Organic–Inorganic Double-Gel System Thermally Insulating and Hydrophobic Polyimide Aerogel

Aerogel materials are used in various fields, but there is a shortage of aerogel materials with an excellent combination of mechanical properties, thermal stability, and easy preparation. In this study, polyimide aerogel materials with superior mechanical properties, thermal stability, and low thermal conductivity were prepared by forming a double-gel system in the liquid phase. The amino-modified gel, prepared by coating SiO2 nano-microspheres with GO through a modified sol-gel method (SiO2@GO-NH2), was subsequently homogeneously dispersed with PAA wet gel in water to form a double-gel system. The construction of a double-gel system enabled the PI aerogel to shape a unique honeycomb porous structure and a multi-layered interface of PI/SiO2/GO. The final obtained PI aerogel possessed effective thermal conductivity (0.0309 W/m·K) and a high specific modulus (46.19 m2/s2). In addition, the high thermal stability (543.80 °C in Ar atmosphere) and the ability to retain properties under heat treatment proved its durability in high thermal environments. The hydrophobicity (131.55°) proves its resistance to water from the environment. The excellent performance of this PI aerogel and its durability in thermal working environments make it possible to be applied in varied industrial and research fields, such as construction and energy, where heat and thermal insulation are required.


Introduction
The overconsumption of energy is becoming a critical issue, based on engineering, environmental, and economic challenges in modern industries [1][2][3]. Pursuing a novel material with high performance and energy efficiency is a forefront research topic in terms of industrial applications [4][5][6][7]. Aerogel, as an impressive commercial energy material, plays a crucial role in the thermal insulation field due to its high strength and low density, which can be subdivided into inorganic and organic aerogels. Inorganic aerogels, especially silica-derived aerogels, have emerged as representatives of the heat insulation field because of their highlighted inherent textural properties, and, more importantly, unprecedented thermal stability and multifunctional use [8][9][10][11][12][13]. Despite the promising performance of inorganic aerogels, the obvious intrinsic drawbacks, such as hygroscopicity, brittleness, and lack of compressive modulus, inhibit their applications [14]. Compared with inorganic aerogels, organic aerogels, which have tunable structural properties, are not stable in thermal conditions [15][16][17][18][19]. Therefore, developing organic-inorganic aerogels is imperative to achieve the desired performance. Additionally, designing an effective combination strategy represents a critical first step in the establishment of a bridge between inorganic and organic aerogels [14,20,21].

Preparation of Amino-Modified SiO 2 @GO (SiO 2 @GO-NH 2 ) Wet Gel by Sol-Gel Method
Graphene oxide (GO) was prepared using a modified Hummers' method [33]. SiO 2 @GO -NH 2 wet gels were prepared using the sol-gel method. Specifically, 25 mg of GO was added to 75 mL of 20 vol% TEOS ethanol solution, ultrasonically treated for 30 min, which ensured the system was well dispersed. Afterward, 10.5 mL of 0.1 M hydrochloric acid was added to acidify the system and was then ultrasonically treated for 5 min to fully hydrolyze the TEOS. A total of 4.5 mL of 10.79 wt.% 3ATPES ethanol solution was titrated slowly with slow mechanical stirring to gel the system and stirred for 24 h. The SiO 2 @GO-NH 2 wet gel was then obtained.

Preparation of Polyamic Acid (PAA) Precursors and Wet Gel
Using an ice bath, 5.2630 g of ODA was dissolved in 55 mL of DMAc at 0 • C. After stirring for 5 min, 4.7369 g of PMDA was slowly added to the system in batches. The molar ratio of PMDA-to-ODA was 50:51 to control the length of polymer chains. The product was slowly poured into DI water at 0 • C and washed four times with DI water and freeze-dried at −65 • C to obtain the PAA precursor. The ether bonding of ODA and the non-coplanar PAA chains make the PAA chains more soluble in the chosen solvent (water) and facilitate hydrogen bonding interactions with the inorganic doped material [34,35]. The PAA precursor was mixed with water and TEA in a mass ratio of 1:10:0.5 at room temperature and stirred at high speed for 30 min to obtain the PAA wet gel [36].

Preparation of Polyimide (PI) Aerogels
The SiO 2 @GO-NH 2 wet gel was dispersed in 7.5 g of water by high-speed stirring and ultrasound. This was followed by continuous stirring in an ice bath at 0 • C using a slower stirring speed, while adding a total of 11.5 g of PAA wet gel in five batches. Stirring was continued for 4 h until the system was homogeneous. The obtained organic-inorganic dual system wet gels were poured into molds and frozen in a cold trap at −65 • C for 4 h, before being freeze-dried for 48 h. The molds were pre-cooled in the cold trap in advance, with the lowest cold trap temperature chosen to reduce the size of ice crystals in the gel system. The obtained aerogels were heat-treated at 80 • C for 30 min, 110 • C for 30 min, 195 • C for 120 min, and 290 • C for 60 min under nitrogen protection to obtain PI aerogels and numbered PI-1 (1 represents the addition of 1 g SiO 2 @GO-NH 2 ). Similarly, a series of PI aerogels were prepared with different ratios of SiO 2 @GO-NH 2 wet gels to study the effect of doping amount on the aerogel properties; the prepared PI aerogels were named PI-0, PI-0.5, PI-1.5, and PI-2. The PI aerogels used for mechanical and thermal property tests were prepared using specific molds. The diameter of the obtained PI aerogel was 35 ± 0.5 mm and the height was 10 ± 0.2 mm.

Characterization
The Nicolet 570 FTIR spectrometer (Thermo Fisher Scientific, Agawam, MA, USA) was used to obtain FTIR spectra of SiO 2 @GO-NH 2 . Microstructure pictures and EDX maps of freeze-dried SiO 2 @GO-NH 2 wet gels and PI aerogels were taken with a SUPRA 55 thermal field emission scanning electron microscope (Carl Zeiss, Berlin, Germany) (SEM). ATR-FTIR spectra of PI aerogels were obtained with a Nicolet iS50 Fourier Transform Infrared Spectrometer (Thermo Fisher Scientific, MA, USA). XRD data of PI aerogels were provided by using an ULTIMA III X-ray diffractometer (Nippon Rigaku, Tokyo, Japan). The DXR2Xi Confocal Laser Raman Spectrometer (Thermo Fisher Scientific, MA, USA) provided Raman spectra of PI-0, PI-0.5, and PI-1.5 samples. The photoelectron spectra of PI-0 and PI-1.5 aerogels were measured using the ESCALAB 250 X-ray photoelectron spectrometer (Agawam, VG, USA). The 13 C NMR data of PI-1.5 aerogel were acquired using an AVANCE III 500 Nuclear Magnetic Resonance Spectrometer (Bruker, Fällanden, Switzerland). The stress-strain curves of the aerogel samples were obtained using an INSTRON-1185 (INSTRON, Norwood, MA, USA). The thermal conductivity of the aerogel samples was investigated using a DZDR-S thermal conductivity meter (Dazhan Mechatronics, Singapore). The infrared photographs of PI-0 and PI-1.5 placed on the heating stage were taken using a DS-2TPH10-3AUF infrared camera (HIKVISION, Hangzhou Hikvision Digital Technology Co., Ltd., Hangzhou, China). The thermogravimetric data of each group of PI aerogels were tested using a STA449C/6/G simultaneous thermal analyzer (NETZSCH, Westendstr, Germany). The water contact angle of each group of PI aerogel samples was measured with a SL200A/B dynamic/static contact angle meter (Shanghai Solon, Shanghai, China).

Amino-Modified SiO 2 @GO (SiO 2 @GO-NH 2 ) Wet Gel
The preparation process of SiO 2 @GO-NH 2 wet gel is shown in Figure 1a. The SEM image in Figure 2b shows uniform accumulation of silica particles on the graphene sheets, indicating that the silica in the wet gel system is grown diffusively as nano-spheres with graphene as the base. FT-IR spectra of SiO 2 @GO-NH 2 gels prepared by the above experimental scheme, and general SiO 2 @GO gels with the replacement of 3APTES with ammonia, are shown in Figure 1c. The peaks of both spectra contain -OH (3400 cm −1 ), C=C (1630 cm −1 ), and C=O (1350 and 1230 cm −1 ) from GO, Si-O-Si-(1090 cm −1 ), SiO 2 , and Si-O-C (1130 cm −1 ), which represents the interaction between SiO 2 and GO. In addition to the peaks shared in SiO 2 @GO-NH 2 , -NH 2 (3260 cm −1 ) and C-N (1220 cm −1 ) are also present. Such data indicate that 3ATPES underwent hydrolysis and co-growth with SiO 2 nano-spheres during the sol-gel process in the alcohol-water system. This co-growth process also serves to amino-modify the silica nano-spheres by functionalization through 3APTES [37].

Amino-Modified SiO2@GO (SiO2@GO-NH2) Wet Gel
The preparation process of SiO2@GO-NH2 wet gel is shown in Figure 1a. The SEM image in Figure 2b shows uniform accumulation of silica particles on the graphene sheets indicating that the silica in the wet gel system is grown diffusively as nano-spheres with graphene as the base. FT-IR spectra of SiO2@GO-NH2 gels prepared by the above experi mental scheme, and general SiO2@GO gels with the replacement of 3APTES with ammo nia, are shown in Figure 1c. The peaks of both spectra contain -OH (3400 cm −1 ), C=C (163 cm −1 ), and C=O (1350 and 1230 cm −1 ) from GO, Si-O-Si-(1090 cm −1 ), SiO2, and Si-O-C (113 cm −1 ), which represents the interaction between SiO2 and GO. In addition to the peak shared in SiO2@GO-NH2, -NH2 (3260 cm −1 ) and C-N (1220 cm −1 ) are also present. Such dat indicate that 3ATPES underwent hydrolysis and co-growth with SiO2 nano-spheres dur ing the sol-gel process in the alcohol-water system. This co-growth process also serves to amino-modify the silica nano-spheres by functionalization through 3APTES [37].

Preparation of PI Aerogel with Inorganic-Organic Double-Gel System
The preparation process of the PI aerogel is shown in Figure 2a. Specifically, this w to form a uniformly dispersed SiO2@GO-NH2 system from controlled quality SiO2@GO NH2 gel in water, using high-speed stirring and ultrasonic treatment. Figure 2b shows t excellent processing properties of this aerogel preparation method, which can form fin and special structures with different molds. Figure 2c-h show typical SEM photograp of various PI aerogels. PI-1 to PI-2 formed a unique honeycomb, showing the limitin effect of the SiO2@GO-NH2 inorganic gel network on synergistic growth of the ice cry tal/PAA linkage [38]. For comparison, PI-0 and PI-0.5 displayed a larger needle-like po structure due to the spontaneous growth of polymer chains in the direction of the ice cry tal growth edge during the freezing process. Typically, SiO2@GO-NH2 wet gel introduc in the PI-0.5 system is not enough to form an inorganic gel network, making the micr structure more similar to that of PI-0. The large pore size exhibited in PI-0 and PI-0 proves the difficulty of controlling the microscopic scale of polyimide aerogels prepar using the general freeze-drying method. In particular, the pores of PI-2 showed a lot merging and fragmentation. The abnormal properties of PI-2 might be caused by the e cessive addition of SiO2@GO-NH2 providing an overdone limiting effect, making it mo difficult to form a stable honeycomb structure [39].
The elemental distribution on the microscopic scale of PI-1.5 was measured and an lyzed using EDS. The results showed uniformity of the elements, while the microstructu of the original aerogel can still be observed in the images of each element, especially The results observed by EDS demonstrate the homogeneity of the double-gel system which means that the PI and the SiO2@GO-NH2 form the aerogel honeycomb structure o the nanoscale together.

Preparation of PI Aerogel with Inorganic-Organic Double-Gel System
The preparation process of the PI aerogel is shown in Figure 2a. Specifically, this was to form a uniformly dispersed SiO 2 @GO-NH 2 system from controlled quality SiO 2 @GO-NH 2 gel in water, using high-speed stirring and ultrasonic treatment. Figure 2b shows the excellent processing properties of this aerogel preparation method, which can form finer and special structures with different molds. Figure 2c-h show typical SEM photographs of various PI aerogels. PI-1 to PI-2 formed a unique honeycomb, showing the limiting effect of the SiO 2 @GO-NH 2 inorganic gel network on synergistic growth of the ice crystal/PAA linkage [38]. For comparison, PI-0 and PI-0.5 displayed a larger needle-like pore structure due to the spontaneous growth of polymer chains in the direction of the ice crystal growth edge during the freezing process. Typically, SiO 2 @GO-NH 2 wet gel introduced in the PI-0.5 system is not enough to form an inorganic gel network, making the microstructure more similar to that of PI-0. The large pore size exhibited in PI-0 and PI-0.5 proves the difficulty of controlling the microscopic scale of polyimide aerogels prepared using the general freeze-drying method. In particular, the pores of PI-2 showed a lot of merging and fragmentation. The abnormal properties of PI-2 might be caused by the excessive addition of SiO 2 @GO-NH 2 providing an overdone limiting effect, making it more difficult to form a stable honeycomb structure [39].
The elemental distribution on the microscopic scale of PI-1.5 was measured and analyzed using EDS. The results showed uniformity of the elements, while the microstructure of the original aerogel can still be observed in the images of each element, especially Si. The results observed by EDS demonstrate the homogeneity of the double-gel system, which means that the PI and the SiO 2 @GO-NH 2 form the aerogel honeycomb structure on the nanoscale together.

Chemical Structure of PI Aerogel
The ATR-FTIR infrared spectra of the aerogel materials are shown in Figure 3a. The strong characteristic peaks at 1774 and 1710 cm −1 occurring on all spectra are attributed to the symmetric and asymmetric stretching of the C=O group of the imide double bond. The peak at 1375 cm −1 is attributed to the C-N stretching vibration, and a C=C stretching vibration at 1495 cm −1 was also observed. It can be concluded that the doping of SiO 2 @GO-NH 2 and the double-gel network structure do not affect the chemical process of PI formation by imidization of PAA [40,41]. Figure 3b shows typical XRD data for each PI aerogel. All PI aerogels display characteristic polyimide amorphous structure peaks, but PI-0 and PI-0.5 have higher peak heights compared to other samples, which is related to the oriented structure formed during the aerogel and preparation process [42]. This also proves that the introduction of the SiO 2 @GO-NH 2 gel changes the overall structure of polyimide aerogels. 2022, 14, x FOR PEER REVIEW peak at 1375 cm −1 is attributed to the C-N stretching vibration, and a C=C stretchin tion at 1495 cm −1 was also observed. It can be concluded that the doping of SiO2@G and the double-gel network structure do not affect the chemical process of PI fo by imidization of PAA [40,41]. Figure 3b shows typical XRD data for each PI aero PI aerogels display characteristic polyimide amorphous structure peaks, but PI-0 0.5 have higher peak heights compared to other samples, which is related to the o structure formed during the aerogel and preparation process [42]. This also pro the introduction of the SiO2@GO-NH2 gel changes the overall structure of polyim ogels.  Figure 4a shows the Raman spectra of PI-0, PI-0.5, and PI-1.5. All three spectr ited bright fluorescent signals, and the spectral bands around 1340 cm −1 can be at to the C-N stretching vibration, around 1590 cm −1 to the ring vibration of PI, and 1740 cm −1 to the stretching vibration of C=O from PI and GO [43,44]. This may be by the convergence of the D-band of rGO (1330 cm −1 ) and C-N stretching vibratio cm −1 ) with each other, the overlap of the G-band of rGO (1560 cm −1 ) and the ring v of PI (1590 cm −1 ), and the enhancement of the peak signal with an increase in the Si NH2 [45]. This evidence demonstrates the close association between PI cha SiO2@GO-NH2 within aerogels.  Figure 4a shows the Raman spectra of PI-0, PI-0.5, and PI-1.5. All three spectra exhibited bright fluorescent signals, and the spectral bands around 1340 cm −1 can be attributed to the C-N stretching vibration, around 1590 cm −1 to the ring vibration of PI, and around 1740 cm −1 to the stretching vibration of C=O from PI and GO [43,44]. This may be caused by the convergence of the D-band of rGO (1330 cm −1 ) and C-N stretching vibration (1340 cm −1 ) with each other, the overlap of the G-band of rGO (1560 cm −1 ) and the ring vibration of PI (1590 cm −1 ), and the enhancement of the peak signal with an increase in the SiO 2 @GO-NH 2 [45]. This evidence demonstrates the close association between PI chains and SiO 2 @GO-NH 2 within aerogels.
To further verify the chemical bond structure and elemental composition of the aerogel material, XPS was used to analyze the PI-0 and PI-1.5 materials. The peaks of Si 2p can be deconvoluted into two groups of sub-bands, assigned to O-Si-O (102.9 eV) and O-Si-C (101.2 eV), representing the amino-modification by 3ATPES [37,[46][47][48].
The solid-state 13 C NMR further reveals the chemical structure described by the previous characterizations. The peaks at 118, 123, and 136 ppm belong to graphene and aromatic carbon peaks on the PI chains [49]. The peak at 165 ppm belongs to the carbonyl carbon [50]. The peaks at 155 and 158 ppm, except for the observed double peaks, belong to aromatic carbon forming ether bonds on the PI chains, which are slightly shifted due to their chemical environment [51]. The peaks at 155 and 158 ppm belong to aromatic carbons forming ether bonds in the PI chains.  To further verify the chemical bond structure and elemental composition of the a ogel material, XPS was used to analyze the PI-0 and PI-1.5 materials. The peaks of Si can be deconvoluted into two groups of sub-bands, assigned to O-Si-O (102.9 eV) and Si-C (101.2 eV), representing the amino-modification by 3ATPES [37,[46][47][48].
The solid-state 13 C NMR further reveals the chemical structure described by the p vious characterizations. The peaks at 118, 123, and 136 ppm belong to graphene and a matic carbon peaks on the PI chains [49]. The peak at 165 ppm belongs to the carbo carbon [50]. The peaks at 155 and 158 ppm, except for the observed double peaks, belo to aromatic carbon forming ether bonds on the PI chains, which are slightly shifted due their chemical environment [51]. The peaks at 155 and 158 ppm belong to aromatic c bons forming ether bonds in the PI chains.

Properties of PI Aerogel
In practical applications, mechanical properties are often the key element in meas ing the performance of aerogels. The densities of aerogels were acquired by averaging results of three measurements of a 10 mm × 10 mm × 10 mm sized PI aerogel. In this w we further obtained the specific moduli data of PI aerogels. Although it is generally lieved that limited SiO2 nanoparticles can improve the mechanical properties of polyme materials to a certain extent, an excess can also lead to a decrease in mechanical proper [52,53]. This is because the addition of more SiO2 leads to agglomeration of nanopartic due to van der Waals forces and electrostatic adsorption, resulting in stress concentrat between the SiO2 and the polymer interface [54]. Although a large amount of SiO2@G NH2 was added into the system during the preparation of the PI aerogel in this study, specific modulus of the original PI-1.5 sample still reached a relatively superior 46.19 m while maintaining a low density (0.0441 g/cm 3 ). There are several reasons for this: (1)

Properties of PI Aerogel
In practical applications, mechanical properties are often the key element in measuring the performance of aerogels. The densities of aerogels were acquired by averaging the results of three measurements of a 10 mm × 10 mm × 10 mm sized PI aerogel. In this way, we further obtained the specific moduli data of PI aerogels. Although it is generally believed that limited SiO 2 nanoparticles can improve the mechanical properties of polymeric materials to a certain extent, an excess can also lead to a decrease in mechanical properties [52,53]. This is because the addition of more SiO 2 leads to agglomeration of nanoparticles due to van der Waals forces and electrostatic adsorption, resulting in stress concentration between the SiO 2 and the polymer interface [54]. Although a large amount of SiO 2 @GO-NH 2 was added into the system during the preparation of the PI aerogel in this study, the specific modulus of the original PI-1.5 sample still reached a relatively superior 46.19 m 2 /s 2 while maintaining a low density (0.0441 g/cm 3 ). There are several reasons for this: (1) the high dispersion of SiO 2 @GO-NH 2 , (2) amino groups led to SiO 2 @GO-NH 2 providing crosslinking sites and enhancing material compatibility, (3) the loading effect played by GO, and (4) the improvement in the structural properties of the given substrate by the silica-containing network [39,55]. As shown in Figure 5a, PI-0 and PI-0.5 samples have similar stress-strain curves, probably due to the similarity in microstructure. With the improvement in microstructure, the Young's moduli of the PI-1 and PI-1.5 samples are greatly enhanced. Interestingly, the two groups of aerogels are still maintained in the linear elastic region when strain exceeds 40%. Then, there is a sharp increase in the platform strain behavior and the densification state. This can be attributed to the energy dissipation effect of the cross honeycomb material and graphene with good toughness values [56]. However, the linear elastic region of PI-2 produces a certain decrease with damage to the microstructure and stress concentration, caused by excessive doping. region when strain exceeds 40%. Then, there is a sharp increase in the platform strain behavior and the densification state. This can be attributed to the energy dissipation effect of the cross honeycomb material and graphene with good toughness values [56]. However, the linear elastic region of PI-2 produces a certain decrease with damage to the microstructure and stress concentration, caused by excessive doping.  As a representative of heat-resistant polymers, PI aerogels should maintain good performance even after the influence of high temperatures. The PI aerogel was heat-treated at a constant temperature of 200 • C in the air for 24 h to investigate the property retention of PI aerogel after long-time heat treatment. The stress-strain curves of the heat-treated aerogels shown in Figure 5b, and the Young's moduli, specific moduli, and density data of the two sets of aerogels were statistically analyzed accordingly and are summarized in Figure 5c,d. It is not difficult to see that the moduli and specific moduli of polyimide aerogels decreased after heat treatment, created by the relaxation of internal stress and the destruction of the microstructure of polyimide aerogels after heat treatment [57]. However, PI-1.5 still Polymers 2022, 14, 2818 9 of 14 maintains a good Young's modulus (1.89 MPa) and specific modulus (41.21 m 2 /s 2 ), which proves its durability and mechanical property retention under a long-term thermal environment. PI-1.5 produces huge mechanical property improvement compared with PI-1, based on the increase in filler content for polymeric materials, with constant microstructure and uniform dispersion of fillers [58]. Toughening is achieved. The Young's modulus of PI-2 (2.26 MPa) in PI aerogel preserved at room temperature without heat treatment improves compared with PI-1.5 (2.04 MPa). With an increase in density due to destruction of the microstructure (ρ PI-2 = 0.0536 g/cm 3 ), the specific modulus of PI-2 (42.23 m 2 /s 2 ) was shown to decrease. Instead, the Young's modulus of PI-1.5 and PI-2 (1.97 MPa) became closer after heat treatment. The stress-strain curves showed that the heat-treated PI-2 samples could also be maintained in the linear elastic region at more than 40% of the strain, further indicating the relaxation effect of heat treatment on the concentration of internal stress in the aerogel material.
Thermal insulation is an essential indicator of the energy efficiency of PI aerogels. The thermal resistance analysis of the aerogel material is exhibited in Figure 6. The PI-1.  [59]). As shown in Figure 6a, the flowers placed on top of PI-1.5 showed no significant dehydration after 15 min of burning using an alcohol burner flame. The introduction of graphene generally leads to an increase in the thermal conductivity of the system, with the wrapping of graphene using SiO 2 preventing the formation of thermal pathways between graphene, thus eliminating its thermal conductivity [60,61]. The dispersion interval of the thermal resistance test data of the PI-0 sample has a larger error interval compared to that of the other samples, which may be due to the large pore size of the PI-0 sample. Unlike the similarity of mechanical properties, the thermal resistance of PI-0.5 (original: 0.0387 W/m·K; after heat treatment 0.0390 W/m·K) is much better than PI-0 (original: 0.0426 W/m·K; after heat treatment: 0.0425 W/m·K). The uniform dispersion of the inorganic gel phase in the aerogel system forms a PI/SiO 2 /GO multilayer interface, which enhances the thermal resistance (Kapitza thermal resistance) [62]. The thermal resistance of PI-1 (original: 0.0332 W/m·K; after heat treatment: 0.0344 W/m·K) and PI-1.5 increased significantly compared to other aerogels, suggesting that the double-gel system improves the density and microscopic pore structure of the aerogel system. The increased content of SiO 2 @GO-NH 2 enhanced the Kapitza thermal resistance of the system. As shown in Figure 6d, the infrared images of PI-0 and PI-1.5 are obtained by cutting the original aerogel samples into cylindrical test samples of 2 cm in height and 2 cm in diameter, and then placing them on a 200 • C heating stage for 60 min and photographing them using an infrared camera. PI-1.5 shows much slower thermal diffusion than PI-0, and the thermal resistance is consistent. The superior thermal and mechanical properties make the PI aerogel in this study a possible solution for construction, energy, and other applications [16].
The thermal properties of PI aerogels are discussed further by thermogravimetric analysis. The TGA data of PI aerogels were tested and analyzed, and are shown in Figure 7. PI-0 and PI-0.5 showed similar weight loss changes at 0-500 • C and the curves of both almost overlapped. The aerogel microstructure and other factors produce improvement with the addition of SiO 2 @GO-NH 2 , causing decreased weight loss rates of PI-1 and PI-1.5 at 0-500 • C compared to PI-0 and PI-0.5. The decomposition temperatures of PI-0, PI-0.5, PI-1, and PI-1.5 were 522. 45, 539.30, 548.88, and 543.80 • C, respectively, when the weight loss reached 5% in the Ar atmosphere. While in the air, they were 384.03, 440. 55, 457.22, and 496.03 • C. The improvement in thermal stability demonstrates the strong interaction between organic and inorganic components in the double-gel system [63]. In contrast, the weight loss rate and decomposition temperature (392.12 • C in Ar atmosphere and 248.35 • C in Air atmosphere) of the PI-2 aerogel at 0-500 • C produced a considerable decrease, indicating that the excess SiO 2 @GO-NH 2 for the disruption of the microstructure of the PI aerogel instead reduced the thermal stability of the aerogel [64]. The thermal properties of PI aerogels are discussed further by thermogravime analysis. The TGA data of PI aerogels were tested and analyzed, and are shown in Fig  7. PI-0 and PI-0.5 showed similar weight loss changes at 0-500 °C and the curves of b almost overlapped. The aerogel microstructure and other factors produce improvem with the addition of SiO2@GO-NH2, causing decreased weight loss rates of PI-1 and P at 0-500 °C compared to PI-0 and PI-0.5. The decomposition temperatures of PI-0, PI PI-1, and PI-1.5 were 522. 45, 539.30, 548.88, and 543.80 °C, respectively, when the we loss reached 5% in the Ar atmosphere. While in the air, they were 384.03, 440.55, 457 and 496.03 °C. The improvement in thermal stability demonstrates the strong interac between organic and inorganic components in the double-gel system [63]. In contrast weight loss rate and decomposition temperature (392.12 °C in Ar atmosphere and 24 °C in Air atmosphere) of the PI-2 aerogel at 0-500 °C produced a considerable decre indicating that the excess SiO2@GO-NH2 for the disruption of the microstructure of th aerogel instead reduced the thermal stability of the aerogel [64].  The thermal properties of PI aerogels are discussed further by thermogravim analysis. The TGA data of PI aerogels were tested and analyzed, and are shown in Fi 7. PI-0 and PI-0.5 showed similar weight loss changes at 0-500 °C and the curves of almost overlapped. The aerogel microstructure and other factors produce improvem with the addition of SiO2@GO-NH2, causing decreased weight loss rates of PI-1 and P at 0-500 °C compared to PI-0 and PI-0.5. The decomposition temperatures of PI-0, P PI-1, and PI-1.5 were 522.45, 539.30, 548.88, and 543.80 °C, respectively, when the we loss reached 5% in the Ar atmosphere. While in the air, they were 384.03, 440.55, 45 and 496.03 °C. The improvement in thermal stability demonstrates the strong interac between organic and inorganic components in the double-gel system [63]. In contrast weight loss rate and decomposition temperature (392.12 °C in Ar atmosphere and 24 °C in Air atmosphere) of the PI-2 aerogel at 0-500 °C produced a considerable decr indicating that the excess SiO2@GO-NH2 for the disruption of the microstructure of th aerogel instead reduced the thermal stability of the aerogel [64]. If the PI aerogel can absorb moisture from the environment, the thermal insula property could also reduce. As shown in Figure 8, the hydrophobicity of the PI aer was studied by measuring the water contact angle. Due to the capillarity caused by poor microscopic structure of PI-0, the PI-0 aerogel absorbed all the tested droplets, If the PI aerogel can absorb moisture from the environment, the thermal insulation property could also reduce. As shown in Figure 8, the hydrophobicity of the PI aerogel was studied by measuring the water contact angle. Due to the capillarity caused by the poor microscopic structure of PI-0, the PI-0 aerogel absorbed all the tested droplets, indicating intrinsic hydrophilicity. The PI-0.5 sample had a water contact angle of 27.03 • , indicating that the SiO 2 @GO-NH 2 gel could provide the hydrophobicity of the PI aerogel. The water contact angle of PI-1 and PI-1.5 produced a tremendous leap. More specifically, the water contact angle of PI-1.5 reached 131.55 • , which testifies to its high hydrophobicity. The controlled pore structure of aerogels allows an increase in surface roughness, thus improving the hydrophobicity of the aerogel surface. However, with heat treatment, the water contact angles of the PI aerogels all decreased. The increasing hydrophilicity can be attributed to the introduction of oxygen-containing functional groups or changes in the microstructure [12,65]. the water contact angle of PI-1.5 reached 131.55°, which testifies to its high hydropho city. The controlled pore structure of aerogels allows an increase in surface roughn thus improving the hydrophobicity of the aerogel surface. However, with heat treatme the water contact angles of the PI aerogels all decreased. The increasing hydrophilicity be attributed to the introduction of oxygen-containing functional groups or changes in microstructure [12,65].

Conclusions
In summary, a double-gel system polyimide aerogel material was discussed in t study. A SiO2@GO-NH2 amino-modified wet gel was first fabricated via a novel solmethod, to obtain a SiO2 uniformly coated GO material. Then, the PAA gel and SiO2@G NH2 gel were homogeneously dispersed under the liquid phase, to provide the doub gel system. After a series of subsequent treatments, the prepared PI aerogel had a mic scopic porous honeycomb structure and multiple-layer interface, which provided go thermal resistance. The strong compatibility and binding of SiO2@GO-NH2 with PI a improved the low density (0.0442 g/cm 3 ), high specific modulus (46.19 m 2 /s 2 ), effect thermal insulation properties (0.0309 W/m·K), high thermal stability (543.80 °C in Ar mosphere), high hydrophobicity (131.55°), and ability to maintain performance after p longed exposure to thermal environments. The performance improvement was due to scientific introduction of SiO2@GO-NH2 and the construction of the double-gel syst The easy preparation process and superior performance make it suitable for wide ap cations within various industrial and research fields requiring thermal insulation.

Conclusions
In summary, a double-gel system polyimide aerogel material was discussed in this study. A SiO 2 @GO-NH 2 amino-modified wet gel was first fabricated via a novel solgel method, to obtain a SiO 2 uniformly coated GO material. Then, the PAA gel and SiO 2 @GO-NH 2 gel were homogeneously dispersed under the liquid phase, to provide the double-gel system. After a series of subsequent treatments, the prepared PI aerogel had a microscopic porous honeycomb structure and multiple-layer interface, which provided good thermal resistance. The strong compatibility and binding of SiO 2 @GO-NH 2 with PI also improved the low density (0.0442 g/cm 3 ), high specific modulus (46.19 m 2 /s 2 ), effective thermal insulation properties (0.0309 W/m·K), high thermal stability (543.80 • C in Ar atmosphere), high hydrophobicity (131.55 • ), and ability to maintain performance after prolonged exposure to thermal environments. The performance improvement was due to the scientific introduction of SiO 2 @GO-NH 2 and the construction of the double-gel system. The easy preparation process and superior performance make it suitable for wide applications within various industrial and research fields requiring thermal insulation.