A polyacrylonitrile fiber thermal insulation and cooling aerogel and its self-assembly preparation method

Abstract

The present invention discloses a polyacrylonitrile fiber thermal insulation and cooling aerogel and a self-assembly preparation method thereof, relating to the field of aerogel technology. The method comprises the steps of subjecting polyacrylonitrile fiber to amidoximation modification to obtain amidoxime-based polyacrylonitrile fiber; mixing the amidoxime-based polyacrylonitrile fiber with silica fiber and titanium dioxide, and then deprotonating the mixture under alkaline conditions to obtain a dispersion; freezing the dispersion to obtain an oriented cryogel; and re-protonating the oriented cryogel under acidic conditions to achieve self-assembly to obtain the polyacrylonitrile fiber thermal insulation and cooling aerogel. The method can cause strong crosslinking between polyacrylonitrile fibers, thereby forming a stable structure, ultimately obtaining a polyacrylonitrile fiber thermal insulation and cooling aerogel with good mechanical strength and excellent thermal insulation performance.

Description

Technical Field

[0001] The invention relates to a polyacrylonitrile fiber heat-insulating and cooling aerogel and a self-assembly preparation method thereof, belonging to the technical field of aerogel preparation. Background Art

[0002] Polyacrylonitrile (PAN), also known as acrylic fiber, has the advantages of resistance to organic solvents, wear resistance, high temperature resistance and high mechanical stability.

[0003] The method for preparing PAN nanofiber aerogel generally includes subjecting PAN nanofibers to an infusible treatment, the purpose of which is to convert the linear molecular chains of polyacrylonitrile into a ladder-shaped structure with heat-resistant properties to maintain the fiber morphology under high temperature conditions.

[0004] There are two main types of infusibility reactions: (1) PAN molecular chains undergo a cyclization reaction to form a ladder structure; (2) oxidative cross-linking reactions occur between molecular chains to form a more stable heat-resistant network ladder structure. However, in the actual infusibility process, these two chemical reactions occur almost simultaneously and the process is complex, making it difficult to accurately control the reaction.

[0005] During the infusibility treatment, stretching forces are typically applied to suppress disorientation caused by thermal shrinkage of the fiber molecules along the axial direction. Theoretically, greater thermal shrinkage indicates more severe disorientation. Fiber shrinkage during the infusibility treatment can be categorized as physical and chemical. Within the temperature range of 150–170°C, physical shrinkage primarily occurs due to the release of internal fiber stress. Within the temperature range of 210–280°C, chemical shrinkage occurs due to intermolecular cyclization and cross-linking reactions. To effectively suppress thermal shrinkage of PAN molecules during the infusibility treatment, polyacrylonitrile precursors are typically heat-treated in an inert atmosphere at a suitable temperature. During this process, the fiber molecules first undergo cyclization, forming a highly oriented, rigid, ring-shaped molecular structure along the fiber axis, which also enhances the regularity of the molecular arrangement. Subsequently, the heat-cyclized fibers undergo oxidative cross-linking, further transforming the cyclized, rigid molecular structure into a more stable, regular trapezoidal network, thereby improving the fiber's heat resistance and structural stability. However, the preparation of PAN nanofiber aerogels via high-temperature cyclization and cross-linking is complex and requires high temperature control.

[0006] PAN undergoes alkaline hydrolysis in a sodium hydroxide (NaOH) solution. This process causes the PAN fibers to swell, forming a cross-linked network that allows for the rapid and efficient construction of a network structure. Under alkaline conditions, the nitrile groups (-CN) in PAN fibers hydrolyze to generate reactive groups such as carboxyl groups (-COOH) and amide groups (-CONH2). This enhances the chemical reactivity of PAN, allowing the reactive groups to be further grafted with other functional groups (such as acrylates and citric acid) to prepare multifunctional composites. During alkaline hydrolysis, PAN chains break and recombine, while crosslinkers (such as TiO2 and epoxy resins) form a three-dimensional network structure. Alkaline hydrolysis and chemical crosslinking can enhance the mechanical properties and thermal stability of PAN nanofibers, increasing the breaking strength from 50 MPa to 80 MPa and raising the initial decomposition temperature from 200°C to 320°C.

[0007] Alkaline hydrolysis of PAN nanofibers is simple to perform and offers a controllable swelling rate. By adjusting the alkali concentration (8-12%), the swelling rate of PAN nanofiber membranes can be stabilized below 5%, making them suitable for solvent-resistant separation membranes. However, after NaOH hydrolysis, PAN nanofiber membranes require repeated cleaning (multiple soaking → cleaning → drying), a cumbersome process with high energy consumption. Traditional alkaline hydrolysis requires prolonged, high-temperature treatment (>24 hours). PAN fibers are easily hydrolyzed in strong alkali (-CN → -CONH2 / -COONa), resulting in poor alkali resistance (mechanical properties degrade after <200 hours). Excessive alkali concentrations disrupt the crosslinked network (swelling rate >30%), and increased carboxyl content (>30%) leads to decreased mechanical strength (breaking strength <50 MPa), making it difficult to synergize function and structure.

[0008] There is still a gap in the construction of PAN fiber aerogels with three-dimensional network structures by low-temperature and efficient alkaline hydrolysis (such as rapid reaction below 60°C). Developing a new cross-linking method based on PAN post-alkaline hydrolysis to increase the mechanical strength of PAN aerogels without affecting their thermal insulation properties is of great significance, but it also presents significant challenges. Summary of the Invention

[0009] The purpose of the present invention is to overcome the deficiencies in the prior art and provide a polyacrylonitrile fiber thermal insulation and cooling aerogel and a self-assembly preparation method thereof, which can enable strong cross-linking between polyacrylonitrile fibers to form a stable structure, and ultimately obtain a polyacrylonitrile fiber thermal insulation and cooling aerogel with good mechanical strength and excellent thermal insulation performance.

[0010] To achieve the above object, the present invention is implemented by adopting the following technical solutions:

[0011] In one aspect, the present invention provides a self-assembly preparation method of polyacrylonitrile fiber thermal insulation and cooling aerogel, comprising:

[0012] The polyacrylonitrile fiber is modified by amidoximation to obtain amidoxime-based polyacrylonitrile fiber;

[0013] Amidoxime-based polyacrylonitrile fiber is mixed with silica fiber and titanium dioxide, and then deprotonated under alkaline conditions to obtain a dispersion;

[0014] The dispersion is freeze-formed to obtain an oriented cryogel;

[0015] The oriented cryogel is protonated under acidic conditions to achieve self-assembly to obtain polyacrylonitrile fiber thermal insulation and cooling aerogel.

[0016] Furthermore, the polyacrylonitrile fiber is subjected to amidoximation modification to obtain amidoxime-based polyacrylonitrile fiber, comprising:

[0017] The polyacrylonitrile fiber is added into a hydroxylamine hydrochloride solution for heating reaction. After the reaction is completed, the polyacrylonitrile fiber is taken out, washed multiple times, and dried to obtain the amidoxime-based polyacrylonitrile fiber.

[0018] Furthermore, at least one of the following conditions must be met:

[0019] The concentration of the hydroxylamine hydrochloride solution is 0.5-0.6 mol / L;

[0020] The heating temperature range is 65~70℃;

[0021] The reaction time is 2 to 2.5 hours;

[0022] The drying temperature is 60-70°C.

[0023] Furthermore, the amidoxime group conversion rate in the amidoximation modification of the polyacrylonitrile fiber is 22-30%.

[0024] Furthermore, the amidoxime-based polyacrylonitrile fiber is mixed with silica fiber and titanium dioxide and then deprotonated under alkaline conditions to obtain a dispersion, comprising:

[0025] Adding amidoxime-based polyacrylonitrile fiber and silica fiber into dimethyl sulfoxide solution and homogeneously dispersing them to obtain a fiber suspension;

[0026] Titanium dioxide and potassium hydroxide are added to the fiber suspension, and the mixture is stirred for reaction to obtain a dispersion.

[0027] Furthermore, the mass ratio of the amidoxime-based polyacrylonitrile fiber, silica fiber, and dimethyl sulfoxide solution is 10:(1-6):(5000-6000); the concentration range of the dimethyl sulfoxide solution is 98-100wt%; the mass ratio of the silica fiber and titanium dioxide is (1-1.5):1; the mass ratio of the amidoxime-based polyacrylonitrile fiber and potassium hydroxide is (5-10):1;

[0028] And / or, the homogenizing and dispersing is achieved by a homogenizer, and the conditions thereof include homogenizing and dispersing at a speed of 100,000 to 120,000 rpm for 30 to 35 minutes;

[0029] And / or, the stirring reaction time is 3 to 5 hours.

[0030] Furthermore, the dispersion is freeze-formed to obtain the oriented cryogel, comprising:

[0031] Pour the dispersion into the frozen mold and place it in an insulated box;

[0032] Liquid nitrogen is poured into the insulated box and frozen for 5 to 10 minutes, so that ice crystals grow from the bottom of the dispersion at -196 to -172°C to obtain a directional cryogel.

[0033] Furthermore, the oriented cryogel is protonated under acidic conditions to achieve self-assembly to obtain polyacrylonitrile fiber thermal insulation and cooling aerogel, comprising:

[0034] The oriented cryogel is placed in an aqueous solution of acetic acid, allowed to stand, and then freeze-dried to obtain a polyacrylonitrile fiber heat-insulating and cooling aerogel.

[0035] Furthermore, at least one of the following conditions must be met:

[0036] The pH value of the aqueous solution of acetic acid is in the range of 3 to 4;

[0037] The standing time is 10 to 14 hours;

[0038] The freeze drying comprises first freezing at -75 to -70°C for 4 to 6 hours, and then drying at -75 to -70°C and 1 to 2 Pa for 36 to 48 hours.

[0039] On the other hand, the present invention also provides a polyacrylonitrile fiber thermal insulation and cooling aerogel, which is prepared by the self-assembly preparation method of the polyacrylonitrile fiber thermal insulation and cooling aerogel as described in any of the above items.

[0040] Compared with the prior art, the present invention has the following beneficial effects:

[0041] The present invention uses polyacrylonitrile fiber as a raw material, performs amidoxime modification on the polyacrylonitrile fiber, converts C≡N groups on the surface of the polyacrylonitrile fiber into CO-NH2 groups in a hydroxylamine hydrochloride solution, causes crosslinking inside the amidoxime-based polyacrylonitrile fiber (AOPAN), then alkaline hydrolysis is performed on the amidoxime-based polyacrylonitrile fiber under alkaline conditions. During the alkaline hydrolysis process, a deprotonation reaction occurs between the AOPAN fibers, and the AOPAN fibers are adsorbed and bonded with TiO2 through positive and negative charges. The AOPAN fibers are dissolved and alkaline hydrolyzed to form a gel-like dispersion, so that SiO2 nanofibers and TiO2 particles are evenly distributed on the surface and inside of the gel. The surface of the AOPAN after alkaline hydrolysis contains a large number of hydroxyl groups and carboxyl groups. Finally, it is re-protonated under acidic conditions, and the surface of the AOPAN fiber forms a charge force through hydrogen bonding, resulting in strong crosslinking between the fibers, so that a stable crosslinking system is formed between the fibers. An aerogel material with a stable structure is obtained through a displacement reaction and freeze drying.

[0042] The aerogel of the present invention has a stable, directional porous structure, and SiO2 nanofibers and TiO2 particles are added, evenly distributed throughout the aerogel. The porous structure effectively isolates heat, achieving thermal insulation. The high emissivity and reflectivity of the SiO2 nanofibers and TiO2 particles under visible light reduce thermal radiation when exposed to light. The synergistic effect of the porous structure and radiant cooling gives the aerogel excellent thermal insulation. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 Schematic diagrams of the polyacrylonitrile fiber thermal insulation and cooling aerogel prepared in Example 1 of the present invention are shown using a scanning electron microscope. Schematic diagrams of the scanning electron microscope at a scale of 250 μm are shown in a figure a, a scale of 10 μm are shown in a figure b, and a scale of 1 μm are shown in a figure c.

[0044] Figure 2 This is a schematic diagram of the thermal insulation effect of the polyacrylonitrile fiber thermal insulation and cooling aerogel prepared in Comparative Examples 1 to 5 in Example 1 of the present invention;

[0045] Figure 3 Schematic diagram of reflectivity and emissivity performance testing of the polyacrylonitrile fiber thermal insulation and cooling aerogel prepared in Example 1 of the present invention;

[0046] Figure 4 Schematic diagram comparing the temperature rise curve of the polyacrylonitrile fiber thermal insulation and cooling aerogel prepared in Example 1 of the present invention on the back side of a xenon lamp and the temperature rise curve on the back side of polystyrene foam;

[0047] Figure 5Schematic diagram comparing the backside temperature rise curve of the polyacrylonitrile fiber thermal insulation and cooling aerogel prepared in Example 1 of the present invention and the backside temperature rise curve of polystyrene foam under xenon lamp illumination of different powers;

[0048] Figure 6 This is a schematic diagram comparing the daytime radiation cooling performance test results of the polyacrylonitrile fiber insulation and cooling aerogel prepared in Example 1 of the present invention and polystyrene in a natural environment. DETAILED DESCRIPTION

[0049] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention.

[0050] Example 1

[0051] An embodiment of the present invention provides a self-assembly preparation method of polyacrylonitrile fiber thermal insulation and cooling aerogel, comprising:

[0052] First, polyacrylonitrile (PAN, from McLean, molecular weight 160,000, fiber fineness 200-500 nm) was amidoxime-modified. One gram of PAN fiber was placed in 500 mL of a 0.5 mol / L hydroxylamine hydrochloride solution and subjected to amidoxime modification at 65°C for 2 hours. After the reaction, the PAN fiber was removed and washed several times with distilled water to remove residual chemical residue. The fiber was then dried in an oven at 60°C to produce amidoxime-modified PAN fiber (AOPAN fiber) with an amidoxime group conversion rate of 27%.

[0053] Next, 1g of AOPAN fibers and 0.4g of SiO2 fibers (from Aladdin, fiber fineness 150-550nm) were added to 500g of a 100wt% DMSO solution (from Aladdin, analytical grade) and homogenized in a homogenizer at 100,000rpm for 30 minutes to obtain a fiber suspension. Subsequently, 0.4g of TiO2 and 0.1g of KOH were added, and the mixture was stirred at room temperature for 3 hours. This step was to alkaline hydrolysis of the AOPAN fibers.

[0054] Next, the dispersion is poured into a directional freezing mold, which is then placed in an insulated box filled with liquid nitrogen. The liquid nitrogen (-172°C) causes ice crystals to grow from the bottom of the mold. After freezing for 8 minutes, a freeze-formed directional cryogel is obtained.

[0055] Finally, the cryogel was placed in a mixture of acetic acid and deionized water (pH = 4, mass ratio of cryogel to mixture = 1:10; in this example, the cryogel mass was 1g and the mixture mass was 10g). The mixture was allowed to stand for 12 hours to remove the DMSO under acidic conditions. After DMSO removal, the wet gel was removed from the mixture and freeze-dried in a -70°C freezer for 4 hours, followed by drying in a freeze dryer at -70°C and 2 Pa for 36 hours to obtain the polyacrylonitrile fiber insulation and cooling aerogel.

[0056] like Figure 1 As shown in the scanning electron microscope image, it can be seen that during the directional freezing process, the fibers mainly form a directional skeleton structure ( Figure 1 a), while SiO2 and TiO2 play a reinforcing role in the skeleton structure ( Figure 1 b), thus forming a complex network structure. A certain amount of TiO2 adheres to the AOPAN fibers on the directional pore walls, and TiO2 in this skeleton structure strengthens the fiber strength ( Figure 1 This effectively demonstrates that the alkaline hydrolysis of AOPAN fibers leads to the self-reinforcement of SiO2 and TiO2 by adhesion, as well as the construction of the aerogel's oriented structure, which produces a synergistic effect within the aerogel, resulting in a porous structure within the aerogel and stable mechanical strength.

[0057] Example 2

[0058] An embodiment of the present invention provides a self-assembly preparation method of polyacrylonitrile fiber thermal insulation and cooling aerogel, comprising:

[0059] First, polyacrylonitrile (PAN, from McLean, molecular weight 160,000, fiber fineness 200-500 nm) was amidoxime-modified. 1.2 g of PAN fiber was placed in 505 mL of a 0.6 mol / L hydroxylamine hydrochloride solution and treated at 70°C for 2.5 hours. After the reaction, the PAN fiber was washed several times with distilled water to remove residual chemical residues and then dried in an oven at 70°C to produce amidoxime-modified PAN fiber (AOPAN fiber) with an amidoxime group conversion rate of 30%.

[0060] Next, 1.2g of AOPAN fibers and 0.5g of SiO2 fibers (from Aladdin, with a fiber fineness of 150-550nm) were added to 600g of a 100wt% DMSO solution (from Aladdin, analytical grade). The mixture was homogenized and dispersed in a homogenizer at 120,000rpm for 30 minutes to obtain a fiber suspension. Subsequently, 0.5g of TiO2 and 0.15g of KOH were added, and the mixture was stirred at room temperature for 3 hours.

[0061] Next, the dispersion is poured into a directional freezing mold, which is then placed in an insulated box filled with liquid nitrogen. The liquid nitrogen (-172°C) causes ice crystals to grow from the bottom of the mold. After freezing for 6 minutes, a freeze-formed directional cryogel is obtained.

[0062] Finally, the cryogel was placed in a mixture of acetic acid and deionized water (pH = 3.5, mass ratio of cryogel to mixture = 1:10; in this example, the cryogel mass was 1g and the mixture mass was 10g). The mixture was allowed to stand for 14 hours to remove the DMSO under acidic conditions. After DMSO removal, the wet gel was removed from the mixture and freeze-dried in a -75°C refrigerator for 5 hours, followed by drying in a freeze dryer at -75°C and 2 Pa for 45 hours to obtain the polyacrylonitrile fiber insulation and cooling aerogel.

[0063] Comparative Example 1:

[0064] The only difference between this comparative example and Example 1 is that the stirring reaction time in the alkaline hydrolysis treatment is 1 h.

[0065] Comparative Example 2:

[0066] The only difference between this comparative example and Example 1 is that the stirring reaction time in the alkaline hydrolysis treatment is 2 h.

[0067] Comparative Example 3:

[0068] The only difference between this comparative example and Example 1 is that the stirring reaction time in the alkaline hydrolysis treatment is 4 hours.

[0069] Comparative Example 4:

[0070] The only difference between this comparative example and Example 1 is that the stirring reaction time in the alkaline hydrolysis treatment is 5 h.

[0071] Comparative Example 5:

[0072] The only difference between this comparative example and Example 1 is that the stirring reaction time in the alkaline hydrolysis treatment is 6 h.

[0073] The following is a detailed analysis of the performance of the polyacrylonitrile fiber thermal insulation and cooling aerogels prepared in Example 1 and Comparative Examples 1 to 5: First, the polyacrylonitrile fiber thermal insulation and cooling aerogels prepared in Example 1 and Comparative Examples 1 to 5 were subjected to a thermal insulation test. The test method is as follows:

[0074] The polyacrylonitrile fiber heat insulation and cooling aerogels prepared in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, and Comparative Example 5 were placed on a heating platform (Bangyuan-BY1515 intelligent constant temperature heating platform) and set at 100°C. The surface temperature of Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, and Comparative Example 5 was tested using an infrared thermal imager (Hikvision Micro-Image H21Pro thermal imager). The insulation test results are shown in Figure 2. Figure 2 As shown in the figure, it can be seen that the thermal insulation effect is ranked from good to bad as Example 1, Comparative Example 3, Comparative Example 4, Comparative Example 5, Comparative Example 2, and Comparative Example 1, and the thermal insulation effect of the polyacrylonitrile fiber thermal insulation and cooling aerogel prepared in Example 1 is much better than that of Comparative Example 3, which effectively proves that the thermal insulation performance of the polyacrylonitrile fiber thermal insulation and cooling aerogel prepared in Example 1 is superior.

[0075] The principle of the above test results is based on the fact that if the alkaline hydrolysis treatment time is too short, the cross-linking between the AOPAN fibers will not be in place, resulting in extremely poor mechanical strength of the aerogel finally prepared and affected thermal insulation performance. If the alkaline hydrolysis treatment time is too long, the AOPAN fibers will be excessively hydrolyzed and the fiber structure will be destroyed. Finally, the internal support skeleton of the aerogel obtained after freeze-drying will be reduced, the shrinkage rate will increase, the density will increase, and the porosity will be too low, so its thermal insulation performance will also be affected.

[0076] Secondly, an ultraviolet / visible / near-infrared (UV-VIS-NIR) spectrophotometer equipped with an integrating sphere (Shimadzu Corporation, Japan, model UV3600) was used to test the diffuse reflectivity of the polyacrylonitrile fiber thermal insulation and cooling aerogel prepared in Example 1. The weighted average reflectivity of all reflectivity values ​​was calculated using formula (1).

[0077] A Fourier transform infrared spectrometer with an integrating sphere accessory (from Thermo Fisher Scientific (China) Co., Ltd., instrument number: IS50) was used to first test the mid-infrared reflectivity and mid-infrared transmittance of the sample. Then, the mid-infrared emissivity of the aerogel was obtained using formula (2). Finally, the weighted average emissivity of the aerogel was calculated using formula (3). The expressions of formulas (1) to (3) are as follows:

[0078] (1)

[0079] (2)

[0080] (3)

[0081] Where: represents the weighted average reflectance within the solar spectrum, Indicates the wavelength of the material within the solar spectrum The reflectivity at Represents the standardized ASTM G173 global solar intensity spectrum, represents the total emissivity of the material, R represents the total reflectivity of the material, and T represents the total transmittance of the material. represents the weighted average emissivity of the material in the mid-infrared band, represents the spectral emissivity of the black body, Represents the spectral emissivity of a material.

[0082] from Figure 3 It can be seen that the polyacrylonitrile fiber thermal insulation and cooling aerogel prepared in Example 1 has high reflectivity and emissivity. According to formulas (1) to (3), the average emissivity can be calculated to be 97.0273; the solar spectrum reflectivity is 0.8037; and the visible spectrum reflectivity is 0.7391, which proves that the aerogel can achieve excellent radiation cooling effect.

[0083] Then, the polyacrylonitrile fiber heat insulation and cooling aerogel (size 10×10 cm, thickness 3 mm) prepared in Example 1 and polystyrene foam (from Yue Ning insulation foam board, model 18k, cut to size 10×10 cm, thickness 3 mm) were placed under the light of a xenon lamp for a period of time and then taken out and combined. Figure 4 The back temperature of the polyacrylonitrile fiber thermal insulation and cooling aerogel of Example 1 is always lower than that of the polystyrene foam.

[0084] Furthermore, the polyacrylonitrile fiber heat insulation and cooling aerogel prepared in Example 1 was placed together with polystyrene foam under xenon lamps of different powers for irradiation. The results are as follows: Figure 5 As shown in the figure, the back temperature of polyacrylonitrile fiber thermal insulation and cooling aerogel and polystyrene foam continues to increase with the increase of light intensity, but the back temperature of polyacrylonitrile fiber thermal insulation and cooling aerogel is always lower than that of polystyrene foam.

[0085] Finally, the polyacrylonitrile fiber heat insulation and cooling aerogel prepared in Example 1 was placed together with polystyrene foam under outdoor sunlight from 11:00 to 15:00. The results are as follows: Figure 6 As shown in the figure, the back surface temperature of polyacrylonitrile fiber thermal insulation and cooling aerogel and polystyrene foam always increases and decreases to the same extent, but the back surface temperature of polyacrylonitrile fiber thermal insulation and cooling aerogel is always lower than that of polystyrene foam.

[0086] Therefore, comprehensive Figure 3 、 Figure 4 、 Figure 5 and Figure 6This effectively demonstrates that the polyacrylonitrile fiber insulation and cooling aerogel prepared in Example 1 has superior thermal insulation performance compared to polystyrene foam, while also exhibiting a moderate radiative cooling effect under sunlight. The excellent radiative cooling properties of the SiO2 and TiO2 materials, combined with the microporous structure of the aerogel that regulates internal heat transfer, contribute to the excellent thermal insulation performance of the polyacrylonitrile fiber insulation and cooling aerogel.

[0087] The reaction mechanism of the present invention is:

[0088] The polyacrylonitrile fiber is modified by amidoxime, and the C≡N group on the surface of the polyacrylonitrile fiber is converted into a CO-NH2 group in a hydroxylamine hydrochloride solution. Then, the amidoxime-based polyacrylonitrile fiber undergoes alkaline hydrolysis under alkaline conditions, and a deprotonation reaction occurs between the AOPAN fibers. The surface of the AOPAN fiber after alkaline hydrolysis contains a large number of hydroxyl and carboxyl groups. The fibers can be cross-linked through hydrogen bonds, giving the aerocoagulant a stable structure.

[0089] Titanium dioxide is added during the deprotonation process. Titanium dioxide has stable chemical properties and the polarity of the Ti-O bond is relatively high. Water adsorbed on the surface dissociates due to polarization, easily forming hydroxyl groups. This surface hydroxyl group can improve the performance of TiO2 as an adsorbent and various monomers, providing convenience for surface modification. At the same time, the hydroxyl groups on the surface of TiO2 can be adsorbed and combined with other substances in the reaction system. TiO2 nanoparticles are used as fillers in the SiO2 / AOPAN reaction system. Since the AOPAN surface is positively charged and the TiO2 surface is negatively charged under alkaline conditions, during the alkaline hydrolysis process, the AOPAN fibers and TiO2 are adsorbed and combined through positive and negative charges. Specifically:

[0090] In the initial stages of the reaction, TiO2 particles adhere to the surface of the pore wall fibers, and the AOPAN fibers and SiO2 fibers become entangled and cross-linked. After a period of reaction, the AOPAN rapidly dissolves, and the TiO2 particles are evenly distributed in the pore walls. This process occurs because under alkaline conditions, AOPAN and TiO2 particles bind to each other through positive and negative charges. When the AOPAN dissolves and undergoes alkaline hydrolysis, a gel-like dispersion is formed, in which the SiO2 nanofibers and TiO2 particles are evenly distributed on the surface and inside of the gel.

[0091] Finally, it is reprotonated under acidic conditions, and the surface of the AOPAN fiber forms a charge force through hydrogen bonding, resulting in strong cross-linking between the fibers, forming a stable cross-linking system between the fibers, and obtaining an aerogel material with a stable structure through replacement reaction and freeze drying.

[0092] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A self-assembly preparation method of polyacrylonitrile fiber thermal insulation and cooling aerogel, characterized in that: include: The polyacrylonitrile fiber is modified by amidoximation to obtain amidoxime-based polyacrylonitrile fiber; A dispersion is obtained by mixing amidoxime-based polyacrylonitrile fibers with silica fibers and titanium dioxide and then deprotonating the fibers under alkaline conditions, comprising: adding the amidoxime-based polyacrylonitrile fibers and the silica fibers together into a dimethyl sulfoxide solution, homogenously dispersing the fibers to obtain a fiber suspension; and adding titanium dioxide and potassium hydroxide to the fiber suspension, stirring the mixture to react, and obtaining a dispersion. The dispersion is freeze-formed to obtain an oriented cryogel; The oriented cryogel is protonated under acidic conditions to achieve self-assembly to obtain a polyacrylonitrile fiber thermal insulation and cooling aerogel, comprising: placing the oriented cryogel in an aqueous solution of acetic acid, allowing it to stand and then freeze-drying to obtain the polyacrylonitrile fiber thermal insulation and cooling aerogel; The mass ratio of the amidoxime-based polyacrylonitrile fiber, silica fiber, and dimethyl sulfoxide solution is 10:(1-6):(5000-6000); the concentration range of the dimethyl sulfoxide solution is 98-100wt%; the mass ratio of the silica fiber and titanium dioxide is (1-1.5):1; the mass ratio of the amidoxime-based polyacrylonitrile fiber and potassium hydroxide is (5-10):1; The stirring reaction time is 3 to 5 hours.

2. The self-assembly preparation method of polyacrylonitrile fiber thermal insulation and cooling aerogel according to claim 1, characterized in that: The polyacrylonitrile fiber is subjected to amidoximation modification to obtain amidoxime-based polyacrylonitrile fiber, comprising: The polyacrylonitrile fiber is added into a hydroxylamine hydrochloride solution for heating reaction. After the reaction is completed, the polyacrylonitrile fiber is taken out, washed multiple times, and dried to obtain the amidoxime-based polyacrylonitrile fiber.

3. The self-assembly preparation method of polyacrylonitrile fiber thermal insulation and cooling aerogel according to claim 2, characterized in that: At least one of the following conditions must be met: The concentration of the hydroxylamine hydrochloride solution is 0.5-0.6 mol / L; The heating temperature range is 65~70℃; The reaction time is 2 to 2.5 hours; The drying temperature is 60-70°C.

4. The self-assembly preparation method of polyacrylonitrile fiber thermal insulation and cooling aerogel according to claim 2, characterized in that: The amidoxime group conversion rate in the amidoxime modification of the polyacrylonitrile fiber is 22-30%.

5. The self-assembly preparation method of polyacrylonitrile fiber thermal insulation and cooling aerogel according to claim 1, characterized in that: The homogenizing and dispersing is achieved by a homogenizer, and the condition parameters include homogenizing and dispersing at a speed of 100,000 to 120,000 rpm for 30 to 35 minutes.

6. The self-assembly preparation method of polyacrylonitrile fiber thermal insulation and cooling aerogel according to claim 1, characterized in that: The method of freezing and molding the dispersion to obtain the oriented cryogel comprises: Pour the dispersion into the frozen mold and place it in an insulated box; Liquid nitrogen is poured into the insulated box and frozen for 5 to 10 minutes, so that ice crystals grow from the bottom of the dispersion at -196 to -172°C to obtain a directional cryogel.

7. The self-assembly preparation method of polyacrylonitrile fiber thermal insulation and cooling aerogel according to claim 1, characterized in that: At least one of the following conditions must be met: The pH value of the aqueous solution of acetic acid is in the range of 3 to 4; The standing time is 10 to 14 hours; The freeze drying comprises first freezing at -75 to -70°C for 4 to 6 hours, and then drying at -75 to -70°C and 1 to 2 Pa for 36 to 48 hours.

8. A polyacrylonitrile fiber thermal insulation and cooling aerogel, characterized in that: The aerogel is prepared by the self-assembly preparation method of the polyacrylonitrile fiber thermal insulation and cooling aerogel according to any one of claims 1 to 7.

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