Manufacturing of hydrophobic, mechanically flexible, and optically transparent polyimide aerogel

Abstract

The present disclosure provides a method of manufacturing a physically crosslinked polyimide (PCPI) aerogel having significantly improved properties, performed without the use of a chemical crosslinking agent. The prepared PCPI aerogel shows high moisture resistance, hydrophobic behavior, ultra-low density, ultra-high porosity, excellent thermal stability, increased mechanical strength and high mechanical flexibility. In addition, samples of thin film geometry having high mechanical flexibility and controlled thickness have been successfully manufactured. In addition, some aerogel films produced exhibit an improved optical transparency of greater than 80%, which is the highest transparency of the organic PI aerogels reported so far.

Description

Technical Field

[0001] This disclosure relates to the synthesis of hydrophobic, mechanically flexible and optically transparent physically crosslinked polyimide (PCPI) aerogels. Background Technology

[0002] Aerogels are a type of material prepared by removing their liquid portion without disrupting the solid network of the gel. Given their mesoporous structure and high porosity (greater than 80%), aerogels exhibit unique properties; these properties include, but are not limited to, ultra-low thermal conductivity (as low as 4 mW / m³). -1 K -1 Aerogels possess extremely low density, high porosity, high compressive strength, and large surface area. These properties indicate that aerogels have great potential for a wide range of industrial applications, such as superinsulation, optics, sound insulation, airborne nanoparticle filtration, oil / organic solvent water separation, energy harvesting, energy storage devices, electromagnetic shielding, and even catalysts.

[0003] Aerogels can be prepared from a wide range of materials, as long as these materials can form a gel. Among them, silica is the most studied material. However, its brittleness and hydrophobicity greatly limit its penetration into industrial applications. Given the greater potential of organic polyimide (PI) aerogels in achieving enhanced ductility and higher operating temperatures, they have received more attention from scientists in the past decade. Typically, PI gels are made by the following steps: stepwise growth polymerization of at least diamine monomers and dianhydride monomers, followed by crosslinking of the resulting polymer chains using a chemical crosslinking agent [1]–[4]. The resulting gel is then converted into an aerogel using supercritical or freeze-drying techniques. According to this method, apart from a few studies that control the properties of aerogels by adjusting their morphology [5], [6], most studies have focused on improving their properties by changing the main chain chemical structure of the aerogel by altering its monomer composition or the type of chemical crosslinking agent.

[0004] Against this backdrop, H. Guo et al. used an octa(aminophenyl)silsesquioxane (OAPS) chemical crosslinking agent with a silsesquioxane cage structure (composed of a silicon and oxygen backbone with eight aminophenyl groups) to bond with different monomers (including bisphenyl-p-xyleneamine (BAX), 2,2'-dimethylbenzidine (DMBZ), p-phenylenediamine (PPDA), and 4,4'-diaminodiphenyl ether (ODA) diamine) in the reaction with 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) monomer [2], [4]. The results showed that OAPS crosslinked PI aerogels exhibited slightly lower shrinkage and density compared with previously reported 1,3,5-triaminophenoxybenzene (TAB) crosslinked aerogels with similar backbone chemical structures. In addition, OAPS crosslinked PI aerogels with at least 50% DMBZ mol% in the backbone exhibited enhanced moisture resistance. However, OAPS is not a commercially available product and is also a very expensive chemical crosslinking agent.

[0005] It has been reported that TAB-crosslinked PI aerogels have improved mechanical properties compared to OAPS, but poor moisture resistance. MAB Meador et al. proposed the idea of ​​replacing 50 mol% diamine with poly(propylene glycol) (PPG) to improve the moisture resistance of TAB-crosslinked PI aerogels[7]. According to this study, replacing at least 50 mol% ODA with PPG increased the water contact angle to about 80° and obtained very low hygroscopicity. It has been reported that, according to another study, TAB-crosslinked PI aerogels prepared from DMBZ diamine have a larger surface area and increased modulus (up to 4 times with only a 26% higher density) compared to OAPS-crosslinked aerogels[3]. Given the low cost and commercial availability of 1,3,5-benzenetricarboxylic acid chloride (BTC) crosslinking agents, they have been widely used in recent studies on PI aerogels. Results show that BTC-crosslinked aerogels exhibit compressive modulus comparable to or even higher than that of OAPS-crosslinked aerogels with similar density, as well as a larger surface area. However, a problem with BTC crosslinked PI aerogels is their poor moisture resistance.

[0006] Even the strategy of replacing 100% diamine with hydrophobic DMBZ monomers failed to improve the moisture resistance of BTC crosslinked PI aerogels. In subsequent studies, triisocyanate Desmodur N3300A was used as a cheap alternative for crosslinking PI aerogels [8]. Chemical crosslinking with triisocyanates has been reported to achieve mechanical properties that are the same as or better than those of OAPS, TAB and BTC. However, the lower decomposition initiation temperature of triisocyanates leads to a lower working temperature of the aerogel, which may limit its application. In addition, in a few studies, PI aerogels prepared without the use of chemical crosslinking agents have been limited to those derived from pyromellitic dianhydride (PMDA). Such aerogels have been reported to have very high shrinkage behavior and / or high mechanical brittleness [1], [9],

[10] , which is detrimental to thermal insulation or thermal management system applications in a broader sense.

[0007] Liu et al.

[29] disclosed a method for producing a high molecular weight polyamic acid solution (solid content 5% to 10%, degree of polymerization >39) by long processing time (8 hours to 72 hours), then diluting the solution to a very low solid content (0.1% to 1%), and then adding a chemical imidizing agent to form a wet gel. In addition, to improve stability, the gel was aged at a high temperature of 50°C for 24 hours.

[0008] Given the drawbacks of chemical crosslinking agents on the properties of PI aerogels, their high cost, and environmental impact, it would be highly advantageous to provide a method for synthesizing physically crosslinked polyimide (PCPI) aerogels that exhibit hydrophobicity, mechanical flexibility, and optical transparency, in order to avoid the use of chemical crosslinking reactions. Summary of the Invention

[0009] This disclosure provides a method for fabricating physically crosslinked polyimide (PCPI) aerogels with significantly improved properties without the use of chemical crosslinking agents. The fabricated PCPI aerogels exhibit high moisture resistance, hydrophobic behavior, ultra-low density, ultra-high porosity, excellent thermal stability, increased mechanical strength, and high mechanical flexibility. Furthermore, samples with film geometries exhibiting high mechanical flexibility and controlled thickness were successfully fabricated. Additionally, some of the fabricated aerogel films exhibited improved optical transparency of over 80%, the highest transparency reported to date for organic PI aerogels. Overall, the method introduced in this study shows great potential for the convenient fabrication of cost-effective PI aerogels for a wide range of practical applications, namely flexible and moisture-resistant thermal insulation, durable optics, moisture aerogel sensors, and water filtration. Given the relatively rapid gelation rate of this process, the simplified physical crosslinking pathway developed in this study helps pave the way for the large-scale production of organic aerogels with improved properties, and the developed method system is not limited to the materials used and can be applied to a wide range of monomers and chemical backbones in the field of organic aerogels.

[0010] This method has been successfully used to fabricate thin-film aerogel-based structures with high mechanical flexibility, high thermal insulation properties, and controlled thickness. Such structures can be embedded in electronic devices (e.g., laptops, smartphones, desktop computers, and microelectronic devices) for thermal management purposes. These thermal management measures include reducing heat in electronic devices, minimizing hot spots, protecting heat-sensitive components, improving user comfort, and increasing product performance and lifespan. The synthetic methods disclosed herein are not limited to the materials used and can be applied to a wide range of monomers and chemical backbones.

[0011] Therefore, this disclosure provides a polyimide aerogel comprising: Polyimide-based polymers of Formula 1: Formula 1 Where n is an integer equal to or greater than 10, and where the polymer is physically cross-linked through polymer chain entanglement, and is formed by a method including initial gelation of approximately 1 minute to approximately 10 minutes; and Aerogels are not a major source of polyamic amide polymers, or, in some cases, aerogels contain virtually no polyamic amide polymers.

[0012] Polyimide aerogels can have pore sizes ranging from about 2 nanometers to about 100 nanometers.

[0013] Polyimide aerogels can have pore sizes ranging from about 2 nanometers to about 50 nanometers.

[0014] The pore size of polyimide aerogel can be less than about 2 nanometers.

[0015] Polyimide aerogels can have a porosity in the range of about 85% to about 99%.

[0016] Polyimide aerogels can have a porosity in the range of about 95% to about 99%.

[0017] Polyimide aerogel does not contain chemical cross-linking agents.

[0018] Polyimide aerogels can have a density of approximately 0.07 g / cm³. 3 To approximately 0.25 gm / cm 3 The density within the range.

[0019] Polyimide aerogels can have thermal conductivity in the range of about 15 milliwatts per Kelvin-meter to about 50 milliwatts per Kelvin-meter.

[0020] Polyimide aerogels can be characterized by their hydrophobicity in the range of approximately 90 degrees to approximately 140 degrees water contact angle.

[0021] Polyimide aerogels can be characterized by a water absorption rate as low as approximately 1%.

[0022] Polyimide aerogels can be characterized by their thermal decomposition onset temperature in the range of about 500°C to about 700°C.

[0023] Polyimide aerogels can be characterized by their dielectric constant in the range of about 1.5 to about 3.

[0024] Polyimide aerogels can be in the form of a film with a thickness of at least about 80 micrometers, and when in thin film form, they can have optical transparency in the range of about 60% to about 99%.

[0025] Polyimide aerogels can be characterized by their compressive modulus in the range of approximately 17 MPa to 25 MPa.

[0026] Polyimide aerogels can be characterized by their tensile modulus in the range of about 1 MPa to about 5 MPa.

[0027] The integer n can be in the range of approximately 10 to approximately 60, 10 to approximately 50, or approximately 30 to approximately 50.

[0028] This disclosure also provides a method for preparing polyimide aerogel, which includes the following steps: • A wet gel is formed by reacting a diamine monomer (DIAM) with a dianhydride monomer (DIAH) in a reaction solution containing a dipolar aprotic solvent to form a polymer of formula A; where n is an integer equal to or greater than 10. Formula A; • To imidize the resulting polymer; • Allow the formed polymer to gel for approximately 1 to 10 minutes; • Use a dry solvent instead of an aprotic solvent; and • Remove the dry solvent from the formed wet gel.

[0029] The imidization of the formed polymer can be carried out by thermal imidization or chemical imidization.

[0030] The imidization step can be carried out by chemical imidization, which can be achieved by adding a chemical dehydrating agent to the reaction solution. The chemical dehydrating agent can be a mixture of acetic anhydride and pyridine.

[0031] The method may further include steps of rapidly gelling and aging the wet gel to improve polymerization yield and increase physical crosslinking. Physical crosslinking is accomplished through the physical entanglement of the formed polymer chains and can be described morphologically. Rapid gelling can be carried out over a period of approximately 1 minute to approximately 10 minutes. Aging of the wet gel can be carried out over a period of 90 minutes to 48 hours.

[0032] The aprotic solvent can be N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or a mixture of NMP and tetrahydrofuran (THF). However, typically, the solvent can include polar organic solvents such as amides, sulfones, and ethers. Since the solvent will be washed away without participating in any chemical reaction with the polymer matrix, the type of solvent does not significantly affect the overall process.

[0033] The method may additionally include the step of adding a chemical crosslinking agent to the reaction solution.

[0034] The step of reacting DIAM monomers with DIAH monomers is carried out without the use of chemical crosslinking agents.

[0035] First, dissolve the DIAM monomer and DIAH monomer separately, then combine the DIAM monomer solution and the DIAH monomer solution.

[0036] First, dissolve the DIAM monomer, then add the DIAH monomer to the DIAM solution.

[0037] The diamine monomer (DIAM) can be 2,2'-dimethylbenzidine (DMBZ) or 4,4'-diaminodiphenyl ether (ODA).

[0038] The diamine monomer (DIAM) can be 2,2'-dimethylbenzidine (DMBZ).

[0039] The diamine monomer (DIAM) can be 4,4'-diaminodiphenyl ether (ODA).

[0040] The diamine monomer (DIAM) can be a combination of 2,2'-dimethylbenzidine (DMBZ) and 4,4'-diaminodiphenyl ether (ODA).

[0041] The dianhydride monomer (DIAH) can be biphenyl-tetracarboxylic dianhydride (BPDA).

[0042] The drying solvent can be removed by supercritical drying, freeze-drying, or ambient pressure drying, and supercritical drying is particularly effective. The drying solvent can be a solvent soluble in liquid CO2. The drying organic solvent can be ethanol, acetone, toluene, tetrahydrofuran (THF), cyclohexane, or a combination thereof.

[0043] The drying solvent can be removed by freeze drying, and the drying solvent can be water or alcohol.

[0044] The drying solvent can be removed by environmental drying, and the drying solvent can be alcohol or acetone.

[0045] The wet gel formation process can be carried out within a timeframe of 1 minute to 24 hours.

[0046] The method may further include the step of pouring the reaction solution into a block mold or a film mold.

[0047] The method may further include the step of adding an additive to the reaction solution. The additive may be a polymer stabilizer, a functionalizing agent or a combination thereof, other polymers, other aerogels, carbon nanotubes, metal fillers or particles, organic or inorganic fibers, or organic or inorganic fillers or particles, and nonwoven or woven fiber reinforcements composed of carbon precursor fibers, glass fibers, polymer organic fibers, ceramic fibers or biopolymer fibers.

[0048] The additives may be selected from the group consisting of: polymers, aerogels, carbon nanotubes, metal fillers or particles, organic or inorganic fibers, organic or inorganic fillers or particles, and nonwoven or woven fiber reinforcements consisting of carbon precursor fibers, glass fibers, polymer organic fibers, ceramic fibers, biopolymer fibers or any combination thereof.

[0049] The integer n can be in the range of approximately 10 to approximately 60, 10 to approximately 50, or approximately 30 to approximately 50.

[0050] The functionality and advantages of this disclosure can be further understood by referring to the following detailed embodiments and accompanying drawings. Attached Figure Description

[0051] This disclosure will be more fully understood by referring to the accompanying drawings, which form part of this application, and wherein: Figure 1 A method for synthesizing physically crosslinked polyimide (PCPI) aerogels without the use of chemical crosslinking agents is shown according to this disclosure.

[0052] Figure 2 A reaction scheme for synthesizing a crosslinked DMBZ-BPDA PI aerogel with a solid / solvent ratio of 7.5% by weight is shown. This aerogel is used as a reference for chemically crosslinked PI aerogels (CCPI), i.e., for comparison with the PCPI aerogels synthesized in this study.

[0053] Figure 3a The porosity (%) of PCPI aerogel and the previously reported chemically cross-linked PI aerogel based on DMBZ-BPDA backbone varies with density (g / cm³). 3 A comparison chart of the changes.

[0054] Figure 3b This is the FTIR spectrum of the manufactured PCPI aerogel.

[0055] Figure 3c These are micrographs of the fabricated aerogels: (C1-C2) PCPI-1; (D1-D2) PCPI-2; (E1-E2) PCPI-3; (F1-F2) PCPI-4; (G1-G2) PCPI-5; (H1-H2) PCPI-6; (I1-I2) PCPI-7; (J1-J2) CCPI Figure 4a The top panel shows photos of the samples before the test (water absorption test) (physical state), the middle panel shows photos of the samples after the test (physical state), and the bottom panel shows photos of the samples after drying (physical state). From left to right, the sample names are: PCPI-4, PCPI-5, PCPI-6, PCPI-3, and PCPI-7. Figure 4b The histograms show the diameter (mm) of five (5) different aerogels before testing (leftmost histogram), after testing (middle histogram), and after drying (rightmost histogram).

[0056] Figure 4c The histograms show the weight (in grams) of the five (5) different aerogels identified before testing (leftmost histogram), after testing (middle histogram), and after drying (rightmost histogram).

[0057] Figure 5a This is a graph showing the change in WCA (moisture resistance of aerogel) over time.

[0058] Figure 5b This is a series of photographs showing the wettability behavior of aerogels over different time ranges, (up) PCPI-6 aerogel (down) CCPI aerogel.

[0059] Figure 5c The image above shows a comparison of the wettability of PCPI-6 aerogel and CCPI aerogel. The image shows that water droplets are absorbed very quickly, causing the surface of PCPI-6 to disintegrate, while this does not happen with CCPI aerogel (bottom).

[0060] Figure 6a This is a graph showing the change in temperature as a percentage of weight in the TGA analysis of aerogels.

[0061] Figure 6b This is a histogram of the decomposition temperatures of aerogels.

[0062] Figure 6c This is a graph showing the change in frequency (Hz) of the aerogel with respect to its dielectric constant.

[0063] Figure 6d The dielectric constant of PCPI aerogel and other aerogels varies with density (g / cm³). 3 A comparison chart of the changes.

[0064] Figure 7a This is a curve showing the change in compressive strain (mm / mm) of aerogel with compressive stress (MPa).

[0065] Figure 7b The density (g / cm³) of PCPI aerogel compared to other aerogels. 3 A comparison chart showing the variation of compression modulus (MPa).

[0066] Figure 7c This is a curve showing the change in strain (%) of aerogel with stress (MPa).

[0067] Figure 7d These are a series of photographs demonstrating the flexibility of aerogels.

[0068] Figure 8aThe transparency (%) of PCPI aerogel (in this study) compared to cellulose-based aerogels, polyimide aerogels, and silica-based aerogels as a function of density (g / cm³) is shown. 3 A comparison chart of the changes.

[0069] Figure 8b This is a graph showing the change in transparency (%) of the aerogel as a function of its thickness (mm).

[0070] Figure 9 This is a histogram showing the density of PCPI aerogels prepared using ODA monomers with various configurations.

[0071] Figure 10 This is a histogram showing the shrinkage rate of PCPI aerogels prepared using ODA monomers with various configurations.

[0072] Figure 11 The figures show the thermal conductivity results, illustrating the insulation properties of PCPI aerogels prepared using ODA monomers with various configurations.

[0073] Figure 12 This is a graph showing the change of dielectric constant with frequency, illustrating the electrical properties of PCPI aerogels prepared using ODA monomers with various configurations.

[0074] Figure 13 The graph shows the change in weight percentage with temperature, illustrating the thermal stability of PCPI aerogels prepared from ODA monomers with various configurations.

[0075] Figure 14 These are photographs showing the changes in the wettability properties of PCPI aerogels prepared using ODA monomers with various configurations over time. Detailed Implementation

[0076] This document discloses the synthesis of hydrophobic, mechanically flexible, and optically transparent polyimide aerogels. While embodiments of the invention are disclosed herein, they are merely exemplary, and it should be understood that the invention relates to various alternative forms, including different shapes and sizes. Furthermore, the drawings are not to scale, and some features may be enlarged or reduced to a minimum to show detail of specific features, while related elements may have been omitted to avoid obscuring novel aspects. Therefore, the specific structural and functional details disclosed herein should not be construed as limiting, but merely as the basis for the claims and a representative basis enabling those skilled in the art to utilize the invention in various ways.

[0077] As used herein, the terms “comprising,” “including,” “containing,” and “comprising” should be interpreted as inclusive and open-ended, rather than exclusive. Specifically, when the terms “comprising,” “including,” “containing,” and “comprising,” and variations thereof are used in this specification including the claims, they mean to include the specified features, steps, or components. These terms should not be construed as excluding the presence of other features, steps, or components.

[0078] As used herein, when the terms “about” and “approximately” are used in conjunction with size ranges, the composition of mixtures, or other physical properties or characteristics, they refer to covering the slight variations that may exist in the upper and lower limits of the size range, so as not to exclude embodiments in which, on average, most size requirements are met, but statistically, sizes may exist outside that range. This invention is not intended to exclude embodiments such as these.

[0079] As used in this article, the coordinating conjunction “and / or” indicates a choice between logical disjunction and logical conjunction of adjacent words, phrases, or clauses. Specifically, the phrase “X and / or Y” is intended to be interpreted as “one or both of X and Y,” where X and Y are any words, phrases, or clauses.

[0080] As used in this article, the phrase “physical crosslinking” refers to intermolecular bonds or interactions between polymer chains without involving any chemical reaction. In this mechanism, the polymer chain network is held together by entanglement due to the long polymer chains with high molecular weights and various weak forces (such as van der Waals forces, hydrogen bonds, or dipole-dipole interactions).

[0081] Conversely, "chemical crosslinking" refers to the formation of chemical bonds between polymer chains via covalent bonds, and typically requires the use of specific chemical reagents. Chemical crosslinking agents can be selected from the group consisting of: triamines, aliphatic amines containing three or more amines, aliphatic triamines, aromatic amines containing three or more amine groups, aromatic triamines, 1,3,5-tris(aminophenoxy)benzene, and silica gel structures modified with three or more amines, to name just a few non-limiting examples.

[0082] As used herein, the phrase “aerogel” refers to a type of mesoporous solid material with an open-pore structure, wherein the average pore size ranges from about 2 nm to about 50 nm, and such material is prepared by removing the liquid contents of a wet gel while maintaining the integrity of the solid network.

[0083] As used herein, the phrase “diamine” refers to a type of polyamine having exactly two amino groups. Other diamines, besides those used in the non-limiting examples below, include, but are not limited to, 1. m-phenylenediamine (m-PDA), 2. p-phenylenediamine (p-PDA), 3. 4,4'-methylenediphenylamine (MDA), and 4. 4,4'-diaminodiphenyl ether (ODA).

[0084] This disclosure will now be illustrated by the following non-limiting examples.

[0085] experiment Material 2,2'-Dimethylbenzidine (DMBZ) and biphenyl-tetracarboxylic dianhydride (BPDA) were selected as the diamine monomer and dianhydride monomer, respectively. In our previous study

[11] , the DMBZ-BPDA combination was considered a promising main-chain chemical structure for the fabrication of aerogels with improved moisture resistance, mechanical strength, and thermal stability. However, chemically crosslinked DMBZ-BPDA has been reported to exhibit high rigidity and therefore low mechanical flexibility [1]. N-methylpyrrolidone (NMP) was selected as the solvent due to its highly basic aprotic properties. Pyridine and acetic anhydride were used to catalyze the imidization reaction and to remove the water byproducts of the condensation reaction, respectively. For the chemically crosslinked aerogel, 1,3,5-tris(4'-aminophenyl)benzene (TAPB) was selected as the crosslinking agent. 100% ethanol was used in the solvent exchange step. All reagents were purchased from Sigma Aldrich and were ready for use without further purification.

[0086] Characterization The density of the aerogel was measured using calipers and an electronic balance. Shrinkage was calculated by comparing the retained diameter and initial diameter of the aerogel. Porosity was measured using a helium hydrometer (Quantachrome Instrument Ultra-Foam 1000) according to ASTM D6226. Morphological characteristics of the samples were evaluated using an electron microscope (field emission SEM, Quanta, model FEG-250) operated at 5 kV to 15 kV. Fourier transient infrared (FTIR) analysis of the PCPI aerogel was performed using a Perkin Elmer spectrometer to identify the chemical structure of the PCPI aerogel by spectral transmittance measurements and chemical bond assessment. A 4000 cm⁻¹ section was used. -1 The spectral range up to 650 cm⁻¹ was used for spectral transmittance collection, with a spectral resolution of 4 cm⁻¹. -1 .

[0087] The surface wettability of PCPI aerogels was characterized using an Ossila contact angle goniometer (L2004A1). Water droplets were suspended on the sample surface using a micropipette, and the captured images were analyzed and the WCA was recorded using analysis software. Further wettability analysis was performed to assess the water absorption capacity of the aerogels. Similar to the previously reported method

[12] , the prepared samples were completely immersed in distilled (DI) water at 25°C for 24 hours. Then, to remove excess water, the wet samples were wiped and their weight was assessed using a balance. The water absorption rate of the aerogels was calculated using Equation (1):

[0088] W here i and W f These are the initial (before testing) weight and the final (after testing) weight of the sample, respectively.

[0089] To analyze the thermal stability of the aerogel and determine its decomposition initiation temperature, thermogravimetric analysis (TGA) was performed on a TA-Instrument Q50. Under nitrogen atmosphere, the temperature was increased at 10℃ / min increments. -1 The temperature was increased from 25°C to 700°C, and sample weight loss was monitored. The compressive properties of the monolithic aerogel, including compressive modulus and strength, were evaluated based on ASTM guideline D695-02a. An Instron 5848 miniature tester was used for this purpose. All compression tests were performed at 0.05 inmin. -1 The tests were conducted at a rate of [missing information]. Furthermore, tensile strength tests were performed using a dynamic mechanical analyzer (DMA, Q800, TA Instrument). The dielectric constant of the aerogel was measured using an Alpha-A high-performance conductivity analyzer (Novocontrol Technologies GmbH & Co. KG, Germany), and measurements were taken at 10 [missing information]. -1 Hz to 10 5 The measurements were performed within the Hz frequency range. The optical properties of the samples (transmittance and haze) were measured using a PerkinElmer Lambda 1050 UV-Vis spectrophotometer equipped with a 150 mm integrating sphere in the spectral range of 200 nm to 2500 nm.

[0090] Preparation of physically cross-linked polyimide aerogel Our preliminary studies indicate that monomer reaction time plays a significant role in the formation of high-viscosity polyamic acid (PAA) precursors and the generation of physically crosslinked PI aerogels. With a solids / solvent ratio of 14 wt% and a theoretical repeating unit number of 35 (n=35), when diamine (DMBZ) was dissolved in NMP (within 30 minutes) and dianhydride (BPDA) was added to the solution, PAA reached high viscosity after 2 hours of monomer reaction, forming a gel network even in the absence of a chemical crosslinking agent (PCPI-1). Subsequently, to improve monomer dispersion quality, both DMBZ and BPDA were dissolved in 50% NMP. Therefore, a parametric study was designed to evaluate the importance of monomer dispersion time and the diamine-dianhydride reaction strategy to fully understand the contribution of processing parameters to PCPI aerogel formation (see Table 1).

[0091] Table 1. Material composition and processing details of the samples As an example, the formulation PCPI-6 from Table 1 (consisting of 15.8 mmol% DMBZ and 16.25 mmol% BPDA, with a solid / solvent ratio of 14% by weight and n=35) was prepared as follows (e.g. Figure 1 (As shown): DMBZ (3.35 g, 15.8 mmol) and BPDA (4.78 g, 16.25 mmol) were dissolved separately in 25 mL of NMP and stirred for 2 hours until dissolved. Subsequently, the monomer solutions were reacted using a mechanical stirrer at 200 rpm. Acetic anhydride and pyridine were then added and mechanically stirred for 2 minutes. Finally, the solutions were poured into cylindrical and rectangular molds for processing monolithic wet gels and thin-film wet gels, respectively. After aging for 24 hours, the wet gels were solvent-exchanged with ethanol and transferred to a supercritical drying chamber for drying.

[0092] from Figure 1 It was observed that PCPI aerogels with both bulk monolithic and thin-film geometries were obtained. To investigate the effect of the combination of physical and chemical crosslinking mechanisms on the properties of PI aerogels, PCPI-7 was prepared using a method similar to that used for PCPI-3, with similar repeating unit number and solid / solvent ratio, but with TAPB (0.109 g) added as a crosslinking agent in the synthesis pathway. Furthermore, according to... Figure 2 A TAPB-crosslinked DMBZ-BPDA PI aerogel with a solid / solvent ratio of 7.5% by weight was prepared. Chemically crosslinked PI aerogel (CCPI) was used as a reference for comparison purposes in this study.

[0093] Results and discussion Density, shrinkage and porosity Figure 3a The density-porosity relationship of PCPI aerogels is compared with that of chemically cross-linked PI aerogels based on the DMBZ-BPDA backbone. Figure 3b The FTIR spectrum of the fabricated PCPI aerogel is shown.

[0094] Figure 3a The shrinkage behavior of the fabricated PCPI aerogel was revealed by examining the density and porosity of the samples. Similar to previously reported aerogels, the density and porosity of the fabricated physically cross-linked samples exhibited an inverse trend, with porosity decreasing with increasing density. Furthermore, Figure 3a This study summarizes the density and porosity of chemically crosslinked DMBZ-BPDA aerogels reported in previous studies and presents the results of this study [1], [3], [4], [8]. Given the relatively high solids / solvent ratio (14 wt%), the physically crosslinked sample produced showed a density of 0.142 g cm⁻¹. -3 Up to 0.178 g cm -3 The density is slightly higher within the range. However, both PCPI-3 and PCPI-6 exhibit a density of approximately 0.14 g / cm³. -3 The density of PCPI-3 and PCPI-6 is similar to that of chemically cross-linked DMBZ-BPDA aerogels with 35 repeating units and a relatively low solids / solvent content (7.5 wt%). The similarity in density between PCPI-3 and PCPI-6 and the chemically cross-linked DMBZ-BPDA aerogels at the low solids / solvent content demonstrates that PCPI aerogels exhibit lower shrinkage and higher porosity.

[0095] In reactions with BPDA dianhydride monomers, rigid DMBZ diamines are known to produce higher porosity compared to other diamines such as PPDA and ODA [3]. The resulting PCPI aerogels exhibit even higher porosity compared to chemically crosslinked PI aerogels, with observed porosities ranging from 95.09% to 98.46%. High porosity will be beneficial for improving a wide range of aerogel properties, including but not limited to reduced weight, improved filtration efficiency, and enhanced thermal insulation.

[0096] According to previous studies on chemically crosslinked PCPI aerogels, the density increases with the increase of the number of repeating units and the increase of molecular weight [1], [8]. Based on this trend, the increased density of the PCPI aerogel with higher porosity compared to the chemically crosslinked aerogel can indicate that the PCPI aerogel has a larger molecular weight. This high molecular weight and the resulting longer polymer chains are key factors in crosslinking PCPI samples without the use of chemical crosslinking agents.

[0097] aerogel morphology Figure 3c SEM micrographs of the fabricated aerogel are shown. Figure 3c As shown, the morphology of the manufactured physically or chemically crosslinked aerogels is primarily formed by crosslinked polymer chains, thus exhibiting a fibrous morphology. However, due to different processing techniques and crosslinking methods, the samples exhibit various morphological parameters, including pore size, average fiber thickness, crosslinking density, and morphological texture. Among these samples, PCPI-2, prepared with a shorter monomer dissolution time and a shorter reaction time, exhibits a larger pore size and coarser fibers, each of which is formed by a combination of parallel-arranged finer fibers. This is believed to be the reason behind the higher density and lower porosity of PCPI-2 compared to the other samples manufactured. Although PCPI-1 was manufactured with a very short monomer dissolution time, even with BPDA added to the DMBZ solution in powder form and then mixed for only 10 minutes, the longer reaction time (2 hours) before the addition of chemical reagents compared to PCPI-2 resulted in the formation of finer fibers and smaller pore sizes and a more uniform pore structure. In PCPI-3, although the reaction time was relatively short (20 minutes), increasing the monomer dissolution time to 60 minutes resulted in reduced fiber thickness and smaller pore sizes.

[0098] This observation is consistent with the porosity and density trends of PCPI-1, PCPI-2, and PCPI-3 samples. (Based on data from...) Figure 3c (F1) to Figure 3c The observations of (H2) show that, compared to PCPI-1 to PCPI-3, PCPI-4, PCPI-5, and PCPI-6 exhibit relatively similar physically entangled fiber morphologies, with increased fiber aspect ratios, decreased fiber thickness, and smaller pore sizes. This can be attributed to their longer monomer dissolution times or longer reaction times. Given the highly sensitive relationship between aerogel properties and their morphology, this indicates that the monomer dissolution, dispersion, and reaction processes are crucial for the intermolecular forces during the gelation stage, and consequently, the morphology and properties of the resulting aerogel.

[0099] Because the onset time of physical crosslinking (1 to 4 minutes) is significantly faster than the time required for gelation using TAPB chemical crosslinking agents (0.5 to 1 hour), the primary crosslinking and gelation mechanism in PCPI-7 is still considered to be physical crosslinking. However, as Figure 3c (I1) to Figure 3cAs shown in (J2), introducing the TAPB chemical crosslinking agent into the DMBZ-BPDA solution caused a significant change in its morphology, transforming it into the chemically crosslinked DMBZ-BPDA (CCPI) morphology. This can be attributed to the role of TAPB molecules in altering intermolecular forces during gelation. This confirms the idea of ​​adjusting the properties of aerogels by introducing other elements, molecules, or even ions into the solution system to change intermolecular forces and thus control the morphology of the aerogel [6]. These elements can react with the aerogel backbone or be washed away through solvent exchange without changing the chemical structure of the aerogel, but they will play a role in altering intermolecular forces, thereby changing the morphology of the aerogel.

[0100] Chemical structure FTIR was performed to identify the main characteristic bands and functional groups, and to verify the successful imidization of the PCPI aerogel. Observation Figure 3b The FTIR results shown in the figure indicate that at 739 cm⁻¹, [the following can be observed]. -1 The four main peaks at 1370 cm⁻¹, 1720 cm⁻¹, and 1775 cm⁻¹ correspond to the vibrations of the imide ring, the CN band, the C=O symmetric band, and the C=O asymmetric band, respectively. The CH bond from the benzene ring can be observed at 1150 cm⁻¹. -1 Nearby analysis indicated moderate strength, and 1500 cm... -1 The nearby CH bonds correspond to the C=C stretching vibration. The absence of anhydride C=O stretching vibration (corresponding to the BPDA-initiated dianhydride) and amine NH stretching vibration (corresponding to the DMBZ-initiated diamine) confirms successful imidization of the PCPI aerogel.

[0101] Hydrophobicity and moisture resistance The wettability of PCPI aerogels was evaluated by measuring water absorption rate and water contact angle (WCA) to elucidate the hydrophobicity and moisture resistance of the prepared aerogels. Our previous research showed that pristine PI aerogels synthesized via a chemical crosslinking pathway exhibit superhydrophilicity resulting from the polar structure of PI; therefore, the resulting PI aerogels cannot be used in moisture-sensitive environments. In this study, to evaluate the moisture resistance of the fabricated PCPI aerogels, water absorption rate tests were performed, where the water absorption rate ratio of the samples was calculated by comparing the weights of the samples before and after the test. Figure 4a The changes in the physical state of the samples over time are shown in the water absorption test. The samples from left to right are named: PCPI-4, PCPI-5, PCPI-6, PCPI-3, and PCPI-7. Figure 4b The variation in sample diameter during the water absorption test is shown. Figure 4c The change in sample weight (D) during the water absorption test is shown. i : Initial diameter of the sample before testing; Wi : Initial weight of the sample before testing; D f : The final diameter of the sample after testing; W f : The final weight of the sample after testing; D 48h Diameter of the sample after 48 hours of drying; W 48h (Weight of the sample after 48 hours of drying).

[0102] from Figure 4a As observed, monolithic block aerogels were prepared, and their physical states were monitored after testing and after drying at room temperature for 48 hours. Clearly, except for PCPI-6 (intermediate sample), the other aerogels absorbed a significant amount of water (water absorption rates ranging from 300% to 551%), and their color turned a deep yellow, confirming the presence of water. For example, PCPI-3, with an initial weight of 0.96 g and a diameter of 25.6 mm, experienced a weight increase of approximately 551% (water absorption rate = 551%), resulting in a final weight of approximately 6.26 g. Observing the dimensional stability of PCPI-3, it can be seen that the diameter of the aerogel increased slightly due to rapid water penetration (swelling behavior). However, as expected, the samples underwent considerable shrinkage after drying (~42% for PCPI-3), and significant structural deformation was observed. This observation was consistent across all tested aerogels; see [link to relevant documentation]. Figure 4b and Figure 4c .

[0103] Surprisingly, PCPI-6 exhibited an ultra-low water absorption rate of approximately 15%, significantly lower than that of previously reported PI aerogels, including poly(maleic anhydride) crosslinked PI aerogels (up to 700%)

[13] , bis(trimethoxysilylpropyl)amine (BTMSPA) crosslinked ODA-BPDA aerogels (up to 500%)

[14] and 1,3,5-triaminophenoxyphenyl (TAB) crosslinked ODA-BPDA aerogels (up to 383%)

[15] . PCPI-6’s excellent moisture resistance was also comparable to that of other chemically modified PI aerogels, namely those incorporating 1,12-dodecyldiamine (DADD)

[16] , 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FAPB)

[15] and 2,2-bis[4-(4-aminophenoxy)phenyl]propane (BAPP)

[14] . PCPI-6, thanks to its ultra-low water absorption, demonstrated the ability to maintain its physical structure during testing without collapsing, likely due to the strong physical cross-linking of the aerogel network.

[0104] To better understand the moisture resistance and ultra-low water absorption of PCPI-6, the influencing factors in the water absorption process of PI aerogels should first be discussed. A key factor to consider is the hydrophobicity / hydrophilicity of the polymer chain. When the polymer backbone contains -O- (as in ODA), the resulting PI aerogel is more likely to be moisture-sensitive, as observed in previous studies. DMBZ has been reported to exhibit improved hydrophobicity compared to the ODA structure due to its chemical structure. Typically, the wettability coefficient (WCA) is measured to characterize surface wettability. A WCA greater than 90° indicates unfavorable surface wetting.

[0105] Figure 5a The wettability behavior of the fabricated aerogel, the WCA of PCPI-6, and its stability over time are shown. Figure 5b The wettability behavior of PCPI-6 aerogels with hydrophobic features (ascending) and chemically cross-linked PI aerogels with superhydrophilic features (descending) in different frameworks is compared. Figure 5c The surface wettability behavior of PCPI-6 and CCPI aerogels was demonstrated. Observation Figure 5a The WCA measurements in the study notably show that PCPI-6 exhibits hydrophobicity, with a WCA of approximately 110°. While this material is not superhydrophobic (greater than 150°), the obtained WCA is almost unobservable in PI aerogels, as they are typically superhydrophilic. Figure 5b A comparison of the wettability of PCPI-6 aerogel and CCPI aerogel shows that for CCPI aerogel, water droplets are quickly absorbed, leading to surface disintegration. This can... Figure 5c This is clearly evident. Furthermore, water droplets are quite stable on the surface of PCPI-6 aerogel, and no structural damage was observed, confirming the hydrophobicity of PCPI-6 aerogel. Therefore, it can be concluded that due to the hydrophobicity of the material, water penetration into the pores is effectively hindered, resulting in an ultra-low water absorption rate.

[0106] Another influencing factor is the morphological characteristics of the aerogel, including the degree of crosslinking and mesoporous structure. It is well known that the pore size and pore structure of aerogels have a considerable influence on their water absorption. As explained by Guo et al.

[13] , aerogels with larger pore sizes allow water to enter more easily, resulting in higher water absorption. This phenomenon is not only observed in PI aerogels but also applies to other aerogels, such as cellulose-based aerogels

[17] . However, more importantly, the physical crosslinking of PCPI aerogels plays a crucial role in their moisture resistance. When mechanical stirring is applied in the fabrication of PCPI-6, the physical crosslinking of the wet gel network is enhanced, accompanied by an increase in the degree of entanglement and a decrease in water absorption. Furthermore, similar hydrophobic behavior observed in smaller cuts of the samples suggests that the moisture resistance of the fabricated aerogels is not due to their surface (skin) properties.

[0107] thermal stability Figure 6a The thermogravimetric analysis (TGA) results of the fabricated aerogel are shown; Figure 6b The decomposition initiation temperature of the aerogel is shown; Figure 6c The dielectric constant of the studied aerogel at different frequencies is shown; and Figure 6d A comparison of the dielectric constant-density relationship between PCPI aerogel and other aerogels is shown (data reference benchmarks: SiO2 aerogel

[18] ,

[19] , BTFB / ODPA / POSS aerogel

[20] , ODA-BPDA-BTC aerogel

[11] , DMBZ-BPDA-BTC aerogel

[11] ).

[0108] Figure 6a and Figure 6b The TGA results and decomposition onset temperatures of the fabricated aerogels are shown. Despite the different processing methods, the fabricated PCPI aerogels exhibited similar decomposition temperatures (T0). d (Approximately 530℃). This indicates that compared to chemically cross-linked PI aerogels with similar repeating unit numbers and main chain chemical structures, T d Increase, and T d The range is 510℃ to 520℃. The lower decomposition temperature of chemically crosslinked PI aerogels is related to the type of chemical crosslinking agent. The high decomposition temperature of PCPI aerogels and their temperature range at T... d The relatively low weight loss (ranging from 2.7 wt% to 9.75 wt%) indicates improved thermal stability compared to chemically crosslinked aerogels. Furthermore, adjusting the processing parameters from PCPI-4 to PCPI-6 allows for improved thermal stability at T... d The following weight loss was further reduced. In these samples, PCPI-6 showed improved performance at T... dThe following weight loss is minimal, at only 2.7% by weight. A weight loss of less than 5% by weight at the decomposition temperature can also indicate complete imidization.

[0109] like Figure 6a As shown, chemically crosslinked DMBZ-BPDA aerogel (CCPI) and PCPI-7 containing a similar TAPB crosslinking agent both exhibited a weight loss trend between 330 °C and 517 °C. In addition, PCPI-1, PCPI-2, PCPI-3 and CCPI samples showed an initial weight loss of less than 200 °C. This weight loss trend may be due to slightly incomplete imidization or moisture content. However, above 200 °C and below the decomposition initiation temperature of 530 °C, PCPI-1 to PCPI-3 showed a decomposition trend relatively similar to PCPI-4 to PCPI-6, with a weight loss of about 3 wt%. PCPI aerogels showed a char residue of 64 wt% to 72 wt% in a nitrogen atmosphere, which is within the range of previously reported chemically crosslinked PI aerogels with similar main chain chemical structures [4].

[0110] Electrical properties Due to their nanoscale structure and highly porous network, aerogels typically exhibit ultra-low dielectric constants, making them ideal for applications such as microelectronic devices, antennas, and microwave circuits. In this study, the dielectric constant of the prepared PCPI aerogels was measured using an Alpha-A high-performance conductivity analyzer. All aerogel samples were equilibrated under atmospheric conditions, and the dielectric constant was measured at 298 K. -1 Hz to 10 5 The measurement is performed within the Hz frequency range. Both the real part (also known as the dielectric constant) and the imaginary part (also known as the loss factor) are measured. The dielectric constant of PCPI aerogel at different frequencies is shown below. Figure 6c As shown in the figure, the dielectric constant falls within the range of 2.36 to 2.83, with PCPI-4 and PCPI-7 having the lowest and highest values, respectively.

[0111] Generally, it is well known that the dielectric constant of aerogels depends primarily on the chemical backbone structure, and more importantly, on the free volume fraction (i.e., porosity) of the aerogel network. Based on this principle, for PCPI-1 to PCPI-6 samples with similar compositions, it is reasonable to assume that the dielectric constant is closely related to the porosity (or density) of the sample; the lower the density, the higher the porosity, and the lower the dielectric constant. This relationship is further elaborated in... Figure 6d This is described in the text. As an example, it can be seen that it has the highest density (0.178 g / cm³). -3PCPI-2 exhibits the highest dielectric constant (2.72). This dependence is not only present in PCPI aerogels, but also from... Figure 6d It is evident that density is a crucial factor in the dielectric properties of aerogels, a phenomenon also present in other inorganic and organic aerogels. Surprisingly, in the aerogels fabricated in this study, although the density of PCPI-7 with the addition of a crosslinking agent was lower, the dielectric constant exhibited the highest value of 2.83. This high value, compared to PCPI-6 with a similar density, reflects the influence of different chemical structures and / or morphologies of PCPI-7 on the resulting dielectric properties. The dielectric constant achieved in this study is comparable to that of ODA-BPDA-BTC aerogel and slightly higher than that of DMBZ-BPDA-BTC aerogel. Furthermore, with similar dielectric constant values, the PCPI aerogels exhibited a density several times lower, ensuring their application in next-generation electronic devices to meet the requirements of low dielectric constant and lightweight design.

[0112] Mechanical properties Figure 7a The variation of compressive stress with compressive strain in the fabricated aerogel is shown, while Figure 7b This is a comparison of the compressive modulus-density of PCPI aerogel with previously reported PI aerogels (data reference benchmarks: PPDA-PMDA [5],

[10] , ODA-PPDA-PMDA-BPDA [5], ODA-BPDA [5],

[11] , PPDA-BPDA [5], ODA-PMDA [5], ODA-BPDA-TPU [6], ODA-PPDA-PMDA

[21] , PPDA-BPDA

[21] , ODA-PPDA-BPDA

[21] , DMBZ-BPDA

[11] ). Figure 7c It is a curve showing the change of tensile stress as a function of strain in the manufactured aerogel, and Figure 7d The diagram illustrates the flexibility, bendability, and torsion properties of the thin-film aerogel.

[0113] The compressive modulus and strength of PCPI aerogels were characterized using a mechanical compression test based on ASTM D695-02a. Cylindrical aerogels were compressed at 0.05 in min... -1 The rate is subjected to compressive stress, and the modulus E is recorded by monitoring the stress-strain curve. c (Initial slope of the curve) and strength (yield point reached after the linear elastic region). Observation Figure 7aThe stress-strain curves in the figure show three deformation states: elastic region, plateau region, and densification region. PCPI aerogel exhibits good toughness due to its strong physical cross-linked 3D network and is able to absorb compressive energy. After compression with more than 70% strain, it achieves a compressive strength of up to 2.2 MPa, which is about 40 times the measured value of PPDA-PMDA PI aerogel

[10] and about 10 times the measured value of ODA-BPDA PI aerogel

[22] . Its compressive strength is also superior to other aerogels (including cellulose aerogel

[23] ,

[24] , carbon aerogel

[25] and silica aerogel

[26] ,

[27] ).

[0114] The synthetic strategy disclosed in this paper also improves the elastic modulus of the developed aerogel, making it superior to other PI aerogels. This enhancement is achieved in... Figure 7b The superior mechanical properties of PCPI aerogels are highlighted in this study. A compressive modulus exceeding 20 MPa was obtained, significantly greater than that of previously reported PCPI aerogels [5],

[10] ,

[11] ,

[21] . PCPI aerogels exhibit superior properties even compared to hybrid aerogels and aerogels containing thermoplastic polyurethane (TPU) nanofibers. They also maintain a low density (0.142 g cm⁻¹). -3 Up to 0.178 g cm -3 This can be attributed to the higher molecular weight strengthening the physical cross-linking network by enhancing polymer chain entanglement. Indeed, the longer polymer chains of PAA (as observed through increased solution viscosity) provide more network entanglement in PCPI aerogels. Therefore, it is easier to form more cross-linking points, resulting in improved mechanical properties compared to chemically cross-linked aerogels.

[0115] For all the compositions listed in Table 1, thin-film PCPI aerogels can also be manufactured using the film casting method. For example... Figure 7d As shown, all fabricated thin-film aerogels exhibited excellent mechanical flexibility. No significant structural damage was observed when subjected to partial and / or complete bending, and even under complete twisting, they maintained structural integrity and could recover to their original shape without collapse. This indicates that chain entanglement in the physically crosslinked network of PCPI aerogels plays a crucial role in achieving this formability and flexibility. Typically, DMBZ is considered a rigid diamine (with more brittle characteristics) and should be partially / completely replaced by other flexible diamines (e.g., ODA) to improve network flexibility. However, we can recognize that the physical crosslinking strategy proposed in this study can even be used to develop highly flexible and scalable PI aerogels based on rigid monomers.

[0116] In fact, developing flexible thin-film aerogels with improved thermal and mechanical properties using simple strategies has been a long-standing challenge. While flexible aerogel films based on graphene, cellulose, silica, and even PI have been reported in recent years, their preparation methods are either complex or their high cost limits large-scale production. Against this backdrop, developing more cost-effective and simpler strategies (such as those proposed in this study) could be significant for further expanding the applications of PI aerogels in practical fields such as aerospace, building insulation, microelectronics, and even winter clothing.

[0117] Typical stress-strain curves of PCPI aerogel membranes from tensile tests are shown in... Figure 7c Tensile moduli ranging from 1 MPa to 1.8 MPa were achieved. Notably, among the tested aerogel membranes, only PCPI-7 F (containing a crosslinking agent) ruptured during testing, while the other PCPI aerogels continued to stretch until the DMA device reached its ultimate force capacity. This observation is consistent with the compression test results, where PCPI-7 exhibited the lowest compressive strength. Based on the tensile test results, it is clear that the physical crosslinking strategy is effective in improving the tensile modulus and strength of the aerogel membranes. This can be explained by the entangled polymer network in the PCPI aerogel forming tight connections between the molecular chains. As discussed above, the tensile properties are improved due to stress concentration at these connection points.

[0118] Optical transparency Figure 8a The transparency of PCPI aerogel is compared with other reported aerogels, and... Figure 8b The effect of film thickness on the transparency of PCPI aerogels is shown. PI aerogels with improved transmittance could be a promising alternative to currently available optical polymers (e.g., polycarbonate), which typically suffer from low operating temperatures and high densities. However, only a few studies in the literature have demonstrated the feasibility of achieving PI aerogels with improved transparency, and this topic remains under-explored. To our knowledge, previous studies have not yielded PI aerogels with transmittance comparable to silica-based aerogels (typically exceeding 80%). Therefore, novel and simple strategies are needed to develop PI aerogels with enhanced transparency.

[0119] For the selected PCPI aerogel membrane, transmittance was measured using a UV-Vis spectrophotometer by placing the sample on the transmission port of an integrating sphere. The transmittance of the aerogel membrane and their corresponding densities are shown in [the table / image / image]. Figure 8aAs shown in the figure, this study achieved a transparency of up to 85%, the highest value reported to date for organic PI aerogels. Although the density of the aerogel developed in this study is slightly higher than that of cellulose-based aerogels, the transparency is improved. More importantly, the optical transparency is comparable to that of previously reported silica-based inorganic aerogels. However, we should emphasize that PCPI aerogels exhibit significantly better mechanical flexibility compared to silica-based aerogels, thus making them effective for practical applications.

[0120] Compared to previously reported PI aerogels, the PCPI aerogel membrane in this study exhibits improved transparency, which can be attributed to the adoption of a physical crosslinking strategy and the elimination of crosslinking agents. All these factors, aside from slight differences in chemical composition and molecular weight, contribute to the improved transparency. Furthermore, a more uniform and smaller pore size distribution also contributes to such high transparency. Another important factor rarely discussed in the literature is the thickness of the aerogel membrane. This is crucial from an application perspective, especially when thermal insulation properties are required.

[0121] like Figure 8b As shown, thickness has a considerable effect on the optical transparency of PI aerogel films. When the thickness is reduced from 0.95 mm to 0.22 mm, the transparency is significantly improved, from 63.5% to 84.8%. It has been previously reported that the transparency of aerogels is so dependent on their thickness. In the case of cross-linked aerogels, it has been suggested that the presence of pores affects the transmittance (visibility) of aerogels because their thickness changes. Zhao et al.

[28] further investigated this phenomenon using theoretical and experimental evaluations. Theoretical (using a radiative transfer model) and experimental measurements of the total transmittance and haze of transparent aerogels both demonstrate the importance of aerogel thickness. For various particle sizes, it was observed that in all cases, an increase in thickness has an adverse effect on optical transparency.

[0122] However, this dependence is minimized for aerogels with smaller particle sizes, and high transparency is maintained across a wide range of thicknesses. Nevertheless, the underlying reasons for the transmittance of aerogels remain not fully understood, and other sample parameters (i.e., density, molecular weight, and chain stacking ability) may also play a positive role, requiring further investigation. Finally, it should be noted that not all aerogels can be fabricated into thin-film configurations with controlled thicknesses while maintaining mechanical flexibility. For example, silica aerogels, due to their inherent brittleness, cannot be transformed into thin-film aerogels without structural disintegration. Therefore, the physical crosslinking pathway introduced in this study can be considered to have great potential in fabricating thin-film aerogels with controlled thicknesses and modulated properties (crucial for adhesive applications).

[0123] It is noteworthy that, according to FTIR results, aerogels are not the primary source of polyamide-amide polymers. According to Irvin et al., polyamide-amide polymers are prepared by reacting intermediate polyamic acid derivatives with free amines, rather than by cyclization and dehydration reactions, to form polyimides. The following schematic diagram illustrates the repeating structural units of polyamide-amide polymers disclosed in Irvin et al.'s publication (US 2017 / 0355829 A1): X can be a first organic group having at least two carbon atoms, Y can be a second organic group having at least two carbon atoms, and Z can be a nitrogen-containing hydrocarbon compound containing at least one secondary nitrogen atom. According to the method described in this application, polyimide is formed by a cyclization dehydration reaction (imidization), i.e., by converting polyamic acid (a polyimide precursor) into polyimide using a chemical dehydrating agent and a catalyst.

[0124] According to Table 1 of Irvin et al.'s patent (US 2017 / 0355829 A1), the FTIR peaks between 1415 and 1440 indicate the presence of amide-amide bonds with C=N stretching vibrations. However, the comparable FTIR results of this application are the same as those for polyimide, and there are no significant peaks between the 1415 and 1440 frequencies. Therefore, it is concluded that the aerogel is not a major source of polyamide-amide polymers, or that, to a person skilled in the art, the aerogel does not contain polyamide-amide polymers.

[0125] Alternative implementation plan The following section describes the applicability of physical crosslinking methods for preparing physically crosslinked polyimide aerogels (PCPIs) using other monomers. 4,4'-Diaminodiphenyl ether (ODA) was used as the starting diamine monomer, and 3,3,4,4-biphenyltetracarboxylic dianhydride (BPDA) was used as the dianhydride to prepare physically crosslinked polyimide aerogels (PCPIs). As an example, ODA-PCPI-2 was prepared as follows: ODA and BPDA were dissolved separately in a solvent (NMP) for 60 minutes to ensure complete dissolution. The resulting solutions were mixed and added to another beaker, and thoroughly mixed at 200 rpm for 2 hours. Acetic anhydride and pyridine were then added for a chemical imidization process. After pouring the final solution into a mold, the sample was solvent-exchanged and supercritically dried. To further demonstrate the practicality of the novel physical crosslinking strategy for hybrid backbones, ODA and DMBZ (1:1) were used as diamine monomers and BPDA was used as a dianhydride monomer to synthesize polyimide aerogel (ODA-PCPI-3).

[0126] See Figure 9 and Figure 10Clearly, ODA-PCPI aerogels exhibit ultra-low density (as low as 0.125 g / cm³). 3 And low shrinkage (as low as 5.5%). Similar to previously reported aerogels, the density and shrinkage of the fabricated ODA-PCPI aerogels exhibit similar trends. Aerogels with higher shrinkage actually fill more of the same volume, resulting in higher density. Figure 11 The thermal stability results show that no significant weight loss can be observed at temperatures up to 200 °C, which reflects complete imidization. All aerogel samples exhibited relatively similar decomposition reactions in the range of 506 °C to 553 °C. The excellent thermal stability of PI aerogels is thought to be due to the presence of multiple aromatic units. From the results, ODA-PCPI-3 has a slightly lower decomposition temperature, which may be due to the presence of DMBZ in its main chain. As previously shown, the presence of methyl groups in the DMBZ structure is the reason for this behavior

[30] .

[0127] See Figure 12 As can be seen, ODA-PCPI aerogel benefits from its ultra-low porosity and exhibits high thermal insulation performance, with a thermal conductivity as low as 0.034 W / mK. Thermal conductivity is directly related to the density of the aerogel. The total thermal conductivity consists of gas terms, solid terms and radiation thermal conductivity terms

[31] . Since the solid conductivity is directly affected by the density, the conduction is greatly suppressed.

[0128] See Figure 13 Observations revealed that the dielectric constant increased with frequency from 0 Hz to 10 Hz. 5 The dielectric constant decreases with increasing frequency. This behavior is due to the frequency dependence of the polarizable units of the polymer, which can align with the applied electric field

[32] . The dielectric constant of ODA-PCPI aerogels ranges from 2.6 to 2.85. Notably, aerogels containing DMBZ have a lower dielectric constant, which may be due to the effect of DMBZ on chain rigidity and mobility

[33] .

[0129] See Figure 14 The wettability of the aerogel was characterized. It was evident that water droplets rapidly spread across the entire surface within a few seconds, and the water contact angle (WCA) reached zero in all cases. This indicates that the ODA-PCPI aerogel exhibits superhydrophilic behavior, which may be due to their polar structure containing CO bonds, C=O bonds, CN bonds and NH bonds

[30] .

[0130] Comparison of this disclosure with Liu et al. (CN111253614 A) To distinguish this disclosure from that of Liu et al. (CN111253614_A), the inventors conducted experiments comparing the products of Liu et al.'s invention with those of the claimed invention. The inventors obtained aerogel samples for comparison following the same procedure outlined by Liu et al. Detailed synthesis procedures, documentation of each step, observations, and final results are included below.

[0131] Material 2,2'-Dimethylbenzidine (DMBZ; TCI America) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA; Sigma Aldrich) were selected as the monomeric diamine and dianhydride, respectively. 1-Methyl-2-pyrrolidone (NMP; Sigma Aldrich) was selected as the solvent. Pyridine (Sigma Aldrich) and acetic anhydride (Sigma Aldrich) were used to catalyze the imidization reaction and to remove the aqueous byproducts of the condensation reaction.

[0132] Synthesis program The inventors have repeated the same procedure outlined in Example 4 of Liu et al. as described below: Step 1: Prepare 2.94 g (10 mmol) dianhydride and 2.12 g (10 mmol) diamine, and dissolve them in the polymerization solvent NMP (44 mL). Polycondensation is carried out at 25 °C. After 24 hours, a high molecular weight polyamic acid solution is obtained. It is reported that the solid content of the reaction solution in this step is 10%.

[0133] Step 2: Add 583 mL of NMP diluent to the reaction system. It has been reported that after stirring at 25°C for 5 minutes, the solid content of the diluted solution was 0.78%.

[0134] Step 3: Add the chemical imidizing reagent (16 mL, acetic anhydride, and 13 mL pyridine) to a polyamic acid solution system with a low solids content. After thorough mixing, pour the solution into a mold and allow it to gel. According to the synthesis protocol of Liu et al., after the reaction system gels, the temperature should be raised to 50°C for 24 hours for aging. Solvent exchange and supercritical drying are then performed.

[0135] Observations and Results After adding the chemical imidizing agent to a polyamic acid solution system with a low solids content, the solution was mixed thoroughly, poured into a mold, and allowed to gel. Since Liu et al. did not report the gelation time, the samples were monitored until gelation occurred. It was observed that gelation did not occur even after 24 hours. At this stage, following the synthesis protocol of Liu et al., a portion of the sample was transferred to an oven and aged at 50°C for 24 hours. Notably, gelation did not occur even after this stage. It should be further noted that for those samples outside the oven, gelation did not occur even after 96 hours. Because the samples did not gel, completing solvent exchange and supercritical drying, and thus preparing aerogel samples for characterization, was not feasible.

[0136] Differences between Liu et al.

[29] and the present invention in terms of synthesis scheme The method employed by Liu et al. involves generating high-molecular-weight polyamic acid solutions (long polymer chains, degree of polymerization >39, solids content 5% to 10%), which requires a long processing time (8 to 72 hours). The solutions are then diluted to very low solids content (only 0.1% to 1%), followed by the addition of chemical imidizing agents to form a wet gel. Due to the very low solids content, this process theoretically takes a very long time to complete. However, unlike Liu et al.'s approach, which sacrifices processing time by focusing on high molecular weight and very low solids content, our inventive strategy constructs a relatively rapid process by rapidly activating the system at high solids content and using specific catalysts (acetic anhydride and pyridine) for controlled polymerization. This allows for very rapid gelation (1 to 10 minutes), meeting the core requirement for large-scale continuous production of aerogels in block, membrane, or fibrous geometries. The differences in the synthetic schemes are illustrated in the following examples.

[0137] Following the procedure reported by Liu et al., the inventors were unable to obtain any gelling products. In addition to the above facts, there are many other points that need to be emphasized: (i) The lowest density reported by Liu et al. is 2 mg / cm³. 3 This is within the scope of the present invention (0.07 g / cm). 3 Up to 0.25 g / cm 3 Compared to [previous methods], the density of the gel of this invention is increased by 35 to 125 times. Given the direct relationship between density and mechanical properties, the aerogel produced by Liu et al. is expected to have extremely low mechanical properties and exhibit brittleness.

[0138] (ii) Given the ultra-low density of the aerogels reported by Liu et al., it is expected that their aerogels will have ultra-high porosity. Since their surface area is in the range of typical aerogels, it is expected that their pore size will be large, and thus their thermal conductivity will increase to the range of conventional polymer foams.

[0139] (iii) Overall, such aerogels are expected to have low mechanical properties and high thermal conductivity, and they are not suitable for practical applications.

[0140] Useful implementation plan In one embodiment, this disclosure provides a polyimide aerogel comprising a polyimide-based polymer of formula 1: Formula 1 Where n is an integer equal to or greater than 10, and where the polymer is physically cross-linked through polymer chain entanglement, and is formed by a method including initial gelation of approximately 1 minute to approximately 10 minutes; and The aerogel is not the primary source of the polyamide amide polymer, or, to those skilled in the art, the aerogel does not contain the polyamide amide polymer.

[0141] In one embodiment, the aerogel is free of polyamide polymers, as will be apparent to those skilled in the art.

[0142] In the embodiments, the aerogel has a pore size in the range of about 2 nanometers to about 100 nanometers, or in the range of about 2 nanometers to about 50 nanometers, or a pore size less than about 2 nanometers.

[0143] In the embodiments, the aerogel has a porosity in the range of about 85% to about 99%, or a porosity in the range of about 95% to about 99%.

[0144] In the implementation plan, the aerogel does not contain chemical cross-linking agents.

[0145] In the implementation scheme, the aerogel has a density of approximately 0.07 g / cm³. 3 To approximately 0.25 gm / cm 3 The density within the range.

[0146] In the implementation, the aerogel has a thermal conductivity in the range of about 15 milliwatts / Kelvin-meter to about 50 milliwatts / Kelvin-meter.

[0147] In the implementation scheme, the aerogel is characterized by its hydrophobicity in the range of approximately 90 degrees to 140 degrees water contact angle.

[0148] In the implementation scheme, the aerogel is characterized by a water absorption rate as low as about 1%.

[0149] In the embodiments, the aerogel has a thermal decomposition initiation temperature in the range of about 500°C to about 700°C.

[0150] In the embodiments, the aerogel has a dielectric constant in the range of about 1.5 to 3.

[0151] In the implementation, the aerogel is in the form of a membrane having a thickness of at least about 80 micrometers.

[0152] In the implementation, the aerogel has an optical transparency in the range of about 60% to about 99%.

[0153] In the implementation, the aerogel is characterized by its compressive modulus in the range of about 17 MPa to 25 MPa.

[0154] In the implementation, the aerogel is characterized by a tensile modulus in the range of about 1 megapascal (MPa) to 5 megapascal.

[0155] In the implementation scheme, the integer n can be in the range of about 10 to about 60, 10 to about 50, or about 30 to about 50.

[0156] In one embodiment, a method for preparing polyimide aerogel is provided, comprising the following steps: • A wet gel is formed by reacting a diamine monomer (DIAM) with a dianhydride monomer (DIAH) in a reaction solution containing a dipolar aprotic solvent to form a polymer of formula A; where n is an integer equal to or greater than 10. Formula A; • To imidize the resulting polymer; • Allow the formed polymer to gel for approximately 1 to 10 minutes; • Use a dry solvent instead of an aprotic solvent; and • Remove the dry solvent from the formed wet gel.

[0157] In embodiments of the method, other diamines and dianhydrides may be applicable. However, properties including hydrophobicity and mechanical properties may be different properties as described below, for example, non-limiting alternatives to DIAM include m-phenylenediamine (m-PDA), p-phenylenediamine (p-PDA), 4,4'-methylenediphenylamine (MDA), and 4,4'-diaminodiphenyl ether (ODA).

[0158] In the implementation scheme, alternatives to DIAH include, but are not limited to, pyromellitic dianhydride (PMDA), 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA), 2,2'-bis[4-(3,4-dicarboxyphenoxy)phenyl]propane-dianhydride (BPADA), benzophenone tetracarboxylic dianhydride (BTDA), and 4,4'-oxobisphthalic anhydride (ODPA).

[0159] In one embodiment, the imidization step of the formed polymer is performed by thermal imidization or chemical imidization. Thermal imidization is not a common method for imidizing PI aerogels because PI aerogels require temperatures up to 300 degrees Celsius, which can lead to solvent evaporation and significant shrinkage. However, thermal imidization can be performed after supercritical drying when the solvent has been extracted, or by heating using a pressure vessel to prevent solvent evaporation (a more complex process).

[0160] In one embodiment, the imidization step is performed by chemical imidization.

[0161] In one embodiment, chemical imidization is carried out by adding a chemical dehydrating agent to the reaction solution.

[0162] In one embodiment, the chemical dehydrating agent may be a mixture of alkyl anhydrides and organic bases. Alkyl anhydrides may include acetic anhydride, propionic anhydride, benzoic anhydride, n-butyric anhydride, trifluoroacetic anhydride, or any combination thereof. Organic bases may include pyridine, monoalkylamines, dialkylamines and trialkylamines, isoquinoline, piperazine, morpholine, piperidine, or any combination thereof.

[0163] In one embodiment, the chemical dehydrating agent is a mixture of acetic anhydride and pyridine.

[0164] In one embodiment, the method includes the steps of rapid gelation and aging of the wet gel to improve polymerization yield and increase physical crosslinking degree. Physical crosslinking is accomplished through the physical entanglement of the formed polymer chains and can be described morphologically. However, for polymer chains of very small size (tens of nanometers), calculating the degree of physical crosslinking using image processing would be challenging. Nevertheless, it is still possible to compare the properties, such as mechanical properties, of different physically crosslinked aerogels to compare their degree of crosslinking.

[0165] In one implementation, rapid gelation takes place over a period of 1 to 10 minutes.

[0166] In one implementation, the aging of the wet gel takes place over a period of 90 minutes to 48 hours.

[0167] In one embodiment, the aprotic solvent is N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO), or a mixture of NMP and tetrahydrofuran (THF).

[0168] In one embodiment, the method includes the step of adding a chemical crosslinking agent to the reaction solution. Non-limiting examples of chemical crosslinking agents include triamines, aliphatic amines containing three or more amines, aliphatic triamines, aromatic amines containing three or more amine groups, aromatic triamines, 1,3,5-tris(aminophenoxy)benzene, and silica gel structures modified with three or more amines.

[0169] In one embodiment, the step of reacting the DIAM monomer with the DIAH monomer is carried out without the use of a chemical crosslinking agent.

[0170] In one implementation, the DIAM monomer and DIAH monomer are first dissolved separately, and then the DIAM monomer solution and the DIAH monomer solution are combined.

[0171] In one implementation, the DIAM monomer is first dissolved, and then the DIAH monomer is added to the DIAM solution.

[0172] In one embodiment, the aprotic solvent is N-methylpyrrolidone (NMP).

[0173] In one embodiment, the diamine monomer (DIAM) is 2,2'-dimethylbenzidine (DMBZ) or 4,4'-diaminodiphenyl ether (ODA).

[0174] In one embodiment, the diamine monomer (DIAM) is 2,2'-dimethylbenzidine (DMBZ).

[0175] In one embodiment, the diamine monomer (DIAM) is 4,4'-diaminodiphenyl ether (ODA).

[0176] In one embodiment, the diamine monomer (DIAM) is a combination of 2,2'-dimethylbenzidine (DMBZ) and 4,4'-diaminodiphenyl ether (ODA).

[0177] In one embodiment, the dianhydride monomer (DIAH) is biphenyl-tetracarboxylic dianhydride (BPDA).

[0178] In one implementation, the drying solvent is removed by supercritical drying, freeze drying, or ambient pressure drying.

[0179] In one embodiment, the drying solvent is a solvent soluble in liquid CO2 and may be any or a combination of ethanol, acetone, toluene, tetrahydrofuran (THF), cyclohexane, or a combination thereof.

[0180] In one embodiment, the drying solvent is removed by freeze drying, and the drying solvent is water or alcohol.

[0181] In one implementation, the drying solvent is removed by environmental drying.

[0182] In one embodiment, the drying solvent is an alcohol or acetone.

[0183] In one embodiment, the wet gel formation step is performed over a period of 1 minute to 24 hours.

[0184] In one embodiment, the method further includes the step of pouring the reaction solution into a block-shaped monolithic mold or a film mold.

[0185] In one embodiment, the method further includes the step of adding an additive to the reaction solution. In one embodiment, the additive is a polymer stabilizer, a functionalizer, or a combination thereof, other polymers, other aerogels, carbon nanotubes, metal fillers or particles, organic or inorganic fibers, organic or inorganic fillers or particles, or a nonwoven or woven fiber reinforcement composed of carbon precursor fibers, glass fibers, polymer organic fibers, ceramic fibers, or biopolymer fibers.

[0186] In one embodiment, the additive is selected from the group consisting of: polymers, aerogels, carbon nanotubes, metal fillers or particles, organic or inorganic fibers, organic or inorganic fillers or particles, nonwoven or woven fiber reinforcements consisting of carbon precursor fibers, glass fibers, polymer organic fibers, ceramic fibers, biopolymer fibers or any combination thereof.

[0187] In the implementation scheme, the integer n can be in the range of about 10 to about 60, 10 to about 50, or about 30 to about 50.

[0188] in conclusion Polyimide (PI) aerogels, as an important class of organic aerogels, have attracted considerable attention from academia and industry over the past decade due to their excellent mechanical, thermal, and chemical properties. Recently, numerous attempts have been made to modify the properties of PI aerogels, primarily by altering the monomer composition or the type of chemical crosslinking agent. However, a major drawback of PI aerogels is the high cost and environmental impact of chemical crosslinking agents. In this study, the inventors, for the first time, introduced a novel physical crosslinking pathway for PI aerogel synthesis by effectively controlling process parameters, including monomer dispersion time and reaction time. Compared to chemically crosslinked PI aerogels, the synthesized physically crosslinked aerogels exhibited higher porosity, with observed porosity ranging from 95.09% to 98.46%. Morphological characterization of the PCPI aerogels revealed that, due to their longer monomer dissolution and reaction times, physically entangled fibers with increased fiber aspect ratio and reduced fiber thickness were formed. This robust physical crosslinking network showed a significant impact on the mechanical properties of the PCPI aerogels.

[0189] A compressive modulus exceeding 20 MPa was achieved, significantly higher than that of previously reported PI aerogels. Interestingly, the PCPI aerogel exhibited excellent moisture resistance. When the sample was fully immersed in water, a water absorption rate as low as 15% and a water absorption capacity (WCA) exceeding 110° were achieved. Considering that superhydrophilicity has long been a drawback of PI aerogels, this study could pave the way for the fabrication of moisture-resistant PI aerogels without sacrificing other properties. Studies of the thermal and electrical properties of the PCPI aerogel support this phenomenon. TGA results show that the sample possesses excellent thermal stability, with a decomposition temperature of approximately 530 °C. Furthermore, the dielectric constant remains in a low range of 2.3 to 2.8, comparable to other low-dielectric polymers.

[0190] A significant advantage of the method of this invention is that it uses industrially available polyimide monomers to manufacture the chemical formulation at a price comparable to that of polyimide foam per unit weight. However, given that the density of the manufactured material is as low as 1 / 10 to approximately 1 / 5 compared to polymer foam, a significant reduction in material cost per unit volume is expected. Furthermore, unlike previously introduced polyimide aerogels, a novelty of the introduced material lies in the avoidance of any chemical crosslinking agents, which are very expensive (up to $80 / gram) and not environmentally friendly. In addition to the chemical backbone material and chemical reagents, the processing solvent, including the liquid CO2 used in the drying process, can be reused during the filtration cycle. The manufacturing method disclosed herein involves simple solution mixing and casting, where the gelation process is relatively rapid, followed by supercritical CO2 drying. Moreover, the relatively rapid gelation and film geometry mean that it can be easily scaled up according to the requirements of the target application.

[0191] Another advantage is that organic aerogels can be recycled through various methods, including (i) pyrolysis (converting aerogels into oils and fuels through thermal degradation); (ii) solvent extraction (using aqueous or non-aqueous media to dissolve aerogels and recover valuable components); and (iii) mechanical milling (reusing aerogel powder as an absorbent in a series of continuous processes for oil / chemical cleaning and insulation purposes).

[0192] References

Claims

1. A polyimide aerogel comprising: Polyimide-based polymers of Formula 1: Formula 1 Where n is an integer equal to or greater than 10, and wherein the polymer is physically cross-linked through polymer chain entanglement and formed by a method including initial gelation of approximately 1 minute to approximately 10 minutes; and The aerogel is not a primary source of polyamide amide polymer, or the aerogel does not contain polyamide amide polymer.

2. The polyimide aerogel of claim 1, wherein the aerogel has a pore size in the range of about 2 nanometers to about 100 nanometers.

3. The polyimide aerogel according to claim 1 or 2, wherein the aerogel has pores with a pore size of less than about 2 nanometers.

4. The polyimide aerogel according to claim 1, 2 or 3, wherein the aerogel has a porosity in the range of about 85% to about 99%.

5. The polyimide aerogel according to any one of claims 1 to 4, wherein the polyimide aerogel is free of chemical crosslinking agents.

6. The polyimide aerogel according to any one of claims 1 to 5, wherein the aerogel has a density of about 0.07 g / cm³. 3 To approximately 0.25 g / cm 3 The density within the range.

7. The polyimide aerogel according to any one of claims 1 to 6, wherein the aerogel has a thermal conductivity in the range of about 15 mW / Kelvin-meter to about 50 mW / Kelvin-meter.

8. The polyimide aerogel according to any one of claims 1 to 7, wherein the aerogel is characterized by hydrophobicity in the range of about 90 degrees water contact angle to about 140 degrees water contact angle.

9. The polyimide aerogel according to any one of claims 1 to 8, wherein the aerogel is characterized by a water absorption rate of less than to about 1%.

10. The polyimide aerogel according to any one of claims 1 to 9, wherein the aerogel has a thermal decomposition initiation temperature in the range of about 500°C to about 700°C.

11. The polyimide aerogel according to any one of claims 1 to 10, wherein the aerogel has a dielectric constant in the range of about 1.5 to about 3.

12. The polyimide aerogel according to any one of claims 1 to 11, wherein the aerogel is in the form of a film having a thickness of at least about 80 micrometers, and wherein the aerogel has an optical transparency in the range of about 60% to about 99%.

13. The polyimide aerogel according to any one of claims 1 to 12, wherein the aerogel is characterized by a compressive modulus in the range of about 17 MPa to 25 MPa.

14. The polyimide aerogel according to any one of claims 1 to 13, wherein the aerogel is characterized by a tensile modulus in the range of about 1 MPa to 5 MPa.

15. The polyimide aerogel according to any one of claims 1 to 14, wherein the integer n is in the range of about 10 to about 60, about 10 to about 50, or about 30 to about 50.

16. A method for preparing the polyimide aerogel according to any one of claims 1 to 15, comprising the following steps: • A wet gel is formed by reacting a diamine monomer (DIAM) with a dianhydride monomer (DIAH) in a reaction solution containing a dipolar aprotic solvent to form a polymer of formula A; where n is an integer equal to or greater than 10. • To imidize the resulting polymer; • Allow the formed polymer to gel for approximately 1 to 10 minutes; • Replace the aprotic solvent with a dry solvent; and • Remove the dried solvent from the formed wet gel.

17. The method of claim 16, wherein the step of imidizing the formed polymer is performed by thermal imidization or chemical imidization.

18. The method of claim 16 or 17, further comprising the step of aging the wet gel obtained by initial gelation to improve the polymerization yield and increase the degree of physical crosslinking.

19. The method according to claim 16, 17 or 18, wherein the aprotic solvent is N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO) or a mixture of NMP and tetrahydrofuran (THF).

20. The method according to any one of claims 16 to 19, wherein the step of reacting the DIAM monomer with the DIAH monomer is carried out without the use of a chemical crosslinking agent.

21. The method according to any one of claims 16 to 20, wherein the DIAM monomer and the DIAH monomer are first dissolved separately, and then the DIAM monomer solution and the DIAH monomer solution are combined.

22. The method according to any one of claims 16 to 21, wherein the DIAM monomer is first dissolved, and then the DIAH monomer is added to the DIAM solution.

23. The method according to any one of claims 16 to 22, wherein the drying solvent is removed by supercritical drying, freeze drying or ambient pressure drying.

24. The method according to any one of claims 16 to 23, wherein the wet gel formation step is performed over a period of about 1 minute to about 24 hours.

25. The method according to any one of claims 16 to 24, wherein the integer n is in the range of about 10 to about 60, 10 to about 50, or about 30 to about 50.

26. The method according to any one of claims 16 to 25, wherein the diamine monomer (DIAM) is 2,2'-dimethylbenzidine (DMBZ) or 4,4'-diaminodiphenyl ether (ODA).

27. The method of claim 26, wherein the diamine monomer (DIAM) is 2,2'-dimethylbenzidine (DMBZ).

28. The method of claim 26, wherein the diamine monomer (DIAM) is 4,4'-diaminodiphenyl ether (ODA).

29. The method according to any one of claims 16 to 28, wherein the dianhydride monomer (DIAH) is biphenyl-tetracarboxylic dianhydride (BPDA).

30. The method according to any one of claims 16 to 25, wherein the diamine monomer (DIAM) is a combination of both 2,2'-dimethylbenzidine (DMBZ) and 4,4'-diaminodiphenyl ether (ODA).

31. The method according to any one of claims 16 to 30, wherein the integer n is in the range of about 10 to about 60, 10 to about 50, or about 30 to about 50.

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