Aerogels are three-dimensional porous materials with high porosity, high specific surface area, and low density. They possess many unique mechanical, thermal, electrical, and chemical properties, and show promising applications in thermal insulation, adsorption, sensors, catalysis, energy storage, etc.[1–3] Various flexible aerogels including compressible, bendable, and stretchable aerogels have been developed by optimizing their porous microstructures.[4–6] Among them, stretchable aerogels are especially attractive because of their potential applications in flexible strain/pressure sensors, stretchable conductors, flexible batteries, flexible supercapacitors, stretchable electromagnetic interference (EMI) shielding materials, stretchable thermal management materials, etc. [6–8] While the compressible and bendable aerogels have been extensively studied and reported, there are much fewer reports on stretchable aerogels. Many specially designed structures such as cellular and layered structures can endow aerogels with high compressibility and elasticity.[4,5,7,8,9−12] However, the highly porous structures of aerogels usually tend to be broken when they are stretched. It is a great challenge to achieve highly stretchable aerogels.
Some chemically or physically crosslinked polymer aerogels are stretchable benefiting from the excellent deformability and recoverability of the flexible polymer networks. The reported poly(isocyanurat-urethane) aerogels could be stretched 120% without fracture. [13] The stretchable polymer aerogels based on poly(caprolactone) exhibited elongations at break of approximately 80–275%. [14] The stretchable (25% elongation at break) polyimide aerogel fibers have been developed by a sol-gel confined transition method for thermal insulation.[15] The regenerated stretchable all-cellulose aerogel fibers with a graded aligned nanostructure showed a maximum tensile strain of 20–50%.[16] The cellulose nanofibers/polyurethane (PU) aerogels exhibited an elongation at break of 10–25%.[17] The chemically crosslinked poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) networks could endow the PEDOT:PSS aerogels with good stretchability with an elongation at break of 10–100% and a reversible tensile strain of 40–60%.[18]
The combination of flexible polymers and inorganic building blocks such as graphene and MXene can afford stretchable organic-inorganic hybrid aerogels. The reported MXene/polyimide aerogels exhibited an elongation at break of 25–35% and a reversible tensile strain of 20%.[19] The “layer-strut” skeletons of reduced graphene oxide (rGO)/polyimide facilitated load transfer between rGO sheets and polyimide during deformation, allowing the rGO/polyimide aerogels to exhibit an elongation at break of 16%. [20] The aerogel fibers based on aramid nanofiber/carbon nanotube (CNT) could withstand a 20–26% tensile strain. [21] Besides, we have reported stretchable (~ 100% elongation at break) rGO/PU aerogels with a gradient porous structure for high-performance pressure-sensitive wearable electronics. [22]
Special porous microstructures such as lamellar and honeycomb-like structures may effectively reduce stress concentration and contribute to stress transfer, which may make the resultant aerogels stretchable. Carbon aerogels with a long-range lamellar multi-arch porous structure obtained by bidirectional freezing can achieve a maximum tensile strain of 80%. [23] Graphene aerogels with a hyperboloid structure by hydroplastic foaming exhibited a break elongation of ~ 20%.[24] Graphene aerogels with a highly crimped and crosslinked network could achieve a reversible elongation of 400%. [25]
The aerogels with fibrous structures may be stretchable because of the flexible fibers that compose the networks. Nanofibrous polymer aerogels with a hyperconnective network of aramid nanofiber composites were stretchable (20–30% elongation at break) and could be used for thermal insulation, wearable electronics, and filtration.[26] The silica nanofiber aerogels prepared by combining electrospinning and freeze drying possessed good stretchability (~ 220% elongation at break) and showed potential applications in thermal insulation and radiative cooling. [27] The ceramic aerogels composed of curly SiC-SiOx nanofibers showed a reversible elongation of 20%.[7] The interwoven crimped nanofibrous structure endowed the ceramic nanofibrous aerogels with good stretchability up to 100% tensile strain (40% reversible tensile strain).[28] Stretchable (withstand a 40% tensile strain) hypocrystalline zircon nanofibrous aerogels with a near-zero Poisson’s ratio have been achieved by constructing a zig-zag architecture.[29] The Si3N4 nanofiber sponge with an interlocked nanostructure showed a break elongation of ~ 80% and a reversible elongation of 20%.[30]
Some special macroscopic structures such as the structures of lattice, serpentine, spring, and wrinkle show high deformability and elasticity, which can make the aerogels with these kinds of structures stretchable. The stretchable graphene/CNT aerogel lattices prepared by ink-printing possessed a reversible 200% elongation and could be used as strain sensors. [6] The shape memory polymer/graphene aerogel lattices could withstand a 100% tensile strain. [31] While the graphene aerogels with a serpentine structure showed high stretchability up to 1200% tensile strain, the same graphene aerogels without the special macroscopic structures only exhibited an elongation at break of 6%, showing low intrinsic stretchability.[8] The intrinsic stretchability of the reported aerogels is far from satisfactory. The elongations at break and reversible elongations of the reported intrinsically stretchable aerogels were usually lower than 200% and 100%, respectively. To our knowledge, there is no report on intrinsically stretchable aerogels with reversible elongations higher than 400%. In addition, the elasticity and durability of the stretchable aerogels need to be further improved.
Metamaterials with a negative Poisson’s ratio contract laterally when compressed and expand laterally when stretched, which can exhibit excellent deformability and toughness.[29,32−34] They have attracted a lot of attention for their unusual mechanical properties and promising applications in shock-absorbing materials, air filters, fasteners, etc. Recently, aerogels with negative Poisson’s ratios during compression have been reported. The elastic ceramic aerogels with a hyperbolic architecture showed negative Poisson’s ratios during compression. [4] Compressible polyimide aerogels with negative Poisson’s ratios could be achieved by directional and tridirectional freezing strategies. [35, 36] Graphene aerogels with Poisson’s ratios in the range of -0.95-1.64 during compression can be obtained by constructing meta-structures via laser engraving. [8] However, to our knowledge, there is no report on intrinsically highly stretchable aerogels with negative Poisson’s ratios during stretching.
Herein, we report unprecedented intrinsically highly stretchable rGO/polymer nanocomposite aerogels with low or negative Poisson’s ratios achieved by uniaxial, biaxial, and triaxial hot-pressing strategies. Highly compressible aerogels with positive Poisson’s ratios can be converted into super-stretchable meta-aerogels with zero or negative Poisson’s ratios via these hot-pressing strategies. The uniaxially hot-pressed aerogels with compressed and folded porous structures exhibited record-high stretchability with an elongation at break of 1250% and a reversible elongation larger than 800%. Besides, the meta-aerogels with reentrant porous structures obtained by biaxial (or triaxial) hot pressing showed high biaxial (or triaxial) stretchability and a negative Poisson’s ratio. To our knowledge, this is the first time to achieve negative Poisson’s ratios upon stretching for intrinsically highly stretchable aerogels. We demonstrated that the resulting aerogels could be applied for ultrabroad-range-response strain (0-1200%) and pressure (0-9.5 MPa) sensors. In addition, they could be applied for smart thermal management and EMI shielding, which were achieved by regulating the porous microstructures simply via stretching. This work opens a new way to highly stretchable and negative-Poisson-ratio aerogels with new application possibilities in flexible electronics, thermal management, EMI shielding, energy storage, etc.
Aerogel preparation
The foldable Chinese lantern can be both highly compressed and stretched because of its foldable accordion- or honeycomb-like structure (Fig. 1a). Inspired by the foldable lantern, highly stretchable rGO/polymer nanocomposite aerogels have been developed via uniaxial, biaxial, and triaxial hot-pressing strategies (Fig. 1b-d). Graphene oxide (GO) was used as the raw material of rGO, while PU foam (PUF), PU (dispersed in water), polyvinyl alcohol (PVA), or melamine foam (MF) was used as the toughening polymer (Fig. S1). Ethanediamine or (3-aminopropyl)triethoxysilane was used as the crosslinker and reductant of GO. Polypyrrole (PPy) was introduced in the aerogels via in-situ oxidation polymerization of pyrrole to further enhance their electrical conductivities. The aerogels were prepared via either freeze drying or ambient pressure drying (APD). The starting compositions of typical rGO/polymer aerogels were listed in Tables S1-S3. The reaction and microstructure variation during preparation were schematically presented in Fig. 1b and Fig. S2.
Hot pressing of the rGO/polymer aerogels was readily performed using a home-made apparatus (Fig. 1c, Fig. S3-S8). In the case of uniaxial hot pressing, a monolithic rGO/polymer aerogel was first sandwiched with two pieces of glass and then compressed with 66.7–87.5% strain in x direction using two other pieces of glass, followed by fixing and heat treatment at 120 or 140 ℃ (Fig. S3 and Fig. S6). In the case of biaxial hot pressing, a monolithic aerogel was first compressed 50% in x direction and then compressed 50% in y direction, followed by fixing and heat treatment (Fig. S4 and Fig. S7). In the case of triaxial hot pressing, a monolithic aerogel was first compressed 50% in z direction and then compressed 50% in both x and y directions, followed by fixing and heat treatment (Fig. S5 and Fig. S8). After hot pressing, the aerogels maintained the compressed shapes without springing back when they were released.
Structural characterization
The appearances of typical rGO/polymer aerogels before and after hot pressing were shown in Fig. 2a-h and Fig. S6-S8. The rGO/polymer aerogels obtained by introducing different toughening polymers (PUF, PU, PVA, and MF) exhibited highly porous structures with different morphologies (Fig. 2i-t, Fig. S9-S11). The interconnected rGO nanosheets were observed in the aerogels rGO/PPy/PUF1, rGO/PPy/PUF2, and rGO/PVA/MF (Fig. S9,S10). Besides, there are many irregular spherical particles on the skeletons of the aerogels rGO/PPy/PUF1, rGO/PPy/PUF2, and rGO/PPy/PU, which were well preserved after hot pressing (Fig. 2i-p and Fig. S10). These particles are supposed to be PPy polymers, which were deposited by in-situ oxidation polymerization of pyrrole.
The uniaxially hot-pressed aerogels (rGO/PPy/PUF1-UHP, rGO/PPy/PUF2-UHP, rGO/PPy/PU-UHP, and rGO/PVA/MF-UHP) exhibited compressed and folded porous structures in the xy and xz planes with smaller pores compared with those of aerogels without hot pressing (Fig. 2j,n,q and Fig. S9-S11). The biaxially hot-pressed aerogels (rGO/PPy/PUF1-BHP and rGO/PPy/PU-BHP) presented reentrant porous structures in the xy plane and compressed and folded porous structures in the xz and yz planes, which are quite different from those of the aerogels without hot pressing (Fig. 2k,o,r and Fig. S11). The triaxially hot-pressed aerogels (rGO/PPy/PUF1-THP and rGO/PPy/PU-THP) showed the similar compressed and reentrant porous structures in the three planes (xy, xz, and yz planes) (Fig. 2l,p,s,t).
Softening and plastic deformations of the polymers (PUF, PU, PVA, or MF) in the skeletons of the rGO/polymer aerogels would occur when the aerogels were compressed and heated at 120–140 ℃. The shapes of the polymers would be fixed permanently after the aerogels were cooled to room temperature. Therefore, the shapes of the aerogels could be fixed without springing back when they were released after hot pressing. Besides, as shown in the optical microscope and SEM images (Fig. 2 and Fig. S9-S11), new contacts between the neighboring pore walls of the aerogels could be produced after hot pressing, which would result in further physical crosslinking (such as hydrogen bonds) at these contact points. This may also contribute to the fixing of the aerogels after hot pressing.
The chemical structures of the rGO/polymer aerogels were investigated by X-ray diffraction (XRD) patterns, Raman spectra, and Fourier transform infrared (FTIR) spectra (Figure S12). These investigations confirmed the successful incorporation of rGO and PPy in the rGO/polymer aerogels. [20, 22, 37, 38] Detailed analyses on chemical structures are shown in supplementary information.
Mechanical properties
Benefiting from their unique porous structures, the hot-pressed rGO/polymer aerogels exhibited high stretchability and high elasticity (Fig. 3 and Fig. S13, S14). The uniaxially hot-pressed aerogels rGO/PPy/PUF1-UHP and rGO/PPy/PUF2-UHP exhibited high stretchability with elongations at break of 810% and 1250%, respectively, and reversible elongations larger than 700% and 800%, respectively (Fig. 3a,d,e, Fig. S13, Movies S1-S4). After stretching-releasing with 500% strain for 1000 cycles, rGO/PPy/PUF1-UHP nearly recovered its original shape, indicating the excellent elasticity and fatigue resistance of the aerogels (Fig. 3f). rGO/PPy/PU-UHP and rGO/PVA/MF-UHP also exhibit high stretchability in x direction with elongations at break of 470% and 112%, respectively, and reversible elongations of 400% and 100%, respectively (Fig. 3d,g and Fig. S14). The stretchability of the hot-pressed aerogels was significantly higher than that of the pristine aerogels without hot-pressing (Fig. S15, S16).
In addition, the biaxially hot-pressed rGO/polymer aerogel possessed high biaxial stretchability. For example, rGO/PPy/PUF1-BHP exhibited elongations at break of 335–340% and reversible elongations of 300% in both x and y directions (Fig. 3b,d,h and Movies S5,S6). Furthermore, the triaxially hot-pressed rGO/polymer aerogels presented high triaxial stretchability. The elongations at break of rGO/PPy/PUF1-THP were in the range of 310–340% in x, y, and z directions (Fig. 3d, Movie S7).
In order to further evaluate the stretchability of the hot-pressed aerogels, the theoretical values of the elongations at break of typical aerogels were calculated. The calculation method and results were illustrated in Fig. S17-S19. The calculated theoretical values of the elongations at break of rGO/PPy/PUF1-UHP, rGO/PPy/PUF2-UHP, and rGO/PPy/PUF1-BHP were 820%, 1308%, and 360%, respectively, which were generally consistent with the measured values (810%, 1250%, and 335–340%, respectively).
There are mainly three reasons that probably contribute to the high stretchability and elasticity of the hot-pressed rGO/polymer aerogels. First, the flexibility of the polymers (PUF, PU, PVA, or MF) endow the rGO/polymer networks with excellent reversible deformability, which can allow the aerogels to be compressed with a large strain without fracture during hot pressing. Second, the compressed rGO/polymer networks can be fixed without springing back after being cooled to room temperature. Third, the obtained folded or reentrant porous rGO/polymer structures after hot pressing are highly stretchable.
To our knowledge, the stretchability of our aerogels significantly surpassed all those of the reported intrinsically stretchable aerogels (Fig. 3i and Table S4). The maximum tensile strains of all the previously reported intrinsically stretchable aerogels were no larger than approximately 400%.[7,13−30] Moreover, the stretchability of our aerogels were higher than those of the reported intrinsically stretchable foams and sponges based on inorganic building blocks [39] and various polymers including polydimethylsiloxane (PDMS) [40–43], PU [44–49], poly(vinylidene fluoride) (PVDF) [50], and polyacrylate [51] (Fig. 3j and Table S5).
Because of their unique compressed and folded porous structures, the uniaxially hot-pressed rGO/polymer aerogels showed zero or low Poisson’s ratios (ν) over a wide range of tensile strains during stretching (Fig. 3a and Fig. S20). The Poisson’s ratios of rGO/PPy/PUF1-UHP were zero and 0-0.016 with tensile strains in the range of 0-100% and 100–800%, respectively, the values of which were much lower than those of rGO/PPy/PUF1 without hot pressing (0.305–0.316) (Fig. S21). For rGO/PPy/PUF2-UHP, the Poisson’s ratios were zero and 0-0.015 with tensile strains in the range of 0-200% and 200–1200%, respectively. For rGO/PPy/PU-UHP, the Poisson’s ratios were zero and 0-0.014 with tensile strains in the range of 0-100% and 100–400%, respectively, the values of which were much lower than those of rGO/PPy/PU (0.268–0.312) (Fig. S21). It should be noted that the biaxially and triaxially hot-pressed rGO/polymer aerogels exhibited negative Poisson’s ratios during stretching because of their reentrant porous structures. The Poisson’s ratios of rGO/PPy/PUF1-BHP in x and y directions were in the range of -(0.072–0.174) and − (0.032–0.120), respectively (Fig. 3b,k and Fig. S22), while those of rGO/PPy/PU-BHP were in the range of -(0.091–0.110) and − (0.059–0.106), respectively (Fig. S22). Besides, the Poisson’s ratios of rGO/PPy/PUF1-THP in x, y, and z directions were in the range of -(0.042–0.158), -(0.058–0.150), and − (0.039–0.118), respectively (Fig. S23), while those of rGO/PPy/PU-THP were in the range of -(0.100–0.120), -(0.095–0.096), and − (0.067–0.094), respectively (Fig. S24).
Moreover, the hot-pressed rGO/polymer aerogels could be reversibly compressed 80% in z direction, showing high compressibility and elasticity (Fig. S25, S26). The compressive stresses of the aerogels have been significantly enhanced by hot pressing (Fig. S25-S27). Furthermore, the hot-pressed rGO/polymer aerogels exhibited high bendability (Fig. S28). The high compressibility, bendability, and elasticity of the hot-pressed aerogels are probably attributed to the reversible deformability of PUF1, PUF2, PVA, and MF as well as the synergistic effect of rGO and the flexible polymers (Fig. S16,S29).
The in-situ morphology observation of the hot-pressed rGO/polymer aerogels during stretching and compression was performed to investigate the deformation mechanism of the porous structures. The pore size became larger and the folded pore walls became unfolded along x direction for the uniaxially hot-pressed aerogels (rGO/PPy/PUF1-UHP and rGO/PPy/PUF2-UHP) upon stretching in x direction within 0-800% tensile strains (Fig. 4a and Fig. S30). The morphologies of the hot-pressed aerogels after stretching were similar to those of the pristine aerogels without hot pressing. In the case of the biaxially hot-pressed aerogel (rGO/PPy/PUF1-BHP), the reentrant pores became unfolded and the pore size became larger along both x and y directions upon stretching in x or y direction within 0-250% tensile strains (Fig. 4b). It was noteworthy that the pores moved away from the center along y direction upon stretching in x direction (Fig. 4b), confirming the negative Poisson’s ratios of the biaxially hot-pressed rGO/polymer aerogels in terms of microstructure variations. Besides, it was observed that the pores of the hot-pressed aerogels were further compressed and the pore size became smaller upon compression in z direction (Fig. S31).
In order to further understand the deformation mechanism of the porous microstructures of the hot-pressed rGO/polymer aerogels, the mechanical simulations during hot pressing and stretching were performed via finite element analysis (FEA) (Fig. 4c,d and Fig. S32, S33). In the case of hot-pressing simulations, the pores of the aerogels were compressed and the pore walls were folded gradually upon uniaxial hot pressing (Fig. 4c and Movie S8). Reentrant porous structures of the aerogels were formed after biaxial hot pressing (Fig. 4c and Movie S9). The simulated folded and reentrant structures after uniaxial and biaxial hot pressing were generally consistent with the morphologies of the corresponding hot-pressed rGO/polymer aerogels as presented in Fig. 2 and Fig. S9-S11.
In the case of stretching simulations, the pore size became larger and the folded pore walls became unfolded along x direction for the uniaxially hot-pressed aerogel upon stretching in x direction (Fig. 4d). According to the simulation, the calculated Poisson’s ratios of the uniaxially hot-pressed aerogel at tensile strains of 100%, 200%, 300%, and 400% were − 0.027, 0, 0.026, and 0.057, respectively, indicating the zero or low Poisson’s ratios of this kind of porous structures during stretching. For the biaxially hot-pressed aerogel, the reentrant porous structure became unfolded and the pore size became larger upon stretching in x direction (Fig. 4d). The calculated Poisson’s ratios of the simulated reentrant porous structure of the biaxially hot-pressed aerogel at tensile strains of 50%, 100%, 150%, and 200% were − 0.59, -0.48, -0.21, and − 0.054, respectively, confirming the negative Poisson’s ratios via simulation. The simulated microstructure variations of the uniaxially and biaxially hot-pressed aerogels during stretching were generally consistent with the morphology variations obtained by in-situ observation (Fig. 4a,b and Fig. S30). More importantly, this simulation result verifies the versatility of the triaxial and biaxial hot-pressing strategies for constructing folded and reentrant porous structures with high stretchability and low or negative Poisson’s ratios. Highly compressible aerogels with positive Poisson’s ratios can be converted into super-stretchable aerogels with negative Poisson’s ratios via these hot-pressing strategies.
Sensing properties
Benefiting from their high stretchability, high compressibility, and high elasticity, the hot-pressed rGO/polymer aerogels can be used for ultrabroad-range-response strain and pressure sensors, which can’t be achieved by traditional aerogels. The introduction of rGO and PPy makes the rGO/polymer aerogels electrically conductive, allowing the resultant aerogel-based strain/pressure sensors to be able to work in a resistive mode (Fig. 5a). The resistance of all the rGO/polymer aerogels increased with the increase of tensile strain and decreased with the increase of compressive strain (or pressure) (Fig. 5b,c). As shown in Fig. 4a,b, the pore size of the hot-pressed aerogels became larger and the amount of contact points of the neighboring pore walls decreased upon stretching, which resulted in the decreased conductive paths and increased resistance. On the contrary, the pores of the hot-pressed aerogels were compressed and more contact points were produced upon compression (Fig. S31), which led to the increased conductive paths and decreased resistance.
The maximum detectable tensile strains of the strain sensors based on rGO/PPy/PUF1-UHP, rGO/PPy/PUF2-UHP, rGO/PPy/PU-UHP, rGO/PVA/MF-UHP, rGO/PPy/PUF1-BHP, and rGO/PPy/PUF1-THP reached 800%, 1200%, 450%, 100%, 300%, and 300%, respectively (Fig. 5b and Fig. S34). The maximum detectable pressures of the pressure sensors based on rGO/PPy/PUF1-UHP, rGO/PPy/PUF2-UHP, rGO/PPy/PU-UHP, rGO/PPy/PUF1-BHP, and rGO/PPy/PUF1-THP reached 4.7 MPa, 4.4 MPa, 2.8 MPa, 8.2 MPa, and 9.5 MPa, respectively (Fig. 5c). The response of the rGO/PPy/PUF1-UHP-based strain sensor remained nearly unchanged during stretching-releasing with 400% strain for 1000 cycles, indicating its excellent durability and fatigue resistance (Fig. 5d). Besides, the rGO/PPy/PUF1-UHP-based pressure sensor can withstand compression-decompression with 80% strain for 1000 cycles, demonstrating its excellent durability against compression (Fig. S35).
To our knowledge, the maximum detectable tensile strain (1200%) of the strain sensor based on rGO/PPy/PUF2-UHP significantly surpasses all those of the previously reported strain sensors based on intrinsically stretchable aerogels (Fig. 5e and Table S4).[6,18−25] Moreover, its maximum detectable tensile strain was larger than those of the previously reported strain sensors based on intrinsically stretchable conductive foams and sponges (Fig. 5e and Table S5).[41–44, 46, 49]
Because of their ultrabroad detection ranges, the strain/pressure sensors based on the hot-pressed rGO/polymer aerogels can be used as strain- or pressure-sensitive wearable electronics. Finger and wrist bending and muscular movement could be monitored in real time by attaching the rGO/PPy/PUF1-UHP-based sensor on the surfaces of a finger, wrist, and muscle, respectively, demonstrating its application potentials in monitoring human body motions (Fig. 5f and Fig. S36). More importantly, the hot-pressed aerogel-based strain sensors can be used for monitoring large tensile strains in the range of 0-1200%. For example, the rGO/PPy/PUF1-UHP-based sensor could monitor the large tensile strain (0-600%) of a balloon that was blown up by attaching it on the surface of the balloon (Fig. 5g). Besides, it could monitor the large tensile strain (0-300%) of a chest developer during exercise (Fig. 5h and Movie S10). Furthermore, we demonstrated that the hot-pressed aerogel-based strain sensors showed potential applications in robots and prostheses. Five rGO/PPy/PUF1-UHP-based strain sensors were fixed on five fingers of a bionic hand. The strain sensor would be stretched when the finger was bent, resulting in the increased resistance of the sensor. Different gestures of the bionic hand could be monitored in real time by recording the response of each strain sensor on the fingers (Fig. 5i).
Smart thermal management
Because of their highly porous structures, aerogels can be used as thermal insulators for thermal management. Traditional thermal insulation materials usually showed fixed thermal insulation performance. The pore sizes of traditional thermal insulation materials usually can’t be reversibly tuned by stretching. However, the pore sizes of the hot-pressed rGO/polymer aerogels were highly tunable over a wide range simply via stretching with different strains (Fig. 4a,b and Fig. S30). Since the hot-pressed aerogels showed high stretchability, high elasticity, and reversibly tunable pore sizes, they are expected to be able to achieve reversibly tunable thermal insulation for smart thermal management via stretching.
The thermal insulation performances of the hot-pressed rGO/polymer aerogels under different tensile strains were investigated by infrared thermography. The infrared thermal images of rGO/PPy/PUF1-UHP (1 mm thick) with different tensile strains on a hot plate (51 ℃) were observed in real time (Fig. 6a). The temperature of the top surface of the aerogel increased with the increase of tensile strains in spite of the larger size of the stretched aerogel. The top surface temperature of pristine rGO/PPy/PUF1-UHP stabilized at 42.4 ℃, the value of which was lower than those of the stretched aerogel with 50–200% tensile strain (Fig. 6b). The larger pore size and lower apparent density under a larger tensile strain may result in the higher thermal conductivities of gas and radiation, leading to the lower thermal insulation of the hot-pressed aerogel with larger tensile strains (Fig. 6c). [5, 52, 53] As expected, the hot-pressed rGO/polymer aerogels exhibited tunable thermal insulation and could be applied for smart thermal management, which was achieved by regulating the porous microstructures simply via stretching.
Smart EMI shielding
Smart EMI shielding materials with reversibly tunable EMI shielding performances were attractive for next-generation EMI shielding devices. However, the traditional EMI shielding materials usually exhibited fixed EMI shielding performance.[9, 54, 55] To our knowledge, there is no report on highly stretchable aerogels with reversibly tunable EMI shielding performances via stretching. Here, reversibly tunable EMI shielding has been achieved by the highly stretchable hot-pressed rGO/polymer aerogels simply via stretching with different strains.
The total shielding effectiveness (SET) and absorption effectiveness (SEA) of the uniaxially hot-pressed aerogel rGO/PPy/PUF1-UHP in z direction obviously decreased with the increase of tensile strain (Fig. 6d,e). The SET of pristine rGO/PPy/PUF1-UHP was approximately 40 dB, the value of which was higher than the requirement (> 20 dB) of the standard EMI shielding. Therefore, pristine rGO/PPy/PUF1-UHP can be regarded as a kind of EMI shielding materials. By contrast, the SET of rGO/PPy/PUF1-UHP at 100%, 200%, and 300% tensile strains were only approximately 17.3, 9.8, and 7.5 dB, respectively. As we can see, EM wave shielding for the hot-pressed rGO/polymer aerogel can be converted to EM wave transmission simply via stretching. The reversible switch between EM wave shielding and transmission for the hot-pressed aerogel can be achieved by repeatedly stretching and releasing the aerogel (Fig. 6f).
EMI shielding is influenced by the reflection, multiple reflections, and absorption of the EM wave.[55, 56] Since the pore size of the stretched rGO/polymer aerogels becomes larger, there are fewer contact points between neighboring conductive rGO nanosheets and PPy particles, resulting in less electron transport paths. As a result, the electrical conductivities of the hot-pressed rGO/polymer aerogels became lower upon stretching (Fig. 6g). In addition, the larger pores of the aerogels may result in lower permittivity. [56] Therefore, the impedance matching became better and the dielectric loss and attenuation of EM waves inside the aerogel were decreased for the stretched rGO/polymer aerogel, leading to the reduced reflection and increased transmission of EM waves (Fig. 6h). This kind of aerogels with reversibly tunable EMI shielding performances show potential applications in smart EMI shielding devices.
