A type of cellulose nanofibers from reed stems, their preparation method and application

Abstract

This invention relates to the field of cellulose nanofiber technology, and discloses a reed stem cellulose nanofiber, its preparation method, and its application. Specifically, using bromine functional groups grafted onto reed stem cellulose nanofibers as initiators, an atom transfer radical polymerization reaction is carried out on the surface of the reed stem cellulose nanofibers under the action of cuprous halide. Subsequently, the ethyl sulfonate of the monomer undergoes a substitution reaction with sodium azide to obtain azide-modified reed stem cellulose nanofibers. Polylactic acid (PLA) undergoes a click reaction with the azide groups on the surface of the nanofibers through an alkynyl functional group to generate triazole groups, achieving covalent bonding between PLA and the nanofibers. Silver ions are loaded through the strong coordination of the triazole groups with silver ions. Finally, a reed stem cellulose nanofiber-modified PLA membrane is obtained by a coating method. This membrane achieves significant improvements in both antibacterial and mechanical properties.

Description

Technical Field

[0001] This invention relates to the field of cellulose nanofiber technology, and in particular to a reed stem cellulose nanofiber, its preparation method, and its application. Background Technology

[0002] Plastic film is one of the most commonly used materials for food packaging. Common base materials include polyolefins such as polyethylene (PE) and polypropylene (PP). However, these polyolefin films are difficult to degrade in the natural environment, posing a serious threat to the ecological environment. Polylactic acid (PLA), as a novel biodegradable material, has excellent biocompatibility, biodegradability, and thermoplastic processability, making it an ideal food packaging material. It provides a closed environment on the food surface and incorporates additives with antibacterial and antioxidant functions to inhibit microbial growth, thereby achieving food preservation. However, PLA suffers from drawbacks such as slow crystallization rate, high brittleness, and poor thermal stability, limiting its widespread application in the packaging field.

[0003] Cellulose nanofibers are a type of natural material with nanoscale properties, widely found in lignocellulose. Due to their renewable raw materials, high specific surface area, high aspect ratio, good biocompatibility, and biodegradability, they can be used as reinforcing phases in nanocomposite materials. Reed stem cellulose nanofibers, prepared from reed stems through chemical, physical, biological, or combined methods, can be introduced into the PLA matrix as nucleating agents and toughening agents to improve the crystallization rate and crystallinity of PLA, thereby enhancing its mechanical properties.

[0004] However, the cellulose nanofibers of reed stems contain a large number of hydroxyl groups, which have good hydrophilicity and are very easy to form hydrogen bonds. This makes the cellulose nanofibers of reed stems prone to severe flocculation in PLA-type hydrophobic polymer materials.

[0005] Research has found that by improving the surface properties of cellulose nanofibers from reed stems, the compatibility between cellulose nanofibers and PLA polymer matrix can be enhanced, thereby forming a good interfacial interaction. Summary of the Invention

[0006] Based on molecular design mechanisms, this invention designs and synthesizes a novel alkenylated compatibilizing monomer. This monomer is introduced into reed stem cellulose nanofibers to obtain azidolated reed stem cellulose nanofibers. Through an azido-alkynylated polylactic acid (PLA) "click" reaction, the reed stem cellulose nanofibers are covalently grafted onto the PLA backbone. Furthermore, they coordinate with silver ions. Finally, a composite antibacterial film for food packaging is obtained by a coating method. This film exhibits excellent antibacterial and mechanical properties.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing a polylactic acid membrane modified with cellulose nanofibers from reed stems includes the following steps: Step 1: Prepare 2-bromoisobutyryl bromide-functionalized reed stem cellulose nanofibers, and use the bromine functional groups grafted on the reed stem cellulose nanofibers as initiators. Under the catalysis of cuprous halide, the alkenylated compatibilized monomer undergoes an atom transfer radical polymerization reaction on the surface of the reed stem cellulose nanofibers. Subsequently, the ethyl sulfonate of the monomer undergoes a substitution reaction with sodium azide to obtain azide-modified reed stem cellulose nanofibers. Step 2: Prepare alkynylated polylactic acid (PLA). PLA undergoes a click reaction with the azide groups on the surface of the nanofibers via alkynyl functional groups to generate triazole groups, thereby achieving covalent bonding between PLA and reed stem cellulose nanofibers. Silver ions are loaded through the strong coordination of the triazole groups with silver ions. The resulting composite material is then used to prepare a reed stem cellulose nanofiber modified PLA membrane by a coating method.

[0008] Preferably, the method for preparing the alkenylated compatibilized monomer is as follows: Based on the substitution reaction mechanism, 1 molar equivalent of protected diethylenetriamine reacts with 1 molar equivalent of 1-chlorooctadecane in the presence of a base catalyst to prepare a long alkyl chain diethylenetriamine intermediate. Based on the quaternization reaction mechanism, 1 molar equivalent of a long alkyl chain diethylenetriamine intermediate reacts with 1 molar equivalent of 11-chloro-1-undecene to prepare a diethylenetriamine intermediate containing an alkenyl functional group. Based on the Schiff base reaction mechanism, a diethylenetriamine intermediate containing an alkenyl functional group reacts with p-hydroxymethylbenzaldehyde in a molar ratio of 1:2-2.04 to prepare a diethylenetriamine intermediate containing a hydroxyl group. Based on the substitution reaction mechanism, a diethylenetriamine intermediate containing a hydroxyl group reacts with ethylsulfonyl chloride in a molar ratio of 1:2-2.03 to prepare an alkenylated compatibilized monomer.

[0009] Preferably, the alkaline catalyst is one of potassium hydroxide and sodium hydroxide.

[0010] Preferably, the preparation method of the 2-bromoisobutyryl bromide-functionalized reed stem cellulose nanofibers is as follows: Reed cellulose nanofibers were prepared by a pre-hydrolysis synergistic eutectic solvent method. By reacting the hydroxyl groups in reed stem cellulose nanofibers with the acyl bromide groups in 2-bromoisobutyryl bromide, bromine functional groups are grafted onto the surface of reed stem cellulose nanofibers, thus obtaining 2-bromoisobutyryl bromide-functionalized reed stem cellulose nanofibers.

[0011] Preferably, the preparation method of the alkynylated polylactic acid is as follows: the polar functional group at the end of the polylactic acid molecular chain reacts with the bromine functional group in 3-bromopropyne to obtain alkynylated polylactic acid.

[0012] Preferably, the thickness of the polylactic acid membrane modified with reed stem cellulose nanofibers is 0.2-0.4 mm.

[0013] Preferably, the cuprous halide is one of cuprous bromide, cuprous chloride, and cuprous iodide.

[0014] Preferably, the mass ratio of alkynylated polylactic acid to azidated reed stem cellulose nanofiber modified polylactic acid membrane is 100:1-8.

[0015] Preferably, the antibacterial rate of the polylactic acid membrane modified with reed stem cellulose nanofibers is >99%.

[0016] Preferably, the polylactic acid film modified with reed stem cellulose nanofibers is used in food packaging to provide antibacterial protection.

[0017] The beneficial effects of this invention are as follows: Based on the mechanisms of substitution reaction, quaternization reaction and Schiff base reaction, an alkenylated compatibilizing monomer was prepared using diethylenetriamine, 1-chlorooctadecane, 11-chloro-1-undecene, p-hydroxymethylbenzaldehyde and ethylsulfonyl chloride as raw materials. Based on the atom transfer radical polymerization mechanism, using alkenylated compatibilized monomers as raw materials, bromine functional groups grafted onto reed stem cellulose nanofibers were used as initiators, and azide-treated reed stem cellulose nanofibers were prepared under the catalysis of cuprous bromide. Based on the azido-alkynyl "click" reaction mechanism, under the action of cuprous bromide, alkynylated polylactic acid and azidoated reed stem cellulose nanofibers were used as raw materials to prepare reed stem cellulose nanofibers / polylactic acid composite material. Then, the strong coordination between the triazole group and silver ions was used to load silver ions. Finally, the composite antibacterial film was prepared by the coating method. Experimental results demonstrate that the antibacterial film prepared by combining azide-modified reed stem cellulose nanofibers with polylactic acid can be used in food packaging. This film achieves significant improvements in both antibacterial and mechanical properties. Detailed Implementation Example 1

[0018] The preparation of alkenylated compatibilized monomers includes the following steps: Step S1: Preparation of long alkyl chain diethylenetriamine intermediate. The preparation mechanism is as follows: the long alkyl chain diethylenetriamine intermediate is obtained by the substitution reaction between the secondary amine group in the protected diethylenetriamine and the chlorine functional group in 1-chlorooctadecane. The specific experimental steps are as follows: 6.8 g imidazole was added to 50 mL of dichloromethane, and 21.82 g of ditert-butyl dicarbonate was added dropwise at room temperature. The mixture was stirred evenly and refluxed for 2 h. After the reaction was completed, the mixture was cooled to room temperature, the solvent was concentrated, 20 mL of petroleum ether was added, and the solid was precipitated under freezing. The solid was filtered to obtain N-tert-butyloxycarbonyl imidazole. 13.46 g of N-tert-butoxycarbonylimidazolium was added to 20 mL of toluene and mixed well. 4.12 g of diethylenetriamine was added dropwise under ice bath conditions. The system temperature was raised to 65 °C and the reaction was stirred for 5 h. After cooling, the solid was filtered off, the solvent was removed, and the mixture was recrystallized and dried to obtain the protected diethylenetriamine. 2.9 g of 1-chlorooctadecane and 50 mL of N,N-dimethylformamide were added to a 250 mL three-necked flask and mechanically stirred until homogeneous. Then, 3.1 g of protected diethylenetriamine was added, the temperature was raised to 70 °C, and the mixture was stirred for 24 h. After the reaction was completed, the solvent was removed by vacuum distillation and the mixture was dried under vacuum at 50 °C for 10 h to obtain a long alkyl chain diethylenetriamine intermediate. Step S2: Preparation of a diethylenetriamine intermediate containing an alkenyl functional group. The preparation mechanism is as follows: the tertiary amine group in the long alkyl chain diethylenetriamine intermediate undergoes a quaternization reaction with the chlorine functional group in 11-chloro-1-undecene, followed by the removal of the protecting group of the primary amine, to obtain the diethylenetriamine intermediate containing an alkenyl functional group. The specific experimental steps are as follows: In a 250 mL four-necked flask equipped with a thermometer, reflux condenser, and stirrer, 5.6 g of the long alkyl chain diethylenetriamine intermediate is added. Ethylenetriamine intermediate, 1.9 g of 11-chloro-1-undecene, and 60 mL of toluene were mechanically stirred until homogeneous. The system temperature was raised to 50 °C and the reaction was carried out for 54 h. After the reaction was completed, the solvent was removed by rotary evaporation. The solution was then added to 50 mL of 12% HCl solution (ethanol / water = 1:1, v / v) and shaken and soaked at room temperature for 24 h. After soaking, the solution was filtered, washed, and vacuum dried at 60 °C to constant weight to obtain diethylenetriamine intermediate containing alkenyl functional groups. Step S3: Preparation of diethylenetriamine intermediate containing hydroxyl groups. The preparation mechanism is as follows: the amino group in the diethylenetriamine intermediate containing alkenyl functional groups reacts with the aldehyde group in p-hydroxymethylbenzaldehyde through a Schiff base reaction to obtain the diethylenetriamine intermediate containing hydroxyl groups. The specific experimental steps are as follows: In a 250mL three-necked flask equipped with a stirrer, condenser, and thermometer, 2.8g of p-hydroxymethylbenzaldehyde, 50mL of toluene, and 0.3g of anhydrous calcium chloride are added sequentially and mixed well. Then, 5.5g of diethylenetriamine intermediate containing alkenyl functional groups is added and stirred evenly. The mixture is stirred and reacted at 75℃ for 8h. After the reaction is completed, the solvent is removed by rotary evaporation and dried to obtain the diethylenetriamine intermediate containing hydroxyl groups. Step S4: Preparation of alkenylated compatibilized monomer. The preparation mechanism is as follows: alkenylated compatibilized monomer is obtained by the substitution reaction of the hydroxyl group in the diethylenetriamine intermediate containing the hydroxyl group with ethylsulfonyl chloride. The specific experimental steps are as follows: 7.8g of the diethylenetriamine intermediate containing the hydroxyl group is added to 60mL of toluene and mechanically stirred until homogeneous. 2.6g of ethylsulfonyl chloride is added dropwise under ice-water bath conditions. After the addition is complete, the ice bath is removed and the reaction is carried out at room temperature for 24h. After the reaction is completed, the toluene is removed by rotary evaporation, washed, and vacuum dried at 40℃ for 5h to obtain the alkenylated compatibilized monomer. The chemical structural formula of the alkenylated compatibilized monomer is: ; The 1H NMR characterization of the alkenylated compatibilized monomer is as follows: 1 H NMR (CDCl3, 400MHz) δ: 0.88-0.90 (t, 3H), 1.08-1.12 (t, 6H), 1.27-1.38 (m, 42H), 1.73-1.79 (m, 4H), 2.01-2.05 (m, 2H), 3.20-3.25 (m, 4H), 3 .68-3.71(t, 4H), 3.80-3.84(m, 8H), 4.86-4.99(dd, 2H), 5.11(s, 4H), 5.72-5.81 (m, 1H), 7.38-7.40 (d, 4H), 7.65-7.67 (d, 4H), 8.40 (s, 2H). Example 2

[0019] The preparation of azide-modified reed stem cellulose nanofibers includes the following steps: Step 1: Preparation of cellulose nanofibers from reed stems. The specific experimental steps are as follows: Hydrothermal pre-hydrolysis: The pre-hydrolysis treatment was carried out in an electric rotary cooker. Each tank contained 60g of reed stems (after air-drying, cutting, and impurity removal), with a liquid-to-liquid ratio of 1:6 (g:mL). The maximum temperatures for hydrothermal pre-hydrolysis were 135℃, 145℃, 155℃, and 165℃, and the holding times were 5min, 10min, 20min, 30min, 50min, and 70min, respectively. The heating process started at 40℃ and increased at a rate of 1.25℃ / min. At 105℃, a small amount of gas was released for 5min. After reaching the maximum temperature, the tank was held. After the pre-hydrolysis was completed, the reaction tank was removed and cooled. The reed stems and pre-hydrolysate were separated using a Buchner funnel. The reed stems were air-dried and sealed for later use. The pre-hydrolysate was centrifuged for 10min (4000r / min), and the supernatant was collected and refrigerated at 4℃ for later use to obtain the pre-hydrolyzed reed stem sample. Weigh 0.2 mmol / g of ferric chloride hexahydrate, oxalate dihydrate, and choline chloride (mass ratio 1:2:1) in a eutectic solvent (DES) and react them in an oil bath at 80℃ for 2 h to form a homogeneous solution. Add 1 g of pre-hydrolyzed reed stem sample and react at 300 r / min for 5 h. After the reaction is complete, cool to terminate the reaction, dilute with 100 mL of hot deionized water, centrifuge at 8000 r / min for 10 min, wash the precipitate with deionized water at 10000 r / min for 5 min, and repeat the above operation 3 times. Place the centrifuged suspension in a treated dialysis bag and dialyze for 5-7 days, changing the water every 6 h. After dialysis, store at 4℃ to obtain reed stem cellulose nanofibers. Step 2: Preparation of brominated reed stem cellulose nanofibers. The preparation mechanism is as follows: the hydroxyl groups in the reed stem cellulose nanofibers react with the acyl bromide groups in 2-bromoisobutyryl bromide to graft bromine functional groups onto the surface of the reed stem cellulose nanofibers, thus obtaining brominated reed stem cellulose nanofibers. The specific experimental steps are as follows: 10g of reed stem cellulose nanofibers, 1mL of triethylamine and 0.6g of 2-bromoisobutyryl bromide are added to a 100mL three-necked flask. The mixture is stirred in an ice-water bath for 3h, and then stirred at room temperature for 20h to obtain brominated reed stem cellulose nanofibers. Step 3: Preparation of azide-modified reed stem cellulose nanofibers. The preparation mechanism is as follows: using an alkenylated compatibilizing monomer as the polymerization monomer, brominated reed stem cellulose nanofibers as the initiator, and cuprous bromide as the catalyst, an atom transfer radical polymerization reaction first occurs, followed by reaction with sodium azide to obtain azide-modified reed stem cellulose nanofibers. The specific experimental steps are as follows: 2g of brominated reed stem cellulose nanofibers are added to 50mL of toluene and stirred evenly. 0.25g of cuprous bromide and 10g of alkenylated compatibilizing monomer are added. Under nitrogen protection, 0.3g of N,N,N',N'',N''-pentamethyldiethylenetriamine is added. The system temperature is raised to 60℃ and stirred for 24h. After the reaction is completed, toluene is removed by rotary evaporation to obtain ethyl sulfonate type reed stem cellulose nanofibers. 2g of ethyl sulfonate-type reed stem cellulose nanofibers were added to 50mL of deionized water and mechanically stirred until homogeneous. Then, 5.4g of sodium azide was added, the temperature was raised to 50℃, and the reaction was carried out for 24h. After the reaction was completed, sodium hydroxide solid was added to adjust the pH value to 11, thus obtaining azide-treated reed stem cellulose nanofibers. Example 3

[0020] The preparation of composite antibacterial film I includes the following steps: Step 1: Preparation of alkynylated polylactic acid. The preparation mechanism is as follows: alkynylated polylactic acid is obtained by reacting the polar functional groups at the ends of the polylactic acid molecular chains with the bromine functional groups in 3-bromopropyne. The specific experimental steps are as follows: Under nitrogen protection, 10g of polylactic acid (brand name 4032D) and 0.3g of potassium hydroxide were added to 60mL of dichloromethane. The mixture was stirred for 1h under ice bath conditions. Then, 1.2g of 3-bromopropyne was added. After the addition was complete, the temperature was raised to 80℃ and the mixture was refluxed for 20h. After the reaction was completed, the mixture was filtered, washed, and dried at 60℃ for 10h to obtain alkynylated polylactic acid. Step 2: Preparation of reed stem cellulose nanofiber / polylactic acid composite material. The preparation mechanism is as follows: the reed stem cellulose nanofiber / polylactic acid composite material is obtained by a click reaction between the azide groups in the azidolated reed stem cellulose nanofiber and the alkynylated polylactic acid functional groups. The composite material comprises the following raw materials in parts by weight: 100 parts of acetylated polylactic acid; Five portions of azide-treated reed stem cellulose nanofibers; 4.5 parts cuprous bromide; 5.5 parts of N,N,N',N'',N''-pentamethyldivinyltriamine; 60 parts dichloromethane; The preparation method of reed stem cellulose nanofiber / polylactic acid composite material includes the following steps: azide-modified reed stem cellulose nanofiber and alkynylated polylactic acid are added to dichloromethane and ultrasonically dispersed for 30 min. Cuprous bromide and N,N,N',N'',N''-pentamethyldivinyltriamine are added and stirred evenly. The system is subjected to three cycles of freezing-vacuuming-thawing. The system is sealed under vacuum and refluxed at 60°C for 2 h. After the reaction is completed, the mixture is filtered and vacuum dried to obtain the reed stem cellulose nanofiber / polylactic acid composite material. Step 3: Preparation of composite antibacterial film I. The preparation mechanism is as follows: Silver is loaded onto the reed stem cellulose nanofibers / polylactic acid composite material by the strong coordination between the triazole group in the reed stem cellulose nanofibers / polylactic acid composite material and silver ions. The specific experimental steps are as follows: 5g of reed stem cellulose nanofibers / polylactic acid composite material is placed in a mixed solution of 60mL of dichloromethane and deionized water (the volume ratio of dichloromethane and deionized water is 1:1), stirred evenly, and then 4mL of 1mol / L AgNO3 solution is added. The mixture is stirred and reacted for 10h, filtered, washed, and vacuum dried to obtain the silver-loaded reed stem cellulose nanofibers / polylactic acid composite material. 50 parts by weight of silver-loaded reed stem cellulose nanofibers / polylactic acid composite material were added to 50 mL of a mixed solution of dichloromethane and acetone (volume ratio of dichloromethane to acetone was 9:1). The mixture was magnetically stirred for 2 h, allowed to stand for degassing treatment for 4 h, and then the casting solution was coated with an automatic coating machine. The film was dried in a fume hood at room temperature for 48 h to obtain a composite antibacterial film I with a thickness of 0.3 mm. Example 4

[0021] The preparation of composite antibacterial film II includes the following steps: Step 1: Prepare alkynylated polylactic acid, the preparation method of which is the same as in Example 3; Step 2: Preparation of reed stem cellulose nanofiber / polylactic acid composite material, comprising the following raw materials in parts by weight: 100 parts of acetylated polylactic acid; One part of azide-treated reed stem cellulose nanofibers; 4.5 parts cuprous bromide; 5.5 parts of N,N,N',N'',N''-pentamethyldivinyltriamine; 60 parts dichloromethane; The preparation method of reed stem cellulose nanofiber / polylactic acid composite material includes the following steps: azide-modified reed stem cellulose nanofiber and alkynylated polylactic acid are added to dichloromethane and ultrasonically dispersed for 30 min. Cuprous bromide and N,N,N',N'',N''-pentamethyldivinyltriamine are added and stirred evenly. The system is subjected to three cycles of freezing-vacuuming-thawing. The system is sealed under vacuum and refluxed at 60°C for 2 h. After the reaction is completed, the mixture is filtered and vacuum dried to obtain the reed stem cellulose nanofiber / polylactic acid composite material. Step 3: Preparation of composite antibacterial film II. The specific experimental steps are as follows: 5g of reed stem cellulose nanofiber / polylactic acid composite material was placed in a 60mL mixed solution of dichloromethane and deionized water (the volume ratio of dichloromethane and deionized water was 1:1), stirred evenly, and then 4mL of 1mol / L AgNO3 solution was added. The mixture was stirred and reacted for 10h, filtered, washed, and vacuum dried to obtain silver-loaded reed stem cellulose nanofiber / polylactic acid composite material. 50 parts by weight of silver-loaded reed stem cellulose nanofibers / polylactic acid composite material were added to 50 mL of a mixed solution of dichloromethane and acetone (volume ratio of dichloromethane to acetone was 9:1). The mixture was magnetically stirred for 2 h, allowed to stand for degassing treatment for 4 h, and then the casting solution was coated with an automatic coating machine. The film was dried in a fume hood at room temperature for 48 h to obtain a composite antibacterial film II with a thickness of 0.3 mm. Example 5

[0022] The preparation of composite antibacterial film III includes the following steps: Step 1: Prepare alkynylated polylactic acid, the preparation method of which is the same as in Example 3; Step 2: Preparation of reed stem cellulose nanofiber / polylactic acid composite material, comprising the following raw materials in parts by weight: 100 parts of acetylated polylactic acid; 8 portions of azide-treated reed stem cellulose nanofibers; 4.5 parts cuprous bromide; 5.5 parts of N,N,N',N'',N''-pentamethyldivinyltriamine; 60 parts dichloromethane; The preparation method of reed stem cellulose nanofiber / polylactic acid composite material includes the following steps: azide-modified reed stem cellulose nanofiber and alkynylated polylactic acid are added to dichloromethane and ultrasonically dispersed for 30 min. Cuprous bromide and N,N,N',N'',N''-pentamethyldivinyltriamine are added and stirred evenly. The system is subjected to three cycles of freezing-vacuuming-thawing. The system is sealed under vacuum and refluxed at 60°C for 2 h. After the reaction is completed, the mixture is filtered and vacuum dried to obtain the reed stem cellulose nanofiber / polylactic acid composite material. Step 3: Preparation of composite antibacterial film III. The specific experimental steps are as follows: 5g of reed stem cellulose nanofiber / polylactic acid composite material was placed in a 60mL mixed solution of dichloromethane and deionized water (the volume ratio of dichloromethane and deionized water was 1:1), stirred evenly, and then 4mL of 1mol / L AgNO3 solution was added. The mixture was stirred and reacted for 10h, filtered, washed, and vacuum dried to obtain silver-loaded reed stem cellulose nanofiber / polylactic acid composite material. 50 parts by weight of silver-loaded reed stem cellulose nanofibers / polylactic acid composite material were added to 50 mL of a mixed solution of dichloromethane and acetone (volume ratio of dichloromethane to acetone was 9:1). The mixture was magnetically stirred for 2 h, allowed to stand for degassing treatment for 4 h, and then the casting solution was coated with an automatic coating machine. The film was dried in a fume hood at room temperature for 48 h to obtain a composite antibacterial film III with a thickness of 0.3 mm.

[0023] Performance testing:

[0024] I. Antibacterial properties: Preparation of liquid culture medium: Add 18g of nutrient broth to 1L of distilled water, stir and heat at 100℃ until the nutrient broth is completely dissolved, pour into an Erlenmeyer flask, seal with gauze and autoclave (temperature 120℃, time 30min), cool to room temperature after sterilization, and transfer to a sterile operating table for later use. Solid culture medium: Weigh 18g each of nutrient broth and solid agar and add them to 1L of distilled water. Stir and heat at 100℃ until the nutrient broth is completely dissolved. Pour into an Erlenmeyer flask, seal with gauze, and autoclave (sterilization temperature 120℃, time 30min). Before the nutrient solution in the Erlenmeyer flask solidifies, remove it and pour it into a petri dish. Then place it on a sterile operating platform for later use. Preparation of bacterial suspension: Before use, the bacterial strain needs to be activated on a solid culture medium in a biochemical constant temperature incubator. The activated strain is then incubated on a fresh solid culture medium at 37°C for 24 hours to rejuvenate the strain. This process is repeated once more. Individual colonies from the two rejuvenated colonies are picked from the solid culture medium and placed into a liquid culture medium. The inoculated liquid culture medium is then placed in a constant temperature shaker for incubation to obtain 10... 8 CFU / mL bacterial suspension; Determination of antibacterial activity: Staphylococcus aureus (Gram-positive bacterium, ATCC 6538) and Escherichia coli (Gram-negative bacterium, ATCC 25922) were used as representative test species. First, the film prepared in this invention was made into a 5mm × 5mm sample and placed in a 100mL Erlenmeyer flask containing phosphate buffer. Then, 1mL of diluted solution containing 10% phosphate buffer was added. 6CFU / mL of Escherichia coli, Staphylococcus aureus, or Aspergillus niger bacterial suspensions were cultured in conical flasks on a biochemical shaker at 37°C for 24 hours. 0.1 mL of the sample supernatant was then evenly spread onto a nutrient agar plate, and the plate was placed in a biochemical incubator at 37°C for 24 hours. The bacterial colony count was then performed, and compared with a blank sample. The antibacterial rate of the film, T(%), was calculated as (M0-M1) / M0, where M0 is the number of colonies on the plate after the polylactic acid film came into contact with the test bacteria, and M1 is the number of colonies on the plate after the film prepared in this invention came into contact with the test bacteria. The test results are shown in Table 1 below; Table 1. Test results of the antibacterial properties of the composite antibacterial film.

[0025] II. Mechanical Properties: The tensile strength of the composite antibacterial film prepared in this invention was tested using an electronic universal testing machine according to GB / T1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets". The composite antibacterial film sample was 200 mm long and 20 mm wide, and the tensile speed was 500 mm / min. The test results are shown in Table 2 below; Table 2. Test results of mechanical properties of composite antibacterial film

[0026] The comparative formulation is: 100 parts by weight of polylactic acid (brand name 4032D); 5 parts by weight of reed stem cellulose nanofibers (preparation method is the same as in Example 2), the preparation method of which is described in Composite Antibacterial Film I; As can be seen from the experimental results in Tables 1 and 2, the composite antibacterial film prepared by the present invention has achieved significant improvements in both antibacterial and mechanical properties, which are beneficial technical effects.

Claims

1. A method for preparing a polylactic acid membrane modified with cellulose nanofibers from reed stems, characterized in that, Includes the following steps: Step 1: Prepare 2-bromoisobutyryl bromide-functionalized reed stem cellulose nanofibers, and use the bromine functional groups grafted on the reed stem cellulose nanofibers as initiators. Under the catalysis of cuprous halide, the alkenylated compatibilized monomer undergoes an atom transfer radical polymerization reaction on the surface of the reed stem cellulose nanofibers. Subsequently, the ethyl sulfonate of the monomer undergoes a substitution reaction with sodium azide to obtain azide-modified reed stem cellulose nanofibers. The chemical structural formula of the alkenylated compatibilized monomer is: ； The preparation method of the 2-bromoisobutyryl bromide-functionalized reed stem cellulose nanofibers is as follows: Reed cellulose nanofibers were prepared by a pre-hydrolysis synergistic eutectic solvent method. By reacting the hydroxyl groups in reed stem cellulose nanofibers with the acyl bromide groups in 2-bromoisobutyryl bromide, bromine functional groups are grafted onto the surface of reed stem cellulose nanofibers, thus obtaining 2-bromoisobutyryl bromide-functionalized reed stem cellulose nanofibers. Step 2: Prepare alkynylated polylactic acid. Polylactic acid undergoes a click reaction with the azide groups on the surface of the nanofibers through alkynyl functional groups to generate triazole groups, thereby achieving covalent bonding between polylactic acid and reed stem cellulose nanofibers. Silver ions are loaded through the strong coordination of the triazole groups with silver ions. The resulting composite material is used to prepare a polylactic acid film modified with reed stem cellulose nanofibers by a coating method. The preparation method of the alkynylated polylactic acid is as follows: alkynylated polylactic acid is obtained by reacting the polar functional groups at the end of the polylactic acid molecular chain with the bromine functional groups in 3-bromopropyne.

2. The method for preparing a polylactic acid membrane modified with reed stem cellulose nanofibers according to claim 1, characterized in that, The preparation method of the alkenylated compatibilized monomer is as follows: Based on the substitution reaction mechanism, 1 molar equivalent of protected diethylenetriamine reacts with 1 molar equivalent of 1-chlorooctadecane in the presence of a base catalyst to prepare a long alkyl chain diethylenetriamine intermediate. Based on the quaternization reaction mechanism, 1 molar equivalent of a long alkyl chain diethylenetriamine intermediate reacts with 1 molar equivalent of 11-chloro-1-undecene to prepare a diethylenetriamine intermediate containing an alkenyl functional group. Based on the Schiff base reaction mechanism, a diethylenetriamine intermediate containing an alkenyl functional group reacts with p-hydroxymethylbenzaldehyde in a molar ratio of 1:2-2.04 to prepare a diethylenetriamine intermediate containing a hydroxyl group. Based on the substitution reaction mechanism, a diethylenetriamine intermediate containing a hydroxyl group reacts with ethylsulfonyl chloride in a molar ratio of 1:2-2.03 to prepare an alkenylated compatibilized monomer.

3. The method for preparing a polylactic acid membrane modified with reed stem cellulose nanofibers according to claim 2, characterized in that, The alkaline catalyst is one of potassium hydroxide and sodium hydroxide.

4. A polylactic acid membrane modified with reed stem cellulose nanofibers prepared according to any one of claims 1-3, characterized in that, The thickness of the polylactic acid membrane modified with cellulose nanofibers from reed stems is 0.2-0.4 mm.

5. The polylactic acid membrane modified with reed stem cellulose nanofibers according to claim 4, characterized in that, The cuprous halide is one of cuprous bromide, cuprous chloride, and cuprous iodide.

6. The polylactic acid membrane modified with reed stem cellulose nanofibers according to claim 4, characterized in that, The mass ratio of alkynylated polylactic acid to azidated reed stem cellulose nanofibers in the polylactic acid membrane modified with reed stem cellulose nanofibers is 100:1-8.

7. The polylactic acid membrane modified with reed stem cellulose nanofibers according to claim 4, characterized in that, The antibacterial rate of the polylactic acid membrane modified with cellulose nanofibers from reed stems is >99%.

8. The polylactic acid membrane modified with reed stem cellulose nanofibers according to claim 4, characterized in that, The polylactic acid film modified with cellulose nanofibers from reed stems is used in food packaging and has an antibacterial effect.

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