Hierarchically porous polyimide/Ti₃C₂T_x film with stable electromagnetic interference shielding after resisting harsh conditions

Hierarchical polyimide/Ti₃C₂T_x film enables highly efficient and stable electromagnetic interference shielding.

Abstract

Polymer-based conductive nanocomposites are promising for electromagnetic interference (EMI) shielding to ensure stable operations of electronic devices and protect humans from electromagnetic radiation. Although MXenes have shown high EMI shielding performances, it remains a great challenge to construct highly efficient EMI shielding polymer/MXene composite films with minimal MXene content and high durability to harsh conditions. Here, hierarchically porous polyimide (PI)/Ti₃C₂T_x films with consecutively conductive pathways have been constructed via a unidirectional PI aerogel–assisted immersion and hot-pressing strategy. Contributed by special architectures and high conductivities, PI/Ti₃C₂T_x films with 2.0 volume % Ti₃C₂T_x have high absolute EMI shielding effectiveness up to 15,527 dB cm² g⁻¹ at the thickness of 90 μm. Superior EMI shielding performance can be retained even after being subjected to hygrothermal or combustion environments, cryogenic (−196°C) or high (250°C) temperatures, and rapid thermal shock (∆T = 446°C), demonstrating high potential as high-performance EMI shielding materials resisting harsh conditions.

INTRODUCTION

The continued advancement of electronic telecommunication technology and portable/wearable electronic devices entails the development of highly efficient electromagnetic interference (EMI) shielding materials to ensure the stable operation of devices and protect humans from electromagnetic radiation (1, 2). Commercial metal-based EMI shielding materials are gradually hard to satisfy the requirements of aerospace-used electronic devices with sophisticated configurations and harsh working conditions owing to their high density and corrosion susceptibility. The currently emerged MXene materials are regarded as promising EMI shielding alternatives (3–6). In particular, Ti₃C₂T_x film (7) offers an EMI shielding effectiveness (SE) of 92 dB with a thickness of 45 μm, and Ti₃CNT_x (8) film exhibits even higher EMI SE of 116 dB with a thickness of 40 μm. However, the pristine MXene films typically suffer from poor mechanical performance with tensile strength of 35 to 50 MPa and fracture strain down to 1.5% (9–11), as well as unavoidable oxidization in humid air or water (12, 13), poor processability, and high cost. Therefore, it remains a great challenge to develop EMI shielding materials that simultaneously meet the comprehensive requirements of high shielding performance, strong mechanical performance, environmental stability, low density, easy processability, and cost efficiency.

The construction of conductive networks of MXenes in polymeric matrix is regarded as an effective method to obtain highly efficient EMI shielding films while meeting the above requirements (14–17). Generally, improving conductivity and constructing architectures with multiple reflection interfaces are regarded as two effective pathways to enhance EMI shielding performances. So far, construction of uniformly distributed MXene architectures (18–26) and alternating multilayer MXene architectures (27–30) in polymeric matrix are two main strategies to produce conductive polymer/MXene films for highly efficient EMI shielding. Uniformly distributed architectures are usually constructed via the casting of solution/melt with polymer and MXene flakes, in which the polymers are capable of acting as binders to enhance mechanical performance as well as antioxidative coatings to protect the MXenes from degradation (31–33). However, high MXene content up to 40 to 90 weight % (wt %) [19.9 to 65.4 volume percent (volume %)] (27, 34, 35) is usually required to overcome the insulated polymer barriers between MXene flakes to increase the conductivity and achieve satisfactory EMI SE. It massively increases the fabrication cost owing to the expensive MXene materials, while deteriorating both processability and mechanical properties because of mass agglomeration of MXene flakes. The construction of alternating multilayer MXene architectures in polymer-based composite films is capable to reduce the MXene loading while maintaining high conductivity, in which MXene flakes are gathered between two polymeric layers to form consecutively conductive pathways (36, 37). Meanwhile, the incident electromagnetic waves (EMWs) are subjected to multiple reflection loss between two MXene layers in these alternating multilayer architectures, resulting in improved EMI shielding performance. Nevertheless, the lowest MXene content of the alternating films is still up to 19.5 (12 volume %), and the density is up to 1.76 g/cm³ (37), which are not advanced enough to meet the comprehensive requirements of low cost and low density in the large-scalable applications in aerospace field. In addition, most polymer-based composite films still suffer from high-temperature decomposition, cryogenic brittleness, poor thermal shock resistance, and flammability, which seriously restrict their usefulness in aerospace applications, such as the deep-space probes, lunar rovers, and mars rovers (38–41). Thus, it is still a great challenge to achieve highly efficient EMI shielding polymer/MXene films with minimal amounts of MXenes, while having extreme temperature resistance, thermal shock stability, and nonignitability. Among polymeric materials, polyimide (PI) can be an ideal candidate as matrix to protect MXene from being damaged and contribute stable mechanical performance in harsh working conditions for the sake of its high durability even in high temperature (>250°C) and deep cryogenic temperature (−269°C) (42).

To this end, we designed and constructed hierarchically porous PI/Ti₃C₂T_x (MXene) composite films with consecutively conductive pathways of Ti₃C₂T_x flakes via a unidirectional PI aerogel–assisted immersion and hot-pressing approach. Consecutively, conductive pathways of Ti₃C₂T_x flakes endow the composite films with highly efficient reflection loss for high electrical conductivity. The hierarchically porous architecture creates multiple interfaces to increase propagation paths of penetrated EMWs and promote interfacial polarization loss, giving rise to EMW attenuation inside the PI/Ti₃C₂T_x film. Benefiting from the above synergistic effects, the PI/Ti₃C₂T_x film with 2.0 volume % Ti₃C₂T_x has high absolute EMI SE (SSE/t) up to 15,527 dB cm² g⁻¹ at a thickness of 90 μm. Meanwhile, protected by PI matrix with high thermal and cryogenic resistance, the PI/Ti₃C₂T_x film can retain high EMI shielding performance after resisting in hygrothermal environment, high temperature (250°C), cryogenic temperature (−196°C), and rapidly thermal shock (∆T = 446°C). Furthermore, the mechanically strong PI matrix endows the multilayer PI/Ti₃C₂T_x films with satisfactory temperature-invariant mechanical performance over a wide temperature range of −100° to 250°C, little dimensional variation (<0.5%) between −70° and 250°C, as well as tensile and bending durability over 10,000 cycles at 250°C. Notably, the obtained EMI shielding films are capable to be easily processed into various shapes without geometric constraints, revealing unique advantages of scalable applications in electronic devices and instruments with complex configurations.

RESULTS

Fabrication of PI/Ti₃C₂T_x films

The fabrication process of PI/Ti₃C₂T_x composite films via a unidirectional PI aerogel–assisted immersion and hot-pressing method consists of three main steps: fabrication of unidirectional PI aerogels, fabrication of Ti₃C₂T_x aqueous suspension, and assembly of these elements. PI was obtained through chemical imidization at room temperature by adding acetic anhydride and triethylamine into poly(amic acid) precursors, which were synthesized from 4,4′-oxydianiline (ODA) and 4,4′-oxydiphthalic anhydride (ODPA) in dimethyl sulfoxide (DMSO) solvent (fig. S1A). Subsequently, a DMSO crystal template–assisted freeze-casting and freeze-drying method was applied to produce unidirectional PI aerogels (fig. S1B). The obtained PI aerogels have a pore size of around 200 μm, ultrahigh porosities of up to 99.08%, and highly ordered cellular architectures (fig. S2), which are beneficial to the infiltration and well arrangement of Ti₃C₂T_x flakes. In addition, the Ti₃C₂T_x flakes were obtained through the selective etching of Al layers from the MAX phase of Ti₃AlC₂ in the HCl/LiF mixture to produce multilayer Ti₃C₂T_x, followed by ultrasonication (fig. S3).

The successfully exfoliated Ti₃C₂T_x was clarified by the shift of the prominent (002) peaks from 9.5° to 6.3° in the x-ray diffraction (XRD) patterns (fig. S4), the transparent feature in transmission electron microscopy (TEM) images (fig. S5), and ultrathin size around 2 nm in the scanning probe microscopy image (fig. S6). In addition, the Ti₃C₂T_x flakes have a size distribution of 300 to 100 nm (fig. S7), which is beneficial for their access into the unidirectional PI aerogel with much larger pore size (≈200 μm). As shown in Fig. 1, through the immersion process under vacuum, the Ti₃C₂T_x aqueous suspension with controlled concentrations of 0, 2, 4, 6, 8, and 10 mg/ml infiltrated into the PI aerogels, and the obtained samples were designated by PIM-n (n = 0, 2, 4, 6, 8, and 10). Benefiting from the good hydrophilia (fig. S8) and highly ordered cellular architecture of PI aerogels, the Ti₃C₂T_x flakes gradually adhered and became well aligned on the interfaces of the internal walls of PI aerogels via the van der Waals and hydrogen bonding interaction between C═O in PI molecular chains and ─OH in Ti₃C₂T_x flakes. The internal morphologies and scanning electron microscope (SEM) mapping results in fig. S9 prove the successful access of Ti₃C₂T_x flakes into the unidirectional PI aerogels with uniform distribution. After freeze-drying followed by hot pressing perpendicular to the z direction of the unidirectional PI aerogels in vacuum, the PI/Ti₃C₂T_x aerogels were compressed into thin films with multilayer structures. Upon releasing, the compressed PI/Ti₃C₂T_x films were slightly resilient to generate some voids between two Ti₃C₂T_x layers, forming hierarchically porous architectures in the composite film. The thickness of the PI/Ti₃C₂T_x films could be adjusted with PI/Ti₃C₂T_x aerogels of various thicknesses before hot pressing, and the thicknesses of PIM-10 before and after hot pressing were shown in table S1.

Fig. 1. Fabrication process of hierarchically porous PI/Ti₃C₂T_x composite films with consecutively conductive pathways of Ti₃C₂T_x flakes.

Structure and morphology of PI/Ti₃C₂T_x films

The chemical structures and components of the PI/Ti₃C₂T_x films were then thoroughly investigated. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectra are adopted to verify the successful fabrication of PI/Ti₃C₂T_x films and investigate the possible interaction between PI and Ti₃C₂T_x flakes. As shown in Fig. 2A, the characteristic peaks at 1780 cm⁻¹ (imide C═O asymmetric stretching vibration), 1720 cm⁻¹ (imide C═O symmetric stretching vibration), 1378 cm⁻¹ (C─N stretching vibration), and 1016 and 745 cm⁻¹ (C─N─C stretching vibration) are assigned to pure PI, while the characteristic peaks at 1630 cm⁻¹ (C═O) and 3428 cm⁻¹ (─OH) are assigned to Ti₃C₂T_x (43). All the above characteristic peaks can be observed in the spectrum of PI/Ti₃C₂T_x, while the peaks of C═O and ─OH slightly shift to lower wave numbers. These shifts are attributed to the formation of hydrogen bonding between the ─OH in Ti₃C₂T_x and the C═O in PI chains, which reveals the strong interaction on the interface between them (19, 44, 45). As confirmed by x-ray photoelectron spectroscopy (XPS) broad survey spectra (Fig. 2B), the significantly increased C 1s and O 1s signals along with an additional peak of N 1s for PI/Ti₃C₂T_x further prove the successful fabrication of composite films in accordance with the ATR-FTIR results. Meanwhile, obvious peaks at 458.8 and 464.2 eV for TiO₂ 2p_3/2 and TiO₂ 2p_1/2 are observed in the Ti 2p core level XPS spectra of Ti₃C₂T_x, but no obviously increasing intensity is detected in the spectrum of PI/Ti₃C₂T_x (fig. S10). Furthermore, in the XRD patterns (fig. S11), the increasing intensity of typical diffracted peaks at 6.3° for the crystal (002) plane assigned to Ti₃C₂T_x can be found with the increased Ti₃C₂T_x content, while the diffracted peaks for TiO₂ are too weak to be detected. These results illustrate that no conspicuous oxidization of Ti₃C₂T_x occurs during the assembly process of PI and Ti₃C₂T_x owing to the vacuum environment as well as highly effective protection of PI to the Ti₃C₂T_x flakes, thus guaranteeing the final EMI properties of PI/Ti₃C₂T_x films.

Fig. 2. Structural characterization of PI/Ti₃C₂T_x films.

(A) FTIR spectra of PI, Ti₃C₂T_x, and PI/Ti₃C₂T_x films. a.u., arbitrary units. (B) XPS broad survey spectra of PI, Ti₃C₂T_x, and PI/Ti₃C₂T_x films. (C) Ti₃C₂T_x content and density of PI/Ti₃C₂T_x composite films. (D) Cross-sectional SEM images of PI/Ti₃C₂T_x films. (E) EDX mapping images of PI/Ti₃C₂T_x film surface. (F) The formation mechanism of hierarchical porous architectures in PI/Ti₃C₂T_x films during hot pressing. Photo credit: Yang Cheng, Fudan University.

The well arrangement of Ti₃C₂T_x flakes and the morphologies of the final PI/Ti₃C₂T_x films are crucial to the EMI shielding performance, mechanical performance, stability, and low-weight feature. As shown in Fig. 2C, the proportion of Ti₃C₂T_x flakes in the PI/Ti₃C₂T_x film increases from 0.2 to 2.0 volume % with the densifying of Ti₃C₂T_x aqueous suspension from 2 to 10 mg/ml. However, as the Ti₃C₂T_x content increases to 2.0 volume %, the density of the PI/Ti₃C₂T_x film tends to decrease from 1.23 to 0.39 g/cm³, which is much lower than the densities of both pure PI (PIM-0, 1.23 g/cm³) and single Ti₃C₂T_x flake (≈4.00 g/cm³) (46), revealing the presence of abundant voids in the composite film. The cross-sectional SEM images of PI/Ti₃C₂T_x films (Fig. 2D) demonstrate the hierarchically porous architectures and gradually expanded voids between layers with the increase of Ti₃C₂T_x content, verifying the gradually decreasing density property. In addition, the energy-dispersive x-ray (EDX) mapping of specific Ti elements in Ti₃C₂T_x is adopted to present the distribution of Ti₃C₂T_x flakes in PI/Ti₃C₂T_x films (Fig. 2E). Obviously, with the rise of Ti₃C₂T_x content, the Ti₃C₂T_x flakes are increasingly serried and even overlapped with each other to create consecutively conductive pathways of Ti₃C₂T_x flakes in the composite films. The overlapped Ti₃C₂T_x flake layers, PI layers, and air layers (voids) compose of the unique hierarchical consecutively conductive porous architectures in the PI/Ti₃C₂T_x films, and the formation mechanism is illustrated in Fig. 2F. When the PI aerogels were immersed into Ti₃C₂T_x aqueous suspension, the Ti₃C₂T_x flakes were well arranged to cover on the internal walls forced by the highly aligned cellular architectures in unidirectional PI aerogels. The covered areas enlarged with the densification of Ti₃C₂T_x aqueous suspension until the highly conductive Ti₃C₂T_x flakes overlap with each other on the internal walls. During hot pressing, the internal walls of pure PI will adhere to each other, but the Ti₃C₂T_x flakes covering on the internal walls can obstruct this adhesion. Upon releasing, the pure PI layers without the obstruction of Ti₃C₂T_x flakes stay totally conglutinate to each other and so formed a dense film (PIM-0). However, owing to the good resilience of unidirectional PI aerogels (fig. S12), the internal walls of the aerogels partially covered with Ti₃C₂T_x flakes were somewhat resilient to generate some voids (partial adhesion) after removing the pressure. The internal walls covered with consecutively overlapped Ti₃C₂T_x flakes were resilient to complete separation (no adhesion), resulting in hierarchically porous architectures and consecutively conductive pathways composed of consecutive Ti₃C₂T_x flakes in the composite films. Moreover, a small amount of residual bound water in the Ti₃C₂T_x-coated PI aerogel after lyophilization turned into vapor during hot pressing at 300°C and evolved out, which may also promote the separation of contacted layers to generate voids in the PI/Ti₃C₂T_x composite films. To this end, the unique architectures are equivalent to splitting one conductive layer in conventional polymer/Ti₃C₂T_x alternating composite films into two layers, creating the Ti₃C₂T_x/air/Ti₃C₂T_x structures. The porous architectures in the films will not only result in a much lower composite film density but also maximize the reflection interfaces for penetrated EMWs to largely increase the multiple reflection loss. With the hierarchical consecutively conductive porous architectures, it is greatly promising for construction of lightweight polymer-based composite films with high EMI shielding performance.

Mechanical performance of PI/Ti₃C₂T_x films

The stable thermal and mechanical properties of PI/Ti₃C₂T_x film are of great significance for the design of high-performance EMI shielding materials for aerospace applications. Figure 3A illustrates that the PIM-10 is capable of being folded and withstanding a load of 2 kg, which is nearly 340,000 times of its own weight, visually revealing satisfactory flexibility and strong mechanical properties. As shown in Fig. 3 (B and C), PI/Ti₃C₂T_x films exhibit strong tensile strength and large fracture strain. However, the incorporation of Ti₃C₂T_x may impair mechanical performance owing to the gradually enlarged voids in the composite films with the increase of Ti₃C₂T_x content (Fig. 2D). Despite the negative effects of the massive voids in the composite films, PIM-10 still retains the tensile modulus of 2.1 GPa, the tensile strength of 129 MPa, and the fracture strain of 25%, which are far superior to the self-supported Ti₃C₂T_x films (47, 48). In addition, the mechanical performance of PI/Ti₃C₂T_x films displays remarkable environmental stability owing to its high glass transition temperatures (T_g) of up to 255°C (fig. S13), thermal decomposition temperatures of up to 563°C (fig. S14), and ultrahigh storage modulus of up to 3.1 GPa (fig. S15). For example, PI/Ti₃C₂T_x films display improved dimensional stability with the increase of Ti₃C₂T_x content, and even the dimensional variation of PIM-10 is less than 0.5% versus temperatures from −70° to 250°C, which is capable to rival aluminum and much better than polyamide (Fig. 3D) (49, 50). PIM-10 is presented as an example for further investigation of mechanical stability. As shown in Fig. 3E, the tensile tests were carried out at −100°, 25°, and 250°C, respectively. It displays remarkable temperature-invariant mechanical performance, which is important for EMI shielding applications in extreme environments. Furthermore, after resisting rapid cyclic thermal shock 30 times between −196° and 250°C (∆T = 446°C), PIM-10 still maintains a similar mechanical performance, displaying notable mechanical stability (Fig. 3F). In addition, to determine the negative effects of creep deformation and stress relaxation on the long-term service of polymer-based materials, we carried out the cyclic tensile tests (strain, 1%) and the bending tests with the frequency of 1 Hz to estimate the mechanical durability. As shown in Fig. 3 (G and H), no significant deteriorations of the mechanical performance of PIM-10 are observed, even after 10,000 stretching-releasing and bending-releasing cycles, indicating high mechanical stability for long-term service.

Fig. 3. Thermal and mechanical properties of PI/Ti₃C₂T_x films.

(A) Digital images of the strong and flexible performance of PI/Ti₃C₂T_x films. (B) Tensile stress-strain curves of the PI/Ti₃C₂T_x films. (C) Tensile strength and modulus of the PI/Ti₃C₂T_x films. (D) Thermal expansion of PI/Ti₃C₂T_x films. (E) Tensile stress-strain curves of PIM-10 at different temperatures. (F) Tensile stress-strain curves of PIM-10 before and after rapid thermal shock for 15 and 30 cycles. (G) The tensile durability test under 1% strain of PIM-10 at 25° and 250°C. (H) The bending durability test of PIM-10 at 25° and 250°C. Photo credit: Yang Cheng, Fudan University.

EMI shielding performance of PI/Ti₃C₂T_x films

The hierarchical porous PI/Ti₃C₂T_x film filled with consecutively conductive pathways of Ti₃C₂T_x flakes in the PI matrix are expected to have enhanced EMI shielding performance. As shown in Fig. 4A, the light-emitting diode lamp connected with the PI/Ti₃C₂T_x films becomes brighter with higher Ti₃C₂T_x content, corresponding to the increased electrical conductivity. The surface conductivity of PIM-10 can be as high as 1.6 × 10³ S/cm tested by a four-point probe method attributed to the formation of numerous junction points and highly efficient Ti₃C₂T_x conductive networks on the surface of PI. High electrical conductivity is a prerequisite for the improved EMI SE of PI/Ti₃C₂T_x films. Thus, with the increase of Ti₃C₂T_x content from 0.2 to 2.0 volume %, the EMI SE of PI/Ti₃C₂T_x film exhibits a significant ascending trend from 19 to 77.4 dB (Fig. 4B) in the X band with a thickness of 210 μm. In addition, Fig. 4C demonstrates that the EMI SE decreases from 77.4 to 54.5 dB as the thickness of PI/Ti₃C₂T_x films varies from 210 to 90 μm, revealing the crucial role of thickness in tuning EMI shielding performances. In comprehensive consideration of the thickness, density, and EMI SE, the highest achievable absolute SE value of SSE/t (SE divided by density and thickness of samples) can reach 15,527 dB cm² g⁻¹ at a thickness of 90 μm with a low MXene content of 2.0 volume %. As illustrated in Fig. 4D, the layer-by-layer attenuation process of the incident EMWs in the PI/Ti₃C₂T_x films was simulated via finite element method (FEM) for a relative comparison of EMI shielding performance of PI/Ti₃C₂T_x composite film with various thicknesses and Ti₃C₂T_x contents. In addition, the EMI SE values at 8.2 GHz of PI/Ti₃C₂T_x films are growing with the increase of thickness and Ti₃C₂T_x content, which are in accordance with the experimentally measured results.

Fig. 4. EMI shielding properties and mechanism of PI/Ti₃C₂T_x films.

(A) Electrical conductivities of PI/Ti₃C₂T_x films. (B) EMI SE of the PI/Ti₃C₂T_x films with various Ti₃C₂T_x contents in X band. (C) EMI SE of the PIM-10 with various thicknesses in X band. (D) Simulated EMI SE of PI/Ti₃C₂T_x films with various Ti₃C₂T_x contents at 8.2 GHz via finite element method. (E) SE_total, SE_A, SE_R, A, and R of the PI/Ti₃C₂T_x films in X band. (F) Synergistic reaction mechanism of hierarchical porous architectures and consecutively conductive pathways in PI/Ti₃C₂T_x films. (G) Comparison of EMI SE performance of PIM-10 with PI/Ti₃C₂T_x films fabricated by uniformly compounding method. (H) Comparison of $\left(\frac{\text{SSE}}{t}\right)/V_M$ values of PIM-10 with polymer/MXene films in previous literatures. (I) EMI SE of the PI/Ti₃C₂T_x films with various Ti₃C₂T_x contents in Ku band. Photo credit: Yang Cheng, Fudan University.

To investigate the EMI shielding mechanisms of PI/Ti₃C₂T_x composite film, we shall individually analyze the absorption coefficient (A) and reflection coefficient (R). As shown in Fig. 4E, R is much higher than A for all PI/Ti₃C₂T_x composite films, indicating that they are highly reflective materials due to the impedance mismatch. It means that the predominate EMI shielding mechanism of PI/Ti₃C₂T_x composite films is reflection loss, and a small part of incident EMWs can penetrate into the composite film to be absorbed. In addition, the total values of R and A for all PI/Ti₃C₂T_x composite films are much close to 100%, revealing that all the penetrated EMWs have been absorbed, especially PIM-2 that absorbs 21% of incident EMWs. Besides, the dominate contribution of SE_A to the overall EMI SE (65 to 79%) also testifies the highly efficient attenuation of penetrated EMWs within the PI/Ti₃C₂T_x films. The reflection loss for the incident EMWs and highly efficient attenuation for penetrated EMWs are mainly attributed to the satisfying electrical conductivity and hierarchical porous architectures of PI/Ti₃C₂T_x films. Forced by the highly ordered PI aerogels, the Ti₃C₂T_x flakes tend to form consecutively conductive pathways, which greatly enhance the electrical conductivity. Thus, as shown in Fig. 4F, the consecutive Ti₃C₂T_x flakes serve as improved efficient reflection loss for incident EMWs owing to the increased electrical conductivity (σ) according to Simon’s formula (35, 51, 52)

$$\text{EMI} \text{SE}=50+10 \text{lg}(\mathrm{\sigma }/f)+1.7t\sqrt{\mathrm{\sigma }f}$$ (1)

where f (MHz) is the frequency of incident EMWs, σ (siemens per centimeter) is the electrical conductivity, and t (centimeters) is the thickness of the EMI shielding film. Moreover, different from the reported two-component alternating multilayer structure of polymer/MXene, air layers are incorporated into the PI/Ti₃C₂T_x films forming multiple interfaces to increase propagation paths of penetrated EMWs and promote polarization loss, largely giving rise to the EMW attenuation inside the PI/Ti₃C₂T_x film. On the one hand, the different permittivity of PI (κ = 2.5 to 3.5) and air (κ = 1) creates two types of impedance mismatch between PI and Ti₃C₂T_x, as well as air and Ti₃C₂T_x. During multiple reflection, the initial planar wavefronts are inclined to miss the phase coherence owing to the phase shifts for various propagation paths in two types shielding layers of Ti₃C₂T_x/PI/Ti₃C₂T_x and Ti₃C₂T_x/air/Ti₃C₂T_x, and the phase mismatching causes EMW attenuation like another absorption mechanism. Thus, the SE_A in can be described for PI/Ti₃C₂T_x films constructed with hierarchical consecutively conductive porous architectures by the following equation

$${\text{SE}}_A={\displaystyle {\sum }_{i=1}^n}{\text{SE}}_{A1i}+{\displaystyle {\sum }_{i=1}^{n-1}}{\text{SE}}_{A2i}$$ (2)

where SE_A1i and SE_A2i are the SE of reflection within each Ti₃C₂T_x/PI/Ti₃C₂T_x layer and each Ti₃C₂T_x/air/Ti₃C₂T_x layer, respectively, and i stands for the number of PI layers. On the other hand, local dipoles between Ti and terminating groups (─F, ═O, and ─OH) especially highly electronegative ─F on the Ti₃C₂T_x flakes surfaces may be created when subjected to an alternating electromagnetic field, which promotes the attenuation of penetrated EMWs due to interfacial polarization loss (7). The multiple interfaces in the hierarchically porous PI/Ti₃C₂T_x films are capable of further promoting polarization loss to enhance EMI shielding performance due to much increased polarized interfaces, especially when the films are low electrical conducive (4, 53–55). All in all, the structural advantages of consecutively conductive pathways of Ti₃C₂T_x flakes and hierarchical porous architectures shall be responsible for the extraordinary EMI shielding performance of PI/Ti₃C₂T_x film in the X band with such a low Ti₃C₂T_x content down to 2.0 volume %. To demonstrate the advantages of the above assembly, we set PI/Ti₃C₂T_x film produced by the directly compounding method as a comparison. Because of the insulating PI matrix between the isolated Ti₃C₂T_x flake, the composite film produced with the directly compounding method displays inferior electrical conductivity, resulting in 56-dB lower EMI SE than that of PIM-10 with the same thickness of 210 μm and Ti₃C₂T_x content of 2.0 volume % in the X band (Fig. 4G). The definition of $\left(\frac{\text{SSE}}{t}\right)/V_M$ is introduced to evaluate the performance and economic efficiency of polymer/MXene films for EMI shielding, where V_M is the volume fraction of Ti₃C₂T_x in the polymer-based composite films. As shown in Fig. 4H, the PI/Ti₃C₂T_x film in this work has obvious advantage compared to previous literatures (7, 21, 29, 36, 37, 56–61). It demonstrates that the special designed architectures give full play to the EMI shielding ability of Ti₃C₂T_x in polymeric matrix, which is beneficial to construct polymer/MXene films with high EMI shielding performance at low MXene content. To determine the further possible application environments, we have also measured EMI shielding performance of PI/Ti₃C₂T_x films in the Ku band with various Ti₃C₂T_x contents and thicknesses. Similar to the variation tendency in the X band, the EMI SE of PI/Ti₃C₂T_x films display an obviously ascending trend with the increase of Ti₃C₂T_x content and thickness (Fig. 4I and fig. S16). PIM-10 (210 μm) has an EMI SE of 83 dB in the Ku band, demonstrating the similar high EMI shielding performance as that in the X band.

Environmentally stable EMI SE and processability of PI/Ti₃C₂T_x films

The complex morphology and harsh working environments present a great challenge for the stable shielding performance and easy processability of EMI shielding materials. In view of the easy oxidization of Ti₃C₂T_x in the humid air, the long-term stability of EMI shielding performance of PI/Ti₃C₂T_x films has been investigated in the X band. As shown in Fig. 5A, only 1.8- to 9.4-dB declines of EMI SE have been observed during 30 days in a hygrothermal environment (70°C, 60 to 85% humidity) among all PI/Ti₃C₂T_x films with various Ti₃C₂T_x contents, revealing satisfying hydrothermal stability. When the PI/Ti₃C₂T_x film was treated in the hydrothermal environment, the surface Ti₃C₂T_x flakes were easy to be oxidatively degraded owing to the direct reaction with hydrothermal air. Nevertheless, the predominant internal Ti₃C₂T_x flakes are still intact according to the Ti2p core level XPS spectra of internal surfaces (fig. S17). As shown in Fig. 5B, the dense PI layer under the surface Ti₃C₂T_x flakes is capable of preventing the further erosion of O₂ and H₂O to the internal Ti₃C₂T_x flakes owing to its thermal and oxygen stability and thermal insulation. Besides, despite the presence of voids, the side gaps of the films are less than 2 μm, which brings huge obstacles for the entry of O₂ and H₂O into the film and ameliorates the oxidative degradation of internal Ti₃C₂T_x flakes. Thus, after treatment in the hydrothermal environment for 30 days, the intact internal Ti₃C₂T_x flakes still contribute to the high conductivity up to 1.4 × 10³ S/cm and decreased only 12.5% as shown in Fig. 5C. PIM-10 film was also been treated in many other harsh conditions, such as ultrasonic washing, cryogenic temperature, high temperature, cyclic rapid thermal shock, and flame, but it retains satisfying electrical conductivity of 1.4 × 10³ to 1.6 × 10³ S/cm after treatment providing the crucial premise for the high EMI shielding durability (fig. S18). In view of the weak hydrogen bonding and van der Waals interaction between the Ti₃C₂T_x flakes and the PI matrix, the binding stability of them usually raises some concerns. Through the comparison of EDX mapping results before (Fig. 2E) and after (fig. S19) ultrasonic washing (300 W and 40 kHz) for 1 hour, very little decline of Ti₃C₂T_x contents is detected, revealing the strong bonding between Ti₃C₂T_x flakes and PI matrix. Furthermore, Fig. 5D demonstrates that PIM-10 displays almost no decline of EMI shielding performance in the X after ultrasonic treatment, demonstrating high EMI shielding stability. Besides, aiming at the extreme working conditions of some electronic devices in spacecrafts, such as low- or high-temperature environments, the EMI shielding retainability of PIM-10 before and after treatment in −196° and 250°C has been detected. As shown in Fig. 5E, PIM-10 has almost identical EMI shielding performance after 30 bending cycles in liquid N₂ (−196°C) and only a tiny decline in EMI SE after treatment in 250°C for 1 hour (Fig. 5F). Besides, PIM-10 can even retain inconspicuous decline of EMI SE in the X band even after rapid cyclic thermal shock between above temperatures (∆T = 446°C) 30 times (Fig. 5G). PI is not only born with high thermal stability and cryogenic tolerance but also has ultralow thermal conductivity of 0.1 to 0.5 W m⁻¹ K⁻¹, displaying high thermal insulation performance (42, 62). While the hierarchically porous PI/Ti₃C₂T_x composite film was resisting the thermal attack, the outer PI layers and void layers are capable of insulating the most ambitious thermal attack, allowing inner Ti₃C₂T_x layers to confront much lower temperature than ambient temperature. Thus, only few outer Ti₃C₂T_x flakes were oxidatively degraded, but most of inner Ti₃C₂T_x flakes avoided being attacked and kept high EMI shielding performance. Moreover, after putting PIM-10 in a flame for 7 s, it exhibits not only good retainability of EMI shielding performance but also satisfying flame resistance, which is very important for electrical safety (Fig. 5H). The easy shape processability of EMI shielding materials without shape restrictions is usually contradictory to their resistance to extreme conditions. However, the flat PI/Ti₃C₂T_x films are capable to be processed into various shapes, such as circle, dodecagon, hexagon, square, triangle, and rhombus, via a scalable heat-squeezing method with designed molds (Fig. 5I), which is of great significance to fit the modern electronic devices with complex morphologies. The PI/Ti₃C₂T_x films with highly stable performance in extreme environments and easy processability display huge potential in EMI shielding applications for electronic devices with harsh working conditions and complex morphologies.

Fig. 5. Stable EMI shielding performance after resisting harsh conditions and processability of PI/Ti₃C₂T_x films.

(A) EMI SE of PI/Ti₃C₂T_x films treated in hygrothermal environment. (B) Illustration for the protective role of PI layer in hydrothermal environment. (C) Conductivity and decrement of conductivity of PI/Ti₃C₂T_x films after treatment in hygrothermal environment. (D) EMI SE of the PIM-10 before and after ultrasonic cleaning for 1 hour. (E) EMI SE of the PIM-10 before and after bending in liquid N₂ (−196°C) 30 times. (F) EMI SE of the PIM-10 before and after treatment at 250°C for 1 hour. (G) EMI SE of the PIM-10 before and after thermal shock between −196° and 250°C 30 times. (H) EMI SE of the PIM-10 before and after putting on fire for 7 s. (I) Processability of PIM-10. Photo credit: Yang Cheng, Fudan University.

DISCUSSION

In summary, a unique hierarchically porous PI/Ti₃C₂T_x film with stable high EMI shielding performance and strong mechanical performance in extreme conditions has been designed and produced using a unidirectional PI aerogel–assisted immersion and hot-pressing method. During immersion, the Ti₃C₂T_x nanosheets were forced into an orderly arrangement by the highly aligned cellular structure in the PI aerogel, forming consecutively conductive pathways that serve as highly conductive reflection walls for incident EMWs. Owing to the consecutive nonsticky nature of Ti₃C₂T_x flakes and superelasticity of unidirectional PI aerogels, the compressed aerogel is slightly resilient to construct hierarchical porous architectures with consecutively conductive layers in PI/Ti₃C₂T_x films after hot processing. The synergistic effect of the above specially designed architecture endows PI/Ti₃C₂T_x composite film with a high absolute EMI SE of up to 15,527 dB cm² g⁻¹ at a thickness of 90 μm with only 2.0 volume % Ti₃C₂T_x. It mainly attributed to consecutively conductive pathways and hierarchical porous architectures endowing the PI/Ti₃C₂T_x films with high reflection loss of incident EMWs at the surfaces and highly efficient attenuation of penetrated EMWs within the films. Meanwhile, benefiting from the protection of the PI matrix with thermal and cryogenic resistance, such high EMI shielding performance and strong mechanical performance of PI/Ti₃C₂T_x film can be maintained after treatment in harsh environment. Moreover, the easy processability of the PI/Ti₃C₂T_x films is of great significance to the scalable applications in modern electronic devices with complex morphologies. In short, the special designed PI/Ti₃C₂T_x film can satisfy the comprehensive requirements and displays huge potential in scalable aerospace applications for the sake of its superior EMI shielding performance and mechanical performance resistant to extreme conditions, low cost (2.0 volume % Ti₃C₂T_x), lightweight (0.39 g/cm³), and easy processability.

MATERIALS AND METHODS

Materials

ODA (99.5%) and ODPA (99.5%) were purchased from Changzhou Sunlight Pharmaceutical Co. Ltd. DMSO was purchased from Shanghai Taitan Technology Co. Ltd. and dried with molecular sieves before use. Acetic anhydride (analytical reagent), triethylamine (analytical reagent), hydrochloric acid (HCl, 6.0 to 38.0%), and lithium fluoride (LiF, analytical reagent) were purchased from Sinopharm Chemical Reagent Co. Ltd. The MAX phase of Ti₃AlC₂ was provided by Jilin 11 Technology Co. Ltd.

Fabrication of unidirectional PI aerogels

First, a mixture of 133.5 g of DMSO, 3.0036 g of ODA (15 mmol), and 4.6532 g of ODPA (15 mmol) was stirred in a 250-ml three-necked flask equipped with a nitrogen inlet to carry out the polymerization. After stirring for 12 hours at room temperature, PI/DMSO solution with a solid content of 6 wt % was obtained by adding 3.0627 g (30 mmol) of acetic anhydride and 3.3393 g (30 mmol) of triethylamine into the poly(acrylic acid)/DMSO solution and stirring for 1 hour. The PI/DMSO solution was diluted to 1.0 wt % by adding more DMSO solvent. A unidirectional freeze-casting process was carried out by adding PI/DMSO solution into a cubical mold on a freezing stage of −60°C. After the solution was frozen entirely, the frozen gel was kept in the refrigerator for 24 hours. Then, the sample was freeze-dried for 4 days in a freeze dryer with temperatures of −110°C in the cold trap and − 3°C in the sample chamber, while the pressure was kept at 1 Pa. The dried samples were treated at 250°C in a vacuum oven for 3 hours to obtain the unidirectional PI aerogels. As a comparison, the unidirectional PI/Ti₃C₂T_x composite aerogel with directly compounded PI and 2.0 volume % Ti₃C₂T_x was fabricated (freeze casting of the mixture followed by the freeze-drying process, instead of immersing pure PI aerogel in Ti₃C₂T_x aqueous suspension).

Fabrication of Ti₃C₂T_x suspension

Two-gram MAX phase of Ti₃AlC₂ was added into the uniform mixture with 40 ml of HCl (9 M) and 2.0 g of LiF in a polytetrafluoroethylene container. After magnetic stirring of the mixture for 48 hours at 40°C to remove the Al layer, the processes of centrifugation at 10,000 rpm for 10 min and washing with deionized water were carried out alternately until the mixture became neutral. The precipitate is collected and dried in a vacuum oven at 60°C for 12 hours to obtain multilayer Ti₃C₂T_x powder. After undergoing ultrasonic exfoliation in water for 1 hour at 5°C, the mixture was centrifuged again for 10 min under 4500 rpm to collect the upper suspension with exfoliated Ti₃C₂T_x flakes. The Ti₃C₂T_x colloidal suspensions with controlled concentrations (2, 4, 6, 8, and 10 mg/ml) were obtained after freeze drying and redispersion in water.

Fabrication of PI/Ti₃C₂T_x composite films

With the aid of a vacuum, the unidirectional PI aerogels were immersed into the Ti₃C₂T_x suspensions with various concentrations (0, 2, 4, 6, 8, and 10 mg/ml) for 24 hours at room temperature. After freeze-drying for 24 hours, dried PI aerogels adhered with Ti₃C₂T_x flakes were obtained. Subsequently, the hot pressing was carried out by clamping the PI/Ti₃C₂T_x aerogels between two clamped stainless steel plates with releasing agent and clamping force of 200 N in a vacuum oven at 300°C for 30 min. For context, note that, the hot pressing was also performed on the unidirectional PI/Ti₃C₂T_x composite aerogel with a uniform composition of 2.0 volume % Ti₃C₂T_x to produce a PI/Ti₃C₂T_x film with directly compounded Ti₃C₂T_x flakes.

Characterizations

The contents of Ti₃C₂T_x were measured by weighing the increased mass of PI/Ti₃C₂T_x film as compared with the PI aerogel before immersion. In addition, the volume fraction V_M = (ρ_C/ρ_M)W_M, where the W_M is the Ti₃C₂T_x mass fraction, ρ_C is the density of the PI/Ti₃C₂T_x composite films, and ρ_M is the density of Ti₃C₂T_x. The density of the PI/Ti₃C₂T_x films were measured by weighing films with precise dimensions. Five parallel measurements of contents and density were performed on each series of samples. ATR-FTIR spectroscopy was recorded on a Nicolet is10 spectroscope with the range of 4000 to 600 cm⁻¹ by averaging 32 scans. The crystalline structures of Ti₃AlC₂, Ti₃C₂T_x, and the PI/Ti₃C₂T_x films were examined by XRD (Bruker D8 ADVANCE x-ray diffractometer, Cu Kα radiation). XPS (PHI5000C and PHI5300) was adopted to investigate the surface elements of pure PI, Ti₃C₂T_x, and the PI/Ti₃C₂T_x films. The thickness of the Ti₃C₂T_x flakes was measured with a scanning probe microscope (Bruker, A27). The transparent feature of Ti₃C₂T_x flake was characterized by TEM (JEOL JEM-2100 LaB6) with an acceleration voltage of 200 kV, and the diluted Ti₃C₂T_x suspension was dropped on carbon substrate for testing. The microstructure of the aerogels was observed using a field-emission SEM (TESCAN MAIA3) at an accelerating voltage of 15 kV. The water contact angle test was carried out on a contact angle meter (DataPhysics OCA 40 Micro, Germany) with 2 μl of each drop of water. The tensile tests were carried out on an Instron 5966 material testing instrument, and five parallel tests were performed on each series of samples. The dimensional variations were measured with a thermal mechanical analyzer (TMA) of Mettler Toledo TMA/SDTA 2+ LN/600. The fatigue tests were performed on a TA ElectroForce 3220 Mechanical Test Instrument equipped with a heating oven. Differential scanning calorimetry (DSC) was performed with a Netzsch DSC 404F3 at a scan rate of 10°C/min in flowing nitrogen. The thermal conversion process was analyzed using a Netzsch TG 209 F1 Thermogravimetric Analyzer at a heating rate of 10°C/min in flowing nitrogen. Storage modulus were tested by DMA Q800 (TA Instruments) with a crosshead speed of 0.5 mm/min. The electrical conductivity was measured using a four-point probe resistance measurement system (MCP-T610, Mitsubishi Chemical). The measurements of EMI shielding performance were performed on a vector network analyzer (PNA, Keysight, N5227,10 MHz to 67 GHz) equipped with two waveguide-to-coaxial adaptors connected face-to-face in 8.2 to 12.4 GHz (X-band) and 11.9 to 18 GHz (Ku-band). EMI SE, R, and A were calculated from the S parameters according to the following equations

$$R={\mid S_{11}\mid }^2$$ (3)

$$A=1-{\mid S_{11}\mid }^2-{\mid S_{21}\mid }^2$$ (4)

$${\text{SE}}_R=-10\text{lg}(1-{\mid S_{11}\mid }^2)$$ (5)

$${\text{SE}}_A=-10\text{lg}\left(\frac{{\mid S_{21}\mid }^2}{1-{\mid S_{11}\mid }^2}\right)$$ (6)

$$\text{EMI} \text{SE}={\text{SE}}_R+{\text{SE}}_A$$ (7)

Simulations based on FEM

The attenuation process of incident EMWs in the PI/Ti₃C₂T_x composite films is implemented via COMSOL Multiphysics software. A simplified two-dimensional model of the hierarchically porous PI/Ti₃C₂T_x film cross section was created according to the SEM images. The physical field is based on EMWs (frequency domain), and planar EMWs with rated power and a frequency of 8.2 GHz were adopted. The model boundaries parallel to the propagated direction of EMWs were set as perfect magnetic conductor, and the vertical boundaries were given scattering conditions to make a boundary transparent for a scattered wave. The propagation of EMWs in the shielding blocks is described using following equation (63)

$$\nabla \times ({\mathrm{\mu }}_r^{-1}\nabla \times E(r))-k_0^2{\mathrm{\epsilon }}_{r}E(r)=0$$ (8)

where ∇ is the Laplace operator, μ_r is the permeability of materials, ε_r is the electrical conductivity of materials, and E(r) is the electrical field in the shielding blocks at a distance of r from the field source. k₀ is the wave number of free spaces, which is defined as

$$k_0=\mathrm{\omega }\sqrt{{\mathrm{\epsilon }}_0{\mathrm{\mu }}_0}=\frac{\mathrm{\omega }}{c_0}$$ (9)

where ω is the angular frequency and c₀ is the light speed in vacuum. Thus, EMI SE is described in decibels and defined as follows

$$\text{EMI} \text{SE}=20\text{lg}\left(\frac{E_0}{E(r)}\right)$$ (10)

where E₀ is the electrical field without shielding blocks.

Supplementary Materials

This PDF file includes:

Figs. S1 to S19

Table S1