A method for preparing cellulose nanocrystals with excellent dispersibility

Abstract

The application discloses a preparation method of cellulose nanocrystals with excellent dispersity, and belongs to the technical field of dispersion of nanocellulose. The method comprises the following steps: uniformly stirring and mixing CNC suspension and a polar organic solvent, and then performing ultrasonic treatment to obtain a composite suspension; performing an evaporation-induced self-assembly process on the composite suspension to obtain a cellulose nanocrystal-based composite iridescent film; diluting the cellulose nanocrystal-based composite iridescent film to form a re-dispersed composite suspension, and performing freeze-drying on the composite suspension; and then re-dispersing the dried product to obtain cellulose nanocrystals with excellent dispersity. The method introduces an organic solvent, such as N, N-dimethylformamide, to overcome the problems that bacteria are easy to grow and the cellulose nanocrystals have strong hygroscopicity during storage.

Description

Technical Field

[0001] This invention relates to the field of nanocellulose dispersion technology and the optical properties of chiral photonic crystal films, specifically a method for preparing cellulose nanocrystals with excellent dispersibility. Background Technology

[0002] Cellulose nanocrystals (CNCs) are nanoscale cellulose extracted from natural fibers. They are usually rod-shaped, spherical, or disc-shaped. They not only have the characteristics of nanoparticles, but also have some unique strength and optical properties. However, preparing CNC-based chiral materials with excellent optical properties is quite challenging, which also makes them have broad application prospects.

[0003] CNC particles can generally be divided into two types: solid powder and suspension. However, the solid powder form has disadvantages such as low thermal stability and poor dispersibility. To improve its dispersion, high-performance CNC powder can usually be obtained through simple mixing and modification with ionic liquids. Currently, the solid content concentration of well-dispersed unmodified CNC suspensions is low, resulting in high storage and transportation costs for CNC, and making it difficult to prevent the growth of bacteria and fungi in the suspension. In addition, high solid content is required when CNC is dispersed in solvents for modification and the preparation of composite materials. Therefore, CNC suspensions must be dried to reduce storage and transportation costs and prevent the growth of bacteria and fungi in the suspension. However, the CNC surface contains a large number of hydroxyl groups, which makes it easy to form intermolecular or intramolecular hydrogen bonds, causing aggregation when it is redispersed after drying.

[0004] Therefore, to improve the redispersion effect of CNC particles, the surface charge density of CNC can be increased to increase the repulsive force between fibers (such as carboxylation, sulfonation, phosphorylation, quaternization, etc.). Alternatively, macromolecules compatible with solvents can be introduced onto its surface to form steric hindrance, thereby increasing interfacial compatibility and meeting the requirements for large-scale applications. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention provides a method for preparing cellulose nanocrystals with excellent dispersibility, which improves the dispersibility of CNC suspension particles by controlling the mass ratio of DMF to CNC.

[0006] This invention is achieved through the following technical solution:

[0007] A method for preparing cellulose nanocrystals with excellent dispersibility includes the following steps:

[0008] Step 1: Mix the CNC suspension with a polar organic solvent until homogeneous, and then perform ultrasonic treatment to obtain a composite suspension;

[0009] Step 2: Obtain a cellulose nanocrystal-based composite iris film by evaporation-induced self-assembly of the composite suspension;

[0010] Step 3: Dilute the cellulose nanocrystal-based composite iris film to form a composite suspension, freeze-dry the composite suspension, and then redisperse the dried product to obtain cellulose nanocrystals with excellent dispersibility.

[0011] Preferably, the polar organic solvent is a DMF solution, a DMSO solution, or an NMP solution.

[0012] Preferably, the mass ratio of the CNC suspension to the polar organic solvent is (5-10):1.

[0013] Preferably, the concentration of the composite suspension in step 2 is 1-5 wt%.

[0014] Preferably, the temperature of the composite suspension in step 2 is 20-40℃.

[0015] Preferably, the humidity of the composite suspension in step 2 is 45-65% RH.

[0016] Preferably, the freeze-drying temperature in step 3 is -40°C.

[0017] Preferably, the freeze-drying time in step 3 is 24 hours.

[0018] A cellulose nanocrystal with excellent dispersibility is prepared by the aforementioned preparation method.

[0019] Compared with the prior art, the present invention has the following beneficial technical effects:

[0020] This application provides a method for preparing cellulose nanocrystals with excellent dispersibility. CNC is placed in a DMF solution without any other surfactants or chemical modifications, providing a simple strategy for better integration of CNC with other functional materials. By controlling the mass ratio of DMF to CNC, not only can the dispersibility of the CNC suspension particles be improved, but the optical properties of the CNC film can also be effectively adjusted, resulting in films with uniform appearance and smooth surfaces. Secondly, since the redispersibility of dried samples deteriorates due to irreversible hydrogen bonding, a co-solvent strategy using DMF and water is employed to reduce hydrogen bonds between CNC particles, thereby improving the colloidal stability of the redispersed suspension. This provides a reference for the preparation of CNC-based uniform structural color materials. Testing shows that this method effectively improves the aggregation phenomenon of CNC suspensions, and the dried product overcomes the problems of bacterial growth and high hygroscopicity of CNC during storage, marking an important step towards its functional applications in various fields. Attached Figure Description

[0021] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 Atomic force microscopy (AFM) images of the cellulose nanocrystal suspension and composite suspension prepared in this invention.

[0023] Figure 2 Polarized light microscope (POM) images of the pure cellulose nanocrystal membrane and composite membrane prepared in this invention.

[0024] Figure 3 The UV-Vis spectra of the pure cellulose nanocrystal membrane and composite membrane prepared in this invention are shown.

[0025] Figure 4 This is a scanning electron microscope (SEM) image of the composite membrane prepared in this invention.

[0026] Figure 5 The zeta potential (Zeta potential), dispersion value (PDI), and hygroscopicity (RH) test curve of the dried redispersed sample prepared in this invention are shown.

[0027] Figure 6 This is a bacterial detection diagram of the dried redispersed sample prepared according to the present invention. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. The components of the embodiments of this application described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0029] Therefore, the following detailed description of the embodiments of this application provided in the accompanying drawings is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the application. All other embodiments obtained by those skilled in the art based on the embodiments of this application without inventive effort are within the scope of protection of this application.

[0030] The surface of cellulose nanocrystals contains a large number of hydroxyl groups, which makes them prone to forming intermolecular or intramolecular hydrogen bonds. This is also the reason why they tend to aggregate when redispersed after drying. Therefore, to improve the redispersibility of CNC particles, the surface charge density of CNC can be increased to increase the repulsive force between fibers (such as carboxylation, sulfonation, phosphorylation, quaternization, etc.). Alternatively, solvent-compatible macromolecules can be introduced onto their surface to form steric hindrance and increase interfacial compatibility.

[0031] A method for preparing cellulose nanocrystals with excellent dispersibility includes the following steps:

[0032] Step 1, Preparation of CNC suspension and composite suspension:

[0033] The CNC suspension with a concentration of 5 wt% was diluted with deionized water, and a certain concentration of CNC suspension was added with a polar organic solvent. The mixture was stirred in a magnetic stirrer and then sonicated in an ice-water bath to obtain a composite suspension.

[0034] Dilute the 5wt% CNC suspension to a concentration of 2-3wt% CNC suspension.

[0035] The CNC suspension with a concentration of 2.2 wt% was selected and obtained by sulfuric acid treatment, and the surface was covered with functional groups such as sulfonic acid groups and hydroxyl groups, and it was in the form of rods.

[0036] The polar organic solvent is DMF solution, DMSO solution or NMP solution.

[0037] The CNC suspension and polar organic solvent were stirred for 2 hours and sonicated for 15 minutes.

[0038] When the mass ratio of CNC:DMF is 10:1, the dispersion performance of slightly higher concentration CNC suspensions can be significantly improved.

[0039] Step 2, Preparation of CNC pure membrane and hybrid membrane:

[0040] A certain volume of composite suspension was subjected to concentration, temperature and humidity control, and cellulose nanocrystal-based composite iris film was obtained through an evaporation-induced self-assembly process.

[0041] Step 3, Preparation of redispersible solution and solid powder:

[0042] A cellulose nanocrystal-based composite iris film was placed in a beaker and water was added to obtain a redispersed composite suspension. The composite suspension was then dried in a freeze dryer. The dried sample was then dispersed in separate beakers to obtain powder, thus obtaining the regulated cellulose nanocrystals.

[0043] The freeze-drying temperature was -40℃ for 24 hours. After freeze-drying, the samples were ground and redispersed. The DMF solution played a positive role in the redispersibility of the CNC nanoparticles. After two months of storage, the dried samples exhibited slight hygroscopicity and no bacterial contamination was detected.

[0044] This method places CNC in a DMF solution without any other surfactants or chemical modifications, and improves redispersibility after drying. By introducing organic solvents such as N,N-dimethylformamide (DMF), the problems of easy bacterial growth and strong hygroscopicity of CNC during storage are overcome. At the same time, chiral structural color materials with excellent optical properties are obtained, providing an effective means for the construction and functional application of CNC-based composite materials.

[0045] Example 1

[0046] A method for preparing cellulose nanocrystals with excellent dispersibility includes the following steps:

[0047] Step 1: Dilute the 5wt% CNC suspension with deionized water to a concentration of 2.2wt%, then add DMF solution at a mass ratio of CNC:DMF = 10:1, stir in a magnetic stirrer for 2 hours, and then sonicate in an ice-water bath for 15 minutes to obtain a composite suspension.

[0048] Step 2: 5 mL of a 1 wt% composite suspension was evaporated and self-assembled into a film at 25°C and 45% RH. The composite suspension was then poured into a 35 mm diameter circular culture dish and evaporated and self-assembled into a cellulose nanocrystal-based composite iridescent film.

[0049] Step 3: Dilute the cellulose nanocrystal-based composite iris film to form a CNC suspension, obtain a redispersed composite suspension, and place the composite suspension in a freeze dryer to dry at -40℃ for 24 hours. Place the dried sample in beakers to redisperse it to obtain powder, and obtain the regulated cellulose nanocrystals, labeled as C2.2D.

[0050] Example 2

[0051] A method for preparing cellulose nanocrystals with excellent dispersibility includes the following steps:

[0052] Step 1: Dilute the 5wt% CNC suspension with deionized water to a concentration of 2.2wt%, then add DMF solution at a mass ratio of CNC:DMF = 5:1, stir in a magnetic stirrer for 2 hours, and then sonicate in an ice-water bath for 15 minutes to obtain a composite suspension.

[0053] Step 2: 5 mL of a 1.4 wt% composite suspension was evaporated and self-assembled into a film at 25 °C and 45% RH. The composite suspension was then poured into a 35 mm diameter circular culture dish and evaporated and self-assembled into a cellulose nanocrystal-based composite iridescent film.

[0054] Step 3: Dilute the cellulose nanocrystal-based composite iris film to form a CNC suspension, obtain a redispersed composite suspension, and place the composite suspension in a freeze dryer to dry at -40℃ for 24 hours. Place the dried sample in beakers to redisperse it to obtain powder, thus obtaining the regulated cellulose nanocrystals.

[0055] Example 3

[0056] A method for preparing cellulose nanocrystals with excellent dispersibility includes the following steps:

[0057] Step 1: Dilute the 5wt% CNC suspension with deionized water to a concentration of 2.2wt%, then add DMSO solution at a mass ratio of CNC:DMSO = 10:1, stir in a magnetic stirrer for 2 hours, and then sonicate in an ice-water bath for 15 minutes to obtain a composite suspension, labeled as C-DMSO.

[0058] Step 2: 5 mL of a 1.8 wt% composite suspension was evaporated and self-assembled into a film at 25°C and 45% RH. The composite suspension was then poured into a 35 mm diameter circular culture dish and evaporated and self-assembled into a cellulose nanocrystal-based composite iridescent film.

[0059] Step 3: The composite suspension formed by diluting the cellulose nanocrystal-based composite iris film is placed in a freeze dryer and dried at -40℃ for 24 hours. The dried sample is then placed in beakers and dispersed to obtain powder. The regulated cellulose nanocrystals are then labeled as C-DMSO.

[0060] Example 4

[0061] A method for preparing cellulose nanocrystals with excellent dispersibility includes the following steps:

[0062] Step 1: Dilute the 5wt% CNC suspension with deionized water to a concentration of 2.2wt%, then add NMP solution at a mass ratio of CNC:NMP = 10:1, stir in a magnetic stirrer for 2 hours, and then sonicate in an ice-water bath for 15 minutes to obtain a composite suspension.

[0063] Step 2: 5 mL of a 2.2 wt% composite suspension was evaporated and self-assembled into a film at 25 °C and 45% RH. The composite suspension was then poured into a 35 mm diameter circular culture dish and evaporated and self-assembled into a cellulose nanocrystal-based composite iridescent film.

[0064] Step 3: The composite suspension formed by diluting the cellulose nanocrystal-based composite iris film is placed in a freeze dryer and dried at -40℃ for 24 hours. The dried sample is then placed in beakers and dispersed to obtain powder. The regulated cellulose nanocrystals are then labeled as C-NMP.

[0065] Example 5

[0066] A method for preparing cellulose nanocrystals with excellent dispersibility includes the following steps:

[0067] Step 1: Dilute the 5wt% CNC suspension with deionized water to a concentration of 2.2wt%, then add NMP solution at a mass ratio of CNC:NMP = 10:1, stir in a magnetic stirrer for 2 hours, and then sonicate in an ice-water bath for 15 minutes to obtain a composite suspension.

[0068] Step 2: 5 mL of a 3 wt% composite suspension was evaporated and self-assembled into a film at 25 °C and 45% RH. The composite suspension was then poured into a 35 mm diameter circular culture dish and evaporated and self-assembled into a cellulose nanocrystal-based composite iridescent film under the environmental conditions.

[0069] Step 3: The composite suspension formed by diluting the cellulose nanocrystal-based composite iris film is placed in a freeze dryer and dried at -40℃ for 24 hours. The dried sample is then placed in beakers and dispersed to obtain powder, thus obtaining the regulated cellulose nanocrystals.

[0070] Example 6

[0071] A method for preparing cellulose nanocrystals with excellent dispersibility includes the following steps:

[0072] Step 1: Dilute the 5wt% CNC suspension with deionized water to a concentration of 1wt%, then add DMF solution at a mass ratio of CNC:DMF = 10:1, stir in a magnetic stirrer for 2 hours, and then sonicate in an ice-water bath for 15 minutes to obtain a composite suspension.

[0073] Step 2: 5 mL of a 5 wt% composite suspension was evaporated and self-assembled into a film at 25 °C and 45% RH. The composite suspension was then poured into a 35 mm diameter circular culture dish and evaporated and self-assembled into a cellulose nanocrystal-based composite iridescent film.

[0074] Step 3: Dilute the cellulose nanocrystal-based composite iris film to form a CNC suspension, obtain a redispersed composite suspension, and place the composite suspension in a freeze dryer to dry at -40℃ for 24 hours. Place the dried sample in beakers to redisperse it to obtain powder, thus obtaining the regulated cellulose nanocrystals.

[0075] The following is an experimental analysis of the regulated cellulose nanocrystals prepared in Examples 1-6:

[0076] A CNC suspension with a concentration of 2.2 wt% is denoted as C2.2. Figure 1 AFM images of the microstructure of CNC (C2.2) and CNC / DMF composite suspension (C2.2D) at different magnifications clearly show that the CNC nanoparticles are rod-shaped (i.e., needle-shaped). When the concentration of the CNC suspension is 2.2 wt%, some aggregation occurs, forming partially overlapping and aggregated nanoparticles. When the concentration is further increased to 3-5 wt%, severe aggregation occurs, thus requiring effective adjustment of its dispersion.

[0077] Figure 2 The properties of the CNC membrane solid phase were characterized using POM. The films formed at different concentrations exhibited a relatively uniform and bright blue color, indicating strong long-range orientation of the CNC domains. Among them, the C2.2 membrane showed a relatively saturated structural color and did not exhibit the coffee ring phenomenon.

[0078] Figure 2 The C2.2D cellulose nanocrystal-based composite iris film was characterized using POM (Polymer Oxidation Membrane), revealing significant birefringence and a relatively uniform bright blue pattern. Notably, in the pure CNC film, photonic crystals converge, exhibiting a large micro-gamut and resulting in an uneven color distribution. However, after introducing DMF (Dimethyl Fluoride), the blue photonic crystals aggregated in a smaller micro-gamut and were evenly distributed throughout the composite film. The C2.2D cellulose nanocrystal-based composite iris film showed a more uniform color, which was clearly observable to the naked eye.

[0079] Figure 2POM was used to characterize C-DMSO and C-NMP films. Compared with the C2.2D film, the C-DMSO cellulose nanocrystal-based composite iris film and the C-NMP cellulose nanocrystal-based composite iris film also showed obvious birefringence. However, neither of them presented a single-color pattern. In addition to the coffee ring phenomenon, the surface of the cellulose nanocrystal-based composite iris film was uneven and wrinkled.

[0080] DMF and CNC interact through hydrogen bonds. Figure 3 It can be seen that, compared with the cellulose nanocrystal-based composite iris film with a mass ratio of CNC:DMF=5:1, the cellulose nanocrystal-based composite iris film with a mass ratio of CNC:DMF=10:1 has a higher reflection peak, indicating that its optical performance is better, and also showing that an appropriate amount of DMF can improve the optical performance of the film.

[0081] The uniformity of color in cellulose nanocrystal-based composite iris films is determined by the orderliness of the pitch. Figure 4 Scanning electron microscopy (SEM) revealed that the cross-section of the cellulose nanocrystal-based composite iris film exhibited a clear, regular, layered helical structure and a consistent chiral nematic sequence after the addition of DMF. This explains the structural color of the cellulose nanocrystal-based composite iris film. This indicates that the addition of an appropriate amount of DMF does not hinder CNC self-assembly, and the formed helical structure and its orientation are not significantly affected.

[0082] Figure 5 (a) It can be seen that the ζ-potential values ​​of the R-C2.2 and R-C2.2D samples after redispersibility treatment were -17.9 mV and -35.6 mV, respectively, and the PDI dispersion indices were 0.306 and 0.270, respectively. This indicates that the absolute value of the ζ-potential value of the sample without DMF solution was smaller compared to the sample with added DMF solution, suggesting that DMF solution plays a positive role in the redispersibility of CNC nanoparticles. Furthermore, both samples exhibited low PDI dispersion indices, indicating that they could still be redispersed after freeze-drying.

[0083] The CNC suspension redispersed solution was labeled R-C2.2, and the C2.2D redispersed solution was labeled R-C2.2D. After storage for a period of time, R-C2.2 showed signs of severe aggregation, indicating incomplete redispersibility, while the latter exhibited slight floating, indicating more complete dispersion. This result is attributed to the effective prevention of hydrogen bonding between CNCs by hydrogen bonds formed between DMF and water, thus reducing the number of hydrogen bonds between CNCs and improving redispersibility.

[0084] Figure 5 (b) It can be seen that the dried samples DG-C2, DG-C2.2D and DG-C5 have a weak hygroscopic effect after being stored for two months, which may limit their application. Figure 6 It can be seen that no bacterial contamination was detected in the sample.

[0085] This application, by controlling the mass ratio of DMF to CNC, not only improves the dispersibility of CNC suspension particles but also effectively adjusts the optical properties of the CNC film, resulting in a film with uniform color and a smooth surface. Since dried samples exhibit irreversible hydrogen bonding, leading to poor redispersibility, a co-solvent strategy using DMF and water is employed to reduce hydrogen bonds between CNC particles, thereby improving the colloidal stability of the redispersed suspension. This is beneficial for sample preservation and transportation.

[0086] The above content is only for illustrating the technical concept of the present invention and should not be construed as limiting the scope of protection of the present invention. Any modifications made to the technical solution based on the technical concept proposed in this invention shall fall within the scope of protection of the claims of this invention.

Claims

1. A method for producing cellulose nanocrystals having excellent dispersibility, characterized by, The method comprises the following steps: Step 1, uniformly mixing CNC suspension with polar organic solvent, and then performing ultrasonic treatment to obtain a composite suspension; The polar organic solvent is DMF solution; The CNC suspension is a CNC suspension obtained by sulfuric acid treatment; The mass ratio of the CNC to the polar organic solvent is (5-10):1; Step 2, obtaining a cellulose nanocrystal-based composite iridescent film through an evaporation-induced self-assembly process from the composite suspension; Step 3, diluting the cellulose nanocrystal-based composite iridescent film to form a composite suspension, freeze-drying the composite suspension, and then grinding and redispersing the dried product to obtain cellulose nanocrystals with excellent dispersibility.

2. The method for producing cellulose nanocrystals having excellent dispersibility according to claim 1, characterized by, The concentration of the composite suspension in step 2 is 1-5 wt%.

3. The method of claim 1, wherein the cellulose nanocrystals have excellent dispersibility. The temperature of the composite suspension in step 2 is 20-40℃.

4. The method of producing cellulose nanocrystals with excellent dispersibility according to claim 1, characterized by, The humidity of the composite suspension in step 2 is 45-65% RH.

5. The method of claim 1, wherein the cellulose nanocrystals have excellent dispersibility. The freeze-drying temperature in step 3 is -40℃.

6. The method of claim 1, wherein the cellulose nanocrystals have excellent dispersibility. The freeze-drying time in step 3 is 24h.

7. A cellulose nanocrystal having excellent dispersibility, characterized by, The cellulose nanocrystals are prepared by the preparation method in any one of claims 1-6.

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