From Monomers to a Lasagna-like Aerogel Monolith: An Assembling

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From Monomers to a Lasagna-like Aerogel Monolith: An Assembling Strategy for Aramid Nanofibers Chunjie Xie,† Lianyuan He,† Yifei Shi,† Zhao-Xia Guo,† Teng Qiu,*,‡ and Xinlin Tuo*,†

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†

Key Laboratory of Advanced Materials (MOE), Department of Chemical Engineering, Tsinghua University, No.1, Tsinghua Garden, Haidian District, Beijing 100084, P.R. China ‡ Key Laboratory of Carbon Fiber and Functional Polymers, Ministry of Education, Beijing University of Chemical Technology, No.15, North Third Ring Road, Chaoyang District, Beijing 100029, P.R. China S Supporting Information *

ABSTRACT: The manipulation of nanobuilding blocks into a 3D macroscopic monolith with ordered hierarchical structures has been much desired for broad and large-scale practical applications of nanoarchitectures. In this paper, we demonstrate a fully bottom-up strategy for the preparation of aramid aerogel monoliths. The process starts from the synthesis of poly(p-phenylene terephthalamide) (PPTA) through the polycondensation of p-phenylenediamine and terephthaloyl chloride, with the assistance of a nonreactive dispersing agent (polyethylene glycol dimethyl ether), which helps the dispersal of the as-synthesized PPTA in an aqueous medium for the formation of p-aramid nanofibers (ANF). Then the vacuum-assisted self-assembly (Vas) technique is skillfully connected with the ice-templated directional solidification (I) technique, and the combined VasI method successfully tailors the self-assembly of ANF to transform the 1D nanofibers into a 3D aerogel monolith with a specific long-range aligned, lasagna-like, multilaminated internal structure. The study of the aerogel microstructure revealed the dependence of the lamina orientation on the direction of the freezing front of ice crystals. This direction should be parallel to the deposition plane of the Vas process if a long-range aligned lamellar structure is desired. The anisotropy of the multilaminated aerogel was proven by the different results in the radial and axial directions in the compression and thermal conductivity tests. As a kind of organic aerogel, the ANF monolith has typical low density, high porosity, and low thermal conductivity. Additionally, the ANF monolith exhibits high compressive stress and excellent thermal stability. Considering its high performance and facile preparation process, potential applications of the ANF aerogel monolith can be expected. KEYWORDS: aramid nanofiber, aerogel, anisotropy, thermal stability, compression, vacuum-assisted self-assembly, ice-templated technique including chemical cross-linking,7 physical cross-linking,9 the ice-templated technique,6 and 3D printing.10 The use of nanobuilding blocks is expected to endow aerogels with improved performances with wide applications for adsorption,11 filtration,12 separation,13 catalysis,14 or thermal/acoustic insulation.8,15 However, there are still great challenges: One challenge involves the special group of organic aerogels, for which the desired organic building blocks with the superior mechanical and thermal stability required to ensure material

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ith the rapid development of cellular solids (foams), monolithic aerogel has drawn a lot of interest since the initial work of Kistler1 as a result of its superior characteristics, including low density, high porosity, and a large specific surface area. The further evolution of this type of material was inspired by the great successes achieved with nanomaterials and nanotechnology. To date, numerous kinds of nanomaterials have been prepared and utilized for aerogel fabrication, such as metal and metallic oxide,2 ceramics,3 carbon materials,4 synthetic organic polymers,5 natural polymers,6,7 and nanocomposites.8 Various methods have been required to assemble nanoparticles or nanodots, nanofibers, and nanosheets into designed 3D architectures, © XXXX American Chemical Society

Received: March 12, 2019 Accepted: July 9, 2019 Published: July 9, 2019 A

DOI: 10.1021/acsnano.9b01955 ACS Nano XXXX, XXX, XXX−XXX

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Cite This: ACS Nano XXXX, XXX, XXX−XXX

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ACS Nano Scheme 1. Schematic Diagram of the VasI Strategya

a

The formation processes of the ANF hydrogel are shown in (a−c), during which the ANF was transformed into a multilayer stacking structure. The detail descriptions in (a,b) indicate the hydrophilicity of ANF. The procedures of the directional solidification are shown in (d−f). The red arrow and the black arrow in (e) represent the moving direction of water molecules and the ANF network layer during the solidification process. Notes: The coordinate axes are given for a better understanding of the direction; the ice-templated directional solidification was conducted after the vacuum-assisted self-assembly.

contribute to more superior mechanical characteristics such as toughness, flexibility, and ductility when compared with a monolith composed mainly of irregularly stacked particles.26−28 Furthermore, as a special kind of 1D building block, nanofibers have been found with the capability of being manipulated when some properly induced self-assembly technique is applied. This capability is attractive in the aerogel field due to the possibility of ordered microstructures.26,29,30 The ice-templated technique plays an important role for this purpose, and it has been utilized as an effective assembly technology to prepare macroscopic 3D materials with ordered internal structure from various nanomaterials, owing to its good controllability of the material structure. Recently, aerogels with multilaminated, honeycomb-like, or hierarchical architectures have been prepared via the self-assembly of nanofibers or nanosheets tailored by the anisotropic growth of ice crystals.2,30−33 After the pioneering work of Kotov’s group on the preparation of aramid nanofiber (ANF) with the cleavage method,34 ANF has emerged as a new kind of nanobuilding block for the preparation of films, papers, and different kinds of composites. This material shows impressive performances in the applications of batteries and supercapacitors,35−39 engineering composite material,40−42 adsorption material,43 electromagnetic interference shielding material,44 optoelectronic material,45 infrared thermal stealth material,46 electrical insulating nanomaterial,47 and biomimetic composites.48 Recently, ANF obtained with the cleavage method has also been applied in the building of aerogels.49 The whole process can be seen as a “top-down” (the cleavage of micron-sized aramid fibers into nano blocks) and then an “up” (the assembly of nanoblocks into a 3D monolith) method. Due to the work needed in the production of aramid fibers from their

safety in applications are still being determined. Another challenge involves the subtle control of the fine structure of the aerogel in order to produce long-range regularities.6,16−18 Well-developed high-performance polymers shed light on the choice of nano-organic building blocks with superior strength and environmental resistance. The most typical type is aromatic polyamide (aramid), which is famous for its excellent strength, modulus, and fire resistance in its fiber form with the commercial name of Kevlar or Twaron. Although the poor solubility of aramid remains a problem in its applications as a porous monolith, special molecular structures and processing methods have been exploited in attempts to resolve the problem.19−22 Different methods, including the introduction of branched or hyperbranched points, the copolymerization of flexible monomers, and/or the application of strong dissolution conditions, have been utilized in recent literature for the preparation of solutions of different kinds of “aramid material”, followed by the induced phase separation for the formation of the micropores and mesopores in the consolidation of the monolith.19−22 For microstructures, the porous solids built by the methods mentioned above are generally composed of many stacking particles on the microscopic scale,19−21 which suggests weak points at the interface of the particles when force is applied. Instead of particles, inspiring work has been done using nanofibrous materials as building blocks.23−29 Commonly, nanofibers with high aspect ratios usually possess the flexibility to twist and entangle with each other.23 In addition, strong supramolecular interactions like hydrogen bonding forces have pronounced contributions to the connection strength at the joints of the adjacent nanofibers.9,13 All these characteristics are beneficial for the formation of strong 3D nanofibrous aerogels12,24,25 as the junctures in the nanofibrous network B


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Figure 1. (a) TEM image of ANF. (b) FTIR spectra of ANF and Kevlar 29. (c) DSC curves of ANF, Kevlar 29, and DME. (d) X-ray diffraction patterns of ANF and Kevlar 29.

large quantities. Then we utilized the ice-templated directional solidification (I) technique to further tailor the inner structure through the directional growth of ice crystals, which was important for the long-range aligned structure and the properties of the products. Traditionally, a vacuum-assisted self-assembly technique is usually applied in the preparation of 2D film or paper, and the ice-templated directional solidification technique is usually applied in the preparation of 3D aerogels. In this work, we demonstrate a combination strategy of vacuum-assisted self-assembly with ice-templated directional solidification. (This term is simplified as the VasI strategy in the following section. The schematic diagram of the VasI strategy is demonstrated in Scheme 1.) The multilayer stacking structure formed by vacuum-assisted self-assembly could guild the growth direction of the ice crystals in the ANF hydrogel, and the strong association of the ANF in-plane provides enough strength to limit the irregular growth of the ice crystals in other directions. The combined effects are helpful for the stable anisotropic growth of the ice crystals, providing the monolithic aerogel with a specific anisotropic, long-range aligned lamellar structure. The laminae orientation inside our ANF aerogel could reach a range as long as the centimeter scale. The product had the typical characteristics of an aerogel such as high porosity but with special anisotropic features. Additionally, the product exhibited excellent comprehensive performances, including low density, high compressive stress, excellent thermal stability, and heat insulation. A detailed study of the dependence of the aerogel structures and properties on the VasI conditions is also illustrated in this work. As far as we know, these two methods were seldom used together for the preparation of 3D aerogels with long-range aligned lamellar structure, especially for an anisotropic, aramid-based aerogel

monomers, this top-down and up procedure might be timeconsuming. There should be the possibility of the development of more efficient preparation methods intended to decrease the operation difficulty, save costs, and modulate the specific geometric parameters of ANF.50 Because the lightweight aerogels with ordered microstructures achieved from the icetemplated technique have shown properties better than those with disordered microstructures obtained by the nondirectional freezing technology,32,33,51 further methods for producing sophisticated microstructure control would also be desirable in special areas for the development of aramid aerogel. In our previous work, we demonstrated a bottom-up method for the preparation of ANF, which provided a simple and direct method for producing ANF from monomers through polymerization-induced self-assembly (PISA).52−54 Two-dimensional structures such as membranes and papers have been prepared using our ANF product as a building block.52−54 However, the addition of dihydroxyl- or monohydroxyl-terminated polyethylglycol (PEG) would affect the properties of the ANF products, especially the thermal stability. In order to prepare an aramid aerogel composed of “pure” poly(p-phenylene terephthalamide) (PPTA), we used nonreactive PEG, which could be removed after polymerization at the fiber collection stage through washing and filtration, instead of using the reactive hydroxyl-terminated PEG as the tailoring agent in the preparation of ANF. Moreover, in order to obtain the 3D porous monolith with well-controlled structures, first, we introduced the vacuum-assisted self-assembly (Vas) technique to direct the inner structure of the hydrogel fabricated by ANF, which also avoided the possible large volume shrinkage from hydrogel to aerogel as the dispersion medium was removed in C


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Figure 2. Preparation procedures of the ANF aerogel. Note: The black arrows in the images indicate the main growth direction of the ice crystals.

Figure 1d demonstrates the X-ray diffraction characterization results of ANF and Kevlar 29. The diffraction peaks of ANF at 20.6, 23.1, and 28.2° correspond to the (110), (200), and (004) lattice planes in the PPTA crystal.55 The almost undifferentiated location of the three peaks in both profiles suggested that the addition of DME would not disturb the crystallization of PPTA. The four characterization results prove that the nonreactive DME could induce the formation of ANF, but it had the least impact on the composition and crystallization of the polymer, which guaranteed the “purity” of the PPTA nanofibers. Preparation of the Aerogel. The preparation procedures from monomers to an ANF aerogel are illustrated in Figure 2. After fibrillation, the solid content of the dispersion was as low as 0.1%. As indicated by Figure 2, a 3D hydrogel was built via Vas, and the shape of this hydrogel was maintained well during the subsequent freezing and drying. A small shape change was found between the ANF hydrogel and the ice-templated frozen hydrogel in the ice crystal growth direction, and almost no shrinkage was observed after the frozen hydrogel was dried to a light aerogel. The density of the ANF aerogel was about 25 ± 2 mg/cm3, which is similar to that of some cellulose nanofiber aerogels.7,56 The porosity of the ANF aerogel was as high as 98.2 ± 0.1%, calculated by the liquid filling method.57 The microstructure of the ANF aerogel prepared through the VasI process is shown in Figure 3. As we can see in Figure 3a, the ANF aerogel was characterized by its multilaminated structure in the z-direction. The multilaminated structure could be stripped into a single lamina of about 0.6 μm in thickness, which was composed of numerous ANF layers (Figure 3b). The surface layer of the lamina, as shown in Figure 3c, was built with irregularly overlapped sheets. Figure 3d shows the porous fibrous network of ANF on the sheets. During the filtration process, the nanofibers “spread out”, and they were entangled together with their longitudinal axes orienting along the xy-plane, so that the sheet-like layers could be formed. Figure 3e shows the statistics of the interlamellar spacing of the laminae in Figure 3a. The average interlamellar spacing was about 28.4 ± 10.2 μm. In Figure 3f, it can be determined that the “bridges” existed between the adjacent laminae, which also consisted of a nanofiber network (Figure S3). It should be noted the self-woven ANF network provided the strength needed by the monolith to sustain its original shape after the removal of water in the hydrogel, so the organic aerogel with low density and high porosity could then be obtained. It is worth emphasizing that the VasI method could

with comprehensive performances. We believe that the VasI strategy could also be applied in the preparation of other aerogel materials with ordered lamellar structure.

RESULTS AND DISCUSSION Preparation of ANF. Being a typical type of all-aromatic fiber-forming polymer, PPTA itself has a strong tendency to orient under a proper shear stress field, especially in solution. However, the association between the oriented molecules is also strong, which is conducive to the formation of fiber in macro sizes. Based on our already reported work,52−54 the introduction of polyethylene glycol in the polymerization of PPTA can impair the strong H-bonding forces between macromolecules and tailor the fibrillation process because of the feature of mPEG as a hydrophilic H-bonding acceptor. In this research, in order to produce a nano product without an obvious deterioration of properties, instead of reactive methoxypolyethylene glycol (mPEG), we introduced the nonreactive polyethylene glycol dimethyl ether (DME) in the polymerization. DME could also compete with the PPTA molecules in the formation of the H-bonding and retard the size growth of the fibrils in the radial direction. The interaction between DME and PPTA was weaker than that of the supramolecular forces inside the PPTA and was easier to be destroyed through the solvation of DME when dispersed in aqueous conditions. As seen in Figure 1a, the ANF in the aqueous medium had a fibrous form with a large aspect ratio (more than 200, estimated from the image) and a uniform radial size (20 ± 5 nm). The zoomed-in view of ANF is displayed in Figure S1. The junctures and entanglements among the ANF were plentiful in the observation field because of the removal of water. The Fourier transform infrared (FTIR) spectra of ANF and the aramid fiber (commercial name Kevlar 29) are displayed in Figure 1b. The characteristic PPTA bands at 1647 (CO stretching vibration), 1540, and 1251 cm−1 (N−H deformation and C−N stretching coupled mode) all emerged in the spectrum of ANF, and these bands were consistent with those of Kevlar 29. After the repeated rinsing with water, there was no sign of the characteristic bands of the residual DME in the spectrum of ANF (Figure S2). Further evidence for the removal of DME is provided in Figure 1c, where, in sharp contrast to the obvious crystallization and the melting peak of DME at 30.6 and 52.0 °C, respectively, ANF showed almost no response at 200 °C, the same as the thermal behavior of Kevlar 29. D


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Figure 3. SEM images of the ANF aerogel. (a) Cross-sectional view of the ANF aerogel (xz-plane). (b) Detailed structure of the single lamina of the boxed area in (a). The arrow indicates the thickness of a single lamina. (c) Top-down view of the ANF aerogel (xy-plane). (d) Magnified view of the boxed area in (c). (e) Statistics of the interlamellar spacing of the area of (a). (f) Cross-sectional view of the ANF aerogel with 1000× magnification (xz-plane) and the “bridge” phenomenon (as indicated by the red line). (g) Long-range aligned lamellar structure.

1.6 cm × 1.0 cm, the aligned lamellar planes extended uniformly along the x- or y-directions to no less than 1 cm, as

provide effective control of the long-range regularity of the aerogel. In the monolith block with the section size of about E


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Figure 4. (a) Nitrogen adsorption−desorption isotherms of the ANF aerogel. (b) Pore size distribution of macropores and mesopores. Inset: pore size distribution of the micropores.

Figure 5. (a) TGA and differential thermogravimetry curves of the ANF aerogel, Kevlar 29, and ANF-mPEG aerogel. (b,c) Macroscopic states of the ANF aerogel before and after the heat resistance test (500 °C, 30 min, N2 atmosphere), respectively. (d,e) Inner structures of the ANF aerogel before and after the heat resistance test (500 °C, 30 min, N2 atmosphere), respectively.

Thermogravimetric analysis (TGA) profiles of the ANF aerogel can be seen in Figure 5a, together with the reactive methoxy polyethylene glycol (mPEG)-assisted dispersed ANF (ANF-mPEG) aerogel and Kevlar 29 for comparison. Benefiting from the “pure” PPTA building blocks, our ANF aerogel showed pronounced thermal stability similar to that for Kevlar. This is a big improvement compared with ANF-mPEG aerogel52−54 because of the removal of nonreactive DME (decomposition temperature range = 275−450 °C, Figure S5). The onset temperature of the ANF aerogel is 502 °C, and that of Kevlar 29 is 505 °C; the temperature of the peak decomposition Td of the ANF aerogel is 591 °C, and that of Kevlar 29 is 581 °C. It was noted that Kevlar 29 showed only one rapid decomposition stage at 505−623 °C, whereas our ANF aerogel decomposed in two stages, a rapid stage at 502−610 °C and a slow stage at 610−800 °C. This difference could be mainly caused by three reasons. First, the interfacial effect might happen due to the characteristics of nanomaterials, which means that the nanofibers would be more likely to decompose and carbonize at a relatively lower temperature because of the large surfaces, and the carbonized layer formed on ANF might retard the further decomposition of the material beneath, acting as a protection layer.59−61 Second, the different molecular weights of the two products might account for the difference in TGA curves.22 Third, the residue CaCl2 might also affect the decomposition of the two products.62

shown in Figure 3g, and the orderly stacking of lamellae could also sustain the thickness (z-direction) of 1 cm (Figure S4). It should be noted that the upper limitation of the long-range aligned lamellar structure was “limited” by the characterization method because the diameter of the sample stage used for our scanning electron microscopy (SEM) characterization was about 2.5 cm (Figure S4). Longer-range regularity could be expected as larger examples could be prepared through the VasI method. Figure 4 exhibits the nitrogen adsorption−desorption isotherm and the pore size distribution of the ANF aerogel. A small hysteresis loop can be observed on the nitrogen adsorption−desorption isotherm in Figure 4a, which is consistent with the type-IV isotherms according to the IUPAC classification,58 indicating that the porous structure of the aerogel was dominated by mesopores. The pore size distribution profile in Figure 4b shows that the pore size had a wide distribution in the range of 0−180 nm, with a relatively concentrated distribution in 15−50 nm range. The pore volume of the macropores and mesopores was 0.33 cc/g (Barrett−Joyner−Halenda method). Micropores were also detected, as the inset image demonstrates. However, the pore volume of micropores was only 0.021 cc/g (Saito−Foley method). These results were in accordance with the scenario in Figure 3d, in which macropores and mesopores dominate. The Brunauer−Emmett−Teller (BET) surface area of the ANF aerogel was 62.88 ± 0.50 m2/g. F


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Figure 6. Cross-sectional views of the aerogel prepared by different methods. (a) Inner structure of Sample 2. Inset images (a1), (a2), and (a3) show the macroscopic state of the original hydrogel, the recombinational hydrogel, and Sample 2, respectively. The arrow in (a) indicates the main growth direction of the ice crystals. (b) Detailed morphology of the boxed area of (a). (c,e,g) Inner structure of Sample 3, Sample 4, and Sample 5, respectively. (d,f,h) Corresponding detailed morphology of the boxed area of (c,e,g), respectively. The inset pictures in (c,e,g) show the corresponding macroscopic state of the aerogel. The arrows in all the inset pictures represent the main growth direction of the ice crystals. The size of the cubes of the ANF hydrogel used in this serial experiment was 10 mm × 10 mm × 10 mm.

A detailed description of the various methods is listed in the Experimental Section. VasI was assigned as Method 1, and for comparison, the “ANF aerogel” prepared above was also named “Sample 1”. The association forces (physical entanglement, hydrogen bonds, and π−π stacking) connecting the ANF in the aerogel monolith were physical, which suggested that the hydrogel from the Vas operation unit could be reshaped by Method 2 before the freezing and drying operation. The recombined hydrogel had the same appearance as the original hydrogel. However, obvious deformation happened for its corresponding aerogel, as shown by the inset pictures in Figure 6a. Laminae can also be observed, as indicated by the yellow dashed line, but their continuous orientation was largely twisted and destroyed by the recombination method, leaving massive sheets in the view (Figure 6b). This result indicates that the initial structure of hydrogel from Vas is essential for the formation of a continuous multilaminated structure. The effects of the solidification direction of the ice crystal and the drying methods are summarized in Figure 6c−h, including the microscopic and macroscopic images of the aerogels. In order to compare the influence of the solidification direction, we placed the xy-plane on the surface of a cold center to make the ice crystal grow along the z-direction. With this condition, we finally obtained the results shown in Figure 6c. Nondirectional solidification was also carried out in our experiment by placing the hydrogel sample in a refrigerator, in which the freezing happened indistinguishably on all the sample surfaces, and the ice crystal propagated irregularly in all the directions. For this condition, we obtained the results shown in Figure 6e. The hydrogel sample was also directly dried at ambient temperature. The hydrogel sample was placed in a silica gel dryer (25 °C) for more than 4 days to completely remove the water. In this case, we only obtained a xerogel in the end, as shown in Figure 6g. By comparing the inner structures of the above samples with Sample 1, we found that in our experiment the multilayer stacking structure could be distinguished in all of the samples, although the microstructures were altered depending on the different solid-

For a better demonstration of the thermal stability of the ANF aerogel, a tube furnace was used to carry out the heat resistance test. The ANF aerogel was heated at 500 °C for 30 min in a N2 atmosphere. Figure 5b,d shows the macroscopic state and the inner structure of the ANF aerogel before the heat resistance test. Although surface carbonization was observed, the aerogel maintained its shape after the test, as well as its inner laminar structure (Figure 5c,e). Trying to illustrate the interactions between ANFs and their contribution in the shape maintenance of the ANF aerogel in the heating process, we carried out temperature-dependent FTIR analysis. The characteristic band for the CO of amide (amide I) at ∼1650 cm−1 in Figure S6a demonstrates clearly that the amide groups are in highly hydrogen-bonded association in the ANF network. The association is stable with 84.1% survival at 450 °C, as shown in the intensity retention plot in Figure S6b. The two-dimensional correlation spectroscopy (2D-COS) analysis was also carried out, and the synchronous and asynchronous 2D-COS of the ANF film in the wavenumber range of 1580−1720 cm−1 are shown in Figure S7. They reveal that the disassociation of hydrogen bonds between ANFs would happen when the temperature increases, followed by the appearance of free carbonyl groups and changes of other kinds of molecular interactions. However, the remaining molecular interactions, dominated by hydrogen bond forces, are strong enough to maintain the aerogel framework at the heating temperature below 500 °C. Detailed analysis of Figures S6 and S7 is in the “Changes of hydrogen bonds between ANFs at different temperatures” section of the Supporting Information. Control of the Laminae Orientation. To find out the influence of different preparation conditions on the aerogel monolith, three factors, including the state of the ANF hydrogel, the growth direction of the ice crystals, and the drying methods, were studied in this research. Accordingly, various methods were applied for the preparation of different samples. The different methods were denoted as “Method N”, and the corresponding samples were denoted as “Sample N” (N denotes the method and sample number; N = 1, 2, 3, 4, 5). G


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Figure 7. Compression performance of the ANF aerogel. (a) Stress−strain curves of the ANF aerogel in different directions. (b) Specific compressive strength of the ANF aerogel and other aerogels. Note: For accurate comparison, the aerogels we chose were in the density range of 10−35 mg/cm3 and all of the compressive stress values were estimated at a compression strain of 70%. Samples: CNF;64 PAN-SiO2;65 PVA/CNF/GONS;66 NFC aerogel;67 CNF-1;68 CNF-2;56 BANF aerogel;49 LC-Ncell;26 PVA/CNF;69 MFC.70 (c) Stress retention in the axial direction of the ANF aerogel in fatigue test with 1000 compression−decompression cycles (ε = 30%). The inset pictures show the macroscopic state of the ANF aerogel during one compression−decompression cycle. (d) Macroscopic state of the ANF aerogel before and after compression in the axial direction and the radial direction, respectively (ε = 75%). (e−g) SEM images of the ANF aerogel before compression, after axial compression, and after radial compression. All of the ANF aerogel samples used in this series of tests were prepared using Method 1.

ification directions and the drying methods. In addition, the layers were built by the nanofibers, which were oriented and entangled with each other in the xy-plane (i.e., the filtration plane). Therefore, we drew the conclusion that the multilayer stacking structure was achieved with the Vas process. This process should be related to the interception time of the filter elements. In this way, the differences of the laminae thickness, orientation, and the interlamellar spacing in different samples should be caused by the freezing and drying operation. To study the influence of the freezing and drying operation on the aerogel microstructure, we first examined Sample 1 in Figure 3. The laminae in Sample 1 had the highest degree of orientation, as well as the highest uniformity of the laminae thickness and the interlamellar spacing. In this condition, the growth direction of the freezing front was consistent with the orientation of the layers in the Vas process. Moreover, in comparison with the tightly woven junctures of the nanofibers in the filtration plane, the interplanar connection between

layers was relatively weak. Accordingly, we presumed that the ice crystal would prefer to grow along the interplanar defect. During the freezing process, the existing water between the layers would diffuse toward the developing ice plane to sustain the propagation of the ice crystals. The thickness growth of the ice layer would then gradually expel its adjacent ANF layer, creating a layers-stacking laminae separated by ice (Scheme 1e,f). Thus, the laminae were oriented uniformly in the direction parallel to the growing direction of the freezing front. During the growth of the ice crystal, some layers could be entrapped into the ice crystal63 as a result of the interplanar entanglement, which was caused by the high aspect ratio of ANF. This phenomenon resulted in the formation of “bridges” (Scheme 1f). The schematic diagram for the formation of multilaminated aerogel is demonstrated in Scheme 1. According to the assumption above, when the growth direction of the freezing front was inconsistent with the plane direction of the layers formed through Vas, for example, H


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Figure 8. (a) Thermal conductivity of the ANF aerogel in different directions (25 °C) and the schematic illustration of thermal conduction in different directions (25 °C). (b) Thermal conductivity comparison of the ANF aerogel and other porous materials (25 °C). Note: the thermal conductivity of these samples is accurately quoted or estimated from the literature. Samples: PVPMS;15 PAN/BA-a/SiO2;8 PI aerogel;76 NFC aerogel;67 KNA film;47 KNF aerogel mat;75 PU foam;77 NFA;13 PVA/CNF/GONS;66 PA aerogel.22

also displayed in Figure 7b and Table S1. It can be determined that the specific strength of the ANF aerogel was at a high level in the radial direction. The specific strength of the ANF aerogel in the axial direction was not as high as that in the radial direction, but it was also in the middle level and higher than the specific strengths of some other kinds of aerogels. Moreover, our ANF aerogel demonstrated resilience under small strain in this direction, which is special and seldom seen in the reports of other aramid-based aerogel samples.22,49 As can be seen in Figure 7c and Table S3, in the fatigue test, when the compressive strain was fixed at 30%, the compressive stress of the ANF aerogel remained at 95.07% of the initial value after the compression−decompression process for 1000 cycles. It should be noted that the 5% decrease of compressive stress happened mainly in the first 20 cycles. From then on, there was almost no further change in the compressive stress or the sample height after 1000 cycles. The ANF aerogel during the fatigue test can be viewed in Video S1. The shapes of the ANF aerogel before and after compression (ε = 75%) in different directions are shown in Figure 7d. After being pressed to 25% of the original height (ε = 75%), the ANF aerogel recovered to 59.6 and 52.4% of the original height in the axial direction and the radial direction, respectively. The microstructures of the aerogel before and after compression (ε = 75%) in different directions are revealed in Figure 7e−g. A denser arrangement of laminae with a mean interlamellar spacing of 22.4 ± 9.3 μm can be found in Figure 7f after the axial compression, in comparison with the original sample (28.8 ± 10.6 μm) in Figure 7e. The compression in the radial direction under this strain, however, resulted in the distortion of the ANF laminae as indicated by the dashed line in Figure 7g. The obvious differences in the axial and radial compression tests supported the anisotropy structure of the ANF aerogel. Figure 8a shows the thermal conductivity of the ANF aerogel in the axial and radial directions and the schematic illustration of the thermal conduction in the two directions at 25 °C. The thermal conductivity of the ANF aerogel was 0.0418 ± 0.0006 W/(m·K) in the radial direction and 0.0372 ± 0.0004 W/(m·K) in the axial direction (25 °C). The directional differences of the thermal conductivity could also be related to the anisotropic microstructure.71 In theory, the total thermal conductivity (λtotal) of an aerogel in air can be

perpendicular to the plane direction, the ice crystallization still happened in Sample 3. In this case, the ordered growth of ice crystal along the freezing front was restrained by the ANF layers, which affected the microstructure of the aerogel monolith, as indicated by Figure 6c,d. The orientation of the laminae was not as regular as that in Sample 1, and it deviated from the xy-plane. The spacing between the laminae was obviously enlarged. In the case of Sample 4, almost no deformation was found on the aerogel in the macroscopic state as a result of the nondirectional solidification. However, the degree of order in the microscope was further lost, as indicated by the irregular laminae arrangement, enlarged interlamellar spacing, and laminae thickness, as shown by Figure 6e,f. All of these results imply the existence of some random growth of ice crystals. When the drying of the hydrogel was conducted at ambient temperature conditions, we only obtained a huddled xerogel, as shown in Figure 6g,h. The absolute loss of the porous structure adequately verified the important role of freeze-drying in our aerogel formation. Anisotropic Properties. In order to further illustrate the long-range ordered orientation of the multilaminated ANF aerogel, we carried out direction-related compression and thermal conductivity tests. The results are discussed in this section. The behavior of the ANF aerogel under compression is revealed in Figure 7. The two curves show a different increase tendency in the process of compression (Figure 7a), with final stresses of 0.095 ± 0.003 and 0.165 ± 0.005 MPa in the axial direction (the compression was applied vertical to the lamina plane) and the radial direction (the compression is applied parallel to the lamina plane) direction (ε = 70%), respectively. The specific compressive strength of the ANF aerogel in the radial direction for our work was 6.600 ± 0.200 kN·m/kg (ε = 70%), which was 13.4% larger than that of the already reported branched aramid nanofiber (BANF) aerogel49 (Figure 7b and Table S1). However, the specific strength of the ANF aerogel in the axial direction was 3.800 ± 0.120 kN·m/kg (ε = 70%), which was 34.7% smaller than that of BANF aerogel. The specific strength of the ANF aerogel was also comparable to that of some aromatic polyamide (PA) samples,22 as Table S2 demonstrates. The comparison of the specific strength of the ANF aerogel with other reported aerogels of similar densities is I


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Figure 9. (a) Compressive stress (ε = 70%) and (b) thermal conductivity (25 °C) comparison of different samples.

m/kg, estimated at ε = 70%)67 is not as high as the compressive stress of our aerogel (3.800 ± 0.120 kN·m/kg, ε = 70% in the axial direction). Moreover, from the point of view of organic polymer porous material, the preparation process of the ANF aerogel is simpler than that of PI aerogel76 and the heat resistance of the ANF aerogel is better than that of polyurethane (PU) foam.77 This fact should be put in perspective together with the relatively low density, the superior mechanical strength, the excellent thermal stability, and the facile preparation process of these materials. In general, our ANF aerogel possesses a good comprehensive performance including low density, high compressive stress, and excellent thermal stability. Additionally, as a characteristic of the ANF aerogel, the anisotropy could be utilized in the application of unidirectional heat insulation. The comparisons of the compressive stress (ε = 70%) and thermal conductivity (25 °C) of Sample 1, Sample 2, Sample 3, and Sample 4 are given in Figure 9, Figures S10 and S11, and Table S5. Sample 5 shrank to a huddled xerogel, and it was not suitable for the two tests. Because the multilayer stacking structure of the hydrogel was destroyed by the recombination method, Sample 2 exhibited isotropy. The growth direction of the ice crystals in the preparation of Sample 2 was defined as the radial direction, and the direction perpendicular to it was defined as the axial direction. As a result, there is little difference in the stress− strain curves and the thermal conductivity for the two directions. The small difference in the two directions might be attributed to the growth of ice crystals. The compressive stress of the two directions of Sample 2 was between the values of Sample 1 and so was the thermal conductivity of the two directions. Sample 3 and Sample 4 showed anisotropy in the compressive stress and the thermal conductivity, just like their inner structures. However, when compared with Sample 1, there were still some differences. For Sample 3, the freezing front proceeded perpendicularly to the xy-plane, destroying the parallel structure and promoting the orientation of the laminae along the z-axis direction to some extent. As a result, the compressive stress in the axial direction was larger than that of Sample 1, whereas a reduction of compressive stress was found in the radial direction. For Sample 4, the irregular laminae arrangement and the enlarged interlamellar spacing indicated the loss of the degree of order, which resulted in the reduction of the compressive stress in both directions when compared with that of Sample 1. The thermal conductivities of Sample 3 and Sample 4 were larger than that of Sample 1 in both directions, which could be attributed to the destruction of the parallel structure. The compressive stress and the thermal

approximated by the sum of the heat conduction through the solid phase (λs), the conduction through gas the phase (λg), and the radiative heat transfer through the pores (λr).72 At room temperature, the thermal contribution of λr to the λtotal was small, and it could even be neglected.15 For this condition, the difference in the thermal conductivity for different directions is mainly influenced by λs and λg. Owing to the anisotropic multilaminated structure, the alignment of the solid phase (laminae) contributed to heat conduction in the radial direction.73 As a result, λs in the radial direction was higher than that in the axial direction. Furthermore, the numerous interlamellar spaces were also helpful for λg in the radial direction. In contrast, λs and λg were restricted in the axial direction because heat conduction could also proceed along the laminae and the interlamellar spaces due to the anisotropic structure.71 The difference in the thermal conductivity in the two directions was lessened when the test temperature increased to 200 °C. At this temperature, the thermal conductivities were 0.1075 ± 0.0017 and 0.1056 ± 0.0011 W/(m·K) along the radial direction and the axial direction, respectively. The λr increased rapidly with temperature,72,74 which could account for the reduction in the thermal conductivity difference of the different directions. However, the aerogel maintained its shape as well as its compressive stress after the test at 200 °C, as shown in Figures S8 and S9. This could be beneficial to the possible thermal insulation applications comprehensively considering the low density, excellent thermal stability, and high strength of the ANF aerogel. The thermal conductivity comparison of the ANF aerogel with previously reported aramid-based porous materials and other porous materials is displayed in Figure 8b and Table S4. The thermal conductivity of our ANF aerogel (in the axial direction) is similar to that of Kevlar nanofiber aerogel (KNA) film47 or nanofibrous Kevlar (KNF) aerogel fiber mat,75 and it is much lower than that of PA aerogel.22 In the radial direction, the thermal conductivity of the ANF aerogel is between those of KNA film, KNF aerogel mat, and PA aerogel. In comparison with other kinds of porous materials, it can be determined that the thermal conductivity of the ANF aerogel in the axial direction is above that of several aerogels. However, in these aerogels, polyvinylpolymethylsiloxane (PVPMS) aerogel (200 ± 20 mg/cm3)15 and polyimide (PI) aerogel (110 ± 30 mg/cm3)76 both have a density much higher than that of our ANF aerogel (25 ± 2 mg/cm3), and the compressive stress of polyacrylonitrile/benzoxazine/SiO2 (PAN/BA-a/SiO2) aerogel (1.880 kN·m/kg, estimated at ε = 70%)8 and nanofibrillated cellulose (NFC) aerogel (2.533 kN· J


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shear homogenizer (Wiggens D-500). The speed of the homogenizer was 10000 rpm, and the homogenizing time was 5 min. After being homogenized, an ANF dispersion with a concentration of 0.1% was formed. For comparison, an ANF-mPEG dispersion was also prepared, in which the DME was replaced by the same amount of mPEG. Preparation of the ANF Aerogel. Typically, a certain amount of ANF dispersion was filtered through a sintered filter funnel (diameter = 3−18, variable) under vacuum conditions. For convenience, we defined the filtration plane of the funnel as the xy-plane and the direction perpendicular to the filtration plane (i.e., the filtration direction) as the z-direction (shown in Scheme 1c). After most of the solvent was removed by filtration, a hydrogel was formed. Deionized water (three times the volume of the ANF dispersion) was used to thoroughly rinse the hydrogel. After being rinsed, the hydrogel was transferred onto a homemade freezer (Figure S12) with the xz-plane (refer to Scheme 1d) in contact with the surface of the copper (the temperature of the copper surface was about −60 °C, and the frozen rate of the ANF hydrogel was about 2.5 mm/min−1). Then the frozen sample was quickly transferred to the freeze-drying equipment (BenchTop Pro, Virtis). After 1−5 days, the aerogel was obtained. The aerogel was named “ANF aerogel” or “Sample 1”, and the method described above was denoted as “Method 1”. In this method, as sketched in Scheme 1d−f, the solidification proceeded with the growth direction of the ice crystals, which was parallel to the xy-plane. The ANF-mPEG aerogel was also prepared by the same method. For comparison, other preparation methods were also proposed, as described below. Method 2: Recombination Method. In this method, the ANF hydrogel (4 cm × 4 cm × 2 cm) was cut up into hydrogel slurry by a lab disperser (SFJ-400, Shandong Longxing Chemical Machinery Group Co., Ltd.) for 10 min. The obtained hydrogel slurry was manually reshaped into a cylinder shape inside a 50 mL syringe at 25 °C. Recombination was considered to be complete when the hydrogel slurry was squeezed out from the tip of a needle. Then the recombined hydrogel was taken out from the syringe and cut into the needed size. The hydrogel samples were placed onto the homemade freezer for freezing. Method 3. The freezing direction proceeded perpendicular to the xy-plane. Method 4. The hydrogel samples were placed in a refrigerator (−18 °C), and the ice crystals were grown from each plane of the hydrogel. Method 5. The hydrogel samples were dried directly at ambient temperature. The hydrogel samples used in Methods 3, 4, and 5 were the same samples as those used in Method 1. By using sintered filter funnels of different diameters, combined with cutting, a batch of identical hydrogel samples could be obtained as well as aerogels of different sizes (Figure S13). Characterization. The morphology of the ANF was observed with transmission electron microscopy (TEM, H-7650B, Hitachi). The inner structure of the ANF aerogels was characterized with field emission scanning electron microscopy (JSM-7001F, Japan Electron Optics Laboratory Co., Ltd.). The FTIR spectra of the ANF, Kevlar 29, and DME were accessed with a Nicolet560 spectrometer. Thermal analysis was carried out with a differential scanning calorimeter (DSCQ2000, TA Instrument) with a heating and cooling rate of 10 °C/ min. The crystal structure was characterized by X-ray diffraction (D/ maxIIIB, Rigaku). The density of the ANF aerogel (ρgel) was obtained by dividing the weight by the volume. The porosity was tested with the liquid filling method.64 In brief, the ANF aerogels with specific weights and volumes were immersed in n-butyl alcohol for 12 h, and then the aerogels were removed from the solvent. The redundant solvent on the surface of aerogels was wiped away with filter paper and weighed again. The porosity was calculated using equation 1.

conductivity of the different samples were in good agreement with their structures, and the regular multilaminated structure with smaller interlamellar spacing was conducive to the relatively higher compressive stress (radial direction) and lower thermal conductivity (axial direction).

CONCLUSION ANF aerogel with a lasagna-like multilaminated structure was successfully prepared with a fully bottom-up strategy. The protocol started from the polymerization of monomers, followed by the assembly of the polymers into ANF. After that, the VasI method was used, which directed the transformation of 1D ANF into a 3D aerogel monolith with ordered microstructures. A long-range ordered lamellar structure was also obtained using VasI methods, and the orientation area of the ANF aerogel was on the centimeter scale. The ANF monolith had typical low density and high porosity. The anisotropic development of the ANF into 3D architectures was achieved by the ANF deposition in the Vas process, and the directional growth of the ice crystals had a strong influence on the orientation control of the multilaminated structures and the final product performance. The anisotropy in the ANF aerogel reflected on the properties. The ANF network endowed the ANF aerogel with high compressive stress in the radial direction (σ = 0.165 ± 0.005 MPa, ε = 70%) and with good resilience in the axial direction (95% of the initial compressive stress remained after 1000 compression−decompression cycles, ε = 30%). Being similar to Kevlar fiber, the ANF aerogel showed an excellent thermal stability. The ANF aerogel exhibited a low thermal conductivity at 25 °C (0.0372 ± 0.0004 W/(m·K)) and 200 °C (0.1056 ± 0.0011 W/(m·K)). Considering its good comprehensive performance, the potential applications of the ANF aerogel could be expected in the areas of insulation, absorption, and high temperature filtration. Furthermore, the VasI method utilized in this work, which could subtly tailor the microstructure of organic aerogel, could also be extended to the preparation of other aerogel based on 1D nanomaterials. EXPERIMENTAL SECTION Materials. Terephthaloyl chloride (TPC) was purchased from Shandong Kaisheng New Materials Co., Ltd. p-Phenylenediamine (PPD) (>99%) was purchased from Amino-Chem Co., Ltd. DME (Mn = 2000), and mPEG (Mn = 2000) was purchased from SigmaAldrich Co., Ltd. N-Methyl pyrrolidone (NMP) was purchased from Damao Chemical Reagent Factory and dried over a 4 Å molecular sieve (Sinopharm Chemical Reagent Co., Ltd.) before being used to remove the trace water. CaCl2 (>99%) was purchased from Beijing Chemical Works, and it was heated at 450 °C for 4 h to eliminate the trace water before use. Kevlar 29 was purchased from Dupont. Deionized water, prepared in our lab, was used for all of the experimental processes. Preparation of ANF Dispersion. For the experiment, 100 mL of NMP was added into the reaction vessel and heated to 100 °C under nitrogen. After being stirred for 5 min at 100 °C, certain amounts of CaCl2 and DME (3 wt % of the total weight of PPD and TPC) were added and dissolved at 100 °C for 30 min. After this, the reaction system was cooled to 0 °C, and PPD was added and totally dissolved in the system. The stirring speed in the above procedures was 400 rpm. Finally, TPC was added into the system and the stirring speed was increased to 2000 r/min. In this reaction, the concentrations of TPC and PPD were 0.21 and 0.20 mol/L, respectively. The reaction was stopped when the Weissenberg effect happened. A certain amount of the product was diluted by NMP, and the diluent was added into a certain amount of deionized water with the help of a high

ji m ‐m zy porosity = jjjj 2 1 zzzz × 100% j ρL V z k {

K

(1) DOI: 10.1021/acsnano.9b01955 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano where m1 and m2 represent the weight of the ANF aerogel and the combined weight of the aerogel with the absorbed n-butyl alcohol, respectively. V represents the volume of the ANF aerogel, and ρL represents the density of the n-butyl alcohol. In the calculation of the ρgel and the porosity, five samples were used to achieve an average value. The pore size and the size distribution, as well as the BET surface area, were analyzed with N2 adsorption and desorption using an automated surface area and pore size analyzer (Autosorb-iQ2-MP, Quantachrome). Thermogravimetric analysis was conducted by Q5000IR (TA Instruments) under a nitrogen atmosphere with a heating rate of 10 °C/min from 30 to 800 °C. The compression tests were carried out using a universal testing machine (WDW-100, Changchun Kexin Ltd.). The compression rate was 2 mm/min. The fatigue test was carried out with an electronic dynamic and static fatigue testing machine (Instron, E3000 V1.4) with a frequency of 1 Hz. A thermal conductivity meter (XIATECH TC3200, Xi’an Xia Xi Electronic Technology Co., Ltd.) was used to characterize the thermal conductivity of aerogels at 25 and 200 °C. The heat resistance test was carried out with a tube furnace (SK-G0612K, Tianjin Zhonghuan Experiment Electric Furance Co., Ltd.) under a nitrogen atmosphere with a heating rate of 5 °C/min from 30 to 500 °C, followed by a thermostatic process (500 °C) for 30 min. Temperature-dependent FTIR test was carried out by a Spectrum 100 FTIR spectrometer (PerkinElmer), and the detailed description of the test is in the “Changes of hydrogen bonds between ANFs at different temperatures” section of the Supporting Information.

Zhao-Xia Guo: 0000-0003-3019-9535 Teng Qiu: 0000-0002-0538-5531 Xinlin Tuo: 0000-0003-3220-5898 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful for the financial support by the National Basic Research Program of China (2011CB606102) and the funding of Tsinghua University−Chambroad Research Center for High Performance Polymers. The authors are grateful to Associate Professor Longhai Guo and Professor Xiaoli Sun of the College of Materials Science and Engineering, Beijing University of Chemical Technology, for their help with the temperature-dependent FTIR spectra characterization and the 2D-COS analysis. REFERENCES (1) Kistler, S. S. Coherent Expanded Aerogels and Jellies. Nature 1931, 127, 741. (2) Qian, F.; Lan, P. C.; Freyman, M. C.; Chen, W.; Kou, T.; Olson, T. Y.; Zhu, C.; Worsley, M. A.; Duoss, E. B.; Spadaccini, C. M.; Baumann, T.; Han, T. Y.-J. Ultralight Conductive Silver Nanowire Aerogels. Nano Lett. 2017, 17, 7171−7176. (3) Su, L.; Wang, H.; Niu, M.; Fan, X.; Ma, M.; Shi, Z.; Guo, S.-W. Ultralight, Recoverable, and High-Temperature-Resistant SiC Nanowire Aerogel. ACS Nano 2018, 12, 3103−3111. (4) Worsley, M. A.; Pauzauskie, P. J.; Olson, T. Y.; Biener, J.; Satcher, J. J.; Baumann, T. F. Synthesis of Graphene Aerogel with High Electrical Conductivity. J. Am. Chem. Soc. 2010, 132, 14067− 14069. (5) Katsoulidis, A. P.; He, J.; Kanatzidis, M. G. Functional Monolithic Polymeric Organic Framework Aerogel As Reducing and Hosting Media for Ag Nanoparticles and Application in Capturing of Iodine Vapors. Chem. Mater. 2012, 24, 1937−1943. (6) Pan, Z.-Z.; Nishihara, H.; Iwamura, S.; Sekiguchi, T.; Sato, A.; Isogai, A.; Kang, F.; Kyotani, T.; Yang, Q.-H. Cellulose Nanofiber As a Distinct Structure-Directing Agent for Xylem-Like Microhoneycomb Monoliths by Unidirectional Freeze-Drying. ACS Nano 2016, 10, 10689−10697. (7) Yang, X.; Cranston, E. D. Chemically Cross-Linked Cellulose Nanocrystal Aerogels with Shape Recovery and Superabsorbent Properties. Chem. Mater. 2014, 26, 6016−6025. (8) Si, Y.; Yu, J.; Tang, X.; Ge, J.; Ding, B. Ultralight NanofibreAssembled Cellular Aerogels with Superelasticity and Multifunctionality. Nat. Commun. 2014, 5, 5802. (9) Lu, Y.; Sun, Q.; Yang, D.; She, X.; Yao, X.; Zhu, G.; Liu, Y.; Zhao, H.; Li, J. Fabrication of Mesoporous Lignocellulose Aerogels From Wood via Cyclic Liquid Nitrogen Freezing-Thawing in Ionic Liquid Solution. J. Mater. Chem. 2012, 22, 13548−13557. (10) Zhu, C.; Han, T.-Y.; Duoss, E. B.; Golobic, A. M.; Kuntz, J. D.; Spadaccini, C. M.; Worsley, M. A. Highly Compressible 3D Periodic Graphene Aerogel Microlattices. Nat. Commun. 2015, 6, 6962. (11) Wu, Z.-Y.; Li, C.; Liang, H.-W.; Chen, J.-F.; Yu, S.-H. Ultralight, Flexible, and Fire-Resistant Carbon Nanofiber Aerogels from Bacterial Cellulose. Angew. Chem., Int. Ed. 2013, 52, 2925−2929. (12) Deuber, F.; Mousavi, S.; Federer, L.; Hofer, M.; Adlhart, C. Exploration of Ultralight Nanofiber Aerogels As Particle Filters: Capacity and Efficiency. ACS Appl. Mater. Interfaces 2018, 10, 9069− 9076. (13) Liu, Q.; Chen, J.; Mei, T.; He, X.; Zhong, W.; Liu, K.; Wang, W.; Wang, Y.; Li, M.; Wang, D. A Facile Route to the Production of Polymeric Nanofibrous Aerogels for Environmentally Sustainable Applications. J. Mater. Chem. A 2018, 6, 3692−3704. (14) Wu, Z.-S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Muellen, K. 3D Nitrogen-Doped Graphene Aerogel-Supported Fe3O4 Nano-

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b01955. Figures S1−S13 showing a TEM image of ANF at high resolution; FTIR spectra of ANF and DME; zoomed-in view of the “bridges”; wide-field-of-view SEM image of an intact sample (section size = about 1.6 cm × 1.0 cm); TGA and DTA curves of DME; evolution of the FTIR band for amide I (CO) and the intensity retention of CO at different temperatures; synchronous and asynchronous 2D-COS of ANF film in the wavenumber range of 1580−1720 cm−1; images of ANF aerogels before and after the thermal conductivity test at 200 °C; stress−strain curves of ANF aerogels before and after thermal conductivity test at 200 °C; typical stress−strain curves of samples 1−4; thermal conductivities of samples 1−4 (25 °C); schematic diagram of the directional freezing device; macrophotographs of ANF hydrogel and ANF aerogel samples; Tables S1−S5 giving specific compressive strength comparison of different kinds of aerogels (ε = 70%); specific compressive strength comparison of ANF aerogel and aromatic polyamide aerogel samples (ε = 10%); stress retention of ANF aerogel in the axial direction in the fatigue test (ε = 30%); thermal conductivity comparison of ANF aerogel and other porous materials (T = 25 °C); compressive stress comparison of samples 1−4 (ε = 70%) (PDF) Video S1: Fatigue tests of ANF aerogel (AVI)

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] (T.Q.). *E-mail: [email protected] (X.T.). ORCID

Chunjie Xie: 0000-0003-4842-2096 L


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