A highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film and its preparation method and sensor

Abstract

The present invention discloses a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film and a preparation method thereof. Two monomers with fluorine-containing groups are used to synthesize a polyimide polymer material, thereby increasing the spatial steric hindrance of the movement of the polyimide polymer chain, which helps to overcome the stress relaxation phenomenon of the polymer material. An electrostatic spinning method is used to prepare a polyimide spinning fiber membrane, and the carboxylated carbon nanotubes are chemically grafted onto the polyimide fiber through the silane coupling agent γ-aminopropyltriethoxysilane to form a strong interface bond, which greatly improves the stability and fatigue resistance of the obtained polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film and has long-term sensing reliability. At the same time, the polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film has good mechanical properties, and in the pressure loading test, it has high sensitivity, excellent response speed and recovery speed, and has broad application prospects as a sensitive layer material for flexible pressure sensors.

Description

Technical Field

[0001] The present invention belongs to the field of composite conductive fiber preparation, and relates to a composite conductive pressure-sensitive film and a preparation method thereof, and in particular to a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film and a preparation method and a sensor thereof. The composite conductive pressure-sensitive film has very excellent sensitivity, stability, and fatigue resistance. Background Art

[0002] With the advancement of science and technology, sensor technology has developed rapidly, and its application has penetrated into various aspects such as industrial production, ocean exploration, environmental protection, medical diagnosis, bioengineering, space development, smart home, etc. Compared with traditional rigid sensors, flexible sensors can fit the object being measured, accurately capture input signals, have high sensitivity, and can maintain pressure sensing capabilities under arbitrary deformation conditions, so they are highly favored. However, the flexible pressure sensors currently using polymer materials as sensitive layers often experience stress relaxation, signal drift, interface shear adhesion failure, etc., which seriously affect the accuracy and stability of sensor measurements. Therefore, the preparation of a pressure-sensitive material that is resistant to relaxation, fatigue, and has good stability is of great significance to improving the application prospects of flexible pressure sensors. Summary of the Invention

[0003] The present invention aims to solve the problems of stress relaxation, interface damage and poor stability of the sensitive layer materials of traditional flexible pressure sensors, and provides a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film and its preparation method and sensor. The composite conductive pressure-sensitive film has very excellent sensitivity, stability and anti-relaxation performance, and can be used in high-precision, long-term and other working scenarios.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions:

[0005] A method for preparing a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive voltage-sensitive membrane comprises the following steps: adding a diamine monomer and a dianhydride monomer, both having fluorine-containing groups, to a solvent for reaction to obtain a polyamic acid electrospinning solution; electrospinning to obtain a fiber membrane, followed by heating for thermal imidization to obtain a polyimide fiber membrane; soaking the polyimide fiber membrane in a strong alkaline solution for surface hydrolysis and ring opening, and then soaking the polyimide fiber membrane in a strong acid solution for surface carboxylation; and then soaking the soaked polyimide fiber membrane in a mixed solution containing carboxylated carbon nanotubes and a silane coupling agent, γ-aminopropyltriethoxysilane (KH-550), ultrasonically treating the membrane, removing the membrane, washing, and drying the membrane to obtain a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive voltage-sensitive membrane.

[0006] In the above technical solution, the diamine monomer and dianhydride monomer, both of which have fluorine-containing groups, are 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB) and 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride (6FDA), respectively, with a molar ratio of 6FDA to TFDB of 1:1. By selecting two types of fluorine-containing monomers to synthesize polyimide, the steric hindrance of polyimide polymer chain motion can be increased, thereby effectively enhancing its resistance to stress relaxation, providing a foundation for the long-term, reliable and stable operation of the pressure-sensitive fiber membrane.

[0007] Furthermore, the mass fraction of the monomer in the polyamic acid electrospinning solution is 25-26%.

[0008] Furthermore, the electrospinning voltage is 13-14 kV, the pushing speed is 0.0010-0.0020 mm / s, and the receiving distance is 10-15 cm.

[0009] Furthermore, the thermal imidization temperature is gradually increased from room temperature to 300° C., with each gradient increasing by 50-100° C. at a heating rate of 5-10° C. / min, and the temperature is kept for 30-60 min.

[0010] Furthermore, the solvent in the strong base solution is water, and the concentration of the strong base is 5M.

[0011] Furthermore, the solvent of the strong acid solution is water, and the volume fraction of the strong acid is 50%.

[0012] Furthermore, the solvents in the mixed solution are water and ethanol, and the usage ratio of the two will affect the subsequent chemical grafting effect of carboxylated carbon nanotubes on the polyimide surface. The volume ratio of water, KH-550 and ethanol is usually 1:3:4-8, more preferably 1:3:6.

[0013] Furthermore, the ultrasonic treatment time is 1 to 5 hours, preferably 3 hours. The conductivity of the obtained pressure-sensitive film is in the range of 2.80×10 -3 -3.79×10 -1 S / m.

[0014] At present, flexible pressure sensors made of composite materials often have problems such as relaxation, long stabilization time, difficulty in reaching equilibrium, obvious signal hysteresis before and after loading, and poor repeatability. These problems will affect the reliability of the device operation and thus restrict its application. The present invention uses two fluorine-containing monomers to synthesize polyimide, thereby increasing the steric hindrance of molecular chain movement, thereby enhancing the stress relaxation resistance of the pressure-sensitive material. Electrospinning technology is used to construct the microstructure of the sensitive layer material to improve the sensitivity of the sensitive layer material. At the same time, carboxylated carbon nanotubes are grafted onto polyimide fibers using a chemical grafting method. The interface bonding is firm, avoiding problems such as interface shear adhesion and conductive phase shedding caused by methods such as direct mixing or spraying to cover the conductive phase, thereby greatly improving the stability and fatigue resistance of the pressure-sensitive material. In this way, a pressure-sensitive composite material with very excellent reliability and stability is successfully designed. It can have both signal stability and long-term fatigue resistance while ensuring sensitivity. It can be used for long-term reliable and stable flexible pressure sensing, and can provide new solutions for many fields such as electronic skin, intelligent prostheses and virtual reality, as well as medical monitoring and diagnosis, and has great practical significance. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 The figure is a flow chart for preparing a polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film;

[0016] Figure 2 This is the SEM photo of the polyimide spun fiber membrane;

[0017] Figure 3 The SEM images of polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive films prepared with different solution ratios and different ultrasonic times;

[0018] Figure 4 The conductivity diagram of polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film prepared with different solution ratios and different ultrasonic times;

[0019] Figure 5 is a current-pressure curve of the polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film;

[0020] Figure 6 24,000 pressure loading and unloading cycles for polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film;

[0021] Figure 7 This is the SEM image of the polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film after 24,000 pressure loading and unloading cycles;

[0022] Figure 8 This is a sensitivity performance diagram of the polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film in the pressure range of 0-18 kPa;

[0023] Figure 9 The SEM image and current-pressure curve of the composite conductive pressure-sensitive membrane of polyimide fiber membrane and carboxylated carbon nanotubes without KH-550 grafting but only ultrasonic mixing loading. DETAILED DESCRIPTION

[0024] The technical solution of the present invention is further described in detail below with reference to the accompanying drawings and specific embodiments:

[0025] The highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film of the present invention is prepared as follows: Figure 1 As shown, an electrospinning precursor solution was prepared by first synthesizing polyimide precursor polyamic acid from two fluorine-containing monomers, and the polyamic acid was electrospun to form a fiber membrane. The fiber membrane was then thermally imidized to obtain a polyimide fiber membrane. Carboxylated carbon nanotubes were then chemically grafted onto the polyimide fiber using a silane coupling agent KH-550. The grafting interface was firm, and a polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive membrane was obtained, which had reliable stability and anti-fatigue performance.

[0026] The conductivity of the polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film prepared by the present invention is tested by measuring the resistivity of the film using a four-probe resistivity tester and then converting it into conductivity. The conductivity of the polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film is measured to be within the range of 2.80×10 -3 -3.79×10 -1 S / m.

[0027] The pressure sensing test method of the composite conductive pressure-sensitive film is as follows: the composite conductive pressure-sensitive film is cut into discs with a diameter of 0.7 mm, placed on two semicircular electrodes for packaging, a push-pull force gauge or weight is used to apply pressure to the fiber membrane, and a digital source meter outputs data to draw a current-pressure curve.

[0028] Example 1

[0029] 1. Dissolve 3.20 g TFDB and 4.44 g 6FDA (molar ratio of 1:1) in N,N-dimethylformamide (DMF) to prepare a polyamic acid spinning solution with a monomer mass fraction of 26%, and stir continuously at 0-10 ° C for 12 hours;

[0030] 2. The spinning solution was loaded into a syringe and spun in an electrospinning machine. The spinning voltage was set to 13 kV, the pushing speed was 0.0020 mm / s, and the distance from the receiver was 10 cm. After the spinning was completed, the spinning membrane was removed and placed in an oven to dry.

[0031] 3. The polyamic acid spinning membrane was thermally imidized, and the temperature steps were set as follows: the heating rate was 5°C / min, the temperature was raised from room temperature to 80°C and then kept warm for 60 minutes, the temperature was raised to 100°C and then kept warm for 30 minutes, the temperature was raised to 150°C and then kept warm for 30 minutes, the temperature was raised to 200°C and then kept warm for 30 minutes, the temperature was raised to 250°C and then kept warm for 30 minutes, and the temperature was raised to 300°C and then kept warm for 30 minutes to obtain a polyimide fiber membrane. The scanning electron microscope (SEM) was used for observation. Figure 2 As shown, the obtained fibers are continuous and uniform in thickness;

[0032] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir magnetically for 1 hour, then take it out, wash and dry it;

[0033] 5. Soak the polyimide fiber membrane treated in step 4 in a 50% by volume acetic acid solution, stir magnetically for 10 minutes, then take it out, wash and dry it;

[0034] 6. Dissolve the silane coupling agent γ-aminopropyltriethoxysilane (KH-550) in a mixed solvent of water and ethanol. The ratio of each component in the mixed solution is water: KH-550: ethanol = 1:3:4. After stirring for half an hour, add a certain amount of carboxylated carbon nanotubes to the above solution. Then, place the mixed solution in a container and place it in an ultrasonic cleaner for half an hour.

[0035] 7. Soak the polyimide fiber membrane treated in step 5 in the ultrasonic mixed solution described in step 6, and continue ultrasonication for 3 hours. Then, take out the fiber membrane, wash and dry it to obtain a polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive membrane with excellent reliability and stability. SEM observation is performed, as shown in FIG. Figure 3 As shown in (a), the surface of the polyimide fiber is obviously covered with carboxylated carbon nanotubes. The conductivity is measured by the four-probe method, as shown in Figure 4 As shown, its conductivity is 6.63×10 -2 S / m.

[0036] Example 2

[0037] 1. Dissolve 3.20 g TFDB and 4.44 g 6FDA (molar ratio of 1:1) in N,N-dimethylformamide (DMF) to prepare a polyamic acid spinning solution with a monomer mass fraction of 26%, and stir continuously at 0-10 ° C for 12 hours;

[0038] 2. The spinning solution was loaded into a syringe and spun in an electrospinning machine. The spinning voltage was set to 13 kV, the pushing speed was 0.0020 mm / s, and the distance from the receiver was 10 cm. After the spinning was completed, the spinning membrane was removed and placed in an oven to dry.

[0039] 3. The polyamic acid spinning membrane was thermally imidized, and the temperature steps were set as follows: the heating rate was 5°C / min, the temperature was raised from room temperature to 80°C and kept warm for 60 min, the temperature was raised to 100°C and kept warm for 30 min, the temperature was raised to 150°C and kept warm for 30 min, the temperature was raised to 200°C and kept warm for 30 min, the temperature was raised to 250°C and kept warm for 30 min, and the temperature was raised to 300°C and kept warm for 30 min to obtain a polyimide fiber membrane;

[0040] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir magnetically for 1 hour, then take it out, wash and dry it;

[0041] 5. Soak the polyimide fiber membrane treated in step 4 in a 50% by volume acetic acid solution, stir magnetically for 10 minutes, then take it out, wash and dry it;

[0042] 6. Dissolve the silane coupling agent γ-aminopropyltriethoxysilane (KH-550) in a mixed solvent of water and ethanol. The ratio of each component in the mixed solution is water: KH-550: ethanol = 1:3:6. After stirring for half an hour, add a certain amount of carboxylated carbon nanotubes to the above solution. Then, place the mixed solution in a container and place it in an ultrasonic cleaner for half an hour.

[0043] 7. Soak the polyimide fiber membrane treated in step 5 in the ultrasonic mixed solution described in step 6, and continue ultrasonication for 5 hours. Then, take out the fiber membrane, wash and dry it to obtain a polyimide-carboxylated carbon nanotube composite conductive voltage-sensitive membrane with excellent reliability and stability. SEM observation is performed, as shown in FIG. Figure 3 As shown in (f), the surface of the polyimide fiber is obviously covered with carboxylated carbon nanotubes. The conductivity is measured by the four-probe method, as shown in Figure 4 As shown, its conductivity is 4.74×10 -2 S / m.

[0044] Example 3

[0045] 1. Dissolve 3.20 g TFDB and 4.44 g 6FDA (molar ratio of 1:1) in N,N-dimethylformamide (DMF) to prepare a polyamic acid spinning solution with a monomer mass fraction of 26%, and stir continuously at 0-10 ° C for 12 hours;

[0046] 2. The spinning solution was loaded into a syringe and spun in an electrospinning machine. The spinning voltage was set to 13 kV, the pushing speed was 0.0020 mm / s, and the distance from the receiver was 10 cm. After the spinning was completed, the spinning membrane was removed and placed in an oven to dry.

[0047] 3. The polyamic acid spinning membrane was thermally imidized, and the temperature steps were set as follows: the heating rate was 5°C / min, the temperature was raised from room temperature to 80°C and kept warm for 60 min, the temperature was raised to 100°C and kept warm for 30 min, the temperature was raised to 150°C and kept warm for 30 min, the temperature was raised to 200°C and kept warm for 30 min, the temperature was raised to 250°C and kept warm for 30 min, and the temperature was raised to 300°C and kept warm for 30 min to obtain a polyimide fiber membrane;

[0048] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir magnetically for 1 hour, then take it out, wash and dry it;

[0049] 5. Soak the polyimide fiber membrane treated in step 4 in a 50% by volume acetic acid solution, stir magnetically for 10 minutes, then take it out, wash and dry it;

[0050] 6. Dissolve the silane coupling agent γ-aminopropyltriethoxysilane (KH-550) in a mixed solvent of water and ethanol. The ratio of each component in the mixed solution is water: KH-550: ethanol = 1:3:8. After stirring for half an hour, add a certain amount of carboxylated carbon nanotubes to the above solution. Then, place the mixed solution in a container and place it in an ultrasonic cleaner for half an hour.

[0051] 7. Soak the polyimide fiber membrane treated in step 5 in the ultrasonic mixed solution described in step 6, and continue ultrasonication for 3 hours. Then, take out the fiber membrane, wash and dry it to obtain a polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive membrane with excellent reliability and stability. SEM observation is performed, as shown in FIG. Figure 3 As shown in (c), the surface of the polyimide fiber is obviously covered with carboxylated carbon nanotubes. The conductivity is measured by the four-probe method, as shown in Figure 4 As shown, its conductivity is 3.08×10 -2 S / m.

[0052] Example 4

[0053] 1. Dissolve 3.20 g TFDB and 4.44 g 6FDA (molar ratio of 1:1) in N,N-dimethylformamide (DMF) to prepare a polyamic acid spinning solution with a monomer mass fraction of 26%, and stir continuously at 0-10 ° C for 12 hours;

[0054] 2. The spinning solution was loaded into a syringe and spun in an electrospinning machine. The spinning voltage was set to 13 kV, the pushing speed was 0.0020 mm / s, and the distance from the receiver was 10 cm. After the spinning was completed, the spinning membrane was removed and placed in an oven to dry.

[0055] 3. The polyamic acid spinning membrane was thermally imidized, and the temperature steps were set as follows: the heating rate was 5°C / min, the temperature was raised from room temperature to 80°C and kept warm for 60 min, the temperature was raised to 100°C and kept warm for 30 min, the temperature was raised to 150°C and kept warm for 30 min, the temperature was raised to 200°C and kept warm for 30 min, the temperature was raised to 250°C and kept warm for 30 min, and the temperature was raised to 300°C and kept warm for 30 min to obtain a polyimide fiber membrane;

[0056] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir magnetically for 1 hour, then take it out, wash and dry it;

[0057] 5. Soak the polyimide fiber membrane treated in step 4 in a 50% by volume acetic acid solution, stir magnetically for 10 minutes, then take it out, wash and dry it;

[0058] 6. Dissolve the silane coupling agent γ-aminopropyltriethoxysilane (KH-550) in a mixed solvent of water and ethanol. The ratio of each component in the mixed solution is water: KH-550: ethanol = 1:3:6. After stirring for half an hour, add a certain amount of carboxylated carbon nanotubes to the above solution. Then, place the mixed solution in a container and place it in an ultrasonic cleaner for half an hour.

[0059] 7. Soak the polyimide fiber membrane treated in step 5 in the ultrasonic mixed solution described in step 6, continue ultrasonication for 1 hour, then take out the fiber membrane, wash and dry it to obtain a polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive membrane with excellent reliability and stability. Figure 3 As shown in (d), the surface of the polyimide fiber is obviously covered with carboxylated carbon nanotubes. The conductivity is measured by the four-probe method, as shown in Figure 4 As shown, its conductivity is 2.80×10 -3 S / m.

[0060] Example 5

[0061] 1. Dissolve 3.20 g TFDB and 4.44 g 6FDA (molar ratio of 1:1) in N,N-dimethylformamide (DMF) to prepare a polyamic acid spinning solution with a monomer mass fraction of 26%, and stir continuously at 0-10 ° C for 12 hours;

[0062] 2. The spinning solution was loaded into a syringe and spun in an electrospinning machine. The spinning voltage was set to 13 kV, the pushing speed was 0.0020 mm / s, and the distance from the receiver was 10 cm. After the spinning was completed, the spinning membrane was removed and placed in an oven to dry.

[0063] 3. The polyamic acid spinning membrane was thermally imidized, and the temperature steps were set as follows: the heating rate was 5°C / min, the temperature was raised from room temperature to 80°C and kept warm for 60 min, the temperature was raised to 100°C and kept warm for 30 min, the temperature was raised to 150°C and kept warm for 30 min, the temperature was raised to 200°C and kept warm for 30 min, the temperature was raised to 250°C and kept warm for 30 min, and the temperature was raised to 300°C and kept warm for 30 min to obtain a polyimide fiber membrane;

[0064] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir magnetically for 1 hour, then take it out, wash and dry it;

[0065] 5. Soak the polyimide fiber membrane treated in step 4 in a 50% by volume acetic acid solution, stir magnetically for 10 minutes, then take it out, wash and dry it;

[0066] 6. Dissolve the silane coupling agent γ-aminopropyltriethoxysilane (KH-550) in a mixed solvent of water and ethanol. The ratio of each component in the mixed solution is water: KH-550: ethanol = 1:3:6. After stirring for half an hour, add a certain amount of carboxylated carbon nanotubes to the above solution. Then, place the mixed solution in a container and place it in an ultrasonic cleaner for half an hour.

[0067] 7. Soak the polyimide fiber membrane treated in step 5 in the ultrasonic mixed solution described in step 6, and continue ultrasonication for 3 hours. Then, take out the fiber membrane, wash and dry it to obtain a polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive membrane with excellent reliability and stability. SEM observation is performed, as shown in FIG. Figure 3 As shown in (b) and (e), the surface of the polyimide fiber is obviously covered with carboxylated carbon nanotubes. Compared with Examples 1-4, the covered carboxylated carbon nanotubes are more dense and uniform. The conductivity is measured by the four-probe method, as shown in FIG. Figure 4 As shown, its conductivity is 3.79×10 -1 S / m, higher than the composite conductive film obtained in Examples 1-4.

[0068] Example 6

[0069] 1. Dissolve 3.20 g TFDB and 4.44 g 6FDA (molar ratio of 1:1) in N,N-dimethylformamide (DMF) to prepare a polyamic acid spinning solution with a monomer mass fraction of 25%, and stir continuously at 0-10 ° C for 12 hours;

[0070] 2. The spinning solution was loaded into a syringe and spun in an electrospinning machine. The spinning voltage was set to 13 kV, the pushing speed was 0.0020 mm / s, and the distance from the receiver was 10 cm. After the spinning was completed, the spinning membrane was removed and placed in an oven to dry.

[0071] 3. The polyamic acid spinning membrane was thermally imidized, and the temperature steps were set as follows: the heating rate was 5°C / min, the temperature was raised from room temperature to 80°C and kept warm for 60 min, the temperature was raised to 100°C and kept warm for 30 min, the temperature was raised to 150°C and kept warm for 30 min, the temperature was raised to 200°C and kept warm for 30 min, the temperature was raised to 250°C and kept warm for 30 min, and the temperature was raised to 300°C and kept warm for 30 min to obtain a polyimide fiber membrane;

[0072] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir magnetically for 1 hour, then take it out, wash and dry it;

[0073] 5. Soak the polyimide fiber membrane treated in step 4 in a 50% by volume acetic acid solution, stir magnetically for 10 minutes, then take it out, wash and dry it;

[0074] 6. Dissolve the silane coupling agent γ-aminopropyltriethoxysilane (KH-550) in a mixed solvent of water and ethanol. The ratio of each component in the mixed solution is water: KH-550: ethanol = 1:3:6. After stirring for half an hour, add a certain amount of carboxylated carbon nanotubes to the above solution. Then, place the mixed solution in a container and place it in an ultrasonic cleaner for half an hour.

[0075] 7. Soak the polyimide fiber membrane treated in step 5 in the ultrasonic mixed solution described in step 6, and continue ultrasonicating for 3 hours. Then, take out the fiber membrane, wash and dry it to obtain a polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive membrane with excellent reliability and stability.

[0076] Example 7

[0077] 1. Dissolve 3.20 g TFDB and 4.44 g 6FDA (molar ratio of 1:1) in N,N-dimethylformamide (DMF) to prepare a polyamic acid spinning solution with a monomer mass fraction of 26%, and stir continuously at 0-10 ° C for 12 hours;

[0078] 2. The spinning solution was loaded into a syringe and spun in an electrospinning machine. The spinning voltage was set to 14 kV, the pushing speed was 0.0010 mm / s, and the distance from the receiver was 15 cm. After the spinning was completed, the spinning membrane was removed and placed in an oven to dry.

[0079] 3. The polyamic acid spinning membrane was thermally imidized, and the temperature steps were set as follows: the heating rate was 5°C / min, the temperature was raised from room temperature to 80°C and kept warm for 60 min, the temperature was raised to 100°C and kept warm for 30 min, the temperature was raised to 150°C and kept warm for 30 min, the temperature was raised to 200°C and kept warm for 30 min, the temperature was raised to 250°C and kept warm for 30 min, and the temperature was raised to 300°C and kept warm for 30 min to obtain a polyimide fiber membrane;

[0080] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir magnetically for 1 hour, then take it out, wash and dry it;

[0081] 5. Soak the polyimide fiber membrane treated in step 4 in a 50% by volume acetic acid solution, stir magnetically for 10 minutes, then take it out, wash and dry it;

[0082] 6. Dissolve the silane coupling agent γ-aminopropyltriethoxysilane (KH-550) in a mixed solvent of water and ethanol. The ratio of each component in the mixed solution is water: KH-550: ethanol = 1:3:6. After stirring for half an hour, add a certain amount of carboxylated carbon nanotubes to the above solution. Then, place the mixed solution in a container and place it in an ultrasonic cleaner for half an hour.

[0083] 7. Soak the polyimide fiber membrane treated in step 5 in the ultrasonic mixed solution described in step 6, and continue ultrasonicating for 3 hours. Then, take out the fiber membrane, wash and dry it to obtain a polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive membrane with excellent reliability and stability.

[0084] The present invention uses two fluorine-containing monomers to synthesize polyimide, increasing the steric hindrance of molecular chain movement, thereby improving the creep resistance of the polyimide material and enhancing the stress relaxation resistance of the pressure-sensitive material. Electrospinning technology is used to construct the microstructure of the sensitive layer material, and the conductive phase carboxylation of carbon nanotubes is chemically grafted to make the composite conductive pressure-sensitive membrane have reliable stability and fatigue resistance, thereby obtaining a highly reliable and stable flexible pressure sensor. The fiber membranes obtained in Examples 1-7 all have excellent signal stability, load-unload cycle stability, and high sensitivity; Figure 5 As shown in FIG, the current-pressure curve is the pressure sensing test result of the composite conductive pressure-sensitive film obtained in Example 5. The loading curve and unloading curve of each pressure stage are basically the same, which shows that the composite conductive pressure-sensitive film has excellent loading deformation and unloading recovery. The polyimide-carboxylated carbon nanotube composite conductive film obtained in Example 5 was subjected to pressure loading and unloading cycle test. Figure 6 As shown in the figure, it can be seen that after 24,000 cycles of testing, the output current curve is flat and the output current level is stable. The average output current change between the first ten times and the last ten times is only 3.6%, indicating that the sensor has reliable stability. Figure 7 The SEM photo of the composite conductive pressure-sensitive film after 24,000 loading and unloading cycles shows that the fiber morphology is intact without any breakage. The fiber surface is still evenly covered with carbon nanotubes without any carbon nanotube shedding. This is due to the fact that the carbon nanotubes are attached to the PI fiber through chemical grafting, and the interface bonding is firm, which proves that the sensor has good fatigue resistance and is feasible for long-term use. Figure 8As shown, the composite conductive pressure sensitive film has a pressure of 124.3kPa in the pressure range of 0-2kPa. -1 Sensitivity of 20.6kPa in the pressure range of 2-18kPa -1 The sensitivity is high, which shows that the composite conductive pressure sensitive film has a strong ability to capture weak pressure in pressure detection applications. Figure 9 As shown, for a sample in which KH-550 was not added for chemical grafting, but only the polyimide fiber membrane and carboxylated carbon nanotubes were ultrasonically mixed and loaded in deionized water, SEM images showed that the carboxylated carbon nanotubes were unevenly distributed, and obvious carbon nanotube agglomeration occurred locally on the polyimide fibers. This would lead to uneven reinforcement of the carbon nanotubes on the polyimide fibers, untimely and unstable conversion of mechanical signals to current signals, and thus poor stability and unloading recovery of the composite conductive pressure-sensitive membrane. This can be reflected in the current-pressure curve of the composite conductive pressure-sensitive membrane. When the pressure remains unchanged, the output current value fluctuates greatly, and the loading and unloading curves at each pressure stage are not horizontal. This shows that the polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive membrane chemically grafted with KH-550 has a strong and uniform conductive phase interface, and therefore has reliable long-term stability, fatigue resistance, and excellent sensitivity.

[0085] While the present invention is described through the above-described embodiments, the present invention is not limited to the above-described detailed methods, and implementation of the present invention is not necessarily dependent on the above-described detailed methods. Those skilled in the art will appreciate that any improvements to the present invention, equivalent substitutions for the raw materials of the products of the present invention, and the selection of specific methods and conditions, etc., fall within the scope of protection and disclosure of the present invention.

Claims

1. A method for preparing a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film, characterized in that: The method comprises the following steps: adding a diamine monomer and a dianhydride monomer, both of which have fluorine-containing groups, into a solvent in sequence for reaction to obtain a polyamic acid electrospinning solution; heating the fiber membrane obtained by electrospinning for thermal imidization to obtain a polyimide fiber membrane; soaking the polyimide fiber membrane in a strong alkaline solution for surface hydrolysis and ring opening, and in a strong acid solution for surface carboxylation treatment; the soaked polyimide fiber membrane is then immersed in a mixed solution containing carboxylated carbon nanotubes and a silane coupling agent, γ-aminopropyltriethoxysilane, subjected to ultrasonic treatment, taken out, washed, and dried to obtain a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive voltage-sensitive membrane.

2. The method for preparing a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film according to claim 1, characterized in that: The diamine monomer and dianhydride monomer both having fluorine-containing groups are 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl and 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride, respectively, and the molar ratio of 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride to 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl is 1:

1.

3. The method for preparing a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film according to claim 1, characterized in that: The mass fraction of the monomer in the polyamic acid electrospinning solution is 25-26%.

4. The method for preparing a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film according to claim 1, wherein: The electrospinning voltage was 13-14 kV, the pushing speed was 0.0010-0.0020 mm / s, and the receiving distance was 10-15 cm.

5. The method for preparing a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film according to claim 1, characterized in that: The solvent in the strong base solution is water, and the concentration of the strong base is 5M.

6. The method for preparing a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film according to claim 1, characterized in that: The solvent of the strong acid solution is water, and the volume fraction of the strong acid is 50%.

7. The method for preparing a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film according to claim 1, characterized in that: The solvents in the mixed solution are water and ethanol, and the volume ratio of water, gamma-aminopropyltriethoxysilane and ethanol is 1:3:4-8.

8. The method for preparing a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film according to claim 1, characterized in that: The solvents in the mixed solution are water and ethanol, and the volume ratio of water, gamma-aminopropyltriethoxysilane and ethanol is 1:3:

6.

9. The method for preparing a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film according to claim 1, characterized in that: The ultrasonic treatment time is 1 to 5 hours.

10. The method for preparing a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film according to claim 1, characterized in that: The ultrasonic treatment time is 3 hours.

11. A highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film, characterized in that: The method is prepared by the method according to any one of claims 1 to 10.

12. A flexible pressure sensor, characterized in that: The highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film is used for preparation.

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