PH / temperature dual-responsiveness polyacrylonitrile-based copolymer, preparation method of precursor, preparation method of carbon fiber, carbon fiber and application

Abstract

The invention discloses a pH / temperature dual-responsiveness polyacrylonitrile-based copolymer, a preparation method of a precursor, a preparation method of a carbon fiber, the carbon fiber and application, the preparation method of the pH / temperature dual-responsiveness polyacrylonitrile-based copolymer comprises the following steps: synthesizing the pH / temperature dual-responsiveness polyacrylonitrile-based copolymer; then co-dissolving the copolymer, a cross-linking agent and an initiator in a solvent to prepare a spinning solution; through an innovative'thermally induced phase separation-in-situ crosslinking 'wet spinning technology, fiber forming and crosslinking network construction are synchronously completed in a low-temperature coagulating bath; and finally, carrying out multi-stage thermal crosslinking curing, pre-oxidation and carbonization treatment to obtain the intelligent response type carbon fiber. Through molecular design and process innovation, the field problem that stimuli responsiveness is difficult to retain in the high-temperature carbonization process is successfully solved, and the obtained carbon fiber has excellent mechanical properties and sensitive and reversible pH / temperature dual response characteristics and has great application value in the field of intelligent materials.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of polyacrylonitrile-based carbon fiber preparation, in particular to a pH / temperature dual-responsive polyacrylonitrile-based copolymer, a preparation method of a precursor fiber, a preparation method of carbon fiber, carbon fiber and application. BACKGROUND

[0002] Carbon fiber is widely used in aerospace, sports equipment, automobile industry and other fields due to its high specific strength, high specific modulus, corrosion resistance, good electrical conductivity and other excellent properties. However, the chemical inertness of traditional carbon fiber is high, and its performance is fixed after preparation, which cannot actively respond to external environmental stimuli (such as pH, temperature, light, electric field, etc.), which greatly limits its application in the fields of intelligent structure, bionic actuator, drug controlled release carrier, advanced sensor and other fields.

[0003] Polyacrylonitrile (PAN) is the most mainstream precursor for preparing carbon fiber. At present, some researches have tried to give carbon fiber some functional properties, for example, by surface deposition or grafting to introduce nanomaterials to enhance its electrical conductivity or electromagnetic shielding performance. However, these methods usually have problems such as weak bonding force between modified layer and fiber body, single function, irreversible response or complex preparation process.

[0004] Introducing stimulus-responsive polymers (such as poly (N-isopropyl acrylamide, PNIPAM) into the carbon fiber system is a promising direction. However, direct blending or surface coating faces great challenges: 1) the poor compatibility of responsive polymers such as PNIPAM in PAN spinning dope easily leads to phase separation, affecting spinnability and fiber mechanical properties; 2) during the subsequent high-temperature pre-oxidation and carbonization process, these polymers will completely decompose, and the response characteristics cannot be retained to the final carbon fiber. SUMMARY

[0005] The purpose of the present application is to overcome the above-mentioned deficiencies of the prior art, and to provide a pH / temperature dual-responsive polyacrylonitrile-based copolymer, a preparation method of a precursor fiber, a preparation method of carbon fiber, carbon fiber and application.

[0006] The technical problem of the present application is solved by adopting the following technical solutions.

[0007] The present application provides a pH / temperature dual-responsive polyacrylonitrile-based copolymer, comprising: units from acrylonitrile monomers, units from pH-responsive monomers containing carboxyl groups, and units from temperature-responsive monomers containing amide groups, wherein: the mass ratio of the acrylonitrile monomers, the pH-responsive monomers containing carboxyl groups and the temperature-responsive monomers containing amide groups is (85-92):(3-8):(5-10).

[0008] The application provides a preparation method of polyacrylonitrile-based carbon fiber filaments, comprising the following steps: after preparing the spinning stock solution of the pH / temperature dual-responsive polyacrylonitrile-based copolymer, forming extrusion spinning through a spinneret, and forming into nascent fibers through a coagulation bath.

[0009] The application provides a preparation method of polyacrylonitrile-based carbon fibers with pH / temperature dual responsiveness, comprising the following steps: sequentially performing heat crosslinking solidification treatment, pre-oxidation treatment and carbonization treatment on the nascent fibers to obtain the polyacrylonitrile-based carbon fibers with pH / temperature dual responsiveness.

[0010] The application provides polyacrylonitrile-based carbon fibers with pH / temperature dual responsiveness prepared from the copolymer.

[0011] The application provides applications of the polyacrylonitrile-based carbon fibers with pH / temperature dual responsiveness in intelligent composite materials, sensors, artificial muscles and controllable drug release systems.

[0012] The application has the following beneficial effects: (1) molecular structure innovation: for the first time, the pH response unit (-COOH) and the temperature response unit (-CONH-) are directly bonded to the PAN main chain through molecular copolymerization, which endows the carbon fiber precursor with intrinsic stimulus responsiveness from the molecular source, and solves the fundamental problem of poor physical blending compatibility.

[0013] (2) process innovation - "in-situ crosslinking spinning": the "thermally induced phase separation" coagulation is creatively combined with the "in-situ radical crosslinking" reaction. The low-temperature coagulation bath not only completes the fiber forming, but also ingeniously forms a unique "reactor" in the low-temperature and low-oxygen environment, successfully constructing a stable interpenetrating / crosslinking network at the moment of fiber forming. This network is the key to the preservation of the subsequent responsiveness.

[0014] (3) solving the problem of preserving responsiveness during carbonization: traditional responsive polymers (such as PNIPAM) are completely decomposed above 300 DEG C. Through the multi-stage heat crosslinking solidification process, the stability and heat resistance of the crosslinking network are greatly improved before pre-oxidation, so that it can safely pass through the pre-oxidation and carbonization processes. After carbonization, although the responsive polymer segments will be carbonized and graphitized, the crosslinking point structure and unique disordered carbon structure are preserved, and the original network's stimulus response behavior mechanism is inherited, which is unprecedented.

[0015] (4) function integration and performance synergy: the final carbon fibers not only maintain good mechanical properties (strength can reach 4.5-5.5 GPa, modulus can reach 250-280 GPa), but also have pH and temperature dual response characteristics, and the response is sensitive and reversible, realizing the integration of structure and function.

[0016] (5), wide application prospect: the intelligent carbon fiber prepared by the application provides a key material basis for developing a new generation of intelligent composite materials, sensors, artificial muscles, controllable drug release systems and the like. BRIEF DESCRIPTION OF DRAWINGS

[0017] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed to be used in the embodiments will be briefly introduced as follows. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0018] Fig. 1 The process flow chart of the preparation method of the present application is shown in the figure. Fig. 2 The length change rate curve of the responsive carbon fiber prepared in Example 1 in different pH buffer solutions is shown in the figure. Fig. 3 The length change rate curve of the responsive carbon fiber prepared in Example 1 at different temperatures (pH = 7.4) is shown in the figure. DETAILED DESCRIPTION

[0019] In order to make the purpose, technical solutions and advantages of the embodiments of the present application more clear, the technical solutions in the embodiments of the present application will be clearly and completely described as follows. The specific conditions not mentioned in the embodiments are carried out according to the conventional conditions or the conditions suggested by the manufacturers. The reagents or instruments not mentioned by the manufacturers are all conventional products that can be purchased in the market.

[0020] The preparation method of a pH / temperature dual-responsive polyacrylonitrile-based copolymer, the preparation method of a precursor and the preparation method of a carbon fiber, the carbon fiber and the application provided by the present application will be specifically described as follows.

[0021] In the first aspect, the present application provides a pH / temperature dual-responsive polyacrylonitrile-based copolymer, comprising: units from an acrylonitrile monomer, units from a pH-responsive monomer containing a carboxyl group, and units from a temperature-responsive monomer containing an amide group, wherein: the mass ratio of the acrylonitrile monomer, the pH-responsive monomer containing a carboxyl group and the temperature-responsive monomer containing an amide group is (85-92):(3-8):(5-10).

[0022] When the content of the pH-temperature responsive monomer containing carboxyl is too low, the effective pH responsive site (-COOH) density formed in the fiber is too low. When the pH changes, the change rate of the length or volume of the fiber (ΔL / L) will be very small (for example, <1%), which cannot meet the sensitivity requirements of smart materials for driving or sensing. In addition, a small amount of response groups is not enough to form a continuous and stable response network inside the fiber. Its phase transition behavior will become discontinuous and insignificant, and only has a weak response under very extreme pH conditions (such as pH = 2 or pH = 12), and the response is extremely slow in the range of 4-10, which greatly reduces the practicability. The carboxyl in the pH-temperature responsive monomer containing carboxyl can act as a reaction site during the thermal crosslinking and curing stage, promoting the cyclization or crosslinking reaction between molecular chains, and forming a more stable ladder structure.

[0023] When the content of the pH-temperature responsive monomer containing carboxyl is too high, its hydrophilicity is strong, and a too high content will make the compatibility of the spinning dope and the coagulation bath too high, resulting in a too fast double diffusion process. The fiber skin layer instantaneously forms a too thick and dense structure, which hinders the outflow of the solvent in the inner layer, resulting in the occurrence of huge cavities, finger-like pores or even breakage in the fiber, and the formation of a continuous and dense round fiber. Although the pH response change range may be larger, due to the too poor mechanical properties of the fiber itself, the fiber structure is easily damaged due to insufficient mechanical strength in repeated swelling / shrinking cycles, and the cycle life and durability are poor. Too little pH-temperature responsive monomer containing carboxyl weakens the auxiliary stabilizing effect, slightly reduces the pre-oxidation efficiency, and has an adverse effect on the mechanical properties of the final carbon fiber.

[0024] The content of the temperature responsive monomer containing amide also needs to be accurately controlled. When the content is less than 5%, the temperature responsiveness of the fiber is not significant, the critical phase transition temperature is ambiguous, and effective driving cannot be achieved. When the content is higher than 10%, the mechanical properties and thermal stability of the precursor fiber will be seriously deteriorated, resulting in decomposition and breakage of the precursor fiber during the pre-oxidation and carbonization processes, and the carbon fiber with practical strength cannot be prepared. Therefore, the preferred content range of the present application is 5% to 10%.

[0025] The stimulus-responsive polymer prepared based on direct blending or surface coating faces great challenges. The present application provides a pH / temperature dual-responsive polyacrylonitrile-based copolymer. Through innovative design of the molecular structure, the pH responsive unit (-COOH) and the temperature responsive unit (-CONH-) are directly bonded to the PAN main chain for the first time through molecular copolymerization, the responsive groups are firmly introduced into the PAN molecular chain in the form of chemical bonds, and the stimulus responsiveness is stably integrated in the carbon fiber body. The intrinsic stimulus responsiveness of the carbon fiber precursor is endowed from the molecular source, and the fundamental problem of poor physical blending compatibility is solved.

[0026] In some alternative embodiments, the pH-responsive monomer containing a carboxyl group includes at least one of methacrylic acid (MAA), acrylic acid (AA), and itaconic acid (ITA), and the temperature-responsive monomer containing an amide group is N-isopropylacrylamide (NIPAM).

[0027] Secondly, the present invention provides a method for preparing polyacrylonitrile-based carbon fiber precursor, comprising the following steps: preparing a spinning solution from the above-mentioned pH / temperature dual-responsive polyacrylonitrile-based copolymer, forming an extrusion spinneret through a spinneret, and then solidifying it into nascent fiber through a coagulation bath.

[0028] In some alternative embodiments, the following steps are included: dissolving the polyacrylonitrile-based copolymer in an organic solvent, adding a crosslinking agent and a thermal initiator, stirring vigorously to fully dissolve and disperse it to obtain a uniform and clear spinning solution, and extruding the spinning solution through a spinneret into a low-temperature coagulation bath, while simultaneously performing thermally induced phase separation and in-situ crosslinking reaction to obtain nascent fibers; Preferably, the organic solvent includes dimethyl sulfoxide (DMSO), and the polyacrylonitrile copolymer is dissolved in the organic solvent to form a spinning solution with a concentration of 15-25 wt%. Preferably, the crosslinking agent comprises N,N'-methylenebisacrylamide (MBA), and the crosslinking agent is used in an amount of 1-5% of the total mass of the copolymer; MBA molecules contain two vinyl groups, allowing them to copolymerize with active sites on copolymer chains in the presence of free radicals, thereby connecting different molecular chains to form a three-dimensional network structure. When the thermal initiator is activated in a low-temperature coagulation bath, sulfate radical anions are generated. These free radicals can attack and activate the carbon-carbon double bonds of MBA. The activated MBA molecules then combine with other MBA molecules or macromolecular chain free radicals. Through this reaction, MBA forms covalent bonds between two or more polymer chains, thus constructing a robust three-dimensional network. The double bonds of MBA copolymerize with unreacted monomers or double bonds at the ends of chain segments on the copolymer. More importantly, the amide groups (-CONH-) in its molecule may form hydrogen bonds with the carboxyl groups (-COOH) of MAA, undergoing complex amidation reactions during the high-temperature crosslinking stage, further strengthening the network. In the spinning solution, MBA and the initiator are uniformly dispersed in the polymer solution. During coagulation and in-situ crosslinking, the polymerization of MBA, crosslinking, and phase separation of the polymer phase occur simultaneously. This results in not a simple cross-linking point, but rather a cross-linked network that interpenetrates with the original polymer chain, a structure that is very stable.

[0029] When the MBA content is <1%, the crosslinking density is too low to form a continuous and stable network to "lock in" the temperature-responsive segments (PNIPAM). During the pre-oxidation and carbonization stages, the loose PNIPAM segments decompose and volatilize violently, leading to numerous defects and even breakage in the fiber. Ultimately, the temperature responsiveness cannot be retained in the carbon fiber. When the MBA content is >5%, the excessively high crosslinking density firmly "locks in" the molecular chains, restricting the movement of the segments. Whether it's ionization swelling / shrinkage caused by pH response or segment collapse / stretching caused by temperature response, the space for movement is severely limited. This causes the fiber's response rate to drop sharply, becoming "rigid" and losing its intelligent responsiveness.

[0030] Preferably, the thermal initiator comprises potassium persulfate (KPS), and the amount of the thermal initiator is 0.1-1% of the total mass of the copolymer; When the KPS content is <0.1%, the free radical concentration is too low, resulting in a slow cross-linking reaction rate and a low degree of cross-linking.

[0031] Within the limited spinning time, a sufficiently dense and complete cross-linked network cannot be formed. This "fragile" network cannot effectively lock in responsive polymer segments and will collapse during subsequent pre-oxidation and carbonization processes, leading to loss of responsiveness. Simultaneously, the mechanical properties of the nascent fibers are also poor. Insufficient free radicals cause the cross-linking reaction to occur only in localized areas. Inconsistent cross-linking structures appear within the fiber. This structural inhomogeneity becomes stress concentration points, significantly reducing fiber strength and potentially leading to inconsistent response behavior. When the KPS content >1%, the excessively high free radical concentration causes the cross-linking reaction to proceed explosively within a very short time.

[0032] Spinneret clogging: The cross-linking reaction may occur prematurely inside the spinneret orifice or at the outlet, causing gel to clog the spinneret orifice and preventing continuous spinning. Fiber structure defects: An overly rapid reaction can cause the network to form instantaneously, encapsulating a large amount of solvent and internal stress, resulting in microcracks and voids inside the fiber.

[0033] Preferably, the low-temperature coagulation bath is a mixed solution of DMSO and water at a temperature of 0-10℃.

[0034] This invention provides a method for preparing polyacrylonitrile-based carbon fiber precursor, comprising: degassing and filtering the spinning solution, then conveying it to a spinneret via a metering pump, and extruding it into a low-temperature coagulation bath. In this low-temperature environment, thermally induced phase separation occurs in the solution. Simultaneously, the dissolved oxygen content decreases, the thermal initiator is activated, and the crosslinking agent MBA undergoes an in-situ crosslinking reaction between copolymer molecular chains and with its active groups (such as carboxyl and amide groups), forming a stable three-dimensional network structure while the fiber solidifies. The nascent fibers are drawn by guide rollers and collected on a winding device.

[0035] In some optional embodiments, the preparation of the polyacrylonitrile-based copolymer includes: dissolving acrylonitrile (AN) monomer, a pH-responsive monomer containing carboxyl groups, and a temperature-responsive monomer containing amide groups in an organic solvent under an inert atmosphere, and then adding a free radical initiator to initiate a copolymerization reaction; Preferably, the pH-responsive monomer containing a carboxyl group includes at least one of methacrylic acid (MAA), acrylic acid (AA), and itaconic acid (ITA), and the temperature-responsive monomer containing an amide group includes N-isopropylacrylamide (NIPAM). Preferably, the organic solvent comprises dimethyl sulfoxide (DMSO), and the free radical initiator comprises azobisisobutyronitrile (AIBN). Preferably, the copolymerization reaction is carried out at a temperature of 60-70°C for 12-24 hours. Preferably, the reaction further includes: after the reaction is complete, the product is poured into a mixture of ethanol and water for precipitation, and then filtered, washed, and dried to obtain a polyacrylonitrile-based copolymer.

[0036] Thirdly, the present invention provides a method for preparing pH / temperature dual-responsive polyacrylonitrile-based carbon fiber, comprising the following steps: subjecting the above-mentioned nascent fiber to thermal crosslinking curing treatment, pre-oxidation treatment and carbonization treatment in sequence to obtain the pH / temperature dual-responsive polyacrylonitrile-based carbon fiber.

[0037] In some alternative embodiments, the multi-stage thermal crosslinking curing process includes: treating the nascent fibers in an inert atmosphere at 120-140°C for 30 minutes, followed by treatment at 160-180°C for 30 minutes.

[0038] Phase 1: 120~140℃: Initial strengthening of the "in-situ crosslinking" network. ① The crosslinking network formed in the coagulation bath is not yet perfect, with low bond energy. At this temperature, residual unreacted double bonds (from MBA or polymer chain ends) can react further, increasing crosslinking density and uniformity. ② Promoting the initial reaction of carboxyl groups: Carboxyl groups from pH-responsive monomers containing carboxyl groups, such as MAA, undergo various stabilization reactions at this temperature: Dehydration to anhydride: Adjacent carboxyl groups can react to form a more stable anhydride structure (-CO-O-CO-), which is itself a form of crosslinking. Reaction with amide groups: Reacts with the amide groups (-CONH2) of NIPAM or MBA to form imide bonds, which are very stable crosslinking bonds. These reactions further enhance the rigidity of the network and reduce the number of active groups that decompose at high temperatures. This is a "preheating" and "stabilization" phase, allowing the internal structure of the fiber to smoothly transition to higher temperatures, avoiding violent reactions and internal stress concentration caused by direct exposure to high temperatures.

[0039] Second Stage: 160~180℃: "Deep Crosslinking and Cycling Initiation". Deep crosslinking and formation of a stable structure: ① At this higher temperature, more active molecular motion allows previously inaccessible functional groups to meet and react. Crosslinking and imidization reactions tend to complete at this stage, forming a highly rigid, heat-resistant "interpenetrating network" framework. This framework is the physical basis for preserving responsiveness. ② Ingenious connection with the PAN cyclization reaction: The cyclization reaction of pure PAN usually only begins to occur significantly above 180℃. This invention sets the temperature of the second stage at 160~180℃, precisely at the starting front of the PAN cyclization reaction.

[0040] Preferably, the pre-oxidation treatment includes: heating to 230-280°C at a rate of 1-5°C / min in an air atmosphere and holding at that temperature for 0.5-2 hours.

[0041] Preferably, the carbonization process includes: heating to 800-1200°C at a rate of 2-10°C / min under the protection of high-purity nitrogen or argon, holding at that temperature for 5-20 minutes, and then naturally cooling to room temperature to obtain the responsive polyacrylonitrile carbon fiber.

[0042] This invention provides a method for preparing polyacrylonitrile-based carbon fibers with dual pH / temperature responsiveness. Since a stable three-dimensional network structure is formed during fiber solidification, the nascent fibers with the three-dimensional network structure are subjected to subsequent thermal crosslinking curing, pre-oxidation, and carbonization treatments. The carbon fibers retain a network composed of crosslinked, carbonized, and responsive units, enabling the polyacrylonitrile-based carbon fibers to simultaneously possess temperature responsiveness, pH responsiveness, and dual pH / temperature responsiveness.

[0043] Fourthly, the present invention provides a pH / temperature dual-responsive polyacrylonitrile-based carbon fiber prepared from the above-described copolymer.

[0044] In some alternative embodiments, the pH / temperature dual-responsive polyacrylonitrile-based carbon fiber has a strength of 4.5-5.5 GPa and a modulus of 250-280 GPa.

[0045] In some alternative embodiments, the pH / temperature dual-responsive polyacrylonitrile-based carbon fiber can produce a length change rate of 2%-8% in a temperature range of 25-45°C.

[0046] At lower temperatures, the amide groups (-CONH-) in the fiber network interact strongly with water molecules through hydrogen bonds. Water molecules are tightly bound around the polymer chains, forming a "hydrated" state. At this point, the molecular chains are hydrophilically extended, and the fiber is in a swollen state, but its macroscopic length remains stable. When the temperature rises to near its lowest critical solution temperature (LCST), thermal energy breaks the hydrogen bonds formed between the amide groups and water molecules. The hydrophobic interaction of the molecular chains (isopropyl groups) begins to dominate, and the chain segments become hydrophobic, attempting to escape the aqueous environment. The molecular chains undergo a conformational change, collapsing from an extended random coil state into a compressed spherical structure. This microscopic collapse manifests macroscopically as a rapid contraction of the entire fiber. Above the LCST, the dehydration and collapse of the molecular chains are complete, and the network structure reaches a new hydrophobic equilibrium state. Therefore, the length no longer changes significantly, remaining in a stable contracted state.

[0047] In some alternative embodiments, the pH / temperature dual-responsive polyacrylonitrile-based carbon fibers can produce a length change rate of 3%-10% in the pH range of 4-10.

[0048] In an acidic environment, the H+ in the solution... + The ion concentration is very high. Residual carboxyl groups (-COOH) in the fiber network undergo protonation, forming uncharged -COOH groups. The electrostatic repulsion between molecular chains weakens, but more importantly, due to the H... + Ions disrupt the original hydrophilic-hydrophobic balance of the chain segments and bind with water molecules, causing a large amount of water molecules to penetrate into the fiber network, thus swelling the fiber and macroscopically manifesting as length elongation. At neutral pH, some -COOH groups ionize (-COO₂). - This generates some electrostatic repulsion between molecular chains, but the repulsion is weaker compared to an alkaline environment, and the hydrophilicity is moderate. Therefore, the fiber neither swells nor shrinks drastically. In an alkaline environment, the OH- in the solution... - Ions promote the deprotonation of the carboxyl group (-COOH), forming a negatively charged carboxylate ion (-COO). - These groups with the same charge generate strong electrostatic repulsion, leading to the expansion of the molecular chain network. However, for this cross-linked carbon network, the expansion of the chain segments does not manifest as macroscopic swelling. Instead, the extension of the chain segments may alter the topology of the network, and the strong hydrophilicity of the charged groups may change the structure of the surrounding hydration layer, ultimately resulting in a macroscopic contraction effect.

[0049] In some alternative embodiments, the pH / temperature dual-responsive polyacrylonitrile-based carbon fiber can produce a length change rate of 3%-10% in the pH range of 4-10; and a length change rate of 2%-8% in the temperature range of 25-45°C.

[0050] Dual-response synergy: Fibers can exhibit synergistic response behavior to stimuli from pH and temperature.

[0051] Fifthly, the present invention provides an application of the above-mentioned pH / temperature dual-responsive polyacrylonitrile-based carbon fiber in smart composite materials, sensors, artificial muscles, and controlled-release drug systems.

[0052] The following detailed description, in conjunction with embodiments, illustrates a pH / temperature dual-responsive polyacrylonitrile-based copolymer, a method for preparing precursor fibers, a method for preparing carbon fibers, and the carbon fibers and their applications provided by the present invention. However, these descriptions should not be construed as limiting the scope of protection of the present invention.

[0053] Example 1 This application provides a method for preparing polyacrylonitrile-based carbon fibers with dual pH / temperature responsiveness, comprising the following steps: S1. Synthesis of terpolymer: Dissolve 85g AN, 5g MAA, and 10g NIPAM in 500mL DMSO and purge with N2 for 30min to remove oxygen. Add 0.5g AIBN and react at 65℃ for 18h. Pour the reaction solution into an ethanol / water (1:1) mixture to precipitate, filter, wash three times with ethanol, and vacuum dry at 60℃ for 24h to obtain a white powdered polymer.

[0054] S2. Preparation of spinning solution and in-situ crosslinking spinning: Dissolve 20g of the above polymer in 80g of DMSO, add 0.4g of MBA and 0.04g of KPS, and stir until completely dissolved. After degassing, filter through a 100-mesh filter. Extrude the spinning solution through a 100-hole (0.08mm) spinneret under a pressure of 0.2MPa, and enter a 60% DMSO aqueous solution coagulation bath at 5℃ with a draw ratio of 1.2 to obtain nascent fibers.

[0055] S3. Post-treatment: The nascent fibers are first treated at 130℃ for 30 min in a N2 atmosphere, and then at 170℃ for 30 min. Subsequently, they are pre-oxidized in air by heating to 250℃ at a rate of 2℃ / min and holding for 1 h. Finally, they are carbonized in an Ar atmosphere by heating to 1000℃ at a rate of 5℃ / min and holding for 10 min to obtain black glossy responsive carbon fibers.

[0056] Example 2 Similar to the steps in Example 1, except that the pH-responsive monomer is acrylic acid (AA).

[0057] Example 3 The steps are similar to those in Example 1, except that the pH-responsive monomer is itaconic acid (ITA).

[0058] Comparative Example 1 Similar to the steps in Example 1, except that: carbon fibers are prepared by blending using conventional methods, directly and physically blending stimuli-responsive polymers (PNIPAM, PMAA) with PAN powder, dissolving them in a solvent (such as DMSO) to prepare a spinning solution, and then performing wet spinning.

[0059] Comparative Example 2 Similar to the steps in Example 1, except that: responsive carbon fibers are prepared by surface coating, the carbon fiber surface is activated by plasma treatment, and then PMAA and PNIPAM chains are grafted to fix the responsive polymer on the fiber surface.

[0060] Comparative Example 3 Similar to the steps in Example 1, except that the mass ratio of acrylonitrile, methacrylic acid and N-isopropylacrylamide in the preparation process is 95:2:3.

[0061] Comparative Example 4 Similar to the steps in Example 1, except that the mass ratio of acrylonitrile, methacrylic acid and N-isopropylacrylamide in the preparation process is 78:10:12.

[0062] Comparative Example 5 Similar to the steps in Example 1, except that: acrylonitrile, units from pH-responsive monomers containing carboxyl groups, and unit copolymers from temperature-responsive monomers containing amide groups are directly added to DMSO without crosslinking agents and thermal initiators to prepare spinning solution.

[0063] Comparative Example 6 Similar to the steps in Example 1, except that: instead of multi-stage thermal crosslinking and curing, pre-oxidation and carbonization are performed directly.

[0064] Comparative Example 7 Similar to the steps in Example 1, the only difference is that the temperatures of the multi-stage thermal crosslinking curing process are 160°C and 210°C respectively.

[0065] Test case 1. Response test of polyacrylonitrile-based carbon fiber Performance testing: A single fiber is fixed on the testing device and placed in different environments to measure its length change rate, where: length change rate (%) = (length after treatment - length before treatment) / length before treatment × 100%, a positive result indicates elongation, and a negative result indicates shortening.

[0066] (1) pH response: At 25°C, the prepared polyacrylonitrile-based carbon fiber was placed in a phosphate buffer solution. When the pH of the phosphate buffer solution changed from 4 to 10, the fiber length underwent reversible shrinkage and swelling. The change rate of polyacrylonitrile-based carbon fiber length at pH=4 and pH=10 was recorded (based on pH=7).

[0067] (2) Temperature response: The prepared polyacrylonitrile-based carbon fiber was placed in a phosphate buffer solution with pH=7.4. When the temperature of the phosphate buffer solution increased from 25℃ to 45℃, the fiber length underwent reversible shrinkage. The change rate of polyacrylonitrile-based carbon fiber length was recorded when the temperature increased from 25℃ to 45℃.

[0068] Table 1

[0069] As can be seen from Table 1 above, the polyacrylonitrile-based carbon fibers prepared using the scheme provided in the embodiments of the present invention have good pH and temperature responsiveness. Compared with the embodiments, the length change rate of the polyacrylonitrile-based carbon fibers prepared in the comparative examples is smaller, indicating that their pH and temperature responsiveness is poor. Although the length change rate of the polyacrylonitrile-based carbon fibers in comparative example 4 is greater than that in example 1 at pH=4 and pH=10, its mechanical strength is low, which makes it easy to break when subjected to external force. Considering all factors, its performance is not as good as that of example 1.

[0070] 2. Mechanical property testing of polyacrylonitrile carbon fiber Test method: The polyacrylonitrile carbon fiber prepared in the example was used as a sample, and then the tensile strength and tensile modulus of the sample were tested. The test results are then summarized in Table 2. The test standard is based on GB / T3362.

[0071] Table 2

[0072] See Table 1-2 and Figs. 1-3 As can be seen from the test results of the embodiments, the polyacrylonitrile carbon fibers prepared according to the preparation process provided in the embodiments of this application have excellent pH and temperature response performance, as well as good mechanical properties. Although the mechanical properties of the fiber are higher when the content of the responsive monomer is low (Comparative Example 3), the responsive performance deteriorates; conversely, the responsive performance of Comparative Example 4, which has a higher content, is high, but the mechanical properties are difficult to meet the needs of practical use.

[0073] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A pH / temperature dual-responsive polyacrylonitrile-based copolymer, characterized in that, It comprises: units from acrylonitrile monomer, units from a pH-responsive monomer containing a carboxyl group, and units from a temperature-responsive monomer containing an amide group, wherein the mass ratio of the acrylonitrile monomer, the pH-responsive monomer containing a carboxyl group, and the temperature-responsive monomer containing an amide group is (85-92):(3-8):(5-10).

2. The pH / temperature dual-responsive polyacrylonitrile-based copolymer according to claim 1, characterized in that, The pH-responsive monomer containing a carboxyl group includes at least one of methacrylic acid (MAA), acrylic acid (AA), and itaconic acid (ITA), and the temperature-responsive monomer containing an amide group is N-isopropylacrylamide (NIPAM).

3. A method for preparing polyacrylonitrile-based carbon fiber precursor, characterized in that, The process includes the following steps: preparing a spinning solution from the pH / temperature dual-responsive polyacrylonitrile-based copolymer as described in claim 1 or 2, forming an extrusion spinneret through a spinneret, and then solidifying it into nascent fibers through a coagulation bath.

4. The preparation method according to claim 3, characterized in that, Includes the following steps: The polyacrylonitrile copolymer is dissolved in an organic solvent, and a crosslinking agent and a thermal initiator are added. The mixture is stirred vigorously to ensure complete dissolution and dispersion, resulting in a uniform and clear spinning solution. The spinning solution is then extruded through a spinneret into a low-temperature coagulation bath, where thermal phase separation and in-situ crosslinking reactions are simultaneously carried out to obtain nascent fibers. Preferably, the organic solvent includes dimethyl sulfoxide (DMSO), and the polyacrylonitrile copolymer is dissolved in the organic solvent to form a spinning solution with a concentration of 15-25 wt%. Preferably, the crosslinking agent comprises N,N'-methylenebisacrylamide (MBA), and the crosslinking agent is used in an amount of 1-5% of the total mass of the copolymer; Preferably, the thermal initiator comprises potassium persulfate (KPS), and the amount of the thermal initiator is 0.1-1% of the total mass of the copolymer; Preferably, the coagulation bath is a mixed solution of dimethyl sulfoxide and water, and the temperature is 0-10°C.

5. The preparation method according to claim 3, characterized in that, The preparation of the polyacrylonitrile-based copolymer includes: dissolving acrylonitrile (AN) monomer, a pH-responsive monomer containing carboxyl groups, and a temperature-responsive monomer containing amide groups in an organic solvent under an inert atmosphere, and then adding a free radical initiator to initiate a copolymerization reaction; Preferably, the pH-responsive monomer containing a carboxyl group includes at least one of methacrylic acid (MAA), acrylic acid (AA), and itaconic acid (ITA), and the temperature-responsive monomer containing an amide group includes N-isopropylacrylamide (NIPAM). Preferably, the organic solvent comprises dimethyl sulfoxide (DMSO), and the free radical initiator comprises azobisisobutyronitrile (AIBN). Preferably, the copolymerization reaction is carried out at a temperature of 60-70°C for 12-24 hours. Preferably, the reaction further includes: after the reaction is complete, the product is poured into a mixture of ethanol and water for precipitation, and then filtered, washed, and dried to obtain a polyacrylonitrile-based copolymer.

6. A method for preparing a pH / temperature dual-responsive polyacrylonitrile-based carbon fiber, characterized in that, The process includes the following steps: subjecting the nascent fibers of any one of claims 3-5 to thermal crosslinking curing, pre-oxidation, and carbonization treatments in sequence to obtain the pH / temperature dual-responsive polyacrylonitrile-based carbon fibers.

7. The preparation method according to claim 6, characterized in that, The multi-stage thermal crosslinking and curing process includes: treating the nascent fibers at 120-140℃ for 30 minutes in an inert atmosphere, and then at 160-180℃ for 30 minutes. Preferably, the pre-oxidation treatment includes: heating to 230-280°C at a rate of 1-5°C / min in an air atmosphere and holding at that temperature for 0.5-2 hours; Preferably, the carbonization process includes: heating to 800-1200°C at a rate of 2-10°C / min under the protection of high-purity nitrogen or argon, holding at that temperature for 5-20 minutes, and then naturally cooling to room temperature to obtain the responsive polyacrylonitrile carbon fiber.

8. A pH / temperature dual-responsive polyacrylonitrile-based carbon fiber prepared from the copolymer of any one of claims 6-7.

9. The pH / temperature dual-responsive polyacrylonitrile-based carbon fiber according to claim 8, characterized in that, The pH / temperature dual-responsive polyacrylonitrile-based carbon fiber satisfies one or more of the following conditions: The pH / temperature dual-responsive polyacrylonitrile-based carbon fiber has a strength of 4.5-5.5 GPa and a modulus of 250-280 GPa. The pH / temperature dual-responsive polyacrylonitrile-based carbon fiber can produce a length change rate of 3%-10% in the pH range of 4-10; and / or a length change rate of 2%-8% in the temperature range of 25-45℃.

10. The application of a pH / temperature dual-responsive polyacrylonitrile-based carbon fiber prepared by the preparation method according to any one of claims 6-7 or the preparation method according to any one of claims 8-9 in smart composite materials, sensors, artificial muscles, and controlled-release drug systems.

Images