CN113637234B - Elastic cellulose aerogel and preparation method and application thereof - Google Patents
Elastic cellulose aerogel and preparation method and application thereof Download PDFInfo
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Abstract
The invention relates to an elastic cellulose aerogel and a preparation method and application thereof, and the preparation method comprises the following steps: (1) Freezing the nano cellulose suspension liquid by liquid nitrogen, and then carrying out freeze drying to obtain the submicron cellulose fiber; (2) Dispersing the submicron cellulose fiber in water, freezing for the second time, and then freezing and drying to obtain the elastic cellulose aerogel; the elastic cellulose aerogel is subjected to surface modification by adopting a silane compound through a chemical vapor deposition method to obtain the super-hydrophobic elastic cellulose aerogel, and the super-hydrophobic elastic cellulose aerogel is applied to the field of oil-water separation. The elastic cellulose aerogel obtained by the method has good rebound resilience and shape recovery performance under normal temperature and cold environment, and also has low heat conduction coefficient, the super-hydrophobic elastic cellulose aerogel obtained by surface modification shows super-strong separation performance in oil-water separation, and the adsorption capacity to chloroform can reach about 489 times of the self-mass.
Description
Technical Field
The invention relates to the technical field of high polymer materials, and particularly relates to elastic cellulose aerogel and a preparation method and application thereof.
Background
As the lightest solid material, aerogels have found widespread use in the fields of thermal regulation, energy collection and storage, sensors, environmental remediation, and biomedicine. This diverse use of aerogels is attributed to its numerous advantages, including low density, high porosity, high specific surface area, low thermal conductivity, and biocompatibility. Among the various physical properties of aerogels, mechanical properties are important factors that limit the applications of aerogels. Much effort has been made to improve the mechanical properties of aerogels, mainly focusing on improving the compression resistance and elasticity of organic and inorganic aerogels, and among them, aerogels having high resilience and rapid shape recovery have been further studied because the ability to rapidly recover shape from large compressive strains is extremely important for their applications in electrical signal sensing, water treatment, thermal and acoustic insulation, air filtration and energy storage.
Biologically derived natural polymers, especially cellulose, have attracted considerable interest as a raw material for the preparation of aerogels due to their wide source and ease of processing. Conventional cellulose aerogel synthesis requires a dissolution regeneration process of a large amount of solvent. Recently, nanocellulose, in particular cellulose nanofibers, has been widely used to construct aerogels of high specific surface area and excellent mechanical properties. Due to the high specific surface area and the abundance of polar functional groups, ultra-light cellulose aerogels can be conveniently prepared by freeze-drying nanofiber suspensions, exhibiting excellent structural integrity. Although nanocellulose aerogels have good compressibility and can compress strains of more than 90% without breaking, compressed aerogels generally have poor resilience and cannot recover from a compressed state. The lack of resilience is due to the inelastic microstructure and the strong hydrogen bonds formed between adjacent cellulose fibers during compression. The nanocellulose aerogel prepared by the single freezing method has unsatisfactory resilience performance due to the anisotropic elasticity in the axial direction.
Disclosure of Invention
Aiming at the technical problems that the conventional aerogel is poor in resilience and cannot be recovered from a compressed state, the elastic cellulose aerogel and the preparation method and the application thereof are provided. The elastic cellulose aerogel prepared by the invention has lower density and heat conductivity coefficient, and has better elasticity and shape recovery under normal temperature and cold environment.
In order to achieve the purpose, the invention is realized by the following technical scheme:
a preparation method of elastic cellulose aerogel comprises the following steps:
(1) Freezing the nano cellulose suspension liquid by liquid nitrogen, and then carrying out freeze drying to obtain the submicron cellulose fiber;
(2) And dispersing the submicron cellulose fibers in water, freezing for the second time, and then freezing and drying to obtain the elastic cellulose aerogel.
Further, in the step (1), the nanocellulose suspension is obtained by centrifuging 0.2wt% of nanocellulose dispersion liquid to remove precipitates, collecting an upper layer suspension, wherein the rotation speed of the centrifugation is 3000-800rpm, the centrifugation time is 5-10min, and diluting the upper layer suspension to obtain the nanocellulose suspension with the concentration of 0.05 wt%.
Further, the nano-cellulose dispersion is obtained by oxidizing natural paper pulp by a TEMPO/NaClO oxidation system for 20 min-5 h, in the oxidation treatment process, the pH of the oxidation system is controlled to be 9.8-10.5 by NaOH solution, after the oxidation treatment is finished, a dialysis bag is adopted for dialysis for one week, the nano-cellulose is obtained by treating for 25min by a cell crusher, and the nano-cellulose dispersion with the weight percent of 0.2 is obtained after dilution; the mass ratio of the TEMPO to the NaClO to the paper pulp is 1 (1.4-5) to 1.
Further, in the step (2), the concentration of the submicron cellulose fiber dispersed in water is controlled to be 0.2-2 wt%; preferably, the concentration is controlled to be 0.2wt% to 0.6wt%.
Further, the temperature of the liquid nitrogen freezing in the step (1) is-196 ℃, and the time is 1-2h; freezing for 3 hours at the temperature of minus 20 ℃ by using a refrigerator in the secondary freezing in the step (2); and (3) freezing the freeze drying in the step (1) and the step (2) for 48 hours by adopting a freeze drier with the temperature of-55 to-45 ℃, wherein the freezing pressure is 0.02 to 1Pa.
In another aspect, the invention provides an elastic cellulose aerogel prepared by the preparation method.
The third aspect of the present invention provides the use of the elastic cellulose aerogel prepared by the above preparation method in water-absorbing materials or in heat-insulating materials.
The fourth aspect of the invention provides an application of the elastic cellulose aerogel prepared by the preparation method, the elastic cellulose aerogel is subjected to surface modification by adopting a silane compound through a chemical vapor deposition method to obtain a super-hydrophobic elastic cellulose aerogel, and the super-hydrophobic elastic cellulose aerogel is applied to the field of oil-water separation.
Further, the chemical vapor deposition method comprises the steps of placing the elastic cellulose aerogel above a stainless steel grid, placing two containers respectively containing water and the silane compound below the stainless steel grid side by side, and reacting for 5-7 hours at 70-90 ℃ in a closed vacuum state to obtain the super-hydrophobic elastic cellulose aerogel.
Furthermore, the dosage of the elastic cellulose aerogel, the silane compound and the water is (20-25) mg (1-2) mL:0.5mL; the silane compound is methyl trimethoxy silane or polydimethylsiloxane.
The beneficial technical effects are as follows:
according to the invention, the elastic cellulose aerogel is assembled by a double freezing method of liquid nitrogen freezing and low-temperature refrigerator freezing, and then the super-hydrophobic elastic cellulose aerogel is obtained by surface modification. The density of the elastic cellulose aerogel is 2-20mg/cm 3 The material has better shape recovery, better rebound resilience and super elasticity under normal temperature and extremely cold environment (-196 ℃); the thermal conductivity coefficient is low and is only 0.023W/m.K, and the heat insulation and infrared shielding performances are good; in addition, the super-hydrophobic elastic cellulose aerogel obtained after surface modification shows super-strong separation performance in oil-water separation, and the adsorption capacity of the super-hydrophobic elastic cellulose aerogel on chloroform can reach about 489 times of the self-mass. The material can be used in the fields of high water absorption materials, heat insulation materials, oil-water separation and the like.
Drawings
FIG. 1 is a SEM topography made in example 1, wherein a represents a nanocellulose suspension, b represents sub-micron cellulose fibres, c, d represent an elastic cellulose aerogel; the scale in the abd plot is 500nm and the scale in the c plot is 500. Mu.m.
FIG. 2 is a graph of elastic stress-strain curves of the elastic cellulose aerogel prepared in example 1.
Fig. 3 is an FTIR curve of the superhydrophobic elastic cellulose aerogel of application example 1, wherein a represents the elastic cellulose aerogel of example 2, and b represents the superhydrophobic elastic cellulose aerogel of application example 1.
Fig. 4 is a graph of water contact angle versus time for the superhydrophobic elastic cellulose aerogel of application example 1.
Detailed Description
The technical solutions in the embodiments of the present invention will be clearly and completely described below with reference to the embodiments of the present invention, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. The following description of at least one exemplary embodiment is merely illustrative in nature and is in no way intended to limit the invention, its application, or uses. All other embodiments, which can be obtained by a person skilled in the art without inventive step based on the embodiments of the present invention, are within the scope of protection of the present invention.
Unless specifically stated otherwise, the numerical values set forth in these examples do not limit the scope of the invention. Techniques, methods known to those of ordinary skill in the relevant art may not be discussed in detail, but are intended to be part of the specification where appropriate. In all examples shown and discussed herein, any particular value should be construed as merely illustrative, and not limiting. Thus, other examples of the exemplary embodiments may have different values.
Example 1
A preparation method of elastic cellulose aerogel comprises the following steps:
(1) Putting 0.05wt% of nano cellulose suspension into liquid nitrogen at the temperature of-196 ℃ for liquid nitrogen freezing for 1h, and then putting the nano cellulose suspension into a freeze drier at the temperature of-55 ℃ and the pressure of 0.02Pa for freeze drying for 48h to obtain sub-micron cellulose fibers;
(2) Dispersing the submicron cellulose fiber in water by a vortex mixer to prepare a dispersion liquid with the concentration of 0.2wt%, uniformly stirring, placing in a refrigerator with the temperature of-20 ℃ for secondary freezing for 3h, and then placing in a freeze drier with the temperature of-55 ℃ and the pressure of 0.02Pa for freeze drying for 48h to obtain the elastic cellulose aerogel.
Wherein the method for obtaining the nano-cellulose suspension in the step (1) comprises the following steps: (1) oxidizing natural paper pulp by using a TEMPO/NaClO oxidation system for 2 hours, controlling the pH of the oxidation system to be 10 by using NaOH solution in the oxidation treatment process, dialyzing for one week by using a dialysis bag after the oxidation treatment is finished, treating for 25min by using a cell crusher to obtain nano-cellulose, and diluting to obtain 0.2wt% of nano-cellulose dispersion liquid; the mass ratio of the TEMPO to the NaClO to the pulp is 1.6; (2) and (3) placing the 0.2wt% of nano-cellulose dispersion liquid into a centrifuge tube, centrifuging by using a high-speed centrifuge, wherein the rotating speed of the centrifuge is 5000r/min, the centrifuging time is 10 minutes, removing white precipitate at the bottom of the centrifuge tube after the centrifugation is finished, collecting upper-layer suspension, and diluting the upper-layer suspension to obtain the 0.05wt% nano-cellulose suspension.
Observing an electron microscope on the elastic cellulose aerogel prepared by the submicron cellulose fiber prepared in the step (1) in the embodiment, as shown in fig. 1, wherein a represents a nano cellulose suspension, b represents the submicron cellulose fiber, and c and d represent the elastic cellulose aerogel; the scale in the abd plot is 500nm and the scale in the c plot is 500. Mu.m. As can be seen from FIG. 1, the original state of the nanocellulose suspension in FIG. 1a has a cellulose width of 3-5nm and a cellulose length of at least 500nm; FIG. 1b shows continuous submicron cellulose fibers after freezing with liquid nitrogen, the fibers having a width of about 100-200nm; fig. 1c and 1d show the aerogel obtained after freezing by liquid nitrogen and then performing low-temperature secondary freezing by a refrigerator, and it can be seen that the micro-morphology of the aerogel presents a honeycomb structure after the liquid nitrogen freezing and the secondary freezing by the low-temperature refrigerator.
The elastic cellulose aerogel obtained in this example was subjected to a stress-strain test in which a cylindrical aerogel (21 mm diameter, 23mm height) was placed on an Instron 3345 type material test system and compressed to a strain of 60% using a 2KN load cell. As shown in fig. 2, it can be seen from fig. 2 that the cyclic compressive stress-strain curve of the aerogel of this example shows good elastic behavior after 50 cycles, the aerogel can almost recover to the original size after compression, the unrecoverable strain is only 3.8% after 50 cycles, and the resilience is good. The ultimate stress was maintained at 0.542kPa, near the first cycle, indicating that the aerogel structure remained better.
The elastic aerogel prepared by the invention has better resilience, and the better resilience can be attributed to the existence of submicron fibers. The nano-scale cellulose fibers (with the diameter of 3-5 nm) can be assembled into submicron cellulose fibers with the diameter of 100-200nm in the liquid nitrogen freezing process, the submicron cellulose fibers are dispersed in water to form dispersion liquid, and then in the refrigerator freezing (-20 ℃) process, as large ice crystals grow at a low freezing rate, a honeycomb structure with a plurality of hundreds of micron macropores (as shown in figure 1 c) is formed, in the process of converting the nano-fibers (with the diameter of 3-5 nm) into the submicron fibers (with the diameter of 100-200 nm), due to the fact that the surface area is remarkably reduced, the submicron cellulose is assembled into a structure which is not formed in an aerogel process, hydrogen bonding between adjacent celluloses is inhibited, and the honeycomb structure formed by the mutually connected submicron fiber network provides excellent elastic behavior and shows excellent flexibility and foldability.
Example 2
A preparation method of elastic cellulose aerogel comprises the following steps:
(1) Putting 0.05wt% of nano cellulose suspension into liquid nitrogen at the temperature of-196 ℃ for liquid nitrogen freezing for 2h, and then putting the nano cellulose suspension into a freeze drier at the temperature of-55 ℃ and the pressure of 0.2Pa for freeze drying for 48h to obtain sub-micron cellulose fibers;
(2) Dispersing the submicron cellulose fiber in water by a vortex mixer to prepare a dispersion liquid with the concentration of 0.2wt%, uniformly stirring, placing in a refrigerator with the temperature of minus 20 ℃ for secondary freezing for 3h, and then placing in a freeze dryer with the temperature of minus 55 ℃ and the pressure of 0.2Pa for freeze drying for 48h to obtain the elastic cellulose aerogel.
Example 3
The preparation method of the elastic cellulose aerogel of this example is the same as that of example 2, except that the submicron cellulose fiber is dispersed in water to prepare a dispersion having a concentration of 0.4wt% in step (2).
Example 4
The preparation method of the elastic cellulose aerogel of this example is the same as that of example 2, except that the submicron cellulose fiber is dispersed in water to prepare a dispersion having a concentration of 0.6wt% in step (2).
Example 5
The preparation method of the elastic cellulose aerogel of this example is the same as that of example 2, except that the submicron cellulose fiber is dispersed in water to prepare a dispersion having a concentration of 0.8wt% in step (2).
The aerogels prepared in examples 2-5 all had a honeycomb microstructure morphology.
Comparative example 1
Preparation of conventional cellulose gel:
(1) Oxidizing natural paper pulp for 2 hours by using TEMPO/NaClO, controlling the pH of an oxidation system to be 10 by using NaOH solution in the oxidation treatment process, dialyzing for one week by using a dialysis bag, treating for 25min by using a cell crusher to obtain nano-cellulose, and diluting to obtain 0.4wt% nano-cellulose dispersion liquid; the mass ratio of the TEMPO to the NaClO to the pulp is 1.6.
(2) Freezing the 0.4wt% nano-cellulose dispersion liquid obtained in the step (1) in a refrigerator at the temperature of-20 ℃ for 3 hours to obtain 0.4wt% frozen nano-cellulose;
(3) Freezing the 0.4wt% frozen nano-cellulose in the step (2) in a freeze dryer at-55 ℃ for 48h, wherein the freezing pressure is 0.2Pa.
Comparative example 2
The preparation method of the cellulose aerogel of the comparative example was: and (3) putting the 0.5wt% nano-cellulose suspension into liquid nitrogen at the temperature of-196 ℃ for liquid nitrogen freezing for 5min, and then putting the nano-cellulose suspension into a freeze dryer at the temperature of-91 ℃ and under the pressure of 0.6Pa for freeze drying for 72h to obtain the cellulose aerogel.
The aerogels prepared in examples 1 to 4 and comparative examples 1 to 2 were subjected to the tests of porosity, density, thermal conductivity and resilience, and the test results are shown in Table 1. The method for testing the normal-temperature rebound rate comprises the following steps: the aerogel with the diameter of 21mm multiplied by the height of 23mm is compressed to 80% of the height of the original size by adopting a 200g weight, the rebound rate of the aerogel after the initial compression is calculated and the percentage of the rebound height of the aerogel after the 200g weight is removed to the height of the original size, 50 cycles of compression rebound are carried out on the basis, and the rebound height of the aerogel after the 200g weight is removed after 50 cycles is calculated and the percentage of the rebound height of the aerogel after the original size is the compression rebound rate after 50 cycles. The compression rebound rate at-196 ℃ is measured by placing the aerogel in liquid nitrogen, forcibly compressing the aerogel to 80% of the original size, removing the force, and observing the rebound resilience.
TABLE 1 Properties of aerogels obtained in examples 1-4 and comparative examples 1-2
As can be seen from Table 1, the aerogel prepared by the method has a developed pore structure and very small density, and has good shape recovery, good rebound resilience and super elasticity at normal temperature and extremely cold environment (-196 ℃); and the thermal conductivity coefficient is low, only 0.023-0.028W/m.K, and the heat-insulating and infrared shielding performances are good. Can be used in the fields of high water absorption materials and heat insulation materials.
Application example 1
Carrying out surface modification on the elastic cellulose aerogel prepared in the embodiment 2 by adopting a chemical vapor deposition method and adopting methyltrimethoxysilane (MTMS for short) to obtain a super-hydrophobic elastic cellulose aerogel; the chemical vapor deposition method comprises the steps of placing 22mg of the elastic cellulose aerogel above a stainless steel grid, placing two containers respectively filled with 0.5mL of water and 1mL of methyltrimethoxysilane below the stainless steel grid side by side (the stainless steel grid needs to cover the surface areas of the two containers), and reacting for 6 hours at 80 ℃ in a closed vacuum state to obtain the super-hydrophobic elastic cellulose aerogel.
FTIR tests were performed on the prepared superhydrophobic elastic cellulose aerogel, and the test results are respectively shown in fig. 3. Since the cellulose aerogel is hydrophilic, it can be easily re-dispersed in water under mild agitation. Therefore, in order to improve the stability and hydrophobicity of the elastic cellulose aerogel, the superhydrophobic elastic cellulose aerogel can be obtained by vapor depositing MTMS for 6h at 80 ℃ under vacuum. Successful modification of the silane was confirmed by FTIR spectroscopy. 3328cm -1 、1598cm -1 And 1032cm -1 The characteristic peaks at (a) and (b) were all O-H stretching vibration, C = O/C = C stretching vibration and C-O stretching vibration, and were all present in the elastic cellulose aerogel (product of example 2) and the superhydrophobic elastic cellulose aerogel (product of this application example). In addition to having the typical spectral characteristics of unmodified cellulose aerogel, the MTMS modified elastic cellulose aerogel obtained from the super-hydrophobic elastic cellulose aerogel in Si-CH 3 And 1262cm of Si-O-Si -1 、776cm -1 、798cm -1 All appear characteristic peaks.
The water contact angle of the prepared super-hydrophobic elastic cellulose aerogel is tested, the change of the contact angle along with time is shown in figure 4, and as can be seen from figure 4, when the super-hydrophobic elastic cellulose aerogel obtained by the application example is tested to be in contact with water for the first time, the water contact angle is 164 degrees, the water contact angle is gradually reduced to 157 degrees after 7s, the water contact angle is stable, and the super-hydrophobic elastic cellulose aerogel has good super-hydrophobicity and certain self-cleaning capability.
The super-hydrophobic elastic cellulose aerogel prepared by the application example is applied to oil-water separation, an oily solvent is selectively removed from water, and the data of the oil-water separation performance are shown in table 2. The oil-water separation performance test method comprises the following steps: the oil-water separation performance was evaluated by immersing the superhydrophobic elastic cellulose aerogel in water mixed with an oily solvent to adsorb the oily solvent, and expressing the weight gain (wt%) as the ratio of the weight of the superhydrophobic elastic cellulose aerogel after adsorbing the oily solvent to the weight of the dried aerogel.
TABLE 2 oil-water separation Performance
As can be seen from table 2, the superhydrophobic elastic cellulose aerogel obtained by modifying the surface energy of the elastic cellulose aerogel with low energy shows high adsorption capacity to most of oil solvents, and the weight gains of chloroform, pump oil and rapeseed oil after adsorption respectively reach 48886%, 25765% and 23416%. The super-hydrophobic elastic cellulose aerogel can absorb most of oil solvents by 159-489 times of the weight of the super-hydrophobic elastic cellulose aerogel, and has a good oil-water separation effect.
The above description is only for the preferred embodiment of the present invention, but the scope of the present invention is not limited thereto, and any person skilled in the art should be considered to be within the technical scope of the present invention, and the technical solutions and the inventive concepts thereof according to the present invention should be equivalent or changed within the scope of the present invention.
Claims (8)
1. A preparation method of elastic cellulose aerogel is characterized by comprising the following steps:
(1) Performing liquid nitrogen freezing on the nano cellulose suspension, and then performing freeze drying to obtain submicron cellulose fibers;
the nano-cellulose suspension is prepared by centrifuging 0.2wt% of nano-cellulose dispersion liquid to remove precipitates, collecting an upper suspension, and diluting the upper suspension to obtain a nano-cellulose suspension with the concentration of 0.05 wt%;
the nano-cellulose dispersion is obtained by oxidizing natural paper pulp by a TEMPO/NaClO oxidation system for 20 min-5 h, in the oxidation treatment process, the pH of the oxidation system is controlled to be 9.8-10.5 by NaOH solution, after the oxidation treatment is finished, a dialysis bag is adopted for dialysis for one week, nano-cellulose is obtained by a cell crusher, and the nano-cellulose dispersion with the weight percent of 0.2 is obtained after dilution; the mass ratio of the TEMPO to the NaClO to the paper pulp is 1 (1.4-5) to 1;
the temperature of the liquid nitrogen freezing is-196 ℃, and the time is 1-2h;
(2) Dispersing the submicron cellulose fiber in water, freezing for the second time, and then freezing and drying to obtain elastic cellulose aerogel; the concentration of the submicron cellulose fiber dispersed in water is controlled to be 0.2wt% -2 wt%; the temperature of the secondary freezing is frozen for 3 hours by adopting a refrigerator with the temperature of-20 ℃.
2. The method for preparing elastic cellulose aerogel according to claim 1, wherein the rotation speed of the centrifugation is 5000rpm, and the centrifugation time is 10min; the time for the cell crusher to treat is 25min.
3. The method for preparing elastic cellulose aerogel according to claim 1, wherein the freeze drying in step (1) and step (2) is performed by a freeze drier at-55 to-45 ℃ for 48h, and the freezing pressure is 0.02 to 1Pa.
4. An elastic cellulose aerogel produced by the production method according to any one of claims 1 to 3.
5. Use of the elastic cellulose aerogel produced by the production method according to any one of claims 1 to 3 for water-absorbent materials or heat-insulating materials.
6. The application of the elastic cellulose aerogel is characterized in that the elastic cellulose aerogel is prepared according to the preparation method of any one of claims 1 to 3, the surface of the elastic cellulose aerogel is modified by adopting a silane compound through a chemical vapor deposition method to obtain the super-hydrophobic elastic cellulose aerogel, and the super-hydrophobic elastic cellulose aerogel is applied to the field of oil-water separation.
7. The use of claim 6, wherein the chemical vapor deposition method comprises placing the elastic cellulose aerogel above a stainless steel grid, placing two containers containing water and the silane compound below the stainless steel grid side by side, and reacting at 70-90 ℃ for 5-7h in a closed vacuum state to obtain the super-hydrophobic elastic cellulose aerogel.
8. The use according to claim 6, wherein the amount of the elastic cellulose aerogel, the silane compound and the water is (20-25) mg (1-2) mL:0.5mL; the silane compound is methyl trimethoxy silane or polydimethylsiloxane.
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| WO2008121069A1 (en) * | 2007-03-29 | 2008-10-09 | Swetree Technologies Ab | Magnetic nanoparticle cellulose material |
| CN110724301A (en) * | 2018-07-16 | 2020-01-24 | 中国林业科学研究院木材工业研究所 | Method for preparing cellulose-based aerogel in bidirectional gradient manner |
| CN109824945A (en) * | 2019-01-31 | 2019-05-31 | 清华大学深圳研究生院 | Bi-component honeycomb aerogel material and the preparation method and application thereof |
| WO2021015676A1 (en) * | 2019-07-22 | 2021-01-28 | Krosslinker Pte Ltd | System and method for manufacturing water-based hydrophobic aerogels and aerogel composites |
| CN111621056A (en) * | 2020-06-17 | 2020-09-04 | 陕西科技大学 | Preparation method of cellulose nanofibril aerogel suitable for layer-by-layer self-assembly process |
| CN112662015A (en) * | 2020-12-24 | 2021-04-16 | 中国科学技术大学 | Flame-retardant nano-cellulose composite aerogel with oriented structure and preparation method thereof |
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