Zwitterionic nanofibrillated cellulose and method for its preparation

Abstract

The application discloses zwitterionic nanofibrillated cellulose and a preparation method thereof. Firstly, a chemical pretreatment is performed on cellulose to introduce charged groups on the surface of the cellulose, then a water-based free radical initiator is added to induce a grafting reaction between the pretreated cellulose and a zwitterionic monomer containing an unsaturated double bond to obtain zwitterionic modified cellulose, and finally, the zwitterionic cellulose is subjected to mechanical defibration treatment to obtain zwitterionic nanofibrillated cellulose. The zwitterionic nanofibrillated cellulose prepared by the application has the advantages of small and uniform microfiber diameter, high transparency of a dispersion liquid, good hydrophilicity, excellent antibacterial performance, good biocompatibility, and stable dispersion in a wide pH range.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of high polymer materials, and relates to zwitterionic nanofibrillated cellulose and a preparation method thereof, in particular to high-nanofibrillated zwitterionic nanofibrillated cellulose and a preparation method thereof. BACKGROUND

[0002] Nanofibrillated cellulose (NFC) is a kind of nanocellulose with a high aspect ratio, a diameter of 2-100 nm and a length of several microns, which is obtained by mechanically disassembling and separating cellulose pulp. Compared with nanocrystalline cellulose (NCC) prepared by acid hydrolysis, NFC presents a three-dimensional network morphology of high aspect ratio nanofilament random entanglement. NFC has great potential in many fields such as paper reinforcement, composite materials, packaging materials, adsorption materials, medical materials and rheological modifiers, due to its high aspect ratio, good toughness, low oxygen permeability and thermal expansion coefficient, easy surface chemical modification and thixotropic dispersion liquid. In order to reduce the energy consumption of mechanical treatment, it is necessary to perform enzymatic or chemical pretreatment on the surface of cellulose to weaken the hydrogen bond interaction between cellulose microfibers. The chemical pretreatment includes cationization, carboxylation, phosphatation and sulfonation. However, due to the amphiphilic nature of the cellulose molecular structure in these NFC molecules, the hydrophilicity of the NFC surface is still poor, which easily causes the adhesion of proteins, microorganisms and impurities, and the poor anti-fouling, anti-fog and anti-icing effects, thereby limiting its application in many fields. In addition, due to the occurrence of side reactions and the limitation of heterogeneous reactions, it is difficult to achieve a satisfactory degree of substitution of the grafted functional groups (such as quaternary ammonium salt groups) on the surface of cellulose, which leads to the failure to meet the use requirements in some fields (such as antibacterial) and the possible cytotoxicity caused by the introduction of too high content of functional groups.

[0003] Zwitterionic polymer is a kind of polymer material which is overall electrically neutral and contains both anionic and cationic groups on the same monomer side chain. One of the characteristics of zwitterionic polymer is its strong hydration capacity, which can form a dense hydration layer on the surface of the material by combining water molecules. So far, it has shown good application prospects in many fields such as anti-fouling coating, protein modification, drug delivery and membrane separation material. Zwitterionic nanofibrillated cellulose is a kind of cellulose derivative modified by zwitterions, which has the characteristics of both zwitterionic polymer and NFC. However, due to the electrically neutral surface of zwitterionic cellulose, it is difficult to mechanically fibrillate, the energy consumption is high, the nanofibrillated effect is poor, and the size distribution of microfibers is uneven, so there are few reports on the preparation of zwitterionic nanofibrillated cellulose.

[0004] Therefore, the application provides high-nanometer zwitterionic nanofibrillated cellulose and a preparation method thereof, which has low energy consumption and high content of zwitterionic groups and nanometer degree, and can be stably dispersed in a wide pH range, and has wide application prospects in the fields of antibacterial, anti-fouling and anti-fog coating, separation membrane, biological medicine and the like. SUMMARY

[0005] The first object of the application is to provide a preparation method of high-nanometer zwitterionic nanofibrillated cellulose to overcome the defects of the prior art.

[0006] The preparation method of the zwitterionic nanofibrillated cellulose comprises the following steps:

[0007] S1, performing surface chemical pretreatment on cellulose to obtain charge-functionalized modified cellulose;

[0008] The pretreatment modification is one of phosphoric esterification, quaternary ammonium saltification and carboxylation.

[0009] S2, preparing zwitterionic functionalized modified cellulose:

[0010] Under inert gas protection and acidic conditions, unsaturated double bond-containing zwitterionic monomers and aqueous radical initiator are sequentially added to the chemically pretreated cellulose in step S1, and then mechanical stirring is performed at 25-60 DEG C for a period of time, then the pH of the system is adjusted to 7 by using NaOH solution, and finally the cellulose is washed by using deionized water until the conductivity of the filtrate is less than 20 mu s / cm, so that the zwitterionic functionalized modified cellulose is obtained.

[0011] The radical initiator includes one of cerium ammonium nitrate, ammonium persulfate, potassium persulfate and hydrogen peroxide.

[0012] The unsaturated double bond-containing zwitterionic monomers include one or a combination of any number of [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (i.e., methacryloyl ethyl sulfobetaine), 2-(methacryloyloxy)ethyl-2-(trimethylammonio)ethyl phosphate (i.e., methacryloyloxyethyl phosphocholine), 3-[[2-(methacryloyloxy)ethyl]dimethylammonio]propanoate (i.e., methacryloyl ethyl carboxyl betaine), [2-(acryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide, 2-(acryloyloxy)ethyl-2-(trimethylammonio)ethyl phosphate, 3-[[2-(acryloyloxy)ethyl]dimethylammonio]propanoate.

[0013] S3, preparing zwitterionic modified nanofibrillated cellulose:

[0014] The zwitterionized functional modified cellulose prepared in step S2 is diluted with deionized water to the desired solid content, and then subjected to cyclic mechanical treatment to obtain zwitterionized modified nanofibrillated cellulose.

[0015] As preferred, the phosphate esterification in step S1 specifically comprises the following steps:

[0016] (1) A phosphate reagent and a small molecule compound A are added to the wet pulp, and after dissolution, the mixture is allowed to stand for a period of time, and then dried in an oven at 70-100°C. After drying, the modified pulp is solidified at 150-175°C for 1-30 min to obtain a phosphate ester modified pulp;

[0017] (2) The phosphate esterified pulp is prepared into a suspension with a solid content of 0.5-1.0 wt%, and after stirring for a period of time, unreacted reagents are removed by suction filtration and washing, and then the pH of the system is adjusted to 12 with a NaOH solution. After stirring for a period of time, the pulp is washed by suction filtration with deionized water until the conductivity is less than 20 μs / cm.

[0018] More preferably, the mass ratio of the dry weight of the wet pulp, the phosphate reagent and the small molecule compound A in step (1) is 5:(1-3):(4-6).

[0019] More preferably, the phosphate reagent in step (1) is selected from one or any combination of lithium dihydrogen phosphate, lithium hydrogen phosphate, lithium triphosphate, lithium polyphosphate, sodium dihydrogen phosphate, sodium hydrogen phosphate, sodium polyphosphate, potassium dihydrogen phosphate, potassium hydrogen phosphate, potassium polyphosphate, ammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium polyphosphate.

[0020] As preferred, the small molecule compound A in step (1) is one or any combination of urea, thiourea or melamine.

[0021] As preferred, the quaternary ammonium salt modification (cationic modification) in step S1 can be achieved by nucleophilic addition reaction of cationic reagents with the surface hydroxyl groups of cellulose under strong alkali activation, or by self-activation of cationic reagents to graft onto the surface hydroxyl groups of cellulose under the assistance of heat without the presence of strong alkali.

[0022] The above two methods for preparing quaternary ammonium salt modified cellulose specifically comprise the following steps:

[0023] (1) Quaternary ammonium salt cellulose prepared by cationic reagent under strong alkali activation includes the following steps: adding NaOH and cationic reagent into wet pulp, mixing uniformly, and then placing at 35-75°C for 2-8h to obtain cationized cellulose pulp; Quaternary ammonium salt cellulose prepared by self-activation of cationic reagent under heat assistance in the absence of strong alkali includes the following steps: mixing wet pulp and cationic reagent uniformly, and then placing at 50-100°C for 6-12h. In order to reduce or avoid yellowing or thermal degradation of the prepared quaternary ammonium salt cellulose under the condition of no strong alkali, a small amount of urea can be added as an inhibitor.

[0024] In the above two methods for preparing quaternary ammonium salt cellulose, the strong alkali is one of sodium hydroxide, potassium hydroxide, lithium hydroxide, calcium hydroxide, or any combination of multiple thereof; the cationic reagent is one of 2,3-epoxypropyltrimethylammonium chloride (EPTAC) or 3-chloro-2-hydroxypropyltrimethylammonium chloride, or any ratio combination of the two.

[0025] (2) The pulp after cationization treatment is prepared into a suspension with a solid content of 0.5-1wt%, then the pH of the system is adjusted to 7, and finally the pulp is washed by deionized water filtration until the conductivity is less than 20μs / cm.

[0026] As a preferred, under the presence of strong alkali, the mass ratio of the dry weight of wet pulp, strong alkali, and cationic reagent in step (1) is 5:(0.25-0.5):(14.5-20);

[0027] As a preferred, under the absence of strong alkali, the mass ratio of the dry weight of wet pulp, cationic reagent, and urea in step (1) is 5:(14.5-30):(0.05-5).

[0028] As a preferred, the carboxylation in step S1 adopts a carboxymethylation pretreatment method.

[0029] The carboxymethylation pretreatment method specifically includes the following steps:

[0030] (1) The wet pulp is washed with ethanol multiple times for solvent replacement, and a pulp with a solid content of 30wt% is prepared.

[0031] (2) The pulp with solvent replaced by ethanol is immersed in a mixed solution of chloroacetic acid and isopropyl alcohol for a period of time, and after immersion, the pulp is added into the mixed solvent in batches, and under the condition of heating and stirring, it is refluxed to perform carboxymethylation treatment on the pulp. The mixed solvent is composed of NaOH, methanol, and isopropyl alcohol.

[0032] (3) sequentially washing the slurry after step (2) with deionized water and acetic acid to remove unreacted reagents, then immersing the slurry in a NaHCO3 solution for 1-2 h, and finally washing with deionized water until the conductivity of the filtrate is less than 20 μs / cm, to obtain the carboxymethyl-modified slurry.

[0033] Preferably, the mass ratio of the wet pulp, chloroacetic acid and isopropyl alcohol in step (1) is 1:(0.3-0.5):(10-20).

[0034] Preferably, the immersion time in step (2) is 30-60 min.

[0035] Preferably, the mass ratio of the wet pulp, NaOH, methanol and isopropyl alcohol is 1:0.7:(20-30):(50-60).

[0036] Preferably, the reflux temperature in step (2) is 80-90℃, and the reaction time is 1-2 h.

[0037] Preferably, the mass ratio of the cellulose containing charged groups, the zwitterionic monomer containing unsaturated double bonds and the aqueous radical initiator in step S2 is 5:(4-10):(2-5).

[0038] Preferably, the reaction time in step S2 is 8-24 h.

[0039] Preferably, the concentration of the NaOH solution in step S2 is 1M.

[0040] Preferably, the acidic condition in step S2 is a pH value of 1-3.

[0041] Preferably, the cyclic mechanical treatment in step S3 is specifically a plurality of high-pressure homogenization treatments or a plurality of fine grinding treatments.

[0042] A second object of the present application is to provide a zwitterionized NFC prepared by the above method.

[0043] The beneficial effects of the present application are as follows:

[0044] 1. The zwitterionized NFC prepared by the present application has a high degree of nanocrystallization and a high degree of transparency, and requires fewer cyclic mechanical treatments during preparation.

[0045] 2. The zwitterionized NFC prepared by the present application has a high content of grafted zwitterionic groups and good hydrophilicity, and can be applied in the fields of antifouling, antifogging, protein adhesion resistance and surface self-cleaning materials.

[0046] 3. The zwitterionized NFC prepared by the present application can be dispersed in a wide pH range and has a wide application environment.

[0047] 4、The zwitterionic NFC prepared by the method has excellent antibacterial performance due to the presence of zwitterionic groups, and has good biocompatibility while maintaining high antibacterial performance, and can be applied to many fields such as biological medicine, cosmetics, tissue engineering and wearable electronic devices. BRIEF DESCRIPTION OF DRAWINGS

[0048] Figure 1 (a)-(b) are transmission electron microscope (TEM) photos of Example 8 and Comparative Example 3, respectively.

[0049] Figure 2 The dispersion transmittance test results of Example 8, Example 10, Example 11, Comparative Example 2 and Comparative Example 3.

[0050] Figure 3 The dispersion state photos of the aqueous dispersion of Example 8 and Comparative Example 4 under different pH conditions.

[0051] Figure 4 The antibacterial performance test results of the films of Example 2, Example 3, Example 9, Comparative Example 3, Comparative Example 4 and Comparative Example 5.

[0052] Figure 5 The in vitro cytotoxicity test results of the films of Example 2, Example 3, Example 9, Comparative Example 3, Comparative Example 4 and Comparative Example 5. DETAILED DESCRIPTION

[0053] As described above, in view of the deficiencies of the prior art, the present inventors have long studied and practiced a large amount of experiments, and proposed the technical solution of the present application, which is mainly based on at least including:

[0054] (1) The present application introduces charged groups into cellulose by chemical pretreatment first, and then grafts zwitterionic monomers containing unsaturated double bonds by free radicals. This step can significantly improve the efficiency of the subsequent mechanical defibration process (i.e. lower circulating energy consumption), and the zwitterionic NFC prepared has high nanoscale degree.

[0055] (2) The present application first modifies the cellulose by charge functionalization and then grafts unsaturated zwitterionic monomers. The loading amount of the zwitterionic monomers on the surface of the charge functionalized cellulose can be improved by electrostatic interaction, so that the zwitterionic NFC with high degree of substitution is obtained after grafting reaction, which has more excellent hydrophilicity and can be applied to the fields of antifouling and antifogging, protein adhesion resistance, surface self-cleaning water treatment membrane, etc.

[0056] (3) The present application proposes that the cellulose is first charge functionalized and then grafted with an amphoteric ion monomer containing unsaturated double bond, which can significantly improve the antibacterial performance of NFC while maintaining its biocompatibility, and broaden its application range.

[0057] In order to make the purpose, technical scheme and advantages of the present application more clear, the present application is further described in detail below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application and not to limit the present application.

[0058] The present application is a preparation method of amphoteric ionized NFC, comprising the following steps:

[0059] S1, surface chemical pretreatment of cellulose to obtain charge functionalized cellulose;

[0060] The pretreatment modification adopts one of phosphoric acid esterification, quaternary ammonium saltification and carboxylation;

[0061] S2, preparation of amphoteric ionized functionalized cellulose:

[0062] Under inert gas protection and acidic conditions, unsaturated amphoteric ion monomer and aqueous free radical initiator are sequentially added to the chemically pretreated cellulose of step S1, and then reacted at 25-60℃ under mechanical stirring for a period of time, then the pH of the system is adjusted to 7 with NaOH solution, and finally washed with deionized water until the conductivity of the filtrate is less than 20 μs / cm, to obtain amphoteric ionized functionalized cellulose;

[0063] The free radical initiator includes one of cerium ammonium nitrate, ammonium persulfate, potassium persulfate or hydrogen peroxide;

[0064] The amphoteric ion monomer containing unsaturated double bond includes one or any combination of [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (i.e. methacryloyl ethyl sulfobetaine), 2-(methacryloyloxy)ethyl-2-(trimethylammonio)ethyl phosphate (i.e. methacryloyloxyethyl phosphocholine), 3-[[2-(methacryloyloxy)ethyl]dimethylammonio]propanoate (i.e. methacryloyl ethyl carboxyl betaine), [2-(acryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide, 2-(acryloyloxy)ethyl-2-(trimethylammonio)ethyl phosphate, 3-[[2-(acryloyloxy)ethyl]dimethylammonio]propanoate;

[0065] S3, preparation of amphoteric ionized NFC

[0066] The modified cellulose grafted with zwitterionic monomers obtained in step S2 is diluted with deionized water to the required solid content, and then homogenized under high pressure or finely ground to obtain zwitterionic NFC.

[0067] Preferably, the mass ratio of the modified wet pulp dry weight, unsaturated zwitterionic monomer, and aqueous free radical initiator in step S2 is 5:(4-10):(2-5);

[0068] Preferably, the reaction time in step S2 is 8-24 hours;

[0069] Preferably, the concentration of the NaOH solution in step S2 is 1M;

[0070] Preferably, the acidic condition described in step S2 is a pH value of 1-3;

[0071] Preferably, the multiple high-pressure homogenization operations in step S3 involve multiple high-pressure homogenization processes or multiple fine grinding processes.

[0072] Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0073] Example 1: Preparation of zwitterionic cation NFC

[0074] (1) Quaternization pretreatment:

[0075] Take 5g of dry wet pulp and mix it with 0.4g of NaOH solid powder until homogeneous. Activate the mixture in a 35℃ constant temperature oven for 30 minutes. Then add 14.5g of cationic reagent EPTAC powder, mix and knead until homogeneous, and react at 35℃ for 1 hour. After the reaction is complete, dilute the pulp with deionized water to a suspension with a solid content of 1wt%. Adjust the pH of the suspension to 7 with 1M hydrochloric acid solution, and then filter and wash with deionized water until the conductivity of the filtrate is less than 20μs / cm to obtain cationic cellulose.

[0076] (2) Free radical grafting of unsaturated zwitterionic monomers

[0077] Add 2.7g of concentrated nitric acid to a flask and dilute with deionized water to 250g. Under a nitrogen atmosphere, add 5g of dry weight of modified wet pulp, 4.5g of amphoteric monomer (i.e., methacryloyl ethyl sulfobetaine, chemical structure shown below), and 2.8g of cerium ammonium nitrate to the flask sequentially. After reacting at 40℃ with mechanical stirring for 8h, remove the mixture, adjust the pH of the system to neutral using 1M NaOH solution, and then filter and wash with deionized water until the conductivity of the filtrate is below 20μs / cm. The product obtained is cationic cellulose grafted with sulfobetaine monomer (i.e., amphoteric cationic cellulose).

[0078]

[0079] Chemical structure of methacryloyl ethyl sulfobetaine

[0080] (3) Mechanical defibrillation treatment

[0081] The prepared zwitterionic cationic cellulose (5 g dry weight) was mechanically dispersed in 500 mL deionized water, and then the diluted pulp sample was subjected to nanofibrillation treatment with a high-pressure homogenizer, a total of 4 high-pressure homogenization cycles, the first treatment pressure was 300 bar, the second treatment pressure was 600 bar, the third and fourth treatment pressures were 1000 bar, and the viscous sample obtained after the fourth treatment was the zwitterionic cationic NFC.

[0082] Example 2: Preparation of zwitterionic cationic NFC

[0083] (1) Quaternary ammonium salt pretreatment:

[0084] The cationic cellulose was prepared according to the same procedure as step (1) of Example 1.

[0085] (2) The amount of zwitterionic monomer was changed to 2.25 g, and the rest of the operation and reaction conditions were the same as step (2) of Example 1, and the zwitterionic cationic cellulose was prepared.

[0086] (3) The same as step (3) of Example 1, and finally the zwitterionic cationic NFC was prepared.

[0087] Example 3: Preparation of zwitterionic cationic NFC

[0088] (1) Quaternary ammonium salt pretreatment:

[0089] The cationic cellulose was prepared according to the same procedure as step (1) of Example 1.

[0090] (2) The amount of zwitterionic monomer was changed to 9 g, and the rest of the operation and reaction conditions were the same as step (2) of Example 1, and the zwitterionic cationic cellulose was prepared.

[0091] (3) The same as step (3) of Example 1, and finally the zwitterionic cationic NFC was prepared.

[0092] Example 4: Preparation of zwitterionic cationic NFC

[0093] (1) Quaternary ammonium salt pretreatment:

[0094] The cationic cellulose was prepared according to the same procedure as step (1) of Example 1, except that the amount of cationic reagent was changed to 29 g.

[0095] (2) Same as step (2) of Example 1, zwitterionized cationic cellulose was prepared.

[0096] (3) Same as step (3) of Example 1, zwitterionized cationic NFC was finally prepared.

[0097] Example 5: Preparation of zwitterionized cationic NFC

[0098] (1) Quaternary ammonium saltification pretreatment:

[0099] Except that the reaction time was changed to 8 h after adding the cationic reagent, the rest of the operation and reaction conditions were the same as step (1) of Example 1, and cationic cellulose was prepared.

[0100] (2) Same as step (2) of Example 1, zwitterionized cationic cellulose was prepared.

[0101] (3) Same as step (3) of Example 1, zwitterionized cationic NFC was finally prepared.

[0102] Example 6: Preparation of zwitterionized cationic NFC

[0103] (1) Same as step (1) of Example 5, cationic cellulose was prepared.

[0104] (2) Same as step (2) of Example 2, zwitterionized cationic cellulose was prepared.

[0105] (3) Same as step (3) of Example 1, zwitterionized cationic NFC was finally prepared.

[0106] Example 7: Preparation of zwitterionized cationic NFC

[0107] (1) Except that the reaction temperature was changed to 55°C after adding the cationic reagent, the rest of the operation and reaction conditions were the same as step (1) of Example 3, and cationic cellulose was prepared.

[0108] (2) Same as step (2) of Example 1, zwitterionized cationic cellulose was prepared.

[0109] (3) Same as step (3) of Example 1, zwitterionized cationic NFC was finally prepared.

[0110] Example 8: Preparation of zwitterionized cationic NFC

[0111] (1) Same as step (1) of Example 7, cationic cellulose was prepared.

[0112] (2) Same as step (2) of Example 2, zwitterionized cationic cellulose was prepared.

[0113] (3) Same as step (1) of Example 1, finally preparing the zwitterionic cationic NFC.

[0114] Example 9: Preparation of zwitterionic cationic NFC

[0115] (1) Quaternary ammonium salt pretreatment (without alkali):

[0116] Take 5 g of dry weight of wet pulp, 14.5 g of cationic reagent EPTAC powder and 0.5 g of urea, mix and knead uniformly, and then put into a constant temperature box at 80°C for 8 h. After the reaction is completed, dilute the pulp into a suspension with a solid content of 1 wt% with deionized water, adjust the pH of the suspension to 7 with 1M hydrochloric acid, and then wash with deionized water until the conductivity of the filtrate is less than 20 μs / cm, to obtain cationic cellulose.

[0117] (2) Same as step (2) of Example 1, zwitterionic cationic cellulose is prepared.

[0118] (3) Same as step (3) of Example 1, finally preparing the zwitterionic cationic NFC.

[0119] Example 10: Preparation of zwitterionic phosphate ester modified NFC

[0120] (1) Phosphate ester pretreatment:

[0121] Take 5 g of dry weight of wet pulp, 1.3 g of ammonium dihydrogen phosphate and 4 g of urea, mix and knead uniformly, and then put into a constant temperature box at 70°C for 30 min. After drying to constant weight, put the dried pulp into an oven at 165°C for solidification reaction for 10 min. After the reaction is completed, dilute the pulp into a suspension with a solid content of 0.5 wt% with deionized water, wash with deionized water to remove unreacted residual reagents, adjust the pH of the suspension to 12 with 1M NaOH solution, and then wash with deionized water again until the conductivity of the filtrate is less than 20 μs / cm, to obtain phosphate ester modified cellulose.

[0122] (2) Take 5 g of dry weight of phosphate ester modified pulp, and the rest of the conditions are the same as step (2) of Example 2, to prepare zwitterionic phosphate ester modified cellulose.

[0123] (3) Same as step (3) of Example 1, finally preparing the zwitterionic phosphate ester modified NFC.

[0124] Example 11: Preparation of zwitterionic carboxyl modified NFC

[0125] (1) Carboxymethylation pretreatment:

[0126] The 5 g of dry weight of the wet pulp after the solvent of ethanol was replaced was immersed in a mixed solution of 12.5 g of chloroacetic acid and 90 g of isopropyl alcohol for 30 min. After the immersion, the pulp was added to a mixed solution containing 3.5 g of NaOH, 100 g of methanol, and 250 g of isopropyl alcohol, and reacted at 80°C for 1 h under stirring. Subsequently, the pulp was washed with deionized water and acetic acid in sequence to remove the unreacted reagents, and finally immersed in a 1M sodium bicarbonate solution for 1 h, and then washed with deionized water until the conductivity of the effluent was less than 20 μs / cm, to obtain carboxymethyl-modified cellulose.

[0127] (2) 5 g of dry weight of the carboxymethyl-modified pulp was taken, and the remaining conditions were the same as those in step (2) of Example 2, to obtain zwitterionic carboxymethyl-modified cellulose.

[0128] (3) The same as step (3) of Example 1, to finally obtain zwitterionic carboxymethyl-modified NFC.

[0129] Example 1: Preparation of zwitterionic carboxymethyl-modified NFC

[0130] (1) 2.7 g of concentrated nitric acid was added to a flask and diluted with deionized water to 250 g. Under the atmosphere of N2, 5 g of dry weight of the modified wet pulp, 9 g of zwitterionic monomer (methacryloyl ethyl sulfobetaine), and 2.8 g of cerium ammonium nitrate were sequentially added to the flask. After reaction at 40°C for 8 h under mechanical stirring, the mixture in the flask was taken out, the pH of the system was adjusted to neutral with a 1M NaOH solution, and then the pulp was washed with deionized water until the conductivity of the effluent was less than 20 μs / cm, to obtain the product of cellulose grafted with sulfobetaine monomer (zwitterionic cellulose).

[0131] (2) 5 g of dry weight of the wet pulp grafted with zwitterionic monomer was mixed with 0.9 g of NaOH solid powder and kneaded uniformly, and then placed in a constant-temperature oven at 35°C for activation treatment for 30 min. Then, 14.5 g of cationic reagent EPTAC powder was added, and the mixture was kneaded uniformly and then reacted at 35°C for 8 h. After the reaction, the pulp was diluted into a suspension with a solid content of 1 wt%, and then the pH of the suspension was adjusted to 7 with a 1M hydrochloric acid. Then, the pulp was washed with deionized water until the conductivity of the effluent was less than 20 μs / cm, to obtain zwitterionic cationic cellulose.

[0132] (3) The same as step (3) of Example 1, to finally obtain zwitterionic cationic NFC.

[0133] Example 2: Preparation of zwitterionic carboxymethyl-modified NFC

[0134] (1) The rest of the operations and reaction conditions are the same as those in Step (1) of Comparative Example 1, except that the amount of the zwitterionic monomer is changed to 4.5 g, to prepare zwitterionic cellulose.

[0135] (2) The same as Step (2) of Comparative Example 1, to prepare zwitterionic cationic cellulose.

[0136] (3) The same as Step (3) of Example 1, to finally prepare zwitterionic cationic NFC.

[0137] Comparative Example 3: Direct preparation of zwitterionic NFC

[0138] (1) Take 5 g of wet pulp by dry weight, and the rest of the conditions are the same as those in Step (2) of Example 1, to finally prepare zwitterionic cellulose.

[0139] (2) Disperse the prepared zwitterionic cellulose (5 g by dry weight) in 500 mL of deionized water, and use a high-pressure homogenizer to perform nanofiberization treatment on the treated sample. A total of 15 cycles of high-pressure homogenization treatment are performed, with the pressure being 100 bar for the first 5 cycles, 600 bar for the next 5 cycles, and 1000 bar for the last 10 cycles. After the 15th cycle, the viscosity of the obtained sample is significantly increased, and the zwitterionic NFC is obtained.

[0140] Comparative Example 4: Preparation of high-charge cationic NFC

[0141] (1) Mix 5 g of wet pulp by dry weight with 0.9 g of NaOH solid powder and knead uniformly, and then place it in a 35°C constant-temperature box for activation treatment for 30 min. Then, add 29 g of cationic reagent EPTAC powder, mix and knead uniformly, and then place it in a 35°C constant-temperature box for reaction for 8 h. After the reaction is completed, dilute the pulp into a suspension with a solid content of 1 wt% with deionized water, adjust the pH of the suspension to 7 with 1M hydrochloric acid, and then wash it with deionized water until the conductivity of the filtrate is less than 20 μs / cm, to obtain cationic cellulose. The surface charge of the cationic cellulose is calculated by elemental analysis to be 1.28 mmol / g.

[0142] (2) The same as Step (3) of Example 1, to finally prepare cationic NFC.

[0143] Comparative Example 5: Preparation of low-charge cationic NFC

[0144] (1) The same as Step (1) of Example 1, to prepare cationic cellulose.

[0145] (2) The same as Step (3) of Example 1, to finally prepare cationic NFC.

[0146] Test results:

[0147] ①TEM image analysis: TEM images of uranyl acetate staining can be found that the zwitterionic cationic NFC prepared in Example 8 has smaller diameter, more uniform size distribution and higher nanofiber degree than the NFC directly grafted with zwitterionic monomer in Comparative Example 3. Therefore, the method for preparing zwitterionic NFC disclosed in the present application can not only reduce the mechanical energy consumption in the preparation process, but also achieve higher nanofiber degree of the NFC.

[0148] ②Organic element analysis: The results of organic element analysis of Examples 1-11 and Comparative Examples 1-5 are shown in Table 1.

[0149] Table 1. Results of organic element analysis

[0150]

[0151] Each zwitterionic monomer contains 1 sulfonic acid group (i.e. 1 sulfur element) and 1 quaternary ammonium salt group (i.e. 1 nitrogen element), and each cationic group introduced by quaternary ammonium salt modification also contains 1 quaternary ammonium salt group (i.e. 1 nitrogen element). Therefore, the grafting rate of zwitterionic monomer and the grafting rate of quaternary ammonium salt modification can be calculated by element analysis, respectively, and the calculation formula is as follows.

[0152] The calculation formula of the grafting rate of zwitterionic monomer (unit: mmol / g) is as follows:

[0153]

[0154] In the formula, S is the mass fraction of sulfur element measured by organic element analysis of the NFC sample.

[0155] The calculation formula of the grafting rate of quaternary ammonium salt modification (unit: mmol / g) is as follows:

[0156]

[0157] In the formula, N is the mass fraction of nitrogen element measured by organic element analysis of the NFC sample; and A is the grafting rate of zwitterionic monomer calculated above.

[0158] The grafting rate of the amphoteric monomer is determined by the sulfur element content in the product. As can be seen from the organic element analysis results, the sulfur element content in the zwitterionic cationic NFC prepared by the preparation method used in the application is relatively high, and the more amphoteric monomer reactant is put in, the higher the grafting rate of the amphoteric monomer in the NFC product is. As can be seen from Example 1 and Comparative Example 1 and Example 2 and Comparative Example 2, the raw materials and reactant feeding amounts used in the two methods are the same, but the grafting rates of the amphoteric monomers are quite different. This is because the zwitterionic NFC prepared by first cationizing modification of zwitterionic cellulose and then grafting the amphoteric monomer containing unsaturated double bonds avoids the occurrence of the falling off of the first grafted zwitterionic groups due to the strong alkali or high temperature environment in the later cationization modification reaction, thereby significantly improving the grafting rate of the zwitterionic groups. Therefore, the method of first chemical pretreatment and then grafting the amphoteric monomer disclosed in the application is a more efficient method for preparing zwitterionic NFC.

[0159] ③NFC dispersion liquid transmittance test:

[0160] The Example 8, Example 10, Example 11, Comparative Example 2 and Comparative Example 3 were prepared into water dispersions with a solid content of 0.2wt%, and the transmittance-wavelength curves of the dispersions were measured by using a UV-visible spectrophotometer (the results are shown in Figure 2 As can be seen from the results in the figure, the zwitterionic cellulose with positively charged quaternary ammonium groups, negatively charged phosphate groups and negatively charged carboxylate groups is more likely to be prepared into a zwitterionic NFC water dispersion with better transmittance through high-pressure homogenization circulation. The transmittance at 500nm of Example 8 (quaternary ammonium saltization-zwitterionic NFC), Example 10 (phosphate-zwitterionic NFC) and Example 11 (carboxymethyl-zwitterionic NFC) is more than 80%, while the transmittance at 500nm of Comparative Example 3 (direct grafting of zwitterionic NFC on pulp) is only about 60%, which shows that the zwitterionic NFC prepared by the application has a high nanometerization degree.

[0161] ④Hydrophilicity: water contact angle test:

[0162] The NFCs of Example 1-11 and Comparative Example 1-5 were prepared into thin films with a thickness of 2μm by drying, and the average value of the water contact angles measured by using a contact angle measuring instrument at three different positions of the above-mentioned film samples (the data read after 15 seconds of dropping a liquid drop on the surface of the film) was obtained. The experimental results are shown in Table 2.

[0163] Table 2. Water contact angle test results

[0164] Sample Water contact angle average (°) Example 1 62.3 Example 2 65.6 Example 3 59.7 Example 4 70.1 Example 5 65.4 Example 6 69.4 Example 7 78.7 Example 8 80.3 Example 9 64.6 Example 10 70.5 Example 11 60.6 Comparative Example 1 87.7 Comparative Example 2 90.6 Comparative Example 3 76.5 Comparative Example 4 92.6 Comparative Example 5 82.2

[0165] It can be seen from the water contact angle test results that the zwitterionic NFC films disclosed in the present application have smaller water contact angle values than the cationic NFC, and the higher the zwitterionic content, the smaller the water contact angle. The above results are attributed to the high polarity of the zwitterionic group, which endows the ultrafine NFC fibers with high hydrophilicity. The super strong hydrophilic performance enables it to form a tightly bound hydration layer on the material surface, thereby effectively hindering the adsorption of biological molecules such as proteins.

[0166] (5) Acid-base stability test:

[0167] Example 8 and Comparative Example 4 were diluted with deionized water to a 0.1 wt% dispersion, and their pH was adjusted to 1.0, 7.0, and 14.0 using 1 mol / L NaOH solution and 1 mol / L HCl solution, respectively. The stability of the NFC dispersion was observed. As shown in Table 2, Example 8 (zwitterionic cationic NFC) can be stably dispersed under three pH conditions, while Comparative Example 4 (high-charge cationic NFC) has flocculation in the dispersion at pH = 14. Therefore, the zwitterionic NFC prepared by the method of the present application has the advantage of being stably dispersed under a wide pH range. Figure 3

[0168] (6) Antibacterial performance test: According to the ISO22196:2007 standard, the antibacterial activity of the NFC sample films of Example 2, Example 3, Example 9, Comparative Example 3, Comparative Example 4, and Comparative Example 5 against Staphylococcus aureus and Escherichia coli was quantitatively determined by the film sticking method.

[0169] The experimental process is as follows: 20 mL of activated bacteria solution was poured onto the surface of a square NFC film (4 cm x 4 cm), and a layer of square polyethylene film (3 cm x 3 cm) was used to cover the inoculated sample. The culture dish containing the inoculated film was incubated at 37°C and a relative humidity of greater than 90% for 24 h, then the film was washed with 10 mL of agar powder base medium (SCDLP), and diluted with phosphate buffer to one-tenth of the original concentration. Finally, the diluted solution was mixed with solid medium and incubated at 37°C for 48 h, and the number of colony forming units (CFU) in the solid medium was estimated by the pour plate method to calculate the inhibition rate and evaluate the antibacterial activity. The test results are shown in Table 3. Figure 4

[0170] ​​It can be found from Comparative Example 3, Comparative Example 4 and Comparative Example 5 that both zwitterionic modification and quaternary ammonium salt modification can endow NFC with certain antibacterial ability, and the antibacterial ability of zwitterion is better. It can be found from Example 2, Example 3 and Comparative Example 5 that the higher the zwitterionic grafting rate of NFC is, the stronger the antibacterial ability is. In addition, the antibacterial performance of NFC grafted with zwitterion and quaternary ammonium salt is better than that of NFC only modified by zwitterion or quaternary ammonium salt. This indicates that zwitterion and quaternary ammonium salt have a synergistic effect, which enhances the antibacterial performance, and the antibacterial effect of zwitterionic NFC on E. coli is more significant.

[0171] (7) In vitro cytotoxicity test: According to GB716886.5-2017 standard, the cytotoxicity of the NFC sample films of Example 2, Example 3, Example 9, Comparative Example 3, Comparative Example 4 and Comparative Example 5 was determined.

[0172] In addition to excellent antibacterial activity, good cell compatibility is also important for certain applications. The in vitro cytotoxicity of the sample films of Example 2, Example 3, Example 9, Comparative Example 3, Comparative Example 4 and Comparative Example 5 was evaluated using mouse fibroblasts. The area of 5 cm x 6 cm sample film was taken as 60 cm 2 , 10 ml of the extraction solution was added at a ratio of 6 cm 2 / ml, and the water bath was shaken at 37°C for 24 h. The original solution of the extraction solution was added to the cell suspension at a rate of 100 μL / well, and a positive control (10% dimethyl sulfoxide in MEM medium) and a blank control (MEM medium) were also prepared. The in vitro cells were cultured at 37°C and 5% CO2 for 24 h, and the in vitro cell survival rate of the NFC film was determined as shown in Figure 5 .

[0173] It can be found from the figure that the cell survival rate decreases with the increase of the content of quaternary ammonium salt groups, and the high-charge cationic NFC (Comparative Example 4) has certain cytotoxicity compared with the low-charge cationic NFC (Comparative Example 5). Compared with the zwitterionic NFC (Comparative Example 3), the cell survival rate is higher, which indicates that although the quaternary ammonium salt group endows the NFC with certain antibacterial performance, it increases the cytotoxicity, while the zwitterionic group has good cell compatibility. When the zwitterionic group and the quaternary ammonium salt group are grafted at the same time (Example 2, Example 3 and Example 9), the cell survival rate is higher. This indicates that the introduction of zwitterion weakens the cytotoxicity of the quaternary ammonium salt group on the NFC, and the higher the zwitterionic grafting rate is, the weaker the cytotoxicity is. Therefore, the zwitterionic NFC material disclosed in the present application has excellent antibacterial performance while maintaining good biocompatibility, which is conducive to the growth, adhesion and proliferation of cells.

[0174] The above merely describes preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, any changes or replacements within the technical scope disclosed by the present application, which can be easily thought by those skilled in the art, should be covered within the protection scope of the present application. Therefore, the protection scope of the present application should be subject to the protection scope of the claims.

Claims

1. A process for the preparation of a highly nanofibrillated zwitterionized nanocellulose, characterized by The method comprises the following steps: S1, chemically pretreating cellulose fibers to obtain cellulose containing charged groups on the surface; The chemical pretreatment adopts quaternary ammonium saltization; S2, preparing zwitterionic functional modified cellulose: Under inert gas protection and acidic conditions, zwitterionic monomer containing unsaturated double bonds and aqueous radical initiator are sequentially added into the cellulose containing charged groups on the surface prepared in step S1, and mechanical stirring reaction is carried out at 25-60℃ for a period of time; after the reaction is completed, the pH value of the system is adjusted to neutral, and then the zwitterionic functional modified cellulose is obtained by washing with deionized water. The aqueous radical initiator comprises one of cerium nitrate ammonium, ammonium persulfate, potassium persulfate and hydrogen peroxide; The zwitterionic monomer containing unsaturated double bonds comprises one or a combination of [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide, 2-(methacryloyloxy)ethyl-2-(trimethylammonio)ethyl phosphate, 3-[[2-(methacryloyloxy)ethyl]dimethylammonio]propanoate, [2-(acryloyloxy)ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide, 2-(acryloyloxy)ethyl-2-(trimethylammonio)ethyl phosphate and 3-[[2-(acryloyloxy)ethyl]dimethylammonio]propanoate. S3, preparing zwitterionic modified nanofibrillated cellulose: The zwitterionic functional modified cellulose prepared in step S2 is diluted to the required solid content with deionized water, and then high-pressure homogenization or mechanical grinding is carried out to obtain the zwitterionic modified nanofibrillated cellulose.

2. The process for the preparation of a highly nanofibrillated zwitterionized nanocellulose according to claim 1, characterized in that The cellulose quaternary ammonium salt modification pretreatment in step S1 specifically comprises the following steps: (1) under strong alkali activation, a cationic reagent is added to the wet pulp, and after uniform mixing, it is placed under the condition of 35-75℃ for 2-8h to obtain the quaternary ammonium salt treated pulp; wherein the strong alkali is one or a combination of sodium hydroxide, potassium hydroxide, lithium hydroxide and calcium hydroxide; Or, in the absence of strong alkali, the wet pulp, cationic reagent and urea are uniformly mixed, and then placed under the condition of 50-100℃ for 6-12h to obtain the quaternary ammonium salt treated pulp; wherein the cationic reagent is one or a combination of 2,3-epoxypropyltrimethylammonium chloride and 3-chloro-2-hydroxypropyltrimethylammonium chloride; (2) the quaternary ammonium salt treated pulp is prepared into a suspension with a solid content of 0.5-1 wt%, and then the pH value of the system is adjusted to 7, and finally washed with deionized water until the conductivity of the filtrate is less than 20 μs / cm.

3. A process for the preparation of a highly nanofibrillated zwitterionized nanocellulose according to claim 2, characterized by In the presence of strong alkali, the mass ratio of the dry weight of the wet pulp, strong alkali and cationic reagent in step (1) is 5:(0.25-0.5):(14.5-20); in the absence of strong alkali, the mass ratio of the dry weight of the wet pulp, cationic reagent and urea in step (1) is 5:(14.5-30):(0.05-5).

4. The process for preparing a highly nanofibrillated zwitterionized nanocellulose according to claim 1, characterized in that The pH value of the acidic condition in step S2 is 1-3.

5. The process for preparing a highly nanofibrillated zwitterionized nanocellulose according to claim 1, characterized in that In step S2, the mass ratio of the cellulose containing charged groups, the zwitterionic monomer containing unsaturated double bonds, and the aqueous radical initiator is 5:(4-10):(2-5); the reaction temperature is 25-60°C, and the reaction time is 8-24h.

6. A high-nanofibrillated zwitterionized nanocellulose prepared by the method of any one of claims 1-5.

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