Hybrid polyimide foams with hyperbranched organosilicon for high-temperature sound absorption

doi:10.21203/rs.3.rs-4686609/v1

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Hybrid polyimide foams with hyperbranched organosilicon for high-temperature sound absorption

https://doi.org/10.21203/rs.3.rs-4686609/v1

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Lightweight and flexible polyimide foams (PIFs) with superior thermal and acoustic properties were high demand for advanced aerospace and industrial machinery, however, traditional foams could not offer both satisfactory acoustic and thermal insulation simultaneously. In this study, we successfully developed a new class of flexible PIFs by incorporating hyperbranched polysiloxanes (NH2-HBPSi) into ammonium salt oligomer (PAES) through a thermal foaming process. The complex viscosity, micro-foaming behavior of PAES and cell morphology of the resulting PIFs could be precisely controlled by tuning the ratio of NH2-HBPSi/ODA. Significantly, PIFHBPSi-4, a PIF with a NH2-HBPSi loading of 33.3% and exceptional thermal property (T5% exceeded 480 ℃), exhibited a compressive strength of 149 kPa at room temperature (over 100 times higher than commercial PIFs) and a Noise Reduction Coefficient (NRC) of 0.375, which is substantially improved compared to previously reported polyimide composites. Additionally, NH2-HBPSi could form a dense oxide layer at high temperature, impeding the transmission of heat. This allowed PIFHBPSi-4 to withstand a 10-minute exposure to high heat flux of 50 kW/m2 without any significant erosion. These lightweight, flexible, thermally resistant, and acoustically superior hybrid PIFs had the potential to revolutionize applications in high-tech fields like aerospace, acoustics, and aviation.

polyimide foam

hyperbranched polysiloxanes

hybrid

sound absorption

Noise, unwanted or bothersome sound, derives from the Latin word “nausea”, reflecting its negative connotation since ancient times^[1]. Chronic exposure to excessive noise could impair learning, reduce work efficiency, elevate irritability, disrupt sleep patterns, and even caused permanent hearing damage^[2]. Traditional foams, like PU, PS, and PMI, were successful noise absorbers due to their porous structure and lightweight nature. However, these commercial foams lacked the high-temperature resistance needed for applications exceeding 200°C^[3]. PIFs were a type of high-performance foam that consisted of polyimide as main component, with gas dispersed throughout the microscopic structure^{[4, 5]}. Due to its exceptional heat resistance, mechanical properties, flame retardancy and radiation resistance, PIF had been widely used in aerospace, ships, weapons^[6–10] and other fields^[11–14]. Nevertheless, research on developing PIFs that were simultaneously ultralight, flexible, oxidation-resistant, and possess excellent sound absorption properties remained limited.

Nowadays, several methods had been used to prepare lightweight and flexible PIFs for high-temperature sound absorption, including freeze-drying^[15–17], powder foaming^[18–20], supercritical foaming^[21–23], 3D printing^{[24, 25]}, etc. It was well-established that the performance of PIFs was determined by the inherent properties of their molecular structures and pore structures, which were typically influenced by the foaming techniques used^{[19, 26, 27]}. For instance, Huang et al.^[28] prepared a unique CaCO₃/polyimide composite aerogel with excellent sound absorption, mechanical properties and thermal insulation by freezing-drying method. Importantly, this composite exhibited a noise reduction coefficient of 0.327 and an average sound absorption coefficient of 0.903 in the frequency range of 2000–6300 Hz, showing promising application prospects in ancient architecture protection. Zhang et al.^[29]employed an emulsion-templating strategy to produce a rigid PIF with a densely interconnected open-cell structure. Their results showed that the density, porosity and strength of PIF could be effectively controlled by managing the content of internal phase and surfactants. Furthermore, the sound absorption test revealed the unique PIF had a high NRC of 0.386, making it a promising candidate for acoustic material. Hyperbranched polysiloxanes (HBPSi) were a fascinating class of organic-inorganic hybrid macromolecules with a three-dimensional spherical structure and excellent high-temperature stability^[30–33]. However, there was a scarcity of research exploring the influence of HBPSi on the pore structure and mechanical properties of PIFs designed for sound absorption.

Leveraging the multifunctional properties of HBPSi, this study incorporated NH₂-HBPSi to control the pore structure of PIFs, thereby enhancing their mechanical and acoustic properties. We comprehensively compared the mechanical strength, thermal insulation, acoustic performance, crystalline structures, rheological properties, and microstructures of the PIFs. The unique structure of NH₂-HBPSi endowed hybrid PIFs with a distinctive pore structure, which was crucial for enhancing the thermal, mechanical properties and sound absorption. The mechanism behind their thermal insulation, thermal oxidation resistance, and pore structure was elucidated. This work successfully fabricated lightweight, flexible, heat-resistant and hydrophobic PIFs with NH₂-HBPSi, which had the potential to significantly broaden the applications of PIFs in thermal protection, sound insulation, aerospace industry and marine sector.

Materials. Phenyltriethoxysilane (PhOTSi) and Aminopropyltriethoxysilane (APTS) was supplied by Aladdin (Nanjing, China). 4,4'-diaminodiphenyl ether (ODA) and 3,3′,4,4′-benzophenone tetracarboxylic acid dianhydride (BTDA) were supplied by yuanye Bio-Technology Co., Ltd (Shanghai, China), BTDA and ODA were vacuum dry at 120 ℃ for 12 h prior to use. Polydimethylsiloxane-polyol copolymer (DC-193) was provided by dowcorning (Shanghai, China). Methanol (CH₃OH), tetrahydrofuran (THF) and ethanol (CH₃CH₂OH) were supplied by Macklin (Nanjing, China). Deionized water (H₂O) was made in the laboratory.

Preparation of NH ₂ -HBPSi.The hyperbranched amino siloxane (NH₂-HBPSi) was synthesized via a sub-stoichiometric ratio hydrolytic condensation between PhOTSi and APTS, following the previously report methods as depicted in Fig S1^[34]. The amino content of NH₂-HBPSi was controlled by the amount of APTS during the process, in this work, the amino content was 13.409 mmol/g. The yield of NH₂-HBPSi was approximately 90%, and the entire process took around 9 h.

Preparation of PEAS-n. The PAES with different repeating units (n) were synthesized by using various ODA and NH₂-HBPSi. Taking n = 4 as an example, BTDA (32.23 g, 0.1 mol), CH₃OH (32 g, 1 mol) and THF (36.05 g, 0.5 mol) were charged into a boiling flask-3-neck with a mechanical stirring under N₂ atmosphere, the system was reacting at 70 ℃ for 2 h till the reaction clarified. Afterwards, ODA (16.02 g, 0.8 mol) and NH₂-HBPSi (29.8 g, 0.4 mol) were added into the above solution and reacted for another 2 h until the solution became transparent. Subsequently, the above solution was vacuum dry at 60 ℃ for 8 h to obtain the PAES_HBPSi-3 precursor. Then, the precursor was crushed into powder with diameters of 0-0.2 µm for subsequent use. The other precursors were prepared by varying the ratio of ODA/NH₂-HBPSi (10:0, 8.89:1.11, 8.57:1.43, 8:2, 6.67:3.33) and the specific ratios were shown in Table S1.

Preparation of various PIFs. A series of PIFs were obtained by heating PAES, PAES_HBPSi-1, PAES_HBPSi-2, PAES_HBPSi-3, PAES_HBPSi-4 powders in a mold (40 mm×40 mm×15 mm). The molds, loaded with various 2.2 g PAES powders, were preheated at 70 ℃ for 30 min in an oven. Afterwards, the molds were placed on a hot press for 1 h, with the hot plates setting at 170 ℃ and a pressure of 10 MPa. After hot pressing, the mold was transferred into an oven, keeping at 250 ℃ for 1 h, 300 ℃ for 30 min. Following the molds descent to ambient temperature, the PIFs were acquired through the process of demolding, as illustrated in Fig. 1.

Characterization. FTIR spectra were measured with a Nicolet 670 Fourier transform infrared spectrometer (FT-IR) between 4000 and 400 cm^− 1 by averaging 64 scans. Nuclear magnetic resonance (¹H and ²⁹Si) of NH₂-HBPSi were performed on a Bruker Avance 500 spectrometer. Powder X-ray diffraction spectra (PXRD) were acquired on a Bruker D8 Advance diffractometer, the testing range was from 5 ° to 80 ° and the scan speed was 2 °/min. Solid state nuclear magnetic spectra (SNMR) was acquired on Bruker Avance NEO operating at 400M. The X-ray photoelectron spectroscopy (XPS) were collected using AXIS Ultra with a monochromatic Al-Kα ray source, where the voltage and current of tube were 40 kV, 40 mA. Rheology was performed on an TA AR2000 rheo-meter, the test temperature was from 50 to 250 ℃ at 10 ℃/min. The micro-foaming morphology of PEAS were observed using an Eclipse E400 optical microscope equipped with hot stage, where the temperature range for observation was 50–250 ℃ with a speed of 30 ℃/min. The images of scanning electron microscopy (SEM) were obtained on a S4800, the average pore size and thickness of pore were analyzed using Nano measurer 1.2. The density of various foams was characterized according to GB/T 6343 − 2009. The compressive tests were performed on Instron-5567 mechanical tester, the dimensions of samples were 30 mm×30 mm×15 mm and the speed of compression was 2 mm/min. The thermal stability spectra (TG) were acquired on TGA under N₂ and air atmosphere where the temperature was 30-1000 ℃ with 10 ℃/min. The thermal conductivity of PIFs was determined using a constant temperature heating platform, the surface temperature was recorded by a thermal infrared imager and the dimension of PIFs was 40×40×15 mm. The limiting oxygen index (LOI) were measured by FTT0077 oxygen index tester according to ISO 4589-2:17. The cone calorimeter (CC) tests were performed on a FTT0007 cone calorimeter with a heat flux of 50 kW/m², the size of various samples was 100 mm×100 mm×10 mm in accordance with ISO 5660-1. The acoustical properties of different PIFs were measured with impedance tubes according to GB/T 18696.1–2004, which the thickness of the samples was 10 mm.

The Characterization of chemical structure. The chemical structure of NH₂-HBPSi had been characterized through FTIR and NMR, as illustrated in Fig S2(a), a novel peak of asymmetric stretching for Si-O-Si at 1125 cm^− 3 appeared, attributed to the silsesquioxane groups in NH₂-HBPSi. The characteristic peak at 3350 cm^− 3 represented the successful functionalization of amino groups in NH₂-HBPSi. Additionally, the ¹H NMR spectrum in Fig S2(b) represented the aromatic proton (~ 7.6) and terminal amine proton (~ 2.5 pmm) from PhOTSi and APTS, respectively. The ¹³C spectrum in Fig S2(c) shown the peaks at around 17.3 ppm, 26 ppm and 43 ppm, attributed to aminopropyl group carbon atoms in APTS. The ²⁹Si NMR spectrum in Fig S2(d) indicated the presence of three different silicon atoms in NH₂-HBPSi, located to around − 68 ppm, -74.05 ppm and − 81.4 ppm, associated with the silicon atoms from siloxane prepolymers with different degrees of hydrolysis. Thus, the FTIR, ¹H NMR, ¹³C NMR and ²⁹Si NMR spectra proved the successfully synthesized of NH₂-HBPSi. Moreover, after verifying the structure of NH₂-HBPSi, it was used as an amine-containing cross-linker to prepared PAES, as detailed in Fig. 2(a), the infrared absorption peaks for all PAES were identified at 2956 cm^− 1 (CH₃), 1718 cm^− 1 (O = C-C), 1605 cm^− 1 (N-H) and 1240 cm^− 1 (C-O-C), indicating successful reactions between diamine and diester dicarboxylate. Moreover, the novel absorption peaks of PAES_HBPSi-1, PAES_HBPSi-2, PAES_HBPSi-3 and PAES_HBPSi-4 were observed at 1100 cm^− 1 (Si-O-Si), displaying the PAES were modified by organic-silicone. When all PAESs were subjected to high temperature treatment, as illustrated in Fig. 2(b), various of PIFs shown the new absorption peaks at 1778 cm^− 1, 1716 cm^− 1 (the asymmetric, symmetric stretching vibration of C = O in imide ring) and 1371 cm^− 1 (C-N). The ¹³C SNMR spectra of PIF and PIF_HBPSi-4 were depicted in Fig. 2(c), all spectra displayed peaks at 124 ppm (the carbonyl carbon in BTDA), 131 ppm (carbon in benzene ring), 157 ppm (aromatic ether carbon in C-O-C), 166 ppm (imide carbonyl carbon in O = C-N) and 194 ppm (carbonyl carbon in anhydride). In particular, compared to PIF, PIF_HBPSi-4 exhibited novel peaks at 5–55 ppm (the saturated carbonyl in NH₂-HBPSi). PXRD diffraction analysis was availed to examine the crystal characteristics of PAES and PIFs, as illustrated in Fig. 2(d), one distinctive diffraction peak at 2θ = 22.83° for PAES was observed, belonging to the crystalline structure of precursor powder due to the easily movement of macromolecular^[35]. In addition, all hybrid PIFs displayed an ordinary amorphous broad peak at around 17.77° after thermal imidization. Under such condition, the molecular weight of chain segment rose dramatically due to the removal of small molecules like CH₃OH, THF and H₂O, resulting in the mobility of chain restricted, consequently, the structure of chain segment transformed into amorphous. A novel diffraction peaks at 2θ = 6.84° assigned to hybrid silicon was appeared in PAES_HBPSi-1, PAES_HBPSi-2, PAES_HBPSi-3 and PAES_HBPSi-4, shown in Fig. 2(e), affiliating with the long-range order of NH₂-HBPSi. As displayed in Fig. 2(f), the XPS spectra of PIF and PIF_HBPSi-4 exhibited distinct peaks at 283.17 eV, 399.73 eV, and 531.33 eV, related to the C1s, N1s, and O1s, respectively. However, novel peaks of PIF_HBPSi-4 were appeared at 102.08 eV and 153 eV, attributing to Si2p and Si2s in NH₂-HBPSi^[36]. Furthermore, as displayed in Fig. 2(g) (h), the narrow spectrum of C1s for PIF_HBPSi-4 could be spilt to various parts at 283.3 eV, 284.6 eV, 285.1 eV, 286.2 eV and 288.1 eV, corresponding to C-Si, C-C, C-N, C-O and C = O, respectively^[37], the high-resolution narrow spectra of Si2p consisted C-Si-O₃ (101.8 eV) and C-Si-O₂ (101.0 eV)^[36], demonstrating the successful synthesis of hybrid PIFs.

Rheology behavior and foaming process. The complex viscosity(η*) of PAESs as a function of temperature were plotted in Fig. 3(a). The value showed that the η* of all PAESs initiated dropping at approximately 110 ℃, indicating the precursor powder began to melt. Subsequently, the η* of PAES reached rock-bottom, during these two stages, the THF and CH₃OH bonded with hydrogen bonds in PAES molecular chain evaporated. As the temperature continued rising, the η* value of PAESs rapidly increased, implying partial imidization occurred during this period. Interestingly, compared to other PAESs, the temperature for PAES_HBPSi-4 exhibiting a minimum η* was higher, largely due to the highly crosslinked structure of NH₂-HBPSi. Additionally, the relatively lower minimum values of η* was 340 Pa·s, indicating that the pore wall strength for PIF was inadequate to resist the stress during the foaming process. Consequently, the PAES was unable to form an integrated PIF with uniform pore size. Throughout the thermal-foaming process, the pore structure and diameter were significantly influenced by the melt viscosity of different PAESs, indicating that PAES_HBPSi-1, PAES_HBPSi-2, PAES_HBPSi-3 and PAES_HBPSi-4 possessed favorable cellular morphology, proved to be advantageous in enhancing the mechanical properties of foams^[38]. Furthermore, as depicted in Fig S3, the trends of storage modulus (G') were close to η* for various PAESs, the minimum G' of PAES_HBPSi-1, PAES_HBPSi-2, PAES_HBPSi-3 and PAES_HBPSi-4 were 10075, 11961, 11773, 13410 Pa, respectively, showing a consistent upward trend, reconfirming the difference in viscosity of PAES crosslinked by NH₂-HBPSi at different temperature.

The stage optical microscope was exploited to investigate the foaming process of diverse PAESs, as shown in Fig. 3, the foam behavior of all PAES precursors were similar, which could be divided into three stages. Firstly, the bubble nucleus formed in PAES-n precursor at 50–110 ℃ which caused by the volatilization of free THF and CH₃OH. Subsequently, the bubble grown rapidly at 110–170 ℃, In such stage, the THF and CH₃OH undergone a phase change, transforming into a gas phase, consequently causing the bubble to expand. Following this, the thermal imidization of PAES precursors was observed at 250–280 ℃. Moreover, the DSC spectra in Fig. 3(b) displayed three exothermic peaks for PAES precursor. Interestingly, the temperature of peak related to bubble nucleus formation for PAES_HBPSi-4 with large molecular weight was higher, compared to PAES_HBPSi-3 (90.3 ℃), PAES_HBPSi-2 (80.3 ℃), PAES_HBPSi-1 (78.3 ℃) and PAES (60.3 ℃). Consequently, the minimal volatilization of the free solvent in PAES_HBPSi-4 necessitated an increase in temperature to commence this phase. Additionally, the increased enhancement of molecular chain rigidity regulated by NH₂-HBPSi, led to a higher temperature requirement for promoting chain movement, resulting the highest foaming temperature for PAES_HBPSi-4 (153℃).

Cell structure. The images of microstructure for various PIFs were illustrated in Fig. 4 and nano measurements were conducted to analyze the average pore size, shown in Fig S4. As displayed in Fig. 4, PIF_HBPSi-1, PIF_HBPSi-2, PIF_HBPSi-3, and PIF_HBPSi-4 exhibited a nearly hexagonal honeycomb pore structure with the majority of them displaying a semi-closed configuration. However, the pore structure of PIF appeared disordered, attributing to the low complex viscosity depicted in Fig. 3 (a). The complex viscosity of PAESs with NH₂-HBPSi was diverse, leading to the average pore sizes of PIF_HBPSi-1, PIF_HBPSi-2, PIF_HBPSi-3 and PIF_HBPSi-4 were 560, 690, 460 and 440 µm, respectively. Additionally, the individual cell walls thickness of PIF, PIF_HBPSi-1, PIF_HBPSi−2, PIF_HBPSi-3, and PIF_HBPSi-4 were measured to be 0.18 µm, 0.27 µm, 0.4 µm, 1.41 µm, and 2.17 µm, individually. It was observed that PIF_HBPSi-4 containing a higher concentration of NH₂-HBPSi, exhibited a reduced pore diameter and a thick cell wall, which resulted by elevated crosslinking density with a low expansion ratio.

Mechanical properties. The curves of compress stress versus compress strain for different PIFs were plotted in Fig. 5. All PIFs exhibited three typical compressive process, linear elastic region, planar plastic yield zone, and densification zone^[39]. The linear elastic zone corresponded to elastic deformation was typically observed at compress strain less than 10% and exhibited reversible recovery. When the compressive strain surpassed 10%, the PIFs displayed a planar plastic yield zoom, with the cell structure bending and unable to restore original state. Moreover, in densification zoom, further compaction and irreversible damage occurred when strain exceed 30%. As illustrated in Fig S5(a)-(d) and Fig. 5(b), all PIFs could maintain over 80% of maximum compress stress at 10% compression and exhibited virtually no volume shrinkage during cycles of successive compression-release tests, lay in all foams processed exceptional flexibility. The apparent density, water content angle, thermal conductivity, compressive strength and compressive modulus were summarized in Table 1. The compressive strength of PIF, PIF_HBPSi-1, PIF_HBPSi-2, PIF_HBPSi-3, and PIF_HBPSi-4 were 8.11 kPa, 14.59 kPa, 20.66 kPa, 42.71 kPa, and 149.08 kPa at strain of 10%, which was proved that the amended molecular design with NH₂-HBPSi entailed an enhancement in crosslink density and foam rigidity. As expected, the compressive strain of various PIFs at 200 ℃ were lower than those at ambient temperature, attributed to the easily movement of molecular chain. As shown in Fig. 5(c), the compression strength at 200 ℃ for PIF, PIF_HBPSi-1, PIF_HBPSi-2, PIF_HBPSi-3, and PIF_HBPSi-4 were 5.403 kPa, 9.802 kPa, 20.663 kPa, 42.71 kPa, and 128.36 kPa, respectively. However, it was noteworthy that the compression strength and modulus at 200 ℃ exhibited minimal decline approximately 13.90% and 35.6%, respectively, compared to others, indicating the excellent compressive performance in the realm of flexible high-temperature resistance.

Table 1

The characteristic properties of various PIFs
Sample	Density (kg·m^− 3)	Water contact angle(°)	Thermal conductivity (mW/(m·k))	Compressive strength (kPa)		Compressive modulus (kPa)
Sample	Density (kg·m^− 3)	Water contact angle(°)	Thermal conductivity (mW/(m·k))	RT	200℃	RT	200℃
PIF	24± 1.2	98.3± 1.4	38± 13	8.11± 1.23	5.4± 1.36	14.86± 2.10	9.63± 2.2
PIF_HBPSi-1	31.1± 1.0	102.3± 1.2	34± 9	14.59± 1.45	9.8± 1.81	36.56± 4.53	21.36± 3.78
PIF_HBPSi-2	34.6± 1.5	127.4± 1	36.7± 12	20.66± 2.36	16.94± 3.82	72.6± 5.8	68.3± 6.3
PIF_HBPSi-3	35.4± 2	126.2± 0.9	32.5± 2	42.71± 2.57	24.63± 4.58	149.94 ± 5.24	118.46± 5.8
PIF_HBPSi-4	43± 4	120.3± 0.8	30± 2	149.08 ± 5.23	128.36± 6.81	1085.8 ± 8.4	698.42± 7.2

Thermal stability properties. The thermal stability properties of PIFs were illustrated in Fig. 6, the initial decomposition temperature(T_5%), highest thermal degradation temperature (T_max) and char residual at 1000 ℃ (R₁₀₀₀) were summarized in Table S2. Figure 6 showed that the absence of substantial mass loss of PIFs was prior to 500 ℃ in both N₂ and air atmospheres, indicating the excellent thermal stability of all PIFs. PIF_HBPSi-4 contained more NH₂-HBPSi, exhibited the lowest T_5% (521.08 ℃), as illustrated in Table 4, attributing to the large amount of aminopropyl in NH₂-HBPSi. As demonstrated in Fig S6, the T_max of all PIFs was in range of 577–595 ℃, relating to the decomposition of imide rings, phenyl, carbonyl and so on. However, the T_max of different PIFs with NH₂-HBPSi were superior than that of PIF (577 ℃), on account of the formation of dense oxide layer by NH₂-HBPSi at elevated temperature, blocking the further heat entry^[40]. As displayed in Fig. 6(b), the T_5% range in air atmosphere was 477.2-501.16 ℃, slightly lower than in N₂ atmosphere. Furthermore, the R₁₀₀₀ in air atmosphere increased with the content of NH₂-HBPSi for various PIFs, PIF_HBPSi-4 displayed the highest R₁₀₀₀, 12.24%, surpassing other PIFs reported in literatures^{[4, 19]}, which was related to the formation of SiO₂ in air atmosphere at high temperature. These results suggested that construction of silicon/PIF hybrid system cloud enhance the char formation and maximal decomposition temperature of foams.

Thermal insulation properties. The superior thermal insulation behavior of various PIFs was linked to the unique pore structure and micro/nano multiscale architectures. As illustrated in Table 1, the thermal conductivity of PIF, PIF_HBPSi-1, PIF_HBPSi-2, PIF_HBPSi-3, and PIF_HBPSi-4 was 38, 34, 36.7, 32.5, 30 mW/(m·K), respectively. The thickness of cell wall and the closed-cell ratio could affect the heat transfer path and gas flow within the foam^[41]. The PIF_HBPSi-4, characterized by its thick cell wall and high closed-cell content, diminished the thermal conduction path length and curtails the likelihood of gas movement, thereby enhancing its thermal insulation properties^[42]. Furthermore, the thermal insulation properties were observed by the infrared camera in Fig. 7, the temperature on the upper surface of various PIFs placed on a hotplate for 30 min were 93.4, 76, 84, 74.9, 62 ℃, respectively. The slight fluctuation in temperature of different foams was caused by the experimental environment. Curiously, the PIF_HBPSi-4 displayed the lowest temperature indicating the excellent thermal insulation behavior, consistent with the thermal conductivity value listed in Table 1.

Combustion behavior. The flame retardancy of PIFs was investigated by the LOI tests, with the result listed in Table S3. The value showed that with addition of NH₂-HBPSi, the LOI of PIF_HBPSi-4 increased to 45.9%, demonstrating the flame-retardant function of NH₂-HBPSi. Moreover, the cone calorimeter was exploited to discuss the effect of NH₂-HBPSi for PIFs in flame retardancy, results shown in Fig. 8 and Table S3. Compared with the PIF, the THR and PHRR of PIF_HBPSi-3 exhibited a reduction of 35.71% and 9%, however, these values increased by 41.72% and 24.10% for PIF_HBPSi-4. These results demonstrated that the appropriate introduction of NH₂-HBPSi could improve the flame resistance performance of PIFs. As displayed in Fig. 8 (a), the normalized weight loss of PIF_HBPSi-4 was 21.2%, far lower than that of other PIFs. Additionally, the overall integrity after ablation in indicative of the material’s high-temperature oxidation resistance performance, the morphology after ablation were illustrated in Fig. 8(e). The PIF_HBPSi-4 with high content of NH₂-HBPSi exhibited the minimum volume shrinkage, and the surface of product after ablation appeared white, resulting by the NH₂-HBPSi generating silica at high temperature. SEM and EDS of the surface for PIFs which measured after cone calorimeter were illustrated in Fig S7. The porous char morphology of all sample appeared, due to the volatile components generated during the combustion process. Moreover, with an increased addition of NH₂-HBPSi, the surface pore count of the residual carbon decreased, which could effectively block the external heat flow and isolate oxygen leading to exceptional flame retardant. In contrast, the EDS of the surface for residual char revealed that PIF_HBPSi-4 had a higher Si content than other PIFs, indicating the NH₂-HBPSi could accelerate the densification of the ceramic on surface for PIF_HBPSi-4.

Acoustical properties of various PIFs. The impedance tubes were employed to ascertain the acoustic properties of various PIFs. The resulting data on the acoustic absorption coefficient across different frequencies were depicted in Fig. 9 (a). In the range of 100 and 1000 Hz, the acoustical absorption coefficient of all PIFs were lower than 0.2, notably, these coefficients exhibited a rapid increase in the frequency band extending from 1000 to 6500 Hz. The acoustical property could be further evaluated by noise reduction coefficient at four frequencies at 250, 500, 1000, and 2000 Hz^[1]. The NRC of different PIFs were 0.215, 0.282, 0.337, 0.28 and 0.378, respectively in Fig. 9 (c). In general, the NRC of material higher than 0.2 could be used for acoustical absorption^[43], thus, the PIFs prepared by this work presented excellent sound absorption properties, rendering them as potential candidates for use as sound-absorbing materials. Interestingly, the NRC of PIF_HBPSi-4 with reduced average cell size was higher than that of other modified PIFs in literature^{[44] [1, 28, 45–50]}(Fig. 9(d)), as the sound waves encountered the surface of the smaller pores, the increased absorption and reflection, which in turn converted the sound energy into heat or dissipates it through viscous action. Furthermore, when PIF_HBPSi-4 was subjected to 200 ℃ for 6 h, interestingly, the NRC managed to maintain its value at exactly 0.370 in Fig. 9 (c), suggesting that PIF_HBPSi-4 could absorb sound efficiently without degradation under such thermal conditions, making it a potentially reliable choice for applications requiring both thermal resilience and effective noise reduction.

In this work, we have successfully fabricated mechanically flexible PIFs with varying NH₂-HBPSi contents via a thermo-foaming process. The results revealed that PIFs with different content of NH₂-HBPSi exhibited diverse melt viscosity, chain rigidity, foaming temperature, and 3D porous structures. The prepared PIFs demonstrated excellent thermal characteristics with initial thermal degradation temperatures ranging between 479–550 ℃ and a char residue of 54%-59% at 1000 ℃. Mechanical strength increased with NH₂-HBPSi content due to the high cross-linking density, while the flexibility was maintained. PIF_HBPSi-4 with a highly porous and 3D micron-scale cellular structure exhibited superior hydrophobicity, outstanding thermal insulation and excellent thermo-oxidative stability. Additionally, the sound adsorption test showed the PIF_HBPSi-4 had a relatively high NRC of 0.378, exceeding that of commercial PIFs. In summary, this work presents a viable method for preparing lightweight and mechanically flexible hybrid PIFs with high temperature thermal insulation and acoustical properties. This advancement holds the potential to expand the use of foams in the field of high-temperature acoustics applications.

the authors declare no competing interests

Author contribution: Yang Xu prepared the material, conducted most of measurements and wrote the main manuscript text. Luwei Bo coordinated the overall project and prepared all figures. Yandong Wang, Haocheng Xu, Xiaolin Jiang, and Zhaobo Wei revised the manuscript and coordinated the overall project. Kia Xi edited the main manuscript text and supported funding. All authors reviewed the manuscript.

Funding: this work was supported by the Fundamental research funds for the central university (020514380274).

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Hybrid polyimide foams with hyperbranched organosilicon for high-temperature sound absorption

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