Superlight, Mechanically Flexible, Thermally Superinsulating, and

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Superlight, mechanically flexible, super thermally insulating and anti-frosting anisotropic nanocomposite foam based on hierarchical graphene oxide assembling Qingyu Peng, Yuyang Qin, Xu Zhao, Xianxian Sun, Qiang Chen, Fan Xu, Zaishan Lin, Ye Yuan, Ying Li, Jianjun Li, Weilong Yin, Chao Gao, Fan Zhang, Xiaodong He, and Yibin Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14604 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Materials & Interfaces

Superlight, mechanically flexible, super thermally insulating and anti-frosting anisotropic nanocomposite foam based on hierarchical graphene oxide assembling Qingyu Peng1,4*,&,Yuyang Qin1,&,Xu Zhao1, Xianxian Sun1,4, Qiang Chen1, Fan Xu1, Zaishan Lin1, Ye Yuan1, Ying Li1,4, Jianjun Li1, Weilong Yin1,4, Chao Gao2, Fan Zhang3, Xiaodong He1,4*, and Yibin Li1,4* 1. Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, P. R. China 2. MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Hangzhou 310027, P. R. China 3. School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, P. R. China 4. Shenzhen Strong Advanced Materials Institute Incorporation, Shenzhen 518057, P. R. China *Corresponding authors. Email: [email protected], [email protected], [email protected] &

These authors contributed equally to this work Abstract

Lightweight, high-performance thermally insulating, and anti-frosting porous materials are in increasing demand to improve the energy efficiency in many fields, such as aerospace, wearable devices. However, traditional thermally insulating materials (porous ceramics, polymer-based sponge) could not simultaneously meet these demands. Here, we propose a hierarchical assembling strategy for producing nanocomposite foams with lightweight, mechanically flexible, super-insulating and anti-frosting properties. The nanocomposite foams consist of highly anisotropic reduced graphene oxide/polyimide (abbreviated as rGO/PI) network and hollow graphene oxide spheres with micro size. The hierarchical nanocomposite foams are ultralight (density of 9.2 mg·cm-3), exhibit an ultra-low thermal conductivity of 9 mW·m-1·K-1, which is about one-third that of traditional polymer-based insulating materials. Meanwhile, the nanocomposite foams show excellent icephobic performance. Our results show that the hierarchical nanocomposite foams have promising applications in aerospace, wearable devices, refrigerator, liquid nitrogen/oxygen transportation. Keywords: thermally insulating, hierarchical, graphene oxide, anti-frosting, lightweight

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Introduction With the increasing focus on the improvement of energy efficiency,1 great efforts have been undertaken to design high-performance thermally insulating materials, especially in the application fields of aerospace and personal wearable devices.2, 3 However, the lightweight, thermally insulating and anti-frosting properties of traditional insulating materials could not simultaneously meet these demands. Considering the urgent requirement for super thermally insulating materials, pursuing lightweight, mechanically flexible, anti-frosting, and super-insulating materials with ultra-low thermal conductivity (λ) is becoming vitally important. Numerous efforts have been made to fabricate thermally insulating materials. Traditional biopolymer-based materials (such as natural wood) were modified and subsequently used as insulating materials. However, the density and thermally insulating performance of the wood composite are unsatisfying due to a high level of density (greater than 600 mg·cm-3) and thermal conductivities (λ mW·m-1·k-1, λ

radial

axial

= 320

= 150 mW·m-1·k-1).4 Traditional insulation materials such as

polyurethane (λ= 20-30 mW·m-1·k-1), porous aramids (λ= 28 mW·m-1·k-1), expanded polystyrene (λ= 30-40 mW·m-1·k-1), and polymer/clay aerogel (λ= 45 mW·m-1·k-1) also exhibit high thermal conductivities.5-8 The emergence of nanomaterials like nanotubes, nanowires, nanorods and nanosheets have opened a new route to fabricate more efficient insulating materials.9-11 Importantly, these nanosized one- and two-dimensional (1D and 2D) materials can significantly reduce solid heat conduction on account of phonon scattering effects.12,13 Previous studies have shown that silica aerogels can be used as nanoporous insulating materials with a low thermal conductivity (λ= 17-21 mW·m-1·k-1), but their fragile mechanical performance are susceptible to thermal insulation failure.14 Besides, anti-frosting property of insulation materials should be taken into consideration, especially in the humid and cold environment.15 A challenge, however, for the thermal insulation is to explore versatile and accessible approaches to fabricate efficient insulating materials with lightweight,


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mechanically flexible, super-insulating and anti-frosting properties. Here, we propose a hierarchical assembling strategy to produce lightweight, mechanically flexible, highly orientated and super-insulating graphene-based nanocomposite foams with hollow spherical structures. This insulating monoliths are facilely fabricated by freeze-casting of graphene oxide (GO)16 and water-soluble polyimide (PI) precursors,17 using modified polystyrene (PS) spheres as sacrificial templates.18 The introduction of modified PS templates can generate hollow spherical structures on the honeycomb walls of reduced graphene oxide/polyimide (rGO/PI) nanocomposite, which can strongly impede the thermal transport and lead to an ultra-low thermal conductivity. Meanwhile, GO nanosheets contribute to the low-density of insulating monolith. PI is incorporated as a reinforcement for improving the low mechanical strength of GO aerogels, and also enhancing thermal stability.19,

20

21

structures.

Freeze-casting endows the nanocomposite foams with hierarchical

Particularly, the strong orientation in the resulting structures offered

exceptionally anisotropic thermally insulating and mechanical properties.22 The remarkable properties of GO and PI, together with hierarchical structures with hollow units, endow the nanocomposite with great potential applications in the field of thermal insulation.

Results and Discussion The synthetic route to anisotropic rGO/PI nanocomposite foams with hollow spherical structures is schematically illustrated in Figure 1. Based on the LBL self-assembly technique, PS spheres that were modified with aqueous suspensions of polyethylenimine (PEI) and GO nanosheets, acted as the sacrificial templates for the graphene-based hollow spheres.18 Subsequently, an aqueous suspension of GO, water-soluble PI precursor and modified PS spheres was subjected to freeze-casting in a liquid nitrogen bath, followed by freeze drying and thermal annealing under argon (Ar) atmosphere. During the thermal annealing process, water-soluble PI precursor was imidized into PI and GO was reduced to rGO.17, 23 Simultaneously, PS spheres employed as the sacrificial templates in core/shell composite can be removed to form the hollow spherical structures on the cell walls of the nanocomposite. Finally, a



lightweight, mechanically flexible, anti-frosting and hierarchically structured rGO/PI nanocomposite with hollow spherical structures was obtained. To further investigate the property of thermally insulating, rGO/PI nanocomposite foams with different amounts of modified sacrificial templates were fabricated for comparison by the same procedure. The structure and morphologies of the graphene-based hollow sphere precursor and the obtained nanocomposite foams were systematically characterized using microscopy techniques by a combination of scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Figure S1a displays a typical SEM image of bare PS spheres that exhibit the smooth spherical surface with homogenous size ranging from 1µm to 1.5 µm. After the self-assembling of PEI on PS spheres, the as-prepared precursors still exhibited the original morphology without any distinct features (Figure S1b). In contrast, the PS spheres modified by GO nanosheets revealed a spherical surface with a notable rough texture, manifesting the successful encapsulation of GO nanosheets on the polymer templates (Figure S1c). After heat treatment in Ar atmosphere, the PEI moiety was removed and the templates of PS were decomposed into exhaust gas, which resulted in hollow spheres as secondary structures on the walls of the continuous 3D network (Figure 2e). In addition, there are some occasional ruptured hollow spherical structures on the cell walls of the thermal treated nanocomposites, further proving the existence of binary hollow spherical configuration (Figure 2f, Figure S1d). In order to further identify the resulting spherical structure of graphene-based hollow spheres in the nanocomposite, TEM analysis was adopted. Compared with modified PS spheres in the nanocomposite before thermal treatment (Figure 2g), TEM image clearly revealed the hollow spherical configuration with graphene-shelled structure, indicative of the removal of the PS templates (Figure 2h). Corresponding TEM observations also showed the characteristics of wrinkles and folds on the rough spherical surface of the hollow structure, indicating the encapsulation of rGO nanosheets. Moreover, high-magnification TEM imaging reveals the uniform size of shell layers ranging


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from 40 to 50 nm (Figure S1f). Microstructural analysis of the thermally insulating nanocomposite foam exhibits significant anisotropic microstructures,24 which can be vividly defined as a structure in the direction along (A direction) and perpendicular to (P direction) the growth direction of ice, respectively (Figure 2). As illustrated in Figure 2a, typical top-view of the nanocomposite shows a honeycomb-like structure with aligned tubular pores, in which the micro-channels are along the direction of ice front. The pore dimensions of the cellular structure are estimated to be tens of micrometers with a cell wall thickness in the order of 0.1-0.3µm (Figure 2c). Furthermore, it can be clearly seen from the SEM images that the longitudinal section of the nanocomposite reveals ordered cell walls in an almost parallel manner (Figure 2b), which are similar to vertically aligned channels in wood.25 Upon magnifying a well oriented columnar microstructure of the nanocomposite, it is discovered that the channel size is approximate 25µm, which is in agreement with the cell dimension in the A direction (Figure 2d). The high magnification SEM images of the monolith in different directions reveal that the GO modified hollow spherical configurations are attached to the cell walls of the nanocomposite (Figure 2c, d and Figure S1e). These binary hollow structures, resembling “air pockets” in the layered structure of the free-standing rGO films, can strongly impede thermal transport between the cell walls in the P section.26 With the increasing amounts of sacrificial templates, it can be observed from SEM images that more hollow spherical microstructures are attached to the cell walls of the nanocomposites (Figure S2). The possible chemical bonding between the modified PS spheres and the graphene-based thermally insulating monolith were investigated by Fourier transform infrared (FT-IR) studies. As depicted in Figure S3a, the FT-IR spectrum of the PS spheres coated with PEI and GO shows three characteristic peaks, which are attributed to C-H aromatic stretch vibration (3081-3000 cm-1), C-C aromatic stretch (1470 cm-1), and C-H out of plane bending vibration (765 cm-1). As for the hollow spheres, the prominent characteristic absorptions bands of PS are absent, reflecting



the successful decomposition of the polymers after thermal treatment. This can be further confirmed by the FT-IR spectrum of the thermally insulating nanocomposite foams (Figure 3a). The prominent characteristic peaks of the nanocomposite with hollow spherical structures (1722 cm-1 for C=O stretching vibration, 1500 cm-1 for C=C stretching vibration, and 1377 cm-1 for C-N stretching vibration) are consistent with the rGO/PI nanocomposite.17 Furthermore, the characteristic absorptions of the oxidation groups of GO decrease sharply, which implies the partial reduction of GO.23 X-ray photoelectron spectroscopy (XPS) was performed to investigate the chemical composition and morphology of the graphene-based thermally insulating monolith. In the wide-scan XPS survey spectroscopy (Figure S3b), the significant peaks located at 284.6 eV, 400 eV, and 532 eV were observed in the nanocomposite, corresponding to C 1s, N 1s and O 1s, respectively. A Gaussian fit to the C 1s spectrum of the monolith exhibits four peaks at 284.6, 285.5, 286.3, and 288.6 eV, which is assigned to sp2 and sp3 C, C-N single bond, C-O single bond, and C=O double bond, respectively (Figure 3b).17 To evaluate the thermal stability of the modified PS spheres and the graphene-based thermally insulating monolith, thermogravimetric analysis (TGA) data was collected. For pristine PS spheres, it can be seen from the TGA curve that the onset temperature for decomposition is around 367 °C. However, after encapsulated by the rGO nanosheets, the modified PS spheres exhibit an increase in thermal stability with the onset temperature of degradation at 390 °C (Figure S3c). The process of decomposition continues until 455 °C with the residual weight of approximate 1%, indicating that the binary hollow spherical configuration can be obtained upon facile removal of the sacrificial polymer templates by thermal annealing. Compared with rGO/PI sponge, the nanocomposite exhibits a similar pattern of weight loss. While the onset decomposition temperature (around 460 °C) is higher than that of the rGO/PI nanocomposite, suggesting that the addition of hollow spherical structures enhances its thermal stability to a certain extent (Figure 3c). To access the potentially anisotropic mechanical properties of the thermally insulating monolith, a set of compressive tests was performed in different directions.


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As revealed in Figure 3 panels d and e, the compressive stress (σ) of the monolith that loaded in the A direction and P direction, was measured as a function of strain (ε) at a series of set strains (10, 30, 50%).27 Notably, the anisotropic nanostructure of the monolith leads to significant differences in stress-strain (σ-ε) curves. Regarding the loading process of the A direction deformation, three characteristic deformation regions can be observed in the σ-ε curves: a nearly linear elastic region (ε 30%) with stress rising steeply, due to the impingement of the cell walls. This distinct stages are similar to the open-cell and other elastomeric foams.28, 29 In contrast, the σ-ε diagram in the P direction can be divided into two stages with a Hookean elastic regime at ε < 20% followed by an abrupt stress increasing regime. The hysteresis loop obtained during the process of loading and unloading revealed a typical σ-ε curve, which has been observed in foam-like CNT and graphene materials.30, 31 Furthermore, the compressive stress in the A direction is relatively higher than that in the P direction at the same strain (16 vs 8.5 kPa at ε = 50%), which can be ascribed to the significantly higher stiffness and strength of the honeycomb structures along the cellular axis. The σ of unloading curves can almost returns to their original points for each ε, indicating the rapid and complete recovery of the nanocomposite. Such behaviors are consistent with the digital images of the recovering process after unloading, which further reveal excellent mechanical flexibility and resilience of the thermally insulating monolith (Figure 3f). In addition, the surface wettability of the nanocomposite were investigated by the contact angle measurement. Water droplets exhibited in a near-spherical shapes on the nanocomposite interface with a contact angle of 138°, suggesting its intrinsic hydrophobicity (Figure 3g, 3h).28 The anisotropic thermal properties, which is along and perpendicular to the tubular pores of the nanocomposite foam, were investigated using the transient plane source (TPS) technique. The strong orientation effect of the controlled structures and the amounts of hollow spherical structures can engender significant anisotropic



thermal-insulating properties, which can be observed from the apparent thermal conductivities in different orientations of the rGO/PI nanocomposites with different content of hollow spherical configurations. As depicted in Figure 4a, the thermal conductivity in the P direction is remarkably lower than that in the A direction for the above-mentioned nanocomposites. This interesting feature is attributed to the anisotropic cellular architecture and the integration of orientation-dependent radiation and solid conduction in the cellular channels,32 together with the possible natural convection along the honeycomb cell axis direction.33 With the introduction of hollow spherical configurations to the cellular walls of the rGO/PI nanocomposite, the thermal conductivity in the P direction decreased from 19 mW·m-1·K-1 to as low as 9 mW·m-1·K-1. This reduction in the value of thermal conductivity can be ascribed to the appearance of the unique hollow structures attached to the cell walls, which are analogous to the “air pockets” structures formed by the heat treatment in the rGO films.26 The thermal transport in the P direction is significantly impeded by the hollow spherical structures in the cellular walls. However, there is no obvious impact on the thermal conduction in the A direction. Furthermore, the phonon scattering effects play an important role in reducing the solid conduction of the nanosized materials in the thermally insulating nanocomposite foam.9 This result is in agreement with the conjecture that the thermal transport in the P direction of the nanocomposite monolith is primarily affected by the microstructure of the cell walls. Compared with the rGO/PI nanocomposite, the thermally insulating rGO/PI foam with hollow structures exhibits higher thermal conductivity in the A direction. This may attributed to the introduction of graphene nanosheets shells of the hollow structures and residue of the PS spheres after heat treatment. In view of the comparison between rGO/PI nanocomposite with hollow spherical structures and other insulating materials on thermal conductivity and density, the nanocomposite exhibits far better thermally insulating capacity (Figure 4b). The schematic diagram concerning the synergistic effect of conduction (gas conduction λ and solid conduction λ), convection (λ ) and radiation (λ ) to


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the thermal transport of the thermally insulating nanocomposite foam is vividly illustrated in Figure 4c.22 The thermal conduction of the graphene-based thermally insulating nanocomposite can be divided into two parts: the gas conduction (λ) and the solid conduction (a weighted average of the effective solid conduction values ∗ of the individual components of the thermally insulating monolith). The λ within the channels of the nanocellulose based foams can be determined from the following equation λ =

(1)

Where λ is defined as the gaseous conductivity in free space (25 mW·m-1·K-1), Π is the porosity of the monoliths, and (≈ 2) is the air in the foams. K denotes the Knudsen number K = Where

!

(2)

and " are the mean free path of a gas molecule and the pore diameter,

respectively.

!

is estimated to be 10 nm in the cell walls of the thermally insulating

monolith, while that value is 75 nm in free space. The significant decrease in -1

-1 34

markedly reduces the value of gaseous conductivity (below 1 mW·m ·K ).

!

The

effective solid conduction values ∗ is given by ∗ =

#

%$(3)

& #$% ' (

Where λ and d are defined as the solid conduction values and the particle size of the various components of the thermally insulating nanocomposite. ) , namely Kapitza resistance,35 denotes the interfacial thermal resistance of the individual components (GO and PI) of the nanocomposite (Table S1).22,

36

As for the bulk

composite material, whose composition is identical to the thermally insulating nanocomposite, the overall thermal conductivity is estimated to 3000 mW·m-1·K-1. Notably, the phonon scattering effects in the nanostructured composite foam lead to a significant reduction in the solid conduction of the cell walls, as is estimated by equation (3). When calculated by a weighted average of the ∗ value of the individual components, the thermal conductivity of graphene-based thermally ACS Paragon Plus Environment


insulating nanocomposite reduces to as low as 40 mW·m-1·K-1. This value is of the same order of magnitude as the measured value. The anisotropic thermally insulating property of the nanocomposite foam in different directions was visually illustrated by the thermographic images (Figure 4d). Temperature distribution in the P direction is significantly different from that in the A orientation. As for the nanocomposite foam with the cellular walls oriented normal to the heat source, a highly ordered honeycomb-like cell structure with hollow spherical configuration can endow the foam with efficient thermal insulation in the P direction. The anti-frosting property of the nanocomposite in a cold and humid environment was investigated using an open-air cold plate equipment. As depicted in Figure 4e, a metal block and an insulating nanocomposite were placed on the surface of the cold plate. A water droplet froze in 15s when dripped on the surface of the metal block, while the same volume water droplet remained a near-spherical liquid shapes on the surface of nanocomposite. In addition, the metal block was coated with ice from the water vapor in the air after 24h, while the nanocomposite remained its original morphology, showing excellent anti-frosting property (Figure 4f). This confirms that the rGO/PI nanocomposite foams are applicable and effective as a high-performance thermally insulating material with excellent anti-frosting property in the cold and humid environment.

Conclusion In summary, we have shown that highly anisotropic, porous nanocomposite foam composed of hollow rGO nanospheres, rGO/PI network not only display super-low density of 9.2 mg·cm-3, ultralow thermal conductivity of 9 mW·m-1·K-1. Besides, the nanocomposite foams demonstrate excellent mechanical flexibility and good icephobicity, which is very useful when applied in the cold and humid environment. The thermal conductivity of our nanocomposite foams is substantially lower than traditional insulating polymer-based materials. The high-performance thermally


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insulating property is contributed to highly anisotropic structure and the addition of hollow GO nanospheres. This novel GO-based architectures may provide new insights into the improvement of thermal insulation materials and our nanocomposite foam with anisotropic heat conduction properties would be enormously beneficial for the future applications in thermal management.

Experimental Section Material PEI was purchased from Alfa Aesar and used as received. PS spheres were prepared as previously reported.37 GO nanosheets were synthesized based on a modified Hummers method from expandable graphite (EG) powder.16 Water-soluble polyimide precursor solution was prepared using our previously reported method.17 Fabrication Procedure of Graphene-based Hollow Sphere Precursor Graphene-based core/shell composites were synthesized by the LBL self-assembly method.18 PS spheres and GO nanosheets were employed as templates and inorganic shell building blocks respectively. In a typical protocol, 1 g of PS spheres were homogeneously dispersed in 200 mL of PEI suspension (2.5 mg·mL-1) at pH 9.0 under vigorous stirring. The obtained suspension was then treated by sonication for 15 min to enhance the adsorption of PEI to the surface of PS spheres. Next, the PEI-coated PS spheres were obtained by centrifugation and wash cycles with deionized water. Then, a portion of GO suspension (0.025 mg·mL-1) at pH 9.0 was added to the PS suspension (5 mg·mL-1) under stirring. The resulting precipitate was suffered by a process of separation and washing. The assembly procedure between PEI and GO was repeated to obtain PS spheres with different bilayers of (PEI/GO). The obtained product was dispersed in deionized water by sonication for further use. Preparation of rGO/PI Nanocomposite with Hollow Sphere Structure The rGO/PI nanocomposite with graphene-based hollow spheres was fabricated by means of two steps of Freeze-casting and thermal annealing method. Freeze-casting,



as a versatile and simple solution-phase materials shaping technique, can be used to fabricate a sophisticated and continuous three-dimensional (3D) network with highly anisotropic cellular architectures.21 During the controlled unidirectional freezing process, ice crystals present strong anisotropic growth kinetics, which are pronounced along the movement of the water-ice front. Meanwhile, the solid particles in suspension were concentrated and subsequently piled up within channels between the neighboring crystal boundaries, leading to the formation of a highly ordered lamellar assembly parallel to the direction of the moving solidification front. After sublimation of the ice by freeze drying, an anisotropic cellular architecture was produced, whose microstructure is a negative replica of the unidirectional oriented ice crystals template.21 The detailed fabrication procedure is as follows. The PEI-coated PS spheres (500 mg) were added to the mixed aqueous solution of GO (5 mg·mL-1) and water-soluble polyimide precursor (20 mg·mL-1) under stirring. Then, the composite suspensions were subjected to freeze-casting by controlled freezing in Teflon molds immersed in a liquid-nitrogen bath. The obtained monolith was treated by freeze drying for 36 h followed by thermal annealing at 460 °C in argon atmosphere for 2h.

Supporting Information Further SEM analysis and additional characterization data. The Supporting Information is available free of charge via the Internet at http://pubs.acs.org..

Author Contributions &

These authors contributed equally.

Acknowledgments This work was financially supported by the National Natural Science Foundation of China (NSFC, Grant No. 51503052, 51772063), the Young Elite Scientists Sponsorship Program by CAST (Grant No.2016QNRC001), the Fundamental


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Research Funds for the Central Universities (Grant No. HIT. BRETIII.201507), the China Postdoctoral Science Foundation (Grant No. 2015M580259 2016T90281) and the Heilongjiang Postdoctoral Fund (Grant No. LBH-Z15058).

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Figure 1. Structural design of the anisotropic thermally insulating nanocomposite foam with secondary hollow spherical configuration. Schematic illustration of the fabrication process of LBL assembly modified PS spheres consisting of PEI and GO nanosheets and the formation mechanism of the nanocomposite foam by freeze-casting. The aqueous suspension of GO, water-soluble PI precursor and modified PS spheres were directional freezing in a liquid nitrogen bath, followed by thermal annealing to reduce GO and generate hollow spherical structures.



Figure 2. Microstructure of the thermally insulating nanocomposite foams. (a, b) SEM images of the nanocomposite foams (top-view and side-view). (c, d) High-magnification SEM images of the nanocomposite foams in different directions. (e, f) SEM images of the secondary hollow spherical structures. (g, h) TEM images of the sacrificial PS templates in the nanocomposite foams before and after thermal treatment.


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Figure 3. Chemical structure and mechanical properties of the thermally insulating nanocomposite foam. (a-c) FT-IR, XPS, and TGA data of the nanocomposite foam. (d, e) Stress-strain curves of the nanocomposite foam with different set strains in the A and P directions. (f) Digital images exhibit the recovering process of a compressed nanocomposite foam. (g) A photograph of a water droplet (labeled by red ink) on the surface of nanocomposite foam. (h) Contact angle measurement of the nanocomposite, which exhibits high hydrophobicity with a contact angle of 138°.



Figure 4. Thermal insulation properties of the anisotropic nanocomposite foam. (a) The comparison of the thermal conductivities in the A and P directions of rGO/PI nanocomposite with different contents of modified sacrificial PS spheres. Inset: schematic illustrations of the measurement with the sensor and insulating samples. (b) Comparison of thermal conductivity and density of the thermally insulating nanocomposite foam with other selected insulating materials. (c) Schematic illustration of the synergistic effect of conduction, convection and radiation to the thermal conductivity of the foam in different directions. (d) Thermographic images of the foam in the A orientation and P orientation with corresponding schematic illustrations (insert pictures). (e, f) Photos of anti-frosting property of the nanocomposite foams under cold and humid environment (compared with an Aluminum block).


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Superlight, Mechanically Flexible, Thermally Superinsulating, and

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