A liquid crystal polyimide aerogel with thermotropic liquid crystal behavior and its preparation method

Abstract

This invention discloses a polyimide aerogel exhibiting thermotropic liquid crystal behavior and its preparation method, belonging to the field of polymer aerogel materials. The polyimide aerogel exhibiting thermotropic liquid crystal behavior of this invention has a thermal conductivity of 0.07–0.08 W / m² at room temperature. ‑ 1 K ‑1 The thermotropic liquid crystal-like polyimide aerogel is obtained by directional freezing, thermal crosslinking, and imidization of an aqueous solution of polyamic acid. This thermotropic liquid crystal-like polyimide aerogel possesses advantages such as low density, high porosity, relatively high thermal conductivity, and structural stability. It fills the gap in the application of liquid crystal polyimide materials in the aerogel field, improves the thermal conductivity of polyimide aerogels, and provides a foundation for the application of polyimide aerogel materials in the field of high-temperature intelligent deformable materials.

Description

Technical Field

[0001] This invention relates to a liquid crystal polyimide aerogel with thermotropic liquid crystal behavior and its preparation method, belonging to the field of polymer aerogel materials. Background Technology

[0002] Aerogels are porous materials formed by the aggregation of colloidal particles or polymer molecules. They have a continuous three-dimensional framework network, an open and interconnected porous structure, low density, high porosity and high specific surface area, and can be applied to thermal insulation materials, sound insulation materials, energy storage devices, aerospace and other fields.

[0003] Polyimide is a class of organic polymers containing imide rings in its main chain. It possesses advantages such as high mechanical strength, good thermal stability, and wear resistance, and its significant superiority as both a structural and functional material is well-recognized. In recent years, research on polyimide aerogels has received considerable attention. These aerogels combine the excellent properties of both polyimide and aerogels, exhibiting not only the superior characteristics of polyimide but also the outstanding features of aerogels, such as lightweight, ultra-low density, high specific surface area, low thermal conductivity, and low dielectric constant. However, precisely because of its low thermal conductivity, the application of polyimide aerogels may be limited in certain situations, particularly in fields requiring efficient heat conduction. For example, in some industrial or research environments, materials with high thermal conductivity are needed for rapid heat transfer. The low thermal conductivity of polyimide aerogels can be a disadvantage in these situations, as it hinders effective heat transfer. Furthermore, in high-temperature environments, although polyimide aerogels exhibit certain stability at high temperatures, their low thermal conductivity can make thermal management in high-temperature environments more challenging. For example, polyimide aerogel is not the best choice for high-temperature equipment or systems requiring rapid heat dissipation; similarly, it is not ideal for applications requiring rapid response to temperature changes, such as temperature sensors or thermistors. The low thermal conductivity of polyimide aerogel may slow down its response, thus limiting its application in these situations.

[0004] Currently, the main strategy for improving the thermal conductivity of polyimide aerogels is to add a certain amount of high thermal conductivity fillers to the polyimide aerogel matrix to obtain polyimide aerogel thermally conductive composite materials. The advantages of this strategy are its simplicity, convenience, and ease of industrialization. However, the disadvantages are that obtaining polyimide aerogel materials with high thermal conductivity often requires filling with a large amount of thermally conductive fillers, which can easily lead to interfacial thermal barriers and other problems. Furthermore, the excessive addition of thermally conductive fillers inevitably causes a sharp deterioration in the mechanical properties of the polyimide aerogel material, posing significant challenges to material processing and production costs. (Invention Content)

[0005] [Technical Issues]

[0006] In the existing technology, polyimide aerogel has low thermal conductivity, which severely limits its application in heat conduction, thermal management and thermal response. Although the thermal conductivity of polyimide aerogel can be improved to some extent by filling it with high thermal conductivity fillers, it inevitably causes the mechanical properties of polyimide aerogel materials to deteriorate sharply, which brings great challenges to material processing and production costs.

[0007] [Technical Solution]

[0008] To address the shortcomings and deficiencies of existing technologies, this invention provides a polyimide aerogel with thermotropic liquid crystal behavior and its preparation method. This polyimide aerogel with thermotropic liquid crystal behavior has high thermal conductivity without affecting mechanical properties, enabling its application in specific scenarios such as efficient heat conduction, rapid response to temperature changes, and thermal management in high-temperature environments.

[0009] Specifically, a polyamic acid solution is obtained by condensation polymerization of diamine and liquid crystal dianhydride monomers, with the addition of a thermal crosslinking agent. The polyamic acid solution is then slowly poured into ultrapure water and stirred to perform solvent exchange. After freezing and drying, polyamic acid filaments are obtained. The polyamic acid filaments are then mixed with triethylamine in ultrapure water to obtain an aqueous polyamic acid solution. The aqueous polyamic acid solution is injected into a special mold (with a copper plate at the bottom and polytetrafluoroethylene at the top). After freeze-drying, high-temperature thermal crosslinking, and imidization, a liquid crystal polyimide aerogel with relatively high thermal conductivity is obtained.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] The first objective of this invention is to provide a thermotropic liquid crystal polyimide aerogel, wherein the thermotropic liquid crystal polyimide aerogel has a thermal conductivity of 0.07–0.08 W m⁻¹ K at room temperature. -1 The thermotropic liquid crystal behavior polyimide aerogel is obtained by directional freezing, thermal crosslinking and imidization of polyamic acid aqueous solution.

[0012] A second objective of this invention is to provide a method for preparing the above-described liquid crystal polyimide aerogel with thermotropic liquid crystal behavior, the method comprising the following steps:

[0013] (1) The diamine monomer and the liquid crystal dianhydride monomer are subjected to polycondensation reaction in N-methylpyrrolidone (NMP), and then a thermal crosslinking agent is added to continue the reaction to obtain a polyamic acid solution; the polyamic acid solution is placed in ultrapure water for solvent exchange, and then frozen and freeze-dried to obtain polyamic acid dry filaments;

[0014] (2) Add the polyamic acid dried filaments obtained in step (1) and triethylamine to ultrapure water and stir well to obtain a polyamic acid aqueous solution;

[0015] (3) Inject the polyamic acid aqueous solution obtained in step (2) into the directional mold, and place the mold in a freezer for directional freezing and molding. After molding, freeze dry it to obtain polyamic acid aerogel.

[0016] (4) The polyamic acid aerogel obtained in step (3) is thermally crosslinked and imidized to obtain a polyimide aerogel with thermo-induced liquid crystal behavior, which is a liquid crystal polyimide aerogel.

[0017] In one embodiment, the diamine monomer in step (1) includes one or both of 1,4-bis(4-aminophenoxy)benzene (TPE-Q) and 4,4'-diaminodiphenyl ether (ODA).

[0018] In one embodiment, the liquid crystal dianhydride monomer in step (1) is 4,4'-terephthalic anhydride (HQDPA).

[0019] In one embodiment, the thermal crosslinking agent in step (1) includes one or more of 4-ethynylaniline (4-EA), 4-phenylethynyl phthalic anhydride (PEPA), and nadic anhydride (NA).

[0020] In one embodiment, the diamine monomer in step (1) is 1,4-bis(4-aminophenoxy)benzene and 4,4'-diaminodiphenyl ether, and the molar ratio of the two is 1:(0-1); the molar ratio of the diamine monomer and the liquid crystal dianhydride monomer is 1:(0.8-1.5), preferably 1:1.01-1.20.

[0021] In one embodiment, the molar ratio of the diamine monomer to the thermal crosslinking agent in step (1) is 1:(0-0.1), preferably 1:0.02.

[0022] In one embodiment, the polycondensation reaction in step (1) is carried out in an ice-water bath for 3 to 6 hours.

[0023] In one embodiment, the reaction time after adding the thermal crosslinking agent in step (1) is 3 to 6 hours.

[0024] In one embodiment, the polyamic acid solution in step (1) has a solid content of 8-15 wt%.

[0025] In one embodiment, the solvent replacement time in step (1) is 0.5 to 3 hours.

[0026] In one embodiment, the freezing and freeze-drying process in step (1) specifically involves freezing in a freezer at -60 to -80°C for 2 to 5 hours, followed by drying in a freeze dryer at -30 to -50°C for 60 to 72 hours.

[0027] In one embodiment, the concentration of polyamic acid in the polyamic acid aqueous solution in step (2) is 70-90 mg / ml.

[0028] In one embodiment, the stirring speed in step (2) is 500-1000 rpm and the time is 3-6 hours.

[0029] In one embodiment, the orientation mold in step (3) is a specially made cylindrical mold with a copper plate at the bottom and polytetrafluoroethylene at the top.

[0030] In one embodiment, the freezing temperature in step (3) is -80 to -20°C, and the time is 1 to 24 hours.

[0031] In one embodiment, the freeze-drying temperature in step (3) is -50 to -20°C, and the time is 48 to 96 hours.

[0032] In one embodiment, the temperature of the thermal crosslinking in step (4) is 100-350°C, preferably 220-260°C, and the time is 1-3 hours.

[0033] In one embodiment, the imidization in step (4) can be specifically selected from the following temperature processes: treatment at 100-150°C for 0.5-1.5 hours, treatment at 200-250°C for 0.5-1.5 hours, and treatment at 300-350°C for 0.5-1.5 hours.

[0034] In one embodiment, the thermal crosslinking and imidization in step (4) are carried out in an inert atmosphere.

[0035] In one embodiment, the thermal crosslinking and imidization in step (4) are carried out in a tube furnace under a nitrogen atmosphere.

[0036] The present invention also provides the application of the above-mentioned liquid crystal polyimide aerogel with thermotropic liquid crystal behavior in thermal management, thermal insulation materials, intelligent robots, aerospace vehicles, and intelligent wearable devices.

[0037] Compared with the prior art, the present invention has the following advantages:

[0038] (1) This invention is the first to prepare polyimide aerogel with thermotropic liquid crystal behavior. Its preparation method is relatively simple and environmentally friendly, with low cost, good formability, and stable aerogel structure. It can be designed into various shapes and is suitable for a variety of application environments.

[0039] (2) This invention utilizes a directional freezing strategy to generate a relatively ordered pore structure to prepare liquid crystal polyimide aerogel. It utilizes the microscopic order of liquid crystal molecules to moderately improve the intrinsic thermal conductivity of polyimide aerogel, which to some extent alleviates the contradiction of slow response rate caused by slow heat transfer in the polyimide aerogel skeleton. This provides a way to prepare polyimide aerogel that can respond quickly to temperature changes and lays the foundation for expanding the application of polyimide aerogel in specific scenarios such as high-efficiency heat conduction, rapid response to temperature changes, and high-temperature environment thermal management.

[0040] (3) The thermal crosslinking agent added in this invention causes the polyimide aerogel to crosslink and solidify during the high-temperature treatment process, which helps the liquid crystal aerogel to form a stable structure and provides a prerequisite for improving the formability of polyimide aerogel. Attached Figure Description

[0041] Figure 1 This is a schematic diagram of the structure of the specific oriented mold used in this invention;

[0042] Figure 2 This is a microstructure diagram of the surface of the polyimide aerogel with thermotropic liquid crystal behavior prepared in Example 1 of the present invention;

[0043] Figure 3 DSC curves of the polyimide aerogel with thermotropic liquid crystal behavior prepared in Example 1 of the present invention and the polyimide aerogels of Comparative Examples 1 to 3 during the heating process;

[0044] Figure 4 The image shows a POM (Polyimide Oxide) image of the polyimide aerogel with thermotropic liquid crystal behavior prepared in Example 1 of this invention during the heating process.

[0045] Figure 5 This is a POM image of the polyimide aerogel prepared in Comparative Example 1 of the present invention during the heating process. Detailed Implementation

[0046] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. The specific embodiments described below further illustrate the present invention.

[0047] Example 1

[0048] A method for preparing a liquid crystal polyimide aerogel with thermotropic liquid crystal behavior includes the following steps:

[0049] (1) Dissolve 1.0963g of 1,4-bis(4-aminophenoxy)benzene and 0.2503g of 4,4'-diaminodiphenyl ether in 18.69ml of N-methylpyrrolidone, add 2.0317g of 4,4'-terephthalic anhydride, and react in an ice-water bath for 3h; then add 0.0118g of 4-ethynylaniline and 5ml of N-methylpyrrolidone, and continue to react for 3h to obtain a polyamic acid solution with a solid content of 13.8%; precipitate the prepared polyamic acid solution with ultrapure water, freeze it in a -80℃ freezer for 2h, and then dry it in a -50℃ freeze dryer for 72h to obtain polyamic acid dried filaments;

[0050] (2) Dissolve 0.8g of the polyamic acid filaments obtained in step (1) in 10ml of ultrapure water, and weigh 0.24g of triethylamine and add it to accelerate dissolution. Stir thoroughly to obtain a polyamic acid aqueous solution with a concentration of 80mg / ml.

[0051] (3) Pour the polyamic acid aqueous solution obtained in step (2) above into a special mold (the bottom is a copper plate and the top is polytetrafluoroethylene). Figure 1 As shown in the figure, the mold is then placed in a freezer at -40°C for 6 hours. After freezing, the mold is placed in a freeze dryer at -50°C for 72 hours to obtain polyamic acid aerogel.

[0052] (4) The polyamic acid aerogel obtained in step (3) above is placed in a tube furnace and imidized and thermally crosslinked under nitrogen atmosphere. The high temperature treatment program is 100℃ for 1 h, 200℃ for 1 h, 250℃ for 0.5 h, and 280℃ for 0.5 h, thus obtaining a polyimide aerogel with thermotropic liquid crystal behavior, which is liquid crystal polyimide aerogel, denoted as LCPIA.

[0053] Example 2

[0054] The only difference from Example 1 is that the concentration of polyamic acid in step (2) is adjusted to 60 mg / ml, the mass of polyamic acid filaments is 0.6 g, and the amount of triethylamine added is 0.18 g; other conditions and parameters are the same as in Example 1.

[0055] The properties of the obtained liquid crystal polyimide aerogel were measured, and the results showed that the aerogel had poor morphology and surface cracking due to its low solid content.

[0056] Example 3

[0057] The only difference from Example 1 is that the concentration of polyamic acid in step (2) is adjusted to 120 mg / ml, the mass of polyamic acid filaments is 1.2 g, and the amount of triethylamine added is 0.36 g; other conditions and parameters are the same as in Example 1.

[0058] The properties of the obtained liquid crystal polyimide aerogel were measured, and the results showed that due to the high solid content, it was difficult to remove air bubbles during the preparation process, resulting in defects such as foaming and irregular growth of the aerogel.

[0059] Comparative Example 1

[0060] The only difference from Example 1 is that the special mold in step (3) (the bottom is a copper plate and the top is polytetrafluoroethylene) is changed to a conventional silicone mold. Other conditions and parameters are the same as in Example 1. The resulting polyimide aerogel is denoted as PIA1.

[0061] Comparative Example 2

[0062] The only difference from Example 1 is that the diamine added in step (1) is 1.0012g of 4,4'-diaminodiphenyl ether (the amount of substance is the same as the total amount of diamine in Example 1), and the dianhydride added is 1.4858g of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (the amount of substance is the same as the amount of dianhydride in Example 1). Other conditions and parameters are the same as in Example 1. The resulting polyimide aerogel is denoted as PIA2.

[0063] Comparative Example 3

[0064] The only difference from Example 1 is that the diamine added in step (1) is 1.0012g of 4,4'-diaminodiphenyl ether (the amount of substance is the same as the total amount of diamine in Example 1), and the dianhydride added is 1.4858g of 3,3',4,4'-biphenyltetracarboxylic acid dianhydride (the amount of substance is the same as the amount of dianhydride in Example 1). At the same time, the special mold in step (3) (the bottom is a copper plate and the top is polytetrafluoroethylene) is adjusted to a conventional silicone mold. Other conditions and parameters are the same as in Example 1. The resulting polyimide aerogel is denoted as PIA3.

[0065] Performance testing

[0066] 1. Characterization of the structure of liquid crystal polyimide aerogel

[0067] Figure 2 Here is a SEM image of the surface of the liquid crystal polyimide aerogel prepared in Example 1, as shown. Figure 2 As shown, the liquid crystal polyimide aerogel has a porous structure inside, which is due to the pore structure left after the ice crystals sublimate during the freeze-drying process.

[0068] 2. Determination of thermal conductivity of liquid crystal polyimide aerogel

[0069] Thermal conductivity tests were conducted on the polyimide aerogels prepared in Example 1 and Comparative Examples 1, 2, and 3. The thermal conductivity constants were determined using a thermal conductivity meter (TPS2500S). Aerogel samples with a length and width of approximately 3 cm and a thickness of approximately 1 cm were taken. A 5501 probe (1 cm in diameter) was placed between two samples, and the thermal conductivity was measured in isotropic mode. At least three samples were tested in each group, and the average value was taken to obtain the thermal conductivity value. The results are shown in Table 1.

[0070] Table 1 Thermal conductivity of polyimide aerogel at different temperatures

[0071] Temperature (°C) <![CDATA[Thermal conductivity (W m -1 K -1 )]]> Example 1 25 0.0706 Comparative Example 1 25 0.0529 Comparative Example 2 25 0.0631 Comparative Example 3 25 0.0613 Example 1 250 0.0920 Comparative Example 1 250 0.0814 Comparative Example 2 250 0.0775 Comparative Example 3 250 0.0770

[0072] Due to the low density and high porosity of aerogels, their thermal conductivity is generally low, typically ranging from 0.01 to 0.05 W / m². -1 K -1 As shown in Table 1, the liquid crystal polyimide aerogel prepared in Example 1 exhibits relatively high thermal conductivity. Furthermore, it can be observed that the thermal conductivity increases more significantly closer to the liquid crystal formation temperature. This is because the generated liquid crystal structure is microscopically ordered, which reduces phonon scattering and forms thermally conductive pathways. This liquid crystal polyimide aerogel holds promise for expanding the application of polyimide aerogels in specific scenarios requiring efficient heat conduction, rapid response to temperature changes, and thermal management in high-temperature environments.

[0073] 3. Characterization of the liquid crystal structure of liquid crystal polyimide aerogel

[0074] Figure 3 The DSC curves of the liquid crystal polyimide aerogel prepared in Example 1 and the polyimide aerogel of Comparative Example 1 during the heating process are shown below. Figure 3 It can be seen that the aerogel of Example 1 exhibited a complete endothermic peak (peak at 326℃) during the heating process, which is attributed to the gradual transformation of Example 1 from an amorphous phase to a liquid crystal phase; simultaneously combined with Figure 4 The POM image of Example 1 showed that no bright spots appeared in the field of view during the heating process from 100°C to 300°C, but after 350°C, bright areas began to appear in the field of view. This is because Example 1 has entered the liquid crystal generation temperature range.

[0075] And in Figure 3 It can be seen that Comparative Example 1 only exhibited a glass transition process (approximately 229℃), without the transition from an amorphous phase to a liquid crystal phase. This is attributed to the fact that liquid crystal formation requires a relatively ordered environment, while Comparative Example 1, without directional freezing, did not form an ordered structure, which is unfavorable for liquid crystal formation; simultaneously, combined with Figure 5 The POM images of Comparative Example 1 during the heating process showed no bright spots in the temperature range of 100℃-350℃, proving that no liquid crystal phase was generated in Comparative Example 1 without directional freezing during the heating process.

[0076] And from Figure 3 It can be seen that Comparative Examples 2 and 3 only show the glass transition process, without the amorphous phase to liquid crystal phase transition process. In summary, the preparation of this type of polyimide aerogel with thermotropic liquid crystal behavior requires specific monomers to be directionally frozen using a special mold (with a copper plate at the bottom and polytetrafluoroethylene at the top).

[0077] The embodiments provided above are not intended to limit the scope of the invention, nor are the described steps intended to limit the order of execution. Any obvious modifications made to the invention by those skilled in the art based on existing common knowledge also fall within the scope of protection defined by the claims.

Claims

1. A thermotropic liquid crystal polyimide aerogel, characterized in that, The thermally induced liquid crystal polyimide aerogel exhibits a thermal conductivity of 0.07~0.08 W / m² at room temperature. -1 K -1 The thermotropic liquid crystal behavior polyimide aerogel is obtained by directional freezing, thermal crosslinking and imidization of polyamic acid aqueous solution; The preparation of the thermotropic liquid crystal polyimide aerogel specifically includes the following steps: (1) Diamine monomer and liquid crystal dianhydride monomer in N A polyamic acid solution is obtained by polycondensation in methylpyrrolidone, followed by the addition of a thermal crosslinking agent to continue the reaction. The polyamic acid solution is then placed in ultrapure water for solvent exchange, followed by freezing and freeze-drying to obtain polyamic acid filaments. The diamine monomer is 1,4-bis(4-aminophenoxy)benzene and 4,4'-diaminodiphenyl ether; the liquid crystal dianhydride monomer is 4,4'-terephthalodioxybisphthalic anhydride; the thermal crosslinking agent is 4-ethynylaniline; (2) Add the polyamic acid dried filaments obtained in step (1) and triethylamine to ultrapure water, stir well, and obtain a polyamic acid aqueous solution; The concentration of polyamic acid in the polyamic acid aqueous solution is 70~90 mg / ml; (3) Inject the polyamic acid aqueous solution obtained in step (2) into the directional mold, and place the mold in a freezer for directional freezing and molding. After molding, freeze dry it to obtain polyamic acid aerogel. The directional mold is a specially made cylindrical mold with a copper plate at the bottom and polytetrafluoroethylene at the top; (4) The polyamic acid aerogel obtained in step (3) is thermally crosslinked and imidized to obtain a polyimide aerogel with thermotropic liquid crystal behavior, which is a liquid crystal polyimide aerogel.

2. A method for preparing a liquid crystal polyimide aerogel with thermotropic liquid crystal behavior as described in claim 1, characterized in that, The preparation method includes the following steps: (1) The diamine monomer and liquid crystal dianhydride monomer are subjected to polycondensation reaction in N-methylpyrrolidone, and then a thermal crosslinking agent is added to continue the reaction to obtain a polyamic acid solution; the polyamic acid solution is placed in ultrapure water for solvent exchange, and then frozen and freeze-dried to obtain polyamic acid dry filaments; The diamine monomer is 1,4-bis(4-aminophenoxy)benzene and 4,4'-diaminodiphenyl ether; the liquid crystal dianhydride monomer is 4,4'-terephthalic anhydride; the thermal crosslinking agent is 4-ethynylaniline; (2) the polyamic acid dry filaments obtained in step (1) and triethylamine are added to ultrapure water and stirred until a polyamic acid aqueous solution is obtained; The concentration of polyamic acid in the polyamic acid aqueous solution is 70~90 mg / ml; (3) Inject the polyamic acid aqueous solution obtained in step (2) into the directional mold, and place the mold in a freezer for directional freezing and molding. After molding, freeze dry it to obtain polyamic acid aerogel. The directional mold is a specially made cylindrical mold with a copper plate at the bottom and polytetrafluoroethylene at the top; (4) The polyamic acid aerogel obtained in step (3) is thermally crosslinked and imidized to obtain a polyimide aerogel with thermotropic liquid crystal behavior, which is a liquid crystal polyimide aerogel.

3. The preparation method according to claim 2, characterized in that, The diamine monomer in step (1) is 1,4-bis(4-aminophenoxy)benzene and 4,4'-diaminodiphenyl ether, and the molar ratio of the two is 1:(0-1); the molar ratio of the diamine monomer and the dianhydride monomer is 1:(0.8-1.5).

4. The preparation method according to claim 2, characterized in that, The molar ratio of the diamine monomer to the thermal crosslinking agent in step (1) is 1:(0 to 0.1).

5. The preparation method according to claim 2, characterized in that, The solid content of the polyamic acid solution in step (1) is 8-15 wt%.

6. The preparation method according to claim 2, characterized in that, The temperature for thermal crosslinking in step (4) is 100-350°C; the time is 1-3 hours.

7. The application of the liquid crystal polyimide aerogel with thermotropic liquid crystal behavior as described in claim 1 or the liquid crystal polyimide aerogel with thermotropic liquid crystal behavior prepared by any of the preparation methods described in claims 2 to 6 in the fields of thermal management, thermal insulation materials, intelligent robots, aerospace vehicles, and intelligent wearable devices.

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