Sound absorption polyimide composite aerogels for ancient architectures’ protection

Ancient architectures are an important part of immovable cultural heritage and the largest surviving amount of tangible cultural heritage in the world. However, the increasingly serious noise pollution will not only affect the sanctity of ancient architectures, but also damage the internal structure caused by continuous mechanical vibration, and affect their lifetime. In this paper, diaminodiphenyl ether and pyromellitic dianhydride were used as monomers, modified by triethylamine to synthesize water-soluble polyamide acids, and calcium carbonate (CaCO3) was used as filler to prepare CaCO3/polyimide (CaCO3/PI) composite aerogels by homogeneous mixing, freeze-drying, and thermal imidization. CaCO3 can effectively adjust the pore wall roughness of CaCO3/PI composite aerogels, so as to improve their sound absorption performance. When the amount of CaCO3 is 4 wt%, CaCO3/PI composite aerogels exhibit optimal sound absorption performance, excellent mechanical properties, thermal insulation, and heat resistance. The corresponding noise reduction coefficient is 0.327, and the average sound absorption coefficient is 0.903 in the frequency range of 2000 ~ 6300 Hz. Young’s modulus is 4.03 kPa, stress loss and plastic deformation after 100 compression cycles with a maximum strain of 50% are 3.27% and 2.17%, respectively. The energy loss coefficient is 0.248, the thermal conductivity is 0.038 W/(m·K), and the heat resistance index is 334.1 °C. The CaCO3/PI composite aerogels show momentous application prospects in the field of ancient architectures protection. Above exhibited the sound absorption coefficient and noise reduction performance of CaCO3/PI composite aerogels


Introduction
Ancient architectures are an important component of immovable cultural heritages and the most numerous surviving tangible cultural heritage in the world [1][2][3][4][5]. However, they are frequently threatened by natural and human factors in the course of their continuation [6][7][8]. In recent years, accelerated urbanization has made noise pollution increasingly serious [9][10][11]. Not only does it affect the sanctity of the interior of ancient architectures, but the continuous mechanical vibrations would also cause damage to their internal structure and affect their lifespan [12,13]. Therefore, the development of materials with excellent noisereduction properties is of great significance for the protection of ancient architectures.
According to the different noise reduction mechanisms, noise reduction materials can be mainly divided into two categories: sound insulation materials and sound absorption materials [14][15][16]. Sound insulation materials, represented by fiber mats, have excellent noise reduction performance by reflecting sound waves to reduce noise hazards [17,18]. However, fiber mats have the disadvantages of being dense and prone to slagging during long-term use, limiting their wider application in the field of ancient architectures protection [19][20][21].
In contrast, foam-based sound absorption materials have the advantages of being lightweight and having great structural stability [22,23]. Currently, foam-based sound absorption materials mainly include metal foams, inorganic foams, polymer foams, and their composite foams [24,25], which dissipate the sound wave energy through resonance and multiple reflections within the materials to achieve the noise mitigation effect. Xu et al. [26] prepared 304 stainlesssteel foams by polymer foams impregnation process. When the foam's thickness was 1.8 mm and the porosity was 93.7%, the sound absorption coefficient was 0.7 at 4000 Hz. It could be ascribed to the large air gap inside the stainless-steel foams, which dissipated sound energy through air damping. Although metal foams have excellent mechanical and sound absorption properties, it is still relatively dense and has disadvantages such as being prone to rusting, which limits their application in the protection of ancient architectures [27][28][29].
Compared with metal foams, polymers and their composite foams have the merits of being lightweight and having good corrosion resistance [30][31][32]. Mohammad et al. [33] prepared vinyl-vinyl acetate composite foams filled with nano-silica, nano-clay, and graphene nanosheets through the method of uniform mixing, blow molding, and hot pressing. The ethylene vinyl acetate composite foams showed the best sound absorption performance with a noise reduction coefficient of 0.225 at a dosage of 2 phr for both silica and nano-clay and 0.2 phr for graphene nanosheets. It was mainly attributed that the porous structures of the ethylene vinyl acetate foams could dissipate sound energy through multiple reflections, and the introduction of nano-silica, nano-clay, and graphene nanosheets could improve the roughness of the foams pore wall and air damping. Xue and Zhang [34] added formaldehyde into polyvinyl alcohol, and a series of polyvinyl formaldehyde (PVF) composite foams were then prepared by high-temperature reaction and freezedrying under the catalysis of sulfuric acid. PVF composite foams presented open-cell porous structures. And the PVF-3 showed the best sound absorption performance when the hydroxyl ratio of formaldehyde to polyvinyl alcohol was 3. The maximum absorption coefficient was 0.98 at 2000 Hz, mainly due to that adjusting the amount of formaldehyde effectively modulated the pore structures and improved the sound absorption properties. The above polymer composite foams have promising noise-reduction features. However, due to the inferior heat resistance of the matrix (PS, PVF) and the release of toxic gases (styrene, formaldehyde, etc.) at high temperatures, it is detrimental to the protection of ancient architectures and their interior personnel, and it can also corrode ancient architectures structures [35][36][37].
Compared with PS and PVF, polyimide (PI) has superior mechanical and thermal properties and has extensive application prospects in high-quality sound absorption and ancient architectures protection [38]. Sun et al. [39] used aramid fiber core (ARHC) as the matrix, isocyanate-based polyimide (IBPIF) as the fillers, and water as the carrier to prepare the IBPIF-reinforced ARHC composite foams (IBPIF/ARHC) by free foaming method. IBPIF/ARHC showed the best sound absorption capacity at the water dosage of 7.2 g. At 300 ~ 600 Hz, the average absorption coefficient of IBPIF/ARHC was 0.42, at 1500 Hz, the absorption coefficient exceeded 0.9. Owing to that the water dosage effectively modulated the pore structures of the IBPIF/ ARHC composite foams and boosted the sound absorption performance. Malakooti et al. [40] obtained carbon fiber fabric-reinforced PI composite aerogels by impregnation and supercritical foaming. The PI composite aerogels exhibited the optimal sound absorption performance when the thickness of the carbon fabric-reinforced PI composite aerogels was 7.37 mm and the density was 0.12 g/cm 3 . The sound absorption coefficient of PI composite aerogels was 0.82 at 2000 Hz, 3 times higher than that of pure PI aerogels. It was primarily attributed that the insertion of the carbon fiber fabric allowed the sound wave energy to vibrate sympathy with carbon fiber, which further dissipated the sound wave energy. The above-mentioned studies indicate that the sound absorption properties of PI aerogels have been improved by the fillers [41]. Calcium carbonate (CaCO 3 ) is cheap and has excellent mechanical and heat resistance properties [42][43][44], which is anticipated to improve the sound absorption performance of PI aerogels.
In this work, diaminodiphenyl ether and pyromellitic dianhydride are used as monomers, modified by triethylamine to synthesize water-soluble polyamide acids (PAA), and calcium carbonate (CaCO 3 ) is performed as fillers to prepare the corresponding CaCO 3 /polyimide (CaCO 3 /PI) composite aerogels by homogeneous mixing, freeze-drying, and thermal imidization. Scanning electron microscopy (SEM) is used to characterize the microscopic morphology, number of pores, and thickness of pore wall for CaCO 3 / PI composite aerogels. On this basis, the effects of CaCO 3 dosage on the pore structures (pore wall, number of pores, etc.), sound absorption properties, heat resistance, thermal insulation, flame retardancy, and mechanical properties of the CaCO 3 /PI composite aerogels are investigated in detail.

Preparation of CaCO 3 /PI composite aerogels
Four gram of ODA and 48.00 g of DMAc were mixed. Subsequently, 4.42 g of PMDA was added into the above solution, and then the obtained mixtures were stirred for 2 h in an ice-water bath until the reaction product climbed. Water-soluble polyamide acid (PAA) precursor solution was prepared by further adding 2.02 g TEA and stirring for 5 h. The solution was collected by solvent exchange in an ice-water bath, then frozen and freeze-dried to obtain the water-soluble PAA [45][46][47]. Furthermore, 7.5 g of water-soluble PAA was added into a beaker with 2.1 g of TEA and 500 g of deionized water. After water-soluble PAA was completely dissolved, 0.153 g of CaCO 3 was added to the above mixtures. When CaCO 3 was completely dispersed and bubbles disappeared, PAA solution containing 2 wt% of CaCO 3 was then poured into molds and frozen for 24 h. And CaCO 3 /PAA mixtures were placed in a freeze dryer for 72 h to sublimate ice crystals, followed by heat treatment of 300 °C to obtain 2 wt% CaCO 3 /PI composite aerogels. The corresponding schematic diagram of preparation for CaCO 3 /PI composite aerogels is shown in Fig. 1. Pure PI aerogels and CaCO 3 /PI composite aerogels with 4 wt% and 6 wt% CaCO 3 were also prepared by the same method, respectively. According to the different amounts of CaCO 3 , samples were named x wt% CaCO 3 /PI composite aerogels. Figure 2 shows SEM photographs of pure PI aerogels and 4 wt% CaCO 3 /PI composite aerogels. From Fig. 2a, pure PI aerogels show a disordered arrangement and smooth lamellar pore wall porous structures formed by the sublimation of ice crystals with uniform pore size distribution [48][49][50]. With the addition of CaCO 3 , the pore structure of the CaCO 3 / PI composite aerogels remains disordered, and the obtained pore size is basically unchanged (Fig. 2b). However, the pore wall becomes rough (Fig. 2b', Fig. S1a) from the smoothness (Fig. 2a') of the pure PI aerogels, and CaCO 3 adheres to the pore wall. It can be attributed that CaCO 3 dispersing in the aqueous PAA solution is repelled between the PAA as the ice crystals grow. After freeze-drying, as the ice crystals sublimate, CaCO 3 will adhere to the PAA pore wall and increase their roughness. When the CaCO 3 dosage is 6 wt%, the pore wall roughness of the CaCO 3 /PI composite aerogels does not change compared to that of the 4 wt% CaCO 3 /PI composite aerogels (Fig. S1b) but the significant sedimentation phenomenon of CaCO 3 at the bottom of the aerogels. It is mainly attributed that the inability of the CaCO 3 /PI composite aerogels can hardly carry too much CaCO 3 . Figure 3 shows the pore density and average pore wall thickness of the CaCO 3 /PI composite aerogels. With the addition of CaCO 3 , the pore density of the CaCO 3 /PI composite aerogels is all in the range of 1.06 × 10 6 to 1.21 × 10 6 pores/cm 3 (Fig. 3a). The reason is that CaCO 3 only attaches to the surface of the PI pore wall and does not form a separate pore wall structure. With the addition of CaCO 3 , the average pore wall thickness of the CaCO 3 /PI composite aerogels changes, increasing from 1.72 μm of pure PI aerogels to 2.11 μm of 4 wt% CaCO 3 /PI composite aerogels (Fig. 3b). It can be mainly attributed that a small amount of CaCO 3 disperses in the PI pore wall during the preparation process, resulting in an increase in the average pore wall thickness of the CaCO 3 /PI composite aerogels.

Results and discussion
From Fig. 4a, the absorption coefficient of the pure PI aerogels is less than 0.90 in the frequency range from 100 to 6300 Hz. With the addition of CaCO 3 , the sound absorption performance of CaCO 3 /PI composite aerogels gradually increases. When the CaCO 3 amount is 4 wt%, the CaCO 3 / PI composite aerogels show the best sound absorption performance, with better absorption coefficients than pure PI aerogels in the frequency range from 100 to 6300 Hz. As can be seen from Fig. 4b, the sound absorption coefficient (NRC, arithmetic mean of the absorption coefficients at 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz) of the CaCO 3 /PI Fig. 1 Schematic diagram of preparation for CaCO 3 /PI composite aerogels composite aerogels gradually increases from 0.201 to 0.327 in the pure PI aerogel after the addition of CaCO 3 . In the frequency range of 2000 ~ 6300 Hz, the average absorption coefficient of the pure PI aerogels is about 0.810, while the average absorption coefficient of the 4 wt% CaCO 3 /PI composite aerogels increases to 0.903. The reason is that, when the incident sound wave arrives at the surface of the CaCO 3 / PI composite aerogels, its own ultra-high porosity and lowdensity properties enable most of the sound waves to enter the internal of the CaCO 3 /PI composite aerogels and only a little of the sound waves is reflected. The incoming sound waves are reflected multiple times in CaCO 3 /PI composite aerogels pore structures and create the frictional effect with the air, converting the sound wave energy into thermal energy for dissipation. Whereas the pore wall roughness of the CaCO 3 /PI composite aerogels increases compared to the pure PI aerogels, enlarging the air damping, increasing the viscous and frictional interaction with the air, and performing better sound absorption properties. When the CaCO 3 mass fraction is 6 wt%, the CaCO 3 /PI composite aerogels have a comparable absorption coefficient to the 4 wt% CaCO 3 /PI composite aerogels in the frequency range of 100 ~ 6300 Hz. It can be ascribed to the small variation in pore wall roughness of the two CaCO 3 /PI composite aerogels. It is illustrated that the excessive addition of CaCO 3 does not enhance the sound absorption performances of the CaCO 3 /PI composite aerogels.
Moreover, the sound absorption materials may be repeatedly subjected to external stresses and strains in the sophisticated service condition. In this work, the sound absorption performance of 4 wt% CaCO 3 /PI composite aerogels is tested undergoing 100 compression cycles (the maximum  (Fig. 4c). It can be observed that the sound absorption performance of the CaCO 3 /PI composite aerogels declines marginally after 100 compression cycles but still has satisfactory sound absorption performance. The NRC drops from 0.327 to 0.306 with a retention rate of 93.6% in the frequency range of 100 to 2000 Hz, and the average absorption coefficient reduces from 0.903 to 0.848 with a retention rate of 93.9% in the frequency range of 2000 to 6300 Hz. This is mainly attributed to the excellent fatigue resistances of the CaCO 3 /PI composite aerogels, which have achieved positive sound absorption stability in complex operating conditions. From Fig. 4e, the 4 wt% CaCO 3 /PI composite aerogels exhibit a relatively lower sound pressure level (SPL) compared to that of 3 M foams, reducing the average SPL of noise from 72.9 to 54.5 dB, while the 3 M foams can only reduce it to 63.2 dB (Fig. 4f). Since the 4 wt% CaCO 3 /PI composite aerogels have relatively rougher pore wall structures, which can increase the air damping and facilitate the dissipation of sound wave energy, exhibiting superior macroscopic noise reduction performance. Figure 5 shows the TGA curves, heat resistance index (T HRI ) [51][52][53], thermal conductivity (λ) of the pure PI aerogels and CaCO 3 /PI composite aerogels, combustion experimental images of the polystyrene (PS) foams, and 4 wt% CaCO 3 /PI composite aerogels. As shown in Fig. 5a and Table S2, with the addition of CaCO 3 , the temperature corresponding to 5% thermal weight loss (T 5 ) gradually increases from 521.8 °C of pure PI aerogels to 577.3 °C of 4 wt% CaCO 3 /PI composite aerogels. When the amount of CaCO 3 is 6 wt%, T 5 of CaCO 3 /PI composite aerogels is 675.2 °C, significantly higher than that of the 4 wt% CaCO 3 /PI composite aerogels. Because the CaCO 3 attaches to the surface of the PI pore wall prevents further heat diffusion. From Fig. 5b, with the addition of CaCO 3 , the T HRI increases from 277.5 °C of pure PI aerogels to 334.1 °C of 4 wt% CaCO 3 /PI composite aerogels. On one hand, there are "six elementsfive elements" conjugate ring structures consisting of the benzene ring and amide ring within PI molecules. On the other hand, the CaCO 3 attached to the surface of the PI pore wall is in favor of hindering heat diffusion. The above two endow the CaCO 3 /PI composite aerogels with excellent heat wall resistance. From Fig. 5c, with the addition of CaCO 3 , the λ values of the CaCO 3 /PI composite aerogels are in the range of 0.035 ~ 0.039 W/(m·K), showing excellent thermal insulation performance. It is mainly attributed that the introduction of CaCO 3 has little influence on the pore density of CaCO 3 /PI composite aerogels (Fig. 3a), causing λ values to fluctuate within a small range. From Fig. 5d, after 60-s combustion experiments on PS foams and 4 wt% CaCO 3 /PI composite aerogels, the PS foams are completely degraded and could no longer be molded (Fig. S2a), and the pungent smell is produced during the combustion process. However, 4 wt% CaCO 3 /PI composite aerogels are only carbonized on the surface and still remained structurally intact (Fig. S2b). It is attributed that the CaCO 3 attached to the surface of the PI pore wall prevents the high temperature and flame from spreading into CaCO 3 /PI composite aerogels. Figure 6 shows the mechanical properties of the pure PI aerogels and CaCO 3 /PI composite aerogels. As shown in Fig. 6a, Young's modulus of the CaCO 3 /PI composite aerogels gradually increases with the addition of CaCO 3 . When the amount of CaCO 3 is 4 wt%, Young's modulus of CaCO 3 /PI composite aerogels reaches 4.03 kPa, about 1.36 times that of pure PI aerogels (3.13 kPa). The reason is that CaCO 3 disperses inside the PI pore wall, which can increase the thickness of the pore wall. Young's modulus of the 6wt% CaCO 3 /PI composite aerogels is 4.09 kPa, which changes little compared to that of 4 wt% CaCO 3 /PI composite aerogels. From Fig. 6b, stress-strain curves in the compression process show a two-stage deformation similar to open-hole foams: the first stage is a linear elastic region (ε < 3%), and the stress in this stage increases linearly with a gradual increase of compressive strain; the second stage is a structural compaction region (3% < ε < 50%), and the stress in this stage increases rapidly with the increase of strain. After 100 cycles of compression, the plastic deformation of 4 wt% CaCO 3 /PI composite aerogels is only 2.17%. From  Fig. 6c, the maximum compressive stress corresponding to 100 cycles of 50% strain in the 4 wt% CaCO 3 /PI composite aerogels decreases by 3.27% (from 7.34 to 7.10 kPa). It can be attributed that the addition of CaCO 3 modulates the pore wall roughness for CaCO 3 /PI composite aerogels and exhibits little influence on pore wall structures. A pore wall composed of PI can support 4 wt% CaCO 3 /PI composite aerogels to finish compression and rebound, which endow aerogels with excellent compressive resilience. For the seismic protection of ancient architectures, the ability to dissipate external vibration energy is particularly important. Results of the calculated energy loss coefficient [54,55] of 4 wt% CaCO 3 /PI composite aerogels (Fig. 6d) show that the energy loss coefficient exhibits a trend of first decreasing and then stabilizing during the 100 compression cycles. Because in the initial compression cycle, repeated loading and unloading will consume the residual stress in the CaCO 3 /PI composite aerogels, leading to a rapid decline in the energy loss coefficient. After residual stress is completely eliminated, the energy loss coefficient tends to be stable 4 wt% CaCO 3 /PI composite aerogels exhibit an energy loss coefficient of 0.248 after 100 compression cycles, indicating excellent energy dissipation ability. In addition, 4 wt% CaCO 3 /PI composite aerogels also exhibit lightweight, with a density of only 0.026 g/cm 3 , which is smaller than that of commercial polymer foams (0.05 ~ 0.14 g/cm 3 ) [56][57][58] and can be placed on the petals without significant deformation (Fig. S3), which will not affect the ancient architectures due to their self-weight.

Conclusion
CaCO 3 /PI composite aerogels have been successfully prepared by homogeneous mixing, freeze-drying, and thermal imidization. The addition of CaCO 3 can effectively increase the pore wall roughness of the CaCO 3 /PI composite aerogels and improve their sound absorption performance.
When the amount of CaCO 3 is 4 wt%, CaCO 3 /PI composite aerogels exhibit the best sound absorption performance with an NRC of 0.327 and an average absorption coefficient of 0.903 in 2000 ~ 6300 Hz. In addition, the CaCO 3 /PI composite aerogels present excellent mechanical properties (Young's modulus of 4.03 kPa), fatigue resistance (stress loss of 3.27% for 100 compression cycles 50% strain, plastic deformation of 2.17% and energy loss coefficient of 0.248), thermal insulation (λ value of 0.038 W/m·K), and heat resistance (T HRI of 334.1 °C). The fabricated CaCO 3 / PI composite aerogels show important applications in the protection of ancient architectures.

Competing interests
The authors declare no competing interests. Fig. 6 Young's modulus (a) of PI and CaCO 3 /PI composite aerogels, stress-strain curves of 4 wt% CaCO 3 /PI composite aerogels under 100 cycles with 50% strain compression (b), the variation of maximum compression stress (c), and energy loss coefficient (d) corresponding to 50% strain during compression cycles for 4 wt% CaCO 3 /PI composite aerogels