Ultrafine aramid nanofiber and method for preparing the same

Abstract

The present application relates to superfine aramid nanofiber and a method for preparing the same. The superfine aramid nanofiber of the present application comprises a polyamide obtained by copolymerization of a p-phenylenediamine monomer, a heterocyclic diamine monomer and p-phthaloyl chloride, wherein the heterocyclic diamine monomer comprises one or more structures selected from benzoxazole, benzofuran or benzothiophene, and the superfine aramid nanofiber has an average diameter of 5-20 nm. The superfine aramid nanofiber of the present application has a small diameter and excellent dispersion stability in various media.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials, and particularly relates to ultrafine aramid nanofibers and their preparation methods. Background Technology

[0002] Aramid is a high-performance organic fiber material with advantages such as heat resistance, flame retardancy, corrosion resistance, and good mechanical properties at high and low temperatures. It is widely used in aerospace, defense, and many civilian fields. However, due to the chemical structure of aromatic polyamides, aramid is difficult to dissolve and melt, resulting in poor processability, which is a major challenge limiting the expansion of aramid material forms and applications. Aramid nanofibers are a new form of aramid fiber that has emerged in recent years. They not only retain the excellent basic properties of aramid but also offer better processability and superior composite properties with other materials compared to micron-sized aramid fibers. Currently, they show promising application prospects in fields such as papermaking and composite materials.

[0003] There are currently two main strategies for preparing aramid nanofibers. One is the "top-down" method, which primarily targets the liquid crystal molecular properties of para-aramid. This method breaks the intermolecular hydrogen bonds of para-aramid, causing the micron-sized fibers to gradually split into nanofibers. The most typical example of this is the chemical splitting method. In the chemical splitting method, para-aramid fibers undergo a deprotonation reaction in the organic solvent dimethyl sulfoxide (DMSO) and a strongly alkaline medium (such as NaOH). This breaks the intermolecular hydrogen bonds of the poly(p-phenylene terephthalamide) (PPTA) molecules that make up the para-aramid fibers, weakening the intermolecular forces and leading to fiber splitting, ultimately forming fibers with diameters ranging from a few nanometers to tens of nanometers. Nanofibers prepared by the chemical splitting method can have diameters as low as a few nanometers and exhibit good dispersibility. However, this method suffers from drawbacks such as long preparation cycles, low efficiency, difficulty in solvent recovery, and challenges in industrial production. Furthermore, this method disrupts the PPTA molecular structure, significantly affecting the nanofiber properties (such as thermal stability). Patent document 1 discloses an accelerated method for preparing para-aramid nanofibers using the chemical splitting method. This method utilizes ice-water baths and ultrasound to accelerate the formation of aramid nanofibers, but it remains difficult to apply to industrial production. The second method is the "bottom-up" approach, primarily the induced self-assembly method. This method involves rapidly dispersing an aramid solution in a coagulation bath, utilizing the solvent affinity effect and the supramolecular chemical interaction of rigid aramid molecules to drive the aramid molecules to automatically assemble into nanofibers. Patent document 2 discloses a one-step method for preparing para-aramid nanofibers using polymerization-induced self-assembly. This method boasts high efficiency, does not damage the chemical structure of aramid molecules during nanofiber formation, and is currently the only method for achieving industrial-scale production of aramid nanofibers. However, the induced self-assembly method also has a drawback: the diameter of the currently prepared PPTA nanofibers is between 20 and 60 nm, and the fibers are severely entangled. This causes the para-aramid nanofibers prepared by this method to easily flocculate in aqueous media, adversely affecting many production processes (such as paper dewatering) and applications.

[0004] To address the aforementioned drawbacks of the induced self-assembly method, Patent Document 3 reports a scheme combining polymerization and chemical splitting methods to prepare ultrafine para-aramid nanofibers. This scheme involves a secondary treatment of the polymerized para-aramid nanofibers (20-60 nm) with an alkaline medium. The strong alkali disrupts the hydrogen bonds between the nanofibers, thereby producing ultrafine aramid nanofibers with an average diameter of several nanometers. This method offers high preparation efficiency.

[0005] References:

[0006] Patent Document 1: CN110055797A;

[0007] Patent Document 2: CN105153413A;

[0008] Patent document 3: CN115928242A. Summary of the Invention

[0009] The problem the invention aims to solve

[0010] Current methods for preparing ultrafine aramid nanofibers have the following drawbacks:

[0011] Chemical splitting can be used to prepare ultrafine nanofibers, but the preparation efficiency is low. Although the preparation process of nanofibers can be shortened by improvements such as those in Patent Document 1, the improved method also makes the preparation process more complicated. Moreover, chemical splitting destroys the molecular structure of aramid, which affects the performance and long-term stability of nanofibers.

[0012] For example, the para-aramid nanofibers prepared by polymerization-induced self-assembly method disclosed in Patent Document 2 are prone to flocculation in aqueous media, which has an adverse effect on many production processes and applications.

[0013] For example, although the method of secondary alkali treatment after polymerization described in Patent Document 3 can produce ultrafine aramid nanofibers, the process is relatively complex and must be stored in an alkaline medium, which is not conducive to subsequent use.

[0014] Therefore, there is still an urgent need to develop an ultrafine aramid nanofiber with excellent dispersion stability in a variety of solvents and a simple, low-cost, and efficient preparation method.

[0015] Solution for solving the problem

[0016] To address the problems existing in the prior art, this invention proposes to introduce heterocyclic diamine monomers into the polymer molecular chain of ultrafine aramid nanofibers for copolymerization, and also proposes to prepare the corresponding ultrafine aramid nanofibers using a polymerization-induced self-assembly method, thereby solving the above-mentioned problems.

[0017] Specifically, the present invention solves the problems of the present invention through the following solutions.

[0018] [1] An ultrafine aramid nanofiber comprising polyamide, wherein the polyamide is obtained by copolymerization of p-phenylenediamine monomer, heterocyclic diamine monomer and terephthaloyl chloride, wherein the heterocyclic diamine monomer includes one or more structures selected from benzoxazole, benzofuran or benzothiophene, and the ultrafine aramid nanofiber has an average diameter of 5 to 20 nm.

[0019] [2] The ultrafine aramid nanofibers according to [1], wherein the heterocyclic diamine monomer is selected from one or more of the following:

[0020]

[0021] The p-phenylenediamine monomer is selected from one or more of p-phenylenediamine, 5-chloro-p-phenylenediamine, and 2,5-dichloro-p-phenylenediamine.

[0022] [3] According to the ultrafine aramid nanofibers described in [1] or [2], wherein in the polyamide, the ratio of the total number of moles of structural units derived from p-phenylenediamine monomers and heterocyclic diamine monomers to the number of moles of structural units derived from terephthaloyl chloride (diamine / acyl chloride) is 1:(0.9~1.1); the ratio of the number of moles of structural units derived from heterocyclic diamine monomers to the number of moles of structural units derived from p-phenylenediamine monomers is 1:(0.25~4).

[0023] [4] The method for preparing ultrafine aramid nanofibers according to any one of [1] to [3] includes the following steps:

[0024] (a) A polymerization reaction is carried out between p-phenylenediamine monomers, heterocyclic diamine monomers and terephthaloyl chloride in a composite solvent to obtain a polymerization solution;

[0025] (b) The polymer solution is mixed with a diluent, and the resulting mixture is added to a coagulant to obtain an ultrafine aramid nanofiber dispersion;

[0026] The composite solvent includes an organic solvent and a co-solubilizing salt. The organic solvent is one or more selected from N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide. The co-solubilizing salt is one or more selected from alkali metal salts or alkaline earth metal salts.

[0027] [5] According to the preparation method described in [4], wherein,

[0028] The diluent is the composite solvent or an organic solvent in the composite solvent;

[0029] The coagulant is selected from one or more of water, acetone, methanol, ethanol, propanol, butanol, N,N-dimethylformamide, and N,N-dimethylacetamide;

[0030] The volume ratio of the composite solvent in step (a) to the diluent in step (b) is 1:(2~20).

[0031] In step (b), the volume ratio of diluent to coagulant is 1:(2~15).

[0032] [6] According to the preparation method described in [4] or [5], in step (a), the total concentration of p-phenylenediamine monomer, heterocyclic diamine monomer and terephthaloyl chloride in the composite solvent is 0.1~0.8 mol / L.

[0033] [7] According to the preparation method described in [4] or [5], in step (b), the mixture is added to the coagulant while the coagulant is being stirred.

[0034] [8] The preparation method according to [4] or [5] further includes the following steps:

[0035] (c) The ultrafine aramid nanofiber dispersion obtained in step (b) is neutralized with an alkaline aqueous solution, then filtered and washed to obtain an ultrafine aramid nanofiber gel.

[0036] [9] According to the preparation method described in [8], the alkaline solution is characterized in that the alkaline is selected from one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate;

[0037] In this step, the ultrafine aramid nanofiber dispersion obtained in step (b) is neutralized with an alkaline aqueous solution to a pH of 5-8.

[0038]

[10] Ultrafine aramid nanofibers obtained by any one of the preparation methods described in [4] to [9].

[0039] The effects of the invention

[0040] The present invention has the following beneficial effects:

[0041] The ultrafine aramid nanofibers of the present invention have small diameters (average fiber diameter less than 20 nm) and excellent dispersion stability in a variety of media (e.g., water, ethanol and amide organic solvents).

[0042] The preparation method of the present invention is simple, low-cost, and highly efficient, and can be implemented using existing industrial equipment, making it easy to promote on a large scale. Attached Figure Description

[0043] Figure 1 This is a photograph of the ultrafine aramid nanofiber hydrogel obtained in Example 1.

[0044] Figure 2 This is a transmission electron microscope (TEM) image of the ultrafine aramid nanofibers obtained in Example 1.

[0045] Figure 3 This is a transmission electron microscope (TEM) image of the ultrafine aramid nanofibers obtained in Example 2.

[0046] Figure 4 This is a transmission electron microscope (TEM) image of the ultrafine aramid nanofibers obtained in Example 3.

[0047] Figure 5 This is a statistical diagram showing the diameter distribution of the ultrafine aramid nanofibers obtained in Example 1.

[0048] Figure 6 This is a photograph of the ethanol dispersion of the ultrafine aramid nanofibers obtained in Example 1 after 7 days. Detailed Implementation

[0049] The present invention will now be described in detail. The description of the technical features described below is based on representative embodiments and specific examples of the present invention, but the present invention is not limited to these embodiments and specific examples.

[0050] <Terminology and Definitions>

[0051] In this specification, the range of values ​​referred to as "value A to value B" refers to the range including the endpoint values ​​A and B.

[0052] In this specification, the numerical range indicated by "above" or "below" refers to the numerical range that includes the stated number.

[0053] In this specification, the word "may" has two meanings: to perform a certain process and not to perform a certain process.

[0054] In this specification, the terms "optionally" or "optionally" are used to indicate the use or non-use of certain substances, components, procedures, application conditions, etc.

[0055] All unit names used in this manual are international standard unit names, and unless otherwise stated, the "%" indicates weight or mass percentage.

[0056] In this specification, references to "preferred embodiments," "implementation methods," etc., mean that a specific element (e.g., feature, structure, property, and / or characteristic) related to that embodiment is included in at least one of the embodiments described herein, and may or may not be present in other embodiments. Furthermore, it should be understood that the elements may be combined in any suitable manner in various embodiments.

[0057] <Ultrafine Aramid Nanofibers>

[0058] The purpose of this invention is to provide an ultrafine aramid nanofiber comprising a polyamide, wherein the polyamide is obtained by copolymerization of a p-phenylenediamine monomer, a heterocyclic diamine monomer and terephthaloyl chloride, wherein the heterocyclic diamine monomer comprises one or more structures selected from benzoxazole, benzofuran or benzothiophene, and the ultrafine aramid nanofiber has an average diameter of 5-20 nm.

[0059] This invention introduces a heterocyclic diamine monomer as a comonomer into polyamide to disrupt the conjugated structure of polyamide and reduce its intermolecular hydrogen bond density, thereby reducing the intermolecular forces of polyamide and enabling it to spontaneously form an ultrafine nanofiber structure during the induced self-assembly into nanofibers.

[0060] In some implementations, the average diameter of the ultrafine aramid nanofibers is 8–18 nm, such as 10 nm, 12 nm, 14 nm, and 16 nm. In this paper, the average diameter of the fibers can be obtained statistically from transmission electron microscopy (TEM) images. Specifically, TEM images can be imported into the "Nano Measurer 1.2" software, where all nanofibers in the image can be automatically identified or manually labeled, and the software will automatically provide the average diameter of the labeled nanofibers.

[0061] In some embodiments, the aspect ratio of the ultrafine aramid nanofibers is 100 to 500, preferably 200 to 300.

[0062] In some embodiments, the heterocyclic diamine monomer is selected from one or more of the following:

[0063] .

[0064] In some embodiments, the p-phenylenediamine monomer is selected from one or more of p-phenylenediamine, 5-chloro-p-phenylenediamine, and 2,5-dichloro-p-phenylenediamine; from the perspective of production cost and raw material availability, p-phenylenediamine is preferred.

[0065] In some embodiments, the ratio of the total number of moles of structural units derived from p-phenylenediamine monomers and heterocyclic diamine monomers to the number of moles of structural units derived from terephthaloyl chloride (diamine / acyl chloride) in the polyamide is 1:(0.9~1.1), preferably 1:(0.95~1.05).

[0066] In some embodiments, the molar ratio of structural units derived from heterocyclic diamine monomers to structural units derived from p-phenylenediamine monomers (heterocyclic diamine / p-phenylenediamine) is 1:(0.25~4), preferably 1:(0.5~2).

[0067] In some embodiments, the ultrafine aramid nanofibers of the present invention are prepared by polymerization self-assembly.

[0068] The ultrafine aramid nanofibers of the present invention exhibit excellent dispersion stability in a variety of solvents, such as water, ethanol and amide solvents (e.g., N,N-dimethylformamide, N,N-dimethylacetamide) for more than 7 days.

[0069] Another objective of this invention is to provide a method for preparing the ultrafine aramid nanofibers of this invention, which includes the following steps:

[0070] (a) A polymerization reaction is carried out between p-phenylenediamine monomers, heterocyclic diamine monomers and terephthaloyl chloride in a composite solvent to obtain a polymerization solution;

[0071] (b) The polymer solution is mixed with a diluent, and the resulting mixture is added to a coagulant to obtain an ultrafine aramid nanofiber dispersion;

[0072] The composite solvent includes an organic solvent and a co-solubilizing salt. The organic solvent is one or more selected from N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide. The co-solubilizing salt is one or more selected from alkali metal salts or alkaline earth metal salts.

[0073] In some embodiments, the preparation method of the present invention further includes the following steps:

[0074] (c) The ultrafine aramid nanofiber dispersion obtained in step (b) is neutralized with an alkaline aqueous solution, then filtered and washed to obtain an ultrafine aramid nanofiber gel.

[0075] The preparation method of the present invention will be described in detail below.

[0076] Step (a)

[0077] Step (a) is the polymerization reaction step, in which p-phenylenediamine monomers, heterocyclic diamine monomers and terephthaloyl chloride are polymerized to obtain a polymerization solution.

[0078] The polymerization reaction in step (a) is carried out in a composite solvent, which includes an organic solvent and a co-solvent. The organic solvent is one or more selected from N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide. The co-solvent is one or more selected from alkali metal salts or alkaline earth metal salts.

[0079] In some embodiments, the solubilizing salt is one or more halides selected from alkali metals or alkaline earth metals, preferably chlorides, examples of which include, but are not limited to, lithium chloride, sodium chloride, potassium chloride, calcium chloride, and magnesium chloride. Calcium chloride and lithium chloride are further preferred.

[0080] In some embodiments, the mass ratio of the co-solvent salt to the organic solvent in the composite solvent is (0.01~0.10):1, preferably (0.02~0.08):1, for example 0.03:1, 0.04:1, 0.05:1, 0.06:1, 0.07:1.

[0081] In some embodiments, the method of the present invention further includes the step of preparing a composite solvent. Specifically, the co-solvent salt is dissolved in an organic solvent, the dissolution preferably being carried out under heating and / or stirring conditions, the heating temperature preferably being 70-110°C, more preferably 80-105°C.

[0082] In addition to organic solvents and co-solvents, the composite solvent may also contain trace amounts of impurities such as water. Since water can significantly affect the polymerization reaction, it is preferable to control the water content of the composite solvent to be below 300 ppm, more preferably below 200 ppm, even more preferably below 150 ppm, and particularly preferably below 100 ppm.

[0083] Preferably, the preparation method of the present invention further includes a dehydration operation on the composite solvent. This dehydration operation can be performed before the composite solvent is prepared (e.g., dehydrating an organic solvent) or after the composite solvent is prepared. The present invention does not particularly limit the specific dehydration method; suitable methods known in the art can be used, such as dehydration using molecular sieves.

[0084] In a specific implementation plan, p-phenylenediamine monomers and heterocyclic diamine monomers are dissolved in a composite solvent, the temperature of the solution is then lowered to the polymerization temperature, and terephthaloyl chloride is added to carry out the polymerization reaction.

[0085] In some embodiments, dissolution is carried out at temperatures below 20°C (e.g., 5–20°C, 10–15°C). Preferably, the temperature of the composite solvent is first adjusted to the dissolution temperature before the p-phenylenediamine monomer and heterocyclic diamine monomer are added. Preferably, dissolution is carried out with stirring at a speed of 200–800 r / min, preferably 250–500 r / min. Preferably, dissolution is carried out in an inert gas atmosphere, such as a nitrogen or argon atmosphere.

[0086] In some embodiments, the polymerization temperature is -10°C to 10°C, preferably 0°C to 8°C, where "polymerization temperature" refers to the temperature of the system at the start of the polymerization reaction.

[0087] In some embodiments, the polymerization reaction is carried out under stirring conditions, with a stirring speed of 500-1500 r / min, preferably 800-1200 r / min.

[0088] In some implementations, the polymerization reaction is carried out in an inert gas atmosphere, such as a nitrogen or argon atmosphere.

[0089] In a specific implementation scheme, the polymerization reaction in step (a) is carried out as follows: p-phenylenediamine monomers and heterocyclic diamine monomers are dissolved in a composite solvent at a temperature of 5–20°C (preferably 6–15°C). The system is then cooled to -10–10°C (preferably 0–8°C), and terephthaloyl chloride is added. The polymerization reaction is carried out under stirring. The dissolution of the p-phenylenediamine monomers and heterocyclic diamine monomers can optionally be carried out under stirring conditions, with a stirring speed of 200–800 r / min. After adding terephthaloyl chloride, the system can be rapidly stirred to ensure rapid and uniform mixing of the monomers. The rapid stirring speed can be 500–1500 r / min, preferably 700–1200 r / min.

[0090] In some implementations, the polymerization reaction time is 30 to 90 minutes, where "polymerization reaction time" refers to the time from the complete addition of terephthaloyl chloride to the cessation of stirring.

[0091] Preferably, in step (a), the total concentration of p-phenylenediamine monomers, heterocyclic diamine monomers, and terephthaloyl chloride in the composite solvent is 0.1~0.8 mol / L, more preferably 0.2~0.6 mol / L.

[0092] Preferably, in step (a), the total concentration of p-phenylenediamine monomers and heterocyclic diamine monomers in the composite solvent is 0.05~0.4 mol / L, more preferably 0.1~0.3 mol / L. By keeping the monomer concentration within the above range, both production efficiency and the smooth progress of the polymerization reaction are achieved.

[0093] Step (b)

[0094] In step (b), the polymer solution obtained in step (a) is mixed with a diluent, and then the resulting mixture is added to a coagulant to obtain an ultrafine aramid nanofiber dispersion. After the mixture is added to the coagulant, the polyamide self-assembles to form ultrafine aramid nanofibers.

[0095] In some embodiments, the diluent is the composite solvent or an organic solvent within the composite solvent. From a solubility perspective, the composite solvent is preferred.

[0096] The coagulant is a poor solvent for the polyamide polymer obtained in step (a). In some embodiments, the coagulant is one or more selected from water, acetone, methanol, ethanol, propanol, butanol, N,N-dimethylformamide, and N,N-dimethylacetamide; preferably one or more selected from water, ethanol, and N,N-dimethylacetamide. More preferably, the coagulant is water, a water / ethanol mixture, or a water / N,N-dimethylacetamide mixture. In the water / ethanol mixture, the volume fraction of ethanol is preferably 15-40% by volume, more preferably 20-30% by volume. In the water / N,N-dimethylacetamide mixture, the volume fraction of N,N-dimethylacetamide is preferably 5-20% by volume, more preferably 10-15% by volume.

[0097] In some embodiments, the volume ratio of the composite solvent in step (a) to the diluent in step (b) is 1:(2~20), preferably 1:(2~10).

[0098] In some embodiments, the volume ratio of diluent to coagulant in step (b) is 1:(2~15), preferably 1:(2.5~12).

[0099] In some embodiments, in step (b), the mixture is added to the coagulant while the coagulant is being stirred. Preferably, the stirring speed is 10,000 to 25,000 r / min, more preferably 15,000 to 22,000 r / min. Applying high-speed stirring is beneficial for obtaining nanofibers with small fiber diameters and high aspect ratios, resulting in nanofiber dispersions with better dispersibility and longer stability time, and also helps to shorten preparation time and improve production efficiency.

[0100] Step (c)

[0101] Step (c) is a post-processing step, in which the ultrafine aramid nanofiber dispersion obtained in step (b) is neutralized with an alkaline aqueous solution, then filtered and washed to obtain an ultrafine aramid nanofiber gel.

[0102] In some embodiments, the alkali in the alkaline aqueous solution is one or more of an alkali metal / alkaline earth metal hydroxide or carbonate, preferably selected from one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate; more preferably sodium hydroxide, potassium hydroxide, and sodium carbonate.

[0103] Preferably, the ultrafine aramid nanofiber dispersion obtained in step (b) is neutralized with an alkaline aqueous solution to a pH of 5-8, more preferably 6-7.

[0104] In some embodiments, the washing is performed using one or more selected from water and polar solvents. Preferred polar solvents are one or more selected from methanol, ethanol, tetrahydrofuran, N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and dimethyl sulfoxide. Those skilled in the art can select a suitable washing solvent based on the target application of the ultrafine aramid nanofibers.

[0105] The present invention also relates to ultrafine aramid nanofibers, ultrafine aramid nanofiber dispersions, or ultrafine aramid fiber gels obtained by the preparation method of the present invention.

[0106] Example

[0107] The following specific embodiments further illustrate the present invention. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Furthermore, it should be understood that after reading the contents of this invention, those skilled in the art can make various alterations or modifications to the invention, and these equivalent forms also fall within the scope defined by this invention.

[0108] All raw materials used in the following examples were commercially available. Unless otherwise specified, they were reagent-grade and used directly after purchase.

[0109] Example 1

[0110] Prepare a 5 L N-methylpyrrolidone (NMP) / lithium chloride (LiCl) composite solvent and remove water using a 4 Å molecular sieve. The composite solvent contains 3 wt% LiCl and 90 ppm water.

[0111] Under nitrogen protection, 1 L of NMP / LiCl composite solvent was added to the reaction vessel. The temperature of the reaction system was lowered to below 10°C with mechanical stirring at 250 r / min. Then, 10.81 g of p-phenylenediamine and 22.52 g of 2-(4-aminophenyl)-5-aminobenzoxazole were added sequentially. After they were completely dissolved, the temperature of the reaction system was lowered to 0°C. Subsequently, 41.41 g of terephthaloyl chloride was added to the reaction system, and the stirring speed was increased to 1000 r / min. The reaction was continued for 45 min to obtain a copolymerized aramid polymer solution.

[0112] The above copolymer aramid polymer solution was diluted with 4 L of NMP / LiCl composite solvent and mixed evenly. Then it was slowly poured into a 25 L water coagulation bath under high-speed stirring (18000 r / min) and stirred at high speed for 10 min to obtain an ultrafine nanofiber dispersion.

[0113] The pH of the dispersion was adjusted to 6-7 using a 1 mol / L NaOH aqueous solution, followed by centrifugation and filtration. The filter residue was washed with 10 L of pure water and centrifuged and filtered again, repeated three times to obtain a pure ultrafine aramid nanofiber hydrogel. Figure 1 Photograph of this pure, ultrafine aramid nanofiber hydrogel.

[0114] Example 2

[0115] A 3 L N,N-dimethylacetamide (DMAc) / lithium chloride (LiCl) composite solvent was prepared and water was removed using a 4 Å molecular sieve. The composite solvent contained 5 wt% LiCl and 106 ppm water.

[0116] Under nitrogen protection, 1 L of DMAc / LiCl composite solvent was added to the reaction vessel. The reaction system temperature was lowered to 15°C with mechanical stirring at 250 r / min. Then, 10.81 g of p-phenylenediamine and 45.04 g of 2-(4-aminophenyl)-6-aminobenzoxazole were added sequentially. After they were completely dissolved, the reaction system temperature was lowered to 5°C. Subsequently, 61.52 g of terephthaloyl chloride was added to the reaction system, and the stirring speed was increased to 800 r / min. The reaction was continued for 60 min to obtain a copolymerized aramid polymer solution.

[0117] The above copolymer aramid polymer solution was diluted with 2 L of DMAc / LiCl composite solvent and mixed evenly. Then it was slowly poured into an 18 L coagulation bath under high-speed stirring (18000 r / min). The coagulation bath was an ethanol / water solution with a volume ratio of 1:4. The mixture was stirred at high speed for 10 min to obtain an ultrafine nanofiber dispersion.

[0118] The pH of the dispersion was adjusted to 6-7 using a 1 mol / L sodium carbonate aqueous solution, followed by centrifugation and filtration. The filter residue was washed with 10 L of pure water and centrifuged and filtered again, repeated three times to obtain a pure ultrafine aramid nanofiber hydrogel.

[0119] Example 3

[0120] A 4 L N,N-dimethylacetamide (DMAc) / calcium chloride (CaCl2) composite solvent was prepared and water was removed using a 4 Å molecular sieve. The mass fraction of CaCl2 in the composite solvent was 2 wt%, and the water content was 73 ppm.

[0121] Under nitrogen protection, 0.5 L of DMAc / CaCl2 composite solvent was added to the reaction vessel. The temperature of the reaction system was lowered to 10°C with mechanical stirring at 250 r / min. Then, 2.71 g of p-phenylenediamine, 2.80 g of 5-amino-2-(4-aminophenyl)benzofuran, and 3.02 g of 2-(4-aminophenyl)-6-aminobenzothiazole were added sequentially. After complete dissolution, the temperature of the reaction system was lowered to -5°C. Subsequently, 10.45 g of terephthaloyl chloride was added to the reaction system, and the stirring speed was increased to 1000 r / min. The reaction was continued for 75 min to obtain a copolymerized aramid polymer solution.

[0122] The above copolymer aramid polymer solution was diluted with 3.5 L of DMAc / CaCl2 composite solvent and mixed evenly. Then it was slowly poured into a 10 L coagulation bath under high-speed stirring (18000 r / min). The coagulation bath was a DMAc / water solution with a volume ratio of 1:9. The mixture was stirred at high speed for 5 min to obtain an ultrafine nanofiber dispersion.

[0123] The pH of the dispersion was adjusted to 6-7 using a 1 mol / L KOH aqueous solution, followed by centrifugation and filtration. The filter residue was washed with 10 L of pure water and centrifuged and filtered again, repeated three times to obtain a pure ultrafine aramid nanofiber hydrogel.

[0124] <Evaluation>

[0125] 1. The microstructure of the ultrafine aramid nanofibers obtained in Examples 1-3 was observed using a transmission electron microscope (Hitachi H-7650B). The specific operating steps are as follows:

[0126] Sample preparation: Dilute the obtained ultrafine aramid nanofiber hydrogel with water to a solid content of approximately 0.05%~0.1%; place the copper mesh sample stage for transmission electron microscopy on filter paper, use a dropper to draw up the diluted sample and add two drops to the copper mesh, then place the sample stage and filter paper together in a forced-air drying oven and dry at 80℃ for more than 1 hour, then take it out and use tweezers to place the copper mesh in a special sample box for storage.

[0127] Imaging: The copper mesh was mounted on the sample holder of the transmission electron microscope and then inserted into the sample chamber. The vacuum pump was started to evacuate to a high vacuum. The electron gun was turned on, and the spot size, brightness, and astigmatism were adjusted to center the electron beam. The sample stage was moved to find the copper mesh area, and the magnification was gradually increased to a scale bar of around 200 nm. The focus knob was adjusted until the image was clear. After adjusting the exposure and contrast, the TEM image of the sample was obtained.

[0128] TEM images of the ultrafine aramid nanofibers in Examples 1-3 are shown below. Figures 2-4 As shown.

[0129] 2. While observing the microstructure of the ultrafine aramid nanofibers obtained in Example 1 using a transmission electron microscope, the diameter of the samples was statistically analyzed using the equipped "Nano Measurer 1.2" software. The average diameter of the ultrafine aramid nanofibers was obtained, as shown in Table 1 below. The diameter distribution statistics of the ultrafine aramid nanofibers obtained in Example 1 are shown in the figure below. Figure 5 As shown.

[0130] The average diameters of the ultrafine aramid nanofibers of Examples 2 and 3 were obtained using the same method, as shown in Table 1 below.

[0131] Table 1

[0132]

[0133] 3. Dispersion evaluation

[0134] Take 40 g of the ultrafine aramid nanofiber hydrogel obtained in Example 1 and add it to 300 ml of anhydrous ethanol. Stir mechanically at 400 rpm for 5 min to obtain a stable ultrafine aramid nanofiber ethanol dispersion. Place the ethanol dispersion at room temperature and observe its dispersion state. The ethanol dispersion can maintain a uniform dispersion state for more than 7 days. Figure 6 This is a photograph of the ethanol dispersion taken on day 7.

[0135] The dispersion performance of the ultrafine aramid nanofibers obtained in Examples 2 and 3 in ethanol was tested using the same method, and both were able to maintain a uniform dispersion state for more than 7 days.

[0136] The dispersion properties of the ultrafine aramid nanofibers obtained in Examples 1-3 in water and N,N-dimethylformamide were tested in the same manner, and they were able to maintain a uniform dispersion state for more than 7 days.

[0137] The above results indicate that the ultrafine aramid nanofibers obtained by the preparation method of the present invention have excellent dispersibility.

[0138] Industrial availability

[0139] The ultrafine aramid nanofibers of the present invention have a wide range of applications due to their excellent dispersion stability, such as aerospace, defense and military industries and many civilian fields.

Claims

1. An ultrafine aramid nanofiber, characterized in that, The invention comprises a polyamide obtained by copolymerization of a p-phenylenediamine monomer, a heterocyclic diamine monomer, and terephthaloyl chloride, wherein the heterocyclic diamine monomer includes one or more structures selected from benzoxazole, benzofuran, or benzothiophene, and the ultrafine aramid nanofibers have an average diameter of 5-20 nm.

2. The ultrafine aramid nanofiber according to claim 1, characterized in that, The heterocyclic diamine monomer is selected from one or more of the following: The p-phenylenediamine monomer is selected from one or more of p-phenylenediamine, 5-chloro-p-phenylenediamine, and 2,5-dichloro-p-phenylenediamine.

3. The ultrafine aramid nanofibers according to claim 1 or 2, characterized in that, In the polyamide, the ratio of the total number of moles of structural units derived from p-phenylenediamine monomers and heterocyclic diamine monomers to the number of moles of structural units derived from terephthaloyl chloride (diamine / acyl chloride) is 1:(0.9~1.1). The molar ratio of structural units derived from heterocyclic diamine monomers to structural units derived from p-phenylenediamine monomers is 1:(0.25~4).

4. The method for preparing ultrafine aramid nanofibers according to any one of claims 1 to 3, characterized in that, Includes the following steps: (a) A polymerization reaction is carried out between p-phenylenediamine monomers, heterocyclic diamine monomers and terephthaloyl chloride in a composite solvent to obtain a polymerization solution; (b) The polymer solution is mixed with a diluent, and the resulting mixture is added to a coagulant to obtain an ultrafine aramid nanofiber dispersion; The composite solvent includes an organic solvent and a co-solubilizing salt. The organic solvent is one or more selected from N-methylpyrrolidone, N,N-dimethylformamide, and N,N-dimethylacetamide. The co-solubilizing salt is one or more selected from alkali metal salts or alkaline earth metal salts.

5. The preparation method according to claim 4, characterized in that, The diluent is the composite solvent or an organic solvent in the composite solvent; The coagulant is selected from one or more of water, acetone, methanol, ethanol, propanol, butanol, N,N-dimethylformamide, and N,N-dimethylacetamide; The volume ratio of the composite solvent in step (a) to the diluent in step (b) is 1:(2~20). In step (b), the volume ratio of diluent to coagulant is 1:(2~15).

6. The preparation method according to claim 4 or 5, characterized in that, In step (a), the total concentration of p-phenylenediamine monomers, heterocyclic diamine monomers and terephthaloyl chloride in the composite solvent is 0.1~0.8 mol / L.

7. The preparation method according to claim 4 or 5, characterized in that, In step (b), the mixture is added to the coagulant while the coagulant is being stirred.

8. The preparation method according to claim 4 or 5, characterized in that, It also includes the following steps: (c) The ultrafine aramid nanofiber dispersion obtained in step (b) is neutralized with an alkaline aqueous solution, then filtered and washed to obtain an ultrafine aramid nanofiber gel.

9. The preparation method according to claim 8, characterized in that, The alkali in the alkaline aqueous solution is selected from one or more of sodium hydroxide, potassium hydroxide, sodium carbonate, and potassium carbonate. In this step, the ultrafine aramid nanofiber dispersion obtained in step (b) is neutralized with an alkaline aqueous solution to a pH of 5-8.

10. The ultrafine aramid nanofibers obtained by the preparation method according to any one of claims 4 to 9.

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