CN110373814B

CN110373814B - Para-aramid nanofiber membrane and preparation method thereof

Info

Publication number: CN110373814B
Application number: CN201910686371.2A
Authority: CN
Inventors: 陈枫; 汤伟; 傅强
Original assignee: Sichuan University
Current assignee: Sichuan University
Priority date: 2019-07-29
Filing date: 2019-07-29
Publication date: 2020-07-10
Anticipated expiration: 2039-07-29
Also published as: CN110373814A

Abstract

The invention relates to a para-aramid nano-fiber film and a preparation method thereof, belonging to the field of special fiber materials. The invention provides a preparation method of a para-aramid nanofiber membrane, which comprises the following steps: firstly, adding a spinning aid into a para-aramid fiber solution to prepare a uniform spinning solution, and then preparing a para-aramid nanofiber membrane from the obtained spinning solution by an electrostatic spinning method; the spinning aid is one of polyethylene oxide, polyvinylpyrrolidone or polyvinyl alcohol, and the mass ratio of the spinning aid to the para-aramid fiber in the para-aramid fiber solution is as follows: 10 parts of para-aramid fiber and 1-12 parts of spinning aid. The invention successfully realizes the preparation of the continuous para-aramid nano-fiber film by using the electrostatic spinning method for the first time; and by changing the collecting device of electrostatic spinning, the film with random distribution and two structural forms with certain orientation degree is obtained.

Description

Para-aramid nanofiber membrane and preparation method thereof

Technical Field

The invention relates to a para-aramid nano-fiber film and a preparation method thereof, belonging to the field of special fiber materials.

Background

The development of modern science and technology puts higher and higher requirements on the performance of materials, and with the emergence of nanotechnology, the properties and application of nanomaterials begin to become hot spots of research of scholars. The nano material and the nano technology are one of the most potential fields for material research and transformation at present, and are the basis for forming a new generation of advanced fiber and composite material, the application of the micro-nano fiber structure is also the core of many emerging industrial technologies, and the research from the macro to the micro is bound to be the following trend. Continuous nano-fibers represent an emerging class of nano-materials, have key advantages in the combination of structure and application of the materials, and how to push the materials to structural nano-crystallization and functional nano-crystallization is one of the research hotspots at present.

Kevlar fiber, that is, para-aramid fiber, has been drawing attention because it has the characteristics of high strength, high modulus, low density, and good thermal stability, but PPTA fiber is prepared by the steps of condensation polymerization or low-temperature solution polymerization of monomers of terephthaloyl chloride and p-phenylenediamine (in a strong polar system of NMP containing L iCl, or strong acid solvents such as concentrated sulfuric acid, chlorosulfonic acid, and fluoroacetic acid) in strict equal mass ratio in a strong polar amide solvent or a strong acid solvent, and spinning the solution to obtain highly crystalline and highly oriented micron-sized as-spun fiber.

In the prior art, no report related to the para-aramid nano-fiber film with better structure and performance exists.

Disclosure of Invention

Aiming at the defects, the invention provides the preparation method of the para-aramid nano-fiber film for the first time, the para-aramid nano-fiber film with good structure and performance is successfully prepared by the method, and the preparation method has simple process, is easy to operate and has breakthrough guiding significance for the research of the aramid nano-fiber.

The technical scheme of the invention is as follows:

the first technical problem to be solved by the invention is to provide a preparation method of a para-aramid nanofiber membrane, which comprises the following steps: firstly, adding a spinning aid into a para-aramid fiber solution to prepare a uniform spinning solution, and then preparing a para-aramid nanofiber membrane from the obtained spinning solution by an electrostatic spinning method; the spinning aid is one of polyethylene oxide, polyvinylpyrrolidone or polyvinyl alcohol, and the mass ratio of the spinning aid to the para-aramid fiber in the para-aramid fiber solution is as follows: 10 parts of para-aramid fiber and 1-12 parts of spinning aid.

Further, the weight average molecular weight of the spinning aid is 30-700 ten thousand; if the molecular weight is too small, a spindle structure appears in the fiber; if the molecular weight is too large, unevenness in thickness of the fiber occurs, and a structure of the spindle is accompanied.

Further, in the preparation method, the mass concentration of the para-aramid fiber solution is 0.5-2.5 wt%; if the concentration of the solution is too low, the electrostatic spinning process cannot be carried out due to the fact that the viscosity of the spinning solution is too low; if the solution concentration is too high, the solution viscosity is too high, and the electrostatic field stretching force during electrospinning is insufficient to cause the fibers to have too large a diameter and uneven thickness.

Further, in the above preparation method, the process conditions of the electrostatic spinning are as follows: the electrostatic spinning voltage is 7-15 kv, the spinning speed is 0.1-0.5 ml/h, the distance between the spinning nozzle and the receiving device is 10-15 cm, and the spinning temperature is 35-70 ℃.

Further, in the above method, when the receiving device in the electrospinning method is a flat receiver, the microstructure of the obtained para-aramid nanofiber film appears as follows: the nano-fibers in the film are in a random distribution state; when the receiving device is a roller collector, the microstructure of the obtained para-aramid nanofiber film is represented as follows: the nanofibers in the film have a degree of orientation along the direction of rotation of the drum.

Further, in the above method, the method for preparing the uniform spinning solution by adding the spinning aid into the para-aramid fiber solution comprises the following steps: adding a spinning aid into the para-aramid fiber solution, heating and stirring to completely dissolve the spinning aid to prepare uniform spinning solution; wherein the heating temperature is 25-70 deg.C, preferably 50 deg.C.

Further, in the above preparation method, the para-aramid fiber solution is prepared by the following method: stirring the para-aramid micron fibers under the action of an organic solvent and a cosolvent to dissolve the para-aramid micron fibers, and then removing the redundant cosolvent and the incompletely dissolved para-aramid fibers to obtain a uniform para-aramid fiber solution.

Further, in the preparation method of the para-aramid fiber solution, the organic solvent is one of dimethyl sulfoxide, N-dimethylacetamide, N-dimethylformamide, tetrahydrofuran or acetone.

Further, in the preparation method of the para-aramid fiber solution, the cosolvent is one of potassium hydroxide, sodium hydroxide, potassium tert-butoxide or potassium sec-butoxide.

Further, in the preparation method of the para-aramid fiber solution, the method for dissolving the para-aramid microfiber by stirring under the action of the organic solvent and the cosolvent comprises the following steps: adding the para-aramid micron fibers into an organic solvent, adding a cosolvent, sealing, magnetically stirring for 7-12 days or heating by a water bath at 30-60 ℃ for 6-8 days. In the invention, the adding amount of the cosolvent and the solvent is required to ensure that the para-aramid fiber can be fully dissolved, and the mass ratio of the para-aramid fiber to the cosolvent is as follows: 1: 1 to 2.

Further, in the preparation method of the para-aramid fiber solution, a centrifugal treatment mode is adopted to remove redundant cosolvent and incompletely dissolved para-aramid fibers, wherein the centrifugal rate is 5000-10000 rad/min (preferably 8000rad/min), the temperature is 18-30 ℃ (preferably 22 ℃), and the centrifugal time is 30-60 min (preferably 45 min).

Further, in order to improve the mechanical property of the obtained para-aramid nanofiber membrane, the membrane prepared by the electrostatic spinning method is subjected to heat treatment, and the heat treatment method comprises the following steps: and (3) hot-pressing the para-aramid nano-fiber film for 5-60 min (preferably 30min) under the conditions of 60-120 ℃ (preferably 100 ℃) and 0.2-2 MPa (preferably 0.4 MPa).

The second technical problem to be solved by the invention is to provide a para-aramid nanofiber membrane, which is prepared by adopting the method.

Furthermore, the diameter of the nano-fiber in the para-aramid nano-fiber film is 80-200 nm.

Further, the porosity of the para-aramid nanofiber membrane is as follows: 50-90%. When the membrane is a randomly distributed static spinning membrane, the porosity is 80-90%; when the film is a static spinning film with a certain orientation degree, the porosity is 50-60%.

Further, the water contact angle of the para-aramid nanofiber membrane is as follows: 17 to 27 degrees.

The invention has the beneficial effects that:

1. the invention successfully realizes the preparation of the continuous para-aramid nano-fiber film by using the electrostatic spinning method for the first time; and by changing the collecting device of electrostatic spinning, the film with random distribution and two structural forms with certain orientation degree is obtained. The method not only avoids the traditional concentrated sulfuric acid spinning system of aramid fiber, but also successfully prepares the continuous para-aramid nanofiber film with excellent structural morphology with the assistance of the spinning aid, thereby realizing zero breakthrough; has breakthrough guiding significance for the research of the aramid nano-fiber.

2. Compared with the traditional electrostatic spinning film, the para-aramid nano-fiber electrostatic spinning film disclosed by the invention has the advantage of high performance of aramid fiber; the film with random distribution or certain orientation degree has good heat resistance and excellent mechanical property, and simultaneously still maintains good structural form, and has good fiber structure, uniform thickness, high porosity and uniform pore diameter.

3. The para-aramid nano-fiber film prepared by the method is subjected to certain heat treatment, so that the mechanical property of the film can be greatly improved under the condition of keeping small change of the fiber structure form and high porosity, and the mechanical strength of the film is far higher than that of the conventional electrostatic spinning film.

4. The water contact angle of the para-aramid nanofiber membrane obtained by the invention is as follows: 17-27 degrees, and has good hydrophilicity; the obtained para-aramid nano-fiber film has good heat resistance; tests show that partial decomposition can only occur at 320 ℃; and (3) directly heating the film for 10-30 min by using a heating table at 200 ℃, so that the film cannot deform and still keeps a complete structural form.

Description of the drawings:

fig. 1 is a transmission electron microscope image of the ANF/DMSO solution obtained in step (2) of example 1, in which the presence of nanofibers in the ANF solution can be clearly observed and a good structural morphology is maintained.

FIG. 2 is SEM images of the surface and cross-section of a flat collecting electrostatic thin film obtained in example 3; (c) the figure (d) is SEM image of the surface and the cross section of the roller collecting static spun film obtained in example 2.

FIG. 3 is SEM images of the hot-pressed surface and cross-section of a collected flat-plate electrostatic thin film obtained in example 3; (c) FIG. d is SEM images of the hot-pressed surface and cross-section of the collected electrostatic spinning film obtained in example 2.

FIG. 4 is a graph showing a comparison of the stretching of a collected flat-sheet electrostatic film obtained in example 3 before and after hot pressing; (b) the drawing shows the comparison of the stretching before and after hot pressing of the roll collected electrospun film obtained in example 2.

In FIG. 5, (a) is an SEM photograph of the surface of the collected static spinning film obtained in example 1, and (b) is an enlarged view of (a).

In FIG. 6, (a) is an SEM photograph of the surface of the collected static spinning film obtained in example 4, and (b) is an enlarged view of the photograph (a).

In FIG. 7, (a) is an SEM photograph of the surface of the collected static spun-film obtained in comparative example 2, and (b) is an enlarged view of (a).

Detailed Description

The invention provides a preparation method for preparing the para-aramid nano-fiber film with good structure and performance by an electrostatic spinning method for the first time, and provides a new idea for the current research on the para-aramid nano-fiber. The continuous para-aramid nano-fiber membrane is prepared by dissolving para-aramid fibers in an organic solvent/alkali system, adding a certain spinning aid, mechanically stirring and mixing uniformly, and regulating and controlling various parameters of electrostatic spinning. The obtained membrane has the advantages of uniform thickness, good structure, nanofiber shape, high porosity, uniform pore diameter, good heat resistance, excellent mechanical property and potential application in many aspects.

The invention provides a preparation method for preparing a para-aramid nano-fiber film with excellent structure and performance by an electrostatic spinning method, which comprises the following specific steps: the method comprises the following steps: dissolving micron-sized aramid fibers in a DMSO/KOH system, stripping the micron-sized aramid fibers into nano fibers under the action of deprotonation of KOH to obtain an ANF suspension containing the aramid nanofibers, and centrifuging to obtain an ANF solution; step two: then mixing a certain amount of PEO with the obtained ANF solution, and uniformly stirring and mixing the PEO and the ANF solution through a water bath heating machine to obtain a spinning solution; because the molecular main chain of the para-aramid fiber is provided with benzene ring groups, the molecular chain is rigid, the intermolecular entanglement force is weak, and the spinning is difficult under the action of an electrostatic field, and the addition of PEO with certain mass and molecular weight can greatly increase the entanglement among the molecular chains of the para-aramid fiber, thereby helping to realize the electrostatic spinning of the para-aramid fiber; step three: preparing a para-aramid nano-fiber film through electrostatic spinning; step four: and carrying out certain heat treatment on the prepared para-aramid nano-fiber film to obtain the para-aramid nano-fiber static spinning film with more excellent performance.

In the invention, the addition of the spinning aid such as PEO can greatly improve the entanglement among Kevlar 49 molecular chains and improve the viscosity and the conductivity of the solution, thereby ensuring the smooth implementation of the electrostatic spinning process; and then carrying out a certain heat treatment process on the prepared para-aramid nano-fiber static spinning film, wherein the bonding effect of PEO is utilized to greatly improve the lapping effect between the fiber layer and the fiber layer of the static spinning film, thereby greatly improving the mechanical property of the static spinning film.

Embodiments of the present invention will be described in detail below with reference to examples, but it will be understood by those skilled in the art that the following examples are only illustrative of the present invention and should not be construed as limiting the scope of the present invention.

Example 1

(1) Respectively weighing 7g of Kevlar 49 and 7g of KOH, adding the Kevlar 49 and the KOH into 500ml of DMSO, sealing the DMSO by using a sealant, and then magnetically stirring the mixture at room temperature for about one week to obtain a deep red ANF suspension; in the experimental process, the aramid fiber is found to be in different states at different time periods in the dissolving process, the aramid fiber is gradually dissolved, the color of the system is gradually deepened, and finally the color is deep red, so that an ANF suspension is obtained;

(2) centrifuging the ANF suspension in the step (1) for 45min at the rotation speed of 8000rad/min and the temperature of 25 ℃, removing undissolved KOH and aramid fibers in the suspension, and preparing an ANF/DMSO solution with the concentration of 2.159 wt%; FIG. 1 is a transmission electron micrograph of the ANF/DMSO solution obtained in example 1, in which the presence of nanofibers in the ANF solution can be clearly observed and the structural morphology is maintained well; showing that part of amido bonds between para-aramid fiber molecules are damaged due to the deprotonation effect of KOH in a DMSO/KOH system, so that the stripping of the micro-aramid fiber is realized, and the para-aramid nano-fiber is obtained in an ANF solution;

(3) diluting the ANF solution with the concentration of 2.159 wt% obtained in the step (2) to 0.7 wt%, and adding PEO with the molecular weight of 200 ten thousand, the weight ratio of p-aramid fiber: the mass ratio of PEO was 10: 3, heating and mechanically stirring the mixture in a water bath at 50 ℃ to obtain a uniformly mixed spinning solution;

(4) taking a proper amount of spinning solution obtained in the step (3) by using a 5ml syringe with the diameter of 12.5mm, and performing electrostatic spinning by using a needle head with the inner diameter of 0.2mm and the outer diameter of 0.80 mm; the parameters of electrospinning were set as follows: voltage: 7kv, spinning rate: 0.1ml/h, the distance between the spinning nozzle and the collector is 150mm, the spinning temperature: 45 ℃, spinning humidity: 3 percent. Setting the spinning time to be 60 hours under the condition, and collecting by adopting a flat plate to prepare and obtain a randomly distributed para-aramid nano-fiber static spinning film;

(5) putting the static spinning film prepared in the step (4) into a vacuum oven at 60 ℃ for drying for 12h, and removing redundant solvent and a small amount of water in the film; FIG. 5 is an SEM image of the surface of the electrospun membrane obtained in example 1, and it can be seen that the morphology and structure of the fiber are very good, the thickness of the fiber is uniform, and the diameter of the fiber is between 150 nm and 250 nm.

Example 2

(1) 8g of Kevlar 49 and 10g of KOH were weighed out and added to 500ml of DMSO, respectively, and after sealing with a sealant, the mixture was magnetically stirred at room temperature for about one week to obtain a dark red suspension of ANF.

(2) And (2) centrifuging the ANF suspension in the step (1) for 45min at the rotation speed of 8000rad/min and the temperature of 22 ℃, removing the KOH and aramid fibers which are not dissolved in the suspension, and preparing an ANF/DMSO solution with the concentration of 2.365 wt%.

(3) Diluting the 2.365 wt% ANF solution obtained in the step (2) to 1.5 wt% concentration, and adding PEO with the same mass as the ANF and 100 ten thousand molecular weight, namely para-aramid fiber: the mass ratio of PEO was 1: 1, heating and mechanically stirring the mixture in a water bath at 50 ℃ to obtain a uniformly mixed spinning solution.

(4) Taking a proper amount of spinning solution obtained in the step (3) by using a 5ml syringe with the diameter of 12.5mm, and performing electrostatic spinning by using a needle head with the inner diameter of 0.2mm and the outer diameter of 0.80 mm; the parameters of electrospinning were set as follows: voltage: 10kv, spinning rate: 0.3ml/h, the distance between the spinning nozzle and the collector is 150mm, the spinning temperature: 45 ℃, spinning humidity: 3 percent; and setting the spinning time to be 40h under the condition, and collecting by using a roller to prepare the para-aramid nano-fiber electrostatic spinning film with a certain orientation degree.

(5) And (4) putting the static spinning film prepared in the step (4) into a vacuum oven at 60 ℃ for drying for 12h, and removing the redundant solvent and a small amount of water in the film.

(6) And (4) hot-pressing the static spinning film dried in the step (5) for 30min at the temperature of 100 ℃ and under the pressure of 0.4MPa, so that the mechanical property of the film is further improved.

Example 3

(4) Taking a proper amount of spinning solution obtained in the step (3) by using a 5ml syringe with the diameter of 12.5mm, and performing electrostatic spinning by using a needle head with the inner diameter of 0.2mm and the outer diameter of 0.80 mm; the parameters of electrospinning were set as follows: voltage: 10kv, spinning rate: 0.3ml/h, the distance between the spinning nozzle and the collector is 150mm, the spinning temperature: 45 ℃, spinning humidity: 3 percent; and setting the spinning time to be 20h under the condition, and collecting by adopting a flat plate to prepare the randomly distributed para-aramid nano-fiber static spinning film.

(5) Putting the static spinning film prepared in the step (4) into a vacuum oven at 60 ℃ for drying for 12h, and removing redundant solvent and a small amount of water in the film; the film was tested to have a water contact angle of 17 °.

(6) Hot-pressing the static spinning film dried in the step (5) for 30min at 100 ℃ and 0.4MPa, and further improving the mechanical property of the film; the film is obtained by testing, the water contact angle of the film is 27 degrees at the moment, the film is heated for 30min by a hot table at 200 ℃, the film cannot deform, and the complete structural form is still kept; the obtained para-aramid nano-fiber film has good heat resistance.

FIGS. 2(a) and 2(b) are SEM images of the surface and cross-section of a collected static spun-flat film obtained in example 3, respectively; FIGS. 2(c) and 2(d) are SEM images of the surface and cross-section of a collected static spun film obtained by a roller in example 2; as shown in figure 2, the para-aramid nano-fiber film with random distribution and certain orientation degree prepared by the method has good fiber morphology, uniform fiber thickness and uniform fiber thickness, is mainly distributed between 100-200 nm, the porosity of the static spun film obtained by flat plate collection reaches about 87%, and the porosity of the static spun film obtained by roller collection also has about 50%; and the static spinning film that the cylinder was collected has certain orientation degree very obviously, and the overlap joint between the fibrous layer is more inseparable than the film that the flat board was collected obtained, this is because during cylinder collection, the cylinder rotational speed is higher, has reached 3000rad/min, and spinning liquid still experiences the shearing force effect along cylinder direction of rotation after being stretched into the silk by electrostatic field force blowout from the spinneret, and the distance between fibrous layer and the fibrous layer also can be littleer like this, and the overlap joint between fibrous layer and the fibrous layer also can be inseparable, and mechanical properties also can be better.

FIGS. 3(a) and 3(b) are SEM images of the hot-pressed surface and cross-section of a collected electrospun thin film obtained in example 3, respectively; FIGS. 3(c) and 3(d) are SEM images of the hot-pressed surface and cross-section of the collected electrostatic thin film obtained in example 2; FIG. 3 shows that, regardless of whether the electrostatic spinning film is collected by a flat plate or a roller, after hot pressing, the fiber shape in the film is basically kept unchanged, the fiber thickness is uniform, the porosity of the electrostatic spinning film collected by the flat plate after hot pressing is calculated to be about 82%, and the porosity of the electrostatic spinning film collected by the roller is calculated to be about 50%, and compared with the porosity before hot pressing, the porosity is basically unchanged; in general, the structural morphology of the electrospun film did not substantially change before and after hot pressing as shown in FIGS. (a), (c). However, the lap joints between the fiber layers become more compact after the hot pressing of the two films, and almost no gaps are formed as shown in (b) and (d), and it can be seen from (a) and (b) that the hot-pressed films show nodes-like points at the lap joints between the fibers, because during the hot pressing at high temperature, part of PEO flows out of the fibers, and then the cold pressing process converts PEO into a condensed state, so that the so-called joint points are formed; the connecting points promote the lap joint between the fiber layers of the static spinning film, so that the static spinning film with high porosity becomes more compact, and the mechanical property of the film is greatly improved compared with the prior art.

In addition, the invention also compares the tensile properties of the films obtained in example 2 and example 3, and the structure is shown in FIG. 4; FIG. 4 is a graph showing a comparison of the stretching of a collected flat-sheet electrostatic film obtained in example 3 before and after hot pressing; (b) FIG. is a graph showing a comparison of the draw before and after hot-pressing of the collected roll-spun film obtained in example 2; as can be seen from FIG. 4, for the flat-collected electrospun film, the tensile strength without the hot pressing treatment is 10MPa, the tensile strength after the hot pressing treatment reaches 35MPa, which is improved by 3.5 times, and the elongation at break is reduced but still 10%; for the roller-collected static spinning film, the tensile strength without hot pressing treatment is 36MPa, the tensile strength after hot pressing treatment reaches 66MPa, the tensile strength is improved by nearly two times, and the elongation at break is improved to nearly 4% from the original 3%; because the roll collected electrospun film has a degree of orientation, the tensile strength is relatively high and the elongation at break is relatively low compared to flat sheet collected electrospun film. Therefore, the para-aramid nano-fiber electrostatic spinning film prepared by electrostatic spinning has excellent mechanical properties, but after hot pressing treatment, the mechanical properties of the film are greatly improved due to the bonding effect of PEO, and the mechanical properties of the film are far higher than those of the conventional electrostatic spinning film.

Example 4

(1) 7g of Kevlar 49 and 7g of KOH were weighed out and added to 500ml of DMSO, respectively, and after sealing with a sealant, the mixture was magnetically stirred at room temperature for about one week to obtain a dark red suspension of ANF.

(2) And (2) centrifuging the ANF suspension in the step (1) for 45min at the rotation speed of 8000rad/min and the temperature of 25 ℃, removing the KOH and aramid fibers which are not dissolved in the suspension, and preparing the ANF/DMSO solution with the concentration of 2.159 wt%.

(3) Diluting the ANF solution with the concentration of 2.159 wt% obtained in the step (2) to the concentration of 2.0 wt%, and adding PEO with the molecular weight of 400 ten thousand, para-aramid fiber: the mass ratio of PEO was 5: and 6, heating and mechanically stirring the mixture in a water bath at 50 ℃ to obtain the uniformly mixed spinning solution.

(4) Taking a proper amount of spinning solution obtained in the step (3) by using a 5ml syringe with the diameter of 12.5mm, and performing electrostatic spinning by using a needle head with the inner diameter of 0.2mm and the outer diameter of 0.80 mm; the parameters of electrospinning were set as follows: voltage: 15kv, spinning rate: 0.5ml/h, the distance between the spinning nozzle and the collector is 150mm, the spinning temperature: 45 ℃, spinning humidity: 3 percent; and setting the spinning time to be 15h under the condition, and collecting by using a roller to prepare the para-aramid nano-fiber electrostatic spinning film with a certain orientation degree.

(5) Putting the static spinning film prepared in the step (4) into a vacuum oven at 60 ℃ for drying for 12h, and removing redundant solvent and a small amount of water in the film; FIG. 6 is an SEM image of the surface of the electrostatic spinning film obtained in example 4, which can be seen from the SEM image. The fiber morphology remained substantially good.

Example 5

(6) And (3) soaking the static spinning film dried in the step (5) for 6 hours under the condition of heating in a water bath at the temperature of 80 ℃ and mechanical stirring, and removing part of PEO to obtain the para-aramid nano fiber static spinning film with the ANF content of 75%.

(7) And (4) hot-pressing the static spinning film dried in the step (6) for 30min at the temperature of 100 ℃ and under the pressure of 0.4MPa, so that the mechanical property of the film is further improved. The porosity of the obtained hot-pressed film is 80%, the tensile strength is 28MPa, and the elongation at break is 7.2%.

Example 6

(4) Taking a proper amount of spinning solution obtained in the step (3) by using a 5ml syringe with the diameter of 12.5mm, and performing electrostatic spinning by using a needle head with the inner diameter of 0.2mm and the outer diameter of 0.80 mm; the parameters of electrospinning were set as follows: voltage: 10kv, spinning rate: 0.3ml/h, the distance between the spinning nozzle and the collector is 150mm, the spinning temperature: 45 ℃, spinning humidity: 3 percent; and setting the spinning time to be 10h under the condition, and collecting by adopting a flat plate to prepare the randomly distributed para-aramid nano-fiber static spinning film.

(6) And (4) hot-pressing the static spinning film dried in the step (5) for 30min at the temperature of 100 ℃ and under the pressure of 0.4MPa, so that the mechanical property of the film is further improved. The porosity of the obtained hot-pressed film is 78%, the tensile strength is 30MPa, and the elongation at break is 8.2%.

Example 7

(3) Diluting the 2.365 wt% ANF solution obtained in the step (2) to 1.5 wt% concentration, and adding PEO with the same mass as the ANF and 100 ten thousand molecular weight, namely para-aramid fiber: PEO is 1: 1, heating and mechanically stirring the mixture in a water bath at 50 ℃ to obtain a uniformly mixed spinning solution.

(7) And (4) hot-pressing the static spinning film dried in the step (6) for 30min at the temperature of 100 ℃ and under the pressure of 0.4MPa, so that the mechanical property of the film is further improved. The porosity of the obtained hot-pressed film is 75%, the tensile strength is 24MPa, and the elongation at break is 6.4%.

Comparative example 1 no spin aid PEO was added

(3) Diluting the 2.365 wt% ANF solution obtained in step (2) to 1.5 wt% concentration, and thermally stirring in a water bath at 50 deg.C to obtain a spinning solution.

(4) And (3) taking a proper amount of spinning solution obtained in the step (3) by using a 5ml syringe with the diameter of 12.5mm, and carrying out electrostatic spinning by using a needle with the inner diameter of 0.2mm and the outer diameter of 0.80 mm. The parameters of electrospinning were set as follows: voltage: 10kv, spinning rate: 0.3ml/h, the distance between the spinning nozzle and the collector is 150mm, the spinning temperature: 45 ℃, spinning humidity: 3 percent.

Because the molecular chains of the para-aramid fiber are rigid and basically have no entanglement, the para-aramid fiber cannot be well filamentized under the action of electrostatic field force in the electrostatic spinning process, and the para-aramid fiber mostly drops in the form of liquid drops and cannot be well continuous, the continuous and complete para-aramid nanofiber electrostatic spinning film cannot be obtained at all.

Comparative example 2

(1) 15g of Kevlar 49 and 15g of KOH were weighed out and added to 500ml of DMSO respectively, and after sealing with a sealant, the mixture was magnetically stirred at room temperature for about one week to obtain a dark red suspension of ANF.

(2) And (2) centrifuging the ANF suspension in the step (1) for 45min at the rotation speed of 8000rad/min and the temperature of 22 ℃, removing the KOH and aramid fibers which are not dissolved in the suspension, and preparing an ANF/DMSO solution with the concentration of 3.128 wt%.

(3) Diluting the 3.128 wt% ANF solution obtained in the step (2) to 2.7 wt% concentration, and adding 700 ten thousand molecular weight of PEO which is the same mass as the ANF, namely the para-aramid fiber: the mass ratio of PEO was 1: 1, heating in a water bath at 50 ℃, heating, mechanically stirring and uniformly mixing to obtain the spinning solution.

(4) Taking a proper amount of spinning solution obtained in the step (3) by using a 5ml syringe with the diameter of 12.5mm, and performing electrostatic spinning by using a needle head with the inner diameter of 0.2mm and the outer diameter of 0.80 mm; the parameters of electrospinning were set as follows: voltage: 10kv, spinning rate: 0.3ml/h, the distance between the spinning nozzle and the collector is 150mm, the spinning temperature: 45 ℃, spinning humidity: 3 percent.

Because the concentration of the ANF solution is too high and the viscosity is too high, the electrostatic field force in the electrostatic spinning process cannot well ensure the filament discharging state, the prepared fiber has uneven thickness and is accompanied with the occurrence of a spindle structure, and occasionally, the solution cannot be formed into filaments in time due to the too high viscosity and can drop in the form of liquid drops. FIG. 7 is a SEM image of the surface of the electrospun membrane obtained in comparative example 2, and it can be seen that the thickness of the fibers in the membrane is not uniform and the spindle structure appears, indicating that the concentration of the ANF solution is too high, resulting in the morphology of the membrane structure not reaching the ideal state.

Claims

1. A preparation method of a para-aramid nanofiber membrane is characterized by comprising the following steps: firstly, adding a spinning aid into a para-aramid fiber solution to prepare a uniform spinning solution, and then preparing a para-aramid nanofiber membrane from the obtained spinning solution by an electrostatic spinning method; the spinning aid is one of polyethylene oxide, polyvinylpyrrolidone or polyvinyl alcohol, and the mass ratio of the spinning aid to the para-aramid fiber in the para-aramid fiber solution is as follows: 10 parts of para-aramid fiber and 1-12 parts of spinning aid; the mass concentration of the para-aramid fiber solution is 0.5-2.5 wt%; the weight average molecular weight of the spinning aid is 30-700 ten thousand.

2. The preparation method of the para-aramid nanofiber membrane as claimed in claim 1, wherein the process conditions of the electrostatic spinning are as follows: the electrostatic spinning voltage is 7-15 kv, the spinning speed is 0.1-0.5 ml/h, the distance between the spinning nozzle and the receiving device is 10-15 cm, and the spinning temperature is 35-70 ℃.

3. The method for producing a para-aramid nanofiber film as claimed in claim 1 or 2, wherein when the receiving device in the electrospinning method is a flat receiver, the microstructure of the obtained para-aramid nanofiber film appears as: the nano-fibers in the film are in a random distribution state; when the receiving device is a roller collector, the microstructure of the obtained para-aramid nanofiber film is represented as follows: the nanofibers in the film have a degree of orientation along the direction of rotation of the drum.

4. The preparation method of the para-aramid nanofiber membrane as claimed in claim 1 or 2, wherein the method for preparing the uniform spinning solution by adding the spinning aid into the para-aramid fiber solution comprises the following steps: adding a spinning aid into the para-aramid fiber solution, heating and stirring to completely dissolve the spinning aid to prepare uniform spinning solution; wherein the heating temperature is 25-70 deg.C.

5. The method for preparing the para-aramid nanofiber membrane as claimed in claim 4, wherein the heating temperature is 50 ℃.

6. The method for preparing the para-aramid nanofiber membrane as claimed in claim 1 or 2, wherein the para-aramid fiber solution is prepared by the following method: stirring the para-aramid micron fibers under the action of an organic solvent and a cosolvent to dissolve the para-aramid micron fibers, and then removing the redundant cosolvent and the incompletely dissolved para-aramid fibers to obtain a uniform para-aramid fiber solution.

7. The method for preparing a para-aramid nanofiber membrane as claimed in claim 6, wherein in the method for preparing a solution of para-aramid fibers, the organic solvent is one of dimethyl sulfoxide, N-dimethylacetamide, N-dimethylformamide, tetrahydrofuran or acetone.

8. The method for preparing the para-aramid nanofiber membrane as claimed in claim 6, wherein in the method for preparing the para-aramid fiber solution, the cosolvent is one of potassium hydroxide, sodium hydroxide, potassium tert-butoxide or potassium sec-butoxide.

9. The preparation method of the para-aramid nanofiber membrane as claimed in claim 6, wherein in the preparation method of the para-aramid fiber solution, the method of stirring and dissolving the para-aramid microfiber under the action of the organic solvent and the cosolvent comprises the following steps: adding the para-aramid micron fibers into an organic solvent, adding a cosolvent, sealing, magnetically stirring for 7-12 days or heating by a water bath at 30-60 ℃ for 6-8 days.

10. The preparation method of the para-aramid nanofiber membrane as claimed in claim 6, wherein in the preparation method of the para-aramid fiber solution, excess cosolvent and incompletely dissolved para-aramid fibers are removed by adopting a centrifugal treatment mode, wherein the centrifugal rate is 5000-10000 rad/min, the temperature is 18-30 ℃, and the centrifugal time is 30-60 min.

11. The method for preparing a para-aramid nanofiber membrane as claimed in claim 1 or 2, wherein the obtained para-aramid nanofiber membrane is subjected to a heat treatment, the heat treatment method being: and (3) carrying out hot pressing on the para-aramid nano-fiber film for 5-60 min at the temperature of 60-120 ℃ and under the pressure of 0.2-2 MPa.

12. The para-aramid nanofiber membrane is characterized by being prepared by the method of any one of claims 1 to 11.

13. The para-aramid nanofiber membrane as claimed in claim 12, wherein the diameter of the nanofibers in the para-aramid nanofiber membrane is 80-200 nm.

14. The para-aramid nanofiber membrane of claim 13 having a porosity of: 50-90%.

15. The para-aramid nanofiber membrane of claim 14 having a water contact angle of: 17 to 27 degrees.