Polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse mold and preparation method of polyimide-carbon nanotube composite conductive pressure-sensitive film

Abstract

The invention discloses a reverse mold-based polyimide-carbon nanotube composite conductive pressure-sensitive film and a preparation method thereof. The preparation method comprises the following steps: firstly, obtaining a precursor polyamide acid solution of soluble polyimide, and meanwhile, carrying out functional modification treatment on carboxylated carbon nanotubes by adopting a diamine material to be grafted to obtain modified carbon nanotubes; mixing the precursor polyamide acid solution with the modified carbon nanotubes, and adjusting the viscosity to obtain a membrane preparation solution; and taking a cloth carrying the microstructure as a model, obtaining a surface microstructure mold based on a reverse mold process, adding the film preparation liquid into the mold, drying, and carrying out gradient heating thermal imidization to obtain the polyimide-carbon nanotube composite conductive pressure-sensitive film. The piezoresistive sensor prepared based on the composite conductive pressure-sensitive film has high sensitivity, excellent stability and resilience, and has a wide application prospect in the field of flexible sensing devices.

Description

Technical Field

[0001] The present invention belongs to the field of preparation of pressure - sensitive membranes, and particularly relates to a polyimide - carbon nanotube composite conductive pressure - sensitive membrane based on an inverse mold and a preparation method thereof. The material has uniform mechanical and electrical properties, excellent and stable piezoresistive properties. Background Technique

[0002] With the development of science and technology, wearable electronic devices have attracted much attention due to their great application potential in fields such as health monitoring, human - machine interfaces, and soft robots, and they have flexible and diverse application scenarios. Compared with traditional electronic devices, the unique flexibility and ductility of flexible electronic devices enable them to adapt to more complex working environments. Flexible pressure sensors have the advantages of good flexibility and high sensitivity, can be better fitted with flexible interfaces, and are an excellent choice for realizing pressure detection on special - shaped interfaces. They can adapt to complex non - flat surfaces and withstand large - scale deformations such as stretching, bending, and even folding. As a high - stiffness flexible substrate, polyimide is an excellent choice for industrial flexible sensors. It is stable, has excellent mechanical properties, and its high glass transition temperature makes its anti - creep performance excellent. However, its characteristics of being difficult to dissolve and the difficulty in ensuring the uniformity of its composites make it difficult to be widely applied. Based on this, the present invention provides a pressure - sensitive membrane material based on polyimide with excellent piezoresistive sensing performance. This material is a specific surface microstructure film obtained by mixing a soluble polyimide with carbon nanotubes modified by a specific method and based on an inverse - mold process. It has excellent pressure - sensitive performance and is expected to promote the development of flexible piezoresistive sensor devices. Summary of the Invention

[0003] The purpose of the present invention is to provide, in view of the deficiencies of the prior art, a polyimide - carbon nanotube composite conductive pressure - sensitive membrane based on an inverse mold and a preparation method thereof. This solution can solve the problems of the uniformity of mechanical and electrical properties and the device response speed of polyimide - based conductive composites, thereby improving the sensitivity, repeatability, and long - term use stability of flexible sensors, facilitating industrial popularization and production. Moreover, its high glass transition temperature makes its anti - creep performance better, improving the signal stability and long - term reliability when the conductive pressure - sensitive membrane is subjected to pressure loading. The soluble polyimide substrate makes it convenient for secondary processing.

[0004] To achieve the above - mentioned purpose, the present invention adopts the following technical solutions:

[0005] A preparation method of a polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding. First, a precursor polyamic acid solution of soluble polyimide is obtained. At the same time, the carboxylated carbon nanotubes are functionalized and modified with the diamine material to be grafted to obtain modified carbon nanotubes. The precursor polyamic acid solution and the modified carbon nanotubes are mixed and the viscosity is adjusted to obtain a film-forming solution. Based on the reverse molding process, a surface microstructure mold is obtained with a cloth carrying the microstructure. The film-forming solution is added to the mold, the solvent is dried, and gradient temperature rise thermal imidization is carried out to obtain the polyimide-carbon nanotube composite conductive pressure-sensitive film.

[0006] In the above technical solution, further, for the precursor polyamic acid solution, its preparation method includes: dissolving 1,3-bis(3-aminophenoxy)benzene (BAB) and 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride (ODPA) in N,N-dimethylformamide (DMF), and continuously stirring at 0-10 °C to obtain a precursor polyamic acid (PAA) solution.

[0007] Further, the molar ratio of BAB to ODPA is 1:1, and the reaction solid content is 15 wt%.

[0008] Further, the functionalization and modification treatment specifically includes:

[0009] Disperse the carboxylated carbon nanotubes, catalyst and condensing agent in a solvent to activate the carboxyl group to obtain dispersion liquid A; weigh the diamine material to be grafted and dissolve it in a solvent to obtain solution B; add solution B to dispersion liquid A for condensation reaction; after the reaction ends, filter out the solvent, wash successively with ethanol and pure water, carry out suction filtration, and dry.

[0010] Further, the carboxylation degree of the carboxylated carbon nanotubes does not exceed 3.9 wt%, which is more conducive to obtaining excellent dispersibility, and is preferably 1.3 wt%. The solvent is at least one of N-methylpyrrolidone (NMP) and ethylenediamine (EDA). The condensing agent is 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDCI), and the catalyst is 1-hydroxybenzotriazole (HOBt).

[0011] Further, the diamine material to be grafted is one of 1,3-bis(3-aminophenoxy)benzene (BAB) and ethylenediamine (EDA).

[0012] Further, the proportion of the modified carbon nanotubes in the total mass of the precursor polyamic acid solution monomer and the modified carbon nanotubes is 7-10 wt%.

[0013] After modifying carbon nanotubes by the method of the present invention, especially when carbon nanotubes with a carboxylation degree of 1.3 wt% are selected and graft modification is carried out using BAB, the dispersibility of the obtained modified functionalized carbon nanotubes in the polyimide substrate and the electrical conductivity of the composite material prepared therefrom are much better than those of the carboxylated carbon nanotubes before grafting, and less conductive phase can be added to the pressure-sensitive film without sacrificing electrical conductivity.

[0014] Furthermore, after laying the cloth carrying the microstructure flat in a container and pouring in the defoamed PDMS, leveling, curing, and demolding, a surface microstructure mold is obtained. The cloth used in the casting process is usually any one of sponge cloth, fiber cloth 1 (i.e., plain weave fabric), fiber cloth 2 (i.e., satin weave fabric), and fiber cloth 3 (i.e., twill weave fabric). Using this type of structure has significant advantages over other types of microstructures in terms of the properties of the obtained film, especially when using fiber cloth 3, the effect is very excellent.

[0015] A polyimide-carbon nanotube composite conductive pressure-sensitive film is prepared by the method described in any one of the above.

[0016] A flexible pressure sensor contains the above-mentioned composite conductive pressure-sensitive film.

[0017] The beneficial effects of the present invention are as follows:

[0018] The present invention selects the rigid material polyimide containing a flexible group (ether bond), making the polyimide substrate material have excellent solubility characteristics and improving the problem that polyimide-based devices are difficult to process. At the same time, carbon nanotubes, a carbon material with better electrical conductivity and thermal conductivity, are used to prepare conductive pressure-sensitive materials. In particular, carbon nanotubes are chemically grafted with diamines having a structure similar to the substrate and carrying active groups, obtaining a conductive composite material with better compatibility, dispersibility, and electrical conductivity with the organic substrate, and as little conductive phase as possible can be added without sacrificing the performance of the pressure-sensitive film. In addition, in the present invention, a cloth with a small, uniform, and orderly structure is selected for casting. When pressed, the conductive path can undergo more area changes, effectively improving the sensitivity of the pressure-sensitive material to convert pressure into electrical signals. At the same time, the pressure-sensitive film has excellent anti-creep and anti-relaxation properties. The pressure-sensitive film has excellent pressure-sensitive properties and has very good sensitivity, stability, and recovery as a flexible pressure sensor; in addition, since the present invention uses a flexible sensing substrate with excellent solubility, the problem of repeated processing of flexible sensors is solved. Description of the Drawings

[0019] Figure 1 . Flow chart for the preparation of polyimide-functionalized carbon nanotube composite conductive pressure-sensitive film;

[0020] Figure 2. Structural design and preparation flow chart of functional chemical grafting of high-Tg soluble polyimide and carboxylated carbon nanotubes;

[0021] Figure 3 . Glass transition temperature of polyimide and polyimide-functional carbon nanotube composite conductive films;

[0022] Figure 4 . Cross-sectional SEM photos of polyimide-functional carbon nanotube composite conductive films with carbon tubes of different carboxylation degrees grafted with different diamines;

[0023] Figure 5 . Curve graph of resistivity vs. carbon tube mass fraction of polyimide-functional carbon nanotube composite conductive composites prepared with BAB-grafted functional carbon tubes;

[0024] Figure 6 . SEM photos of polyimide-functional carbon nanotube composite conductive films carrying different microstructures;

[0025] Figure 7 . Current-pressure curves of different cloth reverse mold polyimide-functionalized carbon nanotube composite piezoresistive films. Specific implementation manners

[0026] The technical solutions of the present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments:

[0027] In the preparation method of the soluble polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding of the present invention, for the synthesis of polyamic acid, all raw materials are dissolved and continuously stirred at 0-10 °C for at least 12 hours. When preparing functionalized carbon nanotubes, in order to carry out chemical grafting on all carboxyl groups on the carboxylated carbon nanotubes, the carboxylated carbon nanotubes are added to a solution dissolved with an excessive amount of condensing agent and an appropriate amount of catalyst and dispersed sufficiently to activate the carboxyl groups; the pre-grafted diamine is taken in an excessive amount and fully dissolved in a solvent; the activated carboxylated carbon nanotube suspension is mixed with the solution dissolved with the pre-grafted diamine and stirred evenly, and placed in a water bath at 40 °C and stirred for 24 h. The reaction temperature can be appropriately increased to accelerate the reaction rate. After the reaction is completed, the solvent is filtered off, and the solid is successively suction-filtered and washed with ethanol and pure water for multiple times to remove excess reactants, condensing agent, catalyst and solvent, and dried in an oven at 60 °C and stored sealed. Calculate the addition amount of the required functionalized carbon nanotubes based on the solid content of the polyamic acid solution and the mass fraction of the functionalized carbon nanotubes in the polyamic acid. Mix the polyamic acid solution and the functionalized carbon nanotubes in a certain proportion, disperse them sufficiently with the help of an ultra-high-speed mixer, add N,N-dimethylformamide (DMF) to dilute to the required viscosity, and store in an environment of 0-10 °C. Drop the polyamic acid-functionalized carbon nanotube-based conductive composite material dispersion into a pre-prepared PDMS microstructure mold, dry the solvent on a hot stage at 60 °C, and then perform thermal imidization by gradient heating. According to an example of the present invention, the heating and holding program can be: 90 °C / 1 h, 120 °C / 1 h, 150 °C / 1 h, 180 °C / 1 h, 210 °C / 1 h, 240 °C / 1 h, 270 °C / 1 h, 300 °C / 1 h. After the imidization program is completed, cool to room temperature to obtain the soluble polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding.

[0028] The soluble polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding of the present invention has excellent anti-creep performance and anti-relaxation performance (glass transition temperature is 173-178 °C) and good solubility through the selection of matrix groups and the treatment and dispersion of chemically grafted carbon nanotubes. The conductive composite pressure-sensitive film prepared by reverse molding with a fiber cloth carrying a uniform and orderly microstructure has excellent piezoresistive performance.

[0029] The test method for the resistivity of the composite conductive pressure-sensitive film is: use a four-probe resistivity tester to measure the resistivity of the film. The measured resistivity range of the soluble polyimide-carbon nanotube composite conductive film based on reverse molding is adjusted within 2-140 kΩ·cm.

[0030] The test method for the pressure sensing of the composite conductive pressure-sensitive film is:

[0031] Prepare an electrode layer on the flexible protective layer. Connect the electrode lead wire to a digital source meter to read its electrical signal. Place the microstructure surface of the circular pressure-sensitive layer on the electrode layer in contact with it. Finally, cover it with another layer of flexible protective layer. Glue the edges of the two protective layers with a double-sided adhesive layer to complete the encapsulation and obtain a flexible piezoresistive sensor. Apply pressure to the sensor using a push-pull gauge and record the change in current while changing the applied pressure. By establishing a relationship curve between the electrical signal and the pressure, the performance of a series of piezoresistive sensors can be measured.

[0032] Example 1

[0033] (1) After dissolving 2.923 g of BAB and 3.102 g of ODPA (i.e., the molar ratio of BAB:ODPA is 1:1) successively in 34.142 g of N,N-dimethylformamide (DMF) (solid content 15 wt%), continuously stir at 0 - 10 °C for 12 h to obtain a polyamic acid solution, and store it in an environment of 0 - 10 °C.

[0034] (2) Take the polyamic acid solution and drop it on a glass slide. Let it stand flat on a plane. After the liquid has spread evenly, place it on a hot stage and evaporate part of the solvent at 60 °C for 2 h, then gradually raise the temperature to dry the solvent and perform thermal imidization. The temperature increase program is: 90 °C / 1 h, 120 °C / 1 h, 150 °C / 1 h, 180 °C / 1 h, 210 °C / 1 h, 240 °C / 1 h, 270 °C / 1 h, 300 °C / 1 h. After the imidization program ends, cool it to room temperature to obtain a polyimide film with a film thickness of about 0.1 mm.

[0035] (3) Use a differential scanning calorimeter (DSC) to measure the glass transition temperature of the polyimide film to be 173 °C.

[0036] (4) Dissolve the polyimide film in N-methylpyrrolidone (NMP), N,N-dimethylformamide (DMF), and tetrahydrofuran (THF) respectively, and stir at 40 °C to test its solubility. It can be completely dissolved in several common solvents within 1 h.

[0037] Example 2

[0038] (1) After dissolving 2.923 g of BAB and 3.102 g of ODPA (i.e., the molar ratio of BAB:ODPA is 1:1) successively in 34.142 g of N,N-dimethylformamide (DMF) (solid content 15 wt%), continuously stir at 0 - 10 °C for 12 h to obtain a polyamic acid solution, and store it in an environment of 0 - 10 °C.

[0039] (2) The carbon nanotubes with a carboxylation degree of 1.3 wt% and the condensing agent EDCI and the catalyst HOBt were dispersed in N-methylpyrrolidone (NMP) and stirred well (the molar ratio of carboxyl groups in carbon nanotubes to EDCI was 1:10) to obtain a carbon nanotube dispersion; BAB was weighed and dissolved in N-methylpyrrolidone (NMP), and stirred until completely dissolved to obtain a diamine solution; the diamine solution was added to the activated carboxylated carbon nanotube dispersion, and the mixture was stirred at 40 °C for 12 h for a condensation reaction to obtain chemically grafted functionalized carbon nanotubes; after the reaction, the solvent was filtered off, and the solid was successively suction-filtered and washed with ethanol and pure water, and the process was repeated three times, and then dried at 60 °C and stored in a sealed manner.

[0040] (3) 1.75 g of polyamic acid solution was taken, and 0.026 g of functionalized carbon nanotubes (the functionalized carbon nanotubes accounted for 9 wt% of the polyamic acid solid) was added, and the mixture was uniformly mixed with an ultra-high speed mixer at a rotation speed of 2500 r / min for 5 min. Then, an appropriate amount of N,N-dimethylformamide (DMF) could be added to adjust the viscosity of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion to facilitate controlling the liquid amount for subsequent film formation by a dropper. After stirring evenly, it was stored in an environment of 0-10 °C.

[0041] (4) A clean sponge cloth was laid flat and adhered in a petri dish, PDMS was left to stand for defoaming and then poured into the petri dish with the adhered sponge cloth, and it was placed in a vacuum oven at 60 °C for 24 h until completely cured and taken out of the mold to obtain a PDMS mold with a sponge cloth microstructure.

[0042] (5) 4 ml of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion obtained in step (3) was dropped into the PDMS microstructure mold prepared in step (4). After evaporating part of the solvent at 60 °C for 2 h, the solvent was dried by gradient heating and thermal imidization was carried out. The heating program was: 90 °C / 1 h, 120 °C / 1 h, 150 °C / 1 h, 180 °C / 1 h, 210 °C / 1 h, 240 °C / 1 h, 270 °C / 1 h, 300 °C / 1 h. After the imidization program ended, it was cooled to room temperature to obtain a soluble polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding, and the film thickness was about 0.1 mm.

[0043] (6) The glass transition temperature of the polyimide film was measured to be 177.7 °C by a differential scanning calorimeter (DSC).

[0044] (7) The surface morphology of the film sample was observed by a field emission scanning electron microscope (SEM), and it could be observed that the microstructure prepared by the sponge cloth reverse molding was as shown in Figure 6 (a). The cross-sectional morphology of the film sample was observed by a field emission scanning electron microscope (SEM), and it could be observed that the dispersion of the functionalized carbon nanotubes was as shown in Figure 4 (f). Compared with the carboxylated carbon nanotubes before chemical graftingFigure 4 Comparing with (d) in the figure, it can be seen that its dispersibility is significantly improved and there is no obvious agglomeration.

[0045] (8) The resistivity was measured by the four-probe method to be about 2.8 kΩ·cm. The resistivity-carbon nanotube mass fraction curve obtained by changing the content of functionalized carbon nanotubes is as Figure 5 shown.

[0046] Example 3

[0047] (1) First, 2.923 g of BAB and 3.102 g of ODPA (i.e., the molar ratio of BAB:ODPA is 1:1) were successively dissolved in 34.142 g of N,N-dimethylformamide (DMF) (solid content 15 wt%), and then continuously stirred at 0 - 10 °C for 12 h to obtain a polyamic acid solution, which was stored in an environment of 0 - 10 °C.

[0048] (2) Carbon nanotubes with a carboxylation degree of 1.3 wt% and a condensing agent EDCI and a catalyst HOBt were dispersed in N-methylpyrrolidone (NMP) and stirred well (the molar ratio of carboxyl groups in carbon nanotubes:EDCI is 1:10) to obtain a carbon nanotube dispersion; BAB was weighed and dissolved in N-methylpyrrolidone (NMP), and stirred until completely dissolved to obtain a diamine solution; the diamine solution was added to the activated carboxylated carbon nanotube dispersion, and the mixture was stirred at 40 °C for 12 h for a condensation reaction to obtain chemically grafted functionalized carbon nanotubes; after the reaction, the solvent was filtered off, and the solid was successively filtered and washed with ethanol and pure water three times, and then dried at 60 °C and stored in a sealed manner.

[0049] (3) Take 1.75 g of the polyamic acid solution, add 0.026 g of functionalized carbon nanotubes (the functionalized carbon nanotubes account for 9 wt% of the polyamic acid solid), mix well with an ultra-high speed mixer at a rotation speed of 2500 r / min for 5 min, and then an appropriate amount of N,N-dimethylformamide (DMF) can be added to adjust the viscosity of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion to facilitate controlling the liquid volume of subsequent film formation by a dropper, stir well, and store in an environment of 0 - 10 °C.

[0050] (4) A clean fiber cloth 1 was laid flat and bonded in a petri dish, PDMS was left to stand and defoamed, and then poured into the petri dish with the fiber cloth 1 adhered, placed in a vacuum oven at 60 °C for 24 h, taken out after complete curing, and demolded to obtain a PDMS mold with the microstructure of fiber cloth 1.

[0051] (5) Take 4 ml of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion obtained in step (3) and drop it into the PDMS microstructure mold prepared in step (4). After evaporating part of the solvent at 60 °C for 2 h, the solvent is dried by gradient heating and thermal imidization is carried out. The heating program is as follows: 90 °C for 1 h, 120 °C for 1 h, 150 °C for 1 h, 180 °C for 1 h, 210 °C for 1 h, 240 °C for 1 h, 270 °C for 1 h, 300 °C for 1 h. After the imidization program is completed, it is cooled to room temperature to obtain a soluble polyimide-carbon nanotube composite conductive pressure-sensitive film based on the reverse mold, and the film thickness is about 0.1 mm.

[0052] (6) Use a differential scanning calorimeter (DSC) to measure the glass transition temperature of the polyimide film to be 177.7 °C.

[0053] (7) Use a field emission scanning electron microscope (SEM) to observe the surface morphology of the thin film sample. It can be observed that the microstructure obtained by reverse molding of the fiber cloth 1 is as shown in Figure 6 (b) in the figure. Use a field emission scanning electron microscope (SEM) to observe the cross-sectional morphology of the thin film sample. It can be observed that compared with the carboxylated carbon nanotubes before chemical grafting, the dispersion of the functionalized carbon nanotubes is significantly improved and there is no obvious agglomeration.

[0054] Example 4

[0055] (1) First, dissolve 2.923 g of BAB and 3.102 g of ODPA (i.e., the molar ratio of BAB:ODPA is 1:1) in 34.142 g of N,N-dimethylformamide (DMF) (solid content is 15 wt%) successively, and then continuously stir at 0 - 10 °C for 12 h to obtain a polyamic acid solution, which is stored in an environment of 0 - 10 °C.

[0056] (2) Disperse carbon nanotubes with a carboxylation degree of 1.3 wt% and a condensing agent EDCI and a catalyst HOBt in N-methylpyrrolidone (NMP) and stir well (the molar ratio of carboxyl groups in carbon nanotubes:EDCI is 1:10) to obtain a carbon nanotube dispersion; weigh BAB and dissolve it in N-methylpyrrolidone (NMP), stir until completely dissolved to obtain a diamine solution; add the diamine solution to the activated carboxylated carbon nanotube dispersion, stir at 40 °C for 12 h for a condensation reaction to obtain chemically grafted functionalized carbon nanotubes; after the reaction is completed, filter off the solvent, and the solid is successively filtered and washed with ethanol and pure water three times, and then dried at 60 °C and stored in a sealed manner.

[0057] (3) Take 1.75 g of the polyamic acid solution, add 0.026 g of functionalized carbon nanotubes (the functionalized carbon nanotubes account for 9 wt% of the polyamic acid solid), mix evenly with an ultra-high speed mixer at a rotation speed of 2500 r / min for 5 min. Then, an appropriate amount of N,N-dimethylformamide (DMF) can be added to adjust the viscosity of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion liquid to facilitate controlling the liquid volume for subsequent film formation through a dropper. Stir evenly and store it in an environment of 0 - 10 °C.

[0058] (4) Lay the clean fiber cloth 2 flat and bond it in a petri dish. After the PDMS is left to stand and defoamed, pour it into the petri dish with the fiber cloth 2 adhered, place it in a vacuum oven at 60 °C for 24 h, take it out after complete curing, and demold to obtain a PDMS mold with the microstructure of the fiber cloth 2.

[0059] (5) Take 4 ml of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion liquid obtained in step (3) and drop it into the PDMS microstructure mold prepared in step (4). After evaporating part of the solvent at 60 °C for 2 h, gradually raise the temperature to dry the solvent and perform thermal imidization. The temperature rising program is: 90 °C / 1 h, 120 °C / 1 h, 150 °C / 1 h, 180 °C / 1 h, 210 °C / 1 h, 240 °C / 1 h, 270 °C / 1 h, 300 °C / 1 h. After the imidization program ends, cool it to room temperature to obtain a soluble polyimide-carbon nanotube composite conductive pressure-sensitive film based on the reverse mold, and the film thickness is about 0.1 mm.

[0060] (6) Use a differential scanning calorimeter (DSC) to measure that the glass transition temperature of the polyimide film is 177.7 °C.

[0061] (7) Use a field emission scanning electron microscope (SEM) to observe the surface morphology of the film sample. It can be observed that the microstructure obtained by reverse molding of the fiber cloth 2 is as shown in (c) below. Use a field emission scanning electron microscope (SEM) to observe the cross-sectional morphology of the film sample. It can be observed that the dispersion of the functionalized carbon nanotubes is significantly improved compared with that of the carboxylated carbon nanotubes before chemical grafting, and there is no obvious agglomeration. Figure 6 as shown in (c) below. Use a field emission scanning electron microscope (SEM) to observe the cross-sectional morphology of the film sample. It can be observed that the dispersion of the functionalized carbon nanotubes is significantly improved compared with that of the carboxylated carbon nanotubes before chemical grafting, and there is no obvious agglomeration.

[0062] Example 5

[0063] (1) Dissolve 2.923 g of BAB and 3.102 g of ODPA (i.e., the molar ratio of BAB:ODPA is 1:1) successively in 34.142 g of N,N-dimethylformamide (DMF) (the solid content is 15 wt%), and continuously stir at 0 - 10 °C for 12 h to obtain a polyamic acid solution, and store it in an environment of 0 - 10 °C.

[0064] (2) The carbon nanotubes with a carboxylation degree of 1.3 wt% and the condensing agent EDCI and the catalyst HOBt were dispersed in N-methylpyrrolidone (NMP) and stirred thoroughly (the molar ratio of carboxyl groups in carbon nanotubes to EDCI was 1:10) to obtain a carbon nanotube dispersion; BAB was weighed and dissolved in N-methylpyrrolidone (NMP), and stirred until completely dissolved to obtain a diamine solution; the diamine solution was added to the activated carboxylated carbon nanotube dispersion, and stirred at 40 °C for 12 h for a condensation reaction to obtain chemically grafted functionalized carbon nanotubes; after the reaction, the solvent was filtered off, and the solid was successively suction-filtered and washed with ethanol and pure water, repeated three times, and then dried at 60 °C and stored sealed.

[0065] (3) Take 1.75 g of polyamic acid solution, add 0.026 g of functionalized carbon nanotubes (the functionalized carbon nanotubes account for 9 wt% of the polyamic acid solid), mix evenly with an ultra-speed mixer at a rotation speed of 2500 r / min for 5 min, and then an appropriate amount of N,N-dimethylformamide (DMF) can be added to adjust the viscosity of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion to facilitate controlling the liquid volume of the subsequent film formation by a dropper, stir evenly, and store in an environment of 0-10 °C.

[0066] (4) The clean fiber cloth 3 was laid and adhered in a petri dish, PDMS was left to stand for defoaming and then poured into the petri dish with the adhered fiber cloth 3, placed in a vacuum oven at 60 °C for 24 h, taken out after complete curing, and demolded to obtain a PDMS mold carrying the microstructure of the fiber cloth 3.

[0067] (5) Take 4 ml of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion obtained in step (3) and drop it into the PDMS microstructure mold prepared in step (4). After evaporating part of the solvent at 60 °C for 2 h, the solvent was dried by gradient heating and thermal imidization was carried out. The heating program was: 90 °C / 1 h, 120 °C / 1 h, 150 °C / 1 h, 180 °C / 1 h, 210 °C / 1 h, 240 °C / 1 h, 270 °C / 1 h, 300 °C / 1 h. After the imidization program ended, it was cooled to room temperature to obtain a soluble polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding, and the film thickness was about 0.1 mm.

[0068] (6) The glass transition temperature of the polyimide film was measured to be 177.7 °C using a differential scanning calorimeter (DSC).

[0069] (7) The surface morphology of the film sample was observed using a field emission scanning electron microscope (SEM), and the microstructure obtained by reverse molding of the fiber cloth 3 could be observed as Figure 6As shown in (d-f). The cross-sectional morphology of the thin film sample was observed using a field emission scanning electron microscope (SEM). It can be observed that the dispersibility of the functionalized carbon nanotubes is significantly improved compared to the carboxylated carbon nanotubes before chemical grafting, and there is no obvious agglomeration.

[0070] Example 6

[0071] (1) After dissolving 2.923 g of BAB and 3.102 g of ODPA (i.e., the molar ratio of BAB:ODPA is 1:1) successively in 34.142 g of N,N-dimethylformamide (DMF) (solid content is 15 wt%), continuously stir at 0-10 °C for 12 h to obtain a polyamic acid solution, and store it in an environment of 0-10 °C.

[0072] (2) Disperse carbon nanotubes with a carboxylation degree of 1.3 wt% and the condensing agent EDCI and the catalyst HOBt in ethylenediamine (EDA) and stir well (the molar ratio of carboxyl groups in carbon nanotubes to EDCI is 1:10; the ratio of carboxyl groups in carbon nanotubes to EDA is 100 mg:100 ml), stir at 40 °C for 12 h to carry out a condensation reaction to obtain chemically grafted functionalized carbon nanotubes; after the reaction, filter off the solvent, and the solid is successively filtered and washed with ethanol and pure water, repeated three times, and then dried at 60 °C and stored in a sealed manner.

[0073] (3) Take 1.75 g of the polyamic acid solution, add 0.016 g of functionalized carbon nanotubes (the functionalized carbon nanotubes account for 7 wt% of the polyamic acid solid), mix evenly with an ultra-high-speed mixer at a rotation speed of 2500 r / min for 5 min, and then add an appropriate amount of N,N-dimethylformamide (DMF) to adjust the viscosity of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion liquid to facilitate controlling the liquid volume for subsequent film formation by a dropper, stir evenly, and store it in an environment of 0-10 °C.

[0074] (4) Lay the clean fiber cloth 3 flat and bond it in a petri dish, pour the PDMS after standing and defoaming into the petri dish with the fiber cloth 3 adhered, place it in a vacuum oven at 60 °C for 24 h, take it out after complete curing, demold, and obtain a PDMS mold carrying the microstructure of the fiber cloth 3.

[0075] (5) Take 4 ml of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion obtained in step (3) and drop it into the PDMS microstructure mold prepared in step (4). After evaporating part of the solvent at 60 °C for 2 h, the solvent is dried by gradient heating and thermal imidization is carried out. The heating program is: 90 °C / 1 h, 120 °C / 1 h, 150 °C / 1 h, 180 °C / 1 h, 210 °C / 1 h, 240 °C / 1 h, 270 °C / 1 h, 300 °C / 1 h. After the imidization program ends, it is cooled to room temperature to obtain a soluble polyimide-carbon nanotube composite conductive pressure-sensitive film with a high glass transition temperature, and the film thickness is about 0.1 mm.

[0076] (6) Use a field emission scanning electron microscope (SEM) to observe the cross-sectional morphology of the thin film sample, and the dispersion of the functionalized carbon nanotubes can be observed. Figure 4 In (e). Compared with the carboxylated carbon nanotubes before chemical grafting Figure 4 In (d), it can be seen that its dispersibility has been improved to a certain extent, but there are still obvious agglomeration phenomena. Measure its resistivity with the four-probe method to be about 10 kΩ·cm.

[0077] Example 7

[0078] (1) Dissolve 2.923 g of BAB and 3.102 g of ODPA (i.e., the molar ratio of BAB:ODPA is 1:1) successively in 34.142 g of N,N-dimethylformamide (DMF) (solid content is 15 wt%), and continuously stir at 0-10 °C for 12 h to obtain a polyamic acid solution, which is stored in an environment of 0-10 °C.

[0079] (2) Disperse carbon nanotubes with a carboxylation degree of 3.9 wt% and a condensing agent EDCI and a catalyst HOBt in N-methylpyrrolidone (NMP) and stir well to disperse (the molar ratio of carboxyl groups in carbon nanotubes:EDCI is 1:10) to obtain a carbon nanotube dispersion; weigh BAB (the molar ratio of carboxyl groups in carbon nanotubes:BAB is 1:10), dissolve it in N-methylpyrrolidone (NMP), and stir until completely dissolved to obtain a diamine solution; add the diamine solution to the activated carboxylated carbon nanotube dispersion, stir at 40 °C for 12 h to carry out a condensation reaction to obtain chemically grafted functionalized carbon nanotubes; after the reaction ends, filter off the solvent, and the solid is successively filtered and washed with ethanol and pure water, repeated three times, and then dried at 60 °C and stored in a sealed manner.

[0080] (3) Take 1.75 g of the polyamic acid solution, add 0.024 g of functionalized carbon nanotubes (the functionalized carbon nanotubes account for 9 wt% of the polyamic acid solid), mix evenly with an ultra-high-speed mixer at a rotation speed of 2500 r / min for 5 min, then add an appropriate amount of N,N-dimethylformamide (DMF) to adjust the viscosity of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion to facilitate controlling the liquid volume for subsequent film formation by a dropper, stir evenly, and store in an environment of 0 - 10°C.

[0081] (4) Lay the clean fiber cloth 3 flat and bond it in a petri dish. After standing the PDMS to defoam, pour it into the petri dish with the fiber cloth 3 adhered, place it in a vacuum oven at 60°C for 24 h, take it out after complete curing, and demold to obtain a PDMS mold with the microstructure of the fiber cloth 3.

[0082] (5) Take 4 ml of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion obtained in step (3) and drop it into the PDMS microstructure mold prepared in step (4). After evaporating part of the solvent at 60°C for 2 h, gradually raise the temperature to dry the solvent and conduct thermal imidization. The temperature increase program is: 90°C for 1 h, 120°C for 1 h, 150°C for 1 h, 180°C for 1 h, 210°C for 1 h, 240°C for 1 h, 270°C for 1 h, 300°C for 1 h. After the imidization program ends, cool it to room temperature to obtain a soluble polyimide-carbon nanotube composite conductive pressure-sensitive film with a high glass transition temperature, and the film thickness is about 0.1 mm.

[0083] (6) Use a field emission scanning electron microscope (SEM) to observe the cross-sectional morphology of the thin film sample, and the dispersion of the functionalized carbon nanotubes can be observed Figure 4 in (c). Compared with the carboxylated carbon nanotubes before chemical grafting Figure 4 in (a), it can be seen that its dispersibility is significantly improved, but there is still some agglomeration. Measure its resistivity with the four-probe method to be about 3.2 kΩ·cm.

[0084] Example 8

[0085] (1) Dissolve 2.923 g of BAB and 3.102 g of ODPA (i.e., the molar ratio of BAB:ODPA is 1:1) successively in 34.142 g of N,N-dimethylformamide (DMF) (solid content is 15 wt%), and then continuously stir at 0 - 10°C for 12 h to obtain a polyamic acid solution, and store it in an environment of 0 - 10°C.

[0086] (2) The carbon nanotubes with a carboxylation degree of 3.9 wt% and the condensing agent EDCI and the catalyst HOBt were dispersed in ethylenediamine (EDA) and stirred well (the molar ratio of carboxyl groups in carbon nanotubes to EDCI was 1:10; the ratio of carboxyl groups in carbon nanotubes to EDA was 100 mg:100 ml). Stir at 40 °C for 12 h to carry out the condensation reaction to obtain chemically grafted functionalized carbon nanotubes; after the reaction, filter out the solvent, and the solid was successively filtered and washed with ethanol and pure water, repeated three times, and then dried at 60 °C and stored sealed.

[0087] (3) Take 1.75 g of the polyamic acid solution, add 0.024 g of functionalized carbon nanotubes (the functionalized carbon nanotubes account for 9 wt% of the polyamic acid solid), mix well with an ultra-high-speed mixer at a rotation speed of 2500 r / min for 5 min, and then add an appropriate amount of N,N-dimethylformamide (DMF) to adjust the viscosity of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion to facilitate controlling the liquid volume of the subsequent film formation by a dropper. Stir evenly and store in an environment of 0 - 10 °C.

[0088] (4) Lay the clean fiber cloth 3 flat and bond it in a petri dish. After standing the PDMS to defoam, pour it into the petri dish with the fiber cloth 3 adhered, place it in a vacuum oven at 60 °C for 24 h, take it out until it is completely cured, and demold to obtain a PDMS mold with the microstructure of the fiber cloth 3.

[0089] (5) Take 4 ml of the polyamic acid-functionalized carbon nanotube-based conductive composite dispersion obtained in step (3) and drop it into the PDMS microstructure mold prepared in step (4). After evaporating part of the solvent at 60 °C for 2 h, heat the solvent to dry and carry out thermal imidization in a gradient manner. The heating program is: 90 °C / 1 h, 120 °C / 1 h, 150 °C / 1 h, 180 °C / 1 h, 210 °C / 1 h, 240 °C / 1 h, 270 °C / 1 h, 300 °C / 1 h. After the imidization program is completed, cool to room temperature to obtain a soluble polyimide-carbon nanotube composite conductive pressure-sensitive film with a high glass transition temperature, and the film thickness is about 0.1 mm.

[0090] (6) Use a field emission scanning electron microscope (SEM) to observe the cross-sectional morphology of the film sample, and the dispersion of the functionalized carbon nanotubes can be observed Figure 4 in (b). Compared with the carboxylated carbon nanotubes before chemical grafting Figure 4 in (a), it can be seen that the improvement of its dispersion is limited.

[0091] In the present invention, a rigid material polyimide containing a flexible group (ether bond) is selected to improve its rigidity, so that the polyimide substrate material has excellent solubility, enabling the preparation of the sensor to be processed according to requirements, thereby solving the problem that the polyimide material is difficult to process. The relatively high glass transition temperature endows it with excellent anti-creep and anti-relaxation properties. Carbon nanotubes with excellent electrical conductivity and thermal conductivity are chemically grafted with a diamine similar to the substrate structure and carrying active groups to obtain a conductive composite material with better compatibility with the organic substrate, better dispersibility and electrical conductivity. A cloth with tiny, uniform and orderly microstructures is selected for reverse molding. The piezoresistive sensor prepared from this composite conductive pressure-sensitive film has high sensitivity, excellent signal stability and recoverability.

[0092] As Figure 5 shown, it is a graph of the resistivity of a polyimide-functional carbon nanotube composite conductive composite material grafted with BAB using 1.3 wt% carboxylated carbon nanotubes versus the mass fraction of carbon nanotubes. As the mass fraction increases, the resistivity of the composite material gradually reaches stability, meeting the requirements for preparing a piezoresistive sensor. Therefore, the mass fraction of carbon nanotubes in the composite material in the examples is selected based on this data. In addition, Figure 3 the glass transition temperatures of the composite materials with various contents shown are used as another selection basis. Among them, the composite material with a 9 wt% content has stable electrical conductivity and a high glass transition temperature. For the composite material, less inorganic conductive filler can, to a certain extent, avoid damage to the performance of the substrate material itself, and uniform dispersion can contribute to a uniform strengthening effect ( Figure 3 , 4 ). For Examples 2-5, 1.3 wt% -COOH-BAB is used as the conductive filler, enhancing the glass transition temperature of the material and contributing to a more uniform dispersion of the conductive filler.

[0093] As Figure 7 shown, it is the pressure sensing test results of the composite conductive pressure-sensitive films prepared by reverse molding with cloths carrying different microstructures in Examples 2-5. It can be seen that the sensor based on this pressure-sensitive film has excellent response ability to the loading and unloading of pressure. The response abilities of the pressure-sensitive films with different microstructures are slightly different. The current signal noise of the pressure-sensitive film prepared by reverse molding with sponge cloth is slightly larger and the recoverability is average; the pressure signal of the pressure-sensitive films prepared by reverse molding with Fiber Cloth 1 and 2 reaches stability quickly, but there are disadvantages of slightly poor signal recovery and slightly weak repeatability; the signal of the pressure-sensitive film prepared by reverse molding with Fiber Cloth 3 is stable, the recoverability is good, the repeatability is excellent, and the response step is smooth.

[0094] The present invention illustrates the detailed method of the present invention through the above embodiments, but the present invention is not limited to the above detailed method, that is, it does not mean that the present invention must rely on the above detailed method to be implemented. Those skilled in the art should understand that any improvement of the present invention, the equivalent replacement of each raw material of the product 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 a polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding, characterized in that, First, obtain the precursor polyamic acid solution of soluble polyimide. At the same time, functionalize the carboxylated carbon nanotubes with the diamine material to be grafted to obtain modified carbon nanotubes. Mix the precursor polyamic acid solution with the modified carbon nanotubes and adjust the viscosity to obtain a film-forming solution. Based on the reverse molding process, use the cloth with microstructures as a model to obtain a surface microstructure mold. Add the film-forming solution into the mold, dry the solvent and perform gradient heating thermal imidization to obtain the polyimide-carbon nanotube composite conductive pressure-sensitive film.

2. The preparation method of the polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding according to claim 1, wherein For the precursor polyamic acid solution, its preparation method includes: dissolving 1,3-bis(3-aminophenoxy)benzene (BAB) and 3,3',4,4'-diphenyl ether tetracarboxylic dianhydride (ODPA) in N,N-dimethylformamide (DMF), and continuously stirring at 0-10°C to obtain the precursor polyamic acid solution.

3. The preparation method of the polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding according to claim 2, characterized in that, The molar ratio of BAB to ODPA is 1:1, and the reaction solid content is 15 wt%.

4. The preparation method of the polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding according to claim 1, wherein, The functionalization modification treatment of the carboxylated carbon nanotubes specifically includes: Disperse the carboxylated carbon nanotubes, condensing agent, and catalyst in a solvent to activate the carboxyl groups to obtain dispersion A; weigh the diamine to be grafted and dissolve it in a solvent to obtain solution B; add solution B into dispersion A for condensation reaction; after the reaction, filter out the solvent, wash successively with ethanol and pure water, perform suction filtration, and dry.

5. The preparation method of the polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding according to claim 4, wherein, The carboxylation degree of the carboxylated carbon nanotubes is not more than 3.9 wt%, and the solvent is at least one of N-methylpyrrolidone (NMP) and ethylenediamine (EDA); the condensing agent is 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide (EDCI), and the catalyst is 1-hydroxybenzotriazole (HOBt).

6. The preparation method of the polyimide-carbon nanotube composite conductive pressure-sensitive film based on inverse molding according to claim 4, characterized in that, The diamine material to be grafted is at least one of 1,3-bis(3-aminophenoxy)benzene (BAB) and ethylenediamine (EDA).

7. The preparation method of the polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding according to claim 1, wherein, The proportion of the modified carbon nanotubes in the total mass of the precursor polyamic acid solution monomers and the modified carbon nanotubes is 7-10 wt%.

8. The preparation method of the polyimide-carbon nanotube composite conductive pressure-sensitive film based on reverse molding according to claim 1, characterized in that, Lay the cloth with microstructures flat in a container, pour in the defoamed PDMS, level it, cure it, and demold it to obtain a surface microstructure mold.

9. A polyimide-carbon nanotube composite conductive pressure-sensitive film, characterized in that, Prepared by the method according to any one of claims 1-8.

10. A flexible piezoresistive sensor, characterized in that, Containing the composite conductive pressure-sensitive film as described in claim 9.

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