.

[0029] Preferably, the short-chain alcohol in step (3) is at least one of methanol, ethanol, n-propanol and isopropanol.

[0030] Preferably, the mass ratio of the etherified modified lignin to the short-chain alcohol in step (3) is 1:10-20.

[0031] Preferably, the separation in step (3) is followed by drying, and the drying conditions are 50-80° C. for 12-24 hours.

[0032] Preferably, in the organic solvent-water mixed solution of step (4), the organic solvent is at least one of tetrahydrofuran, acetone, ethanol and dioxane; and the volume ratio of the organic solvent to water is 1:0.10-0.45.

[0033] Preferably, the concentration of the alcohol-insoluble lignin in the organic solvent-water mixed solution in step (4) is 1 to 100 g / L; more preferably 1 to 5 g / L.

[0034] Preferably, the volume ratio of the water added dropwise in step (4) to the volume of the organic solvent in the organic solvent-water mixed solution is 3 to 30:1.

[0035] Preferably, the rate of adding water in step (4) is 0.10 to 50 mL / min; more preferably 2.4 to 5 mL / min.

[0036] Preferably, the centrifugal separation conditions in step (4) are 6000-10000 rpm and the time is 10-30 min.

[0037] Preferably, in order to achieve the particle size regulation of monodisperse lignin colloidal spheres, one or more conditions including the initial concentration of lignin in the organic solvent-water mixed solvent (1-100 g / L) and the water addition rate (0.10-50 mL / min) can be changed simultaneously to obtain monodisperse lignin colloidal spheres of different particle sizes, so that the particle size can be adjusted in the range of 50-2000 nm.

[0038] The monodisperse lignin colloid spheres prepared by the preparation method are described.

[0039] The monodisperse lignin colloid spheres have uniform size distribution, a particle size of 50 to 2000 nm, and a PDI of 0.01 to 0.10.

[0040] The above-mentioned monodisperse lignin colloidal spheres are used as structural color materials.

[0041] The application is preferably the application of monodisperse lignin colloidal spheres as building blocks in structural color materials.

[0042] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0043] (1) The process of the present invention can effectively separate lignin components with uniform and moderate polarity, solving the problem of large polarity differences between lignin molecules. At the same time, it does not require the use of a large amount of organic solvents. The operation is simple, the applicability is strong, and the product yield in each step is high (greater than 70%), which is conducive to industrial production.

[0044] (2) The monodisperse lignin colloidal spheres of the present invention have an extremely narrow size distribution, with a PDI range of 0.01 to 0.10. By changing the initial concentration of the lignin solution, the type of solvent, and other conditions, their size can be precisely controlled within the range of 50 nm to 2000 nm. By constructing their short-range ordered structures, lignin-based structural color materials can be prepared, and the resulting products can effectively broaden the application of lignin colloidal spheres in structural color materials. BRIEF DESCRIPTION OF THE DRAWINGS

[0045] Figure 1 This is the SEM image of the monodisperse lignin colloidal spheres obtained in Example 1.

[0046] Figure 2 This is the particle size distribution diagram of the monodisperse lignin colloidal spheres obtained in Example 1 (the particle sizes are 170 nm, 220 nm, and 260 nm, respectively, for preparing blue, green, and red lignin structural color coatings, respectively).

[0047] Figure 3 This is a photo of the lignin structural color coating prepared from the monodisperse lignin colloidal spheres obtained in Example 1. DETAILED DESCRIPTION

[0048] The present invention will be described in further detail below with reference to examples and drawings, but the embodiments of the present invention are not limited thereto.

[0049] In the examples of the present invention, if the specific conditions are not specified, the experiments were carried out according to conventional conditions or the conditions recommended by the manufacturer. All raw materials and reagents used without specifying the manufacturer are conventional products that can be purchased from the market.

[0050] Scanning electron microscopy (SEM) analysis of the monodisperse lignin colloids prepared in the Examples and Comparative Examples was performed using a Hitachi UHR FE-SEM SU8220 scanning electron microscope. Particle size was measured using a Zeta PLAS laser particle size analyzer from Brookerhaven, USA. The particle size distribution of the lignin colloids in water at 20°C was determined. Each sample was tested three times, and the average particle size was calculated.

[0051] Example 1

[0052] 100 g of enzymatically hydrolyzed lignin was weighed and added to 900 mL of a NaOH aqueous solution with a pH of 9. The solution was cooked at 110 ° C for 1 h, and then separated using an ultrafiltration membrane with a molecular weight cutoff of 10,000. The filtrate was then passed through an ultrafiltration membrane with a molecular weight cutoff of 2,000 to obtain a concentrated solution. After drying at 50 ° C for 12 h, medium molecular weight lignin was obtained.

[0053] 100 g of the above-mentioned medium molecular weight lignin was weighed and added to 900 mL of a NaOH aqueous solution with a pH of 9, 5 g of allyl glycidyl ether was added, and the mixture was reacted at 60°C for 1 h. After centrifugation, the precipitate was taken and dried at 50°C for 12 h to obtain etherified modified lignin.

[0054] 100 g of etherified modified lignin was weighed and dispersed in 1000 g of ethanol. After centrifugation, the precipitate was taken and dried at 50° C. for 12 h to obtain alcohol-insoluble lignin.

[0055] 0.2 g of alcohol-insoluble lignin was weighed and dissolved in 200 mL of an acetone-water mixture (138 mL of acetone). 414 mL of water was added to the mixture at a rate of 5.0 mL / min. The mixture was then centrifuged at 6000 rpm for 10 minutes. The resulting precipitate (yield: 72%) was collected, representing monodisperse lignin colloidal spheres. Characterization using SEM and laser particle size analysis revealed an average particle size of 170 nm and a PDI of 0.03.

[0056] The average particle size of the monodisperse lignin colloid spheres can be controlled within the range of 50 to 1000 nm by changing the initial concentration of alcohol-insoluble lignin and the water dropping rate, and the PDI range is 0.02 to 0.07.

[0057] The following examples take the preparation of 220nm and 260nm monodisperse lignin colloidal spheres as examples:

[0058] 0.4 g of alcohol-insoluble lignin was weighed and dissolved in 200 mL of an acetone-water mixture (138 mL of acetone). 414 mL of water was added to the mixture at a rate of 3.7 mL / min. The mixture was then centrifuged at 6000 rpm for 10 minutes. The resulting precipitate (yield 72%) was collected, representing monodisperse lignin colloidal spheres. Scanning electron microscopy (SEM) and laser particle size analysis revealed an average particle size of 220 nm and a particle size distribution index (PDI) of 0.05.

[0059] 0.4 g of alcohol-insoluble lignin was weighed and dissolved in 200 mL of an acetone-water mixture (138 mL of acetone). 414 mL of water was added to the mixture at a rate of 2.4 mL / min. The mixture was then centrifuged at 6000 rpm for 10 minutes. The resulting precipitate (yield 74%) was collected, representing monodisperse lignin colloidal spheres. Scanning electron microscopy (SEM) and laser particle size analysis revealed an average particle size of 260 nm and a particle size distribution index (PDI) of 0.06.

[0060] Preparation method of lignin-based structural color materials: Monodisperse lignin colloidal spheres are dispersed in water. Use a disposable plastic dropper to draw 1 to 3 mL of 170 nm, 220 nm, and 260 nm monodisperse lignin colloidal sphere suspensions, respectively. Drop them onto a smooth and flat black background plate and spread them evenly. Blue, green, and red lignin-based structural color materials can be obtained, respectively.

[0061] Example 2

[0062] 100 g of wheat straw alkali lignin was weighed and added to 233 mL of a NaOH aqueous solution with a pH of 11. The mixture was steamed at 150°C for 3 h, and then separated using an ultrafiltration membrane with a molecular weight cutoff of 10,000. The filtrate was then passed through an ultrafiltration membrane with a molecular weight cutoff of 2,000 to obtain a concentrated solution. After drying at 80°C for 24 h, a medium molecular weight lignin was obtained.

[0063] 100 g of the above-mentioned medium molecular weight lignin was weighed and added to 233 mL of a NaOH aqueous solution with a pH of 11, 10 g of n-butyl glycidyl ether was added, and the mixture was reacted at 80°C for 3 h. After centrifugation, the precipitate was taken and dried at 80°C for 24 h to obtain etherified modified lignin.

[0064] 100 g of etherified modified lignin was weighed and dispersed in 2000 g of methanol. After centrifugation, the precipitate was taken and dried at 80° C. for 24 h to obtain alcohol-insoluble lignin.

[0065] 1.0 g of alcohol-insoluble lignin was weighed and dissolved in 200 mL of a tetrahydrofuran-water mixture (the volume of tetrahydrofuran was 182 mL). 5460 mL of water was added to the mixture at a rate of 0.1 mL / min. The mixture was then centrifuged at 10,000 rpm for 30 minutes. The resulting precipitate (yield 78%) was collected, representing monodisperse lignin colloidal spheres. Scanning electron microscopy (SEM) and laser particle size analysis revealed an average particle size of 550 nm and a particle size distribution index (PDI) of 0.06.

[0066] The average particle size of the monodisperse lignin colloid spheres can be controlled within the range of 100 to 2000 nm by changing the initial concentration of alcohol-insoluble lignin and the water dropping rate, and the PDI range is 0.02 to 0.10.

[0067] Example 3

[0068] Weigh 100g of wood pulp alkaline lignin and add it to 400mL of NaOH aqueous solution with a pH of 10. Cook it at high temperature at 120℃ for 2h, then separate it with an ultrafiltration membrane with a molecular weight cutoff of 10,000. The filtrate is then passed through an ultrafiltration membrane with a molecular weight cutoff of 2000 to obtain a concentrated solution. After drying at 70℃ for 24h, medium molecular weight lignin is obtained.

[0069] 100 g of the above-mentioned medium molecular weight lignin was weighed and added to 400 mL of a NaOH aqueous solution with a pH of 10, 7 g of octyl glycidyl ether was added, and the mixture was reacted at 70°C for 2 h. After centrifugation, the precipitate was taken and dried at 70°C for 24 h to obtain etherified modified lignin.

[0070] 100 g of etherified modified lignin was weighed and dispersed in 1500 g of n-propanol. After centrifugation, the precipitate was taken and dried at 70° C. for 12 h to obtain alcohol-insoluble lignin.

[0071] 0.2 g of alcohol-insoluble lignin was weighed and dissolved in 200 mL of an ethanol-water mixture (160 mL of ethanol). 3200 mL of water was added to the mixture at a rate of 4.0 mL / min. The mixture was then centrifuged at 8000 rpm for 20 minutes. The resulting precipitate (yield 71%) was collected. Characterization using SEM and a laser particle size analyzer revealed an average particle size of 200 nm and a PDI of 0.05.

[0072] The average particle size of the monodisperse lignin colloid spheres can be controlled within the range of 50 to 2000 nm by changing the initial concentration of alcohol-insoluble lignin and the water dropping rate, and the PDI range is 0.01 to 0.07.

[0073] Example 4

[0074] 100 g of bamboo pulp alkali lignin was weighed and added to 900 mL of a NaOH aqueous solution with a pH of 11. The mixture was steamed at 110 ° C for 1 h, and then separated using an ultrafiltration membrane with a molecular weight cutoff of 10,000. The filtrate was then passed through an ultrafiltration membrane with a molecular weight cutoff of 2,000 to obtain a concentrated solution. After drying at 70 ° C for 24 h, a medium molecular weight lignin was obtained.

[0075] 100 g of the above-mentioned medium molecular weight lignin was weighed and added to 400 mL of a NaOH aqueous solution with a pH of 10, 6 g of allyl glycidyl ether was added, and the mixture was reacted at 70°C for 1 h. After centrifugation, the precipitate was taken and dried at 70°C for 24 h to obtain etherified modified lignin.

[0076] 100 g of etherified modified lignin was weighed and dispersed in 1500 g of isopropanol. After centrifugation, the precipitate was taken and dried at 60° C. for 12 h to obtain alcohol-insoluble lignin.

[0077] 20g of alcohol-insoluble lignin was weighed and dissolved in 200mL of an acetone-water mixture (140mL of acetone). 700mL of water was added to the mixture at a rate of 50mL / min. The mixture was then centrifuged at 10,000rpm for 20min. The resulting precipitate (yield: 73%) was collected, representing monodisperse lignin colloidal spheres. Characterization using SEM and laser particle size analysis revealed an average particle size of 480nm and a PDI of 0.07.

[0078] The average particle size of the monodisperse lignin colloid spheres can be controlled within the range of 100 to 1800 nm by changing the initial concentration of alcohol-insoluble lignin and the water dropping rate, and the PDI range is 0.01 to 0.08.

[0079] Example 5

[0080] Weigh 100g of organic solvent lignin and add it to 900mL of NaOH aqueous solution with a pH of 10. Cook it at high temperature at 110℃ for 1h, then separate it with an ultrafiltration membrane with a molecular weight cutoff of 10,000. The filtrate is then passed through an ultrafiltration membrane with a molecular weight cutoff of 2000 to obtain a concentrated solution. After drying at 70℃ for 24h, medium molecular weight lignin is obtained.

[0081] 100 g of the above-mentioned medium molecular weight lignin was weighed and added to 300 mL of a NaOH aqueous solution with a pH of 11, 8 g of allyl glycidyl ether was added, and the mixture was reacted at 70°C for 2 h. After centrifugation, the precipitate was taken and dried at 70°C for 24 h to obtain etherified modified lignin.

[0082] 100 g of etherified modified lignin was weighed and dispersed in 1200 g of ethanol. After centrifugation, the precipitate was taken and dried at 60° C. for 12 h to obtain alcohol-insoluble lignin.

[0083] 20 g of alcohol-insoluble lignin was dissolved in 200 mL of a dioxane-water mixture (the volume of dioxane was 138 mL). 4140 mL of water was added to the mixture at a rate of 30 mL / min. The mixture was then centrifuged at 10,000 rpm for 30 minutes. The resulting precipitate (yield 78%) was collected, representing monodisperse lignin colloidal spheres. Characterization using SEM and laser particle size analysis revealed an average particle size of 540 nm and a PDI of 0.06.

[0084] The average particle size of the monodisperse lignin colloid spheres can be controlled within the range of 100 to 1600 nm by changing the initial concentration of alcohol-insoluble lignin and the water dropping rate, and the PDI range is 0.01 to 0.07.

[0085] Example 6

[0086] Weigh 50g of enzymatic lignin and 50g of organic solvent lignin and add them to 300mL of NaOH aqueous solution with a pH of 9. Cook at high temperature at 110℃ for 2h, then separate with an ultrafiltration membrane with a molecular weight cutoff of 10,000. The obtained filtrate is then passed through an ultrafiltration membrane with a molecular weight cutoff of 2,000 to obtain a concentrated solution. After drying at 80℃ for 12h, medium molecular weight lignin is obtained.

[0087] 100 g of the above-mentioned medium molecular weight lignin was weighed and added to 400 mL of a NaOH aqueous solution with a pH of 10, 5 g of n-butyl glycidyl ether was added, and the mixture was reacted at 70°C for 3 h. After centrifugation, the precipitate was taken and dried at 70°C for 24 h to obtain etherified modified lignin.

[0088] 100 g of etherified modified lignin was weighed and dispersed in 1000 g of n-propanol. After centrifugation, the precipitate was taken and dried at 60° C. for 24 h to obtain alcohol-insoluble lignin.

[0089] 0.6 g of alcohol-insoluble lignin was weighed and dissolved in 200 mL of an acetone-water mixture (182 mL of acetone). 546 mL of water was added to the mixture at a rate of 4 mL / min. The mixture was then centrifuged at 10,000 rpm for 20 minutes. The resulting precipitate (yield: 71%) was collected, representing monodisperse lignin colloidal spheres. Scanning electron microscopy (SEM) and laser particle size analysis revealed an average particle size of 320 nm and a particle size distribution index (PDI) of 0.07.

[0090] The average particle size of the monodisperse lignin colloid spheres can be controlled within the range of 100 to 1200 nm by changing the initial concentration of alcohol-insoluble lignin and the water dropping rate, and the PDI range is 0.01 to 0.08.

[0091] Comparative Example 1 (Compared with Example 1, colloidal spheres were prepared directly using lignin raw material)

[0092] 0.2 g of enzymatically hydrolyzed lignin was weighed and dissolved in 200 mL of an acetone-water mixture (138 mL of acetone). 414 mL of water was added to the mixture at a rate of 5.0 mL / min. The mixture was then centrifuged at 6000 rpm for 10 minutes. The resulting precipitate (85% yield) was collected. Characterization using SEM and a laser particle size analyzer revealed an average particle size of 160 nm and a PDI of 0.68.

[0093] By changing the initial concentration of enzymatically hydrolyzed lignin and the dripping rate of water, the average particle size of the colloidal spheres can be controlled within the range of 100 to 900 nm, and the PDI range is 0.60 to 0.88.

[0094] Comparative Example 2 (Compared with Example 1, direct etherification modification of lignin raw material)

[0095] 100 g of enzymatically hydrolyzed lignin was weighed and added to 900 mL of a NaOH aqueous solution with a pH of 9, 5 g of allyl glycidyl ether was added, and the mixture was reacted at 60°C for 1 h. After centrifugation, the precipitate was taken and dried at 50°C for 12 h to obtain etherified modified lignin.

[0096] 100 g of etherified modified lignin was weighed and dispersed in 1000 g of ethanol. After centrifugation, the precipitate was taken and dried at 50° C. for 12 h to obtain alcohol-insoluble lignin.

[0097] 0.2 g of alcohol-insoluble lignin was weighed and dissolved in 200 mL of an acetone-water mixture (138 mL of acetone). 414 mL of water was added to the mixture at a rate of 5.0 mL / min. The mixture was then centrifuged at 6000 rpm for 10 minutes. The resulting precipitate (80% yield) was collected. Characterization using SEM and a laser particle size analyzer revealed an average particle size of 165 nm and a PDI of 0.80.

[0098] The average particle size of the colloidal spheres can be controlled within the range of 200 to 900 nm by changing the initial concentration of alcohol-insoluble lignin and the water dropping rate, and the PDI range is 0.30 to 0.85.

[0099] Comparative Example 3 (Compared with Example 1, only using an ultrafiltration membrane with a molecular weight cut-off of 10,000 to separate lignin)

[0100] 100 g of enzymatically hydrolyzed lignin was weighed and added to 900 mL of a NaOH aqueous solution with a pH of 9. The mixture was cooked at 110°C for 1 h, and then separated using an ultrafiltration membrane with a molecular weight cutoff of 10,000. The filtrate was dried at 50°C for 12 h to obtain lignin with a molecular weight of less than 10,000.

[0101] 100 g of the above-mentioned lignin with a molecular weight of less than 10,000 was weighed and added to 900 mL of a NaOH aqueous solution with a pH of 9, 5 g of allyl glycidyl ether was added, and the mixture was reacted at 60° C. for 1 h. After centrifugation, the precipitate was taken and dried at 50° C. for 12 h to obtain etherified modified lignin.

[0102] 100 g of etherified modified lignin was weighed and dispersed in 1000 g of ethanol. After centrifugation, the precipitate was taken and dried at 50° C. for 12 h to obtain alcohol-insoluble lignin.

[0103] 0.2 g of alcohol-insoluble lignin was weighed and dissolved in 200 mL of an acetone-water mixture (138 mL of acetone). 414 mL of water was added to the mixture at a rate of 5.0 mL / min. The mixture was then centrifuged at 6000 rpm for 10 minutes. The resulting precipitate (83% yield) was collected. Characterization using SEM and a laser particle size analyzer revealed an average particle size of 200 nm and a particle size distribution index (PDI) of 0.93.

[0104] The average particle size of the colloidal spheres can be controlled within the range of 130 to 920 nm by changing the initial concentration of alcohol-insoluble lignin and the water dropping rate, and the PDI range is 0.35 to 0.98.

[0105] Comparative Example 4 (Compared with Example 1, only using an ultrafiltration membrane with a molecular weight cut-off of 2000 to separate lignin)

[0106] 100 g of enzymatically hydrolyzed lignin was weighed and added to 900 mL of a NaOH aqueous solution with a pH of 9. The mixture was steamed at 110 °C for 1 h, and then separated using an ultrafiltration membrane with a molecular weight cutoff of 2000. The concentrated solution was dried at 50 °C for 12 h to obtain lignin with a molecular weight greater than 2000.

[0107] 100 g of the above-mentioned lignin with a molecular weight greater than 2000 was weighed and added to 900 mL of a NaOH aqueous solution with a pH of 9, 5 g of allyl glycidyl ether was added, and the mixture was reacted at 60° C. for 1 h. After centrifugation, the precipitate was taken and dried at 50° C. for 12 h to obtain etherified modified lignin.

[0108] 100 g of etherified modified lignin was weighed and dispersed in 1000 g of ethanol. After centrifugation, the precipitate was taken and dried at 50° C. for 12 h to obtain alcohol-insoluble lignin.

[0109] 0.2 g of alcohol-insoluble lignin was weighed and dissolved in 200 mL of an acetone-water mixture (138 mL of acetone). 414 mL of water was added to the mixture at a rate of 5.0 mL / min. The mixture was then centrifuged at 6000 rpm for 10 minutes. The resulting precipitate (82% yield) was collected. Characterization using SEM and a laser particle size analyzer revealed an average particle size of 190 nm and a particle size distribution index of 0.88.

[0110] The average particle size of the colloidal spheres can be controlled within the range of 80 to 600 nm by changing the initial concentration of alcohol-insoluble lignin and the water dropping rate, and the PDI range is 0.52 to 0.90.

[0111] Comparative Example 5 (Compared with Example 1, only ultrafiltration without etherification modification of lignin)

[0112] 100 g of enzymatically hydrolyzed lignin was weighed and added to 900 mL of a NaOH aqueous solution with a pH of 9. The solution was cooked at 110 ° C for 1 h, and then separated using an ultrafiltration membrane with a molecular weight cutoff of 10,000. The filtrate was then passed through an ultrafiltration membrane with a molecular weight cutoff of 2,000 to obtain a concentrated solution. After drying at 50 ° C for 12 h, medium molecular weight lignin was obtained.

[0113] 100 g of the above-mentioned medium molecular weight lignin was weighed and dispersed and dissolved in 1000 g of ethanol. After centrifugation, the precipitate was taken and dried at 50° C. for 12 h to obtain alcohol-insoluble lignin.

[0114] 0.2 g of alcohol-insoluble lignin was weighed and dissolved in 200 mL of an acetone-water mixture (138 mL of acetone). 414 mL of water was added to the mixture at a rate of 5.0 mL / min. The mixture was then centrifuged at 6000 rpm for 10 minutes. The resulting precipitate (yield: 72%) was collected. Characterization using SEM and laser particle size analysis revealed an average particle size of 180 nm and a particle size distribution index (PDI) of 0.73.

[0115] By changing the initial concentration of enzymatically hydrolyzed lignin and the water drop acceleration rate, the average particle size of the colloidal spheres can be controlled within the range of 120 to 630 nm, and the PDI range is 0.56 to 0.83.

[0116] Comparative Example 6 (Compared with Example 1, the lignin separation method is referred to Small 2022, 18: e2200671.)

[0117] 100 g of enzymatically hydrolyzed lignin was weighed and dispersed in 1000 g of ethanol. After centrifugation, the precipitate was added to 1000 g of acetone. After centrifugation, the supernatant was dried at 50°C for 12 h to obtain lignin with uniform polarity.

[0118] 0.2 g of uniformly polar lignin was dissolved in 200 mL of an acetone-water mixture (138 mL of acetone). 414 mL of water was added to the mixture at a rate of 5.0 mL / min. The mixture was then centrifuged at 6000 rpm for 10 minutes. The resulting precipitate (9% yield) was collected, representing monodisperse lignin colloidal spheres. Scanning electron microscopy (SEM) and laser particle size analysis revealed an average particle size of 180 nm and a particle size distribution index (PDI) of 0.10.

[0119] By changing the initial concentration of polar uniform lignin and the drop acceleration rate of water, the average particle size of the colloidal spheres can be controlled within the range of 100 to 1000 nm, and the PDI range is 0.04 to 0.14.

[0120] The surface morphology, particle size distribution and structural color performance of the prepared samples were tested, and the results are shown in Table 1 and Figures 1 to 3 .

[0121] Table 1 shows the particle size controllable range, PDI value, yield and structural color performance of the lignin colloidal sphere samples prepared in Examples 1 to 6 and Comparative Examples 1 to 6.

[0122] Table 1 Comparison of basic performance characteristics of Examples 1 to 6 and Comparative Examples 1 to 6

[0123]

[0124] Table 1 Description:

[0125] The results show that the PDI values ​​of the lignin colloidal spheres of samples in Comparative Examples 1 to 5 are all greater than 0.30, while the PDI values ​​of the lignin colloidal spheres of Examples 1 to 6 are all less than 0.10, and the PDI value of the lignin colloidal spheres of sample 6 is less than 0.15. The reason for this is that Comparative Example 1 directly uses lignin raw material for self-assembly, and the lignin raw material contains strong polar molecules and weak polar molecules. The different polarities cause different shrinkage rates within the molecular chain during the self-assembly process, resulting in different sizes of the prepared lignin colloidal spheres. Comparative Examples 2 to 4 did not improve the uniformity of the lignin molecular weight and only carried out etherification modification and then self-assembly. Since the lignin molecules self-assemble, the number of molecules of lignin with larger molecular weight aggregates more, and the colloidal spheres formed are larger in size; while the number of molecules of lignin with smaller molecular weight aggregates less, and the colloidal spheres formed are smaller in size, resulting in a wider size distribution of the lignin colloidal spheres finally prepared. Comparative Example 5: The lignin raw material was subjected to ultrafiltration treatment but no etherification modification. The molecular weight uniformity of the lignin was effectively improved, but the content of the polar functional groups was not effectively regulated, and its polarity was not uniform, so the PDI value of the obtained colloidal spheres was large. Comparative Example 6: Ethanol and acetone were used to purify and separate the lignin raw material in sequence, and lignin with uniform molecular weight and uniform polarity was obtained, and lignin colloidal spheres with narrow size distribution were prepared. However, this process requires a large amount of organic solvent and the yield is less than 10%, which is not conducive to industrial production. Example 1: The lignin raw material was subjected to ultrafiltration, etherification modification and purification and separation in sequence, and lignin with uniform molecular weight and polarity was obtained. The monodisperse lignin colloidal spheres prepared showed an extremely narrow size distribution (PDI value of 0.02 to 0.07), a wide range of size controllable range (50 to 1000 nm), and a yield of more than 70%, which is easy to achieve industrial production. By constructing its short-range ordered structure, lignin-based structural color materials can be prepared, and the resulting products can effectively broaden the application of lignin colloidal spheres in structural color materials.

[0126] Figure 1 The following are scanning electron micrographs (SEM) of three monodisperse lignin colloidal spheres of different particle sizes obtained in Example 1. The experimental results show that the monodisperse lignin colloidal spheres exhibit uniform morphology and size, and spontaneously arrange themselves into regular arrays.

[0127] Figure 2 The particle size distribution diagram of three monodisperse lignin colloidal spheres with different particle sizes obtained in Example 1. From the experimental results, it can be seen that the average particle sizes of the three monodisperse lignin colloidal spheres are 170, 220, and 260 nm, respectively, and they show an extremely narrow particle size distribution, which corresponds to the SEM results.

[0128] Figure 3The following is a photograph of a lignin structural color coating prepared from three monodisperse lignin colloidal spheres obtained in Example 1. The experimental results show that the lignin structural color coating exhibits bright, vivid, and angle-independent structural colors, making it a bio-based structural color material with great application potential.

[0129] Since the processes used in other embodiments are similar to those in Example 1, after testing, the SEM images and particle size distribution diagrams of the products obtained in other embodiments are basically consistent with those in Example 1, and therefore they are not repeated here.

[0130] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing monodisperse lignin colloidal spheres, characterized in that: The following steps are involved: (1) Cooking lignin under alkaline conditions, and then separating and removing low molecular weight and high molecular weight lignin to obtain uniform medium molecular weight lignin; (2) reacting medium molecular weight lignin with glycidyl ether compounds to obtain etherified modified lignin; (3) dispersing and dissolving the etherified modified lignin in a short-chain alcohol, separating and obtaining the alcohol-insoluble lignin; (4) dissolving the alcohol-insoluble lignin in an organic solvent-water mixed solution, adding water dropwise, and centrifuging to obtain monodisperse lignin colloidal spheres; The glycidyl ether compound in step (2) is at least one of allyl glycidyl ether, n-butyl glycidyl ether and octyl glycidyl ether; The weight average molecular weight of the uniform medium molecular weight lignin in step (1) is in the range of 2000 to 10000.

2. The method for preparing monodisperse lignin colloidal spheres according to claim 1, characterized in that: The mass ratio of the medium molecular weight lignin to the glycidyl ether compound in step (2) is 100:5-10.

3. The method for preparing monodisperse lignin colloidal spheres according to claim 1, characterized in that: In step (4), the concentration of the alcohol-insoluble lignin in the organic solvent-water mixed solution is 1 to 100 g / L; and the rate of adding water is 0.10 to 50 mL / min.

4. The method for preparing monodisperse lignin colloidal spheres according to claim 1, characterized in that: In the organic solvent-water mixed solution of step (4), the organic solvent is at least one of tetrahydrofuran, acetone, ethanol and dioxane; the volume ratio of the organic solvent to water is 1: 0.10-0.45; The volume ratio of the water added dropwise in step (4) to the volume of the organic solvent in the organic solvent-water mixed solution is 3 to 30:

1.

5. The method for preparing monodisperse lignin colloidal spheres according to claim 1, characterized in that: The alkaline condition in step (1) refers to an aqueous solution with a pH of 9 to 11; the mass concentration of the lignin in the aqueous solution with a pH of 9 to 11 is 10 to 30%; The cooking temperature in step (1) is 110-150°C and the cooking time is 1-3 hours; The lignin in step (1) is at least one of enzymatic lignin in a biorefining industrial process, organic solvent lignin in a biorefining industrial process, wood pulp alkali lignin from alkaline pulping black liquor, bamboo pulp alkali lignin from alkaline pulping black liquor, and wheat straw alkali lignin from alkaline pulping black liquor; The method for separating and removing low molecular weight and high molecular weight lignin in step (1) is: firstly separating with an ultrafiltration membrane with a molecular weight cutoff of 10,000, then separating the obtained filtrate with an ultrafiltration membrane with a molecular weight cutoff of 2,000, and finally concentrating and drying.

6. The method for preparing monodisperse lignin colloidal spheres according to claim 1, characterized in that: The reaction temperature in step (2) is 60-80°C and the reaction time is 1-3 hours; The reaction medium in step (2) is an aqueous solution with a pH of 9 to 11, and the mass concentration of the medium molecular weight lignin in the aqueous solution with a pH of 9 to 11 is 10 to 30%.

7. The method for preparing monodisperse lignin colloidal spheres according to claim 1, characterized in that: The short-chain alcohol in step (3) is at least one of methanol, ethanol, n-propanol and isopropanol; The mass ratio of the etherified modified lignin to the short-chain alcohol in step (3) is 1:10-20.

8. Monodisperse lignin colloidal spheres prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the monodisperse lignin colloidal spheres according to claim 8 as structural color materials.

Images