Ultrathin and flexible polyimide/Ti₃C₂T_X MXene composite films for electromagnetic interference shielding with harsh environment tolerance

Abstract

Two-dimensional MXene materials show great potential for electromagnetic interference (EMI) shielding application because of their superior electrical conductivity. However, the susceptibility to oxidation and the mechanical brittleness of MXenes greatly limit their extensive application. Herein, an ultrathin flexible polyimide (PI)/Ti₃C₂T_X composite film with a nacre-like lamellar structure is successfully prepared using a simple vacuum filtration and thermal imidization process. Owing to the addition of PI, the composite film exhibits excellent mechanical properties with a tensile strength of 114.9 MPa and a fracture strain of 3.0%. Moreover, the nacre-like lamellar structure is conducive to multiple internal reflections, giving rise to the absorption of electromagnetic waves. The PI/Ti₃C₂T_X composite film exhibits an outstanding EMI shielding effectiveness of 37 dB in the X-band range with an ultrathin thickness of only 15 μm. Furthermore, PI serves as an antioxidation coating, and its EMI shielding performance is maintained even when the composite film is exposed to various harsh environments. Therefore, the resulting composite film exhibits excellent EMI shielding stability and durability, which enables the potential application of high-performance EMI shielding materials against harsh environments.

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Introduction

With the development of 5G technology, a rising number of high-frequency and high-power electronic devices are widely used in different aspects of life, resulting in increasingly serious electromagnetic interference (EMI) or radiation.^1,2 However, a large number of electromagnetic waves will seriously influence normal working and service life of sophisticated electronic equipment and pose a certain risk to human health.^3,4 Therefore, high-performance EMI shielding materials must be developed to address this issue.^5,6

Traditional metal shielding materials, such as copper, iron, and silver, have been widely used in the prevention of electromagnetic pollution due to their good electrical conductivity. However, high density, difficult processing, and non-corrosion resistance of these materials limit their application.^7–11 In recent years, carbon-based nanomaterials, such as carbon black,¹² carbon nanotubes,¹³ reduced graphene oxide,^14–16 and graphene,^17–20 have been regarded as excellent EMI shielding materials due to their corrosion resistance and low density.^21,22 However, the difficulties in dispersion and low EMI shielding capabilities limit the extensive application of these materials.^23,24 In contrast to metal and carbon materials, two-dimensional transition metal carbides and/or nitrides (MXenes) are expected to be the most promising candidate of high-performance EMI shielding materials and have received extensive attention, owing to their excellent electrical conductivity, hydrophilicity, and chemical activity.^25–27 However, the inherent poor mechanical properties and high susceptibility to oxidation of MXene materials hinder their practical application.^28–31

The most common method for improving the mechanical properties of MXenes is to introduce a polymer matrix to compensate for their deficiencies through the excellent mechanical properties of the polymer itself.^32–40 Gogotsi et al.⁴¹ fabricated polyvinyl alcohol (PVA)/Ti₃C₂T_X composite films with excellent mechanical properties by vacuum-assisted filtration. When the PVA content is 60%, the ultimate tensile strength and fracture strain are 91 ± 10 MPa and 4.0 ± 0.5% for the composite films, respectively, which are 4–5 times higher than those of the pure Ti₃C₂T_X film. Xie et al.⁴² chose aramid nanofibers (ANFs) as the polymer matrix, and when ANF was added at 80%, the ANF/Ti₃C₂T_X composite film had the best tensile strength of 197.4 MPa and a fracture strain of 9.8%. In addition, reasonable structural design is also beneficial to the mechanical and electromagnetic shielding performance of MXene-based electromagnetic shielding materials.⁴³ Sun et al.³³ prepared MXene@polystyrene (PS) nanocomposites with a segregated structure by electrostatic self-assembly and pressing. This structure allows the conductive filler to be isolated within the insulating polymer matrix, forming a conductive network with a low volume fraction, while enhancing the electromagnetic shielding capability. The MXene@PS nanocomposite film has an EMI shielding effectiveness (EMI SE) of 62 dB with only 1.9 vol % Ti₃C₂T_X. Xu et al.³⁵ reported MXene/PVA composite foam with porous structure by the freeze-drying method and rolled the foam to obtain the corresponding MXene/PVA porous composite film. The design of the porous structure produces multiple interfaces, which increases the propagation path of the electromagnetic waves and helps to attenuate them within the material. The MXene/PVA porous composite film has an EMI SE of 26 dB at a thickness of 100 μm. Zhou et al.⁴⁴ prepared alternating multilayer CNF@MXene composite films by a simple alternating vacuum filtration method. The design of the alternating multilayer structure can produce large internal scattering at internal interfaces due to the presence of different impedance phases, which helps to attenuate electromagnetic waves. The CNF@MXene composite film has an ultimate tensile strength of 112.5 MPa and an EMI SE of 40 dB at a thickness of 35 μm. Relatively, the most practical method is the construction of nacre-like lamellar, in which the polymer acts as a bonding agent for the two-dimensional MXene nanosheets, thus enhancing the mechanical strength and flexibility of the composite film. Moreover, this method facilitates multiple internal reflections, resulting in the absorption of electromagnetic waves. Using the vacuum filtration-induced self-assembly technique, Cao et al.⁴⁵ prepared a MXene/CNF composite paper with a nacre-like lamellar structure. The tensile strength and fracture strain of the composite paper are 135.4 MPa and 16.7%, respectively. Moreover, the composite paper exhibited a relatively good absolute shielding efficiency of 2647 dB cm² g⁻¹ at 47 μm thickness. In addition, bacterial cellulose,⁴⁶ epoxy resins,³⁴ polyvinylidene fluoride,³² and sodium alginate^47,48 can also be used as polymeric substrates to prepare MXene-based electromagnetic shielding materials. However, the EMI SE of these polymers is not stable in harsh environments, which can significantly limit their application. Among the polymeric materials, polyimide (PI) can be used as an ideal candidate substrate because of its excellent mechanical properties, high and low temperature resistances, and flame retardancy,^49–54 which will widen its applicability as an electromagnetic shielding material.

Herein, an ultrathin and flexible PI/Ti₃C₂T_X composite film with a nacre-like lamellar structure was successfully designed and fabricated using a simple vacuum filtration and thermal imidization process. The design of nacre-like lamellar structures allows PI to join the Ti₃C₂T_X nanosheets together well, which plays a mechanically reinforcing role. This design also facilitates multiple reflections of electromagnetic waves between adjacent layers, acting as an internal loss. In addition, the PI/Ti₃C₂T_X composite film can maintain a high and stable EMI SE under various harsh environments (high temperature (250 °C), low temperature (−196 °C), ultrasonic, and hygrothermal environment). Compared with other MXene composites, the PI/Ti₃C₂T_X composite film has a simple preparation process with excellent EMI SE, even in harsh environments, providing good application prospects in the field of flexible wearable electronic devices and aerospace fields.

Experimental

Materials and chemicals

3,3′,4,4′-Biphenyltetracarboxylic dianhydride (BPDA, ≥97%), 4,4′-diaminodiphenyl ether (ODA, ≥98%), and lithium fluoride (LiF, ≥99%) were obtained from Shanghai Aladdin Biochemical Technology Co. Triethylamine (TEA, ≥99%), N,N-dimethylacetamide (DMAc, ≥99%) and hydrochloric acid (HCl, AR) were purchased from Shanghai Titan Technology Co. Titanium aluminum carbide (Ti₃AlC₂, 200 mesh) was provided by Beijing Huawei Ruike Chemical Co. All chemicals and materials were used as received.

Preparation of Ti₃C₂T_X MXene nanosheets

Ti₃C₂T_X nanosheets were prepared based on the modified minimally intensive layer delamination method.^55,56 In general, 1.6 g of lithium fluoride was dissolved in 20 mL of hydrochloric acid (9 mol L⁻¹) for 10 min to prepare the in situ HF solution. Then, 1.0 g of Ti₃AlC₂ was added to the aforementioned HF solution and reacted at 40 °C for 24 h. The final reaction mixture was washed 4–5 times with deionized water (by centrifugation at 8000 rpm for 5 min) until the pH of the supernatant reached above 6. Finally, the resulting swollen deposits diluted with deionized water were treated by sonication under argon protection for 1 h and then centrifuged at 3500 rpm for 1 h. The supernatant was obtained as the required few-layer Ti₃C₂T_X at a concentration of approximately 2 mg mL⁻¹.

Preparation of water-soluble polyamide acid (PAA)

Triethylamine-capped polyamide acid (TEA-PAA), the water-soluble PI precursor was synthesized by the polycondensation method reported previously,^17,57 and can play a good protective role similar to other water-based polymers.^58–60 Typically, 4.0796 g of ODA was first dissolved in 30 g of DMAc solution, then 5.9204 g of BPDA was dissolved in 60 g of DMAc solution, and they were mechanically stirred in an ice-water bath for 6 h to obtain a transparent yellowish viscous solution. Subsequently, the solution with a solid content of 10 wt% was dropped into deionized water at 0 °C for precipitation. Finally, the precipitate was repeatedly washed 5 times, and the solid PAA was obtained after freeze-drying. Approximately 1 g of the aforementioned PAA was dissolved in 98.525 g of aqueous solution containing 0.475 g of TEA and stirred for 3 h until dissolved to obtain a TEA-PAA solution with a 1 wt% concentration.

Preparation of PI/Ti₃C₂T_X composite films

The PI/Ti₃C₂T_X composite films were fabricated by vacuum-assisted filtration and thermal imidization process (Fig. 1). Specifically, 15 mL of Ti₃C₂T_X aqueous dispersion (2 mg mL⁻¹) was added dropwise to the 1 wt% TEA-PAA solution with different qualities under continuous agitation. Subsequently, the mixture was sonicated for 10 min and stirred for 3 h to form a homogeneous suspension. Thereafter, PAA/Ti₃C₂T_X films were prepared by vacuum-assisted filtration. Finally, PI/Ti₃C₂T_X composite films were prepared by thermal imidation in a vacuum oven at 100, 150, 200 and 250 °C for 1 h, respectively. The mass ratios of PI to Ti₃C₂T_X were 10 : 90, 20 : 80, 30 : 70, and 40 : 60 and denoted as PM-1, PM-2, PM-3, and PM-4, respectively.

Fig. 1 Schematic illustration for the fabrication of flexible conductive PI/Ti₃C₂T_X composite films.

Characterization and measurements

A transmission electron microscope (TEM, JEM-2100, Japan) and a field emission scanning electron microscope (SEM, S-4800, Japan) were used to characterize the microstructure and morphology of the samples. The phase compositions of the samples were analyzed using X-ray diffractometry (XRD, D/max2550VB/PC, Japan). Dynamic light scattering (DLS, Zetasizer Nano Series, USA) was conducted to characterize the size distribution of Ti₃C₂T_X nanosheets. X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, UK) was performed to analyze the surface chemistry of the samples. The thermal properties were characterized using a thermogravimetric analyzer (TGA, Netzsch TG209, Germany) in nitrogen from 30 °C to 800 °C at a heating rate of 10 °C min⁻¹. The infrared spectra of the samples were measured using Fourier transform infrared spectroscopy (FTIR, Nicolet is50, USA). The tensile properties were measured on an electronic universal testing machine (E42.503, China) by cutting each sample into 30 mm × 5 mm strips at a loading rate of 1 mm min⁻¹. Conductivity was measured by using a four-point probe test system (RTS-8). EMI SE was measured based on the waveguide method with a vector network analyzer (Keysight, N5222B, USA) in the frequency range of 8.2 to 12.4 GHz (X-band). The power coefficients for reflection (R), absorption (A), transmission (T), and the total EMI shielding efficiency (EMI SE_T) are calculated from the scattering parameters (S₁₁ and S₂₁) according to the following equations:^19,45,61,62

R = |S₁₁|² = |S₂₂|² (1)

T = |S₂₁|² = |S₁₂|² (2)

A = 1 − R − T (3)

SE_R = −10 log(1 − R) (4)

SE_T = SE_A + SE_R + SE_M (6)

where SE_R is the microwave reflection, SE_A is the microwave absorption, and SE_M is the microwave multiple internal reflection. When SE_T ≥ 15 dB, SE_M can be neglected.^19,45,61,62

Results and discussion

Characterizations of PI/Ti₃C₂T_X composite films

First, the microstructure and morphology of Ti₃C₂T_X nanosheets was characterized. The SEM result shows that Ti₃AlC₂ has a tightly packed structure (Fig. 2(a)), while the etched Ti₃C₂T_X has an accordion-like multilayer structure (Fig. 2(b)). The size of the Ti₃C₂T_X nanosheets ranging from 500 nm to 2 μm can be confirmed by using TEM (Fig. 2(c)) and DLS (Fig. 2(d)). Meanwhile, owing to the nanometer dimension of Ti₃C₂T_X nanosheets, a typical “Tyndall effect” can be observed (Fig. 2(e)). In addition, the successful preparation of Ti₃C₂T_X nanosheets can be directly demonstrated by the XRD pattern (Fig. 2(f)). The shift of the (002) peak from 9.5° to 7.1° and the significant reduction or disappearance of the diffraction peaks confirm the elimination of Al after etching and the increase in the Ti₃C₂T_X layer spacing after exfoliation. The XPS results show the introduction of a large amount of fluorine on the Ti₃C₂T_X surface after etching (Fig. 2(g)) and the disappearance of the peak on the Al 2p orbital (Fig. 2(h)), further confirming that Ti₃C₂T_X nanosheets are prepared successfully.

Fig. 2 (a) SEM image of Ti₃AlC₂. (b) SEM image, (c) TEM image, (d) DLS image, and (e) the Tyndall effect of Ti₃C₂T_X. (f) XRD image, (g) XPS image, and (h) Al 2p spectra of Ti₃AlC₂ and Ti₃C₂T_X.

PM composite films with different PI contents were obtained by solution blending Ti₃C₂T_X aqueous dispersions with different masses of water-soluble PAA, followed by vacuum filtration and thermal imidization (Fig. S1, ESI†). Herein, PI resins act as a binder to improve the mechanical properties and as antioxidant coatings to protect Ti₃C₂T_X, and the resulting PM composite films have good flexibility (Fig. S2, ESI†). The TGA curves of PM composite films with different PI contents are shown in Fig. S3 (ESI†). Because of the loss of a certain amount of PI and Ti₃C₂T_X in the process of vacuum filtration, the content of PI in the PM composite films changes when compared with the nominal content of PI.

Fig. 3(a) and (b) show the microstructure of pure Ti₃C₂T_X and PM composite films, respectively. Without the addition of PI, the pure Ti₃C₂T_X film shows a tightly packed lamellar structure. In contrast, the PM composite film shows a relatively loose layered structure due to the polymer insertion, which allows the Ti₃C₂T_X nanosheets to be effectively separated and avoids restacking, but nonetheless is still sufficiently connected to form a continuous conductive network. In addition, EDS mapping images show that the Ti₃C₂T_X nanosheets are uniformly distributed in the PM composite film (Fig. 3(c) and (d)). This structure is similar to the structure of a pearl shell, with the Ti₃C₂T_X nanosheets playing the role of inorganic “bricks” and PI playing the role of organic “mortar”, and the biomimetic structure not only enhances the mechanical properties, but also facilitates the reflection and absorption of electromagnetic waves of the PM composite film.⁴⁵

Fig. 3 (a) Cross-sectional SEM image of pure Ti₃C₂T_X films. (b) Cross-sectional SEM and (c) and (d) EDS mapping images of PM composite films. (e) FTIR spectra of PAA and PI. (f) XRD patterns of the PM composite film with different PI contents. (g) XPS survey spectrum of pure Ti₃C₂T_X films and PM composite films. (h) Ti 2p spectra of Ti₃C₂T_X and PM-3.

The FTIR spectra confirm the successful conversion of PAA to PI after thermal imidization (Fig. 3(e)). The absorption peaks at 1662 cm⁻¹ (amidic acid, C=O) and 1541 cm⁻¹ (amidic acid, C–N) appearing in PAA are no longer observed after thermal imidization. These peaks are replaced by absorption peaks at 1717 and 1778 cm⁻¹, which are the symmetric and asymmetric stretching vibration absorption peaks of the carbonyl group on the imide ring of PI, respectively. In addition, absorption peaks at 744 and 1372 cm⁻¹ are observed, which are assigned to carbonyl bending and imide bond stretching vibrations, respectively. These peaks provide sufficient evidence for the presence of an aromatic imide structure. The XRD spectral results show that the (002) peak shifts from 2θ = 7.1° to 5.9° with the increase in PI content (Fig. 3(f)), and the layer spacing between Ti₃C₂T_X nanosheets in the PM composite film increases, indicating that PI is successfully embedded in the interlayer space of Ti₃C₂T_X nanosheets. Meanwhile, the chemical compositions of the pure Ti₃C₂T_X film and the PM composite film have been analyzed by XPS (Fig. 3(g)). It can be found that the surface of the PM composite film is rich in F- and O-groups, which can be well hydrogen bonded with the polymer. Also, due to the introduction of PI, the atomic ratio of C/Ti in the PM composite film (3.40) is higher than that of the pure Ti₃C₂T_X film (2.58). Furthermore, the XPS Ti 2p spectra of Ti₃C₂T_X and PM-3 (Fig. 3(h)) show a slight increase in the TiO₂ percentage from 24.85% for Ti₃C₂T_X to 27.91% for PM-3, which is mainly due to oxidation of Ti₃C₂T_X during high temperature thermal imidization. Notably, the introduction of PI coating well prevents further oxidation of Ti₃C₂T_X nanosheets and effectively enhances their stability,⁶³ which can also be fully demonstrated in later application in harsh environments.

Mechanical properties of PI/Ti₃C₂T_X composite films

The mechanical strength and toughness of the MXene-based electromagnetic shielding film can be improved by the design of the “brick-and-mortar” structure.⁴¹Fig. 4(a) presents the tensile stress–strain curves of pure Ti₃C₂T_X and PM composite films, and the detailed mechanical properties are listed in Table S1 and Fig. S4 (ESI†). As expected, the PM composite films exhibit excellent mechanical properties (Fig. 4(b)). The pure Ti₃C₂T_X film shows a tensile strength of 12.9 ± 0.5 MPa and a fracture strain of 0.58 ± 0.01%. With increasing PI content from 10 wt% to 40 wt%, the tensile strength of the PM composite films show a trend of first increasing and then decreasing. In particular, when the PI content reached 30 wt%, the corresponding tensile strength and fracture strain are 114.9 ± 1.6 MPa and 3.02 ± 0.12%, which are 9 and 5 times higher than those of the pure Ti₃C₂T_X film, respectively.

Fig. 4 (a) Tensile stress–strain curves of the PM composite films with different PI contents. (b) Tensile stresses and strains of the PM composite films with different PI contents. (c) Schematic illustration of the fracture mechanism of the PM composite film.

The schematic fracture mechanism is proposed to explore the synergistic toughening effect of Ti₃C₂T_X nanosheets and PI (Fig. 4(c)). The hydrogen bondings between the Ti₃C₂T_X nanosheets and PI^48,63 are broken at first when stretching starts, and the nanosheets slip in the stretching direction. Meanwhile, PI can afford additional frictional energy loss between adjacent nanosheets as a way to resist the sliding effect until cracks occur. Subsequently, to avoid crack propagation, PI chains are stretched and more energy is consumed until the material is completely pulled off.^45,47

EMI shielding performance of PI/Ti₃C₂T_X composite films

The EMI shielding performance of conductive materials is closely related to their electrical conductivity. As shown in Fig. 5(a), the pure Ti₃C₂T_X film exhibits the highest conductivity of 1865 S cm⁻¹. However, the conductivity of PM composite films decreases accordingly with increasing PI content. Fortunately, when the PI content reaches 40%, the conductivity of the PM composite film still reaches 667 S cm⁻¹, which is much higher than 1 S m⁻¹ for the practical application of EMI shielding materials. Meanwhile, the EMI SE of PM composite films with different PI contents in the X-band region is shown in Fig. 5(b). With increasing PI content, the EMI SE of PM composite films gradually decrease, the average EMI SE of pure Ti₃C₂T_X film can reach 56 dB, and the average EMI SE of PM-1 to PM-4 are 44, 41, 37 and 28 dB, respectively, all of which meet the requirements of commercial grade EMI shielding applications (15 dB).

Fig. 5 (a) Electrical conductivity of PM composite films with different PI contents. (b) The EMI SE of PM composite films with different PI contents in the X-band region. (c) Comparison of SE_T, SE_A, and SE_R at a frequency of 12.4 GHz of the PM composite films with different PI contents. (d) Average power coefficients of A, R, and T. (e) Schematic illustration of the EMI shielding mechanism of the PM composite film.

SE_T, SE_A, and SE_R of PM composite films with different PI contents at 8.2 GHz are investigated to further explore the EMI shielding mechanism of the PM composite film. As shown in Fig. 5(c), SE_A and SE_R have the same trend as SE_T, both decreasing with increasing PI and decreasing MXene contents. Meanwhile, the contribution of SE_A to SE_T is greater than that of SE_R, indicating that the absorption mechanism plays a dominant role within the material. Fig. 5(d) shows the average power coefficients of different PM composite films. It is clear that the value of R is much higher than that of A, revealing that most of the incident waves are attenuated by reflection for the whole PM composite films. Therefore, the main shielding mechanism of the PM composite films is predominantly reflection, and a small fraction of the incident electromagnetic waves can penetrate into the composite films and be absorbed. Additionally, the R values of the PM composite films slightly decrease with the increase in the PI content, while the A values slightly increase. The R-value of PM-4 is 0.95 while the A-value is only 0.05. In all PM composite films, the sum of the R-value and A-value is very close to 100%, indicating that almost all penetrating electromagnetic waves can be absorbed. Therefore, the PM composite films can provide a very good electromagnetic shielding effect.

Fig. 5(e) depicts the transmission process of electromagnetic waves through the PM composite film to better explain the EMI shielding mechanism. First, most of the electromagnetic waves are immediately reflected at the surface of the shielding material because of the impedance mismatch between the surface of the shielding material and the air. The remaining electromagnetic waves then enter the interior of the material and interact with the high charge density Ti₃C₂T_X nanosheets, resulting in a loss of energy from the electromagnetic waves. Meanwhile, the nacre-like lamellar structure of the PM composite films facilitates multiple reflections of electromagnetic waves inside, promoting energy dissipation and absorption of electromagnetic waves, and the resulting composite film can effectively shield more than 99.99% of the electromagnetic waves, playing a very good electromagnetic shielding effect.^63,64

Summary and comparison of the properties of PI/Ti₃C₂T_X composite films

In order to compare the various properties of the PM composite films, we have facilitated this comparison by plotting their mechanical properties, electrical conductivity and EMI SE as radar plots (Fig. 6(a)). Clearly, when the PI content increases, the tensile strength of PM composite films shows a trend of increasing and then decreasing, while the EMI shielding performance and electrical conductivity decrease. Considering all the properties, the PM-3 composite films exhibit excellent mechanical properties, with a tensile strength of 114.9 MPa, and outstanding electromagnetic shielding properties, with an EMI SE of 37 dB at a thickness of only 15 μm.

Fig. 6 (a) A radial plot comparing the tensile strength, strain, Young's modulus, electrical conductivity, and EMI SE for MXene and PI/Ti₃C₂T_X composite films. (b) EMI SE of PM-3 composite films with different thicknesses. (c) Comparison of SE/t of PM composite films with literature studies. The numbers inside Fig. 6(c) are sample numbers listed in Table S2 (ESI†).

The PI/Ti₃C₂T_X composite films in this study are compared with those reported in other studies to highlight their excellent EMI shielding properties. Given that the EMI SE of PM composite films are dependent on thickness (Fig. 6(b)), thicker PM composite films exhibit higher EMI SE. Therefore, to provide a fair comparison, EMI SE is normalized by thickness (SE/t) herein. As shown in Table S2 (ESI†) and Fig. 6(c), PM composite films prepared herein show an ultrathin thickness as well as a high SE/t, ranking high in the graph compared with other shielding materials, and can be used as efficient EMI shielding materials.

Flame retardancy and EMI shielding performance in harsh environments of PI/Ti₃C₂T_X composite films

Due to the combination of excellent mechanical properties and electromagnetic shielding properties, the PM-3 composite film is able to show good promise for use in high-performance EMI shielding materials. In particular, for practical applications, its flame retardancy and EMI shielding performance under harsh conditions are critical. As shown in Fig. 7(a), the PM-3 composite film was burned with the flame of an alcohol lamp for half a minute and the whole process was recorded with a smartphone (Video S1, ESI†). As a result, the PM-3 composite film was never ignited during the whole burning process, which is a great indication of its good flame retardancy. In addition, the PM-3 composite film was treated at high temperature (250 °C) and low temperature (−196 °C) for 1 h (Video S2, ESI†) to check the change in EMI shielding performance before and after the treatment. As shown in Fig. 7(b) and (c), the EMI shielding performance of the PM-3 composite film is almost unchanged after 1 h of treatment at 250 °C, while the EMI shielding performance is slightly reduced after 1 h of treatment at a low temperature of −196 °C, indicating that the material is well suited for application in high and low temperature environments. In addition, the bonding stability of the Ti₃C₂T_X nanosheets and the PI matrix also affects the stability of the EMI shielding performance due to the weak hydrogen bonding and van der Waals interaction.⁶⁵ Based on the comparison of the EMI shielding performance of the PM-3 composite film before and after 1 h of ultrasonic (240 W, 40 kHz) treatment (Fig. 7(d)), obviously, the EMI shielding performance remains almost stable, indicating a strong bonding between the Ti₃C₂T_X nanosheets and the PI matrix. To better understand the long durability of the material in practical application, the change in EMI shielding performance of the PM-3 composite film was observed after 7 days of storage at 70% RH, 70 °C. As shown in Fig. 7(e), the EMI shielding performance of the material only slightly decreases after 3 and 7 days. Moreover, the decrease is insignificant and remains around 1–3 dB. Clearly, the material still maintains good EMI shielding performance. Therefore, the high EMI shielding PI/Ti₃C₂T_X composite film prepared in this study exhibits broad application prospects in harsh external environments and is expected to be used in aerospace and other fields.

Fig. 7 The flame retardancy of the PM-3 composite film and the stable EMI shielding performance after resisting harsh environments. (a) Images of the combustion process at different time. (b) EMI SE of the PM-3 composite film before and after treatment at 250 °C for 1 h. (c) EMI SE of the PM-3 composite film before and after treatment in liquid N₂ (−196 °C) for 1 h. (d) EMI SE of the PM-3 composite film before and after ultrasonic cleaning for 1 h. (e) EMI SE of the PM-3 composite film treated in a hygrothermal environment.

Conclusions

Ultrathin and flexible PI/Ti₃C₂T_X composite films have been designed and prepared by a simple vacuum filtration and thermal imidization process. Through the design of the nacre-like lamellar structure, PI is well intercalated between Ti₃C₂T_X nanosheets, playing a role in mechanical reinforcement. Significantly, when the PI content is 30%, the composite film exhibits excellent mechanical properties, with a tensile strength of 114.9 MPa and a fracture strain of 3.0%, which are 9 and 5 times higher than those of pure Ti₃C₂T_X films, respectively. Moreover, the design of the nacre-like lamellar structure also facilitates the multiple reflections of electromagnetic waves between adjacent layers and promotes the absorption of electromagnetic waves. The as-prepared PI/Ti₃C₂T_X composite film (30 wt% PI) has a 37 dB EMI shielding effect in the X-band range when the thickness is only 15 μm. In addition, owing to the high/low temperature resistance of the PI matrix, the PI/Ti₃C₂T_X composite films can be well adapted to various harsh environments and maintain a stable EMI shielding effect; PI plays the role of an anti-oxidation coating and at the same time has good flame retardancy. Therefore, the ultrathin flexible PI/Ti₃C₂T_X composite film exhibits excellent mechanical performance and stable EMI shielding properties. Thus, this film is expected to be used as a high-performance EMI shielding material to resist harsh environments and provides potential application prospects in flexible wearable electronic devices and aerospace fields.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (51503066, 51873063), the National Basic Research Program of China (2013CB035505), the Shanghai Sailing Program (14YF1404900), and the China Postdoctoral Science Foundation (2019M660082).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3tc03024e

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