Electrospun polylactic acid / polyaniline conductive fiber membrane, its preparation method and flexible pressure sensor

Abstract

The present invention discloses an electrospun poly(lactic acid) / polyaniline conductive fiber membrane, a preparation method thereof, and a flexible pressure sensor. The method includes: preparing a poly(lactic acid) electrospun fiber membrane with controllable morphology by using electrospinning technology; performing surface modification on the poly(lactic acid) fiber membrane by using ultraviolet-ozone or oxygen plasma, and a surface modifier; soaking the modified poly(lactic acid) fiber membrane in an acidic solution of aniline monomer and an oxidant for in-situ polymerization growth of polyaniline; and cleaning and drying the obtained poly(lactic acid) / polyaniline conductive fiber membrane. The process of the present invention is simple, the reaction conditions are mild, and it is easy to operate and control, enabling polyaniline to grow in-situ uniformly on the surface of poly(lactic acid) fibers. The composite fiber conductive membrane prepared thereby has good pressure sensing recovery and stability, ensuring good stability and use value when the composite conductive fiber thin film is applied to fields such as wearable flexible pressure sensing.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of composite conductive fibers, and particularly relates to an electrospun polylactic acid / polyaniline conductive fiber membrane for a flexible pressure sensor and a preparation method thereof, which can be used for a flexible pressure sensor. Background Art

[0002] In the past few decades, conductive polymers have opened up new horizons for polymer applications. In 2000, Alan J. Heeger, Alan G. MacDiarmid, and Hideki Shirakawa won the Nobel Prize for the invention of conductive polymers. Polyaniline (PANI) has attracted extensive attention due to its easy synthesis, low cost, and excellent environmental stability. Due to its easy realization of reversible doping-dedoping processes, polyaniline has been applied to sensors and electrode materials in the fields of electricity and optics, etc., and occupies an important position. In addition, with excellent and adjustable bioelectroactivity and biocompatibility with animal cells, polyaniline has been applied to the regeneration of bone cells in tissue engineering (Jing Chen. Journal of Colloid and Interface Science. 2018, 514: 517 - 527) and the cultivation of fibroblasts (Marija Gidavic-Nikolaidis. Polymer Chemistry. 2011, 49: 4902 - 4910), etc. Polylactic acid is a new type of biodegradable material with excellent biocompatibility and mechanical properties, and its electrospun fiber materials are applied to the preparation of biological scaffolds (Xifeng Liu. RSC Advances. 2015, 5: 100824 - 100833; Anna Magiera. Journal of Nanomaterials. 2017, 9246802). On the other hand, thanks to the large specific surface area of electrospun fibers and a large number of fiber-fiber contact sites, in-situ growth of polyaniline on the surface of electrospun polylactic acid fibers is expected to prepare a high-performance flexible pressure sensor with biocompatibility, which can be used for monitoring local physiological sign signals such as intracranial pressure in the body. However, when directly growing polyaniline in-situ on the surface of polylactic acid fibers, only hydrogen bonds are formed between the two, the binding is weak, it is easy to delaminate, and the morphology of the polyaniline conductive layer is not uniform, resulting in poor stability, short lifespan of the final composite conductive fiber membrane, and poor consistency between the prepared devices. If this problem can be solved, it will have great significance for the industrial application of this composite conductive fiber membrane in the fields of biomedicine, etc. Summary of the Invention

[0003] The present invention aims to solve the problem of uneven in-situ growth of polyaniline on the surface of electrospun fiber membranes, and provides a method for preparing an electrospun poly(lactic acid) / polyaniline composite conductive fiber membrane based on in-situ chemical growth, which can improve the controllability and consistency of the preparation of the composite conductive fiber membrane, and further improve the recovery and stability when used in flexible pressure sensors.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] A method for preparing an electrospun poly(lactic acid) / polyaniline conductive fiber membrane, comprising the following steps:

[0006] (1) Preparation of poly(lactic acid) electrospinning solution

[0007] Dissolve poly(lactic acid) in solvent I and stir to dissolve to prepare an electrospinning solution;

[0008] Preferably, the concentration of the spinning solution can usually be 20 wt%.

[0009] The solvent I can be at least one of dichloromethane (CH 2 Cl 2 ), chloroform (CHCl 3 ), and N,N-dimethylformamide (DMF).

[0010] (2) Preparation of poly(lactic acid) electrospun fiber membrane

[0011] Electrospin the electrospinning solution, collect the electrospun fibers, and after removing the fiber membrane, dry it;

[0012] Preferably, the voltage applied for electrospinning is 12 - 18 kV, the flow rate of the spinning solution jet is 3.6 mL / h, the receiving distance is 12 - 18 cm, the ambient temperature is 10 - 30 °C, and the relative humidity is 20 - 80%.

[0013] (3) Surface modification of poly(lactic acid) electrospun fiber membrane

[0014] Treat the dried poly(lactic acid) electrospun membrane with ultraviolet-ozone or oxygen plasma, and soak the treated membrane in an ethanol solution containing an amino-functional silane coupling agent for 2 hours;

[0015] The amino-functional silane coupling agent includes at least one of 3-aminopropyltriethoxysilane (APTES), 3-aminopropyltrimethoxysilane (APS), etc.

[0016] (4) Preparation of aniline monomer solution and oxidant solution

[0017] Dissolve aniline monomers and doping acid in water to prepare an acidic aniline solution with a certain concentration. Preferably, the concentration of aniline monomers is 0.07 - 0.33 mol / L, and the concentration of doping acid is 0 - 1 mol / L; Weigh a certain mass of oxidant and dissolve it in water. The concentration of the oxidant solution is the same as that of the aniline monomer solution;

[0018] The doping acid is at least one of hydrochloric acid, sulfuric acid, nitric acid, dodecylbenzenesulfonic acid, citric acid, and malic acid; The oxidant is at least one of ammonium persulfate and ferric chloride.

[0019] (5) In-situ polymerization growth of polyaniline on the surface of polylactic acid fibers

[0020] After washing the surface-modified polylactic acid fiber membrane with deionized water, immerse it in the acidic aniline monomer solution and store it in a low-temperature environment. At the same time, store the oxidant solution in a low-temperature environment. Subsequently, mix equal volumes of the oxidant and the aniline monomer solution soaked with the polylactic acid fiber membrane, stir evenly, and store it in a low-temperature environment. Take out the fiber membrane, wash it, and dry it to obtain an electrospun polylactic acid / polyaniline conductive fiber membrane. The resistivity range of the composite conductive fiber membrane is 20 - 130 kΩ·cm.

[0021] The low-temperature environment is usually 0 - 6°C. By carrying out a slow reaction in a low-temperature environment, it is possible to avoid excessive reaction and the generation of a large amount of free impurities that affect the product performance.

[0022] Advantages of the present invention:

[0023] The present invention uses a chemical in-situ polymerization method to grow a polyaniline conductive layer on the surface of polylactic acid electrospun fibers. Through the surface modification of polylactic acid electrospun fibers by ultraviolet ozone treatment and silane coupling agent, the amino group at the end of the silane coupling agent forms a chemical graft bonding with polyaniline, solving the problems of uncontrollable morphology and poor uniformity during the traditional in-situ polymerization growth of the polyaniline conductive layer: After ultraviolet ozone or oxygen plasma treatment, oxygen-containing groups such as hydroxyl groups will be formed on the surface of the polylactic acid fibers, and dehydration bonding will occur with the hydroxyl groups formed after the hydrolysis of the silane coupling agent to form an oxygen-silicon-oxygen chemical bonding network. At the same time, the amino group at the end of the silane coupling agent will form a chemical bond with aniline to achieve the graft growth of polyaniline. In this growth mode, polyaniline will no longer form structures such as tubes and rods, but form a continuous and uniform shell layer on the surface of the polylactic acid fibers, and there is a strong bond between the polyaniline and the polylactic acid fibers, improving the reliability of the interface and being beneficial to improving the stability of the device during mechanical sensing. Generally speaking, the present invention realizes the controllable preparation of the polylactic acid / polyaniline composite conductive fiber membrane and has great potential as a flexible pressure sensing material with high sensitivity and high stability. Description of the Drawings

[0024] Figure 1 This is the flow chart for the preparation of the polylactic acid / polyaniline conductive fiber membrane of the present invention;

[0025] Figure 2 It is a schematic diagram of the in-situ polymerization growth of polyaniline on the surface of polylactic acid fibers after surface modification;

[0026] Figure 3 They are SEM photos of the in-situ polymerization growth of polyaniline on the surface of polylactic acid fibers without surface modification at different aniline monomer concentrations;

[0027] Figure 4 They are SEM photos of the in-situ polymerization growth of polyaniline on the surfaces of polylactic acid fibers with and without surface modification;

[0028] Figure 5 They are SEM photos and resistivity values of the in-situ polymerization growth of polyaniline on the surface of polylactic acid fibers after surface modification at different aniline monomer concentrations;

[0029] Figure 6 They are SEM photos and resistivity values of the prepared polylactic acid / polyaniline composite conductive fiber membranes at the same aniline monomer concentration and different doping acid concentrations;

[0030] Figure 7 They are SEM images of the polylactic acid / polyaniline composite conductive fiber membranes prepared by different surface treatment methods;

[0031] Figure 8 They are the current-pressure curves (a - e) of the prepared polylactic acid / polyaniline composite conductive fiber membranes, and the sensitivity (f) calculated after linear fitting. Specific embodiments

[0032] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments:

[0033] In the following examples, the polylactic acid electrospun fiber membrane is prepared by the following method:

[0034] Dissolve polylactic acid in Solvent 1 and stir to dissolve it to prepare an electrospinning solution; Solvent 1 can usually be dichloromethane (CH 2 Cl 2 ) or chloroform (CHCl 3) and at least one of solutions such as N,N-dimethylformamide (DMF). The mass concentration of the spinning solution can usually be 20%. After spinning is completed, the fiber membrane is taken off and dried in a vacuum drying oven; the parameters can be as follows: the voltage is 12 - 18 kV, the flow rate of the spinning solution ejected from the needle tip is 3.6 mL / h, the distance between the needle tip and the roller receiver is 12 - 18 cm, the ambient temperature is 10 - 30 °C, and the relative humidity is 20 - 80%. In the step of in-situ growth of polyaniline on the fiber surface, it is preferred to immerse the fiber membrane in an acidic aniline solution and store it in a low-temperature environment of 0 - 6 °C for at least 1 hour, and the oxidant solution is also stored in a low-temperature environment of 0 - 6 °C for at least 1 hour. After the two are mixed, they are stored in a low-temperature environment of 0 - 6 °C for at least 12 hours to allow them to react fully.

[0035] According to a specific example of the present invention, the dried polylactic acid electrospun membrane can be treated with ultraviolet-ozone or oxygen plasma, and the treated membrane is then immersed in an ethanol solution of a surface modifier; thereby realizing surface modification of the fiber membrane. The test method for the resistivity of the composite conductive fiber membrane prepared by the present invention is: using a four-probe resistivity tester to measure the resistivity of the thin film. The resistivity range of the electrospun polylactic acid / polyaniline conductive fiber membrane is 20 - 130 kΩ·cm.

[0036] The pressure sensing test method for the composite conductive fiber membrane is:

[0037] After cutting the conductive fiber membrane into 1 cm × 1 cm square pieces, place it on the etched ITO glass (an insulating area with a width of 1 mm is etched on the complete ITO coating, and the two sides of the insulating area are respectively connected to the positive and negative electrodes of the digital source meter). Apply pressure to the conductive fiber membrane using a push-pull force gauge, and use the digital source meter to read the resistance value of the conductive fiber membrane under this pressure. By establishing a resistance-pressure curve and using linear fitting to calculate the slope of the resistance-pressure curve, the sensitivity of the composite conductive fiber membrane as a pressure sensing material can be obtained.

[0038] Example 1

[0039] Comparison with the concentration of aniline monomer as a variable without surface modification

[0040] The preparation method of the polylactic acid / polyaniline composite conductive fiber membrane described above includes the following steps

[0041] (1) Measure 10 mL of ethanol, add 1 mL of aminopropyltriethoxysilane, and stir evenly;

[0042] (2) Prepare three aniline monomer solutions with different concentrations: Weigh citric acid monohydrate and add it to deionized water, stir to dissolve, with concentrations of 0.017, 0.033, and 0.05 mol / L respectively. Then add aniline monomer and stir to dissolve, with concentrations of 0.07, 0.14, and 0.2 mol / L respectively. Additionally, prepare ammonium persulfate solutions with concentrations of 0.07, 0.14, and 0.2 mol / L;

[0043] (3) Take three identical polylactic acid electrospun fiber membranes and immerse them respectively in the aniline monomer solutions (10 mL) prepared in (2), and place them in the refrigerator for 2 hours; Place the ammonium persulfate solution in (2) in the refrigerator for 2 hours;

[0044] (4) Take 10 mL of each refrigerated ammonium persulfate solution, mix them respectively with the aniline monomer solutions with different concentrations in which the polylactic acid fiber membranes are immersed, and place them in the refrigerator for 12 hours to enable in-situ polymerization and growth of polyaniline on the surface of the polylactic acid fibers;

[0045] (5) Take out the polylactic acid / polyaniline composite conductive fiber membranes, wash them with deionized water and then dry them in a vacuum drying oven;

[0046] Cut small pieces from the prepared samples, paste them on carbon adhesive tapes, sputter gold and then place them in a field emission scanning electron microscope to observe the morphology of the in-situ grown polyaniline conductive layer on the surface of the polylactic acid fibers. As Figure 3 shown, with the increase in the concentration of aniline monomer, the polyaniline layer grown on the surface of the unmodified polylactic acid fibers thickens, and there are many free polyaniline nanofibers, and the uniformity of the polyaniline layer is very poor. Conduct pressure sensing tests on the prepared composite conductive fiber membranes (citric acid concentration is 0.05 mol / L, aniline concentration is 0.2 mol / L). As Figure 8 shown in (a) of it, the sensor based on this fiber membrane shows a large signal drift, and the signal stabilizes slowly. It still cannot stabilize after more than 30 s after loading. In addition, the loading-unloading recovery is also poor.

[0047] Example 2

[0048] Comparison of surface modification of polylactic acid fiber membranes with and without aminopropyltriethoxysilane

[0049] The preparation method of the polylactic acid / polyaniline composite conductive fiber membrane described above includes the following steps:

[0050] (1) Measure 10 mL of ethanol, add 1 mL of aminopropyltriethoxysilane, and stir evenly;

[0051] (2) Take two identical electrospun poly(lactic acid) fiber membranes. After treating both sides of one membrane with ultraviolet-ozone for 4 minutes each, soak it in the solution prepared in step (1) for 2 hours, then wash it with deionized water. Do not perform surface modification on the other membrane.

[0052] (3) Weigh citric acid monohydrate and add it to deionized water, stir to dissolve, with a concentration of 0.05 mol / L. Then add aniline monomer and stir to dissolve, with a concentration of 0.2 mol / L. Weigh another 0.45 g of ammonium persulfate and dissolve it in 10 mL of water to prepare a solution with a concentration of 0.2 mol / L.

[0053] (4) Immerse the surface-modified and un-surface-modified poly(lactic acid) fiber membranes in the aniline monomer solution (10 mL) prepared in (3) respectively, and place them in the refrigerator for 2 hours; place the ammonium persulfate solution in (3) in the refrigerator for 2 hours.

[0054] (5) Take 10 mL of the refrigerated ammonium persulfate solution respectively, mix them with the aniline monomer solutions in which the surface-modified and un-surface-modified poly(lactic acid) fiber membranes are immersed, and place them in the refrigerator for 12 hours to enable in-situ polymerization and growth of polyaniline on the surface of the poly(lactic acid) fibers.

[0055] (6) Take out the poly(lactic acid) / polyaniline composite conductive fiber membranes, wash them with deionized water and then dry them in a vacuum drying oven.

[0056] Cut small pieces from the prepared samples, paste them on carbon adhesive tapes, sputter gold and then place them in a field emission scanning electron microscope to observe the morphology of the in-situ grown polyaniline conductive layer on the surface of the poly(lactic acid) fibers. As Figure 4 shown, after treatment with ultraviolet-ozone and silane coupling agent ( Figure 4 (b) in), there is no free polyaniline in the prepared poly(lactic acid) / polyaniline composite conductive fiber membranes, which greatly improves the uniformity and preparation controllability of the composite conductive fiber membranes compared with the samples without surface treatment ( Figure 4 (a) in).

[0057] Example 3

[0058] Use FeCl 3 as an oxidant to in-situ grow polyaniline on the surface-modified poly(lactic acid) fiber membranes

[0059] The preparation method of the poly(lactic acid) / polyaniline composite conductive fiber membranes described above includes the following steps:

[0060] (1) Measure 10 mL of ethanol, add 1 mL of APTES, and stir evenly.

[0061] (2) Take a poly(lactic acid) electrospun fiber membrane, treat both sides with ultraviolet-ozone for 4 minutes each, soak it in the solution prepared in step (1) for 2 hours, and then wash it with deionized water;

[0062] (3) Weigh citric acid monohydrate, add it to deionized water and stir to dissolve it to a concentration of 0.05 mol / L. Then add aniline monomer and stir to dissolve it to a concentration of 0.2 mol / L; separately weigh 0.54 g of ferric chloride (FeCl 3 ·6H 2 O), dissolve it in 10 mL of water, and prepare a solution with a concentration of 0.2 mol / L;

[0063] (4) Immerse the surface-modified poly(lactic acid) fiber membrane in the aniline monomer solution (10 mL) prepared in (3), and place it in the refrigerator for 2 hours; place the ferric chloride solution in (3) in the refrigerator for 2 hours;

[0064] (5) Take 10 mL of the refrigerated ferric chloride solution, mix it with the aniline monomer solution in which the surface-modified poly(lactic acid) fiber membrane is immersed, and place it in the refrigerator for 12 hours to allow in-situ polymerization and growth of polyaniline on the surface of the poly(lactic acid) fiber;

[0065] (6) Take out the poly(lactic acid) / polyaniline composite conductive fiber membrane, wash it with deionized water, and dry it in a vacuum drying oven.

[0066] Example 4

[0067] Comparison with variable aniline monomer concentration under surface modification

[0068] The method for preparing the poly(lactic acid) / polyaniline composite conductive fiber membrane includes the following steps:

[0069] (1) Measure 10 mL of ethanol, add 1 mL of aminopropyltriethoxysilane, and stir evenly;

[0070] (2) Take four identical poly(lactic acid) electrospun fiber membranes, treat both sides with ultraviolet-ozone for 4 minutes, then soak them in the solution prepared in step (1) for 2 hours, and wash them with deionized water;

[0071] (3) Prepare three aniline monomer solutions with different concentrations: Weigh citric acid monohydrate, add it to deionized water and stir to dissolve it to concentrations of 0.033, 0.05, 0.067, and 0.082 mol / L respectively. Then add aniline monomer and stir to dissolve it to concentrations of 0.14, 0.2, 0.27, and 0.33 mol / L respectively; separately prepare ammonium persulfate solutions with concentrations of 0.14, 0.2, 0.27, and 0.33 mol / L;

[0072] (4) Immerse the surface-modified polylactic acid fiber membrane in the aniline monomer solution (10 mL) prepared in (3) respectively, and place it in the refrigerator for 2 hours; place the ammonium persulfate solution in (3) in the refrigerator for 2 hours;

[0073] (5) Take 10 mL of the refrigerated ammonium persulfate solution respectively, mix it with the aniline monomer solutions of different concentrations in which the polylactic acid fiber membrane is immersed, and place it in the refrigerator for 12 hours to enable in-situ polymerization and growth of polyaniline on the surface of the polylactic acid fiber;

[0074] (6) Take out the polylactic acid / polyaniline composite conductive fiber membrane, wash it with deionized water and then dry it in a vacuum drying oven;

[0075] Cut small pieces from the prepared samples, paste them on carbon tape, sputter gold and then place them in a field emission scanning electron microscope to observe the morphology of the polyaniline conductive layer grown in-situ on the surface of the polylactic acid fiber. As Figure 5 shown, during the process of in-situ growth of the polyaniline conductive layer on the surface of the polylactic acid fiber after surface modification, the concentration of the aniline monomer has no significant effect on the morphology of the polyaniline layer, but the conductivity of the final conductive fiber membrane changes greatly. Theoretically, the increase in the concentration of the aniline monomer will largely increase the generation of free polyaniline, while after surface modification, the generation of free polyaniline on the surface of the polylactic acid fiber is greatly inhibited, and there is no free aniline on the fiber surface even at high concentrations, which is in sharp contrast to the situation without surface modification ( Figure 3 ).

[0076] Example 5

[0077] Only use ultraviolet-ozone to perform surface modification on the polylactic acid fiber

[0078] The preparation method of the polylactic acid / polyaniline composite conductive fiber membrane described above includes the following steps:

[0079] (1) Take a polylactic acid electrospun fiber membrane and treat the front and back sides with ultraviolet-ozone for 4 minutes each;

[0080] (2) Prepare a doped acid solution of aniline: Weigh citric acid monohydrate and dissolve it in deionized water with a concentration of 0.05 mol / L, add aniline monomer and stir to dissolve with a concentration of 0.2 mol / L; separately prepare an ammonium persulfate solution with a concentration of 0.2 mol / L;

[0081] (3) Immerse the surface-modified polylactic acid fiber membrane in the doped acid solution of aniline (10 mL) prepared in (2) respectively, and place it in the refrigerator for 2 hours; place the ammonium persulfate solution in (2) in the refrigerator for 2 hours;

[0082] (4) Take 10 mL of the ammonium persulfate solution after refrigeration, mix it with the aniline monomer solution soaked with the polylactic acid fiber membrane, and place it in the refrigerator for refrigeration for 12 hours to enable in-situ polymerization and growth of polyaniline on the surface of the polylactic acid fiber;

[0083] (5) Take out the polylactic acid / polyaniline composite conductive fiber membrane, wash it with deionized water, and dry it in a vacuum drying oven;

[0084] Observe the morphology of the prepared composite conductive fiber membrane under a scanning electron microscope. As shown in (a) of Figure 7 , it can be seen that many free polyanilines are attached to the fiber surface, and the uniformity is poor. Conduct pressure sensing tests. As shown in (b) of Figure 8 , compared with the un-surface-modified one (Figure 8(a) in the figure), the signal drift problem of the sensor has been improved, but problems such as long stabilization time and poor recoverability still exist.

[0085] Example 6

[0086] Only use APTES for surface modification

[0087] The preparation method of the polylactic acid / polyaniline composite conductive fiber membrane described above includes the following steps:

[0088] (1) Measure 10 mL of ethanol, add 1 mL of aminopropyltriethoxysilane, and stir evenly;

[0089] (2) Take a polylactic acid electrospun fiber membrane, soak it in the solution prepared in step (1) for 2 hours, and then wash it with deionized water;

[0090] (3) Prepare a doped acid solution of aniline: Weigh citric acid monohydrate and dissolve it in deionized water with a concentration of 0.05 mol / L. Add aniline monomer and stir to dissolve with a concentration of 0.2 mol / L; Prepare an ammonium persulfate solution with a concentration of 0.2 mol / L;

[0091] (4) Immerse the surface-modified polylactic acid fiber membrane in the doped acid solution of aniline (10 mL) prepared in (3), and place it in the refrigerator for refrigeration for 2 hours; Place the ammonium persulfate solution in (3) in the refrigerator for refrigeration for 2 hours;

[0092] (5) Take 10 mL of the ammonium persulfate solution after refrigeration, mix it with the aniline monomer solution soaked with the polylactic acid fiber membrane, and place it in the refrigerator for refrigeration for 12 hours to enable in-situ polymerization and growth of polyaniline on the surface of the polylactic acid fiber;

[0093] (6) Take out the polylactic acid / polyaniline composite conductive fiber membrane, wash it with deionized water, and dry it in a vacuum drying oven;

[0094] Observe the morphology of the prepared composite conductive fiber membrane under a scanning electron microscope. AsFigure 7 As shown in (b), it can be seen that a thick layer of polyaniline has grown on the surface of some fibers, while there is no growth on the surface of some fibers, and the uniformity is poor. Pressure sensing tests were carried out, as Figure 8 shown in (c). Compared with only using ultraviolet-ozone for surface modification ( Figure 8 as shown in (b)), the signal recovery problem of the sensor has been improved, but at 0.4 kPa, the signal recovery and stability have deteriorated.

[0095] Example 7

[0096] A method for preparing a polylactic acid / polyaniline composite conductive fiber membrane by simultaneously using ultraviolet-ozone and APTES for surface modification and growing polyaniline in an environment of 20 °C includes the following steps:

[0097] (1) Measure 10 mL of ethanol, add 1 mL of aminopropyltriethoxysilane, and stir evenly;

[0098] (2) Take a polylactic acid electrospun fiber membrane, treat the front and back sides with ultraviolet-ozone for 4 minutes each, then soak it in the solution prepared in step (1) for 2 hours, and wash it with deionized water;

[0099] (3) Prepare a doped acid solution of aniline: Weigh citric acid monohydrate and dissolve it in deionized water to a concentration of 0.05 mol / L, add aniline monomer, stir and dissolve to a concentration of 0.2 mol / L; separately prepare an ammonium persulfate solution with a concentration of 0.2 mol / L;

[0100] (4) Immerse the surface-modified polylactic acid fiber membrane in the aniline monomer solution (10 mL) prepared in (3) and soak it at room temperature of 20 °C for 2 hours; also place the ammonium persulfate solution in (3) at room temperature of 20 °C for 2 hours;

[0101] (5) Take 10 mL of the refrigerated ammonium persulfate solution, mix it with the aniline monomer solution soaked with the polylactic acid fiber membrane, and react at room temperature of 20 °C for 12 hours to enable in-situ polymerization growth of polyaniline on the surface of the polylactic acid fiber;

[0102] (6) Take out the polylactic acid / polyaniline composite conductive fiber membrane, wash it with deionized water and dry it in a vacuum drying oven;

[0103] The morphology of the prepared composite conductive fiber membrane was observed under a scanning electron microscope. As Figure 7 shown in (c), it can be seen that there are large or small polyaniline particles or polyaniline flakes on the surface of many fibers, and the overall growth uniformity is still poor. Pressure sensing tests were carried out, as Figure 8 shown in (d). Compared with no surface modification ( Figure 8Compared with (a), the signal stability and recoverability of the sensor have been improved, but the signal drift problem still exists.

[0104] Example 8

[0105] Comparison with the variable doping acid concentration under surface modification

[0106] The preparation method of the polylactic acid / polyaniline composite conductive fiber membrane described above includes the following steps:

[0107] (1) Measure 10 mL of ethanol, add 1 mL of aminopropyltriethoxysilane, and stir evenly;

[0108] (2) Take four identical polylactic acid electrospun fiber membranes, treat the front and back sides with ultraviolet-ozone for 4 minutes each, then soak them in the solution prepared in step (1) for 2 hours, and wash them with deionized water;

[0109] (3) Prepare four different concentrations of doped acid solutions: Weigh citric acid monohydrate and dissolve it in deionized water. The concentrations are 0, 0.025, 0.05, 0.1, and 0.2 mol / L respectively, corresponding to the molar ratios of doped acid to aniline monomer of 0, 0.125, 0.25, 0.5, and 1. Add aniline monomer and stir to dissolve, with a concentration of 0.2 mol / L; Prepare an ammonium persulfate solution with a concentration of 0.2 mol / L;

[0110] (4) Immerse the surface-modified polylactic acid fiber membranes in the aniline monomer solutions (10 mL) with different citric acid concentrations prepared in (3) respectively, and refrigerate them in the refrigerator for 2 hours; Refrigerate the ammonium persulfate solution in (3) in the refrigerator for 2 hours;

[0111] (5) Take 10 mL of the refrigerated ammonium persulfate solution each, and mix it with the aniline monomer solutions with different citric acid concentrations in which the polylactic acid fiber membranes are immersed respectively, and refrigerate them in the refrigerator for 12 hours to enable in-situ polymerization growth of polyaniline on the surface of the polylactic acid fibers;

[0112] (6) Take out the polylactic acid / polyaniline composite conductive fiber membranes, wash them with deionized water and dry them in a vacuum drying oven;

[0113] Cut small pieces from the prepared samples, paste them on carbon tape, spray gold, and then place them in a field emission scanning electron microscope to observe the morphology of the in-situ grown polyaniline conductive layer on the surface of the polylactic acid fibers. As Figure 6 shown, with the increase of the doping acid concentration, the morphology of the polyaniline layer grown on the surface of the composite conductive fiber membrane has not changed and is very uniform. Using the four-probe method to measure the resistivity, with the increase of the citric acid concentration, the resistivity of the conductive fiber membrane decreases accordingly, meeting the expectations.

[0114] The composite conductive fiber membrane with a molar ratio of citric acid (CA) to aniline (ANI) of 0.25 prepared was subjected to pressure sensing tests, as shown in Figure 8 (e) below. Compared with those without surface modification or other surface treatment methods ( Figure 8 (a - d) below), overall, the sensor based on this fiber membrane has better loading - unloading recoverability, lower signal drift, and faster signal - stabilizing response. Linear fitting of its electrical signal shows that the sensor has good linear responses in two different pressure ranges, with sensitivities of approximately 5 kPa -1 (0 - 1 kPa) and 3 kPa -1 (1 - 6 kPa).

[0115] In this invention, ultraviolet ozone is used to hydrophilize the surface of polylactic acid fibers, and surfactants such as silane coupling agents containing amino groups are used to modify the surface of polylactic acid fibers, forming a chemically - bonded silane network on the surface and a uniform thin layer of free amino groups. During the polymerization of aniline, the free amino groups can chemically bond with aniline molecules to complete the grafting of polyaniline. Under the action of an oxidant, polyaniline continues to grow along this graft, and finally, uniform in - situ graft polymerization of polyaniline on the surface of polylactic acid fibers is completed, forming a uniform polyaniline conductive layer. In addition, by adjusting the concentration of the doping acid, the doping concentration of the polyaniline conductive layer can be regulated, thereby realizing the controllable adjustment of the resistivity of the polylactic acid / polyaniline composite conductive fiber membrane. The process of this invention is simple, the reaction conditions are mild, it is easy to operate and control, making the prepared nanomaterials have good electrical conductivity and biocompatibility, and having broad application prospects.

[0116] The present invention uses the above - mentioned embodiments to illustrate the detailed method of the present invention, but the present invention is not limited to the above - mentioned detailed method, that is, it does not mean that the present invention must rely on the above - mentioned detailed method to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of each raw material of the products of the present invention, and the selection of specific methods and conditions, etc., all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A preparation method of an electrospun poly(lactic acid) / polyaniline conductive fiber membrane for a flexible pressure sensor, characterized in that, it comprises the following steps: Step 1: Preparation of a poly(lactic acid) electrospinning solution Dissolve poly(lactic acid) in Solvent 1 and stir to dissolve it to prepare an electrospinning solution; Step 2: Preparation of a poly(lactic acid) electrospun fiber membrane Perform electrospinning on the electrospinning solution to collect electrospun fibers. After removing the fiber membrane, dry it; Step 3: Surface modification of the poly(lactic acid) electrospun fiber membrane Treat the dried poly(lactic acid) electrospun membrane in Step 2 with ultraviolet-ozone or oxygen plasma, and soak the treated membrane in an ethanol solution of a surface modifier; the surface modifier is a silane coupling agent containing an amino group; Step 4: Preparation of an aniline monomer solution and an oxidant solution Dissolve aniline monomer and a doping acid in water to prepare an acidic aniline solution; weigh a certain mass of an oxidant and dissolve it in water. The concentration of the oxidant solution is the same as that of the aniline monomer solution; the doping acid is at least one of hydrochloric acid, sulfuric acid, nitric acid, dodecylbenzenesulfonic acid, citric acid, and malic acid; Step 5: In-situ polymerization growth of polyaniline on the surface of poly(lactic acid) fibers After washing the surface-modified poly(lactic acid) fiber membrane in Step 3 with deionized water, soak it in the acidic aniline solution described in Step 4. Keep this solution and the oxidant solution in an environment at 0 - 6°C. Subsequently, mix and stir evenly an equal volume of the oxidant solution and the acidic aniline solution soaked with the poly(lactic acid) fiber membrane, and keep it in an environment at 0 - 6°C. After taking out the fiber membrane, wash and dry it to obtain an electrospun poly(lactic acid) / polyaniline conductive fiber membrane, which is used as a pressure-sensitive material in a flexible pressure sensor.

2. The preparation method of an electrospun poly(lactic acid) / polyaniline conductive fiber membrane for a flexible pressure sensor according to claim 1, characterized in that, The first solvent in Step 1 is at least one of dichloromethane (CH 2 Cl 2 ), chloroform (CHCl 3 ), and N,N-dimethylformamide (DMF).

3. The preparation method of an electrospun poly(lactic acid) / polyaniline conductive fiber membrane for a flexible pressure sensor according to claim 1, characterized in that, the voltage applied for electrospinning in Step 2 is 12 - 18 kV, the flow rate of the electrospinning solution jet is 3.6 mL / h, the receiving distance is 12 - 18 cm, the environmental temperature is 10 - 30°C, and the relative humidity is 20 - 80%.

4. The preparation method of an electrospun poly(lactic acid) / polyaniline conductive fiber membrane for a flexible pressure sensor according to claim 1, characterized in that, the oxidant in Step 4 is at least one of ammonium persulfate and ferric chloride.

5. The preparation method of an electrospun poly(lactic acid) / polyaniline conductive fiber membrane for a flexible pressure sensor according to claim 1, characterized in that, the fiber diameter of the poly(lactic acid) electrospun fiber membrane prepared in Step 2 is 500 - 1000 nm.

6. The preparation method of an electrospun poly(lactic acid) / polyaniline conductive fiber membrane for a flexible pressure sensor according to claim 1, characterized in that, the resistivity range of the electrospun poly(lactic acid) / polyaniline conductive fiber membrane prepared in Step 5 is 20 - 130 kΩ·cm.

7. A poly(lactic acid) / polyaniline conductive fiber membrane, It is characterized in that It is prepared by the method described in any one of claims 1-6.

8. A flexible pressure sensor It is characterized in that The fiber membrane described in claim 7 is used as the pressure-sensitive material.

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