CN114434885A

CN114434885A - Asymmetric wettability composite film and preparation method thereof

Info

Publication number: CN114434885A
Application number: CN202111391066.4A
Authority: CN
Inventors: 吴晶; 赵昕; 张秀芹
Original assignee: Beijing Institute Fashion Technology
Current assignee: Beijing Institute Fashion Technology
Priority date: 2021-11-23
Filing date: 2021-11-23
Publication date: 2022-05-06

Abstract

The invention discloses an asymmetric composite membrane and a preparation method thereof. The composite membrane has the capability of one-way water conveying, so that water can be conveyed from the hydrophobic polymer nanofiber layer to the hydrophilic polymer substrate layer, but reverse permeation cannot be realized; in addition, the thermal-conductivity-improved thermal-humidity protective clothing has the advantages that when the thermal-conductivity-improved thermal-humidity protective clothing is used for textiles, particularly protective clothing, wearing thermal comfort of the clothing can be guaranteed, and a development thought is provided for research work on improving thermal-humidity comfort of the protective clothing.

Description

Asymmetric wettability composite film and preparation method thereof

Technical Field

The invention belongs to the technical field of membrane preparation, and relates to an asymmetric composite membrane and a preparation method thereof.

Background

The sudden infectious diseases such as the now worldwide new COVID-19 coronavirus pneumonia, the biochemical pollution caused by chemical safety accidents and the like form serious threats to the national safety, the health of people and the social stability. The medical disposable protective clothing has the advantages of good barrier protection effect, greatly improved protection capability and the like, and plays an important role in the protection work of the pneumonia epidemic situation infected by the novel coronavirus. However, the excellent moisture barrier property can cause sweat and moisture generated by human bodies to be difficult to discharge from the human bodies, and the heat and moisture comfort of the protective clothing is seriously influenced. How to improve the perspiration and moisture permeability and the heat and moisture comfort of the protective textile becomes a core problem to be solved on the basis of ensuring the filtration, sterilization, disinfection and high-efficiency protection of the protective textile.

The traditional hydrophilic cotton fabric is easily wetted by sweat, so that the body feels uncomfortable wet and sticky, and even a too cold state can be brought under the condition of low temperature; in contrast, hydrophobic textiles such as dacron are waterproof, but do not work for the examination of body surface perspiration. Therefore, how to construct a fibrous membrane material (asymmetric composite membrane) with one-way liquid and moisture conducting function, which can rapidly transport sweat and moisture from the skin side of a wearer to the outside, and endow the material with air permeability and moisture conducting performance, becomes a focus of attention.

Although the asymmetric infiltrating composite membrane researched by the prior art makes a certain progress, the provided composite membrane has poor moisture permeability and low unidirectional water conductivity, and the protective clothing prepared from the composite membrane is greatly uncomfortable after being worn for a long time. For example, chinese patent CN202005282U discloses a waterproof moisture-permeable composite fabric, which comprises an outer fabric layer, an outer waterproof moisture-permeable layer, a mesh fabric, an inner waterproof moisture-permeable layer and an inner fabric layer. However, the waterproof moisture-permeable composite fabric has poor waterproof moisture permeability. US patent No. 4194041 discloses a waterproof laminate fabric comprising a hydrophobic outer layer having a microporous structure and a hydrophilic inner layer. The waterproof laminated fabric has slightly improved moisture permeability, but still has a problem of poor moisture permeability. When the waterproof laminated fabric is made into waterproof articles such as clothes or tents, people feel stuffy and the comfort degree of the clothes and the waterproof articles is reduced.

Other related inventions, such as a method for preparing a superhydrophobic and superhydrophilic electrospun nanofiber composite membrane disclosed in CN102605554A, a composite fiber membrane with one-way water permeability disclosed in CN102691175A and a preparation method thereof, a composite membrane capable of regulating the one-way liquid permeability disclosed in CN105664730A and a preparation method thereof, have obviously seen that the one-way water conductivity of the composite membrane is still to be improved.

In view of the above, further research on the conventional asymmetric composite membrane is needed, and an asymmetric composite membrane having excellent unidirectional water guiding and moisture guiding properties is researched.

Disclosure of Invention

In order to overcome the above problems, the present inventors have conducted intensive studies to develop an asymmetric composite membrane and a method for preparing the same, in which a hydrophilic polymer substrate layer is obtained by alkali-treating a hydrophilic polymer fabric, and a hydrophobic polymer nanofiber layer is electrospun on one side of the hydrophilic polymer substrate layer by means of electrospinning, thereby obtaining the asymmetric composite membrane. The composite membrane has the capability of one-way water conveying, can convey water from the hydrophobic polymer nanofiber layer to the hydrophilic polymer substrate layer, but cannot realize reverse permeation; in addition, the thermal conductivity is good, when the thermal-insulation material is used for textiles, particularly protective clothing, the wearing thermal comfort of the clothing can be ensured, and a development idea is provided for research work on improving the thermal-humidity comfort of the protective clothing, so that the thermal-insulation material is completed.

Specifically, the present invention aims to provide the following:

in a first aspect, an asymmetric wettability composite membrane is provided, comprising a hydrophilic polymer substrate layer, and a hydrophobic polymer nanofiber layer disposed on a hydrophilic polymer fabric.

In a second aspect, there is provided a method of making an asymmetric wettability composite membrane, said method comprising:

step 1, preparing a hydrophilic polymer substrate layer;

step 2, preparing a hydrophobic polymer spinning solution precursor;

and 3, spinning a hydrophobic polymer spinning solution precursor on one side of the hydrophilic polymer substrate layer to form a hydrophobic polymer nanofiber layer, so as to obtain the asymmetric wettability composite membrane.

In a third aspect, the asymmetric wettability composite film of the first aspect or the asymmetric wettability composite film produced by the method of the second aspect is used for producing textiles, and is particularly suitable for producing protective clothing.

The invention has the advantages that:

(1) the asymmetric wettability composite membrane provided by the invention has one-way water permeability, and the time for water drops to permeate from the hydrophobic polymer nanofiber layer to the hydrophilic polymer substrate layer is only 162 s.

(2) The asymmetric wettability composite membrane provided by the invention has excellent moisture permeability, and the maximum water content difference between the hydrophobic polymer nanofiber layer and the hydrophilic polymer substrate layer reaches 686.9%.

(3) The asymmetric wettability composite film provided by the invention has excellent thermal conductivity, and can ensure the wearing thermal comfort of clothes when being used for textiles, particularly protective clothing.

(4) The asymmetric wettability composite membrane provided by the invention is prepared by electrostatic spinning a hydrophobic polymer nanofiber layer on one side of a hydrophilic polymer substrate layer, and the thickness of the hydrophobic polymer nanofiber layer is controlled by adjusting the electrostatic spinning time, so that the one-way water permeability of the asymmetric wettability composite membrane is adjusted.

(5) The asymmetric wettability composite membrane provided by the invention has the advantages that the raw material sources are wide, the price is low, the raw material sources are easy to obtain, particularly, when polylactic acid is used as a polymer to prepare the composite membrane, the waste composite membrane can be degraded by microorganisms, the environment is friendly, and the asymmetric wettability composite membrane is suitable for large-scale production.

Drawings

FIG. 1- (a) shows an SEM characterization picture of one side of a PLA nanofiber layer in the PLA-NaOH @ PLA composite film of example 1;

FIG. 1- (b) shows SEM characterization picture of section of the PLA-NaOH @ PLA composite film in example 1;

FIG. 2- (a) shows an SEM characterization picture of a PLA fabric in Experimental example 1;

FIG. 2- (b) shows SEM characterization pictures of NaOH @ PLA fabric in Experimental example 1;

FIG. 3- (a) SEM images of PLA-NaOH @ PLA composite films prepared in example 10, example 14, example 2 and example 6 of Experimental example 2 from left to right, respectively;

FIG. 3- (b) SEM images of PLA-NaOH @ PLA composite films prepared in example 9, example 13, example 1 and example 5 of Experimental example 2 from left to right, respectively;

FIG. 3- (c) SEM images of PLA-NaOH @ PLA composite films prepared in example 11, example 15, example 3 and example 7 of Experimental example 2 from left to right, respectively;

FIG. 3- (d) SEM images of PLA-NaOH @ PLA composite films prepared in example 12, example 16, example 4 and example 8 of Experimental example 2 from left to right, respectively;

FIG. 4 shows SEM characterization pictures in Experimental example 3;

fig. 5- (a) shows a photograph of the contact angle of the PLA nanofiber film with water in experimental example 4.1;

5- (b) photo showing the contact angle of NaOH @ PLA fabric with water in Experimental example 4.1;

FIG. 6 is a graph showing an infrared spectrum in Experimental example 5;

FIG. 7- (a) shows an EDS diagram of a PLA fabric in Experimental example 6;

FIG. 7- (b) shows EDS profile of NaOH @ PLA fabric in Experimental example 6;

FIG. 8- (a) is a graph showing a comparison of hydrostatic pressures of PLA-NaOH @ PLA composite films obtained in example 1 and examples 18-21 of Experimental example 7;

FIG. 8- (b) is a graph showing a comparison of hydrostatic pressures of PLA-NaOH @ PLA composite films obtained in examples 1 and 5 of Experimental example 7;

FIG. 9 is a diagram showing the state in which water droplets of the PLA-NaOH @ PLA composite films obtained in examples 1, 17 to 22 and comparative examples 1 to 3 of Experimental example 8 permeate;

FIG. 10- (a) is a graph showing the change of the existing form of water droplets in the hydrophobic side of the PLA-NaOH @ PLA composite films prepared in example 1 and example 5, respectively, with time in Experimental example 8;

FIG. 10- (b) is a graph showing the change of the existing form of water droplets in the hydrophilic side of the PLA-NaOH @ PLA composite films prepared in example 1 and example 5, respectively, with time in Experimental example 8;

FIG. 11- (a) is a graph showing the comparison of water vapor transmission rate at different temperatures between PLA fabric in Experimental example 9 and PLA-NaOH @ PLA composite films prepared in examples 1 to 3;

FIG. 11- (b) is a graph showing the comparison of water vapor transmission rates at different temperatures between the PLA fabric of Experimental example 9 and the PLA-NaOH @ PLA composite films prepared in examples 5 to 7;

FIG. 12- (a) is a graph showing the comparison of the maximum water content when water drops are dropped on both sides of the PLA-NaOH @ PLA composite films obtained in example 1, example 18, example 21 and comparative example 3, respectively, in Experimental example 10;

FIG. 12- (b) is a graph showing the comparison of the maximum water content when water drops are dropped on both sides of the PLA-NaOH @ PLA composite films obtained in examples 1 and 5, respectively, in Experimental example 10;

FIG. 13- (a) shows a photograph of the PLA fabric of Experimental example 11 and the PLA/NaOH @ PLA composite film prepared in example 1, which was thermally imaged on human skin in the infrared;

FIG. 13- (b) shows a photograph of the PLA fabric of Experimental example 11 and the PLA/NaOH @ PLA composite film prepared in example 5, which was thermally imaged on human skin in the infrared.

Detailed Description

In a first aspect, an asymmetric wettability composite membrane is provided according to the present invention, comprising a hydrophilic polymer substrate layer, and a hydrophobic polymer nanofiber layer disposed on a hydrophilic polymer fabric. In the present invention, the hydrophilic polymer substrate layer is obtained by alkali-treating a hydrophilic polymer fabric.

Wherein the hydrophilic polymer fabric is selected from one or more of polysulfone fabric, polyamide fabric, polyamic acid fabric, polyimide fabric, polyurethane fabric, polyacrylic acid fabric, polylactic acid fabric, polyethylene oxide fabric and polyvinylpyrrolidone fabric, preferably polylactic acid fabric, such as polylactic acid fabric produced by Puyang Yurun New Material Co., Ltd, which contains 50 wt% polylactic acid, 18 wt% acetic acid and 32 wt% tencel.

Further, the base is a basic substance, and is a strong base such as sodium hydroxide, potassium hydroxide, magnesium hydroxide, or an alkali salt such as sodium hydrogencarbonate, potassium hydrogencarbonate, or the like, preferably a strong base, and more preferably sodium hydroxide.

According to the invention, the hydrophobic polymer nanofiber layer is any one or more of a fluoropolymer nanofiber layer, a polyethylene glycol terephthalate nanofiber layer, a polymethyl methacrylate nanofiber layer, a polyacrylonitrile nanofiber layer, a polycarbonate nanofiber layer, a polyvinyl acetate nanofiber layer and a polylactic acid nanofiber layer, and is preferably a polylactic acid nanofiber layer.

In the invention, the thickness of the asymmetric wettability composite film is 230-270 μm, preferably 240-265 μm, and more preferably 249.9-262.1 μm; in this case, water droplets can only penetrate from the hydrophobic polymer nanofiber layer to the hydrophilic polymer substrate layer, but not from the hydrophilic polymer substrate layer to the hydrophobic polymer nanofiber layer.

According to the present invention, a hydrophobic polymer nanofiber layer is spun onto a hydrophilic polymer substrate layer by electrospinning. Wherein the thickness of the hydrophobic polymer nanofiber layer is determined by the electrospinning time.

In the invention, the diameter of the nanofiber in the hydrophobic polymer nanofiber layer is 400-1000 nm, preferably 450-930 nm, and more preferably 520-870 nm.

In the invention, the contact angle between the hydrophilic polymer substrate layer and water is less than 5 degrees, preferably 0-2 degrees, more preferably 0 degrees, and the contact angle between the hydrophobic polymer nanofiber layer and water is 115-140 degrees, preferably 120-130 degrees, more preferably 127-128 degrees; the time for the water drops to permeate from the hydrophobic polymer nanofiber layer to the hydrophilic polymer substrate layer is less than 180s, preferably between 150 and 170s, and more preferably between 160 and 165 s.

Further, the hydrophilic polymer substrate layer is 3000-3600 cm^-1A broad peak is formed, namely a stretching vibration peak of-OH, which is the main reason for leading the hydrophilic polymer substrate layer to have hydrophilicity; the hydrophobic polymer nanofiber layer is 3502cm^-1There is a weak O-H stretching peak, which is an important reason why the hydrophobic polymer nanofiber layer has hydrophobicity.

In a second aspect, according to the method for preparing the asymmetric wettability composite membrane provided by the present invention, the hydrophobic polymer nanofiber layer is spun on one side of the hydrophilic polymer substrate layer by an electrostatic spinning manner, and the method specifically includes:

step 1, preparing a hydrophilic polymer substrate layer;

step 2, preparing a hydrophobic polymer spinning solution precursor;

The method for producing the asymmetric wettability composite film is described in detail below.

Step 1, preparing a hydrophilic polymer substrate layer.

In step 1, a hydrophilic polymer fabric is alkali-treated to obtain a hydrophilic polymer substrate layer.

In step 1, the hydrophilic polymer fabric is selected from one or more of polysulfone fabric, polyamide fabric, polyamic acid fabric, polyimide fabric, polyurethane fabric, polyacrylic fabric, polylactic acid fabric, polyethylene oxide fabric and polyvinylpyrrolidone fabric, preferably polylactic acid fabric, such as polylactic acid fabric produced by Puyang yurun new material, wherein 50 wt% polylactic acid, 18 wt% acetic acid and 32 wt% tencel.

The selection of the hydrophilic polymer fabric directly influences the hydrophilicity of the asymmetric wettability composite membrane, and the green and environmental protection are also considered factors. The polylactic acid fabric is taken as a representative, and the polylactic acid is completely biodegradable aliphatic polyester, has no toxicity, no stimulation and good biocompatibility; the fabric made of the polylactic acid fiber has the advantages of comfortable wearing, good elasticity and good drapability.

In the present invention, the base is a basic substance, and is a strong base such as sodium hydroxide, potassium hydroxide, magnesium hydroxide, or an alkali salt such as sodium hydrogencarbonate, potassium hydrogencarbonate, etc., preferably a strong base, and more preferably sodium hydroxide.

The inventor finds that the hydrophilicity of the hydrophilic polymer fabric can be further improved by alkali treatment of the hydrophilic polymer fabric, and particularly the hydrophilicity of the hydrophilic polymer fabric treated by sodium hydroxide is more excellent. The reason for this is probably that the hydrophilic polymer fabric such as polylactic acid fabric is hydrolyzed under alkaline condition, the ester bond contained therein is free hydrolyzed and broken, so that the number average molecular weight of polylactic acid is slowly reduced, when the molecular weight is reduced to a certain degree, the polylactic acid starts to dissolve, soluble degradation products are generated, a large amount of pore structures are formed on the surface of the originally relatively smooth polylactic acid fabric, and the preparation of the super hydrophilic polylactic acid fabric is realized.

Further, the concentration of the hydrophilic polymer fabric treated with sodium hydroxide is 5 to 15g/L, preferably 8 to 12g/L, for example 10 g/L.

In the invention, with the extension of the alkali treatment time, the number of holes on the surface of the hydrophilic polymer fabric is increased, the diameter of the holes is increased, the super-hydrophilic performance of the hydrophilic polymer substrate layer obtained after the alkali treatment is ensured, and the time efficiency is considered, and the soaking time of the hydrophilic polymer fabric in the alkali is 1-8 h, preferably 2-4 h, and more preferably 3 h.

Preferably, the hydrophilic polymer fabric is dried for 18-26 h, for example 24h, after the alkali treatment, so as to obtain the hydrophilic polymer substrate layer.

And 2, preparing a hydrophobic polymer spinning solution precursor.

In step 2, hydrophobic polymer particles are added to a solvent and mixed to obtain a hydrophobic polymer spinning solution precursor.

The hydrophobic polymer particles are selected from one or more of fluoropolymer particles, polyethylene terephthalate particles, polymethyl methacrylate particles, polyacrylonitrile particles, polycarbonate particles, polyvinyl acetate particles and polylactic acid particles, and are preferably polylactic acid particles.

In the invention, the spinning solution precursor is prepared from polylactic acid particles and is spun on the hydrophilic polymer substrate layer, so that the formed polylactic acid nanofiber layer has stronger close binding property with the hydrophilic polymer substrate layer.

In the present invention, the solvent is selected from one or two of a ketone solvent such as acetone and butanone, an ether solvent such as tetrahydrofuran, an amide solvent such as N, N-dimethylformamide and N, N-dimethylacetamide, and a sulfone solvent such as dimethyl sulfoxide, and is preferably a mixed solvent of an ether solvent and an amide solvent, and more preferably a mixed solvent of N, N-dimethylformamide and tetrahydrofuran.

The hydrophobic polymer particles can be well dissolved and dispersed by using the solvent, particularly the mixed solvent of N, N-dimethylformamide and tetrahydrofuran, so that a uniform mixed solvent is formed, and the subsequent electrostatic spinning process is facilitated.

Wherein the weight ratio of the N, N-dimethylformamide to the tetrahydrofuran mixed solvent is (5-9): (1-4), preferably (6-8): (2 to 3.5), for example, 7: 3.

in step 2, hydrophobic polymer particles are added to a solvent, sealed, stirred and mixed. And during mixing, the stirring speed is 300-500 rpm, preferably 400rpm, the stirring time is 1-5 h, preferably 3h, the stirring control temperature is 30-50 ℃, preferably 40 ℃, and a uniform hydrophobic polymer spinning solution precursor is obtained.

Further, the concentration of the hydrophobic polymer particles in the hydrophobic polymer dope precursor is 1 to 15 wt%, preferably 5 to 12.5 wt%, such as 5 wt%, 7.5 wt%, 10 wt% or 12.5 wt%, further preferably 10 to 12.5 wt%, such as 10 wt% or 12.5 wt%, most preferably 10 wt%.

In the present invention, the mass fraction of the hydrophobic polymer particles in the hydrophobic polymer dope precursor is small, and a large number of "bead-knot" structures exist in the obtained hydrophobic polymer nanofiber layer, as shown in (a) and (b) of fig. 3, which is probably because a part of the dissolved hydrophobic polymer is stretched and refined into fibers under the action of electrostatic force, and another part of the dissolved hydrophobic polymer is rapidly retracted to form droplets under the action of surface tension, so that the "bead-knot" structures are caused. When the concentration of the hydrophobic polymer particles is increased to 10-12.5 wt%, especially 10 wt%, the hydrophobic polymer spinning solution precursor obtains a uniform nanofiber structure under the traction of electrostatic force, as shown in (c) and (d) of fig. 3.

In step 3, spinning on one side of the hydrophilic polymer substrate layer by adopting an electrostatic spinning method to prepare the hydrophobic polymer nanofiber layer.

According to the invention, the electrostatic spinning technology is used as a novel spinning method for preparing continuous long fibers, not only can fibers with the diameter of several nanometers to several micrometers be prepared, but also the prepared nano fibers are finer and more uniform in diameter, larger in fiber linear density and specific surface area, and better in interface performance and adsorption performance. In addition, the electrospinning method is simple and convenient to operate, low in cost and less in environmental pollution, and becomes one of the most potential methods in industrial-grade fiber preparation. The preparation of functional materials with special wettability by electrospinning has become one of the hot spots in the wettability research field in recent years. By reasonably designing the shape and distribution of the receiving electrode, the arrangement state of the obtained fibers can be effectively controlled, and the fiber structure with special micro-morphology characteristics is obtained.

Wherein, the electrostatic spinning can be carried out by using the electrostatic spinning equipment which is common in the prior art, for example, ET-2033 electrostatic spinning equipment which is produced by Leye science and technology development Limited.

In the invention, the hydrophobic polymer spinning solution precursor is placed in a syringe for electrostatic spinning, and the inner diameter of a spinneret can be selected from 0.06mm-6G, 0.08mm-8G, 0.1mm-10G, 0.12mm-12G, and is preferably 8G.

Wherein the mass fraction of hydrophobic polymer particles and the spinneret pair with different inner diameters have a direct influence on the diameter of the nanofibers in the hydrophobic polymer nanofiber layer. As the mass fraction of hydrophobic polymer particles increases, the diameter of the nanofibers increases; meanwhile, as the inner diameter of the spinneret increases, the diameter of the obtained nanofiber also increases, and the reason for the phenomenon can be as follows: the larger the mass fraction of the hydrophobic polymer particles is, the higher the viscosity of the spinning solution precursor is, the larger the entanglement among molecular chains is, and when the size and the voltage of a spinning nozzle are the same, the more difficult the spinning solution precursor with the larger mass fraction is to be stretched in the spinning process, and the larger the diameter of the nanofiber is; meanwhile, the larger the inner diameter of a spinning nozzle in the spinning process is, the larger the liquid output amount of the spinning nozzle in the spinning process is, and the larger the average diameter of the obtained nanofiber membrane is after further stretching and refining; furthermore, as the inner diameter of the spinneret and the mass fraction of hydrophobic polymer particles increase, the nanofiber diameter gradually increases while the corresponding water contact angle gradually decreases.

In the invention, the technological parameters adopted by electrostatic spinning have important influence on the appearance and the performance of the hydrophobic polymer nanofiber layer.

Preferably, during electrostatic spinning, the spinning speed of the spinning solution precursor is 0.04-0.12 mm/min, preferably 0.06-0.10 mm/min, and more preferably 0.08 mm/min;

the translation speed of the injection device is 300-600 mm/min, preferably 400-550 mm/min, and more preferably 500 mm/min;

the rotating speed of the roller is 100-300 rpm, preferably 150-250 rpm, and more preferably 200 rpm;

the voltage is 3-12 kV, preferably 6-10 kV, and more preferably 8 kV;

the distance between the needle point of the spinning nozzle and the receiving roller (the hydrophilic polymer substrate layer) is 10-20 cm, preferably 13-16 cm, and more preferably 15 cm;

controlling the electrostatic spinning temperature to be normal temperature, such as 25 ℃; the humidity is between 35 and 45 ℃, for example about 40 ℃.

According to the invention, the spinning solution precursor is extruded by a spinneret and then is stretched and refined under the action of a high-voltage electrostatic field to form a fiber bundle, and in the spinning process, the processes of solvent volatilization and solute solidification are carried out, so that a hydrophobic polymer nanofiber layer formed by hydrophobic polymer nanofibers is collected on one side of a hydrophilic polymer substrate layer.

In the present invention, the thickness of the hydrophobic polymer nanofiber layer is determined by the electrospinning time, and the longer the electrospinning time, the thicker the hydrophobic polymer nanofiber layer is formed.

The inventors found that, in addition to the spinning time which affects the thickness of the hydrophobic polymer nanofiber layer; more importantly, with the increase of the spinning time, the hydrostatic pressures of both sides of the prepared asymmetric wettability composite membrane tend to increase, mainly because the thickness of the hydrophobic polymer nanofiber layer gradually increases with the increase of the spinning time, the water blocking capacity is further increased, water drops are difficult to penetrate through the fabric, and therefore the maximum hydrostatic pressures of both sides of the membrane are increased.

Wherein, when the spinning time is 3-30 min, the water one-way permeability of the asymmetric wetting composite membrane is excellent.

Further preferably, the spinning time is 10-20 min;

still more preferably, the spinning time is 15 min.

According to the invention, in the asymmetric wettability composite membrane, the unidirectional water permeability from the hydrophobic polymer nanofiber layer to the hydrophilic polymer substrate layer is obviously higher than that in the opposite direction, and the asymmetric wettability composite membrane has excellent moisture permeability.

Wherein, the maximum water content difference between the hydrophobic polymer nanofiber layer and the hydrophilic polymer substrate layer reaches 500-750 percent, and can further reach 600-700 percent, such as 686.9 percent.

In the present invention, when water droplets are dropped on the hydrophobic polymer nanofiber layer (hydrophobic side), the hydrophobic side has a lower moisture content, and the hydrophilic polymer substrate layer (hydrophilic side) has a higher moisture content, because the water droplets are transported from the hydrophobic side to the hydrophilic side, resulting in a decrease in the moisture content on the hydrophobic side and an increase in the moisture content on the hydrophilic side. When water droplets are dropped on the hydrophilic side, the moisture content of the hydrophilic side increases and the moisture content of the hydrophobic side decreases, because the water droplets are difficult to transfer to the hydrophobic side because they spread inside the hydrophilic side.

According to the invention, when the electrostatic spinning time is 15min, the moisture permeability of the prepared asymmetric wettability composite membrane is optimal, and the maximum water content difference between the hydrophobic polymer nanofiber layer and the hydrophilic polymer substrate layer reaches 686.9%.

The inventor further researches and discovers that the prepared asymmetric wettability composite membrane is placed on a human arm and tested by an infrared camera, and the temperature of the composite membrane is higher than that of a single-layer hydrophilic polymer substrate layer when a hydrophobic polymer nanofiber layer faces downwards no matter on dry skin or wet skin, because the composite membrane with the asymmetric wettability structure has certain heat conduction performance; when the hydrophilic polymer substrate layer faces downwards, the temperature of the composite membrane is lower than that of the hydrophobic polymer nanofiber layer, because the hydrophobic polymer nanofiber layer facing upwards blocks the heat diffusion.

In a third aspect, the asymmetric wettability composite membrane provided by the invention is used for preparing textiles, and is particularly suitable for manufacturing protective clothing.

According to the invention, the hydrophilic polymer fabric is chemically modified by using an alkaline substance to obtain a porous hydrophilic polymer substrate layer, and a hydrophobic polymer nanofiber layer is electrically imitated on one side of the hydrophilic polymer substrate layer through electrostatic spinning to obtain the asymmetric wettability composite membrane, which has better capability of enabling liquid to be transported in a single direction and has better thermal conductivity than a common fabric. The adopted polymer, particularly polylactic acid, is an environment-friendly material which can be degraded by microorganisms and is pollution-free, can not only convey liquid in a single direction, but also is environment-friendly, and provides a development idea for the research work in the aspect of improving the heat and humidity comfort of the protective clothing.

The present invention is further described in detail below by way of examples.

Examples

Example 1

(1) A polylactic acid fabric (abbreviated as PLA fabric, which is composed of 50 wt% polylactic acid, 18 wt% acetic acid and 32 wt% tencel, manufactured by Puyang yurun new materials Co., Ltd.) was cut into a square of 8cm × 8cm, which was immersed in a NaOH solution having a concentration of 10g/L for 3 hours, and then the PLA fabric was dried in a ventilated place for 24 hours, to prepare a hydrophilic polymer substrate layer, i.e., NaOH @ PLA fabric.

(2) 1.5g of polylactic acid (PLA) particles (produced by Zhejiang Haizhizhen biomaterial Co., Ltd.) were weighed into a 25mL conical flask, and 12.285g of a mixed solvent (N, N-dimethylformamide: tetrahydrofuran ═ 7:3(w/w)) was added to obtain a mixed solution having a PLA particle concentration of 10 wt%, the conical flask was sealed well, and the mixture was stirred for 3 hours on a magnetic stirrer at a temperature of 40 ℃ and a rotation speed of 400rmp until the PLA particles were completely dissolved to obtain a PLA spinning solution precursor.

(3) Attaching the NaOH @ PLA fabric prepared in the step (1) to an aluminum foil with the size of 9cm multiplied by 16.5cm, then fixing the aluminum foil on a rolling shaft of electrostatic spinning equipment (model: ET-2033, manufacturer: Leye scientific and technological development Co., Ltd.), placing the PLA spinning solution precursor prepared in the step (2) into a 5mL disposable sterile injector, fixing the sterile injector on pushing equipment by adopting an 8G spinning nozzle, and setting the parameters of electrostatic spinning as follows: the spinning speed is 0.08mm/min, the translation speed of the injection equipment is 500mm/min, the rotating speed of the roller is 200rpm, the distance between the needle point of the spinning nozzle and the receiving roller is 15cm, and the voltage is 8 kV; and then connecting a pushing and injecting device and a high-voltage power supply, starting electrostatic spinning, stopping operation after 15min, obtaining an asymmetric wettability composite film with one side being a NaOH @ PLA fabric and one side being a PLA nanofiber layer, marking the composite film as a PLA-NaOH @ PLA composite film, and measuring the thickness of the PLA-NaOH @ PLA composite film by a film thickness measuring instrument to be 257.8 mu m.

FIG. 1- (a) shows the SEM representation of one side of the PLA nanofiber layer in the PLA-NaOH @ PLA composite film, and it can be found that the PLA nanofibers are uniformly distributed on the NaOH @ PLA fabric, and FIG. 1- (b) shows the section SEM representation picture of the PLA-NaOH @ PLA composite film, so that the asymmetric wettability composite film formed by tightly combining the PLA nanofiber layer and the NaOH @ PLA fabric can be observed after the PLA nanofibers are electrospun onto the NaOH @ PLA fabric.

Example 2

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the thickness of the PLA-NaOH @ PLA composite film was measured to be 256.6. mu.m using a 6G spinneret.

Example 3

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the thickness of the PLA-NaOH @ PLA composite film was 258.9 μm as measured using a 10G spinneret.

Example 4

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the thickness of the PLA-NaOH @ PLA composite film was 259.6 μm as measured using a 12G spinneret.

Example 5

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: weighing 1.5In a 25mL Erlenmeyer flask, 10.5g of a mixed solvent (N, N-dimethylformamide: tetrahydrofuran ═ 7:3(w/w)) was subsequently added to give a mixed solution having a PLA particle concentration of 12.5 wt%, and the thickness of the PLA-NaOH @ PLA composite film was measured to be 258.5. mu.m.

Example 6

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 5, except that: the thickness of the PLA-NaOH @ PLA composite film was 257.7 μm, which was measured using a 6G spinneret.

Example 7

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 5, except that: the thickness of the PLA-NaOH @ PLA composite film was 259.4 μm as measured using a 10G spinneret.

Example 8

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 5, except that: the thickness of the PLA-NaOH @ PLA composite film was 260.6 μm, as measured using a 12G spinneret.

Example 9

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: 1.5g of polylactic acid (PLA) particles were weighed into a 25mL Erlenmeyer flask, followed by addition of 28.5g of a mixed solvent (N, N-dimethylformamide: tetrahydrofuran ═ 7:3(w/w)) to give a mixed solution having a PLA particle concentration of 5 wt%, and the thickness of the PLA-NaOH @ PLA composite film was measured to be 256.3. mu.m.

Example 10

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 9, except that: the thickness of the PLA-NaOH @ PLA composite film was 254.2 μm as measured using a 6G spinneret.

Example 11

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 9, except that: the thickness of the PLA-NaOH @ PLA composite film was measured to be 256.8. mu.m using a 10G spinneret.

Example 12

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 9, except that: the thickness of the PLA-NaOH @ PLA composite film was 257.6 μm, which was measured using a 12G spinneret.

Example 13

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: 1.5g of polylactic acid (PLA) particles were weighed into a 25mL Erlenmeyer flask, followed by addition of 18.5g of a mixed solvent (N, N-dimethylformamide: tetrahydrofuran ═ 7:3(w/w)) to give a mixed solution having a PLA particle concentration of 7.5 wt%, and the thickness of the PLA-NaOH @ PLA composite film was measured to be 256.8. mu.m.

Example 14

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 13, except that: the thickness of the PLA-NaOH @ PLA composite film was 255.3 μm, as measured using a 6G spinneret.

Example 15

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 13, except that: the thickness of the PLA-NaOH @ PLA composite film was 257.9 μm, as measured using a 10G spinneret.

Example 16

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 13, except that: the thickness of the PLA-NaOH @ PLA composite film was 258.2 μm as measured using a 12G spinneret.

Example 17

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the electrostatic spinning time is 3min, and the thickness of the PLA-NaOH @ PLA composite film is 249.9 mu m.

Example 18

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the electrostatic spinning time is 5min, and the thickness of the PLA-NaOH @ PLA composite film is 253.0 mu m.

Example 19

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the electrostatic spinning time is 10min, and the thickness of the PLA-NaOH @ PLA composite film is 257.5 mu m.

Example 20

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the electrostatic spinning time is 20min, and the thickness of the PLA-NaOH @ PLA composite film is 258.1 mu m.

Example 21

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the electrostatic spinning time is 25min, and the thickness of the PLA-NaOH @ PLA composite film is 258.9 mu m.

Example 22

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the electrostatic spinning time is 30min, and the thickness of the PLA-NaOH @ PLA composite film is 262.1 mu m.

Comparative example

Comparative example 1

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the electrostatic spinning time is 2min, and the thickness of the PLA-NaOH @ PLA composite film is 249.2 mu m.

Comparative example 2

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the electrostatic spinning time is 33min, and the thickness of the PLA-NaOH @ PLA composite film is 263.0 mu m.

Comparative example 3

A PLA-NaOH @ PLA composite film was prepared in a similar manner as in example 1, except that: the electrostatic spinning time is 35min, and the thickness of the PLA-NaOH @ PLA composite film is 264.6 mu m.

Examples of the experiments

Experimental example 1

SEM characterization of untreated PLA fabric (produced by Puyang Yurun New Material Co., Ltd.), and NaOH @ PLA fabric produced in step (1) of example 1, using a voltage of 5kV and a current of 10 μ A, resulted in the results shown in FIG. 2, in which FIG. 2- (a) shows SEM characterization of the untreated PLA fabric and 2- (b) shows SEM characterization of the NaOH PLA @ fabric. It can be seen that the surface of the PLA fabric is relatively smooth, with no apparent defects; a large number of hole structures appear on the surface of the NaOH @ PLA fabric, and the hole structures can be found to be uniformly distributed on the surface of the fabric from a partially enlarged view, and the possible reason of the result is that the ester bonds of the PLA are subjected to free hydrolytic breakage under the action of alkali, so that the number average molecular weight of the PLA is slowly reduced, when the molecular weight is reduced to a certain degree, the PLA begins to dissolve to generate a soluble degradation product, and a large number of hole structures are formed on the surface of the PLA fabric, which indicates that the alkali treatment can realize the preparation of the super-hydrophilic NaOH @ PLA fabric.

Experimental example 2

The PLA-NaOH @ PLA composite films prepared in examples 1-16 were SEM-characterized and the results are shown in FIG. 3.

Wherein, the first row of FIG. 3- (a) shows SEM characterization pictures of PLA-NaOH @ PLA composite films prepared in example 10, example 14, example 2 and example 6 (i.e., using a 6G spinneret, an electrospinning time of 15min, and PLA particle concentrations of 5 wt%, 7.5 wt%, 10 wt% and 12.5 wt%, respectively);

the second line, FIG. 3- (b), shows SEM characterization pictures of PLA-NaOH @ PLA composite films prepared in example 9, example 13, example 1 and example 5 (i.e., using an 8G spinneret, an electrospinning time of 15min, and PLA particle concentrations of 5 wt%, 7.5 wt%, 10 wt% and 12.5 wt%, respectively);

the third line of FIG. 3- (c) shows SEM images of PLA-NaOH @ PLA composite films prepared in example 11, example 15, example 3 and example 7 (i.e., PLA particle concentrations of 5 wt%, 7.5 wt%, 10 wt% and 12.5 wt%, respectively, using a 10G spinneret and an electrospinning time of 15 min);

the fourth row of FIG. 3- (d) shows SEM images of PLA-NaOH @ PLA composite films prepared in example 12, example 16, example 4 and example 8 (i.e., PLA particle concentrations of 5 wt%, 7.5 wt%, 10 wt% and 12.5 wt%, respectively, using a 12G spinneret and an electrospinning time of 15 min);

it can be seen that when the concentration of the PLA particles is 5 wt% and 7.5 wt%, a large amount of "bead-knot" structures exist in the obtained PLA-NaOH @ PLA composite film because when the concentration of the PLA particles is low, a part of the PLA is stretched and refined into fibers under the action of electrostatic force, and another part of the PLA is rapidly retracted to form liquid droplets under the action of surface tension, resulting in the "bead-knot" structures; when the dope concentration was increased to 10 wt% and 12.5 wt%, the dope gave a uniform fiber structure under the traction of electrostatic force.

Experimental example 3

The PLA-NaOH @ PLA composite films prepared in examples 1 to 8 were subjected to SEM characterization and particle size measurement, and the results are shown in FIG. 4. It can be seen from the figure that as the concentration of PLA particles increases, the diameter of the PLA nanofibers increases; at the same time, as the inner diameter of the spinneret increases, the diameter of the resulting PLA nanofibers also increases. The reasons for this may be: the PLA spinning solution precursor with high concentration has high viscosity, the entanglement among molecular chains is large, when the size and the voltage of a spinning nozzle are the same, the solution with high concentration is difficult to stretch in the spinning process, and the diameter of the fiber is large; meanwhile, the larger the inner diameter of the spinning nozzle in the spinning process is, the larger the liquid output amount of the spinning nozzle in the spinning process is, and the larger the average diameter of the obtained nanofiber membrane is after further stretching and refining.

Experimental example 4

Experimental example 4.1

A PLA spinning solution precursor was prepared in the same manner as in step (2) of example 1, and a PLA nanofiber layer was prepared by electrospinning in a similar manner to step (3), except that: aluminum foil with the size of 9cm multiplied by 16.5cm is directly fixed on a roller of electrostatic spinning equipment, and electrostatic spinning operation is carried out by using a PLA spinning solution precursor.

A water contact angle test was performed on the NaOH @ PLA fabric (prepared in step (1) of example 1) and the PLA nanofiber layer described above using a contact angle measuring instrument. Before testing, the two samples are adhered to a glass slide by double-sided adhesive to ensure the flatness of the samples. During testing, the sample is placed on a workbench, after the position is adjusted, 5 μ L of deionized water is respectively dropped on the surfaces of the two samples through an injector, when the water is dropped on the sample for 3s, the state of the water drop on the surface of the material is recorded by taking a picture or recording a video through a camera on a contact angle measuring instrument, and meanwhile, the contact angle of the material and the water is tested by using software in the contact angle measuring instrument, and the obtained result is shown in fig. 5. Wherein, 5- (a) shows that the contact angle of the PLA nanofiber membrane and water is 127.1 degrees, and 5- (b) shows that the contact angle of the NaOH @ PLA fabric and water is 0 degrees, which proves the successful preparation of the asymmetric wettability PLA-NaOH @ PLA composite membrane.

Experimental example 4.2

The PLA-NaOH @ PLA composite films prepared in examples 1 to 8 were subjected to a water contact angle test in the same manner as in example 4.1, and the results are shown in table 1:

TABLE 1

Sample (I)	Experimental example 1	Example 2	Example 3	Example 4
					Contact angle	129.1°	131.9°	121.6°	117.6°
Sample (I)	Experimental example 5	Experimental example 6	Experimental example 7	Experimental example 8
					Contact angle	118.9°	125.0°	113.9°	115.9°

Combining the results of experimental example 3 and table 1, it is evident that: as the inner diameter of the spinneret and the mass fraction of PLA increase, the nanofiber diameter gradually increases while the corresponding water contact angle gradually decreases. This is because the diameter of the nanofiber increases with the increase in the inner diameter of the spinneret, the surface roughness of the nanofiber layer formed decreases, and the water contact angle decreases; similarly, as the mass fraction of the PLA particles increases, the diameter of the nanofibers also increases, so that the water contact angle is larger when the mass fraction of the PLA particles is smaller for the same spinneret size.

Experimental example 5

Fig. 6 shows the ir spectra of PLA particles, PLA nanofiber layer made in experimental example 4.1, PLA fabric and NaOH @ PLA fabric sample made in step (1) of example 1.

It can be found that: the two spectra of PLA granules and PLA nanofiber layer are at 3502cm^-1Weak O-H stretching peaks appear at the positions, which is an important reason for the hydrophobicity of the PLA nano-fibers; the untreated PLA fabric is compounded at 3562-3060 cm due to the fact that the untreated PLA fabric contains part of acetic acid^-1An O-H stretching vibration absorption peak of free carboxylic acid is formed, so that the untreated PLA fabric is in a hydrophilic state; the NaOH @ PLA fabric is 3000-3600 cm^-1A broad peak is formed, and is a stretching vibration peak of-OH, which is the main reason for causing the PLA fabric to be hydrophilic. It is possible that the NaOH @ PLA fabric exhibits a superhydrophilic state because it undergoes hydrolysis under the action of a base, exposing a large amount of-OH and-COOH. The results also show that the asymmetric wettability membrane material is successfully prepared.

Experimental example 6

The PLA fabric and the NaOH @ PLA fabric obtained in the step (1) of example 1 were analyzed for the change in the element types and contents by EDS, and the results are shown in FIG. 7. It can be observed that the surface of the untreated PLA fabric (fig. 7- (a)) contains only two elements, C and O. While the surface C content for the NaOH @ PLA fabric after the alkali impregnation treatment (FIG. 7- (b)) decreased from 50.2 wt% to 44.45 wt%, the O content increased from 49.98 wt% to 53.58 wt%, while Na element also appeared, and the Na element content was 1.97 wt%. This result again verifies that the NaOH solution reacts with the PLA fabric to hydrolyze, exposing a large amount of — OH, increasing the content of O element, while Na element that has not reacted completely remains on the fabric surface.

Experimental example 7

The PLA-NaOH @ PLA composite films prepared in examples 1 and 18-21 were placed between two glass tubes with flanges, respectively, and after the hydrostatic pressure device was fixed with a clamp, deionized water was added dropwise from the glass tube to the sample by a syringe pump, and when a water drop could pass through the sample, the distance between the liquid level and the sample was recorded, which was the hydrostatic pressure of the sample. The results are shown in FIG. 8- (a), and it can be seen that: with the increase of the spinning time, the hydrostatic pressure on two sides of the PLA-NaOH @ PLA composite film tends to increase, mainly because the thickness of the hydrophobic PLA nanofiber layer gradually increases with the extension of the spinning time, the water blocking capacity is further increased, water drops are difficult to penetrate through the fabric, and therefore the maximum hydrostatic pressure on two sides of the film increases. It was also found that the PLA-NaOH @ PLA composite film showed the best water one-way permeability when the spinning time was 15min (corresponding to example 1).

Further, when the PLA-NaOH @ PLA composite film prepared in example 5 was subjected to the above-mentioned hydrostatic pressure test, the hydrostatic pressure of the PLA-NaOH @ PLA composite film prepared in example 1 was compared with that of the PLA-NaOH @ PLA composite film prepared in example 5 as shown in FIG. 8- (b), and it can be seen that the hydrostatic pressure of the PLA/NaOH PLA @ composite film prepared in example 1 was greater than that of the PLA/NaOH @ PLA composite film prepared in example 5 when the water drops were from the hydrophilic side to the hydrophobic side, because the PLA nanofibers in the PLA/NaOH @ PLA composite film prepared in example 1 were more uniformly distributed.

Experimental example 8

The asymmetric wettability composite membrane has the ability to allow unidirectional transport of liquid, i.e., liquid droplets can be transported from the hydrophobic side to the hydrophilic side, but cannot be transported from the hydrophilic side to the hydrophobic side. The water drop one-way permeation experiment was performed as follows:

the PLA-NaOH @ PLA composite films prepared in examples 1 and 17 to 22 and comparative examples 1 to 3 were fixed to the sample by using a test tube holder, and water drops were dropped onto the hydrophobic side (PLA nanofiber layer) and the hydrophilic side (NaOH @ PLA fabric) of the sample by a syringe pump, respectively, and the state of the water drops permeating was observed, and the results are shown in fig. 9.

It can be seen that when the thickness of the hydrophobic side of the PLA/NaOH @ PLA composite film is between 249.9 and 262.1 micrometers (the spinning time is between 3 and 30 minutes), water drops can only permeate from the hydrophobic side to the hydrophilic side, but cannot permeate from the hydrophilic side to the hydrophobic side; when the thickness of the fiber film reaches 263 μm (comparative examples 2-3, the spinning time is 32min and 35min respectively), the unidirectional water drop permeability of the composite film can not be achieved.

Further, the PLA-NaOH @ PLA composite films obtained in examples 1 and 5 were subjected to a water transport rate test. The test method comprises the following steps: dropping water drops on two sides of the composite membrane respectively, and observing the permeation time and the reverse spreading state of the water drops from the hydrophobic side to the hydrophilic side, wherein the result is shown in figure 10, the first line of 10- (a) shows the existence form diagram of the water drops on the hydrophobic side of the PLA-NaOH @ PLA composite membrane prepared in example 1 along with the change of time, and the second line shows the existence form diagram of the water drops on the hydrophobic side of the PLA-NaOH @ PLA composite membrane prepared in example 5 along with the change of time; 10- (b) the first row shows the existing form of water drop on the hydrophilic side of the PLA-NaOH @ PLA composite film prepared in example 1 as a function of time, and the second row shows the existing form of water drop on the hydrophilic side of the PLA-NaOH @ PLA composite film prepared in example 5 as a function of time. It can be seen that: the PLA-NaOH @ PLA composite film prepared in example 1 has a time of 162s for the water drop to completely penetrate and drip when the water drop is dripped on the hydrophobic side, which is lower than that of the PLA/12.5-NaOH @ PLA composite film prepared in example 5 (171 s); when a water droplet was dropped on the hydrophilic side of both samples, the spreading area of the water droplet was not large, because the super-hydrophilicity of the hydrophilic side allowed the droplet to spread out quickly.

The combination of SEM characterization in experimental example 2, particle size testing in experimental example 3, hydrostatic pressure testing in experimental example 7, and the photograph of the unidirectional water drop permeability shown in fig. 10 in this experiment can be seen: when the concentration of PLA particles is 10 wt%, the particle size of PLA nanofibers in the prepared PLA-NaOH @ PLA composite film is more uniform, and the PLA-NaOH @ PLA composite film has more excellent water drop one-way transmittance.

Experimental example 9

(1) The PLA fabric, the PLA-NaOH @ PLA composite film prepared in examples 1-3 and 5-7 were cut into a square of 6cm × 6cm, and the square was covered with a rubber band at a 10mL beaker containing deionized water, with the hydrophobic side (PLA nanofiber layer) of the composite film facing downward and the hydrophilic side (NaOH @ PLA fabric) facing upward; then, the small beaker covered with the sample was placed in an oven at 25 ℃ for 1 hour, the sample was taken out and weighed, and the water vapor transmission rate (WVT/kg. m) was recorded^-2·h^-1). Three replicates of each sample were run and the final results were the average of three runs. Wherein the calculation formula of the water vapor transmission rate is as follows:

in the formula: m is₁、m₂Denotes the total mass of the sample covering the beaker before and after evaporation, i.e. m₁-m₂In order to change the quality of the deionized water in the beaker before and after evaporation,

s is the effective area of vapor passing through the PLA-NaOH @ PLA composite membrane,

and t is the test duration.

(2) The water vapor transmission rate was measured in a similar manner to the above step (1), except that: placing the small beaker covered with the sample in an oven at 40 ℃;

(3) the water vapor transmission rate was measured in a similar manner to the above step (1) except that: placing the small beaker covered with the sample in an oven at 60 ℃;

the above results are shown in fig. 11, and it can be seen that: the PLA-NaOH @ PLA composite film has certain capacity of enabling water vapor to penetrate, and with the increase of water temperature, the volatilization rate of water is increased, and the water vapor transmission rate is increased; in addition, when the temperature is 60 ℃, the water vapor transmission rate is higher, because when the diameter of the spinning head is too small, the aperture of the fiber is smaller, the air permeability is poor, the transmission of water vapor is prevented, and the water vapor transmission rate is lower; when the diameter of the spinning head is too large, the pore diameter of the fiber is larger, and the hydrophobic capacity of the fiber membrane is weaker, so that the air permeability is poorer. Therefore, at the same temperature, when the type of the spinning nozzle is 8G, the water vapor transmission rate of the prepared PLA-NaOH @ PLA composite film is the best, namely when the inner diameter of the spinning nozzle is 0.8mm, the water vapor transmission rate of the prepared PLA-NaOH @ PLA composite film is the best, and meanwhile, the water vapor transmission rate of the PLA/NaOH @ PLA composite film prepared in examples 1-3 is generally better than that of the PLA/NaOH @ PLA composite film prepared in examples 5-7, which shows that when the concentration of PLA particles is 10 wt% and the inner diameter of the spinning nozzle is 0.8mm, the prepared PLA-NaOH @ PLA composite film (corresponding to example 1) has better performance.

Experimental example 10

The PLA-NaOH @ PLA composite films prepared in examples 1, 5, 18, 21 and 3 were tested using a liquid water management apparatus to quantitatively analyze the water transport properties of the PLA-NaOH @ PLA composite films. Prior to testing, all samples were placed in a constant temperature and humidity laboratory at 20. + -. 1 ℃ and 65%. + -. 2% relative humidity for 24 h. The samples were then cut into 8cm x 8cm squares, respectively, and placed horizontally between two concentric sensors, one above the other. When the test is started, the instrument transmits the physiological saline in the center of the upper sensor, the time from the beginning of transmission to the time that water drops are diffused in the composite membrane for 120s is maintained, and the instrument records the change of the saline content on the upper side and the lower side of the composite membrane along with the time and the one-way liquid transportation capacity of the composite membrane according to the resistance change of the upper sensor and the lower sensor. In the testing process, 10 groups of parallel experiments are carried out on each sample, wherein 5 groups of samples are upward in hydrophobic side, the other 5 groups of samples are upward in hydrophilic side, the maximum water content of the two sides of the samples is recorded, and the average value of the maximum water content of the 5 groups is taken to evaluate the one-way liquid conveying capacity of the samples.

FIG. 12- (a) is a graph showing the upper maximum water content of water droplets dropped on both sides of the PLA-NaOH @ PLA composite films obtained in example 1, example 18, example 21 and comparative example 3, respectively. When the hydrophobic side is on the upper layer (water drops on the hydrophobic side), it can be found that the maximum water content of the upper layer is reduced and then increased along with the increase of the spinning time, because the composite membrane has the capability of conveying water in one direction, so that the water can be conveyed from the hydrophobic side to the hydrophilic side, but when the spinning time is increased, the number and the size of the holes in the PLA nanofiber layer are reduced, and the single-item conveyance of the water is not easy; when the hydrophilic side is on the upper layer (water drops on the hydrophilic side), the maximum water content of the upper layer tends to be stable after increasing, because the water drops diffuse inside the hydrophilic side and do not transfer to the hydrophobic side; meanwhile, the spinning time corresponding to the peak value of the maximum water content of the composite membrane is 15min, and the unidirectional permeability is proved to be good when the spinning time is 15 min.

FIG. 12- (b) is a graph showing the comparison of the maximum water content of the upper side of the PLA-NaOH @ PLA composite films obtained in examples 1 and 5, wherein the maximum water content of the upper layer of the PLA/10-NaOH @ PLA composite film is 266.1% and the maximum water content of the lower layer is 953.0%; the maximum water content of the upper layer of the PLA/12.5-NaOH @ PLA composite membrane is 912.5%, the maximum water content of the lower layer of the PLA/12.5-NaOH @ PLA composite membrane is 1179.3%, the difference of the maximum water content of the two sides of the PLA-NaOH @ PLA composite membrane prepared in the example 1 is the largest, and the moisture permeability is better.

Experimental example 11

The NaOH @ PLA fabric prepared in step (1) of example 1 and the PLA-NaOH @ PLA composite film prepared in example 1 were cut into squares of 4cm × 4cm, respectively, and placed on dry and wet skin (for the composite film, group 1 hydrophobic side down, group 2 hydrophilic side down) respectively, to compare the thermal conductivity of the samples on human arms. Meanwhile, the infrared camera is used for recording the change of different wetting states of the sample on the arm of the human body along with the time, and the change of the heat conduction capability of the NaOH @ PLA and the PLA-NaOH @ PLA composite film along with the time is explored.

FIG. 13- (a) shows an infrared thermal image of PLA fabric and the PLA/NaOH @ PLA composite film prepared in example 1 on human skin, demonstrating the conductive effect on heat; FIG. 13- (b) shows the IR thermography of the single-layer PLA fabric of Experimental example 11 and the PLA/NaOH @ PLA composite film prepared in example 5 on human skin, demonstrating the conductive effect on heat, as can be seen from the combination of (a) and (b): when the PLA-NaOH @ PLA composite film is placed with the hydrophobic side down, the temperature of the composite film is higher than that of a single-layer fabric, whether on dry skin or wet skin; when the hydrophilic side of the composite membrane faces downwards, the temperature of the composite membrane is lower than that of the single-layer fabric, but the temperature of the PLA-NaOH @ PLA composite membrane prepared in example 1 is higher when the hydrophobic side of the composite membrane faces downwards than that of the PLA-NaOH @ PLA composite membrane prepared in example 5; the temperature of the PLA-NaOH @ PLA composite membrane prepared in example 1 was lower when the hydrophilic side of the membrane was facing downward than when the PLA-NaOH @ PLA composite membrane prepared in example 5 was facing downward when the hydrophobic side of the membrane was facing downward.

The infrared imaging experiment shows that the heat conductivity of the PLA-NaOH @ PLA composite film is better than that of a single-layer NaOH @ PLA fabric, and the heat conductivity of the PLA/NaOH @ PLA composite film prepared in example 1 is better than that of the PLA/NaOH @ PLA composite film prepared in example 5.

The invention has been described in detail with reference to the preferred embodiments and illustrative examples. It should be noted, however, that these specific embodiments are only illustrative of the present invention and do not limit the scope of the present invention in any way. Various modifications, equivalent substitutions and alterations can be made to the technical content and embodiments of the present invention without departing from the spirit and scope of the present invention, and these are within the scope of the present invention. The scope of the invention is defined by the appended claims.

Claims

1. The asymmetric wettability composite membrane is characterized by comprising a hydrophilic polymer substrate layer and a hydrophobic polymer nanofiber layer arranged on a hydrophilic polymer fabric.

2. The composite film according to claim 1, wherein the asymmetric wettability composite film has a thickness of 230 to 270 μm, preferably 240 to 265 μm; the diameter of the nanofiber in the hydrophobic polymer nanofiber layer is 400-1000 nm, and preferably 450-930 nm.

3. A composite membrane according to claim 1 or 2, wherein the hydrophilic polymer substrate layer has a contact angle with water of less than 5 °, preferably 0-2 °, and the hydrophobic polymer nanofiber layer has a contact angle with water of 115 ° to 140 °, preferably 120 ° to 130 °.

4. The composite membrane according to claim 3, wherein the difference between the maximum water content of the hydrophobic polymer nanofiber layer and the maximum water content of the hydrophilic polymer substrate layer is 500-750%, preferably 600-700%.

5. The composite film according to claim 1,

the hydrophilic polymer substrate layer is obtained by alkali treatment of hydrophilic polymer fabric;

the hydrophilic polymer fabric is selected from one or more of polysulfone fabric, polyamide fabric, polyamic acid fabric, polyimide fabric, polyurethane fabric, polyacrylic acid fabric, polylactic acid fabric, polyethylene oxide fabric and polyvinylpyrrolidone fabric;

the hydrophobic polymer nanofiber layer is any one or more of a fluoropolymer nanofiber layer, a polyethylene glycol terephthalate nanofiber layer, a polymethyl methacrylate nanofiber layer, a polyacrylonitrile nanofiber layer, a polycarbonate nanofiber layer, a polyvinyl acetate nanofiber layer and a polylactic acid nanofiber layer.

6. The composite membrane of claim 1, wherein the composite membrane is obtained by electrospinning a hydrophobic polymer nanofiber layer onto one side of a hydrophilic polymer substrate layer.

7. A method of preparing an asymmetric wettability composite membrane, said method comprising:

step 1, preparing a hydrophilic polymer substrate layer;

step 2, preparing a hydrophobic polymer spinning solution precursor;

8. The method according to claim 7, characterized in that in step 1 the hydrophilic polymer fabric is alkali treated to obtain a hydrophilic polymer substrate layer.

9. The method of claim 7, wherein in step 2, hydrophobic polymer particles are added to a solvent and mixed to prepare a hydrophobic polymer dope precursor;

preferably, the solvent is selected from one or two of ketone solvents such as acetone and butanone, ether solvents such as tetrahydrofuran, amide solvents such as N, N-dimethylformamide and N, N-dimethylacetamide, and sulfone solvents such as dimethyl sulfoxide.

10. Use of the asymmetric composite wettability membrane according to one of claims 1 to 6 or obtained by the process according to one of claims 7 to 9 for the preparation of textiles, particularly suitable for the manufacture of protective clothing.