1 Introduction

With the prosperity of modern electronic devices and wireless telecommunication, the information leakage and electromagnetic pollution caused by infrared target exposure (such as temperature changes generated by operation of electronics and fighter engines) and electromagnetic interference (EMI) are becoming increasingly serious in areas of aerospace, weapons, military and wearable electronics [1,2,3,4,5,6]. They detrimentally effect the information security and operational reliability of precision electronics [7,8,9,10]. Therefore, high-efficiency infrared stealth and EMI shielding materials are tremendously desired to protect the infrared target and attenuate the electromagnetic (EM) waves [11,12,13,14]. Metals, such as aluminum, copper and silver, are typical low infrared emissivity and high EMI shielding materials. However, they display serious disadvantages including heavy weight, difficult processing and high cost. The conductive polymer composites (CPCs) with conductive fillers dispersed in polymer matrix have been investigated for lightweight EMI shielding [15,16,17,18]. Nevertheless, high filler contents are needed to achieve the satisfied electrical conductivity and EMI shielding performances, resulting in the weakened mechanical properties and processability [19,20,21,22,23]. Moreover, the reflection-dominant EMI shielding mechanism of metals and CPCs due to the impedance mismatch between air and shields results in the secondary pollution of EM waves [24,25,26]. The radar stealth also requires the shields to attenuate EM waves via absorption in wide frequency range with low EM reflection. Therefore, it remains a significant challenge to develop lightweight and high-efficiency dual-functional CPCs with integrated capacities of infrared stealth and absorption-dominant EMI shielding.

The introduction of cellular structures into CPCs offers high prospects in fabrication of lightweight polymer-based infrared stealth and EMI shielding composites [27,28,29,30,31]. According to Stefan–Boltzmann law, infrared stealth can be acquired by decreasing the infrared emissivity and/or reducing the surface temperature of protected targets [32, 33]. The cellular structures can not only decrease the mass density of CPCs for lightweight purposes, but also reduce the surface temperatures for infrared stealth based on their thermal insulation features [34, 35]. Xu et al. [36] fabricated the lightweight and thermally insulating PEDOT:PSS@melamine (PPM) foams for infrared stealth with a decreased infrared emissivity of 0.757. The PPM foams covered on the hot stage (80 °C) present a decreased radiation temperature of 44.1 °C with a temperature reduction (∆T) of 35.9 °C. Besides, the cell growth process can promote the formation of effective conductive networks via orientation of conductive fillers, leading to the enhanced multiple internal reflections of EM waves and total EMI shielding efficiency (SE) [37,38,39]. Moreover, the cellular structures can improve the impedance matching between air and shields and decrease the direct reflection of EM waves on the surfaces, leading to the absorption-dominant EMI shielding behaviors [40,41,42,43]. Several approaches such as chemical foaming [44], freeze‑drying [45], sacrificial template [46], 3D printing [40, 47] and supercritical carbon dioxide (SC-CO2) foaming [48] can been used for the fabrication of cellular CPCs. Among these, the SC-CO2 foaming process resembles an environmentally friendly, low-cost and efficient physical-blowing technique with gentle critical conditions (Tc = 31.3 °C and Pc = 7.38 MPa) for the fabrication of microcellular foams with cell sizes less than 100 μm and large cell densities [49, 50]. Park et al. [51] reported the significantly improved absorptivity/reflectivity (A/R) ratio by introducing microcellular structures in the polyvinylidene fluoride/carbon nanotube/SiC nanowire (PVDF/CNT/SiCnw) composites. The obtained microcellular composite foams show absorption-dominated EMI shielding performances with an EMI SE of 22 dB in Ku-band and an A/R ratio of 1.07 (higher than 1.0). Zhang et al. [52] fabricated the microcellular Ni-chain/PVDF foams with an EMI SE of 26.8 dB and specific SE (SSE) of 127.62 dB cm2 g−1 in X band by SC-CO2 foaming. Benefitting from the microcellular structures and Ni-chain conductive-magnetic networks, the microcellular foams exhibit absorption-dominant EMI shielding performances. Nevertheless, the improvement of infrared stealth and EMI shielding performances by single introduction of cellular structures is limited and is insufficient to meet the rigorous demands in high-tech applications.

Constructing conductive segregated structures in CPCs is demonstrated to be an effective strategy to fabricate polymer-based EMI shielding composites with significantly decreased percolation threshold at ultralow filler contents [53,54,55]. The conductive fillers present a selective distribution at the interfaces of neighboring polymer microdomains to form highly efficient conductive networks, resulting in the improved electrical properties and enhanced EMI shielding performances [56,57,58]. The multiple internal reflection of EM waves within segregated structures also can improve the EMI SE via absorption [59,60,61]. Recently, Yan et al. [62] successfully prepared the flexible interface-reinforced segregated carbon nanotube/polydimethylsiloxane (CNT/PDMS) composite with a high EMI SE of 47.0 dB at the CNT content of 2.2 vol% and a high tensile strength of 3.6 MPa. Wang et al. [63] constructed the segregated structures in biodegradable porous multi-walled carbon nanotube/polylactic acid (MWCNT/PLA) composites, achieving the enhanced thermal insulation and absorption-dominant EMI shielding performances. Ma et al. [64] reported the preparation of highly resilient segregated MWCNT/PDMS nanocomposites-based piezoresistive sensors for human motion detection by incorporating the silver coated microcellular thermoplastic polyether-block-amide elastomer (TPAE) beads, which contain the crystalline polyamide hard segments and polyether soft segments in the molecule chains. The microcellular TPAE beads with lightweight, high flexibility and resilience exhibit great potentials in aerospace, weapons, military and wearable electronics. The results provide new strategies for the development of lightweight and high-efficiency polymer-based infrared stealth and EMI shielding materials.

In this work, we report the lightweight and high-efficiency dual-functional segregated microcellular TPAE beads coated with Ti3C2Tx (TPAE@Ti3C2Tx) nanocomposite foams for infrared stealth and absorption-dominant EMI shielding via the efficient and scalable SC-CO2 foaming combined with hydrogen bonding assembly and compression molding. Benefitting from the synergistic effect of highly effective thermal insulation and low infrared emissivity, the segregated nanocomposite foams exhibit outstanding infrared stealth performances. The synchronous construction of microcellular structures and segregated structures endows the segregated nanocomposite foams with lightweight and absorption-dominant EMI shielding performances at low Ti3C2Tx contents. Moreover, the segregated nanocomposite foams exhibit outstanding infrared stealth and EMI shielding stability upon dynamic compression cycles. The convenient and low-cost strategy endows the segregated nanocomposite foams with great prospect of large-scale fabrication. The influences of microcellular TPAE expansion ratio and Ti3C2Tx content on the microstructures, mechanical and electrical properties as well as the infrared stealth and EMI shielding performances have been investigated in detail. The lightweight and high-efficiency dual-functional segregated nanocomposite foams with superior infrared stealth and absorption-dominant shielding performances have promising application potentials in aerospace, weapons, military and wearable electronics.

2 Experimental Section

2.1 Materials

Thermoplastic polyamide elastomer (TPAE) beads (shore D hardness: 35, mass density: 1.01 g cm−3) were provided by Arkema Inc. Ti3AlC2 (MAX) powders (200 mesh) were obtained from Laizhou Kai Kai Ceramic Materials Co., Ltd. CO2 gas with a 99.99% purity was utilized as the physically foaming agent. Other chemicals including lithium fluoride (LiF), hydrochloric acid (HCl, 37 wt%) and formic acid (AC) were supplied by Sinopharm Chemical Reagent Co., Ltd.

2.2 Preparation of Microcellular TPAE Beads

Microcellular TPAE beads were prepared by the environmentally friendly solid-state SC-CO2 foaming process. The solid TPAE beads were firstly placed in the autoclave filled with SC-CO2 at 45 °C and 15 MPa for 5 h, achieving the saturated gas concentration of 135 mg CO2 per gram of TPAE (Fig. S1). After pressure releasing, the saturated TPAE beads were transferred into the preheated kettle at 125 °C under uniform mechanical stirring for microcellular foaming. The microcellular TPAE beads with different expansion ratios (β = ρ/ρf, where ρ and ρf are the mass densities of solid and microcellular TPAE beads, respectively) of 2.5, 4.2 and 5.5 were obtained with the foaming time of 25, 50 and 75 s, respectively.

2.3 Synthesis of Ti3C2Tx MXene

The Ti3C2Tx MXene was obtained by chemical etching and delamination. 1.0 g of Ti3AlC2 was added to the etching solution consisting of 1.0 g LiF and 20 mL HCl solution with a concentration of 9 mol L−1. Etching was conducted at 35 °C upon magnetic stirring for 24 h, obtaining the accordion-like m-Ti3C2Tx. The obtained dispersion was washed with deionized (DI) water by centrifuging at 3500 rpm for 5 min to reach a supernatant pH of approximately 6.0. Subsequently, the dispersion was sonicated at 180 W for 20 min and then centrifuged at 3500 rpm for 1 h to obtain the supernatant containing delaminated Ti3C2Tx MXene.

2.4 Fabrication of Segregated Nanocomposite Foams

The microcellular TPAE beads with different expansion ratios were dip-coated in Ti3C2Tx dispersion to prepare the microcellular TPAE@Ti3C2Tx beads via hydrogen-bond assembly. After compression molding at 50 °C for 10 min in a cylindrical steel mold containing a small amount of formic acid, the TPAE@Ti3C2Tx beads were mutually bonded together by physical entanglement and hydrogen bonding interactions to obtain the lightweight and highly resilient segregated nanocomposite foams. The Ti3C2Tx content of obtained segregated nanocomposite foams was tailored by controlling the Ti3C2Tx dispersion concentrations during dip-coating (Table S1). The microcellular TPAE foams without Ti3C2Tx MXene were also prepared via compression molding for comparison.

2.5 Characterizations

The morphologies of microcellular TPAE beads, microcellular TPAE@Ti3C2Tx beads, and segregated nanocomposite foams were assessed using a VEGA 3 LMH scanning electron microscope (SEM) with an energy-dispersive spectrometry (EDS). The samples were cut with a scalpel to reveal the fracture surfaces and sputter coated with Au/Pd. The microstructures of m-Ti3C2Tx and Ti3C2Tx MXene were observed with a FEI Verios 460 field emission SEM (FE-SEM) and a FEI Tecnai transmission electron microscope (TEM). The Image-Pro Plus software was applied to calculate the statistic cell-size distribution. The Archimedes water displacement method was employed to measure the mass densities. The Fourier-transform infrared spectroscopy (FTIR) analysis was conducted on a Thermo Nicolet spectrophotometer, and the X-ray photoelectron spectroscopy (XPS) analysis was performed on an Axis Ultra DLD spectrometer. The X-ray diffraction (XRD) patterns were obtained on a D8 AdvanceX diffractometer. The electrical conductivities of segregated nanocomposite foams were analyzed using the Princeton 4000 + electrometer. The cycling compression properties were conducted on the CMT8502 universal testing machine with a speed of 5 mm·min−1. The mid-infrared emissivity was obtained using a Nicolet iS50 FTIR spectrometer. The radiation temperatures and infrared images were obtained by a Fluke TiS75 + IR thermometer. The EMI shielding performances including SER, SEA and SET were analyzed using a PNA-N5244A vector network analyzer (Agilent).

3 Results and Discussion

3.1 Design Principle and Preparation of Segregated Nanocomposite Foams

By synchronous construction and optimization of microcellular structures and segregated structures, the lightweight and high-efficiency dual-functional segregated nanocomposite foams with integrated infrared stealth and absorption-dominant EMI shielding capacities are developed via the efficient and scalable supercritical CO2 (SC-CO2) foaming combined with hydrogen bonding assembly and compression molding strategy (Fig. 1). Briefly stated, the highly resilient microcellular TPAE beads with thin solid skins and microcellular cores are prepared by the solid-state SC-CO2 foaming. Subsequently, the conductive Ti3C2Tx MXene is uniformly assembled on the surfaces of microcellular TPAE beads based on the abundant hydrogen bonding interaction between the carbonyl group (C=O) in TPAE molecule chains and hydroxyl group (–OH) on Ti3C2Tx MXene. After compression molding, the lightweight and high-efficiency dual-functional segregated nanocomposite foams are obtained. The resultant segregated nanocomposite foams exhibit excellent interface adhesion and dynamic mechanical properties owing to the physical entanglement and hydrogen bonding interactions and show superior infrared stealth and absorption-dominant EMI shielding performances. Firstly, the synergistic effect of highly effective thermal insulation and low infrared emissivity endows the segregated nanocomposite foams with superior infrared stealth performances upon the infrared object. Secondly, the excellent absorption-dominant EMI shielding performances are achieved owing to the synchronous construction of microcellular structures and segregated structures. Moreover, the segregated nanocomposite foams exhibit outstanding working reliability and stability upon dynamic compression cycles. Therefore, the resultant segregated nanocomposite foams are expected to be used as lightweight and high-efficiency dual-functional infrared stealth and absorption-dominant EMI shielding materials in aerospace, weapons, military and wearable electronics.

Fig. 1
figure 1

Schematic illustration for fabrication of lightweight and high-efficiency dual-functional segregated nanocomposite foams for integrated infrared stealth and absorption-dominant EMI shielding

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3.2 Morphologies of Microcellular TPAE Beads and Ti3C2Tx MXene

Figure 2a-c shows the cellular morphologies of microcellular TPAE beads with expansion ratios of 2.5, 4.2 and 5.5 (corresponding mass densities of 0.40, 0.24 and 0.18) foamed for 25, 50 and 75 s, respectively. After microcellular foaming, the TPAE beads turn from semitransparent and stiff to white opaque and highly resilient (Figs. S4 and S5). It is observed that the microcellular TPAE beads all present skin–core morphologies with uniform foamed cores and thin unfoamed skins (inset in Fig. 2a–c). With increasing foaming time, the microcellular TPAE beads exhibit thinner unfoamed skins and more highly foamed cores with larger cell size, smaller cell density and cell wall thickness. This is because that the dissolved CO2 molecules in TPAE matrix continuously diffuse into the initially nucleated cells, resulting in the larger cell size, smaller cell density and thinner cell wall due to the uniaxial compression and biaxial tension effects during microcellular foaming. The statistically calculated cell diameters of microcellular TPAE beads foamed for 25, 50 and 75 s are 39.5, 65.8 and 93.2 μm with large cell densities of 4.93 × 106, 2.65 × 106 and 1.07 × 106 cells cm−3, and cell wall thicknesses of 16.8, 10.2 and 3.9 μm, respectively. For the formation of unfoamed solid skins, it is deduced that after saturation and pressure release, the CO2 molecules in the skin region begin to diffuse outward, resulting in the relatively lower gas concentration and thus decreased foamability. As shown in Fig. 2h, i, the microcellular TPAE foams with good interfacial adhesion and well-maintained microcellular structures are feasibly fabricated by compression molding of the microcellular TPAE beads, which show unique skin–core morphologies with thin unfoamed skins and highly elastic foamed cores. Specifically, the partially dissolved TPAE molecules on the surfaces of adjacent microcellular TPAE beads diffuse rapidly and tangle with each other during compression molding, forming the strong adhesion interfaces between adjacent microcellular TPAE beads (Fig. S6). Meanwhile, the microcellular TPAE beads show excellent flexibility with adaptable microcellular structures upon the compression deformation and exhibit well-maintained microcellular structures after compression molding due to their outstanding rebound resilience. Figure S7 demonstrates the successful fabrication of large-scale microcellular TPAE foams with bigger dimensions based on the microcellular TPAE beads.

Fig. 2
figure 2

a–c SEM images and d–f cell-size distributions of the microcellular TPAE beads with different expansion ratios. g Digital images of the microcellular TPAE beads. h Digital and i SEM images of the microcellular TPAE foams. j SEM image of the m-Ti3C2Tx. k TEM image of the Ti3C2Tx MXene. l XRD patterns of the Ti3AlC2, m-Ti3C2Tx and Ti3C2Tx MXene

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Figures 2j and S8 show the SEM images of Ti3AlC2 and multilayer Ti3C2Tx (m-Ti3C2Tx). After chemically etching the Al layers, the m-Ti3C2Tx shows accordion-like structures with loosely stacked Ti3C2Tx nanosheets. This facilitates the exfoliation of Ti3C2Tx MXene owing to the weakened interlayer interactions. The obtained few-layer Ti3C2Tx MXene exhibits ultrathin and highly transparent features with a large lateral size of 3.5 μm (Fig. 2k). The strong Tyndall effect of Ti3C2Tx dispersion verifies their colloidal characteristics and the high dispersibility of Ti3C2Tx MXene in DI water owing to the abundant functional groups of –O, –OH and –F. Figure 2l shows the XRD patterns of Ti3AlC2, m-Ti3C2Tx and Ti3C2Tx MXene. The disappearance of (101), (103), (104) and (105) characteristic peaks and left shift of (002) peak from 9.5° to 6.4° demonstrate the successful synthesis of Ti3C2Tx MXene with enlarged interlayer spacing. Importantly, the existence of abundant functional groups is beneficial to the hydrogen bonding assembly of Ti3C2Tx MXene on the surfaces of microcellular TPAE beads by convenient dip-coating process.

3.3 Morphologies of Segregated Nanocomposite Foams

Figure 3a–c shows the surface and interior morphologies of microcellular TPAE@Ti3C2Tx beads with the expansion ratio of 4.2. As can be seen, the Ti3C2Tx MXene is uniformly assembled on the surfaces of microcellular TPAE beads with a black surface, thanks to the abundant hydrogen bonding interaction between the carbonyl group (C=O) in TPAE molecule chains and hydroxyl group (–OH) on the surface of Ti3C2Tx MXene. The corresponding EDS mappings of C, O and Ti elements also demonstrate the uniform assembly of Ti3C2Tx MXene on the surfaces of microcellular TPAE beads (Fig. 3d–f). Figure 3g–i shows the digital, SEM and EDS mapping images of the segregated nanocomposite foams with an expansion ratio of 4.2. They evidently demonstrate the synchronous construction of microcellular structures and segregated structures. The Ti3C2Tx MXene is selectively distributed at the interfaces of adjacent microcellular TPAE beads, forming the highly efficient three-dimensional (3D) continuous conductive networks at ultralow Ti3C2Tx contents. The introduction of microcellular structures endows the segregated nanocomposite foams with lightweight and high resilience. For instance, the segregated nanocomposite foams with an expansion ratio of 5.5 exhibit a low mass density of 0.32 g cm−3 (Fig. S10) and can be floated on the water (Fig. S11). Figure 3j–l shows the interfacial morphologies of the segregated nanocomposite foams. The segregated nanocomposite foams present good interfacial adhesion with orientationally aligned Ti3C2Tx MXene at the adhesion interfaces, which is beneficial to obtain the highly efficient 3D continuous conductive networks at ultralow Ti3C2Tx content. The strong adhesion interfaces of the segregated nanocomposite foams mainly benefit from two reasons. On the one hand, the molecular chains on the surfaces of adjacent microcellular TPAE beads diffuse and entangle with each other during compression molding, leading to the physical anchoring of Ti3C2Tx MXene. On the other hand, the hydrogen bonding interaction between C=O in TPAE molecule chains and   –OH on the surface of Ti3C2Tx MXene strengthens the adhesion interfaces of segregated nanocomposite foams.

Fig. 3
figure 3

a Digital and b, c SEM images of the microcellular TPAE@Ti3C2Tx beads. d–f EDS mapping images of C, O and Ti elements of the microcellular TPAE@Ti3C2Tx beads. g Digital, h SEM and i EDS mapping images of the segregated nanocomposite foams. j Digital image of the fracture surface of segregated nanocomposite foams. k, l SEM images of the interface adhesion of segregated nanocomposite foams

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The chemical structures and hydrogen bonding interactions between TPAE and Ti3C2Tx MXene were investigated by XRD, FTIR and XPS. As shown in Fig. 4a, the microcellular TPAE beads exhibit an enhanced intensity of γ-form crystals at 21.5° compared with the solid beads, owing to the plasticization and rearrangement of molecular chains during SC-CO2 foaming. After assembly of Ti3C2Tx MXene on the surface of microcellular TPAE beads, the diffraction peak at 21.5° weakens, and the diffraction peak corresponding to (002) of Ti3C2Tx MXene appears at 6.0°. Figure 4b shows the FTIR spectra of TPAE, Ti3C2Tx MXene and TPAE/Ti3C2Tx nanocomposites. Compared with the pure TPAE and Ti3C2Tx MXene, the C=O characteristic peak of TPAE/Ti3C2Tx nanocomposites is shifted from 1640 to 1630 cm−1, and the  –OH characteristic peak is shifted from 3452 to 3438 cm−1. Therefore, the chemical environment of C =O and  –OH has been changed, indicating the formation of hydrogen bonding interactions between TPAE and Ti3C2Tx MXene with C=O as proton acceptor and  –OH as proton donor. Figure 4c–f shows the XPS wide-scan spectra and high-resolution spectra of TPAE, Ti3C2Tx MXene and TPAE/Ti3C2Tx nanocomposites. As can be seen, the TPAE/Ti3C2Tx nanocomposites show distinct Ti and F characteristic peaks due to the introduction of Ti3C2Tx MXene. For the TPAE/Ti3C2Tx nanocomposites, the C=O characteristic peak of TPAE shifts from 287.7 to 288.2 eV in the C 1s spectra (Fig. 4d), the C–Ti–OH characteristic peak of Ti3C2Tx MXene shifts from 531.9 to 531.8 eV in the O 1s spectra (Fig. 4e), and the N–H characteristic peak of TPAE shifts from 399.1 to 399.9 eV in the N 1s spectra (Fig. 4f). This indicates that the chemical environments of C=O and N–H in TPAE and C–Ti–OH in Ti3C2Tx MXene have been changed, demonstrating the formation of hydrogen bonding interactions between TPAE and Ti3C2Tx MXene. The synergetic effect of physical entanglement and hydrogen bonding interactions contributes to the enhanced adhesion interfaces and improved mechanical properties of segregated nanocomposite foams.

Fig. 4
figure 4

a XRD patterns of the solid and microcellular TPAE beads, as well as microcellular TPAE@Ti3C2Tx beads. b FTIR and c XPS spectra of the TPAE, Ti3C2Tx MXene and TPAE/Ti3C2Tx nanocomposites. High-resolution XPS spectra of d C 1s for TPAE and TPAE/Ti3C2Tx nanocomposites, e O 1s for Ti3C2Tx MXene and TPAE/Ti3C2Tx nanocomposites, and f N 1s for TPAE and TPAE/Ti3C2Tx nanocomposites

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3.4 Infrared Stealth Performances of Segregated Nanocomposite Foams

The infrared stealth performances of microcellular TPAE foams and segregated nanocomposite foams with the same thickness of 8 mm are evaluated on the hot stage simulating the infrared object at various temperatures. The Ti3C2Tx dispersion concentration used for dip-coating is 20 mg mL−1. Figure 5a shows the radiation temperatures of microcellular TPAE foams and segregated nanocomposite foams (expansion ratio of 4.2) with a consistent object temperature of 100 °C. As can be seen, the radiation temperatures of microcellular TPAE foams and segregated nanocomposite foams gradually rise to the low steady values of 47.5 and 29.8 °C with the ∆T of 52.5 and 70.2 °C, respectively, compared with the object temperature, indicating the infrared stealth capacities of microcellular TPAE foams and segregated nanocomposite foams. Notably, the segregated nanocomposite foams exhibit much better infrared stealth performances with a larger ∆T than the microcellular TPAE foams. From the infrared images in Fig. 5a, it is also observed that the upper surface of segregated nanocomposite foams possesses a lower radiation temperature than that of microcellular TPAE foams. Figure 5b shows that at the different object temperatures of 30, 50, 75 and 100 °C, the segregated nanocomposite foams all present much lower radiation temperatures compared with the microcellular TPAE foams, indicating their superior infrared stealth performances.

Fig. 5
figure 5

a Radiation temperatures of the microcellular TPAE foams and segregated nanocomposite foams with an infrared object temperature of 100 °C. b Infrared images of the microcellular TPAE foams and segregated nanocomposite foams at different object temperatures of 30, 50, 75 and 100 °C. c Radiation temperatures of the microcellular TPAE foams and segregated nanocomposite foams with different expansion ratios. d Thermal conductivities and e infrared emissivity of the microcellular TPAE foams and segregated nanocomposite foams. f Long-term infrared stealth performances of the microcellular TPAE foams and segregated nanocomposite foams. g Cycling compression behaviors of the segregated nanocomposite foams with different expansion ratios for 120 circles. h Infrared stealth stabilities of the microcellular TPAE foams and segregated nanocomposite foams upon repeated compression strains. i Infrared stealth mechanisms of the segregated nanocomposite foams. j Infrared image of the diagonally recombined microcellular TPAE foams and segregated nanocomposite foams. k Infrared stealth of the airplane model covered by segregated nanocomposite foams

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Figures 5c and S12 show that the segregated nanocomposite foams with expansion ratios of 2.5, 4.2 and 5.5 all exhibit better infrared stealth performances compared with the microcellular TPAE foams. With the increasing expansion ratio, the radiation temperatures of both microcellular TPAE foams and segregated nanocomposite foams decrease slightly. According to Stefan–Boltzmann law: E = εσT4, where σ refers to the Stefan–Boltzmann constant, the thermal radiation energy is directly dependent on the surface infrared emissivity (ε) and surface absolute temperature (T) [65]. As shown in Fig. 5d, the microcellular TPAE foams and segregated nanocomposite foams exhibit approximately the similar low thermal conductivities (λ) benefitting from the incorporation of microcellular structures, indicating their outstanding thermal insulating features. For instance, the microcellular TPAE foams and segregated nanocomposite foams with an expansion ratio of 4.2 exhibit low λ values of 0.052 and 0.055 W m−1 K−1, respectively. With the increasing expansion ratio, the λ values of them both decrease gradually. Figure 5e shows that the segregated nanocomposite foams possess an ultralow average infrared emissivity of 0.13 compared with the microcellular TPAE foams (0.88), which may benefit from the low infrared emissivity of Ti3C2Tx MXene [66, 67]. Therefore, compared with the microcellular TPAE foams with only thermal insulation dominated infrared stealth, the segregated nanocomposite foams exhibit superior infrared stealth performances owing to the synergistic effect of highly effective thermal insulation of microcellular structures and low infrared emissivity of Ti3C2Tx MXene (Fig. S13).

Figure 5f shows the long-term infrared stealth performances of microcellular TPAE foams and segregated nanocomposite foams at the object temperatures of about 50, 75 and 100 °C, respectively. As can be seen, the microcellular TPAE foams and segregated nanocomposite foams both present steady surface radiation temperatures during the duration of 3 h at different object temperatures, demonstrating their excellent working stability and reliability in infrared stealth. Figures 5g and S15 show the cycling compression behaviors of segregated nanocomposite foams with different expansion ratios for 120 loading–unloading circles with a maximum strain of 25%. Thanks to the intrinsic high resilience of TPAE and incorporation of microcellular structures, the segregated nanocomposite foams exhibit excellent cyclic mechanical stability during dynamic loadings with nearly coincident stress–strain curves and negligible hysteresis rings. With larger expansion ratio, the segregated nanocomposite foams exhibit improved flexibility with lower compression stress and compression modulus. The outstanding tensile properties of segregated nanocomposite foams (expansion ratio: 4.2) with a high tensile strength of 2.05 MPa and a large tensile strain at break of 296.3% also demonstrate the excellent interfacial adhesion between microcellular TPAE@Ti3C2Tx beads (Fig. S16). The infrared stealth performances of microcellular TPAE foams and segregated nanocomposite foams upon repeated compression strains are evaluated, as shown in Fig. 5h. Note that the segregated nanocomposite foams exhibit superior and steady infrared stealth performances with the radiation temperature maintained at low values even after 120 repeated compression cycles, demonstrating their excellent infrared stealth reliability and stability upon mechanical deformations. Figure 5i illustrates the infrared stealth mechanisms of segregated nanocomposite foams. Benefitting from the incorporation of microcellular structures, the segregated nanocomposite foams covered on the high-temperature infrared object exhibit lower surface temperature owing to their highly effective thermal insulation, which is similar to the microcellular TPAE foams. Meanwhile, the low infrared emissivity of segregated nanocomposite foams with assembled Ti3C2Tx MXene further dramatically decreases the surface radiation temperatures. Therefore, the segregated nanocomposite foams exhibit superior infrared stealth performances owing to the synergistic effect of highly effective thermal insulation and low infrared emissivity. Figure 5j shows the infrared image of diagonally recombined sample by two quarters of microcellular TPAE foams and two quarters of segregated nanocomposite foams. The locally distributed radiation temperatures prove the superior infrared stealth capacities of segregated nanocomposite foams. Figure 5k shows that the airplane model covered by segregated nanocomposite foams can realize selectively concealing under the thermal imager, demonstrating their promising application potentials in aerospace infrared stealth.

3.5 EMI Shielding Performances of Segregated Nanocomposite Foams

Figure 6a–c shows the EMI shielding performances of segregated nanocomposite foams with different microcellular TPAE bead expansion ratios and Ti3C2Tx contents. The segregated nanocomposite foams with tailorable Ti3C2Tx contents are obtained by simply changing the Ti3C2Tx dispersion concentration during dip-coating. With the increasing Ti3C2Tx content, the segregated nanocomposite foams with different expansion ratios (thickness: 8 mm) all exhibit significantly improved EMI SE owing to the more efficient 3D conductive networks and higher electrical conductivity (Fig. S17). The EDS mapping images in Fig. S18 also indicate the formation of continuous segregated conductive networks at ultralow Ti3C2Tx contents. The segregated nanocomposite foams with an expansion ratio of 2.5, for instance, exhibit a total EMI SE of 32 dB at the low Ti3C2Tx content of 2.5 vol%, which is sufficient for the commercial application requirements (> 20 dB). When the microcellular TPAE bead expansion ratio is increased to 4.2, the segregated nanocomposite foams with a lower Ti3C2Tx content of 1.7 vol% exhibit an enhanced total EMI SE of 44 dB to meet the higher demand of high-tech applications although the electrical conductivity is decreased. Figure 6d, e shows the corresponding microwave refection (SER), microwave absorption (SEA) and total EMI SE (SET) of segregated nanocomposite foams with expansion ratios of 2.5 and 4.2, respectively. Note that the segregated nanocomposite foams with the larger expansion ratio of 4.2 and lower Ti3C2Tx content of 1.7 vol% exhibit significantly increased SET (44 dB) and SEA (42 dB) with a decreased SER (2 dB) than those with an expansion ratio of 2.5 and a Ti3C2Tx content of 2.5 vol%. It is because that the introduction of more microcellular structures in segregated nanocomposite foams with larger millimeter-scale segregated conductive networks can improve the impedance matching due to the decreased electrical conductivity, thus allowing more penetration of incident EM waves in the segregated nanocomposite foams with less direct reflection on the surfaces. This consequently induces more multiple internal reflection and scattering of EM waves within the millimeter-scale segregated conductive networks, resulting in the enhanced attenuation of EM waves via absorption and thus absorption-dominant EMI shielding. With the higher expansion ratio of 5.5, nevertheless, the segregated nanocomposite foams exhibit a slightly decreased EMI SE of 35 dB (Fig. 6c, f), which could result from the decreased absorption loss of EM waves within the less segregated conductive networks at the same thickness. Interestingly, all the segregated nanocomposite foams exhibit increased total EMI SE upon the increasing EM wave frequency, indicating that the high‑frequency EM waves attenuate more efficiently within the millimeter-scale segregated conductive networks owing to their shorter wavelength with closer trough–crest distance.

Fig. 6
figure 6

a–c Total EMI SE of the segregated nanocomposite foams with different expansion ratios and Ti3C2Tx contents. d–f SER, SEA and SET of the segregated nanocomposite foams with different expansion ratios and Ti3C2Tx contents. g A, R and T coefficients of the segregated nanocomposite foams with different expansion ratios. h A/R of the segregated nanocomposite foams with different expansion ratios. i Relative SE of the segregated nanocomposite foams upon repeated compression. j EMI shielding mechanism of the segregated nanocomposite foams

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The absorptivity (A), reflectivity (R) and transmissivity (T) coefficients are calculated by the scattering parameters to evaluate the EMI shielding mechanisms of segregated nanocomposite foams. As shown in Figs. 6g and S19, the segregated nanocomposite foams with three expansion ratios of 2.5, 4.2 and 5.5 all exhibit high A values above 0.6 and low R values below 0.4. The larger expansion ratio results in the higher A value and lower R value. The lower R value and the larger A value indicate the more EM power attenuated by absorption within the hierarchical cellular structures [68,69,70]. The segregated nanocomposite foams with the expansion ratio of 4.2 possess a low R value below 0.3, and those with a larger expansion ratio of 5.5 possess an even lower R value around 0.2. Correspondingly, the segregated nanocomposite foams with three expansion ratios exhibit high A/R ratios of 1.62, 2.15 and 3.99, respectively, which are much larger than 1.0 (Fig. 6h). This demonstrates that the segregated nanocomposite foams exhibit absorption-dominant EMI shielding behaviors with most of the incident EM waves attenuated through absorption instead of reflection, which is beneficial to reduce the secondary pollution of EM waves. Figure 6i shows the relative SE (SE/SE0 × 100%) of segregated nanocomposite foams upon the repeated compression. After the 120 repeated compression cycles with a compression strain of 25%, the relative SE still presents a high retention rate above 94.5%, demonstrating the outstanding EMI shielding stability of segregated nanocomposite foams after dynamic mechanical deformations.

The absorption-dominant EMI shielding mechanisms of segregated nanocomposite foams mainly benefit from the synchronous construction of microcellular structures and segregated structures. Figure 6j schematically illustrates the propagation of EM waves across the segregated nanocomposite foams. Thanks to the incorporation of microcellular structures, most of the incident EM waves enter the segregated nanocomposite foams with low direct reflection owing to the improved surface impedance matching. In the segregated structures containing numerous microcellular structures, the EM waves will be attenuated via multiple internal reflection and scattering on the interfaces. Meanwhile, the EM waves can be attenuated by interacting with the electron carriers in the conductive networks, leading to the ohmic losses of EM waves. Moreover, the multiple interfacial reflections occurred between the neighboring Ti3C2Tx nanosheets also contribute to the dissipation of EM waves. In addition, the localized imperfections and terminal groups including –O–, –F and –OH on the surfaces of Ti3C2Tx MXene induce the uneven distribution of charge density, causing the creation of local dipoles upon the EM field and increased polarization loss. The unique hierarchical segregated microcellular structures act as the role of “black hole”, which can efficiently absorb the EM waves and prevent them from escaping. Therefore, the obtained segregated nanocomposite foams exhibit superior absorption-dominant EMI shielding performances. Figure S20 demonstrates that the segregated nanocomposite foams possess certain long-term infrared stealth and EMI shielding working stabilities in the air environment. The results demonstrate that the lightweight and high-efficiency dual-functional segregated nanocomposite foams with integrated infrared stealth and absorption-dominant EMI shielding capacities possess excellent potentials in areas of aerospace, weapons, military and wearable electronics.

4 Conclusions

In summary, this work demonstrates the development of lightweight and high-efficiency dual-functional segregated nanocomposite foams for infrared stealth and absorption-dominant EMI shielding via the efficient and scalable supercritical CO2 foaming combined with hydrogen bonding assembly and compression molding strategy. The chemical structures, hierarchical morphologies, electrical and mechanical properties as well as infrared stealth and EMI shielding performances as functions of microcellular TPAE bead expansion ratio and Ti3C2Tx content are investigated in detail. Benefitting from the synchronous construction of microcellular structures and segregated structures, the nanocomposite foams exhibit lightweight, improved flexibility and resilience, as well as desirable electrical conductivities at the ultralow Ti3C2Tx contents. The synergetic effect of physical entanglement and hydrogen bonding interactions between TPAE and Ti3C2Tx MXene results in the excellent adhesion interfaces and dynamic mechanical properties. The resultant segregated nanocomposite foams show superior infrared stealth performances (with a large radiation temperature reduction of 70.2 °C at the object temperature of 100 °C) thanks to the synergistic effect of highly effective thermal insulation and low infrared emissivity, and excellent absorption-dominant EMI shielding performances (with a high A/R ratio of 2.15) owing to the multiple internal reflections within segregated structures, massive ohmic loss, interfacial reflection and polarization loss of EM waves. Moreover, the segregated nanocomposite foams exhibit outstanding infrared stealth and EMI shielding stability upon dynamic compression cycles. We believe that the lightweight and high-efficiency dual-functional segregated nanocomposite foams with integrated infrared stealth and absorption-dominant EMI shielding capacities have promising potentials for applications in aerospace, weapons, military and wearable electronics.