Nanostructure polyacrylonitrile fiber with dense inside and sparse outside, preparation method and application

Abstract

The invention provides a nano-structure polyacrylonitrile fiber with dense inside and sparse outside and a preparation method thereof, and the method comprises the following steps: S1, dissolving polyacrylonitrile raw material powder in a solvent, and carrying out vacuum defoamation to obtain a polyacrylonitrile spinning solution with a certain concentration; s2, the spinning solution is extruded through a spinneret plate and enters a coagulating bath to be coagulated and formed, the temperature and concentration of the coagulating bath are regulated and controlled, and polyacrylonitrile nascent fibers with the uniform and compact structure are obtained; s3, washing the nascent fiber with water, drafting, drying and densifying to obtain the special polyacrylonitrile fiber with a nano-structure which is dense inside and sparse outside. The polyacrylonitrile fiber with the nano structure is prepared by regulating and controlling the solidification forming and drying densification processes, the average pore size of the center part of the cross section of the fiber is smaller than that of the edge part, and the fiber with the structure is higher in specific heat capacity value and has wide application prospects in thermal insulation materials.

Description

Technical Field

[0001] This invention belongs to the field of carbon fiber preparation technology, and particularly relates to a dense-inner-dense-outer-sparse nanostructured polyacrylonitrile fiber, its preparation method, and its application. Background Technology

[0002] Polyacrylonitrile fiber, also known as acrylic fiber, is a synthetic fiber copolymerized from acrylonitrile monomers and other monomers (such as methyl acrylate, methyl methacrylate, etc.), with an acrylonitrile content typically greater than 85%. Polyacrylonitrile fiber has significant advantages: high strength, high temperature resistance, and good chemical stability, allowing it to maintain its performance in various environments; excellent light and weather resistance, retaining 77% of its original strength after 18 months of outdoor exposure, making it suitable for products requiring prolonged sun exposure; and easy dyeing and vibrant colors, enabling the production of textiles in a variety of bright colors.

[0003] Currently, the main processes for preparing polyacrylonitrile (PA) fibers are polymerization, spinning, and post-treatment. Post-treatment includes steps such as preheating, steam drawing, washing, drying, heat setting, crimping, cutting, and packaging. Slowing down the solidification rate of PA during spinning and controlling the drying and densification process during post-treatment are key processes in preparing nanostructured PA fibers. Drying and densification significantly affects PA fibers by altering their crystallinity, grain size, microstructure, pore structure, and mechanical properties. To a certain extent, it reduces the overall porosity and increases the overall density of PA fibers. The evaporation of moisture can lead to inconsistent shrinkage between the internal and external structures of the fiber, resulting in a more compact molecular chain arrangement inside the fiber and a looser morphology on the outer layer. By controlling the drying and densification process, the fiber structure can be designed.

[0004] Polyacrylonitrile fibers prepared by traditional methods exhibit a distinct "core-sheath structure." This structure, composed of a dense sheath and a loose core, not only directly weakens the overall mechanical strength and structural stability of the fiber but also makes it susceptible to thermal shrinkage and degradation at high temperatures. These problems limit the development of polyacrylonitrile fibers in high-performance applications, necessitating improvements in production processes and structural design to overcome these drawbacks. Summary of the Invention

[0005] The first objective of this invention is to provide a method for preparing polyacrylonitrile fibers with a dense inner structure and a sparse outer structure.

[0006] Therefore, the above-mentioned objective of the present invention is achieved through the following technical solution: A method for preparing polyacrylonitrile fibers with a dense inner structure and a sparse outer structure includes the following steps: S1. Dissolve polyacrylonitrile raw material powder in a solvent and degas under vacuum conditions to obtain a polyacrylonitrile spinning solution of a certain concentration. S2. The polyacrylonitrile spinning solution obtained in step S1 is extruded through a spinneret into a coagulation bath to solidify and form a shape. The temperature and concentration of the coagulation bath are adjusted to obtain polyacrylonitrile nascent fibers with uniform and dense structure. S3. Wash the nascent polyacrylonitrile fibers obtained in step S2 with water to remove residual solvent and coagulant, and then stretch, dry and densify them to obtain polyacrylonitrile fibers with a dense inner and sparse outer nanostructure. During the drying and densification process: The temperature range is 1 to 3, with the first range being 55℃ to 75℃. Humidity is 20%–40%; Tension is 0.05 cN / tex to 0.5 cN / tex; The temperature and time for each segment after segmentation are 0–120 s, and the total drying and densification time is 150 s–300 s. The wind speed is 0.5 m / s to 0.8 m / s.

[0007] While adopting the above technical solutions, the present invention may also adopt or combine the following technical solutions: As a preferred embodiment of the present invention: in step S1, the molecular weight of the polyacrylonitrile raw material powder is 5 × 10⁻⁶. 4 ~20×10 4 g / mol.

[0008] As a preferred technical solution of the present invention: in step S1, the solvent for dissolving polyacrylonitrile is at least one of dimethyl sulfoxide, dimethylformamide, and dimethylacetamide.

[0009] As a preferred technical solution of the present invention: in step S1, the concentration of the spinning solution is 5 wt% to 22 wt%.

[0010] As a preferred technical solution of the present invention: in step S2, the temperature of the coagulation bath is -5℃ to 45℃; the coagulating agent used in the coagulation bath is at least one of ethylene glycol, glycerol, diethylene glycol, tetraethylene glycol, and polyethylene glycol; the concentration of the coagulating agent is 40 wt% to 100 wt%.

[0011] As a preferred technical solution of the present invention: in step S3, the washing process takes 1 min to 3 min, and the number of turns around the axis during the fiber washing process is 6 to 13.

[0012] As a preferred technical solution of the present invention: in step S3, the stretching ratio after washing is 1.0 to 1.5.

[0013] The second objective of this invention is to provide nanostructured polyacrylonitrile fibers prepared by the preparation method described above.

[0014] This invention obtains uniform and dense nascent fibers through controlled slow coagulation and molding, and nanostructured fibers through controlled drying and densification processes. By utilizing factors such as drying temperature, time, and tension during the fiber drying and densification process, the cross-sectional morphology of the fibers can be effectively controlled, making the fiber cross-section more regular and dense while achieving layered control, resulting in fibers with a dense inner structure and a loose outer structure. The polyacrylonitrile fibers obtained by controlling the coagulation and drying densification processes exhibit a special structure: a compact internal structure with relatively ordered molecular arrangement, ensuring high strength and stability; and a relatively loose outer structure, giving the fabric good breathability and softness. This dense inner and loose outer structure has significant advantages in thermal insulation fabrics: the dense inner layer, due to its compact fiber arrangement and high crystallinity, effectively reduces the solid heat conduction path and lowers the heat transfer efficiency through the fiber matrix; the loose outer layer, with its numerous closed air cavities and fluffy pores, utilizes the low thermal conductivity of air to suppress heat convection, while also embedding reflective particles or scattering heat radiation through a rough surface, further blocking radiative heat transfer. The external porous design also facilitates moisture diffusion, preventing increased thermal conductivity due to humidity, thus achieving a balance between high-efficiency thermal insulation and lightweight design, as well as comfort. Furthermore, when the internally dense and externally porous polyacrylonitrile fiber is used as a precursor for carbon fiber, it can also facilitate the mixing and wetting of carbon fiber and resin matrix while ensuring the strength of the carbon fiber, thereby improving the interfacial strength and overall performance of the carbon fiber composite material.

[0015] This invention prepares polyacrylonitrile fibers with a special nanostructure by controlling the coagulation and drying densification process. The average pore size of the central part of the fiber cross-section is smaller than that of the edge part. Furthermore, while ensuring the strength of the carbon fiber, the polyacrylonitrile fibers with a special nanostructure can significantly improve the mixing uniformity and wetting performance with the resin matrix, providing favorable conditions for the preparation of high-performance carbon fiber composite materials.

[0016] Furthermore, the use of alcohols in the solidification process is non-toxic and non-irritating, meeting environmental protection requirements. The parameters during the drying and densification process can be stably controlled, meeting the requirement of high feasibility. The preparation method of the internally dense and externally sparse nanostructured polyacrylonitrile fiber described in this invention has the characteristics of being environmentally friendly, having controllable reactions, being highly feasible, and producing more stable fiber quality, thus possessing significant industrial application value. Attached Figure Description

[0017] Figure 1 This is a scanning electron microscope image of the polyacrylonitrile fibers prepared in Example 2.

[0018] Figure 2The diagram shows the pore diameter distribution of the (a) outer layer and (b) inner layer of the polyacrylonitrile fiber prepared in Example 2.

[0019] Figure 3 The image shows a scanning electron microscope image of the polyacrylonitrile fibers prepared in Comparative Example 1.

[0020] Figure 4 The diagram shows the pore diameter distribution of the (a) outer layer and (b) inner layer of the polyacrylonitrile fiber prepared in Comparative Example 1.

[0021] Figure 5 This is a scanning electron microscope image of the polyacrylonitrile fibers prepared in Comparative Example 3.

[0022] Figure 6 The diagram shows the pore diameter distribution of the polyacrylonitrile fibers prepared in Comparative Example 3.

[0023] Figure 7 The graph shows a comparison of the specific heat capacity of the polyacrylonitrile fibers prepared in Example 1, Comparative Example 2, and Comparative Example 3.

[0024] Figure 8 This is a scanning electron microscope image of the polyacrylonitrile fibers prepared in Comparative Example 4.

[0025] Figure 9 This is a scanning electron microscope image of the polyacrylonitrile fibers prepared in Comparative Example 5. Detailed Implementation

[0026] A method for preparing polyacrylonitrile fibers with a dense inner structure and a sparse outer structure includes the following steps: S1. Dissolve polyacrylonitrile raw material powder in a solvent and degas under vacuum conditions to obtain a polyacrylonitrile spinning solution of a certain concentration. The polyacrylonitrile content in the polyacrylonitrile spinning solution is 5 wt% to 22 wt%, preferably 15 wt% to 22 wt%. The polyacrylonitrile spinning solution in the latter concentration range is used to prepare polyacrylonitrile fibers, which makes the fiber surface more uniform and can improve the overall quality of the fibers. At the same time, the amount of solvent used is less, which is more in line with environmental protection requirements.

[0027] The molecular weight of the polyacrylonitrile raw material is 50,000-200,000, preferably 100,000-200,000.

[0028] S2. The polyacrylonitrile spinning solution obtained in step S1 is extruded through a spinneret into a coagulation bath to solidify and form a shape. The temperature and concentration of the coagulation bath are adjusted to obtain polyacrylonitrile nascent fibers with uniform and dense structure. Polyacrylonitrile-based carbon fibers are obtained by spinning using conventional spinning methods in the art. Specifically, in step S2, wet spinning is used, which is beneficial to improve the microstructure of the fiber and enhance its mechanical properties.

[0029] S3. The polyacrylonitrile nascent fibers obtained in step S2 are washed with water, and then stretched, dried and densified to obtain polyacrylonitrile fibers with a dense inner and sparse outer nanostructure. The drying and densification process affects the cross-sectional morphology of fibers by influencing the movement of molecular chain segments, the rate of thermal expansion and contraction, and grain growth and rearrangement. The fiber drying process exhibits a characteristic of "external evaporation rate greater than internal rate": on the one hand, high temperatures accelerate the evaporation of surface moisture, causing the fiber surface to solidify rapidly and form a dense outer shell; on the other hand, this dense shell significantly hinders the outward diffusion of internal moisture, leading to continuous accumulation of internal water vapor and a continuous increase in vapor pressure. This causes a loosening transformation of the fiber's internal structure, resulting in a traditional "core-skin structure" of "dense surface and loose interior." Under appropriate drying conditions, after rapid evaporation of surface moisture, the molecular chains, due to the rapid contraction process, cannot be fully densely arranged, exhibiting a relatively loose state. Subsequently, during internal moisture evaporation, the contraction of the internal molecular chains is strongly constrained by the outer layer, generating inward compressive stress, forcing the internal molecular chains to arrange themselves tightly, ultimately forming a special structure of "dense interior and loose outer layer."

[0030] During the drying and densification process: The temperature range is 1 to 3, with the first range being 55℃ to 75℃. Humidity is 20%–40%; Tension is 0.05 cN / tex to 0.5 cN / tex; The temperature and time for each segment after segmentation are 0–120 s, and the total drying and densification time is 150 s–300 s. The wind speed is 0.5 m / s to 0.8 m / s.

[0031] The present invention will be described in detail below through examples. In the following examples, the mechanical property parameters were measured by the fiber monofilament strength method; the polyacrylonitrile raw material powder was a commercially available product of Shenzhen Huinuo Biotechnology Co., Ltd., with CAS number 25014-41-9.

[0032] In the following embodiments, the wet spinning conditions include: spinneret orifice diameter of 0.06 mm, number of spinneret orifices of 100, length-to-diameter ratio of 2:1, and spinneret material of tantalum.

[0033] Example 1 15 g of polyacrylonitrile powder was added to 85 g of dimethyl sulfoxide solvent, and stirred at 40 °C for 3 h until the polyacrylonitrile powder was completely dissolved. After degassing under vacuum at room temperature, a polyacrylonitrile solution with a concentration of 15 wt% was obtained. The molecular weight of the polyacrylonitrile raw material powder was 20 × 10⁻⁶. 4g / mol. A homogeneous alcohol / dimethyl sulfoxide coagulation bath was prepared, with an alcohol mass fraction of 70 wt%. The polyacrylonitrile solution was fed into the coagulation bath at 25°C through a spinneret to obtain nascent polyacrylonitrile fibers. The nascent fibers were wound 12 times around the shaft, washed with water for 1.5 min, and the stretch ratio after washing was 1.5 times. They were then dried and densified in two chambers at 70°C, 100°C, and 130°C, with 3 temperature sections and a temperature difference of 30°C. The drying and densification humidity was 35%, the tension was 0.1 cN / tex, the time for each temperature section was 50 s, the total drying and densification time was 150 s, and the air velocity was 0.8 m / s.

[0034] After the obtained fibers were quenched with liquid nitrogen and freeze-dried, the microstructure of the fibers was observed using a scanning electron microscope (SEM). The fibers exhibited a structure that was dense inside and sparse outside. The average pore size of the outer layer micropores on the fiber cross-section was about 86 nm, and the average pore size of the inner layer micropores was about 52 nm.

[0035] The crystallinity of the polyacrylonitrile fiber was determined to be 42% using X-ray diffraction analysis.

[0036] The mechanical properties of polyacrylonitrile fibers were tested using the fiber monofilament strength test. The tensile strength was 53.48 MPa, the elastic modulus was 1860 MPa, and the elongation at break was 19%.

[0037] Example 2 20 g of polyacrylonitrile powder was added to 80 g of dimethyl sulfoxide solvent, and stirred at 40 °C for 3 h until the polyacrylonitrile powder was completely dissolved. After degassing under vacuum at room temperature, a polyacrylonitrile solution with a concentration of 20 wt% was obtained. The molecular weight of the polyacrylonitrile raw material powder was 15 × 10⁻⁶. 4 g / mol. A homogeneous alcohol / dimethyl sulfoxide coagulation bath was prepared, wherein the alcohol mass fraction was 80 wt%. The polyacrylonitrile solution was fed into the coagulation bath at 25°C through a spinneret to obtain nascent polyacrylonitrile fibers. The nascent fibers were wound 8 times around the shaft, washed with water for 3 min, and the stretch ratio after washing was 1.5 times. They were then dried and densified in a 70°C chamber with one temperature segment, a drying and densification humidity of 30%, a tension of 0.05 cN / tex, a total drying and densification time of 280 s, and an air velocity of 0.6 m / s.

[0038] The obtained fibers were quenched with liquid nitrogen and freeze-dried, and their microstructure was observed using a scanning electron microscope (SEM). Figure 1 As shown, the fiber exhibits a dense inner and sparse outer structure. The average pore size of the outer layer micropores on the fiber cross-section is approximately 62 nm, and the diameter distribution is shown in the figure. Figure 2 As shown in (a), the average pore size of the inner layer micropores is approximately 27 nm, and the diameter distribution is shown in the figure. Figure 2 As shown in (b).

[0039] The crystallinity of the polyacrylonitrile fiber was determined to be 61% using X-ray diffraction analysis.

[0040] The mechanical properties of polyacrylonitrile fibers were tested using the fiber monofilament strength test. The tensile strength was 58.94 MPa, the elastic modulus was 3156 MPa, and the elongation at break was 59%.

[0041] Example 3 15 g of polyacrylonitrile powder was added to 85 g of dimethyl sulfoxide solvent, and stirred at 40 °C for 3 h until the polyacrylonitrile powder was completely dissolved. After degassing under vacuum at room temperature, a polyacrylonitrile solution with a concentration of 15 wt% was obtained. The molecular weight of the polyacrylonitrile raw material powder was 10 × 10⁻⁶. 4 g / mol. A homogeneous alcohol / dimethyl sulfoxide coagulation bath was prepared, wherein the alcohol mass fraction was 80 wt%. The polyacrylonitrile solution was fed into the coagulation bath at 25°C through a spinneret to obtain nascent polyacrylonitrile fibers. The nascent fibers were wound 10 times around the shaft, washed with water for 2.2 min, and the stretch ratio after washing was 1.3 times. They were then dried and densified in a 60°C chamber with one temperature segment, a drying and densification humidity of 35%, a tension of 0.15 cN / tex, a total drying and densification time of 300 s, and an air velocity of 0.5 m / s.

[0042] After the obtained fibers were quenched with liquid nitrogen and freeze-dried, the microstructure of the fibers was observed using a scanning electron microscope (SEM). The fibers exhibited a dense inner and sparse outer structure. The average pore size of the outer layer micropores on the fiber cross-section was about 68 nm, and the average pore size of the inner layer micropores was about 31 nm.

[0043] The crystallinity of the polyacrylonitrile fiber was determined to be 56% using X-ray diffraction analysis.

[0044] The mechanical properties of polyacrylonitrile fiber were tested using the fiber monofilament strength test. The tensile strength was 43.19 MPa, the elastic modulus was 2013 MPa, and the elongation at break was 54%.

[0045] Comparative Example 1 15 g of polyacrylonitrile powder was added to 85 g of dimethyl sulfoxide solvent, and stirred at 40 °C for 3 h until the polyacrylonitrile powder was completely dissolved. After degassing under vacuum at room temperature, a polyacrylonitrile solution with a concentration of 15 wt% was obtained. The molecular weight of the polyacrylonitrile raw material powder was 10 × 10⁻⁶. 4g / mol. A homogeneous alcohol / dimethyl sulfoxide coagulation bath was prepared, wherein the alcohol mass fraction was 50 wt%. The polyacrylonitrile solution was fed into the coagulation bath at 25°C through a spinneret to obtain nascent polyacrylonitrile fibers. The nascent fibers were wound 7 turns around the shaft, washed with water for 2 min, and the stretch ratio after washing was 1.2 times. They were then dried and densified in an 80°C chamber with one temperature segment, a drying and densification humidity of 20%, a tension of 0.05 cN / tex, a total drying and densification time of 220 s, and an air velocity of 1.2 m / s.

[0046] The obtained fibers were quenched with liquid nitrogen and freeze-dried, and their microstructure was observed using a scanning electron microscope (SEM). Figure 3 As shown, the fiber exhibits a structure that is sparse inside and dense outside. The average pore size of the outer layer micropores on the fiber cross-section is approximately 32 nm, and the diameter distribution is shown in the figure. Figure 4 As shown in (a), the average pore size of the inner layer micropores is approximately 95 nm, and the diameter distribution is shown in the figure. Figure 4 As shown in (b).

[0047] The crystallinity of the polyacrylonitrile fiber was determined to be 38% using X-ray diffraction analysis.

[0048] The mechanical properties of polyacrylonitrile fiber were tested using the fiber monofilament strength test. Its tensile strength was 43.49 MPa, elastic modulus was 2.9 GPa, and elongation at break was 39%.

[0049] Comparative Example 2 20 g of polyacrylonitrile powder was added to 80 g of dimethyl sulfoxide solvent, and stirred at 40 °C for 3 h until the polyacrylonitrile powder was completely dissolved. After degassing under vacuum at room temperature, a polyacrylonitrile solution with a concentration of 20 wt% was obtained. The molecular weight of the polyacrylonitrile raw material powder was 15 × 10⁻⁶. 4 g / mol. A homogeneous alcohol / dimethyl sulfoxide coagulation bath was prepared, with an alcohol mass fraction of 70 wt%. The polyacrylonitrile solution was fed into the coagulation bath at 25°C through a spinneret to obtain nascent polyacrylonitrile fibers. The nascent fibers were wound 9 times around the shaft, washed for 2 min, and the stretch ratio after washing was 1.2 times. They were then dried and densified in two chambers at 90°C and 100°C, with two temperature sections and a temperature difference of 10°C. The drying and densification humidity was 20%, the tension was 0.05 cN / tex, the single temperature section time was 60 s, the total drying and densification time was 120 s, and the air velocity was 1.5 m / s.

[0050] After the obtained fibers were quenched with liquid nitrogen and freeze-dried, the microstructure of the fibers was observed using a scanning electron microscope (SEM). The fibers exhibited a structure that was sparse inside and dense outside. The average pore size of the outer layer micropores on the fiber cross-section was about 68 nm, and the average pore size of the inner layer micropores was about 105 nm.

[0051] The crystallinity of the polyacrylonitrile fiber was determined to be 25% using X-ray diffraction analysis.

[0052] The mechanical properties of polyacrylonitrile fibers were tested using the fiber monofilament strength test. The tensile strength was 48.95 MPa, the elastic modulus was 2670 MPa, and the elongation at break was 26%.

[0053] Comparative Example 3 15 g of polyacrylonitrile powder was added to 85 g of dimethyl sulfoxide solvent, and stirred at 40 °C for 3 h until the polyacrylonitrile powder was completely dissolved. After degassing under vacuum at room temperature, a polyacrylonitrile solution with a concentration of 15 wt% was obtained. The molecular weight of the polyacrylonitrile raw material powder was 10 × 10⁻⁶. 4 g / mol. A homogeneous alcohol / dimethyl sulfoxide coagulation bath was prepared, wherein the alcohol mass fraction was 70 wt%. The polyacrylonitrile solution was fed into the coagulation bath at 25°C through a spinneret to obtain nascent polyacrylonitrile fibers. The nascent fibers were wound 11 times around the shaft, washed with water for 1.8 min, and the stretch ratio after washing was 1. They were then dried and densified in a 130°C chamber with one temperature segment, a drying and densification humidity of 35%, a tension of 0.25 cN / tex, a total drying and densification time of 120 s, and an air velocity of 1 m / s.

[0054] The obtained fibers were quenched with liquid nitrogen and freeze-dried, and their microstructure was observed using a scanning electron microscope (SEM). Figure 5 As shown, the fiber exhibits a dense overall structure, with an average pore size of approximately 54 nm in the fiber cross-section. The diameter distribution is shown in the figure below. Figure 6 As shown.

[0055] The crystallinity of the polyacrylonitrile fiber was determined to be 36% using X-ray diffraction analysis.

[0056] The mechanical properties of polyacrylonitrile fiber were tested using the fiber monofilament strength test. The tensile strength was 63.72 MPa, the elastic modulus was 1021 MPa, and the elongation at break was 9.86%.

[0057] Differential scanning calorimetry (DSC) was used to test the fibers obtained in Example 1 (dense inside, loose outside), Comparative Example 2 (loose inside, dense outside), and Comparative Example 3 (overall dense) to evaluate the thermal insulation performance of fibers with different structures. The specific heat capacity of the fibers in the temperature range of 25℃ to 140℃ is shown below. Figure 7 As shown, the specific heat capacity of fibers with a dense inner structure and a sparse outer structure is significantly higher than that of fibers with a sparse inner structure and a dense outer structure or a dense overall structure, indicating that they have good application prospects in the preparation of thermal insulation materials.

[0058] Comparative Example 4 20 g of polyacrylonitrile powder was added to 80 g of dimethyl sulfoxide solvent, and stirred at 40 °C for 3 h until the polyacrylonitrile powder was completely dissolved. After degassing under vacuum at room temperature, a polyacrylonitrile solution with a concentration of 20 wt% was obtained. The molecular weight of the polyacrylonitrile raw material powder was 15 × 10⁻⁶. 4 g / mol. A uniformly mixed H2O / dimethyl sulfoxide coagulation bath was prepared, wherein the mass fraction of H2O was 80 wt%. The polyacrylonitrile solution was fed into the coagulation bath at 25°C through a spinneret to obtain nascent polyacrylonitrile fibers. The nascent fibers were wound 8 times around the shaft, washed with water for 3 min, and the stretch ratio after washing was 1.5 times. They were then dried and densified in a 70°C chamber with one temperature segment, a drying and densification humidity of 30%, a tension of 0.05 cN / tex, a total drying and densification time of 280 s, and an air velocity of 0.6 m / s.

[0059] The obtained fibers were quenched with liquid nitrogen and freeze-dried, and their microstructure was observed using a scanning electron microscope (SEM). Figure 8 As shown, the fibers exhibit an uneven structure containing large pores.

[0060] Comparative Example 5 15 g of polyacrylonitrile powder was added to 85 g of dimethyl sulfoxide solvent, and stirred at 40 °C for 3 h until the polyacrylonitrile powder was completely dissolved. After degassing under vacuum at room temperature, a polyacrylonitrile solution with a concentration of 15 wt% was obtained. The molecular weight of the polyacrylonitrile raw material powder was 10 × 10⁻⁶. 4 g / mol. A homogeneous H2O / dimethyl sulfoxide coagulation bath was prepared, wherein the mass fraction of H2O was 50 wt%. The polyacrylonitrile solution was fed into the coagulation bath at 5℃ through a spinneret to obtain nascent polyacrylonitrile fibers. The nascent fibers were wound 7 times around the shaft, washed with water for 2 min, and the stretch ratio after washing was 1.2 times. They were then dried and densified in an 80℃ chamber with one temperature segment, a drying and densification humidity of 20%, a tension of 0.05 cN / tex, a total drying and densification time of 220 s, and an air velocity of 1.2 m / s.

[0061] The obtained fibers were quenched with liquid nitrogen and freeze-dried, and their microstructure was observed using a scanning electron microscope (SEM). Figure 9 As shown, the fibers exhibit an uneven structure containing large pores.

[0062] Polyacrylonitrile fibers prepared by the traditional H2O coagulation bath system have a dense outer layer and a finger-like macroporous structure in the inner layer, and it is impossible to obtain a nanostructure by controlling the coagulation and drying densification process.

[0063] The above specific embodiments are used to explain and illustrate the present invention, and are only preferred embodiments of the present invention, not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made to the present invention within the spirit and scope of the claims shall fall within the protection scope of the present invention.

Claims

1. A method for preparing an inner dense and outer sparse nanostructured polyacrylonitrile fiber, characterized by, The method comprises the following steps: S1, dissolving polyacrylonitrile raw material powder in a solvent and degassing under vacuum to obtain a polyacrylonitrile spinning dope of a certain concentration; S2, extruding the polyacrylonitrile spinning dope obtained in step S1 through a spinneret into a coagulation bath to coagulate and form, and adjusting the coagulation bath temperature and concentration to obtain polyacrylonitrile nascent fibers with uniform and dense structure; S3, washing the polyacrylonitrile nascent fibers obtained in step S2, and then performing drawing and drying densification to obtain polyacrylonitrile fibers with a nanostructure of dense inside and sparse outside; During the drying densification process: The number of temperature sections is 1-3, the first section temperature is 55-75℃; The humidity is 20-40%; The tension is 0.05-0.5 cN / tex; The single-section temperature time after sectioning is 0-120 s, and the total drying densification time is 150-300 s; The wind speed is 0.5-0.8 m / s.

2. The production method according to claim 1, characterized by, The molecular weight of the polyacrylonitrile raw material powder in step S1 is 5 x 10 4 ~ 20 x 10 4 g / mol.

3. The preparation method according to claim 1, characterized in that, In step S1, the solvent for dissolving polyacrylonitrile is at least one of dimethyl sulfoxide, dimethylformamide, and dimethylacetamide.

4. The method of claim 1, wherein, In step S1, the spinning dope concentration is 5-22 wt%.

5. The preparation method according to claim 1, characterized in that, In step S2, the coagulation bath temperature is -5-45℃; the coagulant used in the coagulation bath is at least one of ethylene glycol, glycerol, diethylene glycol, tetraethylene glycol, and polyethylene glycol; and the concentration of the coagulant is 40-100 wt%.

6. The method of claim 1, wherein, In step S3, the water washing process time is 1-3 min, and the fiber water washing process has 6-13 turns around the axis.

7. The preparation method according to claim 1, characterized in that, In step S3, the drawing ratio after water washing is 1.0-1.

5.

8. The nanostructured polyacrylonitrile fiber prepared by the method of any one of claims 1-7.

9. The use of the nanostructured polyacrylonitrile fiber of claim 8 in the preparation of thermal insulation materials.

Images