Bio-based high-strength low-expansion long-chain nylon modified cellulose nanofiber and preparation method thereof

Abstract

The invention discloses a preparation method of bio-based high-strength low-expansion long-chain nylon modified cellulose nanofibers. The preparation method specifically comprises the following steps: step 1, synthesizing carboxylated cellulose nanofibers; step 2, preparing aminated cellulose nanofibers; 3, synthesizing carboxyl-terminated long-chain nylon; and step 4, synthesizing the long-chain nylon modified cellulose nanofiber. The invention also aims to provide the bio-based long-chain nylon modified cellulose nanofiber with high strength and low expansion. The long-chain nylon modified cellulose nanofiber solves the problems of poor flexibility and poor dielectric property in the existing long-chain nylon modified cellulose nanofiber.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, and relates to bio-based high-strength, low-expansion long-chain nylon modified cellulose nanofibers. This invention also relates to a method for preparing bio-based high-strength, low-expansion long-chain nylon modified cellulose nanofibers. Background Technology

[0002] Nylon is widely used as a spinning raw material and engineering plastic, mainly due to its excellent mechanical properties, wear resistance, oil resistance, and ease of processing. Limited by polymer synthesis technology, Nylon 6 and Nylon 66 were the earliest short-chain nylon materials developed. It is well known that the chemical structure of nylon materials is characterized by the presence of amide functional groups in its structural units. Because amide bonds readily form hydrogen bonds with water, short-chain nylon products like Nylon 6 and Nylon 66 exhibit strong water absorption, significantly shortening their anti-aging properties and limiting their applications in new energy vehicle cooling pipes, submarine cables, 3D printing, and high-speed rail track components. To address the water absorption problem of short-chain nylons, the current method is to extend the length of the carbon chains in the nylon structure. This is because carbon chains are hydrophobic, and longer carbon chains result in better moisture resistance. However, as the carbon chains in the nylon structure lengthen, nylon molecular crystallization becomes more difficult, severely affecting the mechanical properties and thermal stability of nylon (its coefficient of thermal expansion is as high as 1x10⁻⁶). -4 K -1 This is because the alkane carbon chains in nylon are flexible chains. As the carbon chain lengthens, the free volume of the molecule increases, and the conformation is prone to inversion, which is not conducive to intermolecular stacking, nucleation, and crystallization. Therefore, long-chain nylons have been developed, utilizing their flexibility and good moisture resistance, and incorporating inorganic reinforcing particles such as SiO2, calcium carbonate, and aluminum oxide to improve their mechanical properties. However, the poor compatibility between inorganic nanoparticles and organic long-chain nylons leads to poor dielectric properties in the prepared materials. Furthermore, existing methods have also attempted to introduce aromatic rings into the long-chain alkane structure during the design of long-chain nylons, utilizing the π-π stacking characteristics of aromatic rings to improve the crystallinity and mechanical properties of nylon. However, the increased aromaticity is detrimental to the flexibility of long-chain nylons. Summary of the Invention

[0003] The purpose of this invention is to provide a method for preparing bio-based high-strength, low-expansion long-chain nylon-modified cellulose nanofibers, which solves the problems of poor flexibility and poor dielectric properties in existing long-chain nylon-modified cellulose nanofibers.

[0004] Another objective of this invention is to provide bio-based, high-strength, low-expansion long-chain nylon-modified cellulose nanofibers.

[0005] The first technical solution adopted in this invention is a method for preparing bio-based high-strength, low-expansion long-chain nylon-modified cellulose nanofibers, specifically including the following steps:

[0006] Step 1: Synthesize carboxylated cellulose nanofibers;

[0007] Step 2: Prepare amino-modified cellulose nanofibers;

[0008] Step 3: Synthesize carboxyl-terminated long-chain nylon;

[0009] Step 4: Synthesize long-chain nylon-modified cellulose nanofibers.

[0010] The first technical solution of this invention is further characterized by:

[0011] The specific process of step 1 is as follows:

[0012] Step 1.1: Soak 5-10g of cotton fiber in 1000-2000mL of methanol solution and sonicate for 30-60min. Remove the sonicated cotton fiber and dry it overnight at 45-50℃. Then soak the cotton fiber in 1000-2000mL of deionized water, add citric acid to adjust the pH of the solution to 5-6, and continue to add 0.2-0.3g of cellulase at 40-50℃. Then shake the reaction in a shaking reactor for 1-2h. After the reaction is completed, filter and collect the cotton fiber, put it into boiling water for 10-15min to inactivate it, and then use a high-pressure homogenizer at 120-140MPa to homogenize it 30-50 times to obtain homogenized cotton fiber.

[0013] Step 1.2: Disperse 1.5-3g of homogenized cotton fibers in 100-200mL of deionized water. Cool the reaction solution in an ice-water bath, then add 0.024-0.048g of 2,2,6,6-tetramethylpiperidine-1-oxy radical and 0.15-0.3g of NaBr. Add 0.56-1.12g of NaClO to the reaction solution, followed by dropwise addition of NaOH solution. React for 3-6 hours, maintaining the pH within the range of 10-10.5 throughout the process. After the reaction, add HCl dropwise to adjust the pH of the reaction solution to neutral, obtaining crude carboxylated cellulose nanofibers. Dialyze the crude carboxylated cellulose nanofibers using a 7000Da dialysis bag with 1000-2000mL of deionized water, repeating the dialysis five times to obtain a transparent carboxylated cellulose nanofiber dispersion. Freeze-dry to obtain powdered carboxylated cellulose nanofibers.

[0014] The specific process of step 2 is as follows:

[0015] Step 2.1: Take 1-2g of carboxylated cellulose nanofibers and disperse them in 50-100mL of dimethyl sulfoxide solution, then add 5-10mL of dichlorosulfoxide and stir at room temperature for 30-60min; after the reaction is completed, dry under vacuum at 80-90℃ to obtain acyl cellulose nanofibers.

[0016] Step 2.2: Dissolve 3.4-6.8 g of 1,10-diaminodecane in 100-200 mL of dimethyl sulfoxide solution, then add 3-6 mL of triethylamine and stir until homogeneous; then divide the acylchlorocellulose nanofibers into 10 equal parts and gradually add them to the 1,10-diaminodecane solution, reacting for 10-20 min at each step; after the reaction is complete, dialyze the reaction solution directly through a 7000 Da dialysis bag with 1000-2000 mL of ethanol 5 times, freeze-dry to obtain powdered amino-modified cellulose nanofibers.

[0017] The specific process of step 3 is as follows:

[0018] Step 3.1: Take 16-32g of 4,4'-oxodibutane-1-amine and 21.2-42.4g of sebacic acid, dissolve them in 100-200mL of water and stir at room temperature for 24-48h. After the reaction is complete, remove 75-150mL of deionized water under vacuum at 50-60℃. Let the concentrated solution stand overnight for 48-72h and collect the nylon salt.

[0019] Step 3.2: Dry the nylon salt obtained in step 3.1 at 100-110℃ for 12-24h. Heat the dried nylon salt to 270-280℃ and dehydrate and polycondense it under N2 protection and a negative pressure of 0.05-0.1MPa to obtain carboxyl-terminated long-chain nylon.

[0020] The specific process of step 4 is as follows:

[0021] Add 5-10g of carboxyl-terminated long-chain nylon to 30-60mL of dimethyl sulfoxide solution, then add 0.27-0.54g of N-hydroxy-7-azobenzotriazole, stir at room temperature for 60-120min, then add 0.5-1g of amino-modified cellulose nanofibers and 0.38-0.76g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, raise the temperature to 30-40℃, and continue the reaction for 24-48h. After the reaction is completed, dialyze the reaction solution directly through a 7000Da dialysis bag with 1000-2000mL of ethanol 5 times, and freeze-dry to obtain long-chain nylon modified cellulose nanofibers.

[0022] The second technical solution adopted in this invention is a bio-based high-strength, low-expansion long-chain nylon modified cellulose nanofiber, which is prepared by the above-mentioned preparation method of bio-based high-strength, low-expansion long-chain nylon modified cellulose nanofiber.

[0023] A method for preparing bio-based high-strength, low-expansion long-chain nylon-modified cellulose nanofibers into thin films is as follows: 300-600 mg of long-chain nylon-modified cellulose nanofibers are dispersed in 20-40 mL of hexafluoroisopropanol and allowed to stand at room temperature for 12-24 hours; then, vacuum filtration is performed using a filter membrane at a negative pressure of 0.05-0.06 MPa for 10-20 minutes; the filtered film is then placed in a hot press at 80-90℃ for 10-20 minutes to release stress and prevent film curling; the film thickness is approximately 30-60 g / m². 2 .

[0024] The beneficial effects of this invention are that it uses organic cellulose nanofibers as a framework, grows organic long-chain nylon molecules on its surface to obtain a bio-based nanobrush structure, and constructs long-chain nylon with nanofibers to obtain bio-based long-chain nylon modified cellulose nanofibers with high strength, low coefficient of thermal expansion, high dielectric properties, and flexibility. This invention uses organic cellulose nanofibers as a framework, and its surface is modified by grafting organic long-chain nylon to construct a nanobrush structure, endowing the bio-based long-chain nylon modified cellulose nanofibers with excellent dielectric properties. The cellulose nanofibers not only act as reinforcing particles for the long-chain nylon, improving its mechanical properties, but also effectively promote the crystallization of the long-chain nylon, thereby reducing its coefficient of thermal expansion. This bio-based long-chain nylon modified cellulose nanofiber can be directly used to prepare thin film materials using wet molding, avoiding high-energy-consuming film-forming processes such as high-temperature casting and solvent evaporation. Attached Figure Description

[0025] Figure 1 This is a particle size distribution diagram obtained by laser particle size analyzer from the long-chain nylon modified cellulose nanofibers prepared in Example 1 and Comparative Example 1 of the preparation method of bio-based high-strength, low-expansion long-chain nylon modified cellulose nanofibers of the present invention. Detailed Implementation

[0026] The following detailed description is provided in conjunction with specific implementation methods.

[0027] The present invention discloses a method for preparing bio-based high-strength, low-expansion long-chain nylon-modified cellulose nanofibers, which specifically includes the following steps:

[0028] Step 1, the synthesis of carboxylated cellulose nanofibers, is as follows:

[0029] Step 1.1: Take 5-10g of cotton fiber and soak it in 1000-2000mL of methanol solution, then sonicate it for 30-60min. Remove the sonicated cotton fiber and dry it overnight at 45-50℃. Next, soak the cotton fiber in 1000-2000mL of deionized water, add citric acid to adjust the pH to 5-6, raise the temperature to 40-50℃, and add 0.2-0.3g of cellulase (β-1,4-glucan-4-glucan hydrolase). Then, shake the reaction in a shaker for 1-2h. After the reaction, quickly filter and collect the cotton fiber, immerse it in boiling water at 100℃ for 10-15min to inactivate it, and then homogenize it 30-50 times using a high-pressure homogenizer at 120-140MPa to obtain homogenized cotton fiber.

[0030] Step 1.2: Disperse 1.5-3g of homogenized cotton fibers in 100-200mL of deionized water. Cool the reaction solution in an ice-water bath, then add 0.024-0.048g of 2,2,6,6-tetramethylpiperidine-1-oxo radical (catalyst) and 0.15-0.3g of NaBr. Next, add 0.56-1.12g of NaClO to the reaction solution, followed by dropwise addition of NaOH solution with a pH of 13. React for 3-6 hours, maintaining the pH throughout the process. The pH of the reaction solution was adjusted to 7 by adding HCl dropwise after the reaction was completed, and the crude product of carboxylated cellulose nanofibers was obtained. The crude product was dialyzed with 1000-2000 mL of deionized water using a 7000 Da dialysis bag. The dialyzing was repeated 5 times to remove impurities such as catalyst and inorganic salts from step 1.2, and a transparent dispersion of carboxylated cellulose nanofibers was obtained. The dispersion was freeze-dried at -50 to -40 °C to obtain powdered carboxylated cellulose nanofibers.

[0031] Step 2, prepare amino-modified cellulose nanofibers, as follows:

[0032] Step 2.1: Take 1-2 g of carboxylated cellulose nanofibers and disperse them in 50-100 mL of dimethyl sulfoxide solution. Then add 5-10 mL of dichlorosulfoxide and stir at room temperature for 30-60 min. After the reaction is complete, dry under vacuum (oil pump) at 80-90℃ to remove the solvent and excess dichlorosulfoxide, and obtain acyl chlorocellulose nanofibers.

[0033] Step 2.2: Dissolve 3.4-6.8 g of 1,10-diaminodecane in 100-200 mL of dimethyl sulfoxide solution, then add 3-6 mL of triethylamine and stir until homogeneous; then divide the acylchlorocellulose nanofibers into 10 equal parts and gradually add them to the 1,10-diaminodecane solution, reacting for 10-20 min at each step; after the reaction is complete, dialyze the reaction solution directly through a 7000 Da dialysis bag with 1000-2000 mL of ethanol five times to remove the solvent and excess 1,10-diaminodecane; freeze-dry at -50 to -40 °C to obtain powdered amino-modified cellulose nanofibers.

[0034] Step 3, the synthesis of carboxyl-terminated long-chain nylon, is as follows:

[0035] Step 3.1: Take 16-32g of 4,4'-oxodibutane-1-amine and 21.2-42.4g of sebacic acid (1-2g excess), dissolve them in 100-200mL of water and stir at room temperature for 24-48h. After the reaction is complete, remove 75-150mL of deionized water under vacuum at 50-60℃. Let the concentrated solution stand overnight for 48-72h and collect the nylon salt.

[0036] Step 3.2: The obtained nylon salt is dried at 100-110℃ for 12-24h. The dried nylon salt is heated to 270-280℃ and dehydrated and polycondensed under N2 protection and 0.05-0.1MPa negative pressure to obtain carboxyl-terminated long-chain nylon.

[0037] Step 4, synthesis of long-chain nylon-modified cellulose nanofibers, is detailed below:

[0038] Add 5-10g of carboxyl-terminated long-chain nylon to 30-60mL of dimethyl sulfoxide solution, then add 0.27-0.54g of N-hydroxy-7-azobenzotriazole, stir at room temperature for 60-120min, then add 0.5-1g of amino-modified cellulose nanofibers and 0.38-0.76g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, raise the temperature to 30-40℃, and continue the reaction for 24-48h. After the reaction is complete, dialyze the reaction solution directly through a 7000Da dialysis bag with 1000-2000mL of ethanol five times to remove the solvent and excess catalyst. Freeze-dry at -50 to -40℃ to obtain long-chain nylon-modified cellulose nanofibers.

[0039] Step 5: Wet molding of long-chain nylon-modified cellulose nanofibers to prepare thin films, as detailed below:

[0040] Disperse 300-600 mg of long-chain nylon-modified cellulose nanofibers in 20-40 mL of hexafluoroisopropanol and let stand at room temperature for 12-24 h. Then, vacuum filter the nanofibers using a 1 μm microporous membrane (10 cm diameter) at a vacuum pressure of 0.05-0.06 MPa for 10-20 min. Place the filtered membrane in a hot press at 80-90℃ for 10-20 min to release stress and prevent membrane curling. The membrane thickness should be 30-60 g / m³. 2 .

[0041] Example 1

[0042] Step 1, the synthesis of carboxylated cellulose nanofibers, is as follows:

[0043] Step 1.1: Take 5g of cotton fiber and soak it in 1000mL of methanol solution, then sonicate for 30min. Remove the sonicated cotton fiber and dry it overnight at 45℃. Further soak the cotton fiber in 1000mL of deionized water, add citric acid to adjust the pH to 5, raise the temperature to 40℃, and add 0.2g of cellulase (β-1,4-glucan-4-glucan hydrolase). Then, shake the reaction in a shaker for 1h. After the reaction, quickly filter and collect the cotton fiber, immerse it in boiling water at 100℃ for 10min to inactivate it, and then homogenize it 30 times using a high-pressure homogenizer at 120MPa.

[0044] Step 1.2: 1.5 g of homogenized cotton fibers were dispersed in 100 mL of deionized water. The reaction solution was cooled in an ice-water bath, and then 0.024 g of 2,2,6,6-tetramethylpiperidine-1-oxo radical and 0.15 g of NaBr were added. Then, 0.56 g of NaClO was added stepwise to the reaction solution, followed by dropwise addition of NaOH solution with a pH of 13. The reaction was allowed to proceed for 3 hours, with the pH maintained between 10 and 10.5 throughout the process. After the reaction was complete, HCl was added dropwise to adjust the pH of the reaction solution to 7, yielding crude carboxylated cellulose nanofibers. The crude product was dialyzed five times using a 7000 Da dialysis belt and 1000 mL of deionized water to remove catalyst and inorganic salt impurities, resulting in a transparent dispersion of carboxylated cellulose nanofibers. This dispersion was further freeze-dried at -40 °C to obtain powdered carboxylated cellulose nanofibers.

[0045] Step 2, prepare amino-modified cellulose nanofibers, as follows:

[0046] Step 2.1: Take 1 g of carboxylated cellulose nanofibers and disperse them in 50 mL of dimethyl sulfoxide solution. Then add 5 mL of dichlorosulfoxide and stir at room temperature for 30 min. After the reaction is complete, dry under vacuum (oil pump) at 80 °C to remove the solvent and excess dichlorosulfoxide, thus removing the acylchlorocellulose nanofibers.

[0047] Step 2.2: Dissolve 3.4 g of 1,10-diaminodecane in 100 mL of dimethyl sulfoxide solution, then add 3 mL of triethylamine and stir until homogeneous. Divide the acylchlorocellulose nanofibers into 10 equal portions and gradually add them to the 1,10-diaminodecane solution, reacting for 10 min at each step. After the reaction is complete, dialyze the reaction solution five times directly through a 7000 Da dialysis bag with 1000 mL of ethanol to remove the solvent and excess 1,10-diaminodecane. Further freeze-dry at -40 °C to obtain powdered aminated cellulose nanofibers.

[0048] Step 3, the synthesis of carboxyl-terminated long-chain nylon, is as follows:

[0049] Step 3.1: Take 16g of 4,4'-oxodibutane-1-amine and 21.2g of sebacic acid (1g excess), dissolve them in 100mL of water and stir at room temperature for 24h. After the reaction is completed, remove 75mL of deionized water under vacuum at 50℃. Let the concentrated solution stand overnight for 48h and collect the nylon salt.

[0050] Step 3.2: The obtained nylon salt is dried at 100°C for 12 hours. The further dried nylon salt is heated to 270°C and dehydrated and polycondensed under N2 protection and a negative pressure of 0.05 MPa to obtain carboxyl-terminated long-chain nylon.

[0051] Step 4, synthesis of long-chain nylon-modified cellulose nanofibers, is detailed below:

[0052] 5 g of carboxyl-terminated long-chain nylon was added to 30 mL of dimethyl sulfoxide solution, followed by 0.27 g of N-hydroxy-7-azobenzotriazole. The mixture was stirred at room temperature for 60 min. Then, 0.5 g of amino-modified cellulose nanofibers and 0.38 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride were added. The temperature was increased by 30 °C, and the reaction was continued for 24 h. After the reaction was completed, the reaction solution was dialyzed five times with 1000 mL of ethanol using a 7000 Da dialysis bag to remove the solvent and excess catalyst. The solution was then freeze-dried at -40 °C to obtain long-chain nylon-modified cellulose nanofibers.

[0053] Step 5: Wet molding of long-chain nylon-modified cellulose nanofibers to prepare thin films, as detailed below:

[0054] 300 mg of long-chain nylon-modified cellulose nanofibers were dispersed in 20 mL of hexafluoroisopropanol and allowed to stand at room temperature for 12 h. Then, the nanofibers were vacuum filtered using a 1 μm microporous membrane (10 cm diameter) at a vacuum pressure of 0.05 MPa for 10 min. The filtered membrane was then hot-pressed at 80 °C for 10 min to release stress and prevent membrane curling. The membrane thickness was determined to be 30 g / m³. 2 .

[0055] Example 2

[0056] Step 1, the synthesis of carboxylated cellulose nanofibers, is as follows:

[0057] Step 1.1: Take 10g of cotton fiber and soak it in 2000mL of methanol solution, then sonicate for 60min. Remove the sonicated cotton fiber and dry it overnight at 50℃. Further soak the cotton fiber in 2000mL of deionized water, add citric acid to adjust the pH to 6, raise the temperature to 50℃, and add 0.3g of cellulase (β-1,4-glucan-4-glucan hydrolase). Then, shake the reaction in a shaker for 2h. After the reaction, quickly filter and collect the cotton fiber, immerse it in boiling water at 100℃ for 15min to inactivate it, and then homogenize it 50 times using a high-pressure homogenizer at 140MPa.

[0058] Step 1.2: 3g of homogenized cotton fibers were dispersed in 200mL of deionized water. The reaction solution was cooled in an ice-water bath, and then 0.048g of 2,2,6,6-tetramethylpiperidine-1-oxo radical and 0.3g of NaBr were added. Then, 1.12g of NaClO was added stepwise to the reaction solution, followed by dropwise addition of NaOH solution with a pH of 13. The reaction was allowed to proceed for 6 hours, with the pH maintained between 10 and 10.5 throughout the process. After the reaction was complete, HCl was added dropwise to adjust the pH of the reaction solution to 7, yielding crude carboxylated cellulose nanofibers. The crude product was dialyzed five times using a 7000Da dialysis belt and 2000mL of deionized water to remove catalyst and inorganic salt impurities, resulting in a transparent dispersion of carboxylated cellulose nanofibers. This dispersion was further freeze-dried at -50℃ to obtain powdered carboxylated cellulose nanofibers.

[0059] Step 2, prepare amino-modified cellulose nanofibers, as follows:

[0060] Step 2.1: Take 2g of carboxylated cellulose nanofibers and disperse them in 100mL of dimethyl sulfoxide solution. Then add 10mL of dichlorosulfoxide and stir at room temperature for 60min. After the reaction is complete, dry under vacuum (oil pump) at 90℃ to remove the solvent and excess dichlorosulfoxide, thus obtaining acylchlorocellulose nanofibers.

[0061] Step 2.2: Dissolve 6.8 g of 1,10-diaminodecane in 200 mL of dimethyl sulfoxide solution, then add 6 mL of triethylamine and stir until homogeneous. Divide the acylchlorocellulose nanofibers into 10 equal portions and gradually add them to the 1,10-diaminodecane solution, reacting for 20 min at each step. After the reaction is complete, dialyze the reaction solution five times using a 7000 Da dialysis bag and 2000 mL of ethanol to remove the solvent and excess 1,10-diaminodecane. Further freeze-dry at -50 °C to obtain powdered aminated cellulose nanofibers.

[0062] Step 3, the synthesis of carboxyl-terminated long-chain nylon, is as follows:

[0063] Step 3.1: Take 32g of 4,4'-oxodibutane-1-amine and 42.4g of sebacic acid (2g excess), dissolve them in 200mL of water and stir at room temperature for 48h. After the reaction is completed, remove 150mL of deionized water under vacuum at 60℃. Let the concentrated solution stand overnight for 72h and collect the nylon salt.

[0064] Step 3.2: The obtained nylon salt is dried at 110°C for 24 hours. The further dried nylon salt is heated to 280°C and dehydrated and polycondensed under N2 protection and a negative pressure of 0.1 MPa to obtain carboxyl-terminated long-chain nylon.

[0065] Step 4, synthesis of long-chain nylon-modified cellulose nanofibers, is detailed below:

[0066] 10 g of carboxyl-terminated long-chain nylon was added to 60 mL of dimethyl sulfoxide solution, followed by 0.54 g of N-hydroxy-7-azobenzotriazole. The mixture was stirred at room temperature for 120 min. Then, 1 g of amino-modified cellulose nanofibers and 0.76 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride were added. The temperature was increased to 40 °C, and the reaction was continued for 48 h. After the reaction was completed, the reaction solution was dialyzed five times with 2000 mL of ethanol using a 7000 Da dialysis bag to remove the solvent and excess catalyst. The solution was then freeze-dried at -50 °C to obtain long-chain nylon-modified cellulose nanofibers.

[0067] Step 5: Wet molding of long-chain nylon-modified cellulose nanofibers to prepare thin films, as detailed below:

[0068] 600 mg of long-chain nylon-modified cellulose nanofibers were dispersed in 40 mL of hexafluoroisopropanol and allowed to stand at room temperature for 24 h. Then, the nanofibers were vacuum filtered using a 1 μm microporous membrane (10 cm diameter) at a vacuum pressure of 0.06 MPa for 20 min. The filtered membrane was then hot-pressed at 90 °C for 20 min to release stress and prevent membrane curling. The membrane thickness was determined to be 60 g / m³. 2 .

[0069] Example 3

[0070] Step 1, the synthesis of carboxylated cellulose nanofibers, is as follows:

[0071] Step 1.1: Take 7.5g of cotton fiber and soak it in 1500mL of methanol solution, then sonicate for 45min. Remove the sonicated cotton fiber and dry it overnight at 47.5℃. Further soak the cotton fiber in 1500mL of deionized water, add citric acid to adjust the pH to 5.5, raise the temperature to 45℃, and add 0.25g of cellulase (β-1,4-glucan-4-glucan hydrolase). Then, shake the reaction in a shaker for 1.5h. After the reaction, quickly filter and collect the cotton fiber, immerse it in boiling water at 100℃ for 12.5min to inactivate it, and then homogenize it 40 times using a high-pressure homogenizer at 130MPa.

[0072] Step 1.2: 2.25 g of homogenized cotton fibers were dispersed in 150 mL of deionized water. The reaction solution was cooled in an ice-water bath, and then 0.036 g of 2,2,6,6-tetramethylpiperidine-1-oxo radical and 0.23 g of NaBr were added. Then, 0.84 g of NaClO was added stepwise to the reaction solution, followed by dropwise addition of NaOH solution with a pH of 13. The reaction was allowed to proceed for 4.5 h, with the pH maintained between 10 and 10.5 throughout the process. After the reaction was complete, HCl was added dropwise to adjust the pH of the reaction solution to 7, yielding crude carboxylated cellulose nanofibers. The crude product was dialyzed five times using a 7000 Da dialysis belt and 1500 mL of deionized water to remove catalysts, inorganic salts, and other impurities, resulting in a transparent dispersion of carboxylated cellulose nanofibers. This dispersion was further freeze-dried at -45 °C to obtain powdered carboxylated cellulose nanofibers.

[0073] Step 2, prepare amino-modified cellulose nanofibers, as follows:

[0074] Step 2.1: Take 1.5 g of carboxylated cellulose nanofibers and disperse them in 75 mL of dimethyl sulfoxide solution. Then add 7.5 mL of dichlorosulfoxide and stir at room temperature for 45 min. After the reaction is complete, dry under vacuum (oil pump) at 85 °C to remove the solvent and excess dichlorosulfoxide, thus obtaining acylchlorocellulose nanofibers.

[0075] Step 2.2: Dissolve 5.1 g of 1,10-diaminodecane in 150 mL of dimethyl sulfoxide solution, then add 4.5 mL of triethylamine and stir until homogeneous. Divide the acylchlorocellulose nanofibers into 10 equal portions and gradually add them to the 1,10-diaminodecane solution, reacting for 15 min at each step. After the reaction is complete, dialyze the reaction solution five times using a 7000 Da dialysis bag and 1500 mL of ethanol to remove the solvent and excess 1,10-diaminodecane. Further freeze-dry at -45 °C to obtain powdered aminated cellulose nanofibers.

[0076] Step 3, the synthesis of carboxyl-terminated long-chain nylon, is as follows:

[0077] Step 3.1: Take 24g of 4,4'-oxodibutane-1-amine and 31.8g of sebacic acid (1.5g in excess), dissolve them in 150mL of water and stir at room temperature for 36h. After the reaction is complete, remove 112.5mL of deionized water under vacuum at 55℃. Let the concentrated solution stand overnight for 60h and collect the nylon salt.

[0078] Step 3.2: The obtained nylon salt is dried at 105°C for 18 hours. The further dried nylon salt is heated to 275°C and dehydrated and polycondensed under N2 protection and a negative pressure of 0.075 MPa to obtain carboxyl-terminated long-chain nylon.

[0079] Step 4, synthesis of long-chain nylon-modified cellulose nanofibers, is detailed below:

[0080] 7.5 g of carboxyl-terminated long-chain nylon was added to 45 mL of dimethyl sulfoxide solution, followed by 0.405 g of N-hydroxy-7-azobenzotriazole. The mixture was stirred at room temperature for 90 min. Then, 0.75 g of amino-modified cellulose nanofibers and 0.57 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride were added. The temperature was increased to 35 °C, and the reaction was continued for 36 h. After the reaction was completed, the reaction solution was dialyzed five times with 1500 mL of ethanol using a 7000 Da dialysis bag to remove the solvent and excess catalyst. The solution was then freeze-dried at -45 °C to obtain long-chain nylon-modified cellulose nanofibers.

[0081] Step 5: Wet molding of long-chain nylon-modified cellulose nanofibers to prepare thin films, as detailed below:

[0082] 450 mg of long-chain nylon-modified cellulose nanofibers were dispersed in 30 mL of hexafluoroisopropanol and allowed to stand at room temperature for 18 h. Then, the nanofibers were vacuum filtered using a 1 μm microporous membrane (10 cm diameter) at a vacuum pressure of 0.055 MPa for 15 min. The filtered membrane was then hot-pressed at 85 °C for 15 min to release stress and prevent membrane curling. The membrane thickness was determined to be 45 g / m³. 2 .

[0083] Comparative Example 1 (Carboxylated cellulose nanocrystals were replaced with carboxylated cellulose nanocrystals. The prepared long-chain nylon-modified cellulose nanocrystals had small particle sizes, and the 1µm microporous filter membrane could not retain the flow, resulting in failure of wet molding).

[0084] Step 1, the synthesis of carboxylated cellulose nanocrystals, is as follows:

[0085] Step 1.1: Take 5g of microcrystalline cellulose and soak it in 1000mL of methanol solution, then sonicate for 30min. Remove the sonicated microcrystalline cellulose and dry it overnight at 45℃. Further soak the microcrystalline cellulose in 1000mL of deionized water, add citric acid to adjust the pH of the solution to 5, raise the temperature to 40℃, and add 0.2g of cellulase (β-1,4-glucan-4-glucan hydrolase). Then, shake the reaction in a shaker for 1h. After the reaction, quickly filter and collect the microcrystalline cellulose, inactivate it in boiling water at 100℃ for 10min, and homogenize it 30 times using a high-pressure homogenizer at 120MPa.

[0086] Step 1.2: 1.5 g of homogenized microcrystalline cellulose was dispersed in 100 mL of deionized water. The reaction solution was cooled in an ice-water bath, and then 0.024 g of 2,2,6,6-tetramethylpiperidine-1-oxo radical and 0.15 g of NaBr were added. Then, 0.56 g of NaClO was added stepwise to the reaction solution, followed by dropwise addition of NaOH solution with a pH of 13. The reaction was allowed to proceed for 3 hours, with the pH maintained between 10 and 10.5 throughout the process. After the reaction was complete, HCl was added dropwise to adjust the pH of the reaction solution to 7, yielding crude carboxylated cellulose nanocrystals. The crude product was dialyzed five times using a 7000 Da dialysis belt and 1000 mL of deionized water to remove impurities such as the catalyst and inorganic salts from step 2, resulting in a transparent dispersion of carboxylated cellulose nanocrystals. This dispersion was further freeze-dried at -40 °C to obtain powdered carboxylated cellulose nanocrystals.

[0087] Step 2, amino-modified cellulose nanocrystals, are detailed below:

[0088] Step 2.1: Take 1 g of carboxylated cellulose nanocrystals and disperse them in 50 mL of dimethyl sulfoxide solution. Then add 5 mL of dichlorosulfoxide and stir at room temperature for 30 min. After the reaction is complete, dry under vacuum (oil pump) at 80 °C to remove the solvent and excess dichlorosulfoxide, thus removing the acylchlorocellulose nanocrystals.

[0089] Step 2.2: Dissolve 3.4 g of 1,10-diaminodecane in 100 mL of dimethyl sulfoxide solution, then add 3 mL of triethylamine and stir until homogeneous. Divide the acylchlorocellulose nanocrystals into 10 equal portions and gradually add them to the 1,10-diaminodecane solution, reacting for 10 min at each step. After the reaction is complete, dialyze the reaction solution five times directly through a 7000 Da dialysis bag with 1000 mL of ethanol to remove the solvent and excess 1,10-diaminodecane. Further freeze-dry at -40 °C to obtain powdered amylated cellulose nanocrystals.

[0090] Step 3, the synthesis of carboxyl-terminated long-chain nylon, is as follows:

[0091] Step 3.1: Take 16g of 4,4'-oxodibutane-1-amine and 21.2g of sebacic acid (1g excess), dissolve them in 100mL of water and stir at room temperature for 24h. After the reaction is completed, remove 75mL of deionized water under vacuum at 50℃. Let the concentrated solution stand overnight for 48h and collect the nylon salt.

[0092] Step 3.2: The obtained nylon salt is dried at 100°C for 12 hours. The further dried nylon salt is heated to 270°C and dehydrated and polycondensed under N2 protection and a negative pressure of 0.05 MPa to obtain carboxyl-terminated long-chain nylon.

[0093] Step 4, synthesis of long-chain nylon-modified cellulose nanocrystals, is as follows:

[0094] 5 g of carboxyl-terminated long-chain nylon was added to 30 mL of dimethyl sulfoxide solution, followed by 0.27 g of N-hydroxy-7-azobenzotriazole. The mixture was stirred at room temperature for 60 min. Then, 0.5 g of amino-modified cellulose nanocrystals and 0.38 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride were added. The temperature was increased by 30 °C, and the reaction was continued for 24 h. After the reaction was completed, the reaction solution was dialyzed five times with 1000 mL of ethanol using a 7000 Da dialysis bag to remove the solvent and excess catalyst. The solution was then freeze-dried at -40 °C to obtain long-chain nylon-modified amino-modified cellulose nanocrystals.

[0095] Step 5: Wet molding of long-chain nylon-modified cellulose nanocrystals to prepare thin films, as detailed below:

[0096] 300 mg of long-chain nylon-modified cellulose nanocrystals were dispersed in 20 mL of hexafluoroisopropanol and allowed to stand at room temperature for 12 h. Then, the mixture was vacuum filtered using a microporous membrane with a pore size of 1 μm (membrane diameter of 10 cm) at a vacuum pressure of 0.05 MPa for 10 min. The wet-process molding method for preparing the membrane was unsuccessful.

[0097] Comparative Example 2 (Carboxylated cellulose nanofibers without amine modification, cellulose nanofibers physically doped with long-chain nylon, film thickness after wet molding was only 3 g / m). 2 Long-chain nylon with carboxyl end groups cannot be blocked by microporous filter membranes, mainly cellulose nanofibers.

[0098] Step 1, the synthesis of carboxyl-terminated long-chain nylon, is as follows:

[0099] Step 1.1: Take 23.6g of 3,6,9,12-tetraoxatetradecane-1,14-diamine and 21.2g of sebacic acid (1g excess), dissolve them in 100mL of water and stir at room temperature for 24h. After the reaction is complete, remove 75mL of deionized water under vacuum at 50℃. Let the concentrated solution stand overnight for 48h and collect the nylon salt.

[0100] Step 1.2: The obtained nylon salt was dried at 100°C for 12 hours. The further dried nylon salt was heated to 270°C and dehydrated and polycondensed under N2 protection and a negative pressure of 0.05 MPa to obtain carboxyl-terminated long-chain nylon.

[0101] Step 2, the preparation of a mixed solution of long-chain nylon-doped cellulose nanofibers, is as follows:

[0102] Take 272 mg of carboxyl-terminated long-chain nylon and 28 mg of cellulose nanofibers, disperse them in 20 mL of hexafluoroisopropanol, stir at room temperature for 30 min, and set aside.

[0103] Step 3: Wet molding of long-chain nylon-doped cellulose nanofibers to prepare thin films, as detailed below:

[0104] The above-mentioned long-chain nylon-doped cellulose nanofiber mixture was allowed to stand at room temperature for 12 hours. Then, it was vacuum filtered using a 1µm microporous membrane (10cm diameter) at a vacuum pressure of 0.05MPa for 10 minutes. The filtered membrane was then hot-pressed at 80°C for 10 minutes to release stress and prevent membrane curling. The membrane thickness was determined to be 3 g / m³. 2 .

[0105] Comparative Example 3 (cellulose nanofibers were replaced with silica, resulting in poor mechanical properties).

[0106] Step 1, the preparation of aminated silica, is as follows:

[0107] Take 5g of silica microspheres (2µm in diameter), activate them at 110℃ for 1h, then disperse them in 20mL of deionized water, add silane coupling agent KH550, and stir at room temperature for 2h. After the reaction is complete, centrifuge at 10000r / min for 30min, collect the solid product, wash three times with 50mL acetone, and finally dry at 45℃ for 12h to obtain aminated silica.

[0108] Step 2, the synthesis of carboxyl-terminated long-chain nylon, is as follows:

[0109] Step 2.1: Take 16g of 4,4'-oxodibutane-1-amine and 21.2g of sebacic acid (1g excess), dissolve them in 100mL of water and stir at room temperature for 24h. After the reaction is complete, remove 75mL of deionized water under vacuum at 50℃. Let the concentrated solution stand overnight for 48h and collect the nylon salt.

[0110] Step 2.2: The obtained nylon salt is dried at 100°C for 12 hours. The dried nylon salt is then heated to 270°C and dehydrated and polycondensed under N2 protection and a negative pressure of 0.05 MPa to obtain carboxyl-terminated long-chain nylon.

[0111] Step 3, the synthesis of long-chain nylon-modified silica, is as follows:

[0112] 5 g of carboxyl-terminated long-chain nylon was added to 30 mL of dimethyl sulfoxide solution, followed by 0.27 g of N-hydroxy-7-azobenzotriazole. The mixture was stirred at room temperature for 60 min. Then, 0.5 g of amino-modified silica and 0.38 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride were added. The temperature was increased by 30 °C, and the reaction was continued for 24 h. After the reaction was completed, the reaction solution was dialyzed five times with 1000 mL of ethanol using a 7000 Da dialysis bag to remove the solvent and excess catalyst. The solution was then freeze-dried at -40 °C to obtain long-chain nylon-modified silica.

[0113] Step 4: Wet molding of long-chain nylon-modified silica to prepare thin films, as detailed below:

[0114] 300 mg of long-chain nylon-modified silica was dispersed in 20 mL of hexafluoroisopropanol and allowed to stand at room temperature for 12 h. Then, it was vacuum filtered using a 1 μm microporous membrane (10 cm diameter) at a vacuum pressure of 0.05 MPa for 10 min. The filtered membrane was then hot-pressed at 80 °C for 10 min to release stress and prevent membrane curling. The membrane thickness was determined to be 30 g / m³. 2 .

[0115] Comparative Example 4 (Replacing long-chain nylon with short-chain nylon resulted in decreased hygroscopicity and poor dielectric stability).

[0116] Step 1, the synthesis of carboxylated cellulose nanofibers, is as follows:

[0117] Step 1.1: Take 5g of cotton fiber and soak it in 1000mL of methanol solution, then sonicate for 30min. Remove the sonicated cotton fiber and dry it overnight at 45℃. Further soak the cotton fiber in 1000mL of deionized water, add citric acid to adjust the pH to 5, raise the temperature to 40℃, and add 0.2g of cellulase (β-1,4-glucan-4-glucan hydrolase). Then, shake the reaction in a shaker for 1h. After the reaction, quickly filter and collect the cotton fiber, immerse it in boiling water at 100℃ for 10min to inactivate it, and then homogenize it 30 times using a high-pressure homogenizer at 120MPa.

[0118] Step 1.2: 1.5 g of homogenized cotton fibers were dispersed in 100 mL of deionized water. The reaction solution was cooled in an ice-water bath, and then 0.024 g of 2,2,6,6-tetramethylpiperidine-1-oxo radical and 0.15 g of NaBr were added. Then, 0.56 g of NaClO was added stepwise to the reaction solution, followed by dropwise addition of NaOH solution with a pH of 13. The reaction was allowed to proceed for 3 hours, with the pH maintained between 10 and 10.5 throughout the process. After the reaction was complete, HCl was added dropwise to adjust the pH of the reaction solution to 7, yielding crude carboxylated cellulose nanofibers. The crude product was dialyzed five times using a 7000 Da dialysis belt and 1000 mL of deionized water to remove catalysts, inorganic salts, and other impurities, resulting in a transparent dispersion of carboxylated cellulose nanofibers. This dispersion was further freeze-dried at -40 °C to obtain powdered carboxylated cellulose nanofibers.

[0119] Step 2, synthesizing amino-modified cellulose nanofibers, as detailed below:

[0120] Step 2.1: Take 1 g of carboxylated cellulose nanofibers and disperse them in 50 mL of dimethyl sulfoxide solution. Then add 5 mL of dichlorosulfoxide and stir at room temperature for 30 min. After the reaction is complete, dry under vacuum (oil pump) at 80 °C to remove the solvent and excess dichlorosulfoxide, thus removing the acylchlorocellulose nanofibers.

[0121] Step 2.2: Dissolve 3.4 g of 1,10-diaminodecane in 100 mL of dimethyl sulfoxide solution, then add 3 mL of triethylamine and stir until homogeneous. Divide the acylchlorocellulose nanofibers into 10 equal portions and gradually add them to the 1,10-diaminodecane solution, reacting for 10 min at each step. After the reaction is complete, dialyze the reaction solution five times directly through a 7000 Da dialysis bag with 1000 mL of ethanol to remove the solvent and excess 1,10-diaminodecane. Further freeze-dry at -40 °C to obtain powdered aminated cellulose nanofibers.

[0122] Step 3, the synthesis of carboxyl-terminated short-chain nylon, is as follows:

[0123] Step 3.1: Take 16g of 4,4'-oxodibutane-1-amine and 15.6g of adipic acid (1g excess), dissolve them in 100mL of water and stir at room temperature for 24h. After the reaction is complete, remove 75mL of deionized water under vacuum at 50℃. Let the concentrated solution stand overnight for 48h and collect the nylon salt.

[0124] Step 3.2: The obtained nylon salt is dried at 100°C for 12 hours. The further dried nylon salt is heated to 270°C and dehydrated and polycondensed under N2 protection and a negative pressure of 0.05 MPa to obtain carboxyl-terminated short-chain nylon.

[0125] Step 4, synthesis of short-chain nylon-modified cellulose nanofibers, is detailed below:

[0126] 5 g of carboxyl-terminated long-chain nylon was added to 30 mL of dimethyl sulfoxide solution, followed by 0.27 g of N-hydroxy-7-azobenzotriazole. The mixture was stirred at room temperature for 60 min. Then, 0.5 g of amino-modified cellulose nanofibers and 0.38 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride were added. The temperature was increased by 30 °C, and the reaction was continued for 24 h. After the reaction was completed, the reaction solution was dialyzed five times with 1000 mL of ethanol using a 7000 Da dialysis bag to remove the solvent and excess catalyst. The solution was then freeze-dried at -40 °C to obtain short-chain nylon-modified cellulose nanofibers.

[0127] Step 5: Wet molding of short-chain nylon-modified cellulose nanofibers to prepare thin films, as detailed below:

[0128] 300 mg of short-chain nylon-modified cellulose nanofibers were dispersed in 20 mL of hexafluoroisopropanol and allowed to stand at room temperature for 12 h. Then, the nanofibers were vacuum filtered using a 1 μm microporous membrane (10 cm diameter) at a vacuum pressure of 0.05 MPa for 10 min. The filtered membrane was then hot-pressed at 80 °C for 10 min to release stress and prevent membrane curling. The membrane thickness was determined to be 30 g / m³. 2 .

[0129] Comparative Example 5 (the ether oxygen in the long-chain nylon structure is replaced with conjugated naphthalene, resulting in poor flexibility)

[0130] Step 1, the synthesis of carboxylated cellulose nanofibers, is as follows:

[0131] Step 1.1: Take 5g of cotton fiber and soak it in 1000mL of methanol solution, then sonicate for 30min. Remove the sonicated cotton fiber and dry it overnight at 45℃. Further soak the cotton fiber in 1000mL of deionized water, add citric acid to adjust the pH to 5, raise the temperature to 40℃, and add 0.2g of cellulase (β-1,4-glucan-4-glucan hydrolase). Then, shake the reaction in a shaker for 1h. After the reaction, quickly filter and collect the cotton fiber, immerse it in boiling water at 100℃ for 10min to inactivate it, and then homogenize it 30 times using a high-pressure homogenizer at 120MPa.

[0132] Step 1.2: 1.5 g of homogenized cotton fibers were dispersed in 100 mL of deionized water. The reaction solution was cooled in an ice-water bath, and then 0.024 g of 2,2,6,6-tetramethylpiperidine-1-oxo radical and 0.15 g of NaBr were added. Then, 0.56 g of NaClO was added stepwise to the reaction solution, followed by dropwise addition of NaOH solution with a pH of 13. The reaction was allowed to proceed for 3 hours, with the pH maintained between 10 and 10.5 throughout the process. After the reaction was complete, HCl was added dropwise to adjust the pH of the reaction solution to 7, yielding crude carboxylated cellulose nanofibers. The crude product was dialyzed five times using a 7000 Da dialysis belt and 1000 mL of deionized water to remove impurities such as the catalyst and inorganic salts from step 2, resulting in a transparent carboxylated cellulose nanofiber dispersion. This dispersion was further freeze-dried at -40 °C to obtain powdered carboxylated cellulose nanofibers.

[0133] Step 2, synthesizing amino-modified cellulose nanofibers, as detailed below:

[0134] Step 2.1: Take 1 g of carboxylated cellulose nanofibers and disperse them in 50 mL of dimethyl sulfoxide solution. Then add 5 mL of dichlorosulfoxide and stir at room temperature for 30 min. After the reaction is complete, dry under vacuum (oil pump) at 80 °C to remove the solvent and excess dichlorosulfoxide, thus removing the acylchlorocellulose nanofibers.

[0135] Step 2.2: Dissolve 3.4 g of 1,10-diaminodecane in 100 mL of dimethyl sulfoxide solution, then add 3 mL of triethylamine and stir until homogeneous. Divide the acylchlorocellulose nanofibers into 10 equal portions and gradually add them to the 1,10-diaminodecane solution, reacting for 10 min at each step. After the reaction is complete, dialyze the reaction solution five times directly through a 7000 Da dialysis bag with 1000 mL of ethanol to remove the solvent and excess 1,10-diaminodecane. Further freeze-dry at -40 °C to obtain powdered aminated cellulose nanofibers.

[0136] Step 3, the synthesis of carboxyl-terminated long-chain nylon, is as follows:

[0137] Step 3.1: Take 15.8g of 1,5-naphthyldiamine and 21.2g of sebacic acid (1g excess), dissolve them in 100mL of water and stir at room temperature for 24h. After the reaction is completed, remove 75mL of deionized water under vacuum at 50℃. Let the concentrated solution stand overnight for 48h and collect the nylon salt.

[0138] Step 3.2: The obtained nylon salt is dried at 100°C for 12 hours. The further dried nylon salt is heated to 270°C and dehydrated and polycondensed under N2 protection and a negative pressure of 0.05 MPa to obtain carboxyl-terminated long-chain nylon.

[0139] Step 4, synthesis of long-chain nylon-modified cellulose nanofibers, is detailed below:

[0140] 5 g of carboxyl-terminated long-chain nylon was added to 30 mL of dimethyl sulfoxide solution, followed by 0.27 g of N-hydroxy-7-azobenzotriazole. The mixture was stirred at room temperature for 60 min. Then, 0.5 g of amino-modified cellulose nanofibers and 0.38 g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride were added. The temperature was increased by 30 °C, and the reaction was continued for 24 h. After the reaction was completed, the reaction solution was dialyzed five times with 1000 mL of ethanol using a 7000 Da dialysis bag to remove the solvent and excess catalyst. The solution was then freeze-dried at -40 °C to obtain long-chain nylon-modified cellulose nanofibers.

[0141] Step 5: Wet molding of long-chain nylon-modified cellulose nanofibers to prepare thin films, as detailed below:

[0142] 300 mg of long-chain nylon-modified cellulose nanofibers were dispersed in 20 mL of hexafluoroisopropanol and allowed to stand at room temperature for 12 h. Then, the nanofibers were vacuum filtered using a 1 μm microporous membrane (10 cm diameter) at a vacuum pressure of 0.05 MPa for 10 min. The filtered membrane was then hot-pressed at 80 °C for 10 min to release stress and prevent membrane curling. The membrane thickness was determined to be 30 g / m³. 2 .

[0143] Comparative Example 6 (Aminated cellulose nanofibers and carboxyl-terminated long-chain nylon are physically doped and spin-coated to form a film with a high coefficient of thermal expansion and charge migration).

[0144] Step 1, the synthesis of carboxylated cellulose nanofibers, is as follows:

[0145] Step 1.1: Take 5g of cotton fiber and soak it in 1000mL of methanol solution, then sonicate for 30min. Remove the sonicated cotton fiber and dry it overnight at 45℃. Further soak the cotton fiber in 1000mL of deionized water, add citric acid to adjust the pH to 5, raise the temperature to 40℃, and add 0.2g of cellulase (β-1,4-glucan-4-glucan hydrolase). Then, shake the reaction in a shaker for 1h. After the reaction, quickly filter and collect the cotton fiber, immerse it in boiling water at 100℃ for 10min to inactivate it, and then homogenize it 30 times using a high-pressure homogenizer at 120MPa.

[0146] Step 1.2: 1.5 g of homogenized cotton fibers were dispersed in 100 mL of deionized water. The reaction solution was cooled in an ice-water bath, and then 0.024 g of 2,2,6,6-tetramethylpiperidine-1-oxo radical and 0.15 g of NaBr were added. Then, 0.56 g of NaClO was added stepwise to the reaction solution, followed by dropwise addition of NaOH solution with a pH of 13. The reaction was allowed to proceed for 3 hours, with the pH maintained between 10 and 10.5 throughout the process. After the reaction was complete, HCl was added dropwise to adjust the pH of the reaction solution to 7, yielding crude carboxylated cellulose nanofibers. The crude product was dialyzed five times using a 7000 Da dialysis belt and 1000 mL of deionized water to remove catalysts, inorganic salts, and other impurities, resulting in a transparent dispersion of carboxylated cellulose nanofibers. This dispersion was further freeze-dried at -40 °C to obtain powdered carboxylated cellulose nanofibers.

[0147] Step 2, synthesizing amino-modified cellulose nanofibers, as detailed below:

[0148] Step 2.1: Take 1 g of carboxylated cellulose nanofibers and disperse them in 50 mL of dimethyl sulfoxide solution. Then add 5 mL of dichlorosulfoxide and stir at room temperature for 30 min. After the reaction is complete, dry under vacuum (oil pump) at 80 °C to remove the solvent and excess dichlorosulfoxide, thus removing the acylchlorocellulose nanofibers.

[0149] Step 2.2: Dissolve 3.4 g of 1,10-diaminodecane in 100 mL of dimethyl sulfoxide solution, then add 3 mL of triethylamine and stir until homogeneous. Divide the acylchlorocellulose nanofibers into 10 equal portions and gradually add them to the 1,10-diaminodecane solution, reacting for 10 min at each step. After the reaction is complete, dialyze the reaction solution five times directly through a 7000 Da dialysis bag with 1000 mL of ethanol to remove the solvent and excess 1,10-diaminodecane. Further freeze-dry at -40 °C to obtain powdered aminated cellulose nanofibers.

[0150] Step 3, the synthesis of carboxyl-terminated long-chain nylon, is as follows:

[0151] Step 3.1: Take 16g of 4,4'-oxodibutane-1-amine and 21.2g of sebacic acid (1g excess), dissolve them in 100mL of water and stir at room temperature for 24h. After the reaction is completed, remove 75mL of deionized water under vacuum at 50℃. Let the concentrated solution stand overnight for 48h and collect the nylon salt.

[0152] Step 3.2: The obtained nylon salt is dried at 100°C for 12 hours. The further dried nylon salt is heated to 270°C and dehydrated and polycondensed under N2 protection and a negative pressure of 0.05 MPa to obtain carboxyl-terminated long-chain nylon.

[0153] Step 4, the preparation of the mixed solution of long-chain nylon-doped cellulose nanofibers, is as follows:

[0154] Take 272 mg of carboxyl-terminated long-chain nylon and 28 mg of amino-modified cellulose nanofibers, disperse them in 20 mL of hexafluoroisopropanol, stir at room temperature for 30 min, and set aside.

[0155] Step 5: Solvent evaporation of long-chain nylon-doped aminated cellulose nanofibers to prepare a thin film, specifically as follows: The mixed solution of the above-mentioned long-chain nylon-doped aminated cellulose nanofibers was allowed to stand at room temperature for 12 hours. Then, the solution was poured into a 10cm polytetrafluoroethylene mold and dried at 45℃ for 24 hours. The film thickness was quantitatively 30 g / m. 2 .

[0156] Table 1 Performance test results of Examples 1-3 and Comparative Examples 1-6

[0157]

[0158] The minimum radius r that allows a thin film to freely recover without creases after being rolled up on a cylinder of radius r is the smallest radius r. min Curl rate Dielectric constant fluctuation amplitude under dry and saturated water absorption

[0159] ε 湿 Let ε be the dielectric constant under saturated water absorption conditions. 干 is the dielectric constant under dry conditions.

[0160] Results Discussion

[0161] The films in Examples 1-3 contain cellulose nanofibers, which, as a reinforcing material, significantly improve the tensile strength of the material, reaching 230-236 MPa. The long-chain nylon structure in the films imparts low hygroscopicity. Examples 1-3 exhibit high curl rates, reaching 5. Thanks to the low hygroscopicity of the long-chain nylon in Examples 1-3, water has a relatively small impact on the dielectric constant. Furthermore, the high coefficient of thermal expansion of cellulose nanofibers, when grafted and modified with long-chain nylon, results in films with a low coefficient of thermal expansion.

[0162] In Comparative Example 1, cellulose nanofibers were replaced with cellulose nanocrystals. The resulting long-chain nylon-modified cellulose nanocrystals had small particle sizes, and the 1µm microporous filter membrane could not retain the flow, resulting in failure of wet molding.

[0163] In Comparative Example 2, the cellulose nanofibers were unmodified, and the cellulose nanofibers were physically doped with long-chain nylon. After wet molding, the film thickness was only 3 g / m. 2 Long-chain nylon with carboxyl end groups cannot be blocked by microporous filter membranes; the main component is cellulose nanofibers. This is why the measured coefficient of thermal expansion is low, only 3.5 x 10⁻⁶. -6 In terms of mechanical properties, the film lacks long-chain nylon components, resulting in an elongation at break of only 145 MPa. Because the surface of cellulose nanofibers contains a large number of hydroxyl groups, it possesses a certain degree of water absorption, leading to a high moisture absorption rate of 2.3%. This results in significant fluctuations in its dielectric constant under both dry and wet conditions.

[0164] In Comparative Example 3, replacing cellulose nanofibers with silica resulted in poor mechanical properties. The main reason is that the combination of fibers and polymers can form a structure similar to reinforced concrete, exhibiting good mechanical properties. Silica, being spherical, cannot provide reinforcement. Furthermore, silica is an inorganic material; when hybridized with long-chain nylon, an organic material, defects easily form within the film, causing stress concentration and leading to poor mechanical properties.

[0165] In Comparative Example 4, when long-chain nylon was replaced with short-chain nylon, the number of amide bonds per unit mass of the film differed. The short-chain nylon had a higher amide bond content, resulting in a higher hydrogen bond density and thus a slightly enhanced tensile strength. However, the amide bonds were hygroscopic, leading to increased hygroscopicity and further poor dielectric stability.

[0166] In Comparative Example 5, the ether oxygen in the long-chain nylon structure was replaced with conjugated naphthalene, which easily formed π-π stacking and crystallization of conjugated rings, resulting in improved mechanical properties. However, the introduction of rigid naphthalene rings led to decreased flexibility in the film, with a curl rate of 2 cm. -1 .

[0167] In Comparative Example 6, the amino-modified cellulose nanofibers and carboxyl-terminated long-chain nylon were physically doped and spin-coated. No strong chemical bonds formed between the two, resulting in a slight decrease in mechanical properties. Since both the amino-modified cellulose nanofibers and carboxyl-terminated long-chain nylon exhibit good dispersibility in water, their hygroscopicity was not tested. Furthermore, charge migration occurred in the carboxyl-terminated long-chain nylon component, so the dielectric constant stability under dry and wet conditions was not tested. The physical doping of amino-modified cellulose nanofibers and carboxyl-terminated long-chain nylon is not conducive to the close packing and crystallization of the two components, which is detrimental to a high coefficient of thermal expansion of the film.

[0168] Figure 1 This refers to the particle size distribution of long-chain nylon-modified cellulose nanofibers and nanocrystals in Example 1 and Comparative Example 1. To verify the failure of wet molding of long-chain nylon-modified cellulose nanocrystals in Comparative Example 1, the particle size distribution of long-chain nylon-modified cellulose nanofibers and nanocrystals was determined using a dynamic light scattering particle size analyzer. Figure 1 As shown, the average particle size distribution of long-chain nylon-modified cellulose nanofibers is 2.5 μm, while the average particle size distribution of long-chain nylon-modified cellulose nanocrystals is 560 nm.

Claims

1. A method for preparing bio-based high-strength, low-expansion long-chain nylon-modified cellulose nanofibers, characterized in that: Specifically, the steps include the following: Step 1: Synthesize carboxylated cellulose nanofibers; Step 2: Prepare amino-modified cellulose nanofibers; Step 3: Synthesize carboxyl-terminated long-chain nylon; Step 4: Synthesize long-chain nylon-modified cellulose nanofibers.

2. The method for preparing bio-based high-strength, low-expansion long-chain nylon-modified cellulose nanofibers according to claim 1, characterized in that: The specific process of step 1 is as follows: Step 1.1: Soak 5-10g of cotton fiber in 1000-2000mL of methanol solution and sonicate for 30-60min. Remove the sonicated cotton fiber and dry it overnight at 45-50℃. Then soak the cotton fiber in 1000-2000mL of deionized water, add citric acid to adjust the pH of the solution to 5-6, and continue to add 0.2-0.3g of cellulase at 40-50℃. Then shake the reaction in a shaking reactor for 1-2h. After the reaction is completed, filter and collect the cotton fiber, put it into boiling water for 10-15min to inactivate it, and then use a high-pressure homogenizer at 120-140MPa to homogenize it 30-50 times to obtain homogenized cotton fiber. Step 1.2: Disperse 1.5-3g of homogenized cotton fibers in 100-200mL of deionized water. Cool the reaction solution in an ice-water bath, then add 0.024-0.048g of 2,2,6,6-tetramethylpiperidine-1-oxy radical and 0.15-0.3g of NaBr. Add 0.56-1.12g of NaClO to the reaction solution, followed by dropwise addition of NaOH solution. React for 3-6 hours, maintaining the pH within the range of 10-10.5 throughout the process. After the reaction, add HCl dropwise to adjust the pH of the reaction solution to neutral, obtaining crude carboxylated cellulose nanofibers. Dialyze the crude carboxylated cellulose nanofibers using a 7000Da dialysis bag with 1000-2000mL of deionized water, repeating the dialysis five times to obtain a transparent carboxylated cellulose nanofiber dispersion. Freeze-dry to obtain powdered carboxylated cellulose nanofibers.

3. The method for preparing bio-based high-strength, low-expansion long-chain nylon-modified cellulose nanofibers according to claim 2, characterized in that: The specific process of step 2 is as follows: Step 2.1: Take 1-2g of carboxylated cellulose nanofibers and disperse them in 50-100mL of dimethyl sulfoxide solution, then add 5-10mL of dichlorosulfoxide and stir at room temperature for 30-60min; after the reaction is completed, dry under vacuum at 80-90℃ to obtain acyl cellulose nanofibers. Step 2.2: Dissolve 3.4-6.8 g of 1,10-diaminodecane in 100-200 mL of dimethyl sulfoxide solution, then add 3-6 mL of triethylamine and stir until homogeneous; then divide the acylchlorocellulose nanofibers into 10 equal parts and gradually add them to the 1,10-diaminodecane solution, reacting for 10-20 min at each step; after the reaction is complete, dialyze the reaction solution directly through a 7000 Da dialysis bag with 1000-2000 mL of ethanol 5 times, freeze-dry to obtain powdered amino-modified cellulose nanofibers.

4. The method for preparing bio-based high-strength, low-expansion long-chain nylon-modified cellulose nanofibers according to claim 3, characterized in that: The specific process of step 3 is as follows: Step 3.1: Take 16-32g of 4,4'-oxodibutane-1-amine and 21.2-42.4g of sebacic acid, dissolve them in 100-200mL of water and stir at room temperature for 24-48h. After the reaction is complete, remove 75-150mL of deionized water under vacuum at 50-60℃. Let the concentrated solution stand overnight for 48-72h and collect the nylon salt. Step 3.2: Dry the nylon salt obtained in step 3.1 at 100-110℃ for 12-24h. Heat the dried nylon salt to 270-280℃ and dehydrate and polycondense it under N2 protection and a negative pressure of 0.05-0.1MPa to obtain carboxyl-terminated long-chain nylon.

5. The method for preparing bio-based high-strength, low-expansion long-chain nylon-modified cellulose nanofibers according to claim 4, characterized in that: The specific process of step 4 is as follows: Add 5-10g of carboxyl-terminated long-chain nylon to 30-60mL of dimethyl sulfoxide solution, then add 0.27-0.54g of N-hydroxy-7-azobenzotriazole, stir at room temperature for 60-120min, then add 0.5-1g of amino-modified cellulose nanofibers and 0.38-0.76g of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, raise the temperature to 30-40℃, and continue the reaction for 24-48h. After the reaction is completed, dialyze the reaction solution directly through a 7000Da dialysis bag with 1000-2000mL of ethanol 5 times, and freeze-dry to obtain long-chain nylon modified cellulose nanofibers.

6. Bio-based high-strength, low-expansion long-chain nylon modified cellulose nanofibers, prepared by the preparation method of bio-based high-strength, low-expansion long-chain nylon modified cellulose nanofibers according to any one of claims 1 to 5.

7. The method for preparing thin films from bio-based high-strength, low-expansion long-chain nylon-modified cellulose nanofibers according to claim 6, characterized in that: The specific process is as follows: 300-600 mg of long-chain nylon-modified cellulose nanofibers are dispersed in 20-40 mL of hexafluoroisopropanol and allowed to stand at room temperature for 12-24 hours; then, vacuum filtration is performed using a filter membrane with a vacuum negative pressure of 0.05-0.06 MPa and a filtration time of 10-20 minutes; the filtered membrane is then placed in a hot press at 80-90℃ for 10-20 minutes to release stress and prevent membrane curling. The membrane thickness is 30-60 g / m³. 2 .

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