Method for producing polyacrylonitrile-based fibers having controlled morphology

Abstract

The present disclosure generally relates to a method for producing polymer fibers, typically polyacrylonitrile-based fibers, whose morphology is controlled by using polymer additives to form polymer blends with polyacrylonitrile, which are then subjected to specific coagulation and washing conditions. The present disclosure also relates to carbon fibers produced by processing the produced polymer fibers.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Patent Application No. 63 / 129891, filed December 23, 2020, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure generally relates to a method for producing polymer fibers, typically polyacrylonitrile-based fibers, whose morphology is controlled by using polymer additives to form polymer blends with polyacrylonitrile, which are then subjected to specific coagulation and washing conditions. The present disclosure also relates to carbon fibers produced by processing the produced polymer fibers. [Background technology]

[0003] Carbon fibers have been used in a wide variety of applications due to their desirable properties, such as high strength and stiffness, high chemical resistance, and low thermal expansion. For example, carbon fibers can be molded into structural parts that combine high strength and stiffness while weighing significantly less than metal components of comparable properties. Carbon fibers are increasingly being used as structural components in composite materials, especially for aerospace and automotive applications. In particular, composite materials have been developed in which carbon fibers act as reinforcing materials in resin or ceramic matrices.

[0004] Over 90% of carbon fibers are derived from polyacrylonitrile (PAN)-based precursors. Generally, the process of converting PAN into carbon fibers involves solvent spinning (solution spinning), coagulation, oxidation, stabilization, and then carbonization.

[0005] During solidification, the non-solvent, typically water, flows into the polymer solution and the solvent (typically DMF, DMSO, etc.) flows into the bath, resulting in fiber filaments by interdiffusion. The fiber skin and core structure is first formed during the first few seconds of solidification, and a continuous bath is used to draw the filaments and remove residual solvent. Although the polymer chains can be aligned by drawing to add crystalline domains, it is difficult to manipulate the fiber structure (or morphology) and re-form the skin-core structure or to introduce new features after solidification. Furthermore, there is a growing interest in the carbon fiber industry to introduce porosity into the fiber. Porous fibers can offer the advantage of deeper penetration of resin into the fiber to create larger interphase areas, which can improve mechanical adhesion and conversion properties in composites. Another possible advantage of porous fibers could be applications in gas barrier technology, where the diffusion and / or separation of gases is facilitated by the fiber. Porous fibers can provide lighter and more compact materials suitable for advanced membranes used in greenhouse gas separation, self-supporting energy storage materials, and hydrogen production. Porous fibers also have a lower density, which shows promise for producing lighter carbon fibers and may be an alternative route to hollow fibers.

[0006] However, porous fibers are generally considered to have poor mechanical performance due to the presence of voids and defects in the fiber. The production of porous fibers is possible by targeted selection of solidification conditions that promote interdiffusion of the solvents and quench the fiber structure into a porous state. However, macrovoids formed during solidification can hinder the stretching and drawability of the fiber. Also, macrovoids formed during the nascent stages of spinning can have an amplifying effect on the defects they cause if formed too early.

[0007] Techniques are known for producing porous carbon fibers, such as physical or chemical activation, carbonization of polymer blends, and shaping using nanoparticles and block copolymers. Carbonization of polymer blends requires blending of incompatible polymers that microphase separate into a) a matrix-forming carbon source polymer and b) a dispersed pore-forming sacrificial polymer. Such sacrificial polymers are then typically burned off by pyrolysis during the process of forming the porous carbon material. Not only can the pore-forming sacrificial polymer not be recovered and reused, but the removal of said polymer during oxidation and carbonization makes the carbon material or fiber more susceptible to further damage, resulting in reduced mechanical properties.

[0008] Thus, there is a continuing need to develop processes to control fiber structure (or morphology), such as the introduction and manipulation of porosity, in polymer fibers with reduced impact on the mechanical properties of the produced fibers and, subsequently, carbon fibers produced therefrom. Described herein is a new method for controlling fiber morphology that uses polymer additives to form polymer blends with polyacrylonitrile, which are then subjected to specific coagulation and washing conditions. Summary of the Invention

[0009] Advantageously, it has been discovered that the morphology of carbon fibers can be controlled when polymer additives are used to form polymer blends with polyacrylonitrile. The polymer blends are then subjected to specific coagulation and washing conditions to remove the polymer additives in a controlled manner and to introduce porosity into the resulting fibers with controlled morphology. Such fibers can then be converted into carbon fibers. The polymer additives can be recovered and reused, and the removal of the polymer blends is performed prior to oxidation and carbonization, thus avoiding damage and loss of mechanical properties.

[0010] In a first aspect, the present disclosure relates to a method for producing polyacrylonitrile-based fibers having controlled morphology, the method comprising: a) a polyacrylonitrile-based polymer (polymer A); A polymer (polymer B) different from the polyacrylonitrile polymer, a first liquid comprising a solvent for polymer A; forming a homogeneous solution comprising a first liquid in which polymer B is soluble; b) co-precipitating polymer A and polymer B by contacting the homogeneous solution formed in step a) with a second liquid comprising a solvent for polymer A and a non-solvent for polymer A, in which polymer B is insoluble, thereby forming a polyacrylonitrile-based material comprising polymer A and polymer B; c) selectively removing polymer B from the polyacrylonitrile-based material by contacting the polyacrylonitrile-based material with a third liquid comprising a non-solvent for polymer A, in which polymer B is soluble, thereby producing a polyacrylonitrile-based fiber having a controlled morphology; Includes.

[0011] In a second aspect, the present disclosure relates to polyacrylonitrile-based fibers produced by the methods described herein.

[0012] In a third aspect, the present disclosure relates to a method for producing carbon fibers, the method comprising: (i) producing polyacrylonitrile-based fibers according to the methods described herein; (ii) oxidizing the polyacrylonitrile fiber produced in step (i) to form a stabilized carbon fiber precursor fiber, and then carbonizing the stabilized carbon fiber precursor fiber, thereby producing a carbon fiber. Includes.

[0013] In a fourth aspect, the present disclosure relates to carbon fibers produced by the methods described herein.

[0014] In a fifth aspect, the present disclosure relates to a composite material comprising carbon fibers and a matrix resin produced according to the methods described herein.

[0015] In a sixth aspect, the present disclosure relates to a composite article obtained by curing the composite material described herein. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] As used herein, the terms "a," "an," or "the" mean "one or more" or "at least one," and can be used interchangeably, unless otherwise stated.

[0017] As used herein, the term "and / or" used in a phrase of the form "A and / or B" means A only, B only, or A and B together.

[0018] As used herein, the term "comprises" includes "consists essentially of" and "consists of." The term "comprising" includes "consisting essentially of" and "consisting of." "Comprising" is synonymous with "including," "containing," or "characterized by" and is intended to be inclusive or open-ended and does not exclude additional, unrecited elements or steps. The transitional phrase "consisting essentially of" is inclusive of the specified materials or steps, as well as those that do not essentially affect the basic characteristics or function of the described composition, process, method, or product. The transitional phrase "consisting of" excludes any unspecified elements, steps, or ingredients.

[0019] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains.

[0020] As used herein, unless otherwise indicated, the term "about" or "approximately" refers to an acceptable error for a particular value as determined by one of ordinary skill in the art, which depends in part on how the value is measured or determined. In certain embodiments, the term "about" or "approximately" refers to within 1, 2, 3, or 4 standard deviations. In certain embodiments, the term "about" or "approximately" refers to within 50%, 20%, 15%, 10%, 9%, 8%, 7%, 6%, 5%, 4%, 3%, 2%, 1%, 0.5%, or 0.05% of a given value or range.

[0021] It will also be understood that any numerical ranges described herein are intended to include all subranges subsumed therein. For example, the range "1 to 10" is intended to include all subranges between and including the recited minimum of 1 and the recited maximum of 10, i.e., having a minimum of 1 or more and a maximum of 10 or less. The disclosed numerical ranges are continuous, and therefore include all values ​​between the minimum and maximum. Unless otherwise indicated, the various numerical ranges specified in this application are approximations.

[0022] Throughout this disclosure, various publications may be incorporated by reference. If the meaning of any language in such a publication incorporated by reference conflicts with the meaning of the language in this disclosure, the meaning of the language in this disclosure shall control unless otherwise indicated.

[0023] The process described herein typically uses a polymer blend containing PAN and a polymer additive designed to control the morphology by appropriate control of the coagulation and washing conditions. In particular, the polymer additive is a polymer that has solubility in both the non-solvent and the solvent of the PAN. Solubility in the solvent is necessary to form a homogeneous solution in the viscous spinning "dope" before spinning. This is important because the polymer additive and the PAN cannot form a blend if both are not soluble. Solubility in the non-solvent is unique and provides an opportunity to purposefully control the kinetics of interdiffusion for fibril formation. Furthermore, if the polymer additive blended with the PAN can be dissolved, for example, by changing the conditions of the coagulation or washing bath, it offers the possibility to manipulate the structure beyond the nascent stage of coagulation.

[0024] Accordingly, a first aspect of the present disclosure relates to a method for producing polyacrylonitrile-based fibers having controlled morphology, the method comprising: a) a polyacrylonitrile-based polymer (polymer A); A polymer (polymer B) different from the polyacrylonitrile polymer, forming a homogenous solution comprising a first liquid comprising a solvent for polymer A, in which polymer B is soluble; b) co-precipitating polymer A and polymer B by contacting the homogeneous solution formed in step a) with a second liquid comprising a solvent for polymer A and a non-solvent for polymer A, in which polymer B is insoluble, thereby forming a polyacrylonitrile-based material comprising polymer A and polymer B; c) selectively removing polymer B from the polyacrylonitrile-based material by contacting the polyacrylonitrile-based material with a third liquid comprising a non-solvent for polymer A, in which polymer B is soluble, thereby producing a polyacrylonitrile-based fiber having a controlled morphology; Includes.

[0025] In step a) of the method, a homogeneous solution is formed comprising a polyacrylonitrile-based polymer (polymer A), a polymer different from the polyacrylonitrile-based polymer (polymer B), and a first liquid comprising a solvent for polymer A, in which polymer B is soluble.

[0026] The polyacrylonitrile-based polymer, polymer A, can be any polymer that includes repeat units derived from acrylonitrile. Suitable polyacrylonitrile-based polymers can be homopolymers of repeat units derived from acrylonitrile or copolymers that include repeat units derived from acrylonitrile and one or more comonomers. Such polymers can be obtained from commercial sources or can be prepared according to methods known to those skilled in the art. For example, polymer A can be produced by any polymerization method, including, but not limited to, solution polymerization, dispersion polymerization, precipitation polymerization, suspension polymerization, emulsion polymerization, and variations thereof.

[0027] The polyacrylonitrile-based polymer contains repeating units derived from acrylonitrile and at least one comonomer selected from the group consisting of vinyl acids, vinyl esters, vinyl amides, vinyl halides, ammonium salts of vinyl compounds, sodium salts of sulfonic acids, and mixtures thereof.

[0028] In one embodiment, the polyacrylonitrile-based polymer is a copolymer of acrylonitrile and methacrylic acid (MAA), acrylic acid (AA), itaconic acid (ITA), methacrylate (MA), ethyl acrylate (EA), butyl acrylate (BA), methyl methacrylate (MMA), ethyl methacrylate (EMA), propyl methacrylate, butyl methacrylate, β-hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, 2-ethylhexyl acrylate, isopropyl acetate, vinyl acetate, and at least one comonomer selected from the group consisting of vinyl acrylate (VA), vinyl propionate, vinyl imidazole (VIM), acrylamide (AAm), diacetone acrylamide (DAAm), allyl chloride, vinyl bromide, vinyl chloride, vinylidene chloride, sodium vinyl sulfonate, sodium p-styrenesulfonate (SSS), sodium methallyl sulfonate (SMS), sodium 2-acrylamido-2-methylpropanesulfonate (SAMPS), and mixtures thereof.

[0029] The comonomer ratio (the amount of one or more comonomers relative to the amount of acrylonitrile) is not particularly limited. However, a suitable comonomer ratio is 0-20%, typically 1-5%, more typically 1-3%.

[0030] The molecular weight of polyacrylonitrile-based polymers suitable for use with the above process ranges from 60 to 500 kg / mol, typically from 90 to 250 kg / mol, more typically from 115 to 180 kg / mol.

[0031] The first liquid comprises a solvent for polymer A. At the same time, polymer B is soluble in the first liquid.

[0032] As used herein, the term "solvent" refers to any compound that can typically completely dissolve the respective polymer by itself at the temperature at which the solvent is used. On the other hand, the term "non-solvent" refers to any compound that cannot typically dissolve the respective polymer by itself at the temperature at which the non-solvent is used. It is understood by those skilled in the art that a solvent and a non-solvent, which are typically miscible, can be combined to form a liquid in which the solubility of the respective polymer is different from that of the solvent alone or the non-solvent alone.

[0033] As used herein, the term "soluble" when used to describe a material means that 1% or more, typically 5% or more by weight of the material can be dissolved in a particular solvent or liquid, based on the weight of the solvent or liquid. As used herein, the term "insoluble" when used to describe a material means that less than 1% by weight of the material, typically less than 0.5% by weight of the material can be dissolved in a particular non-solvent or liquid.

[0034] Suitable solvents for polymer A may be selected from the group consisting of dimethylsulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), zinc chloride (ZnCl2) / water, sodium thiocyanate (NaSCN) / water, and mixtures thereof, typically selected from the group consisting of dimethylsulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP).

[0035] In step a), the temperature of the first liquid is kept above room temperature, ie, above 25° C. In one embodiment, the temperature of the first liquid is from about 40° C. to about 85° C.

[0036] The homogeneous solution produced is typically free of gels and / or flocculated polymers. The presence of gels and / or flocculated polymers can be determined using any method known to those skilled in the art. For example, a Hegman gauge may be used to determine the presence of gels and / or flocculated polymers. The homogeneous solution produced is generally stable and does not exhibit gel formation over time.

[0037] The homogeneous solution may have a polymer concentration of at least 10% by weight, typically from about 16% to about 28% by weight, and more typically from about 19% to about 24% by weight, based on the total weight of the solution.

[0038] Step b) is a step of coprecipitating polymer A and polymer B by contacting the homogeneous solution formed in step a) with a second liquid comprising a solvent for polymer A and a non-solvent for polymer A, in which polymer B is insoluble, thereby forming a polyacrylonitrile-based material comprising polymer A and polymer B.

[0039] The second liquid comprises a solvent for polymer A and a non-solvent for polymer A, and polymer B is insoluble in the second liquid. As a result, when the homogeneous solution formed in step a) is contacted with the second liquid, polymer A and polymer B are co-precipitated in the form of a polyacrylonitrile-based material, which is typically in the form of a solid, such as a film, discrete particles, fibers, etc.

[0040] The second liquid used in the process is a mixture of a solvent and a non-solvent for polymer A. Suitable solvents include those described herein. In one embodiment, dimethylsulfoxide, dimethylformamide, dimethylacetamide, or a mixture thereof is used as the solvent. In another embodiment, dimethylsulfoxide is used as the solvent.

[0041] The non-solvent for polymer A can be any compound known to those skilled in the art that does not dissolve polymer A at the temperature used. Typical non-solvents for polymer A include water and C1-C6 alkanols such as methanol, ethanol, n-propanol, isopropanol, etc. In one embodiment, the non-solvent for polymer A is water.

[0042] The ratio of the solvent to the non-solvent and the bath temperature are not particularly limited and can be adjusted according to known methods to achieve a desired solidification rate. However, the second liquid preferably contains 85 wt % or less of a solvent for polymer A and 15 wt % or more of a non-solvent for polymer A based on the total weight of the second liquid.

[0043] In another embodiment, the second liquid comprises 40% to 85% by weight of one or more solvents, the remainder being a non-solvent. In one embodiment, the second liquid comprises 40% to 70% by weight of one or more solvents, the remainder being a non-solvent. In yet another embodiment, the second liquid comprises 50% to 85% by weight of one or more solvents, the remainder being a non-solvent.

[0044] Typically, the temperature of the second liquid is between 0°C and 80°C. In one embodiment, the temperature of the second liquid is between 30°C and 80°C. In another embodiment, the temperature of the second liquid is between 0°C and 20°C.

[0045] In one embodiment, step b) comprises spinning the homogenous solution formed in step a) into or into a coagulation bath carrying a second liquid comprising a solvent for polymer A and a non-solvent for polymer A to form the polyacrylonitrile-based material as one or more fibers.

[0046] In this embodiment, the homogenous solution is spun in or into a coagulation bath. After removing air bubbles by vacuum, the homogenous solution (i.e., "spin dope") can be subjected to conventional wet spinning and / or air gap spinning. In wet spinning, the spinning dope is filtered and extruded through holes in a spinneret (typically made of metal) into a liquid coagulation bath to form filaments. The spinneret holes determine the desired filament count of the fiber (e.g., 3,000 holes for 3K carbon fiber). In air gap spinning, a vertical air gap of 1-50 mm, typically 2-10 mm, is provided between the spinneret and the coagulation bath. In one embodiment, the polymer solution is filtered and extruded in air from the spinneret, and the extruded filaments are then coagulated in a coagulation bath.

[0047] The solvent for polymer A in the first liquid and the solvent for polymer A in the second liquid may be the same or different, and each is selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), zinc chloride (ZnCl2) / water, sodium thiocyanate (NaSCN) / water, and mixtures thereof, and typically is selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), and N-methyl-2-pyrrolidone (NMP).

[0048] In one embodiment, the solvent for polymer A in the first liquid and the solvent for polymer A in the second liquid are the same.

[0049] In step c), polymer B is selectively removed from the polyacrylonitrile-based material by contacting the polyacrylonitrile-based material with a third liquid comprising a non-solvent for polymer A, in which polymer B is soluble.

[0050] The third liquid comprises a non-solvent for polymer A, while polymer B is soluble in the third liquid. Thus, in step c), polymer B can be removed in a selective manner from the PAN polymer fibers, producing polyacrylonitrile-based fibers with controlled morphology.

[0051] The temperature of the third liquid is from 0 to 100°C, typically from 0 to 30°C, more typically from 10 to 25°C.

[0052] The non-solvent for polymer A in the second liquid and the non-solvent for polymer A in the third liquid may be the same or different. In one embodiment, the non-solvent for polymer A in the second liquid and the non-solvent for polymer A in the third liquid are the same.

[0053] In one embodiment, the non-solvent for polymer A in the second liquid and the non-solvent for polymer A in the third liquid are each water.

[0054] In one embodiment, the first liquid comprises a solvent for polymer A.

[0055] In another embodiment, the second liquid comprises a solvent for polymer A and a non-solvent for polymer A.

[0056] In yet another embodiment, the third liquid comprises a non-solvent for polymer A.

[0057] In one embodiment, step c) comprises drawing the one or more fibers through one or more draw baths and wash baths, where at least one bath carries a third liquid comprising a non-solvent for polymer A.

[0058] Drawing of the coagulated polymer fibers is accomplished by conveying the fibers, for example by rollers, through one or more drawing and washing baths. The coagulated polymer fibers are conveyed through one or more washing baths to remove excess solvent, and then drawn in a hot (e.g., 40°C to 100°C) water bath to impart molecular orientation to the filaments as a first step in fiber diameter control. The resulting drawn polymer fibers are substantially free of solvent.

[0059] Thus, in one embodiment, step c) comprises drawing one or more fibers through multiple drawing and washing baths, where the first bath carries a third liquid comprising a non-solvent for the polymer and the temperature of the first bath is between 0 and 30° C., typically between 10 and 25° C. The first bath refers to the bath immediately following the bath used in step b). The baths after the first bath may have a temperature up to 100° C.

[0060] The polymer additive, i.e., polymer B, which is combined with polymer A in the manner described herein to form a homogeneous solution, is a different polymer than polymer A. Suitable polymers for use as polymer B are those that are soluble in the first liquid, insoluble in the second liquid, and soluble in the third liquid at the temperatures used, and may be homopolymers or copolymers. One suitable polymer has the formula (I): [ka] (In the formula: R 1 is hydrogen or methyl; R 2 and R 3 are each independently H or alkyl, typically H or (C1-C6) alkyl.

[0061] As used herein, the terminology "Cx-Cy" or "(Cx-Cy)" (where x and y are each integers) in reference to an organic group means that the group can contain from x carbon atoms to y carbon atoms per group.

[0062] As used herein, the term "alkyl" refers to a monovalent linear or branched saturated hydrocarbon radical, for example, methyl, ethyl, n-propyl, isopropyl, n-butyl, isobutyl, tert-butyl, pentyl, hexyl, and the like.

[0063] As used herein, the term "derived from" means that the repeating units in the polymer are formed by polymerization of the monomers described herein according to methods known to those skilled in the art. In some cases, the polymer may be subjected to subsequent chemical modification. For example, a polymer made by polymerization of an acyl group-containing monomer may be hydrolyzed to form a polymer having a hydroxyl group.

[0064] In one embodiment, polymer B is a homopolymer derived from a monomer according to formula (I).

[0065] In another embodiment, polymer B is poly(N-isopropylacrylamide).

[0066] In one embodiment, polymer B is a copolymer comprising monomeric units derived from a monomer according to formula (I), and more typically, about 50 weight percent ("wt %) or more of the repeat units of the polymer are derived from a monomer according to formula (I).

[0067] Another suitable polymer has formula (II): [ka] (In the formula: R 4 is hydrogen or methyl; R 5 is a polymer that includes one or more repeat units derived from at least one monomer with H, alkyl, or acyl, typically H or acyl.

[0068] As used herein, the term "acyl" refers to a substituent characterized by the formula -(C=O)-R, where R is an alkyl group.

[0069] In one embodiment, polymer B is a homopolymer derived from a monomer according to formula (II).

[0070] In another embodiment, polymer B is polyvinyl alcohol.

[0071] In another embodiment, polymer B is a copolymer comprising monomeric units derived from a monomer according to formula (II), and more typically, about 50 weight percent ("wt %) or more of the repeat units of the polymer are derived from a monomer according to formula (II).

[0072] In step a), the amount of polymer B that is combined with polymer A to form a homogeneous solution is not particularly limited. However, suitable results are obtained when the homogeneous solution contains no more than 50 wt. % of polymer B, typically no more than 20 wt. % and more typically no more than 10 wt. % based on the total weight of the homogeneous solution.

[0073] The process may further comprise step d) of drying the substantially solvent-free drawn polymeric fibers, for example on drying rolls. The drying rolls may consist of a plurality of rotatable rolls arranged in series and in a serpentine configuration over which the filaments pass from roll to roll sequentially and under sufficient tension to cause drawing or relaxation of the filaments on the rolls. At least some of the rolls are heated by pressurized steam circulating therein or through the rolls, or by electric heating elements inside the rolls. A finishing oil may be applied to the drawn fibers before drying to prevent the filaments from sticking to each other in downstream processes.

[0074] The process of the present disclosure can be carried out continuously or batchwise. As used herein, a "continuously carried out" process refers to a process in which the fiber is conveyed through one or more processing steps at a time, a single working unit, without any interruption in time, material, or sequence. This is in contrast to a batch process, which is understood as a process that includes one or more steps in a sequence that are performed in a defined order and at the end of which a finite amount of material is processed or produced, which must be repeated to process or produce another batch of material. In one embodiment, the process is carried out continuously.

[0075] Advantageously, the polymer additive, polymer B, can be recovered and reused. The ability to recover polymer B from the process provides an advantage over "sacrificial polymers" that are volatilized during pyrolysis and lost during downstream carbon fiber formation processes. Thus, in one embodiment, the process further comprises step e) of at least partially recovering polymer B. Any separation method known to those skilled in the art can be used to recover polymer B from any of the steps in the process. For example, vacuum distillation, thin film evaporation, etc. can be used to recover polymer B from any of the liquids described herein.

[0076] In a second aspect, the present disclosure relates to polyacrylonitrile-based fibers produced by the methods described herein, which can be used as precursor fibers, so-called white fibers, for the production of carbon fibers.

[0077] Thus, in a third aspect, the present disclosure relates to a process for producing carbon fibers, the process comprising: (i) producing polyacrylonitrile-based fibers according to the methods described herein; (ii) oxidizing the polyacrylonitrile fiber produced in step (i) to form a stabilized carbon fiber precursor fiber, and then carbonizing the stabilized carbon fiber precursor fiber, thereby producing a carbon fiber. Includes.

[0078] After the polyacrylonitrile-based fibers are produced according to the methods described herein, the polyacrylonitrile-based fibers can be oxidized to form stabilized carbon fiber precursor fibers, which are then carbonized to produce carbon fibers.

[0079] During the oxidation stage, the polymer fibers are fed under tension through one or more specialized ovens, each having a temperature of 150-300°C, typically 200-280°C, more typically 220-270°C, where heated air is supplied to each oven.

[0080] The oxidation process allows oxygen molecules from the air to combine with the fibers and initiate cross-linking of the polymer chains, thereby increasing the fiber density to 1.30 g / cm 3 ~1.45g / cm 3 Such oxidized PAN fibers have an infusible ladder-type aromatic molecular structure and are ready for carbonization.

[0081] Carbonization results in the crystallization of the carbon molecules, resulting in finished carbon fibers having a carbon content of greater than 90 percent. Carbonization of the oxidized or stabilized carbon fiber precursor fibers is carried out in an inert (oxygen-free) atmosphere, typically a nitrogen atmosphere, in one or more specially designed ovens. The oxidized carbon fiber precursor fibers are passed through one or more ovens, each heated to a temperature between 300°C and 1650°C, typically between 1100°C and 1450°C.

[0082] Adhesion between the matrix resin and the carbon fibers is an important criterion in carbon fiber reinforced polymer composites, so during the manufacture of carbon fibers, surface treatments can be performed after oxidation and carbonization to enhance this adhesion.

[0083] Surface treatment can include pulling the carbonized fibers through an electrolytic bath containing an electrolyte, such as ammonium bicarbonate or sodium hypochlorite. The chemicals in the electrolytic bath etch or roughen the surface of the fibers, thereby increasing the surface area available for interfacial fiber / matrix bonding and adding reactive chemical groups.

[0084] The carbon fibers can then be subjected to a sizing process in which a size coating (e.g., an epoxy-based coating) is applied to the fibers. The sizing process can be carried out by passing the fibers through a size bath containing a liquid coating material. The sizing process protects the carbon fibers during handling and processing into intermediate forms, such as dry fabrics and prepregs. The sizing process also reduces fuzz, improves processability, and holds the filaments together in individual tows to increase the interfacial shear strength between the fiber and the matrix resin.

[0085] After sizing, the coated carbon fibers are dried and then wound onto a bobbin.

[0086] Those skilled in the art will appreciate that processing conditions (such as spinning solution and coagulation bath composition, total bath volume, drawing, temperature, and filament speed) are involved in obtaining filaments of the desired structure and denier.

[0087] In a fourth aspect, the present disclosure relates to carbon fibers produced by the methods described herein.

[0088] Carbon fibers made according to the processes described herein can be characterized by mechanical properties such as tensile strength and tensile modulus according to ASTM D4018 test method.

[0089] The carbon fibers produced generally have a tensile strength of 300-1000 ksi, typically 400-600 ksi.

[0090] The carbon fibers produced generally have a tensile modulus of 30-50 msi, typically 35-40 msi.

[0091] The carbon fibers produced can be characterized by their density. Generally, carbon fibers formed according to the methods described herein have a lower density than conventional carbon fibers. Advantageously, the present disclosure provides low density, lightweight carbon fibers. Carbon fibers produced according to the present disclosure have a density of 1.80 g / cm 3 Less than 1.79 g / cm 3 Less than 1.78g / cm 3 In one embodiment, the density is from 1.50 to 1.77 g / cm 3 In another embodiment, the density is 1.70 to 1.77 g / cm 3 or 1.74 to 1.79 g / cm 3 It is.

[0092] The carbon fibers produced herein are suitable for use in the manufacture of composite materials. Thus, in a fifth aspect, the present disclosure relates to a composite material comprising the carbon fibers produced according to the methods described herein and a matrix resin.

[0093] In a number of liquid-molding processes, composites can be produced by molding a preform containing carbon fibers produced according to the methods described herein and infusing the preform with a thermosetting resin. Liquid molding processes that can be used include, but are not limited to, vacuum impregnation (VARTM), in which a vacuum-induced pressure differential is used to infuse the preform with resin. Another method is re-transfer molding (RTM), in which the preform is pressure-injected with resin in a closed mold. A third method is resin film infusion (RFI), in which semi-solid resin is placed under or over the preform, appropriate tooling is placed on the part, the part is bagged, and then the part is melted in an autoclave to infuse the preform with resin.

[0094] The matrix resin for impregnating or infusing the preforms described herein is a curable resin. In this disclosure, "curing" or "cure" refers to the solidification of a polymeric material by chemical cross-linking of polymer chains. The term "curable" in reference to a composition means that the composition can be subjected to conditions that cause the composition to harden or become thermoset. The matrix resin is typically a hardenable or thermosetting resin that contains one or more uncured thermosetting or thermoplastic resins. Suitable thermosetting resins include, but are not limited to, epoxy resins, oxetanes, imides (such as polyimides or bismaleimides), vinyl ester resins, cyanate ester resins, isocyanate modified epoxy resins, phenolic resins, furan resins, benzoxazines, formaldehyde condensation resins (such as urea, melamine, or phenol), polyesters, acrylics, hybrids, blends, and combinations thereof. Suitable thermoplastics include, but are not limited to, polyolefins, fluoropolymers, perfluorosulfonic acids, polyamide-imides, polyamides, polyesters, polyketones, polyphenylene sulfides, polyvinylidene chloride, sulfone polymers, hybrids, blends, and combinations thereof.

[0095] Suitable epoxy resins include glycidyl derivatives of aromatic diamines, aromatic monoprimary amines, aminophenols, polyhydric phenols, polyhydric alcohols, polycarboxylic acids, and non-glycidyl resins produced by peroxidation of olefinic double bonds. Examples of suitable epoxy resins include polyglycidyl ethers of bisphenols such as bisphenol A, bisphenol F, bisphenol S, bisphenol K, and bisphenol Z; polyglycidyl ethers of cresols and phenolic novolacs, glycidyl ethers of phenol-aldehyde adducts, glycidyl ethers of aliphatic dialkyl, diglycidyl ethers, diethylene glycol diglycidyl ethers, aromatic epoxy resins, aliphatic polyglycidyl ethers, epoxidized olefins, brominated resins, aromatic glycidyl amines, heterocyclic glycidyl imides and amides, glycidyl ethers, fluorinated epoxy resins, or combinations thereof.

[0096] Specific examples are the tetraglycidyl derivative of 4,4'-diaminodiphenylmethane (TGDDM), resorcinol diglycidyl ether, triglycidyl-p-aminophenol, triglycidyl-m-aminophenol, bromobisphenol F diglycidyl ether, the tetraglycidyl derivative of diaminodiphenylmethane, trihydroxyphenylmethane triglycidyl ether, polyglycidyl ether of phenol-formaldehyde novolac, polyglycidyl ether of o-cresol novolac or the tetraglycidyl ether of tetraphenylethane.

[0097] Suitable oxetane compounds, which are compounds containing at least one oxetano group per molecule, include, for example, compounds such as 3-ethyl-3[[(3-ethyloxetan-3-yl)methoxy]methyl]oxetane, oxetane-3-methanol, 3,3-bis-(hydroxymethyl)oxetane, 3-butyl-3-methyloxetane, 3-methyl-3-oxetanemethanol, 3,3-dipropyloxetane, and 3-ethyl-3-(hydroxymethyl)oxetane.

[0098] The curable matrix resin may optionally contain one or more additives such as curing agents, curing catalysts, comonomers, rheology control agents, tackifiers, inorganic or organic fillers, thermoplastic and / or elastomeric polymers as reinforcing agents, stabilizers, inhibitors, pigments, dyes, flame retardants, reactive diluents, UV absorbers and other additives known to those skilled in the art to improve the properties of the matrix resin before and / or after curing.

[0099] Examples of suitable curing agents include, but are not limited to, aromatic, aliphatic and alicyclic amines or guanidine derivatives.Suitable aromatic amines include 4,4'-diaminodiphenylsulfone (4,4'-DDS), and 3,3'-diaminodiphenylsulfone (3,3'-DDS), 1,3-diaminobenzene, 1,4-diaminobenzene, 4,4'-diammodiphenylmethane, benzenediamine (BDA);Suitable aliphatic amines include ethylenediamine (EDA), 4,4'-methylenebis(2,6-diethylaniline) (M-DEA), m-xylylenediamine (mXDA), diethylenetriamine (DETA), triethylenetetramine (TETA), trioxatridecanediamine (TTDA), polyoxypropylenediamine, and further homologues, diamino cycloaliphatic amines such as cyclohexane (DACH), isophorone diamine (IPDA), 4,4' diaminodicyclohexylmethane (PACM), bisaminopropylpiperazine (BAPP), N-aminoethylpiperazine (N-AEP); other suitable curing agents include anhydrides, typically polycarboxylic acid anhydrides, such as nadic anhydride, methylnadic anhydride, phthalic anhydride, tetrahydrophthalic anhydride, hexahydrophthalic anhydride, methyltetrahydrophthalic anhydride, methylhexahydrophthalic anhydride, endomethylenetetrahydrophthalic anhydride, pyromellitic dianhydride, chlorendic anhydride, and trimellitic anhydride.

[0100] Still other curing agents are Lewis acid:Lewis base complexes. Suitable Lewis acid:base complexes include, for example, BCl3:amine complexes, BF3:amine complexes, such as BF3:monoethylamine, BF3:propylamine, BF3:isopropylamine, BF3:benzylamine, BF3:chlorobenzylamine, BF 3: Complexes such as trimethylamine, BF3:pyridine, BF3:THF, AlCl3:THF, AlCl3:acetonitrile, and ZnCl2:THF are included.

[0101] Additional curing agents are polyamides, polyamines, amidoamines, polyamidoamines, polycycloaliphatic, polyetheramides, imidazoles, dicyandiamide, substituted ureas and urones, hydrazines and silicones.

[0102] Urea-based hardeners range from materials available under the tradename DYHARD (sold by Alzchem), and urea derivatives such as those sold as UR200, UR300, UR400, UR600, UR700, etc. Uron accelerators include, for example, 4,4-methylenediphenylenebis(N,N-dimethylurea) (available from Onmicure as U52M), etc.

[0103] When present, the total weight of the curing agent is in the range of 1% to 60% by weight of the resin composition. Typically, the curing agent is present in the range of 15% to 50% by weight, more typically in the range of 20% to 30% by weight.

[0104] Suitable toughening agents may include, but are not limited to, homopolymers or copolymers, either alone or in combination, of polyamides, copolyamides, polyimides, aramids, polyketones, polyetherimides (PEI), polyetherketones (PEK), polyetherketoneketones (PEKK), polyetheretherketones (PEEK), polyethersulfones (PES), polyetherethersulfones (PEES), polyesters, polyurethanes, polysulfones, polysulfides, polyphenylene oxides (PPOs) and modified PPOs, poly(ethylene oxide) (PEOs) and polypropylene oxides, polystyrenes, polybutadienes, polyacrylates, polystyrenes, polymethacrylates, polyacrylics, polyphenylsulfones, high performance hydrocarbon polymers, liquid crystal polymers, elastomers, segmented elastomers, and core-shell particles.

[0105] The reinforcing particles or agents, when present, may range from 0.1 wt% to 30 wt% of the resin composition. In one embodiment, the reinforcing particles or agents may be present in the range of 10 wt% to 25 wt%. In another embodiment, the reinforcing particles or agents may be present in the range of 0.1 to 10 wt%. Suitable reinforcing particles or agents include, for example, Virantage VW10200FRP, VW10300FP, and VW10700FRP from Solvay, BASF Ultrason E2020 and Sumikaexcel 5003P from Sumitomo Chemical.

[0106] The reinforcing particles or agents may be in the form of particles having a diameter greater than 20 microns to prevent incorporation into the fibrous layer. The size of the reinforcing particles or agents may be selected so as not to be filtered by the fibrous reinforcement. Optionally, the composition may also include inorganic ceramic particles, microspheres, microballoons, and clays.

[0107] The resin composition may contain conductive particles as described in, for example, WO 2013 / 141916, WO 2015 / 130368, and WO 2016 / 048885.

[0108] The mold for resin infusion can be a two-component sealed type or a single-sided type sealed with a vacuum bag. After the matrix resin is injected into the mold, the mold is heated to cure the resin.

[0109] During heating, the resin reacts with itself to form crosslinks in the matrix of the composite. After the initial heating, the resin gels. Once gelled, the resin no longer flows but rather behaves as a solid. After gelling, the temperature or cure may be increased to a final temperature to complete the cure. The final cure temperature will vary depending on the properties and characteristics of the thermosetting resin selected. Thus, in a preferred method, the composite is heated to a first temperature suitable to gel the matrix resin, and then the temperature is increased to a second temperature and held at the second temperature for a time to complete the cure, thereby resulting in a composite article.

[0110] Methods according to the present disclosure and carbon fibers produced therefrom are further illustrated by the following non-limiting examples. EXAMPLES

[0111] Example 1. PAN / pNIPAM film A polyacrylonitrile-based polymer with repeat units derived from methacrylic acid (MAA) was used. Poly(N-isopropylacrylamide) (pNIPAM; available from Sigma Aldrich, with a number average molecular weight of approximately 40,000 kDa) was used as the polymer additive. Blends of the two polymers were prepared using a Thinky AR-100 centrifugal mixer (2000 rpm) with a sample size of approximately 6 grams using either 1 wt % or 10 wt % pNIPAM relative to the PAN-based polymer concentration in DMSO (approximately 15 wt %). Polymer films were prepared by spreading the solution on a glass plate into a thin film and allowing it to air dry. The films were then extracted by one of the following methods:

[0112] [Table 1]

[0113] Using FTIR, two peaks were observed at approximately 1540 and 1640 cm, which are associated with the CN (stretch) and C=O (stretch) of the amide group, respectively. -1 The presence of pNIPAM was confirmed by approximately 1640 cm -1 The peak at approximately 1540 cm overlaps with the polymer baseline peak associated with carboxylic acid functionality. -1 The peak at is used as a quantitative measure of the presence of pNIPAM in the polymer.

[0114] Method 2 showed the greatest reduction in pNIPAM compared to the other extraction methods. Method 6 was the second most effective extraction method. Wash extractions (methods 1, 3, and 7) were the least effective and retained the most pNIPAM. Of the water-based methods, method 6 was identified as the most efficient in removing pNIPAM.

[0115] To further explore the effect of temperature, water immersions were performed at either 10° C. or 50° C. for 2-3 hours for both the 1 wt % and 10 wt % pNIPAM films (wt % is with respect to the PAN-based polymer). Cold water washes were performed at approximately 1540 cm at both loading levels. -1 A small peak was observed at

[0116] Scanning electron microscopy (SEM) was used to examine the physical characteristics of the films before and after extraction. Before extraction, the film appears smooth, at higher magnification the film appears grainy, and at 50,000x magnification the film appears to be a spongy network. After extraction, the lower magnification images have many surface features that are not present in the control film. The surface is full of small pits or small holes left by the extraction of pNIPAM from the PAN-based polymer. At higher magnification the surface features appear to be submicron cavities.

[0117] Example 2. PAN / pNIPAM White Fiber A polymer blend made using PAN-based polymer and polymer additives was used in Example 1. A 15 gallon Myers mixer was used to dissolve the PAN-based polymer and pNIPAM (3.16 kg PAN-based polymer and 30 g pNIPAM in 14.49 kg DMSO) using a disperser at 500 rpm and a sweeper at 60 rpm. The temperature was raised to 80°C and run for 2 hours before cooling to 45°C. The polymer solution ("dope") was then spun into a coagulation bath (65% DMSO). The coagulation bath was varied from 40 to 50°C, and the first draw bath was either 60°C or cooled to less than 10°C with ice. As a control, the same spinning process was performed with the PAN-based polymer without pNIPAM.

[0118] All of the filaments imaged by the standard sample preparation technique appear to be normal and show no signs of deviation from the control process. Optical images showed no macrovoids. Thus, the introduction of 1 wt. % pNIPAM does not significantly alter the preferred coagulation region from the base process.

[0119] However, by SEM, the fiber structures showed clear differences at the submicron scale. For example, samples taken from the coagulation bath showed filaments with a smooth skin and an internal core structure filled with porosity throughout the cross section. Surprisingly, the skin surface remained intact and showed no surface defects. The core structure appeared more porous and spongy compared to the standard coagulation sample, and in particular the network structure contains many small cavities previously observed in film structures. It was unexpected that the coagulation sample showed such significant porosity when the bath concentration was 65 wt% DMSO (outside the solubility range of pNIPAM), but it is speculated that pNIPAM precipitates at a different rate than PAN-based polymers and phase separates upon coagulation.

[0120] The coagulated filaments were stretched and the porous structure was found to remain intact. The densified spongy core and roughened skin layer as the fibers were stretched. The pore size was reduced, but the core still retained many small indentations in the structure, on the order of 100 nm in size. The pNIPAM concentration in the fibers was found to be greater in the coagulation bath compared to the washing bath and decreased after the washing step, indicating that pNIPAM was removed from the fibers during washing.

[0121] Another difference found for the pNIPAM blended fibers compared to the fibers for the control process is in the swelling behavior. The swelling of the pNIPAM blended fibers and the control fibers was measured according to the following procedure. A sample was taken and first centrifuged at 3000 rpm for 15 minutes to remove any adhering liquid from the filament surface. The collected sample was then submerged in a glass beaker / flask containing deionized water and "washed" for a minimum of 15 minutes. This washing step was then repeated two more times with fresh deionized water to ensure that the sample was completely coagulated and the solvent was removed. Once the final wash was completed, the sample was centrifuged again at 3,000 rpm for 15 minutes and weighed to determine the post-wash weight, or W a The samples were then placed in an air circulating oven at 110° C. for 3 hours. After drying, the samples were removed from the oven and placed in a desiccator for a minimum of 10 minutes. The dried, desiccated samples were reweighed and the final weight was calculated as W f The degree of swelling was then calculated using the following formula: Swelling degree (%) = (W a -W f )×(100 / W f )

[0122] This method relates the porosity of the fiber to the liquid absorption.

[0123] The swelling of the pNIPAM blended sample is similar for the coagulated and first stretched samples (163% vs. 181%) and much lower (183%) versus the control (192%). This difference suggests that pNIPAM may affect the kinetics of solvent interdiffusion into and out of the fiber.

[0124] Interestingly, the mechanical properties of the pNIPAM blended fibers did not appear to be affected by the coagulation bath and first draw bath conditions, although differences in structure, i.e., the presence of porosity, and swelling behavior were noted. Toughness, elongation, and Young's modulus are all within the measurement and process error for these experiments.

[0125] Example 3. Carbon Fibers Made from PAN / pNIPAM White Fibers PAN / pNIPAM white fibers produced according to the procedure described in Example 2 were oxidized and carbonized to form carbon fibers. White fibers produced from PAN-based polymers not containing pNIPAM were oxidized and carbonized to form carbon fibers. It was observed that the addition of pNIPAM did not significantly affect the mechanical properties. For the six carbonization runs that produced the control fibers, the average tensile strength was 482 + / - 32 ksi and the average tensile strength was 39.1 + / - 0.4 Msi, while the carbon fibers produced from the PAN / pNIPAM white fibers exhibited tensile strengths in excess of 500 ksi and modulus in excess of 38.3 Msi. This result indicates that the presence of 1% pNIPAM did not impede the mechanical load carrying capacity of the carbon fibers produced from the PAN / pNIPAM blend. The density of the carbon fibers was typically 1.74-1.79 g / cm 3 was lower than

[0126] Interestingly, strand fractography of the carbon fibers of the present invention showed very fine pores in the cross section of the fiber, all less than 100 nanometers in size and mostly concentrated near the skin surface of the fiber, demonstrating that the porosity created in the spinning is retained through carbonization.

[0127] Example 4. PAN / PVOH white fiber A spin dope was prepared according to the procedure described in Example 2, except that pNIPAM was replaced with polyvinyl alcohol (PVOH; available from Sigma Aldrich). The final spin dope contained about 17.5 wt.% solids (PAN-based polymer + pNIPAM) and about 5 wt.% PVOH (relative to the PAN-based polymer). The zero shear viscosity at 45°C was about 56 Pa*sec.

[0128] The coagulation bath was set at 50° C. and the spin dope was spun to form PAN / PVOH white fibers as in Example 2.

[0129] Fiber samples were taken after coagulation and after the first draw bath. The swelling of the sample taken after coagulation was 241%, which is much higher than is typical for fibers made only from the same PAN-based polymer, which is generally about 190-200%). The swelling of the sample after the first draw was 174%, which is also much higher than is typical for fibers made only from the same PAN-based polymer, which is generally about 120-140%). As with pNIPAM, the differences suggest that PVOH may affect the kinetics of interdiffusion of the solvent into and out of the fiber. Furthermore, the fiber sample extracted after the washing step shows fibers with even greater pore concentration and cavities throughout the core of the fiber compared to Example 2 due to the higher concentration of PVOH relative to the PAN-based polymer. This indicates that, importantly, the pore density and pore volume can be controlled by the polymer blend properties of the polymer blend concentration.

[0130] Example 5. Carbon Fibers Made from PAN / PVOH White Fibers PAN / PVOH white fibers prepared according to the procedure described in Example 4 were oxidized and carbonized to successfully form carbon fibers.

[0131] The tensile strength (according to ASTM method) was 340+ / -12 ksi and the tensile modulus was 31+ / -2.5 Msi. Also, the density of carbon fiber made from PAN / PVOH is typically 1.70-1.77 g / cm. 3 Lower.

[0132] It will be apparent to those skilled in the art that the conditions for carrying out the methods of the present invention described herein may be optimized based on the intended application and environment without departing from the spirit of the present disclosure.

Claims

1. A method for producing a porous polyacrylonitrile fiber, a) Polyacrylonitrile polymer (polymer A), A homopolymer (polymer B) different from the aforementioned polyacrylonitrile-based polymer, A step of forming a homogeneous solution comprising a first liquid containing a solvent for polymer A and in which polymer B is soluble, wherein the homogeneous solution contains polymer B at a concentration of 50% by weight or less relative to the total weight of the homogeneous solution. b) A step of spinning the homogeneous solution in or into a solidification bath containing a second liquid which includes a solvent for polymer A and a non-solvent for polymer A and in which polymer B is insoluble, thereby forming a polyacrylonitrile-based material containing polymer A and polymer B, c) The polyacrylonitrile material removed from the coagulation bath is brought into contact with a third liquid consisting of water in which polymer B is soluble, thereby selectively removing polymer B from the polyacrylonitrile material. This process involves manufacturing porous polyacrylonitrile fibers. Includes, The solvent for polymer A in the first liquid and the solvent for polymer A in the second liquid are each selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), zinc chloride (ZnCl₂) / water, sodium thiocyanate (NaSCN) / water, and mixtures thereof. The second liquid comprises, in proportion to the total weight of the second liquid, the solvent for polymer A at 85% by weight or less, and the non-solvent for polymer A at 15% by weight or more. Polymer B is given by formula (I): 【Chemistry 1】 (In the formula: R1 is hydrogen or methyl, A method for producing a homopolymer derived from a monomer (where R2 and R3 are each independently H or alkyl).

2. The method according to claim 1, wherein the solvent for polymer A in the first liquid and the solvent for polymer A in the second liquid are the same.

3. The method according to claim 1 or 2, wherein the homogeneous solution has polymer concentrations of polymer A and polymer B of 16% to 28% by weight, based on the total weight of the solution.

4. The method according to any one of claims 1 to 3, wherein the non-solvent for polymer A in the second liquid and the non-solvent for polymer A in the third liquid are the same.

5. The method according to any one of claims 1 to 4, wherein the non-solvent for polymer A in the second liquid is water.

6. The method according to any one of claims 1 to 5, wherein the first liquid comprises the solvent for polymer A.

7. The method according to any one of claims 1 to 6, wherein the second liquid comprises the solvent for polymer A and water.

8. Polymer A is composed of acrylonitrile and methacrylic acid (MAA), acrylic acid (AA), itaconic acid (ITA), methacrylate (MA), ethyl acrylate (EA), butyl acrylate (BA), methyl methacrylate (MMA), ethyl methacrylate (EMA), propyl methacrylate, butyl methacrylate, β-hydroxyethyl methacrylate, dimethylaminoethyl methacrylate, 2-ethylhexyl acrylate, isopropyl acetate, vinyl acetate (VA), vinyl propionate, and The method according to any one of claims 1 to 7, comprising a repeating unit derived from nilimidazole (VIM), acrylamide (AAm), diacetone acrylamide (DAAm), allyl chloride, vinyl bromide, vinyl chloride, vinylidene chloride, sodium vinyl sulfonate, sodium p-styrene sulfonate (SSS), sodium methallyl sulfonate (SMS), sodium 2-acrylamido-2-methylpropanesulfonate (SAMPS), and at least one comonomer selected from the group consisting of mixtures thereof.

9. The method according to claim 1, wherein polymer B is poly(N-isopropylacrylamide).

10. A method for producing a porous polyacrylonitrile fiber, a) Polyacrylonitrile polymer (polymer A), A polymer (polymer B) different from the aforementioned polyacrylonitrile-based polymer, A step of forming a homogeneous solution comprising a first liquid containing a solvent for polymer A and in which polymer B is soluble, wherein the homogeneous solution contains polymer B at a concentration of 50% by weight or less relative to the total weight of the homogeneous solution. b) A step of co-precipitating polymer A and polymer B by contacting the homogeneous solution formed in step a) with a second liquid containing a solvent for polymer A and a non-solvent for polymer A, and in which polymer B is insoluble, thereby forming a polyacrylonitrile-based material containing polymer A and polymer B, c) By contacting the polyacrylonitrile material with a third liquid containing a non-solvent for polymer A and in which polymer B is soluble, polymer B is selectively removed from the polyacrylonitrile material. This process involves manufacturing porous polyacrylonitrile fibers. Includes, The solvent for polymer A in the first liquid and the solvent for polymer A in the second liquid are each selected from the group consisting of dimethyl sulfoxide (DMSO), dimethylformamide (DMF), dimethylacetamide (DMAc), ethylene carbonate (EC), N-methyl-2-pyrrolidone (NMP), zinc chloride (ZnCl₂) / water, sodium thiocyanate (NaSCN) / water, and mixtures thereof. The second liquid comprises, in proportion to the total weight of the second liquid, the solvent for polymer A at 85% by weight or less, and the non-solvent for polymer A at 15% by weight or more. Polymer B is given by formula (I): 【Chemistry 1】 (In the formula: R １ is hydrogen or methyl, R ２ and R ３ A method comprising one or more repeating units derived from at least one monomer (where each is independently H or alkyl).

11. The method according to claim 10, wherein polymer B is a copolymer comprising monomer units derived from a monomer according to formula (I), and 50 weight percent ("weight %) or more of the repeating units of polymer B are derived from a monomer according to formula (I).

12. The method according to any one of claims 1 to 11, wherein step c) comprises stretching one or more fibers through one or more stretching baths and washing baths, wherein at least one bath contains the third liquid which contains a non-solvent for polymer A.

13. The method according to any one of claims 1 to 12, wherein in step a), the homogeneous solution contains polymer B in an amount of 10% by weight or less relative to the total weight of the homogeneous solution.

14. The method according to any one of claims 1 to 13, further comprising step d) drying the polyacrylonitrile fiber produced in step c).

15. The method according to any one of claims 1 to 14, further comprising step e) recovering polymer B at least partially.

16. A method for producing carbon fibers having a low density, (i) A step of producing a polyacrylonitrile fiber according to the method of any one of claims 1 to 15, (ii) A step of oxidizing the polyacrylonitrile fiber produced in step (i) to form stabilized carbon fiber precursor fibers, and then carbonizing the stabilized carbon fiber precursor fibers to produce carbon fibers having a density of 1.80 g / cm³ or less. Methods that include...

17. The density of the carbon fiber is 1.50 to 1.77 g / cm³. ３ The method according to claim 16.

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