High-crystallinity nanocellulose thin layer with highly curled and folded structure and preparation method of high-crystallinity nanocellulose thin layer

Abstract

The invention belongs to the technical field of cellulose, and particularly relates to a high-crystallinity nanocellulose thin layer with a highly curled and folded structure and a preparation method of the high-crystallinity nanocellulose thin layer. The method comprises the following steps: 1, treating a macromolecular cellulose raw material by utilizing an acid hydrolysis method to obtain cellulose nanofibers with relatively uniform molecular weight; 2, dissolving cellulose nanofibers to obtain a cellulose solution; 3, dialyzing the cellulose solution in a poor cellulose solvent, and gradually separating out cellulose to obtain a high-crystallinity nano cellulose thin layer material; and 4, performing freeze drying, natural drying and stoving treatment on the high-crystallization nano-cellulose thin-layer material to remove the solvent, thereby obtaining high-crystallization nano-cellulose powder. According to the experimental scheme, cellulose nanofibers are obtained by dissolving acid to hydrolyze macromolecular cellulose, and then a poor solvent is used for dialysis to force molecular chains to perform ordered rearrangement, so that the novel high-crystalline cellulose nanomaterial with a highly curled and folded structure is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of cellulose, and particularly relates to a highly crystalline nanocellulose thin layer having a highly curly and wrinkled structure similar to a "paper ball" and a preparation method thereof. Background Art

[0002] Cellulose is the most widely distributed and largest in reserve natural renewable resource in nature. Cellulose has a wide range of sources and can be extracted not only from plants but also synthesized by microorganisms such as bacteria. Cellulose is the main component of plant cell walls and accounts for more than 50% of the carbon content in the plant kingdom. The cellulose content in cotton is close to 100%, making it the purest natural source of cellulose; in general wood, cellulose accounts for 40-50%. For a long time, cellulose has been an important raw material for papermaking and is also used in fields such as chemical engineering and biology. In recent years, it has been found that nanoscale cellulose not only has natural advantages such as recyclability, natural degradability, etc., but also has good mechanical strength, unique self-assembly, liquid crystal behavior, low thermal expansion coefficient, high specific surface area, and easy modification. These advantages have made nanocellulose widely concerned as a new type of green material and play an important role in many fields such as composite materials, flexible electronic devices, and biomedicine as a nano-enhanced filler, optical film raw material, drug carrier, etc.

[0003] Currently, the most commonly used methods for preparing nanocellulose include acid hydrolysis method, TEMPO oxidation method, mechanical fragmentation method, etc. These methods and the obtained nanocellulose materials have the following characteristics:

[0004] 1. These methods are essentially top-down strategies, that is, by utilizing the difference in the ease of destruction of the amorphous region and the crystalline region of the cellulose raw material under physical or chemical conditions, selectively removing the amorphous region and retaining the fragments with high crystallinity to obtain the so-called cellulose "nanocrystals" or nanofibers.

[0005] 2. Such cellulose nanomaterials usually have fibrous or needle-like external characteristics. Generally speaking, their diameter is less than 50 nm, and the length ranges from several hundred nanometers to several micrometers, with a large aspect ratio. The external characteristics of cellulose nanomaterials are closely related to the raw materials and processing conditions, but the crystal form is generally cellulose I β type. If the cellulose raw material is mercerized, cellulose II type nanocrystals can be obtained, but the shape has changed from rod-shaped to irregular flocculent structure. In addition, in recent years, by using the method of ultrasonic combined with mixed acid hydrolysis, people have also obtained cellulose nanosphere structures. Such cellulose nanomaterials are usually spherical structures with a diameter of dozens to hundreds of nanometers.

[0006] The properties, applications of cellulose nanomaterials are closely related to their size, morphology and surface properties. Theoretically, nanocelluloses with large aspect ratios such as cellulose nanofibers (CNF) and cellulose nanocrystals (CNC) are added to polymer composites as reinforcing phases, which have an obvious strengthening effect on the mechanical strength of the materials, and the strengthening effect becomes more significant as the aspect ratio increases. Cellulose nanospheres with a spherical morphology at the nanoscale are a theoretically isotropic material with a larger specific surface area. This characteristic gives it better dispersibility and surface activity. Manipulating cellulose molecules and supramolecular aggregates at the micro- and nanoscale to obtain new forms of cellulose nanomaterials and endow them with new functions has become one of the frontiers in this field. Obviously, this is an inevitable way to make full use of bio-based materials and continuously expand the applications of bio-based materials. Summary of the Invention

[0007] The purpose of the present invention is to provide a highly crystalline nanocellulose thin layer with a highly curled and wrinkled structure and a preparation method thereof. By dissolving cellulose raw materials in a solvent and then slowly precipitating cellulose in a poor solvent through dialysis for self-assembly, and freeze-drying, a novel highly crystalline nanocellulose is obtained, and a highly crystalline nanocellulose with novel external shape characteristics is obtained.

[0008] The technical solution of the present invention is as follows:

[0009] A highly crystalline nanocellulose thin layer with a highly curled and wrinkled structure, the highly crystalline nanocellulose thin layer is a cellulose II type nanomaterial with a crystallinity ≥ 80%, and its crystal structure is a cellulose II type structure (monoclinic system), and the unit cell parameters are a = 0.81 nm, b = 0.903 nm, c = 1.031 nm, α = β = 90°, γ = 117.1°.

[0010] The described highly crystalline nanocellulose thin layer with a highly curled and wrinkled structure, the external shape characteristic of this nanocellulose thin layer is a film structure, which shrinks into a three-dimensional spherical shape due to curling and wrinkling, and a hollow polyhedron structure is locally formed. The film surface is parallel to the cellulose II type (020) crystal plane, and the thickness is 1 nanometer to 100 nanometers.

[0011] The preparation method of the described highly crystalline nanocellulose thin layer with a highly curled and wrinkled structure includes the following steps:

[0012] Step 1: Treat the macromolecular cellulose raw materials by acid hydrolysis method to obtain cellulose nanofibers with relatively uniform molecular weight;

[0013] Step 2: Swell the cellulose nanofibers described in Step 1 at low temperature in a solvent and stir magnetically until fully dissolved to obtain a uniform and transparent cellulose solution;

[0014] Step 3: Dialyze the cellulose solution described in Step 2 in a cellulose non-solvent, and the cellulose gradually precipitates to obtain a highly crystalline nanocellulose thin film material with a highly curled and wrinkled structure;

[0015] Step 4: After removing the solvent from the highly crystalline nanocellulose thin film material described in Step 3 through freeze-drying, natural drying, and drying, a highly crystalline nanocellulose powder with a highly curled and wrinkled structure is obtained.

[0016] In the preparation method of the highly crystalline nanocellulose thin film with a highly curled and wrinkled structure, in Step 1, the macromolecular cellulose raw material includes refined cellulose or cellulose slurry extracted from plant, animal, or bacterial sources.

[0017] In the preparation method of the highly crystalline nanocellulose thin film with a highly curled and wrinkled structure, in Step 1, the acid used for cellulose hydrolysis includes one or more of sulfuric acid, hydrochloric acid, phosphoric acid, and organic acids.

[0018] In the preparation method of the highly crystalline nanocellulose thin film with a highly curled and wrinkled structure, in Step 1, the molecular weight of the cellulose nanofibers is 10,000 - 200,000 g / mol.

[0019] In the preparation method of the highly crystalline nanocellulose thin film with a highly curled and wrinkled structure, in Step 2, the solvent for the cellulose nanofibers includes sodium hydroxide / urea / water, lithium hydroxide / urea / water, sodium hydroxide / thiourea / water, lithium hydroxide / thiourea / water, dimethyl sulfoxide, or N,N-dimethylacetamide / LiCl.

[0020] In the preparation method of the highly crystalline nanocellulose thin film with a highly curled and wrinkled structure, in Step 2, the low temperature of the solvent for the cellulose nanofibers is -30 to 5 °C.

[0021] In the preparation method of the highly crystalline nanocellulose thin film with a highly curled and wrinkled structure, in Step 3, the cellulose non-solvent includes one or more of water, alcohol, and ketone.

[0022] In the preparation method of the highly crystalline nanocellulose thin film with a highly curled and wrinkled structure, in Step 3, perform dialysis treatment in the non-solvent for 3 - 7 days, change the non-solvent every 6 h to remove the solvent of the cellulose nanofibers, and the molecular weight cut-off of the dialysis bag is 8,000 - 14,000 Da.

[0023] The inventive concept of the present invention is:

[0024] The existing technical solutions for preparing cellulose nanomaterials mainly utilize the differences in the ease of mechanical or hydrolysis damage between the crystalline and amorphous regions of macromolecular cellulose. The fundamental idea is to selectively damage the amorphous region and retain the crystalline region to obtain cellulose nanofibers or nanorods with a large aspect ratio. Guided by the basic idea of "breaking the old and establishing the new", the present invention uses the regulatory effect of organic solvents on the crystallization behavior of polysaccharide molecules for cellulose raw materials. On the basis of hydrolyzing to obtain cellulose nanofibers, a bottom-up material preparation method is used. First, the dissolution of cellulose is achieved by breaking the hydrogen bonds between cellulose molecular chains, and then the anti-solvent method is used to make the cellulose molecular chains slowly and orderly rearrange and gradually precipitate, that is, a nanocellulose thin layer with high crystallinity is obtained. In the solvent, the cellulose thin layer curls and shrinks to reduce the thermodynamic instability of the surface. Therefore, a large number of curly and wrinkled structures are formed. From the morphological point of view, it can be vividly understood as kneading a thin paper into a ball, so it is named the "paper ball" structure. Theoretically, this cellulose nanomaterial has a large specific surface area. Moreover, these thin layers are composed of highly ordered cellulose molecular chain bundles, and its clear layer structure can be observed under a high-resolution transmission electron microscope. This new type of high-crystalline nanocellulose enriches the types of cellulose nanocrystals and can bring more possibilities for the application of nanocellulose, making it a functional material.

[0025] The advantages and beneficial effects of the present invention are as follows:

[0026] The new type of high-crystalline nanocellulose prepared by the present invention uses a bottom-up material preparation method. First, the dissolution is achieved by breaking the hydrogen bonds between cellulose molecular chains, and then dialysis is used to force the molecular chains to rearrange orderly, obtaining a high-crystalline nanocellulose thin layer with a highly curly and wrinkled structure. The difference in this technical solution leads to significant changes in the shape and structure of the obtained nanocellulose material. Calculated based on the input amount of cellulose nanofibers obtained after acid hydrolysis, the yield of the product of the present invention can be as high as over 95%, and it has a thermodynamically stable cellulose II crystal structure. The product preparation method is simple, and the controllability and repeatability of the product are high. In particular, the cellulose nanomaterial obtained by the present invention has completely different shape characteristics from a large number of currently reported cellulose nanomaterials, such as cellulose nanofibers, cellulose nanocrystals, cellulose nanospheres, etc., enriching the types of cellulose nanomaterials. Its thin film shape with high curl and wrinkles endows it with the characteristics of low density and high specific surface area, bringing more possibilities and opportunities for the development of nanocellulose into a functional material. Description of the Drawings

[0027] Figure 1 XRD spectrum of the new type of high-crystalline nanocellulose prepared by the present invention.

[0028] Figure 2TEM images of the novel highly crystalline nanocellulose prepared according to the present invention; among them, (a) low-magnification TEM image, (b) high-resolution TEM image.

[0029] Figure 3 High-magnification TEM images of the novel highly crystalline nanocellulose prepared according to the present invention, and through large-angle tilting; among them, (a) tilting angle is -35°, (b) tilting angle is 0°, (c) tilting angle is 60°. Detailed implementation manners

[0030] In the specific implementation process, the present invention proposes a preparation method of a highly crystalline nanocellulose thin layer with a highly curly and wrinkled structure, including the following steps:

[0031] Macromolecular cellulose raw material

[0032] In the present invention, the "macromolecular" cellulose raw material can be refined cellulose or cellulose slurry separated from plants, animals, bacteria, etc., and the preferred solution is cotton-extracted cellulose or commercially available microcrystalline cellulose.

[0033] Processes 1 and 2: Acid hydrolysis to prepare cellulose nanofibers and then dissolve

[0034] In the present invention, in Process 1, the macromolecular cellulose raw material is treated by an acid hydrolysis method, the purpose is to narrow the molecular weight and molecular weight distribution range of the cellulose raw material, and obtain cellulose nanofibers with relatively uniform molecular weight. One or more of sulfuric acid, hydrochloric acid, phosphoric acid, and organic acids can be used. The temperature range is 45 - 80°C, and the hydrolysis duration is 0.5 - 2 hours.

[0035] Process 2 is to dissolve the cellulose nanofibers obtained in Process 1 to obtain a cellulose solution. The solvents used are various solvent systems reported currently, preferably selected from sodium hydroxide / urea / water, lithium hydroxide / urea / water systems. Other systems such as: sodium hydroxide / thiourea / water, lithium hydroxide / thiourea / water, dimethyl sulfoxide (DMSO), N,N-dimethylacetamide / LiCl can also achieve the dissolution purpose of Process 2 of the present invention.

[0036] Processes 3 and 4: Cellulose reconstruction process

[0037] In Process 3, the cellulose solution obtained in the above Process 2 is dialyzed in a poor solvent, and the cellulose gradually precipitates slowly, so that the cellulose molecular chains are reconstructed under thermodynamic action to obtain a cellulose nanomaterial with high crystallinity. Among them, the poor solvents used are preferably one or more of water, alcohols, and ketones. The dialysis temperature range can be selected from 0 - 30°C, and the dialysis duration is 72 - 168 hours.

[0038] In Process 4, the novel highly crystalline cellulose nanomaterial obtained in the above Process 3 is treated by freeze-drying, natural drying at room temperature or drying in an oven. The main purpose is to remove the excess solvent to obtain a powder of the novel highly crystalline nanocellulose product for long-term storage or subsequent use.

[0039] The present invention will be further illustrated by the following examples and drawings:

[0040] Example 1

[0041] Prepare concentrated sulfuric acid with a mass fraction of 64%. Stir it in a constant temperature water bath at 45°C. After the solution temperature stabilizes, add microcrystalline cellulose, where each gram of microcrystalline cellulose corresponds to 8.75 ml of sulfuric acid. React for 30 min at a stirring speed of 280 rpm. After the reaction is completed, pour it into 5 times the amount of ice water at 0°C to obtain a mixed solution. Let the mixed solution stand for 12 h, pour off the supernatant to obtain a milky white turbid liquid at the bottom layer. Centrifuge the milky white turbid liquid at the bottom layer at 8500 rmp for 20 min to obtain a milky white suspension. Wash it 3 times by centrifugation with deionized water to obtain a milky white sample. Freeze the milky white sample at -20°C for 1 day, take it out and place it in a freeze dryer, and dry for 2 days to obtain cellulose nanofibers with a molecular weight of 40000 g / mol. Take 2 g of cellulose nanofibers and add them to 98 g of a mixed aqueous solution of 10 wt% sodium hydroxide / 8 wt% urea at -20°C. Stir magnetically until completely dissolved to obtain a uniform and transparent solution. Dialyze the solution in water for 5 days, changing the water every 6 h. The molecular weight cut-off of the dialysis bag is 8000 - 14000 Da. Collect the liquid inside the dialysis bag, freeze it at -20°C for 1 day, take it out and place it in a freeze dryer, and dry for 2 days to obtain the novel highly crystalline nanocellulose.

[0042] In this example, the highly crystalline nanocellulose thin layer is a cellulose II-type nanomaterial with a crystallinity of 95%. Its crystal structure is a cellulose II-type structure (monoclinic system), and the unit cell parameters are a = 0.81 nm, b = 0.903 nm, c = 1.031 nm, α = β = 90°, γ = 117.1°. The external shape feature of this nanocellulose thin layer is a film structure, which curls and folds to shrink into a three-dimensional spherical shape, and a hollow polyhedron structure is formed locally. The film surface is parallel to the cellulose II-type (020) crystal plane, and the thickness is 50 nanometers.

[0043] Example 2

[0044] Prepare hydrochloric acid with a mass fraction of 30%, stir it in a constant temperature water bath at 60 °C. After the solution temperature stabilizes, add microcrystalline cellulose, with 15 ml of hydrochloric acid corresponding to each gram of microcrystalline cellulose. React for 60 min at a stirring speed of 280 rpm. After the reaction is completed, pour it into 5 times the amount of ice water at 0 °C to obtain a mixed solution; let the mixed solution stand for 12 h, pour off the upper clear liquid to obtain a lower layer of milky turbid liquid. Centrifuge the lower layer of milky turbid liquid at 8500 rmp for 20 min to obtain a milky suspension, and wash it 5 times with deionized water by centrifugation to obtain a milky white sample; put the milky white sample into a freezer at -20 °C for 1 day, take it out and place it in a freeze dryer, and dry it for 2 days to obtain cellulose nanofibers with a molecular weight of 80000 g / mol. Take 2 g of cellulose nanofibers and add them to 98 g of a mixed aqueous solution of 10 wt% sodium hydroxide / 8 wt% urea at -20 °C, stir magnetically until completely dissolved to obtain a uniform transparent solution. Dialyze the solution for 5 days, change the water every 6 h, and the molecular weight cut-off of the dialysis bag is 8000 - 14000 Da. Collect the liquid inside the dialysis bag, put it into a freezer at -20 °C for 1 day, take it out and place it in a freeze dryer, and take it out after drying for 2 days to obtain novel highly crystalline nanocellulose.

[0045] In this example, the highly crystalline nanocellulose thin layer is a cellulose type II nanomaterial with a crystallinity of 90%. Its crystal structure is a cellulose type II structure (monoclinic system), and the unit cell parameters are a = 0.81 nm, b = 0.903 nm, c = 1.031 nm, α = β = 90°, γ = 117.1°. The external shape feature of this nanocellulose thin layer is a film structure, which curls and folds to shrink into a three-dimensional spherical shape, and a hollow polyhedron structure is locally formed. The film surface is parallel to the cellulose type II (020) crystal plane, and the thickness is 30 nm.

[0046] Example 3

[0047] Prepare hydrochloric acid with a mass fraction of 30%. Stir it in a constant temperature water bath at 60 °C. After the solution temperature stabilizes, add shredded cotton. For each gram of cotton, 15 ml of hydrochloric acid is used. React for 60 min at a stirring speed of 280 rpm. After the reaction is completed, pour it into 5 times the amount of ice water at 0 °C to obtain a mixed solution. Let the mixed solution stand for 12 h, pour off the upper clear liquid to obtain a lower milky turbid liquid. Centrifuge the lower milky turbid liquid at 8500 rmp for 20 min to obtain a milky suspension. Centrifuge and wash it 4 times with deionized water to obtain a milky sample. Place the milky sample in a freezer at -20 °C for 1 day, take it out and place it in a freeze dryer, and dry it for 2 days to obtain cellulose nanofibers with a molecular weight of 80000 g / mol. Take 2 g of cellulose nanofibers and add them to 98 g of a mixed aqueous solution of 10 wt% sodium hydroxide / 8 wt% urea at -20 °C. Stir magnetically until completely dissolved to obtain a uniform and transparent solution. Dialyze the solution for 5 days, changing the water every 6 h. The molecular weight cut-off of the dialysis bag is 8000 - 14000 Da. Collect the liquid inside the dialysis bag, place it in a freezer at -20 °C for 1 day, take it out and place it in a freeze dryer, and dry it for 2 days to obtain novel highly crystalline nanocellulose.

[0048] In this example, the highly crystalline nanocellulose thin layer is a cellulose II-type nanomaterial with a crystallinity of 89%. Its crystal structure is a cellulose II-type structure (monoclinic system), and the unit cell parameters are a = 0.81 nm, b = 0.903 nm, c = 1.031 nm, α = β = 90°, γ = 117.1°. The external shape characteristics of this nanocellulose thin layer are a thin film structure, which curls and folds to shrink into a three-dimensional spherical shape, and a hollow polyhedron structure is locally formed. The thin film surface is parallel to the cellulose II-type (020) crystal plane, and the thickness is 100 nanometers.

[0049] Example 4

[0050] Prepare concentrated sulfuric acid with a mass fraction of 64%, stir it in a 45°C constant temperature water bath. After the solution temperature stabilizes, add microcrystalline cellulose, where 8.75 ml of sulfuric acid corresponds to each gram of microcrystalline cellulose. React for 30 min at a stirring speed of 280 rpm. After the reaction is completed, pour it into 5 times the amount of ice water at 0°C to obtain a mixed solution; let the mixed solution stand for 12 h, pour off the upper clear liquid to obtain the lower milky turbid liquid. Centrifuge the lower milky turbid liquid at 8500 rmp for 20 min to obtain a milky suspension, and centrifuge and wash it 3 times with deionized water to obtain a milky sample; place the milky sample in a freezer at -20°C for 1 day, take it out and place it in a freeze dryer, and dry it for 2 days to obtain cellulose nanofibers with a molecular weight of 40000 g / mol. Take 2 g of cellulose nanofibers and add them to 98 g of an 8 wt% lithium hydroxide / 10 wt% urea mixed aqueous solution at -20°C, stir magnetically until completely dissolved to obtain a uniform transparent solution, perform water dialysis on the solution for 5 days, change the water every 6 h, and the molecular weight cut-off of the dialysis bag is 8000 - 14000 Da. Collect the liquid inside the dialysis bag, place it in a freezer at -20°C for 1 day, take it out and place it in a freeze dryer, and take it out after drying for 2 days to obtain novel highly crystalline nanocellulose.

[0051] In this example, the highly crystalline nanocellulose thin layer is a cellulose II-type nanomaterial with a crystallinity of 88%. Its crystal structure is a cellulose II-type structure (monoclinic system), and the unit cell parameters are a = 0.81 nm, b = 0.903 nm, c = 1.031 nm, α = β = 90°, γ = 117.1°. The external shape feature of this nanocellulose thin layer is a thin film structure, which curls and folds to shrink into a three-dimensional spherical shape, and a hollow polyhedron structure is locally formed. The thin film surface is parallel to the cellulose II-type (020) crystal plane, and the thickness is 70 nanometers.

[0052] Example 5

[0053] Prepare concentrated sulfuric acid with a mass fraction of 64%, stir it in a 45°C constant temperature water bath. After the solution temperature stabilizes, add microcrystalline cellulose, with 8.75 ml of sulfuric acid corresponding to each gram of microcrystalline cellulose. React for 30 min at a stirring speed of 280 rpm. After the reaction is completed, pour it into 5 times the amount of ice water at 0°C to obtain a mixed solution; let the mixed solution stand for 12 h, pour off the upper clear liquid to obtain the lower milky turbid liquid. Centrifuge the lower milky turbid liquid at 8500 rmp for 20 min to obtain a milky suspension, and centrifuge and wash it 4 times with deionized water to obtain a milky white sample; put the milky white sample into a freezer at -20°C for 1 day, take it out and place it in a freeze dryer, and dry for 2 days to obtain cellulose nanofibers with a molecular weight of 40000 g / mol. Take 2 g of cellulose nanofibers and add them to 98 g of a mixed aqueous solution of 10 wt% sodium hydroxide / 8 wt% urea at -20°C, stir magnetically until completely dissolved to obtain a uniform transparent solution, dialyze the solution with ethanol, the dialysis time is 5 days, and change the ethanol every 6 h. The molecular weight cut-off of the dialysis bag is 8000 - 14000 Da. Collect the liquid inside the dialysis bag, place it at room temperature for air-drying, and air-dry at room temperature for 2 days to obtain novel highly crystalline nanocellulose.

[0054] In this example, the highly crystalline nanocellulose thin layer is a cellulose II type nanomaterial with a crystallinity of 92%. Its crystal structure is a cellulose II type structure (monoclinic system), and the unit cell parameters are a = 0.81 nm, b = 0.903 nm, c = 1.031 nm, α = β = 90°, γ = 117.1°. The external shape feature of this nanocellulose thin layer is a thin film structure, which curls and wrinkles and shrinks into a three-dimensional spherical shape, and a hollow polyhedron structure is locally formed. The thin film surface is parallel to the cellulose II type (020) crystal plane, and the thickness is 90 nm.

[0055] Example 6

[0056] Prepare concentrated sulfuric acid with a mass fraction of 64%, stir it in a 45°C constant temperature water bath. After the solution temperature stabilizes, add microcrystalline cellulose, where each gram of microcrystalline cellulose corresponds to 8.75 ml of sulfuric acid. React for 30 min at a stirring speed of 280 rpm. After the reaction is completed, pour it into 5 times the amount of ice water at 0°C to obtain a mixed solution; let the mixed solution stand for 12 h, pour off the upper clear liquid to obtain the lower milky turbid liquid. Centrifuge the lower milky turbid liquid at 8500 rmp for 20 min to obtain a milky suspension, and centrifuge and wash it with deionized water 5 times to obtain a milky white sample; put the milky white sample into a -20°C freezer for 1 day, take it out and place it in a freeze dryer, and dry it for 2 days to obtain cellulose nanofibers with a molecular weight of 40000 g / mol. Take 2 g of cellulose nanofibers and add them to 98 g of a mixed aqueous solution of 10 wt% sodium hydroxide / 8 wt% urea at -20°C, stir magnetically until completely dissolved to obtain a uniform transparent solution, dialyze the solution with acetone for 5 days, change the acetone every 6 h, and the molecular weight cut-off of the dialysis bag is 8000 - 14000 Da. Collect the liquid inside the dialysis bag, put it into an 80°C oven for drying treatment, take it out after drying for 2 hours to obtain novel highly crystalline nanocellulose.

[0057] In this example, the highly crystalline nanocellulose thin layer is a cellulose II type nanomaterial with a crystallinity of 93%. Its crystal structure is a cellulose II type structure (monoclinic system), and the unit cell parameters are a = 0.81 nm, b = 0.903 nm, c = 1.031 nm, α = β = 90°, γ = 117.1°. The external shape characteristics of this nanocellulose thin layer are a thin film structure, which curls and folds to shrink into a three-dimensional spherical shape, and a hollow polyhedron structure is locally formed. The thin film surface is parallel to the cellulose II type (020) crystal plane, and the thickness is 20 nanometers.

[0058] Example 7

[0059] Add 2 g of freeze-dried cellulose nanocrystal powder (molecular weight 100000 g / mol) to 98 g of a mixed aqueous solution of 10 wt% sodium hydroxide / 8 wt% urea at -20°C, stir magnetically until completely dissolved to obtain a uniform transparent solution, dialyze the solution with water for 5 days, change the water every 6 h, and the molecular weight cut-off of the dialysis bag is 8000 - 14000 Da. Collect the liquid inside the dialysis bag, put it into a -20°C freezer for 1 day, take it out and place it in a freeze dryer, and take it out after drying for 2 days to obtain novel highly crystalline nanocellulose.

[0060] In this example, the highly crystalline nanocellulose thin layer is a cellulose II type nanomaterial with a crystallinity of 94%. Its crystal structure is a cellulose II type structure (monoclinic system), and the unit cell parameters are a = 0.81 nm, b = 0.903 nm, c = 1.031 nm, α = β = 90°, and γ = 117.1°. The external shape of this nanocellulose thin layer is a film structure, which shrinks into a three-dimensional mass due to curling and wrinkling, and locally forms a hollow polyhedron structure. The film surface is parallel to the cellulose II type (020) crystal plane, and the thickness is 60 nanometers.

[0061] Example 8

[0062] Add 2 g of freeze-dried cellulose nanocrystal powder (molecular weight of 100,000 g / mol) to 98 g of a mixed aqueous solution of 10 wt% sodium hydroxide / 8 wt% urea at -5 °C, and stir magnetically until completely dissolved to obtain a homogeneous and transparent solution. Dialyze the solution in water for 5 days, changing the water every 6 h. The molecular weight cut-off of the dialysis bag is 8000 - 14000 Da. Collect the liquid inside the dialysis bag, freeze it at -20 °C for 1 day, take it out and place it in a freeze dryer, and take it out after drying for 2 days to obtain a new type of highly crystalline nanocellulose.

[0063] In this example, the highly crystalline nanocellulose thin layer is a cellulose II type nanomaterial with a crystallinity of 94%. Its crystal structure is a cellulose II type structure (monoclinic system), and the unit cell parameters are a = 0.81 nm, b = 0.903 nm, c = 1.031 nm, α = β = 90°, and γ = 117.1°. The external shape of this nanocellulose thin layer is a film structure, which shrinks into a three-dimensional mass due to curling and wrinkling, and locally forms a hollow polyhedron structure. The film surface is parallel to the cellulose II type (020) crystal plane, and the thickness is 80 nanometers.

[0064] Description of the drawings in the specification:

[0065] Figure 1 This is the X-ray diffraction pattern of the new type of highly crystalline nanocellulose prepared in Example 1 of the present invention. From Figure 1 it can be seen that there are obvious multiple diffraction peaks within the measured range corresponding to the diffraction peaks of the cellulose II type crystal plane. The new type of highly crystalline nanocellulose prepared is a cellulose-based substance, and the crystal form is cellulose II type.

[0066] Figure 2 This is the low-magnification and high-resolution TEM photos of the new type of highly crystalline nanocellulose prepared in Example 1 of the present invention. From Figure 2 it can be seen that the new type of highly crystalline nanocellulose can show the regular curling arrangement of molecular chains under high-resolution TEM, which is sufficient to show its crystallinity far higher than that of ordinary cellulose nanomaterials.

[0067] Figure 3 This is the large-angle tilting TEM experimental photo of the novel highly crystalline nanocellulose prepared in Example 1 of the present invention. It can be seen from the large-angle tilting from -35° to 60° that the novel highly crystalline nanocellulose has a thin-layer structure, and most of it forms a three-dimensional cluster through a three-dimensional morphology of high curling and folding, while a hollow polyhedral structure can be seen locally.

[0068] The implementation results show that the present invention obtains cellulose nanofibers by dissolving and acid-hydrolyzing macromolecular cellulose, and then uses a poor solvent dialysis to force the molecular chains to undergo an orderly rearrangement experimental scheme, obtaining a novel highly crystalline cellulose nanomaterial with a highly curled and folded structure. Electron microscopic analysis shows that the novel highly crystalline nanocellulose has a thin-film structure in appearance, shrinks into a three-dimensional cluster due to curling and folding, and forms a hollow polyhedral structure locally, with a thickness of about 1 nanometer to dozens of nanometers. Due to its large surface area and three-dimensional curled and folded structure, the novel highly crystalline cellulose nanomaterial of the present invention is expected to be used as an organic structured catalyst carrier, drug carrier, composite material reinforcement, etc.

[0069] The above embodiments are only used to illustrate the technical concept and characteristics of the present invention, and their purpose is to enable those who are familiar with this technology to understand the content of the present invention and implement it accordingly, and cannot be used to limit the protection scope of the present invention. All equivalent changes or modifications made according to the spirit and essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A highly crystalline nanocellulose thin layer having a highly curled and wrinkled structure, characterized in that: The highly crystalline nanocellulose thin layer is a cellulose II type nanomaterial with a crystallinity of ≥80%. Its crystal structure is a cellulose II type structure (monoclinic system), and its unit cell parameters are a=0.81nm, b=0.903nm, c=1.031nm, α=β=90°, and γ=117.1°.

2. The highly crystalline nanocellulose thin layer with a highly curled and wrinkled structure according to claim 1, characterized in that: The appearance of the nanocellulose thin layer is characterized by a film structure, which shrinks into a three-dimensional mass due to curling and wrinkling, and forms a hollow polyhedron structure locally. The film surface is parallel to the cellulose II type (020) crystal plane, and the thickness is 1 nanometer to 100 nanometers.

3. A method for preparing a highly crystalline nanocellulose thin layer having a highly curled and wrinkled structure according to any one of claims 1 to 2, characterized in that: The following steps are involved: Step 1, treating the macromolecular cellulose raw material by acid hydrolysis to obtain cellulose nanofibers with relatively uniform molecular weight; Step 2, swelling the cellulose nanofibers described in step 1 in a solvent at low temperature, and fully dissolving them by magnetic stirring to obtain a uniform and transparent cellulose solution; Step 3, dialyzing the cellulose solution in step 2 in a poor solvent for cellulose, and gradually precipitating the cellulose to obtain a highly crystalline nanocellulose thin layer material with a highly curled and wrinkled structure; Step 4: The highly crystalline nanocellulose thin layer material described in step 3 is subjected to freeze drying, natural drying and baking treatment to remove the solvent, thereby obtaining a highly crystalline nanocellulose powder having a highly curled and wrinkled structure.

4. The method for preparing a highly crystalline nanocellulose thin layer having a highly curled wrinkled structure according to claim 3, characterized in that: In step 1, the macromolecular cellulose raw material includes refined cellulose or cellulose slurry extracted from plants, animals or bacteria.

5. The method for preparing a highly crystalline nanocellulose thin layer having a highly curled and wrinkled structure according to claim 3, characterized in that: In step 1, the acid used for cellulose hydrolysis includes one or more of sulfuric acid, hydrochloric acid, phosphoric acid and organic acid.

6. The method for preparing a highly crystalline nanocellulose thin layer having a highly curled and wrinkled structure according to claim 3, characterized in that: In the step 1, the molecular weight of the cellulose nanofibers is 10,000 to 200,000 g / mol.

7. The method for preparing a highly crystalline nanocellulose thin layer having a highly curled and wrinkled structure according to claim 3, characterized in that: In step 2, the solvent for the cellulose nanofibers includes sodium hydroxide / urea / water, lithium hydroxide / urea / water, sodium hydroxide / thiourea / water, lithium hydroxide / thiourea / water, dimethyl sulfoxide, or N,N-dimethylacetamide / LiCl.

8. The method for preparing a highly crystalline nanocellulose thin layer having a highly curled and wrinkled structure according to claim 3, characterized in that: In the step 2, the low temperature of the solvent of the cellulose nanofibers is -30 to 5°C.

9. The method for preparing a highly crystalline nanocellulose thin layer having a highly curled and wrinkled structure according to claim 3, characterized in that: In step 3, the poor solvent for cellulose includes one or more of water, alcohol, and ketone.

10. The method for preparing a highly crystalline nanocellulose thin layer having a highly curled and wrinkled structure according to claim 1, characterized in that: In step 3, dialysis treatment is performed in a poor solvent for 3 to 7 days, the poor solvent is replaced every 6 hours, and the solvent of the cellulose nanofibers is removed. The molecular weight cutoff of the dialysis bag is 8000 to 14000 Da.

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