Multifunctional, flexible, and mechanically robust polyimide-MXene nanocomposites: a review

Multifunctional flexible polymer composites have proliferated in different industries. MXenes, as the rising star of 2D materials, offer unique combinations of properties including metallic conductivity, hydrophilicity, high specific capacitance, and solution processability, as well as mechanical flexibility and robustness that accentuate them for the fabrication of multifunctional composites. 2D flake structure and abundant surface terminations of MXene facilitate its integration into polymer matrices to develop high-performance composites. Polyimides (PIs) are high-temperature engineering polymers that have rendered their way into aerospace and electronics industries due to their outstanding mechanical strength, high chemical resistance, high thermal stability, excellent electrical and thermal insulation properties. Amalgamating the outstanding characteristics of these two materials, this paper is the first review to summarize advancements in PI/MXene nanocomposites to address the methods of preparation and the effect of MXene loading on the target application e.g. energy conversion and storage, electromagnetic interference shielding, sensing, and fire-retardancy. The review commences with a critical discussion on PI/MXene nanocomposite fabrication methods. Next, a comprehensive review of the properties and applications of PI/MXene nanocomposites is provided. Lastly, based on the current developments of PI/MXene nanocomposites, this paper is concluded with the prominent characteristics of PI/MXene composites regarding the target application and identifying the gaps and challenges to develop multifunctional composites.


Introduction
The rapid developments in electronic devices in the aerospace and defense industry, electronics and telecommunications, and medical sectors highlight the need for cost-effective multifunctional polymer composites with superior properties. High-performance aromatic polyimides (PIs) have been gaining attention in the microelectronics, optoelectronics, and aerospace industry due to their outstanding properties such as resistance to extreme temperatures, pH, chemicals and radiation, high thermal stability (T > 300 • C) and thermal transition temperature (T g > 200 • C), low dielectric constant (∼3.4), high tensile strength (∼200 MPa) [1][2][3][4]. However, PI-based composites suffer from a few limitations e.g. poor heat dissipation, electrostatic accumulation, and low electrical conductivity. The tribological performance of polymers can be improved by blending with other polymers [5] and nano-fillers. Uniformlydispersed nanomaterials in a polymer matrix improve the thermo-mechanical and physical properties of the composite in terms of dimensional stability [6]. The most economical and effective method to improve the mechanical properties and thermal conductivity of PI is integrating conductive fillers such as alumina [7], silicon carbide [8], CNTs [9], graphene and its derivatives [10][11][12] and non-electrically conductive fillers such as boron nitride (BN) [13,14] and nanoclays [15]. Starting with the latter category, performance improvements depend on the distribution and arrangement of the nanoclay in the polymer matrix. The effective exfoliation of nanoclays maximizes the hydrogen bonding between the layered nanoclay and PI chains resulting in better mechanical and thermal properties [16][17][18]. The thermal conductivity of PI which is in the order of 0.1 W m −1 K −1 cannot meet the requirement of fast heat conduction for advanced electronic devices. PI/BN composites have been investigated as a highly thermally conductive material for applications in electronic devices because of their high thermal stability and insulating properties without a rise in electrical conductivity [19,20]. Next, electrically conductive fillers will be discussed. Incorporating CNT/iron nanoparticles/BN into PI matrix at a 2 wt.% filler reinforcement, increased the in-plane thermal conductivity by 11 430% compared to pure PI [9]. Despite the extremely high thermal conductivity of graphene, its weak interfacial interactions with polymer matrix and strong interlayer interaction hinder its proper dispersion in composites. Thus, surface modification and using derivatives such as graphene oxide (GO) and reduced GO (rGO) plays an important role. Yu et al improved the dispersion of graphene nanosheets in PI by functionalizing with ionic liquids and developed an aerogel with a conductive network to absorb electromagnetic waves [21]. To least affect the heat transport behavior of graphene, the Diels-Alder reaction was used to graft maleimide onto the graphene surface under UV light which next reacted with polyamic acid (PAA) and further went through thermal imidization [22]. This approach provided a new idea to reduce the interfacial thermal resistance between interfaces to enhance the heat dissipation capacity of the composites. Hosseini et al [23] proceeded with in-situ polymerization of PAA in the presence of GO which was reduced to rGO during thermal imidization. Enhanced sheet size led to a decrease in the degradation rate below 200 • C and resistance. Amine-functionalized rGO exhibited a good dispersion in PI matrix and resulted in great reinforcement in dielectric (140 times better than PI) and thermal properties [24]. In comparison to GO, MXene does not require further reduction to present high electrical conductivity. In addition, due to surface functional groups, strong interactions are formed between MXene sheets and polymer chains. Furthermore, the mechanical performance of Ti 3 C 2 was found to be less dependent on flake thickness compared to that of graphene [25].
MXenes, the rising 2D stars, have gained increasing interest since their discovery in 2012 [26]. Ti 3 C 2 T x is a well-investigated MXene family member. Due to its unique physical and chemical properties, it is securing high potential for applications ranging from energy storage [27] and electromagnetic interference (EMI) shielding [28] to water purification [29][30][31], biomedicine [32], and wearable sensors [33]. We recently reviewed the application of the MXene-cellulose nanofiber composites in EMI shielding [34]. While adding MXene to PI resolves the poor conductivity and low density of PI composites, oxidation of Ti 3 C 2 T x is effectively prevented during the preparation process of PI/MXene composite [35]. Furthermore, PI shows remarkable resistance to harsh environments such as acid, base, and salt solutions as well as very high (∼300 • C) to very low temperatures (−196 • C). PI/MXene composites are recently being remarkably recognized as multifunctional materials (figure 1). This review is the first to collect the research studies conducted on PI/MXene nanocomposites designed and manufactured for a wide range of applications. The following section (section 2) presents a detailed discussion on the fabrication methods of the PI/MXene composites. Section 3 is devoted to the comprehensive review of the properties and applications of the PI/MXene composites in energy conversion and storage, sensing, EMI shielding, microwave (MW) absorbance, and flame retardancy. We will conclude this review article by conclusion and prospects for the future of PI/MXene nanocomposites.

Fabrication methods of PI/MXene composites
PIs, as thermally stable polymers, are often with aromatic backbones and are commonly synthesized in two steps. PAA is obtained from the reaction of a dianhydride and a diamine at an ambient condition in a dipolar aprotic solvent such as N, N-dimethylacetamide or N-methyl pyrrolidone. Then, the PAA intermediate is converted to the final PI by either thermal or chemical imidization [36,37]. Alternatively, a one-step high-temperature solution polymerization is used to synthesize PIs that are soluble in organic solvents such as nitrobenzene, m-cresol and dipolar aprotic amides at the polymerization temperatures of 180 • C-220 • C. This method often yields a higher degree of crystallinity compared to the two-step method [38,39].
The properties of the PIs such as T g , melting points, crystallinity, flexibility, thermal stability, and mechanical properties depend on the structure of the PI and can be modified by changing in monomer types and synthesis methods. PIs offer a high degree of versatility in their structure and properties, and understanding the structure-property relationship of PIs is a guideline for the desired end-use properties [40][41][42]. Reinforcing PI with nanomaterials such as silica, titania, alumina, graphene, and CNTs can be found here [43,44].
MXenes are a growing family of transition metal carbonitrides and carbides since they were first introduced [26]. Rapid progress in the research on MXenes has been observed due to their electrical conductivity, which is the highest among all synthetic 2D materials, hydrophilic nature, good dispersion stability, and versatile surface chemistry [45]. Titanium carbide, Ti 3 C 2 T x , is the most representative member of the MXene family and a promising nanomaterial with its high aspect ratio, electrical (∼8000 S cm −1 ) and thermal conductivity, and outstanding hydrophilicity and density (∼4 g cm −3 ) compared with carbon nanomaterials (<0.4 g cm −3 ) [46]. Conventionally, it is synthesized by the successive etching of A-layer atoms from the MAX phase followed by a delamination process that have a significant effect on the number of defects, size of sheets, and the characteristics of MXene [47]. This procedure leaves the surface of MXene with oxygen, hydroxyl, and fluorine groups which are shown with T x in the MXene formula. These terminations play key roles in surface engineering and integration of these 2D materials.
Various scalable MXene synthetic protocols are investigated thoroughly [48,49]. An overview of the MXene synthesis methods related to their 0D, 1D, 2D, and 3D forms can be seen in figure 2 [50]. Due to unique properties and significant interest in MXenes, green and more sustainable methods are in demand [50]. MXene-polymer nanocomposites show an enticing prospect in multiple applications and have been widely explored [51][52][53][54][55][56]. Nevertheless, this review article is the first to accentuate the characteristics and applications of PI/MXene nanocomposites.
PI/MXene nanocomposites offer high mechanical, chemical, radiation, and temperature resistance, electrical conductivity, and flexibility ensuring their candidacy in several fields such as energy conversion and storage, sensing, EMI shielding, and fire retardancy. The components and structure of polymer composites significantly affect their properties. Hydrophilic PAA intercalates between MXene sheets via hydrogen bonding and polar interactions. In PI/MXene composites studied in this review, the most common fabrication method included mixing PAA and MXene dispersion, vacuum filtration and subsequent thermal imidization which resulted in thin films [57][58][59][60][61]. The final step was executed with either heating to high temperatures (up to 300 • C) or hot-pressing. It is worth noting that vacuum filtration is a very common method to prepare thin films imposing a specific surface structure due to directional force generated by vacuum [34]. For instance, MXene/cellulose nanofibers form a 'brick-and-mortar' structure under vacuum filtration. Lightweight aerogels were obtained when the PAA/MXene mixture was first freeze-dried and then subjected to imidization [62][63][64][65][66][67][68][69][70][71]. Hybrid aerogels presented superelasticity with large reversible compressibility. According to the results, when EMI shielding was the target application, aerogels and foams were the preferred structure since multiple reflections and absorption of the absorbed EMW led to the attenuation of the EMW. The porous film was the result of the imidization of the aerogel carried out through hot-pressing [72]. In an alternating procedure, PI aerogel/foam was dip-coated into the MXene dispersion to obtain an aerogel/foam [65,[73][74][75]. In the case of phase change composites (PCCs), polyethylene glycol (PEG) was vacuum-impregnated into the PI-MXene aerogel [76,77]. 3D printing technology was also utilized to fabricate an aerogel array structure consisting of PI/MXene aerogel arrays as the upper layer and PI aerogel lattices as the bottom layer [78]. Spray-coating [79][80][81], drop-casting [82], and spin-coating [83] were utilized to fabricate layeredstructured films. Coating, specifically spray-coating, is known to be a scalable method, therefore is more suitable for industrial applications. Notwithstanding, obtaining a uniform surface may be a challenge that necessitates exploring novel technologies. For sensing applications, an insulating double-sided PI tape was introduced to not only hold the sensitive layer and the electrode layer together to build the device but to also raise the initial contact resistance and guarantee superior sensitivity [84][85][86][87][88][89][90][91]. This approach paves the way to fabricate flexible smart wearables with high mechanical durability.

Energy conversion and storage
PI/MXene nanocomposites have been used as energy storage devices and efficient conversion systems. MXene, intrinsically, exhibits both excellent electrothermal and photothermal conversion with an efficiency of nearly 100% [92]. In addition, as a selfpowered information system, it combines resistance memory characteristics and energy storage properties in a single material system [58]. The summary of PI/MXene composites for energy conversion and storage is presented in table 1.
Phase change materials are a class of materials that can absorb, store, and release a huge amount of latent heat energy within a very narrow temperature range through reversible phase transitions [93]. PEG, as a solid-liquid PCCs, can capture and store solar energy, however, to prevent leakage, ensure shape stability, and stable thermal energy conversion, PCCs are required to composite with a porous material such as MXene or graphene. MXene is not only known to have remarkable light-to-heat conversion efficiency but also has an excellent charring capacity and can improve the flame retardancy of phase change composites (PCCs). Moreover, PI aerogel provides a 3D structure framework with low density and flame retardancy. Cao et al [76] prepared a shape-stable MXene/PI@PEG PCCs by vacuum impregnation of PEG into PI/MXene aerogel. They observed that the aerogel with 50 wt% MXene showed the highest PEG loading (98.1%), and thermal storage density (167.9 J g −1 ) while performing excellent photothermal conversion capacity (99.8%). The same sample showed an outstanding flame retardancy including a peak heat release rate (PHRR) of 529.3 W g −1 at 403.7 • C. Zheng et al [77] used the same composite and preparation method, nevertheless, achieved better values with only 20 wt% MXene loading (figure 3). They obtained 97.68% PEG loading, a high thermal storage density of up to 177.1 J g −1 , and photothermal conversion efficiency of 87.8%. Using the composite in the solar-driven evaporation device, an enhanced seawater evaporation mass of 6.07 kg m −2 and a high evaporation rate of 1.24 kg m −2 h −1 under one-sun illumination were reported. Owing to good reusability, long-term working stability, high productivity, and low energy consumption MXene/PI@PEG showed great potential for sustainable seawater desalination. Aiming the same application, Yang et al [78] utilized 3D printing technology to create 3D PI/MXene evaporator aerogel arrays with MXene the upper layer and PI aerogel lattices as the bottom layer. Owing to the structural design, the model evaporator had a high light absorption (91.3%), energy conversion (99.7%), and evaporation rates (2.17 kg m −2 h −1 ) achieving a higher evaporation rate that Zheng et al [77].
He et al [46] obtained a flexible high-safety PI/MXene nanocomposite with a conductivity of 326 S m −1 and a density of ∼1.12 g cm −3 . For a real-world application, it was used in an open-air environment PVA/MgCl 2 gel electrolyte as a flexible Mg 2+ storage supercapacitor and exhibited remarkable aqueous Mg 2+ storage (∼502.2 F g −1 ), superior rate and ultra-long cycling life over 20 000 cycles. When the weight ratio of MXene to PAA was 1:2, the highest specific capacitance under various scan rates was achieved, while the best Mg 2+ storage was associated with a 3:7 ratio. Huang et al [84] showed that PI/MXene porous hybrid electrode material had superior battery capability. With low few-layer MXene content, the pseudocapacitive contribution reached 69% and 83% at scan rates of 1 and 2 mV s −1 , respectively, while it presented low impedance, fast Li + diffusion, and good reversibility suggesting its potential in developing high-performance PI hybrid electrodes. Liu et al [58] developed a self-powered PI/MXene memristor by casting MXene/PAA on a glass substrate and imidization at various temperatures. The addition of 0.5 wt% MXene resulted in the best degree of orientation and dispersion uniformity leading to high mechanical properties (tensile strength 140.13 MPa and elongation at break 71.64%). The authors explained the information storage mechanism as active oxygen atoms were produced during thermal imidization forming wide band gap aluminum oxide with residual aluminum. This phenomenon alleviated the dielectric band gap between MXene and PI and improved the dielectric reliability of the composite. The integration of PI chains between MXene sheets has led to a strong interacting interface which is the source of memristor performance enhancement and energy storage.

Sensing
MXenes demonstrate great promise in sensing [33]. In comparison to graphene [94][95][96], MXene has the advantage of higher electrical conductivity and various surface termination. As an electrochemical sensor, diverse types of analytes including ions, glucose, hydrogen peroxide, gas molecules, pharmaceuticals, dyes, cancer markers, pesticides, and neurotransmitters have been detected by MXene [97]. Flexible MXene-based sensors have grown into an essential pressure-sensitive material displaying accuracy, detection range, and stability [98]. To tackle the problem of MXene sheets re-stacking and to keep oxidation at bay, polymer composites have illustrated improved mechanical properties due to the flexibility of polymers, thus making them suitable for a variety of applications. In this section, we will review the  PI/MXene nanocomposites as sensors e.g. piezoresistive sensors, fire detection, Braille recognition, and acoustic device. A summary can be found in table 2.
Zhao et al [88] were the first to design a flexible skin-like sensor for smart Braille recognition. It was composed of three layers consisting of poly dimethyl siloxane (PDMS), MXene, and PI tape. The sensitivity of the device was increased by lowering the pressure and reached 507 kPa −1 in the low-pressure region (0.5-5.75 kPa). Gou et al [86] spin-coated MXene on anodic aluminum oxide (AAO) and PI film separately and fabricated a thermoacoustic device. The reason for choosing MXene was its low heat capacity and high thermal conductivity. In addition, due to its larger interlayer, it exhibited a higher sound pressure level (SPL) than that of graphene of the same thickness [100]. MXene-on-AAO achieved a They also tested the film in a commercial earphone which showed good signal matching demonstrating the potential application of MXene in wearable audio equipment (figures 4(e)-(g)). Jandas et al [57] prepared a bioreceptor for realtime acoustic detection of carcinoembryonic antigen (CEA). After hybridizing MXene with gold (Au) NPs and mixing with PAA solution, a thin film was prepared on the surface acoustic wave device via spincoating and heat treatment (600 nm). Further, the mouse monoclonal antibody of CEA was immobilized covalently on the thin film using the thioglycolic acid arm linker. The PI/MXene-Au composite showed a limit of detection (LOD) of 0.001 mg ml −1 and excellent selectivity among the other tumor markers while being stable for 75 d under periodical testing conditions.
Jiang et al [73] dip-coated PI aerogel in MXene dispersion multiple times and dried it in the oven. They observed multiple enhanced characteristics when PAA: MXene weight ratio was 2 and dipcoating was performed once. The composite showed excellent mechanical robustness, stability, and antifatigue performance as well as the best thermoelectric efficiency. It also triggered the fire-warning alarm in less than 5 s which was repeatable and much faster than commercial infrared and smoke alarms (more than 100 s) while preserving its structure during burning. After bending 1000 times, its resistance stayed almost unchanged showing durable and repeatable piezoresistivity (figures 5(a)-(e)). Thus, this composite presented potential in the nextgeneration multifunctional fire-protection aerogels. While Jiang et al [73] achieved a thermal insulation performance of 0.048 W m −1 K −1 , Yang et al [65] obtained a better efficiency of 0.022 W m −1 K −1 by adding GO to the PI/MXene composite. After mixing GO, MXene, and PAA, the aerogel was obtained by directional freeze-drying and subsequent imidization. SEM images showed that increasing MXene/GO content led to a disorder in pore arrangement. They hypothesized that more MXene occupied rGO walls, reducing the π-π interaction between rGO sheets, and leading to an increase in disorder. When MXene/GO was 133 wt%, optimized mechanical properties (stress 29.4 kPa), excellent low-temperature tolerance (−196 • C), significant reversible compression (90%), and fatigue resistance (10 000 cycles) were obtained. In addition, the nanocomposite promptly detected strain indicating its potential application in human health monitoring.
Yang et al [91] developed a smart artificial e-skin by sandwiching MXene-coated tissue paper (MTP) between a PI layer and a printing paper with interdigital electrodes. By increasing the concentration of MXene dispersion from 0.5 to 1.0 mg ml −1 the resistance decreased from 5 MΩ sq −1 to 2.5 kΩ sq −1 . The authors attached the sensor to a face mask and showed its application in remote respiration monitoring (figures 6(a)-(j)). The resulting pressure sensor demonstrated a high sensitivity of 509.5 kPa −1 , a low limit (∼1 Pa), a broad range (100 kPa) of detection, and outstanding stability over 10 000 loading/ unloading cycles. The current response of the sensor in the range of 50-100 kPa dramatically decreased by increasing the PI layer thickness from 35 to 50 and then 60 µm (figure 6(k)). The sensor showed a comparable heart rate signal to electrocardiogram (ECG) (figures 6(l) and (m)). A MXene@PI flexible dry electrode with the electrical conductivity of 742.62 S m −1 showed similar performance to the commercial Ag/AgCl electrode [99]. It could record ECG even when the subject was in motion, indicating the potential in human health monitoring. Du et al [90] assembled the layers in different compositions to obtain a lightweight, breathable, biocompatible, and highly sensitive flexible piezoresistive pressure sensor. The upper layer consisted of MXene impregnated into PVDF porous nanofiber film, and the bottom layer was an Ag-interdigitated PVDF nanofiber electrode. PI double-sided tape was introduced as the middle layer. The optimized sensing properties were obtained when MXene dispersion was 2 mg ml −1 , the immersion time was 7 min, and the thickness of the PI film was 100 µm. The sensitivity of the sensor reached 1970.65 kPa −1 which was 13 times higher than the sensor without the PI layer. While exhibiting fast response/recovery time (10/20 ms) and remarkable cycle stability of 10 000 cycles, it could detect tiny to large human movements.
Sindhu et al [89] reported an IoT-based wireless pressure and level sensor. After vacuum filtration of MXene on cellulose paper, it was cut by laser and then fixed on a PI film. To complete the patch, copper tape was attached to the other side of the PI film. This low-cost, scalable antenna sensor was designed at the operating frequency of 5.8 GHz having a gain of around −6.14 dB. It showed real-time pressure sensing with a sensitivity of approximately 247 MHz kPa −1 as well as level sensing and stability in various temperatures and pH. A flexible ultralow density (0.0998 g cm −3 ) PI/MXene aerogel with low MXene content as a wearable piezoresistive sensor was fabricated [71] demonstrating temperature tolerance ranging from very low (−196 • C) to high temperatures (150 • C), an ultralow detection limit of 0.5% strain (corresponding 10 Pa), recoverability (up to 90% strain corresponding 85.21 kPa), and fatigue resistance over 1000 cycles. In addition to human motion detection, this hydrophobic aerogel presented oil/water separation properties with high adsorption capacity (55.85-135.29 g g −1 ).
Pu et al [66] prepared PAA NFs by electrospinning, and after mixing them with MXene dispersion, freeze-drying, and imidization, a lightweight, robust, and elastic PI/MXene aerogel with 'lamellapillar' microporous architecture was obtained (figures 7(a)-(f)). With 50 wt% MXene content, the composite showed remarkable piezoresistive strain sensing performance with a wide pressure range of 0-8 kPa (50% strain), linear sensitivity (22.32 kPa −1 ), an ultra-low detection limit of 0.1% of strain (corresponding <10 Pa), excellent compression and rebound stability (1500 cycles). In addition to detecting various human motions, prominent wave absorption performance [minimum reflection loss (RL min ) is −40.45 dB at 15.19 GHz, with an effective absorption bandwidth (EAB) of 5.66 GHz (12.34-18 GHz)] makes it a candidate for wearable strain sensor and MW absorber. As consequence of the intercalation of PI nanofibers (PINFs) between MXene flakes that inhibit the restacking of MXene flakes, EMWs can enter the interior space of the composite easily and undergo multiple reflections and scattering  the structure subsequently affecting other properties. The aerogel with PAA: MXene ratio of 2:1 (optimized sample) maintained a lower volume density (0.0113 g cm −3 ) and an electrical conductivity (0.07 mS cm −1 ). It exhibited a wide detection range of 0.1%-80% (60 Pa to 76.5 kPa), short response (100 ms)/recovery (80 ms) times, and excellent longterm cycling stability (30% strain for 1000 cycles). Furthermore, it manifested excellent photothermal evaporation performance with a water evaporation rate of 14.4 kg m −2 h −1 under the irradiation of four sunlight intensities, nominating it as flexible/wearable piezoresistive sensors and solar evaporators.
MXene/PI composite has been showcased in wearable gas sensors. A MXene@PI volatile organic compound (VOC) gas sensor was fabricated [87] via drop-casting MXene dispersion onto a Ptinterdigitated PI substrate. The flexible sensor showed p-type sensing behavior. At room temperature (RT), the average gas response of 0.210, 0.143, 0.115, and 0.075 was observed for ammonia, methanol, ethanol, and acetone, respectively, and the LOD for acetone was 9.27 ppm. The same group [82] used a different member of the MXene family, V 2 CT x , for sensing non-polar gases which is the only paper that used a MXene other than Ti 3 C 2 T x in composite with PI. The nearly free electron state of MXenes and abundance of surface functionalities provide channels for electron and analyte transport, respectively. The 200 nm thick film with 7-10 kΩ at RT exhibited a very low LOD of 2 and 25 ppm for hydrogen and methane, respectively. The gas response of the devices remained almost the same under ambient atmosphere over a month of the operation.
A soft actuator comprised of different layers was developed by Zhao et al [81]. Cellulose paper was first dipped into 10,12-pentacosadiynoic acid (PCDA) solution and then into zinc acetate solution. After the polymerization of PCDA-Zn 2+ , the paper was sprayed with MXene and graphene dispersion separately and finally, topped with adhesive PI tape to obtain an electrothermal smart actuator (ETSA). The complementary PCDA-Zn 2+ and MXene/graphene bilayer enabled real-time sensory feedback (figures 9(a)-(d)).
An electrochemical actuator was fabricated by attaching free-standing MXene film onto a PI tape [85]. The actuating performance of PI/MXene film was tested in a three-electrode set-up in neutral aqueous electrolytes of K 2 SO 4 , Na 2 SO 4 , KCl, and NaCl. Positive or negative electricity, size, and quantity of intercalation ions are the three important factors in the ion intercalation process of MXene. Due to the small size and less quantity of Na + , the actuator generated greater actuation ability in Na + -containing solutions rather than in K + -containing ones.

EMI shielding and MW absorbance
Developing electronic devices and telecommunication technologies generates EMI and pollution which can have adverse effects on human health, the precision of electronic devices, and military security. Usage of traditional metal EMW protection materials in complex and harsh environments is limited due to their high density, poor flexibility, and poor chemical, and corrosion resistance. Through reasonable structural design, conductive PI and MXene-containing materials with tolerance to harsh environments (extreme temperatures, chemicals, long-term cyclic bending) possess promising potential as EMI shielding and MW absorbing materials. Table 3 contains the summary of PI/MXene composites as EMI shielding and MW absorption materials.
Dai et al [70] constructed an anisotropic wavelike lamellar porous PI/MXene composite aerogel by bidirectional-freezing technique. Lamellae were aligned through the Y-Z plane vertical to X-plane in the composite architecture. The impact of this anisotropic architecture on mechanical performance and resistance variation are presented in figure 10. The maximum reflection loss (RL max ) value in the Z-direction (−41.8 dB) was higher than in the Xdirection (−27.2 dB) and Y-direction (−28.0 dB) at a thickness of 4 mm. The optimized EAB performance was 6.5 GHz at the thickness of 1.91 mm for the same composite sample which is one of the best results for MXene-based absorbers. After 1000 compressionrelease cycles, the aerogel has higher sensitivity in the X-direction rather than in the Z-direction.
Incorporating a very low amount of MXene results in lower-cost materials. Liu et al [69] reported a RL max of −45.4 dB at 9.59 GHz, a wide EAB of 5.1 GHz, the fatigue resistance of 1000 cycles at 50% strain, and electrical conductivity of 4.0 S m −1 by integrating 0.084 v% MXene and obtaining an ultralow density (0.0089 g cm −3 ) PI/MXene aerogel. The shape of the aerogel was maintained after burning on an alcohol lamp for the 60 s and its thermal conductivity was measured as 0.032 W m −1 K −1 which is comparable to that of expanded polystyrene foam (0.035 W m −1 K −1 ). An Outstanding EMI shielding effectiveness (SE) was achieved via unidirectional freeze-drying of PI/MXene and hot-pressing method (figure 11) [72]. Directional alignment of the MXene layers provided an intense reflection loss of incident EMWs at the surfaces resulting in the enhancement of the EMI SE of the material. The composite film with density of 0.39 g cm −3 displayed a remarkable EMI SE of 77.4 dB which was maintained at high temperature (250 • C), cryogenic temperature (−196 • C) treatments, and rapid thermal shock (∆T = 446 • C). Mechanical performance and structural dimensions of the composite were consistent over a wide range of temperature from −100 • C to 250 • C.
Sun et al [60] used rather higher MXene content and designed a PI fiber (PIF)/MXene composite film with a tightly lamellar structure using vacuum filtration followed by thermal imidization. The composite film (256 µm thick) displayed an EMI SE of 49.9 dB. The EMI SE value and the structural integrity of the composite were maintained after being soaked into 0.5 M NaCl solution, 0.5 M HCl solution, liquid nitrogen (−196 • C), and calcined at 150 • C for 30 min. Furthermore, the composite film exhibited excellent tailorability, bendability, and foldability owing to the flexibility of the PIF in the structure, with no negative impact on the EMI SE performance. The surface temperature of the film was increased from 25.1 • C to 38, 42.5, 65, 75, 93, and 105 • C under the voltage of 1, 1.2, 1.5, 1.8, 2.3, and 2.5 V in 10 s, respectively, indicating its potential in smart thermal management wearables.
Different nanomaterials such as CTs, Ag NWs, Fe 3 O 4 NPs, and aramid nanofibers (ANFs) have been added to PI/MXene nanocomposites. Cui et al [64] fabricated MXene/CNT/PI (MCP) hybrid aerogels by freezing and casting. As a part of the fabrication process, freezing resulted in the growth of ice peaks either from bottom to top (MCP-1) or from the periphery to the center of the aerogel (MCP-2) and a sieve-like porous surface appeared after the ice template was removed. The hierarchical porous structure of the aerogel and multiple heterogeneous interfaces between MXene, CNTs, and PI endowed it with outstanding EMW absorbing properties. The RL min was measured −50.03 dB@13.7 GHz, and the EAB was 5.6 GHz (11.3-16.9 GHz) for MCP-1 at a thickness of 1.8 mm while it absorbed all the EMW of X-band when the thickness was increased to 2.5 mm. The RL min of MCP-2 was declared as −44.2 dB@14.5 GHz at 2.7 mm. Similar to the MCP-1, if the thickness of MCP-2 was increased to 3.7 mm, it absorbed all the    EMW in X-band. As a candidate in energy harvesting, the surface temperature of MCP-1 reached 110 • C in 70 s. Ag NWs were integrated into PI/MXene composite to form an AgNWs-PI/MXene/AgNWs-PI sandwich-like structure by a layer-by-layer assembly method [79]. The highest EMI SE of the composite film (97 µm thickness) was 40.73 dB. The material maintained sufficient EMI shielding performance under extreme conditions like high temperatures, low pH, multiple bending, and ultrasound treatment. The unique sandwich structure is endowed with low thermal conductivity of 0.03277 W m −1 k −1 and flame retardancy characteristics. Yang et al [63] utilized unidirectional and random freeze-drying to develop anisotropic and isotropic MXene/ANF/PI aerogels with a 'brick/fiber/mortar' structure ( figure 12). They reported that MP-V (vertical alignment) indicated better cyclic compression stability compared to MP-P (parallel alignment) and MP-R (random alignment) relating it with interlayer ANF-connected structures. The MP-P aerogel containing 33.3 wt% MXene with a thickness of 2.7 mm with a density of 0.0314 g cm −3 showed the EAB of 4.2 GHz (covering the whole X-band) at 250 • C.
Zhuo et al [75] used the same components as Yang et al [63] but a different fabrication method in which the PI foam was first fabricated via freeze-drying and imidization of ANFs and PAA. One-time dip-coating in MXene dispersion and second dipping in PAA solution led to the formation of MXene@ANFs/PI and PI@MXene@ANFs/PI, respectively. The hybrid foam exhibited anti-oxidation properties. It was observed that MXene@ANFs/PI displayed EMI SE of 48.9 dB and specific EMI SE (SSE/t) of 1645.5 dB cm 2 g −1 , while PI@MXene@ANFs/PI with the same amount of MXene presented an EMI SE of 46.3 dB and SSE/t of 1402.3 dB cm 2 g −1 ( figure 13(a)). These values were diminished when the samples were heattreated at 250 • C in the air for 100 h. However, the variation was less for PI@MXene@ANFs/PI indicating anti-oxidation performance and outstanding EMI shielding performance in the harsh environment of PI@MXene@ANFs/PI composite ( figure 13(b)). To enhance the hydrophobicity and oxidation stability  of PI/MXene composite, Zeng et al [101] utilized a chemical crosslinking approach. The PI foam was dip-coated in MXene dispersion before crosslinking with methylene diphenyl diisocyanate to obtain crosslinked MXene@PI (C-MXene@PI). The foam with a density of 0.0487 g cm −3 displayed an excellent EMI SE of 62.5 dB. By storing it in an environment with 95% RH and 60 • C, the foam maintained the EMI SE of 44.4 dB in 6 d while MXene@PI showed a remarkable decrease in only 2 d (figures 13(c)-(e)). Furthermore, the C-MXene@PI composite displayed excellent mechanical flexibility even after immersing in liquid nitrogen. The EMI SE and electric signal remained almost constant after bending 1000 times. Combined with the efficient conductive network, the composite foams were able to reach temperatures of over 100 • C in tens of seconds at low voltages (<10 V) with remarkable potential for lightweight and stable electrothermal heaters.
Sang et al [102] developed a multifunctional three-layer polytetrafluoroethylene (PTFE)/MXene/PI electrothermal actuator. When the thickness of free-standing MXene film was adjusted to 12 µm, optimized EMI SE of 44 dB and electrical conductivity of 186 000 S m −1 was observed. The EMI SE exhibited 1-2 dB decrease after 100 cycles of bending. The actuator formed an S-shape by increasing the applied voltage from 1 to 5 V while its temperature increased from 28 • C to 110 • C by applying 6 V. By spray-coating MXene on PVDF film and sticking PI tape on top (PVDF/MXene/PI), Sang et al [80] obtained a wearable heater and EMI shielding material. PVDF/MXene/PI could also monitor human motions such as finger bending, arm bending, drinking water, and talking with good sensibility and repeatability. By applying a voltage of 6.5 V, the temperature of the composite increased to 80 • C indicating the potential application of the material in electronic therapy and melting ice. With 0.165 mg cm −2 MXene content, the EMI SE and electrical conductivity values of the composite reached 40 dB and 4300 S m −1 , respectively. By adding Fe 3 O 4 to the PI/MXene composite, higher EMI SE was achieved. First, Fe 3 O 4 /PAA NFs films were obtained by in-situ polymerization and electrospinning. Then, MXene nanosheets were deposited on the films via vacuumassisted filtration. Subsequently, Janus structured MXene-(Fe 3 O 4 /PI) composite films were obtained by thermal imidization process. The optimized composite film with 80 wt% MXene content and a thickness of 75 µm showed an EMI SE of 66 dB. Tensile strength and Young's modulus of the film were measured as 114.5 MPa and 5.8 GPa, respectively. By the intention of utilizing Janus design, two sides of nanocomposite films displayed different EMW shielding absorption efficiency, and electrothermal and photothermal conversion performances. Absorption SE (SE A ) of the Fe 3 O 4 /PI side of the film was 58 dB while the MXene face showed SE A performance of 39 dB. The MXene surface temperature reached 108 • C when the 4 V was applied and 95 • C when simulated sunlight with a power density of 200 mW cm 2 was exposed. Zou et al [83] measured the THz shielding efficiency of 17 dB for a 12 µm ultra-thin MXene@PI film prepared by spin-coating of MXene flakes on the PI substrate. By increasing the thickness of the film to 25 µm this value increased to 70 dB exhibiting its potential candidacy for the next generation THz shielding materials.

Flame retardancy
The application of PI/MXene composites in electronic devices requires fire resistance. MXene is a promising fire-warning material due to the following reasons: 1-alteration of the output voltage of MXene with the temperature difference, 2-compatibility with polymers because of surface functionalities, 3-high conductivity, 4-synergistic flame-retardant effect for polymers [74]. In addition, since MXene oxidizes to TiO 2 , as a physical protective layer on PI, MXene/PI composites have better flame diffusion speed, HRR, limiting oxygen index, char values compared to PI [62,74,103]. Table 4 includes the summary of this section.
Zhu et al [103] prepared PI/MXene flexible film by freeze-drying MXene/PAA mixture and hotpressing process. It was observed that the flame diffusion speed and burning area of the PI film were larger than those of the PI/MXene film. In addition to the thermal conductivity of 5.12 W m −1 k −1 , the PHRR of PI/MXene containing 40 wt% MXene was measured as 12.8 W g −1 while it was 47.6 W g −1 for PI at 572.7 • C. Wang et al [62] utilized annealing instead of hot-pressing to obtain a hydrophobic PI/MXene aerogel with high oil absorption capacity and excellent fire-resistant properties. When the aerogel with 16.1 wt% MXene content was ignited using an alcohol lamp, no obvious flame was observed during 62 s. The PHRR of 49.8 W g −1 at 623 • C was reported for the same sample. Comparing these two studies shows that higher MXene content enhances PHRR value due to higher char value. By the addition of Ag 2 Se NWs to PI/MXene composite, Zhao et al [74] fabricated a hydrophobic smart fire-retardant composite. The composite was developed by dipcoating a PI aerogel in MXene/Ag 2 Se NW dispersion (3:1) (PI@(MXene/Ag 2 SeNW) and then into a PDMS solution. The addition of Ag 2 Se decreased the PHRR value from 81 W g −1 to 38 W g −1 while the value was 112 W g −1 for PI aerogel. When the nanocomposite was exposed to the fire for 20 cycles, the trigger time stayed similar showing a sensitive, repeatable, and stable fire-warning performance (figures 14(a)-(c)).

Others
In this section, other performance characteristics of PI/MXene nanocomposites such as thermal conductivity and mechanical properties are discussed (table 5). Jian [104] obtained the optimum tensile strength by adjusting carbon fiber (CF) to MXene weight ratio at 15% and 3% in CF/MXene/PI nanocomposite. The thermal conductivity of 0.45 W m −1 K −1 was achieved for the same sample. The tensile strength remained stable by heating  Reprinted from [74], © 2022 Elsevier Ltd. All rights reserved.  [61] investigated the variation in mechanical and tribological properties of polyurea-PI copolymer with low MXene content (0.1-2 w%). As an elastomer with excellent adhesion and lowtemperature brittleness, polyurea improves the mechanical property defect of PI. They observed that the formation of holes in the composite structure was intensified with the increasing amount of MXene that weakened the connections among polymeric chains and decreased the elongation at the break of the composite. Moreover, using MXene as a solid lubricant reduced the friction and wear of the neat copolymer at a wide temperature range. To improve the permselectivity and solvent resistance of the PI membrane, crosslinking with triethylenetetramine was investigated [106]. The crosslinked PI/MXene composite membrane showed excellent resistance to DMF, acetone, and methanol during 18 d of immersion. Furthermore, it performed well with the phase inversion method with a flux of about 268 l m −2 h −1 . The high value of flux was created by two main routes. First, the addition of MXene into the polymer matrix produced water channels due to the phase interface between MXene and PI matrix. Secondly, the interlayer in the MXene particles also provided channels for water molecules. Therefore, this membrane showed potential in wastewater treatment with high salinity.

Conclusion and prospects
In this review, we comprehensively explored the applications of newly rising PI/MXene nanocomposites. We highlighted the common fabrication process of the composites and its effect on the properties and the target application. In the main section, we discussed the application of the composites featuring the MXene content, composite type, and characteristics.
The property enhancement of the nanofillerreinforced polymer nanocomposites depends on different factors such as filler nature, filler dispersion, intercalation of polymer into the spacing of the filler particles, and interface adhesion strength. Therefore, surface engineering of the filler material and controlling the re-stacking/aggregation of the particles are critical parameters in achieving performance enhancements [94,95,[107][108][109][110][111][112][113][114]. The hydrophilic nature of PI and MXene leads to facile integration overcoming the re-stacking challenge of MXene sheets, while MXene enhances the electrical and thermal conductivity of the PI matrix.
(1) In wearable piezoresistive sensors based on PI/MXene, double-sided PI tape ensured flexibility, breathability, and sensitivity. MXene was deposited on the PI film by spray-coating, spincoating, or drop-casting. Dipping a tissue paper into MXene dispersion and attaching it onto PI film has shown to be a low-cost, green, and scalable method to fabricate a sensor. The devices showed a fast response, high sensitivity, low detection limit, short recovery time, and longterm cycling stability. Electrothermal and electrochemical actuators based on PI/MXene nanocomposites exhibited real-time sensory feedback and stability. (2) The thickness of the film and conductive filler loading amount are important parameters in tailoring the properties. For instance, when the composite was targeted for EMI shielding, the thickness was played a critical role, however, in other applications it was recognized less. With very low MXene loading (2 v%) the composites performed effectively in EMI shielding (up to 77 dB) which was maintained in high/low temperatures and humid environments. The studies showed that the main attenuation mechanism of PI/MXene composites is absorption which makes them 'green' materials by reducing secondary EM pollution. Noteworthy, the green MXene synthetic process has gained interest. Efficient MW and THz absorption were also reported for PI/MXene composites. Therefore, excellent resistance in harsh environments, stable electrical and thermal conductivity, and effective wave absorption in a wide range of frequencies secure the potential application of PI/MXene nanocomposites in military aviation equipment and smart thermal management and health monitoring wearables. (3) MXene shows outstanding intrinsic light-toheat conversion characteristics paving its way in water desalination. PI/MXene nanocomposites exhibited photothermal evaporation performance with a high-water evaporation rate comparable to graphene-based nanocomposites. (4) PI aerogels suffer from shrinkage during their birth and service. Adding fibrillar or flaky additives is one of the ways to tackle this issue. The addition of MXene has led to an improvement in the issues in the phase change materials such as reducing leakage and increasing shape and thermal energy conversion stability. (5) Developing effective fire protection strategies is a critical issue in electronic devices. MXene performs better compared to GO since GO is irreversibly reduced in higher temperatures so, it cannot detect fire revival. Moreover, GO-based sensors require external power sources to generate signals leading to the complexity and instability of the system. Due to its thermoelectric effect, MXene has been widely investigated for use in early fire-warning sensors. PI/MXene nanocomposites, in the form of film or aerogel, exhibit comparable or even superior fire detection and retardancy characteristics to commercially available devices.
To recapitulate, PI/MXene nanocomposites have been attracting great attention due to their flexibility, multifunctionality, and tolerance in harsh environments in the past few years paving their way into aerospace, smart wearables, and water desalination.
Here, we would like to address the gaps and challenges in MXene-based composites. Material discovery and development is a slow process. Self-Driven Labs integrate machine learning (ML) and artificial intelligence to accelerate material synthesis [115]. MXenes are a large family and in this review, as expected, almost all the studies, except one, used Ti 3 C 2 T x . ML has been utilized to design MXenebased wearable sensors [116] and should be considered as an advantageous tool in studying the properties of PI/MXene composites other than Ti 3 C 2 T x . From an experimental point of view, other members of MXene family in composite with PI should be investigated for the applications already mentioned in this review and beyond. For instance, Nb 2 C-polyvinyl pyrrolidone colloids showed excellent biocompatibility and physiological stability, and no noticeable toxicity both in vitro and in vivo [117]. The importance of investigation of Ti 3 C 2 and Nb 2 C-polymer composites in biomedical applications is an obvious gap.
With the aim of large-scale production and realworld applications, fabrication methods such as 3D printing should be explored. A major topic for investigation is shelf-life and stabilization of MXene against oxidation which has led to extending the lifetime of V 2 C from less than a day to several months and Ti 3 C 2 from weeks to over a year [118]. Tolerance of PI in harsh environment and the durability and flexibility of PI/MXene composites, specially MXenes other than Ti 3 C 2 , make them a suitable candidate for anti-bacterial wearables with flame resistance which has not been researched. Monolayer Nb 4 C 3 membranes have been reported to possess the highest Young's modulus (386 ± 13 GPa) for the nanoindentation measurements of solution processable 2D materials e.g. GO, rGO, and Ti 3 C 2 MXene indicating its remarkable potential for various mechanical applications [119]. Even though MXene has a dark color, and most PIs are yellowish, developing colorless PIs with low MXene loading can be beneficial in flexible optoelectronics. For applications on, e.g. solar panels, transparency of the coating is essential. However, obtaining photothermal MXene-based transparent coatings is still a challenge. The high transmittance means that little light is absorbed, leading to low photothermal conversion efficiency. In addition, stability of MXene materials, and largescale, environmentally green production are other challenges which need to be investigated. Owing to excellent physical barrier effect and catalytic effect, MXene can significantly improve flame retardancy when used in combination of commercially available flame retardants, such as ammonium poly phosphate in a useful way of meeting industrial requirements.

Data availability statement
No new data were created or analysed in this study