阅读说明：本技术 一种高羧基取代的毛发状纤维素纳米晶体及其制备方法 (High-carboxyl-substituted hairy cellulose nanocrystal and preparation method thereof ) 是由 王兆梅 朱维 金冬冬 季杏迪 于 2021-08-04 设计创作，主要内容包括：本发明涉及生物高分子材料领域,具体公开了一种高取代的羧基纤维素纳米晶体及其制备方法。所述制备方法为：将植物纤维素分散在去离子水中,先用TEMPO/NaClO/NaClO-(2)氧化体系氧化,再超声处理,加入高碘酸盐氧化后,加入氯化钠-亚氯酸钠-过氧化氢多元氧化剂在室温下搅拌反应后,最后进行加热处理,得到最终产物。本发明采用分步氧化法将羧基取代控制在纤维素的无定形区域,从而提高羧基化取代度,再通过热降解将羧基化纤维素在无定形区域断链,形成两端带有无定形羧基化纤维素的毛发状纤维素纳米晶体。纳米纤维素晶体表面羧基含量、羧基取代度等均有显著提高,且操作过程简便、无污染,可大规模推广生产。(The invention relates to the field of biological high polymer materials, and particularly discloses a highly-substituted carboxyl cellulose nanocrystal and a preparation method thereof. The preparation method comprises the following steps: dispersing plant cellulose in deionized water, and first using TEMPO/NaClO 2 Oxidizing in an oxidation system, performing ultrasonic treatment, adding periodate for oxidation, adding a sodium chloride-sodium chlorite-hydrogen peroxide multi-element oxidant, stirring at room temperature for reaction, and finally performing heating treatment to obtain a final product. The invention adopts a step-by-step oxidation method to control carboxyl substitution in an amorphous area of cellulose so as to improve the degree of carboxylation substitution, and then the chain of the carboxylated cellulose is broken in the amorphous area through thermal degradation so as to form the hairy cellulose nanocrystal with amorphous carboxylated cellulose at two ends. The carboxyl on the surface of the nano-cellulose crystal containsThe amount, the substitution degree of carboxyl and the like are obviously improved, the operation process is simple and convenient, no pollution is caused, and the method can be popularized and produced on a large scale.)

1. A method for preparing high carboxyl substituted hairy cellulose nanocrystals, characterized by comprising the steps of:

(1) using plant cellulose as raw material, dispersing cellulose in pH buffer solution, adding 2,2,6, 6-tetramethyl piperidine oxide/NaClO₂An oxidation system, wherein after the reaction is finished, a C6-carboxyl cellulose derivative is obtained, then the C6-carboxyl cellulose derivative is mixed with water to form a dispersion liquid, and then the dispersion liquid is subjected to ultrasonic treatment to obtain C6-carboxyl cellulose nanofibrils;

(2) mixing the C6-carboxyl cellulose nanofibrils obtained in the step (1) with a pH buffer solution, adding periodate, stirring for reaction, adding ethylene glycol to stop the reaction, and obtaining a 2, 3-dialdehyde-6-carboxyl cellulose nanofibril suspension;

(3) mixing the 2, 3-dialdehyde-6-carboxyl cellulose nanofibril suspension obtained in the step (2) with a sodium chloride-sodium chlorite-hydrogen peroxide multi-element oxidant, stirring at room temperature for reaction, and obtaining the cellulose nanofibril suspension substituted by high carboxyl after the reaction is finished;

(4) and (4) heating the cellulose nanofibril suspension with high carboxyl substitution obtained in the step (3), and cooling to room temperature to obtain the hair-like cellulose nanocrystal with high carboxyl substitution.

2. The method of claim 1, wherein: in the step (1), 0.01g to 0.02g of 2,2,6, 6-tetramethyl piperidine oxide and NaClO are added into per gram of oven-dried plant cellulose₂ 8mmol-12mmol，NaClO 1mmol-10mmol。

3. The method of claim 1, wherein: the plant fiber in the step (1) is wood fiber or/and gramineous fiber; the concentration of the plant cellulose in the pH buffer solution in the step (1) is 1-5%, w/v.

4. The method of claim 1, wherein: the reaction in the step (1) is carried out at 35-65 ℃ for 2-10 h; the ultrasonic treatment conditions in the step (1) are as follows: the solid content of the dispersion liquid is 0.1-4%, W/v, the ultrasonic power is 800-.

5. The method of claim 1, wherein: the feed-to-liquid ratio of the C6-carboxyl cellulose nanofibrils in the step (2) to the pH buffer solution is 1g:50mL-1g:250 mL; in the step (2), 1-6g of periodate is added into each gram of cellulose nanofibrils containing C6-carboxyl; the volume-mass ratio of the added amount of the glycol to the periodate in the step (2) is 0.5-2mL:1 g.

6. The method of claim 1, wherein: the stirring reaction in the step (2) is carried out for 6-24h at the temperature of 25-40 ℃.

7. The method of claim 1, wherein: in the step (3), 2-15g of NaCl and 0.9-3.6g of NaClO are added into each gram of 2, 3-dialdehyde-6-carboxyl cellulose nanofibril suspension₂And 0.9-3.3g H₂O₂。

8. The method of claim 1, wherein: stirring at room temperature for 12-36h in the step (3); the heating treatment in the step (4) is heating for 3-8h at the temperature of 60-80 ℃.

9. The method of claim 1, wherein: after cooling to room temperature, purifying the obtained product, specifically, centrifuging the mixed solution containing the final product, removing unreacted micron fiber precipitate, and then adding absolute ethyl alcohol into the supernatant until no further precipitate is generated; and centrifuging at a certain speed again, and collecting white precipitate to obtain the high carboxyl substituted hairy cellulose nanocrystal.

10. A highly carboxyl-substituted hairy cellulose nanocrystal obtainable by a process according to any one of claims 1 to 9.

Technical Field

The invention relates to the field of biological polymer materials, in particular to a high carboxyl substituted hairy cellulose nanocrystal and a preparation method thereof.

Background

Two cellulose units in nano-scale form are distributed in the molecular structure of cellulose: crystalline and amorphous cellulose which, by virtue of their intramolecular and intermolecular hydrogen bonds and weak van der waals forces, maintain self-assembling macromolecular structures and fibrillar morphology. The nano-crystalline cellulose can be obtained by removing amorphous cellulose by a special process, and is generally called as Nano Cellulose Crystals (NCC) (or cellulose nanocrystals, CNC), which is a cellulose-based superfine fiber nano material with the diameter less than 100nm, not only has the properties of non-toxicity, regeneration and degradability of natural cellulose, but also has the typical characteristics of nano materials, such as low density, large specific surface area, strong adsorption capacity, high mechanical strength and the like. Due to its unique self-assembly, liquid crystal behavior, optical properties, superior mechanical properties and potential application value as reinforcing filler in nanocomposites in industry, it has attracted a great deal of attention.

In the preparation process of NCC, the problem that the strong hydrogen bond action in molecules and among molecules in cellulose needs to be overcome in order to separate high-stability and nano-scale cellulose crystals from natural fibers, which always hinders the industrialized preparation of nano-cellulose crystals. The conventional NCC preparation methods mainly include two methods, i.e., an inorganic acid hydrolysis method and an enzyme method. The sulfuric acid hydrolysis method is an early-discovered and well-studied method for preparing NCC. Chinese patent CN108219008A discloses a nano-cellulose crystal prepared by acid hydrolysis of cellulose microfibrils and a method, the method can improve the efficiency of subsequent NCC preparation by disassembling cellulose through homogenization treatment, amorphous regions in a cellulose structure are removed through sulfuric acid pyrohydrolysis, and stable cellulose crystalline regions are remained under the condition of hot acid, so that a large amount of amorphous regions in the cellulose are lost, and the product yield is low. Chinese patent CN105603020A discloses a method for preparing nanocellulose crystal, which is characterized by high yield of NCC prepared by the method, but the obtained NCC has no electric charge on surface, poor dispersion stability of solution, and is not beneficial to subsequent application. Based on the problems of yield and product stability of the NCC preparation technology, Chinese patent CN108484782A discloses a carboxylation modified nano-crystalline cellulose and a preparation method thereof, the method takes microcrystalline cellulose as a raw material, uses a composite oxidant consisting of TEMPO, sodium periodate, sodium hypochlorite and sodium chlorite to carry out one-pot oxidation on the microcrystalline cellulose to obtain carboxylation modified cellulose, and then adopts a micro-jet high-pressure homogenizer to process to obtain the carboxylation modified NCC, the method takes the microcrystalline cellulose as the raw material, and because the microcrystalline cellulose has compact structure and strong hydrogen bond interaction in molecules, the carboxylation modification only occurs on the surface of the microcrystalline cellulose, the carboxylation degree of the product is not high, thereby influencing the stability of the product nano-crystalline cellulose in solution; meanwhile, a plurality of oxidants are simultaneously oxidized by a one-pot method, competition for active hydroxyl exists among a plurality of oxidants, so that the oxidation efficiency is greatly low, and the selectivity of the oxidants on oxidation sites is restricted, so that the carboxylation degree of the product is low; in addition, the "one-pot" oxidation results in a complex composition of the product, affecting the purity of the final product, NCC. Based on the technical defects, the invention takes plant cellulose as a raw material, adopts a step-by-step oxidation method to control carboxyl substitution in an amorphous area of the cellulose, thereby improving the degree of carboxylation substitution, and then carries out thermal degradation to break chains of the carboxylated cellulose in the amorphous area to form the hair-shaped NCC with amorphous carboxylated cellulose at two ends. In the present invention, the amorphous cellulose region becomes a component of NCC after carboxylation, so that the yield of the product is ensured, the degree of substitution of carboxyl groups of the hair-like NCC is high, and a stable dispersion is easily formed in a solution based on electrostatic repulsion.

Disclosure of Invention

To overcome the disadvantages and shortcomings of the prior art, the primary object of the present invention is to provide a method for preparing a highly carboxyl-substituted hairy cellulose nanocrystal.

The invention also aims to provide the high carboxyl substituted hairy cellulose nano-crystal prepared by the method.

The purpose of the invention is realized by the following technical scheme:

a method for preparing high carboxyl substituted hairy cellulose nanocrystals comprising the steps of:

(1) using plant cellulose as raw material, dispersing cellulose in pH buffer solution, adding 2,2,6, 6-tetramethyl piperidine oxide/NaClO₂Oxidation system (TEMPO/NaClO)₂) Obtaining a C6-carboxyl cellulose derivative after the reaction is finished, then mixing the C6-carboxyl cellulose derivative with water to form a dispersion liquid, and carrying out ultrasonic treatment on the dispersion liquid to obtain C6-carboxyl cellulose nanofibrils;

(2) mixing the C6-carboxyl cellulose nanofibrils obtained in the step (1) with a pH buffer solution, adding periodate, stirring for reaction, adding ethylene glycol to stop the reaction, and obtaining a 2, 3-dialdehyde-6-carboxyl cellulose nanofibril suspension;

(3) mixing the 2, 3-dialdehyde-6-carboxyl cellulose nanofibril suspension obtained in the step (2) with sodium chloride-sodium chlorite-hydrogen peroxide multi-element oxidant (NaCl/NaClO)₂/H₂O₂) After mixing, stirring and reacting at room temperature to obtain cellulose nanofibril suspension with high carboxyl substitution after the reaction is finished;

(4) and (4) heating the cellulose nanofibril suspension with high carboxyl substitution obtained in the step (3), and cooling to room temperature to obtain the hair-like cellulose nanocrystal with high carboxyl substitution.

Preferably, the plant fiber in the step (1) is wood fiber or/and gramineous fiber; more preferably, the wood fiber is at least one of softwood fiber, eucalyptus fiber and hardwood fiber, and the non-wood fiber is at least one of sisal fiber, cotton pulp fiber, cornstalk fiber and bamboo fiber.

The concentration of the plant cellulose in the pH buffer solution in the step (1) is 1-5%, w/v;

adding TEMPO/NaClO into the step (1)₂The purpose of the method is to ensure that primary alcohol hydroxyl at the C6 position in a cellulose glucose structural unit is subjected to directional oxidation to form a C6-carboxyl cellulose derivative;

preferably, the plant fiber is absolutely dry per gram in step (1)Adding TEMPO 0.01-0.02 g and NaClO into the vitamin₂8mmol-12mmol, NaClO 1mmol-10 mmol; the NaClO₂The purity of (b) is preferably 70 to 90%, more preferably 80%.

The reaction in the step (1) is carried out at 35-65 ℃ for 2-10 h; more preferably, the pH during the reaction is 4.8-6.8, and stirring is carried out at 400-600rpm until the pH is maintained.

The solid content of the dispersion liquid in the step (1) is 0.1-4%, w/v.

The ultrasonic treatment conditions in the step (1) are as follows: the solid content of the dispersion liquid is 0.1-4%, W/v, the ultrasonic power is 800-.

The pH value of the pH buffer solution in the step (2) is 3-6. The feed-liquid ratio of the C6-carboxyl cellulose nanofibrils to the pH buffer solution is 1g:50mL-1g:250 mL; preferably 1 g/100 mL-1 g/200 mL.

In the step (2), 1-6g of periodate is added into each gram of cellulose nanofibrils containing C6-carboxyl;

the stirring reaction in the step (2) is carried out at 25-40 ℃ for 6-24h, and the stirring speed is 100-300 rpm. Preferably, the stirring reaction is carried out under the condition of keeping out light. The reaction oxidizes the secondary hydroxyl groups at the C2 and C3 positions in the C6-carboxyl cellulose nanofibrillar glucose building block to generate aldehyde groups.

The volume-mass ratio of the added amount of the glycol to the periodate in the step (2) is 0.5-2mL:1g, preferably 0.8mL:1 g;

preferably, the suspension obtained in step (2) is further dialyzed for 3-7 days, and the cut-off molecular weight of the dialysis bag is 2000-10000 Da.

Preferably, in the step (3), 2-15g of NaCl and 0.9-3.6g of NaClO are added to each gram of the 2, 3-dialdehyde-6-carboxyl cellulose nanofibril suspension₂And 0.9-3.3g H₂O₂(ii) a Preferably, the NaClO₂And H₂O₂The molar concentrations were 2 times the molar concentrations of the oxidized nanocelluloses, respectively.

The stirring time at room temperature in the step (3) is 12-36h, preferably 24 h. Preferably, the reaction is carried out at room temperature, with the pH of the reaction being maintained at 5 with NaOH solution.

The highly carboxyl substituted cellulose nanofibril suspension obtained in step (3) is preferably washed to neutrality by water.

The heating treatment in the step (4) is heating for 3-8h at the temperature of 60-80 ℃; cleaving glycosidic bonds of amorphous cellulose regions in the nanocellulose chains;

after cooling to room temperature, preferably purifying the obtained product, namely centrifuging the mixed solution containing the final product, removing unreacted micron fiber precipitate, and then adding absolute ethyl alcohol into the supernatant until no further precipitate is generated; and centrifuging at a certain speed again, and collecting white precipitate to obtain the high carboxyl substituted hairy cellulose nanocrystal.

Preferably, the conditions for the first centrifugation in step (4) are as follows: the centrifugation speed is 25000 to 30000rpm, and the centrifugation time is 5 to 20 min; the conditions for the second centrifugation are: the centrifugal speed is 2500-3500 rpm, and the centrifugal time is 10-20 min.

A highly carboxyl-substituted hairy cellulose nanocrystal prepared by the above method. The carboxyl content of the high carboxyl substituted hairy cellulose nano-crystal is 3.5-6.2mmol/g, the polymerization degree is 700-900, the crystal size length is 130-240nm, and the diameter is 5-10 nm.

A highly carboxyl-substituted hairy cellulose nanocrystal prepared by the above method.

The invention adopts plant fiber as raw material, and uses TEMPO/NaClO₂Oxidation system oxidation cooperating with ultrasonic treatment for nano treatment, periodate and NaCl/NaClO₂/H₂O₂After oxidation, heat treatment and centrifugal treatment, the carboxyl cellulose nanocrystal with high purity, high carboxyl content, uniform dispersion in water phase and high substitution can be prepared.

The design principle of the invention is as follows: (1) TEMPO/NaClO₂The C-6 primary hydroxyl of the cellulose is selectively oxidized into carboxyl, and the negative charge repulsive force and the repulsive effect thereof among the carboxylates greatly reduce the separation difficulty of single microfibril, weaken the hydrogen bond action among molecules and exceedThe relatively weak interface between the microfibrils can be damaged by sound wave impact, and meanwhile, the longitudinal covalent bonds of the cellulose molecular chains are kept complete, so that the C6-carboxyl cellulose nanofibrils are obtained; (2) periodate selectively oxidizes hydroxyl at C2 and C3 positions of C6-carboxyl cellulose nano-fibrils into aldehyde groups, and the oxidation process is hindered by diffusion of ions in crystalline regions of cellulose but easily penetrated in amorphous regions, so that the aldehyde groups are mainly distributed in the outer layers of the crystalline regions and almost all the amorphous regions. The rapid erosion in the oxidation process mainly occurs in an amorphous area, the surface of the crystal slowly reacts along with the prolonging of the reaction time, and finally the oxidation of the crystal core is carried out, so that the integrity of the integral structure and the crystalline area of the fiber can be kept to the maximum extent in the reaction; (3) using NaCl/NaClO₂/H₂O₂Further oxidizing C2 and C3 aldehyde groups of the 2, 3-dialdehyde-6-carboxyl cellulose nanofibrils into carboxyl, stripping the surface of a cellulose crystal area in the oxidation process, decomposing the amorphous area partially, and introducing the carboxyl into the amorphous area to promote the dissolution of cellulose to generate soluble functional cellulose biopolymer and disordered cellulose polymer; (4) thermal degradation leads cellulose to be subjected to chain scission in an amorphous area, so as to form hair-shaped NCC with amorphous aldehyde and carboxyl cellulose at two ends, and single crystal nano particles are attached to the two ends of the hair-shaped NCC; (5) the nanofibers separated from the nanofibers by centrifugation to give the hair-like NCC containing amorphous carboxy cellulose.

The main characteristics of the prior art are as follows:

the traditional preparation of NCC adopts a two-step acidolysis method, wherein sulfuric acid is used firstly to remove the amorphous region of cellulose and reserve the junction region to obtain microcrystalline cellulose, then further acid hydrolysis is carried out to disassemble the microcrystalline cellulose into NCC, a certain amount of sulfuric acid groups are introduced into the surface of the NCC, and the NCC has certain stability. But the amorphous area of the product is removed, and only the crystalline area is remained, so the yield is low; meanwhile, because the reaction efficiency of the microcrystalline cellulose is lower, the NCC sulfate group content of the product is low, the charge quantity is low, the product is not stable enough and is easy to agglomerate; the two-step acid hydrolysis uses a large amount of acid, so that the environment is easily polluted; meanwhile, the use of acid also makes the reaction process have high requirement on the acid resistance of equipment.

The improved NCC preparation method generally takes plant cellulose as a raw material, combines an oxidizing agent with mechanical disassembly, and has the advantages of milder reaction process, less environmental pollution and lower equipment requirement compared with acid hydrolysis, but has certain limitation: the oxidation reaction has lower intensity compared with the acid hydrolysis reaction, and the reaction mainly occurs in an amorphous area, so the requirement on the reaction accessibility of the raw material cellulose is higher, while the oxidizing agent has low oxidation degree on the crystalline raw material, and the raw material needs to contain an amorphous cellulose area; furthermore, in the cellulose glucopyranose structural unit, the oxidation properties of the three active-OH groups are different, and due to steric effects during the reaction, the oxidation agents are required differently.

Compared with the prior art, the invention has the following advantages and beneficial effects:

(1) compared with the prior art, the invention has the following advantages and beneficial effects: the invention takes plant cellulose as raw material, has high content of amorphous cellulose, is beneficial to the selective oxidation of an oxidant and improves the reaction efficiency.

(2) The invention adopts a step-by-step oxidation method, adopts different types of oxidants, and selectively oxidizes different oxidants aiming at different-OH active sites in cellulose, so that carboxyl and aldehyde group substitution sites are mainly controlled at specific sites of an amorphous region of the cellulose, the substitution degree of carboxyl is improved, the charge content of a product is favorably improved, the stability of a product solution is enhanced, and meanwhile, the subsequent degradation of the amorphous region is favorably realized.

(3) The invention adopts thermal degradation and NaCl/NaClO₂/H₂O₂And (3) performing oxidation treatment and thermal degradation to break the cellulose in an amorphous area to form the hair-like NCC with amorphous aldehyde-converted and carboxylated cellulose at two ends. The amorphous cellulose region becomes a component of NCC after carboxylation, so that the product yield and the carboxyl substitution degree are improved, and the hair-like NCC of the carboxylated cellulose is easy to form stable dispersion in solution based on electrostatic repulsion.

(4) The invention has simple treatment process and low requirement on equipment. The oxidation process disassembles the cellulose into nano-cellulose which is looser in the solution, then the nano-cellulose is disassembled into nano-fiber fibrils through ultrasonic treatment of a cell disruption instrument, and the nano-cellulose is shortened through the heating process to reduce the molecular weight of the nano-cellulose. Compared with mechanical equipment such as a homogenizer, the ultrasonic equipment has the advantages of milder treatment process and easy operation.

Drawings

Figure 1 is a structural model of highly carboxyl substituted hairy NCC.

FIG. 2 is a schematic diagram of the process for preparing highly carboxy-substituted hairy NCCs.

FIG. 3 is an AFM image of the highly carboxy-substituted hairy NCC prepared in example 1.

Figure 4 is a DLS plot of the highly carboxy-substituted hairy NCC prepared in example 1.

Detailed Description

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

Those who do not specify specific conditions in the examples of the present invention follow conventional conditions or conditions recommended by the manufacturer. The raw materials, reagents and the like which are not indicated for manufacturers are all conventional products which can be obtained by commercial purchase. The room temperature of the invention is 20-35 ℃. The structural model of the highly carboxyl-substituted hairy NCC of the present invention and the process for its preparation are shown in FIGS. 1 and 2. NaClO used in the examples₂Purchased in the market at 80% purity.

Example 1

Using sisal residue fiber as raw material, dissolving per gram of absolutely dry fiber in 100mL phosphate buffer (0.05mol/L, pH 7) to form 1% (w/v) slurry, and adding TEMPO0.016g, 10mmol NaClO₂And 1mmol NaClO; reacting for 6 hours at 60 ℃ under magnetic stirring of 500rpm, keeping the pH constant, adding 100mL of absolute ethyl alcohol to terminate the reaction, repeatedly washing the obtained oxidized sample until the pH of the solution is neutral, filtering and collecting precipitate slurry, adding oxidized cellulose into deionized water to prepare 0.5% (W/v) suspension, performing ultrasonic treatment for 30min by 800W, performing suction filtration and washing under a vacuum condition until the pH value is neutral. Adding 150mL of pH per gram of C6-carboxyl nano celluloseIn 4.5g acetic acid-sodium acetate buffer, 3g sodium periodate was added to each gram of C6-carboxyl-containing nanocellulose, and the reaction was carried out at 30 ℃ and 200rpm with exclusion of light for 6 hours with stirring, and 5mL ethylene glycol was added to terminate the reaction. Dialyzing the reaction solution for 3 days to obtain 2, 3-dialdehyde-6-carboxyl cellulose nanofibril suspension, adding 2.93g NaCl and 1.41g NaClO into each gram of the suspension containing 2, 3-dialdehyde-6-carboxyl cellulose nanofibril₂And 1.41g H₂O₂Reacting at room temperature at a stirring speed of 100rpm for 24h, controlling the pH of a reaction system to be about 5 by using 0.5M NaOH, repeatedly washing an obtained oxidation sample until the pH of the solution is neutral, heating at 80 ℃ for 6h, cooling to room temperature, centrifuging the solution at a speed of 27000rpm for 10min, removing precipitates, slowly adding absolute ethyl alcohol into a supernatant until no further precipitates are generated, centrifuging the solution at a speed of 3000rpm for 15min, and collecting white precipitates to obtain the hair-shaped NCC containing amorphous carboxyl cellulose.

The yield of high carboxyl substituted hairy NCC prepared in this example was 23%. The particle size was 161nm, the Zeta-potential was-37.5 mV, the crystallinity was 90%, the length was 241nm, the diameter was 5nm, and the NCC surface-COOH content was 4.5 mmol/g.

Example 2

Eucalyptus fibers were used as a raw material, and each gram of absolutely dry fibers was dissolved in 100mL of phosphate buffer (0.05mol/L, pH 7) to form a slurry with a concentration of 1% (w/v), and TEMPO0.016g, 9mmol of NaClO was added thereto₂And 2mmol NaClO; reacting for 6 hours at 55 ℃ under magnetic stirring of 500rpm, keeping the pH constant, adding 100mL of absolute ethyl alcohol to terminate the reaction, repeatedly washing the obtained oxidized sample until the pH of the solution is neutral, filtering and collecting precipitate slurry, adding oxidized cellulose into deionized water to prepare 0.8% (W/v) suspension, performing ultrasonic treatment for 30min by 1000W, performing suction filtration and washing under a vacuum condition until the pH value is neutral. Adding 150mL of acetic acid-sodium acetate buffer solution with the pH value of 4.5 into each gram of C6-carboxyl nano cellulose, adding 3g of sodium periodate into each gram of C6-carboxyl nano cellulose, reacting for 8 hours in a dark place at the temperature of 30 ℃ and the stirring speed of 200rpm, and adding 5mL of glycol to stop the reaction. Dialyzing the reaction solution for 3 days to obtain 2, 3-dialdehyde-6-carboxyl fiberThe suspension of cellulose nanofibrils, each gram of which contains 2, 3-dialdehyde-6-carboxyl cellulose nanofibrils, is added with 3.12g NaCl and 2.56g NaClO₂And 2.45g H₂O₂Reacting at room temperature at a stirring speed of 103rpm for 24h, controlling the pH of a reaction system to be about 5 by using 0.5M NaOH, repeatedly washing an obtained oxidation sample until the pH of the solution is neutral, heating at 75 ℃ for 4h, cooling to room temperature, centrifuging the solution at a speed of 27000rpm for 10min, removing precipitates, slowly adding absolute ethyl alcohol into a supernatant until no further precipitates are generated, centrifuging the solution at a speed of 3000rpm for 15min, and collecting white precipitates to obtain the hair-shaped NCC containing amorphous carboxyl cellulose.

The yield of high carboxy-substituted hairy NCC prepared in this example was 18%. The particle size was 159nm, the Zeta-potential was-35.5 mV, the crystallinity was 86%, the length was 169nm, the diameter was 7nm, and the NCC surface-COOH content was 3.5 mmol/g.

Example 3

Bamboo fiber as raw material was dissolved in 100mL of phosphate buffer (0.05mol/L, pH 7) per gram of absolutely dry fiber to form a slurry with a concentration of 1% (w/v), and 0.018g TEMPO, 9mmol NaClO was added thereto₂And 3mmol NaClO; reacting for 7 hours at 55 ℃ under the magnetic stirring of 450rpm, keeping the pH constant, adding 100mL of absolute ethyl alcohol to terminate the reaction, repeatedly washing the obtained oxidized sample until the pH of the solution is neutral, filtering and collecting precipitate slurry, adding oxidized cellulose into deionized water to prepare 0.6% (W/v) suspension, performing ultrasonic treatment on the suspension for 40 minutes by 900W, performing suction filtration under the vacuum condition, washing and washing until the pH value is neutral. Adding 200mL of acetic acid-sodium acetate buffer solution with the pH value of 4.5 into each gram of C6-carboxyl nano cellulose, adding 5g of sodium periodate into each gram of C6-carboxyl nano cellulose, reacting for 5 hours in a dark place at the temperature of 35 ℃ and the stirring speed of 198rpm, and adding 5mL of glycol to stop the reaction. Dialyzing the reaction solution for 3 days to obtain 2, 3-dialdehyde-6-carboxyl cellulose nanofibril suspension, and adding 6.72g of NaCl and 3.13g of NaClO into each gram of the suspension containing the 2, 3-dialdehyde-6-carboxyl cellulose nanofibril₂And 3.02g H₂O₂The reaction was carried out at room temperature for 24h at a stirring speed of 121rpm, controlled with 0.5M NaOHAnd (3) repeatedly washing the obtained oxidation sample until the pH value of the solution is neutral, heating at 80 ℃ for 4h, cooling to room temperature, centrifuging the solution at 27000rpm for 10min, removing precipitates, slowly adding absolute ethyl alcohol into the supernatant until no further precipitates are generated, centrifuging the solution at 3000rpm for 15min, and collecting white precipitates to obtain the hairy NCC containing amorphous carboxyl cellulose.

The yield of high carboxy-substituted hairy NCC prepared in this example was 36%. The particle size was 182nm, the Zeta-potential was-45.3 mV, the crystallinity was 91%, the length was 198nm, the diameter was 10nm, and the NCC surface-COOH content was 4.8 mmol/g.

Example 4

Cotton pulp fibers were used as a raw material, and each gram of the absolutely dry fibers was dissolved in 100mL of phosphate buffer (0.05mol/L, pH 7) to form a slurry having a concentration of 1% (w/v), and 0.014g of TEMPO, 7mmol of NaClO was added thereto₂And 2mmol NaClO; reacting for 7 hours at 50 ℃ under magnetic stirring of 505rpm, keeping the pH unchanged, adding 100mL of absolute ethyl alcohol to terminate the reaction, repeatedly washing the obtained oxidized sample until the pH of the solution is neutral, filtering and collecting precipitate slurry, adding oxidized cellulose into deionized water to prepare 1.3% (W/v) suspension, performing ultrasonic treatment for 30min by 1200W, performing suction filtration under a vacuum condition, washing and washing until the pH value is neutral. Adding 200mL of acetic acid-sodium acetate buffer solution with the pH value of 4 into each gram of C6-carboxyl nano cellulose, adding 4g of sodium periodate into each gram of C6-carboxyl nano cellulose, reacting for 6 hours at 30 ℃ in a dark place at the stirring speed of 200rpm, and adding 5mL of ethylene glycol to stop the reaction. Dialyzing the reaction solution for 3 days to obtain 2, 3-dialdehyde-6-carboxyl cellulose nanofibril suspension, and adding 3.12g of NaCl and 1.98g of NaClO into each gram of the suspension containing the 2, 3-dialdehyde-6-carboxyl cellulose nanofibril₂And 1.76g H₂O₂Reacting at room temperature at 109rpm for 24h, controlling pH of the reaction system to about 5 with 0.5M NaOH, repeatedly washing the obtained oxidized sample until the pH of the solution is neutral, heating at 80 ℃ for 4h, cooling to room temperature, centrifuging the solution at 27000rpm for 10min, removing precipitate, slowly adding absolute ethyl alcohol into the supernatant,until no further precipitate was formed, the solution was then centrifuged at 3000rpm for 15min and the white precipitate was collected to give a hairy NCC containing amorphous carboxycellulose.

The yield of high carboxyl substituted hairy NCC prepared in this example was 53%. The particle size was 148nm, the Zeta-potential was-48.3 mV, the crystallinity was 87%, the length was 132nm, the diameter was 6nm, and the NCC surface-COOH content was 6.2 mmol/g.

Comparative example 1

Comparative example 1 is a carboxylated modified nanocellulose crystal prepared by a "one-pot" process.

Taking microcrystalline cellulose as a raw material, dissolving microcrystalline cellulose in 10mL of acetic acid-sodium acetate buffer solution with the pH value of 6 per gram, uniformly mixing, adding deionized water to form microcrystalline cellulose dispersion liquid with the concentration of 1% (w/v), and adding 0.08g of TEMPO, 4.5g of sodium periodate and 4.92g of NaClO₂And 2g NaClO; stirring and reacting for 5 hours at 25 ℃ in a dark place, adding 20mL of absolute ethyl alcohol to terminate the reaction, carrying out suction filtration on the obtained oxidation sample under the pressure condition of 0.3psi to obtain a product, repeatedly washing the product with deionized water at 75 ℃ until the pH value of the solution is neutral, and washing the reaction product for 3 times with an ethanol solution with the volume fraction of 50% to obtain the carboxyl modified microcrystalline cellulose; adding the carboxyl modified microcrystalline cellulose into deionized water to prepare 0.1% (w/v) dispersion, and treating for 10 times at 10000psi by using a micro-jet high-pressure homogenizer, wherein the temperature of the dispersion is controlled to be 10 ℃ in the treatment process, so that the carboxyl modified nano-cellulose crystal is obtained.

The performance test analysis of the nanocellulose crystals prepared in each of examples 1 to 4 and comparative example 1 was as follows:

TABLE 1 Properties of nanocellulose crystals obtained in examples 1 to 4 and comparative example 1

As can be seen from Table 1, the nanocellulose crystals prepared by the thermal degradation and step-wise oxidation process of the present invention have smaller particle size (i.e., length in Table 1) and higher carboxyl group content than the "one-pot" process, which is about 8-15 times the carboxyl group content on the surface of the carboxylated nanocellulose crystals prepared by the "one-pot" process.

The above embodiments are preferred embodiments of the present invention, but the present invention is not limited to the above embodiments, and any other changes, modifications, substitutions, combinations, and simplifications which do not depart from the spirit and principle of the present invention should be construed as equivalents thereof, and all such changes, modifications, substitutions, combinations, and simplifications are intended to be included in the scope of the present invention.