Highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film, preparation method thereof and sensor

Abstract

The 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 adopted to synthesize the polyimide polymer material, so that the steric hindrance of movement of a polyimide polymer chain is improved, and the stress relaxation phenomenon of the polymer material can be overcome. A polyimide spinning fiber membrane is prepared by adopting an electrostatic spinning method, carboxylated carbon nanotubes are chemically grafted to polyimide fibers through a silane coupling agent gamma-aminopropyltriethoxysilane, and firm interface bonding is formed; the stability and fatigue resistance of the obtained polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film are greatly improved, and the film has long-term sensing reliability. Meanwhile, the polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film has good mechanical properties, has high sensitivity and excellent response speed and recovery speed in a pressure loading test, and has wide application prospects as a sensitive layer material of a flexible pressure sensor.

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, wherein 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 that currently use polymer materials as sensitive layers often experience stress relaxation, signal drift, interface shear adhesion and detachment damage, etc., which seriously affect the accuracy and stability of sensor measurements. Therefore, preparing a pressure-sensitive material that is resistant to relaxation, fatigue, and good in 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 to provide 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 of which have fluorine-containing groups, into a solvent in sequence to react to obtain a polyamic acid electrostatic spinning solution, heating the fiber membrane obtained by electrostatic spinning to perform thermal imidization to obtain a polyimide fiber membrane, and successively immersing 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 immersed polyimide fiber membrane is then immersed in a mixed solution containing carboxylated carbon nanotubes and a silane coupling agent γ-aminopropyltriethoxysilane (KH-550), subjected to ultrasonic treatment, and washed and dried after being taken out to obtain a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive voltage-sensitive membrane.

[0006] In the above technical solution, further, the diamine monomer and dianhydride monomer both having fluorine-containing groups are 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (TFDB) and 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride (6FDA), respectively, and the molar ratio of 6FDA to TFDB is 1:1. By selecting two types of monomers containing fluorine groups to synthesize polyimide, the steric hindrance of the movement of the polyimide polymer chain can be increased, thereby effectively enhancing its resistance to stress relaxation, providing a basis 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, a heating rate of 5-10°C / min, and the temperature is kept for 30-60min.

[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 surface of polyimide. The volume ratio of water, KH-550 and ethanol is usually 1:3:4-8, and 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, the flexible pressure sensors of composite materials often have such phenomena 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 device operation and thus restrict its application. The present invention uses two fluorine-containing monomers to synthesize polyimide, increase the spatial steric hindrance of molecular chain movement, and thus enhance the stress relaxation resistance of the pressure-sensitive material. The microstructure of the sensitive layer material is constructed by electrospinning technology to improve the sensitivity of the sensitive layer material. At the same time, the carboxylated carbon nanotubes are grafted on the polyimide fiber by combining the chemical grafting method. The interface is firmly bonded, avoiding the problems of interface shear adhesion and conductive phase shedding caused by the covering conductive phase method such as direct mixing or spraying, and greatly improving the stability and fatigue resistance of the pressure-sensitive material. Thus, a pressure-sensitive composite material with excellent reliable stability is successfully designed, which can have both signal stability and long-term fatigue resistance under the premise of ensuring sensitivity, and 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, and medical monitoring and diagnosis, which 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 photos 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 the 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 diagram 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 membrane;

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

[0022] Figure 8 It 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] Fig. 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 ultrasonically mixed and loaded. DETAILED DESCRIPTION

[0024] The technical solution of the present invention is further described in detail below in conjunction with 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 is prepared by first synthesizing polyimide precursor polyamic acid from two fluorine-containing monomers, and the polyamic acid is electrospun to form a fiber membrane. The fiber membrane is then thermally imidized to obtain a polyimide fiber membrane. The carboxylated carbon nanotubes are chemically grafted onto the polyimide fiber using a silane coupling agent KH-550. The grafting interface is firm, and a polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive membrane is obtained, which has reliable stability and anti-fatigue performance.

[0026] The conductivity test method of the polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film prepared by the present invention is: using a four-probe resistivity tester to measure the resistivity of the film, and then converting it into conductivity. The conductivity of the polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film is measured to be in 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, and then 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 3.20g TFDB and 4.44g 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. Load the spinning solution into the syringe and spin in the electrospinning machine. Set the spinning voltage to 13 kV, the pushing speed to 0.0020 mm / s, and the distance from the receiver to 10 cm. After spinning, remove the spinning membrane and place it 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 for 60 minutes, the temperature was raised to 100°C and then kept for 30 minutes, the temperature was raised to 150°C and then kept for 30 minutes, the temperature was raised to 200°C and then kept for 30 minutes, the temperature was raised to 250°C and then kept for 30 minutes, and the temperature was raised to 300°C and then kept for 30 minutes to obtain a polyimide fiber membrane, which was observed by scanning electron microscopy (SEM). 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 it 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 it magnetically for 10 minutes, then take it out, wash it, 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, put the mixed solution in a container and put 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 voltage-sensitive membrane with excellent reliability and stability. SEM observation is performed. 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 3.20g TFDB and 4.44g 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. Load the spinning solution into the syringe and spin in the electrospinning machine. Set the spinning voltage to 13 kV, the pushing speed to 0.0020 mm / s, and the distance from the receiver to 10 cm. After spinning, remove the spinning membrane and place it in an oven to dry.

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

[0040] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir it 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 it magnetically for 10 minutes, then take it out, wash it, 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, put the mixed solution in a container and put 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 reliable 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 3.20g TFDB and 4.44g 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. Load the spinning solution into the syringe and spin in the electrospinning machine. Set the spinning voltage to 13 kV, the pushing speed to 0.0020 mm / s, and the distance from the receiver to 10 cm. After spinning, remove the spinning membrane and place it in an oven to dry.

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

[0048] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir it 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 it magnetically for 10 minutes, then take it out, wash it, 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, put the mixed solution in a container and put 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 voltage-sensitive membrane with excellent reliability and stability. SEM observation is performed. 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 3.20g TFDB and 4.44g 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. Load the spinning solution into the syringe and spin in the electrospinning machine. Set the spinning voltage to 13 kV, the pushing speed to 0.0020 mm / s, and the distance from the receiver to 10 cm. After spinning, remove the spinning membrane and place it in an oven to dry.

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

[0056] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir it 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 it magnetically for 10 minutes, then take it out, wash it, 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, put the mixed solution in a container and put 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, and continue ultrasonication for 1 hour. Then, take out the fiber membrane, wash and dry it to obtain a polyimide-carboxylated carbon nanotube composite conductive voltage-sensitive membrane with excellent reliable stability. SEM observation is performed, as shown in FIG. 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 3.20g TFDB and 4.44g 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. Load the spinning solution into the syringe and spin in the electrospinning machine. Set the spinning voltage to 13 kV, the pushing speed to 0.0020 mm / s, and the distance from the receiver to 10 cm. After spinning, remove the spinning membrane and place it in an oven to dry.

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

[0064] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir it 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 it magnetically for 10 minutes, then take it out, wash it, 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, put the mixed solution in a container and put 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 voltage-sensitive membrane with excellent reliability and stability. SEM observation is performed. 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 uniformly distributed. 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 is higher than that of the composite conductive film obtained in Examples 1-4.

[0068] Example 6

[0069] 1. Dissolve 3.20g TFDB and 4.44g 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. Load the spinning solution into the syringe and spin in the electrospinning machine. Set the spinning voltage to 13 kV, the pushing speed to 0.0020 mm / s, and the distance from the receiver to 10 cm. After spinning, remove the spinning membrane and place it in an oven to dry.

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

[0072] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir it 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 it magnetically for 10 minutes, then take it out, wash it, 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, put the mixed solution in a container and put 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 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 reliable stability.

[0076] Example 7

[0077] 1. Dissolve 3.20g TFDB and 4.44g 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. Load the spinning solution into the syringe and spin in the electrospinning machine. Set the spinning voltage to 14 kV, the pushing speed to 0.0010 mm / s, and the distance from the receiver to 15 cm. After spinning, remove the spinning membrane and place it in an oven to dry.

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

[0080] 4. Soak the polyimide fiber membrane in 5M sodium hydroxide solution, stir it 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 it magnetically for 10 minutes, then take it out, wash it, 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, put the mixed solution in a container and put 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 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 reliable stability.

[0084] The present invention uses two fluorine-containing monomers to synthesize polyimide, increase the steric hindrance of molecular chain movement, thereby improving the creep resistance of polyimide materials and enhancing the stress relaxation resistance of the pressure-sensitive material. The microstructure of the sensitive layer material is constructed by electrospinning technology, and the conductive phase carboxylation carbon nanotubes are chemically grafted, so that the composite conductive pressure-sensitive membrane has 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, loading and unloading cycle stability, and high sensitivity; Figure 5 As shown in FIG. 1 , 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 the unloading curve of each pressure stage are basically the same, and it can be seen 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 a 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 124.3 kPa in the pressure range of 0-2 kPa. -1 The sensitivity is 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. Fig. 9 As shown, for the sample in which KH-550 was not added for chemical grafting, but only the polyimide fiber membrane and the carboxylated carbon nanotubes were ultrasonically mixed and loaded in deionized water, the SEM picture was observed, and the distribution of the carboxylated carbon nanotubes was uneven, and obvious carbon nanotube agglomeration occurred locally in the polyimide fiber, which would lead to uneven reinforcement of the carbon nanotubes on the polyimide fiber, untimely and unstable conversion of the mechanical signal to the current signal, and thus the stability and unloading recovery of the composite conductive pressure-sensitive membrane were poor, which can be reflected in the current-pressure curve of the composite conductive pressure-sensitive membrane. When the pressure remained unchanged, the output current value fluctuated greatly, and the loading curve and the unloading curve of each pressure stage were not horizontal, indicating that the polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive membrane chemically grafted by KH-550 had a firm and uniform conductive phase interface bonding, and therefore had reliable long-term stability, fatigue resistance and excellent sensitivity.

[0085] The present invention illustrates the detailed method of the present invention through the above-mentioned embodiments, 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 of the present invention, equivalent replacement of various raw materials of the product of the present invention, and the selection of specific methods and conditions, etc., all fall within the protection scope and disclosure scope 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 invention comprises the following steps: adding a diamine monomer and a dianhydride monomer, both of which have fluorine-containing groups, into a solvent to react to obtain a polyamic acid electrospinning solution; heating the fiber membrane obtained by electrospinning to perform thermal imidization to obtain a polyimide fiber membrane; soaking the polyimide fiber membrane in a strong alkali solution to perform surface hydrolysis and ring opening, and in a strong acid solution to perform 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 (KH-550), subjected to ultrasonic treatment, taken out, washed and dried, and a highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive voltage-sensitive membrane is obtained.

2. The method for preparing the 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 (TFDB) and 2,2'-bis(3,4-dicarboxylic acid)hexafluoropropane dianhydride (6FDA), respectively, and the molar ratio of 6FDA to TFDB is 1:

1.

3. The method for preparing the 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 the highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive pressure-sensitive film according to claim 1, characterized in that: 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.

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 alkali solution is water, and the concentration of the strong alkali 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, KH-550 and ethanol is 1:3:4-8, preferably 1:3:

6.

8. The method for preparing the 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, preferably 3 hours.

9. 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 8.

10. A flexible pressure sensor, characterized in that: The highly reliable and stable polyimide-carboxylated carbon nanotube composite conductive voltage-sensitive film as claimed in claim 9 is used.

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