Flame-retardant MXene/polyimide film with outstanding thermal and mechanical properties based on the secondary orientation strategy

With the development of multifunction and miniaturization in modern electronics, polymeric films with strong mechanical performance and high thermal conductivity are urgently needed. Two-dimensional transition metal carbides and nitrides (MXenes) have attracted extensive attention due to their tunable surface chemistry, layered structure and charming properties. However, there are few studies on using MXenes as fillers to enhance polymer properties. In this paper, we fabricate a three-dimensional foam by the freeze-drying method to enhance the interfacial interaction between adjacent MXene sheets and polyimide (PI) macromolecules, and then a composite film with a dense and well-ordered layer-by-layer structure is produced by the hot-pressing process. Based on the secondary orientation strategy, the resultant MXene/PI film exhibits an enhanced thermal conductivity of 5.12 ± 0.37 W m−1 K−1 and tensile strength of 102 ± 3 MPa. Moreover, the composite film has good flexibility and flame retardancy owing to the synergistic effect of MXene sheets and PI chains. Hence, the MXene/PI composite film with the properties of flexibility, flame-retardancy, high mechanical strength and efficient heat transmission is expected to be used as the next thermal management material in a variety of applications.


Introduction
With the advent of the 5G era, modern electronics are developing rapidly toward the multifunction and miniaturization direction. [1][2][3] The technical integration of high-power chips, wireless charging, bluetooth and multi-angle folding has caused a severe challenge to the heat dissipation and strength of electronic devices. 4,5 Hence, to effectively dissipate heat in time and transform the brittleness of materials, composites synchronously with excellent thermal and mechanical properties are urgently needed. Compared with the other three material systems of wood, metal and silicate, polymeric materials are attracting more and more attention due to their light weight, exibility, easy processing and excellent corrosion resistance. 6 However, most polymers are inherently poor conductors of heat and electrons, which limits their applications in electronic devices greatly. In addition, these electronic devices also face serious re hazards caused by accidental electrical leakage or aging generally. 7,8 However, it is difficult to restrain the re spread in most polymeric materials once it occurs. Therefore, research and preparation of multifunctional materials with high strength, good thermal conductivity and ame retardancy at the same time have far-reaching academic signicance and wide practical value.
Polyimide (PI) as one of the most popular functional materials has been widely applied in the elds of aerospace, optics and microelectronics, due to its excellent properties, such as chemical resistance, thermal stability and mechanical performance. 9 Recently, incorporating llers of carbonaceous materials (such as carbonaceous carbon bers, graphene and carbon nanotubes), metals (such as Cu nanowires) and insulating thermal materials (such as boron nitride) into a polymer matrix has been considered as one of the most effective and feasible methods to improve the thermal conductivity of composites. [10][11][12][13][14][15][16] For instance, Wei et al. obtained a reduced graphene oxide/PI (rGO/PI) lm with a thermal conductivity of 2.78 W m À1 K À1 when the content of rGO was 8 wt%. 12 Wang et al. obtained an anisotropic thermal conductivity of 2.81 W m À1 K À1 in PI composites with 30 wt% boron nitrides. 13 In 2020, He et al. reported a highly thermally conductive (11.203 W m À1 K À1 ) PI composite lm with graphene oxide (GO) nanosheets and boron nitride (BN) platelets as binary llers. 15 However, in general, carbon llers are incompatible with the polymer matrix, leading to serious aggregation of carbon llers in composites, which increases the interfacial thermal resistance between llers and the matrix and greatly limits the heat transfer performance of composites. Hence, it is still a big problem to achieve a perfect balance between the interfacial compatibility and intrinsic thermal conductivity of llers.
Recently, MXenes, a class of 2D early transition metal carbide and nitride materials, have been discovered, and are fabricated by selectively etching "A" layers from layered ceramics called MAX phases. 17 And the general formula of MAX phases is M n+1 X n , where M represents the early transition metal, X represents carbon or nitrogen and n ¼ 1, 2 or 3. Due to the unique layered atomic structure and adjustable surface functional groups (hydroxyl, oxygen or uorine), MXenes show fascinating properties in many aspects, such as energy storage, electromagnetic interference shielding, adsorption performance, catalytic performance and thermal conductivity. [17][18][19][20][21] In 2016, Zha et al. investigated the theoretical thermal conductivity (472 W m À1 K À1 ) of Sc 2 CT 2 (T ¼ F, OH) MXene using rstprinciples calculations, 21 which provided a new direction for the preparation of thermal management materials. 17,[22][23][24][25][26][27] For instance, Song et al. fabricated a cellulose nanober (CNF)/ Ti 3 C 2 composite lm using a vacuum-assisted ltration method, which exhibited a high thermal conductivity of 11.57 W m À1 K À1 . 17 Jin et al. reported multilayered poly(vinyl alcohol)/ MXene lms with a thermal conductivity of 4.57 W m À1 K À1 . 22 However, there is still a lack of in-depth exploration on using MXenes as llers to enhance the thermal conductivity of the polymer matrix, especially in the aspects of high strength and heat transfer. Hence, in our work, because MXenes are synthesized in uoride-containing aqueous solutions, MXenes exhibit good dispersibility in poly(amic acid) (PAA) solution, which can effectively improve the interfacial compatibility between llers and the matrix and improve the thermal conductivity of composite materials.
Herein, the MXene/PI lm with a dense and well-ordered layer-by-layer structure was fabricated by the interfacial interaction between adjacent MXene sheets and PI macromolecules and the secondary orientation strategy including the freezedrying method and hot-pressing process. The stacked MXene interlocking structure not only increases the in-plane heat transfer path and further reduces the interfacial thermal resistance, but also enhances the friction between sheets to improve the fracture strength of MXene/PI lms. The resultant MXene/PI lm exhibits an enhanced thermal conductivity of 5.12 AE 0.37 W m À1 K À1 and tensile strength of 102 AE 3 MPa. More importantly, the composite lm has good exibility and ame retardancy due to the synergistic effect between MXene sheets and PI chains. Hence, this study provides a feasible and effective scheme for preparing exible, ame-retardant polymeric lms with excellent thermal and mechanical properties.

Preparation process of the MXene/PI lm
The MXene/PI lm was fabricated by means of two steps of the freeze-drying method and hot-pressing process, that is, the secondary orientation strategy. The process of preparing the MXene/PI lm is shown in Fig. 1. Firstly, the poly(amic acid) (PAA) solution is mixed with the MXene (Ti 3 C 2 T x ) suspension to enhance the interfaces between MXene sheets. As described in the section of Materials and methods, the samples with different MXene contents are denoted as MXene-x/PAA, where x was 10, 20, 30, and 40, respectively. Subsequently, the homogeneous mixture is further frozen (cold source at the bottom) and lyophilized in a freeze-dryer. The anisotropic MXene/PAA foam is obtained due to the rearrangement effect of freeze casting. 20 Finally, the MXene/PAA foam is subjected to the hot-pressing process in a (300 C and 25 MPa) vacuum furnace to induce the polymerization of PAA to form PI macromolecules. The thickness of the lm can be well controlled to several tens of micrometers by adjusting the pressure of the vacuum furnace. Thus, ameretardant MXene/PI lms with outstanding thermal and mechanical properties are obtained. And the experimental details can be found in the part of Materials and methods.

Morphology of MXene/PAA foam and MXene/PI lms
To further investigate the secondary orientation effect of the MXene/PI lm, the morphology characterization of the MXene/ PAA foam and MXene/PI lm is performed with scanning electron microscopy (SEM). Owing to the secondary orientation process, the MXene/PI lm exhibits a compact directional structure. On the one hand, the anisotropic MXene/PAA foam is fabricated by freeze-drying. Owing to the rearrangement effect of freeze casting, 28 the MXene/PAA foam shows a porous structure with smooth cell walls ( Fig. 2(a)) and a highly oriented structure similar to the tube bundles ( Fig. 2(b)), different from the MXene foam with a loosely disordered porous structure with weakly interconnected sheets. 29 On the other hand, to further control and improve the MXene sheet distribution and interface bonding of the MXene/PI lm, MXene/PAA foam is subjected to the hot-pressing process and PAA is amidated into PI at the same time. The fabricated MXene/PI lm exhibits a dense and well-ordered layer-by-layer structure without interlayer delamination ( Fig. 2(d)) and a smooth and at surface ( Fig. 2(c)). Some particulate impurities can be observed in the high magnication SEM image in Fig. 2(c), which may be carbon particles on the graphite mold.

Structural characterization of MXene, PI, PAA and MXene/PI lms
The chemical structure and possible interactions of the composite are further conrmed by Fourier transform infrared (FTIR) spectroscopy, Raman spectroscopy and X-ray diffraction (XRD). As shown in Fig. 2(e), the synthesized MXene contains -COOH and Ti-O functional groups, respectively. 30 As for the PI lm, the characteristic peaks at 1726 cm À1 and 1780 cm À1 are assigned to the imide C]O symmetric and asymmetric stretches, and two peaks at 1502 cm À1 and 1379 cm À1 can be observed due to the stretching vibration of C]C and C-N. 31,32 Aer heat imidization treatment, the successful synthesis of the MXene/PI lm is distinctly conrmed by the disappearance of the carboxylic acid and amine groups in PAA foam at 2500-3500 cm À1 and the presence of the characteristic peaks at 1639 cm À1 (C]O), 1379 cm À1 (C]C) and 1226 cm À1 (C-N). 33 The reason for the shi of peaks in the MXene/PI lm is that strong hydrogen bonds are formed by the functional groups of Ti 3 C 2 T x sheets with the carbonyl groups of PAA chains. 29,31 As shown in Fig. 2(f), for the range from 150 cm À1 to 750 cm À1 , the MXene/PI lm and MXene exhibit similar characteristic peaks of the A 1g (205 cm À1 , 715 cm À1 ) band and E g (270 cm À1 , 407 cm À1 , 620 cm À1 ) band. [34][35][36] Simultaneously, the characteristic Raman peaks of the MXene/PI lm at 1380 cm À1 , 1594 cm À1 and 1787 cm À1 can also be observed, which is in agreement with the band of PI according to the literature. 31 Besides, the stretching of sp 3 carbon leads to a peak of MXene at 1582 cm À1 . 37 The structure of the MXene/PI lm is monitored with the XRD pattern, as shown in Fig. 2(g). The diffraction peak around 21.4 is observed in the MXene/PI lm, which corresponds to the amorphous structure of the PI lm. 38 Compared to the MXene, the (002) peak of the MXene/PI lm shis from 6.32 to 6.7 , corresponding to the reduced interlayer spacing from 1.4 to 1.32 nm. 34 This result indicates that the arrangement of MXene sheets in the MXene/PI lm is denser along the in-plane direction, corresponding to the SEM result, which means the fabrication of a compact and well-ordered layer-by-layer structure. Therefore, in this work, the fabricated MXene/PI lm is expected to promote thermal and mechanical properties, owing to the formation of the heat transfer path and strong interaction between MXene sheets and PI chains.

Mechanical properties of MXene/PI lms
To investigate the reinforcing effect of MXene sheets and PI chains in the composite lms, the tensile strengths, tensile modulus and elongation at break of the MXene/PI lm with different MXene contents (10, 20, 30 and 40 wt%) were compared. Apparently, the MXene/PI lm shows enhanced mechanical properties. As shown in Fig. 3(a), the tensile strength of the MXene-10/PI lm (102 AE 3 MPa) is stronger than that of the PI lm (73 AE 2 MPa), because of the synergistic effect of MXene sheets and PI chains. The typical stress-strain curves of PI lms and MXene/PI lms are shown in Fig. S1 of ESI. † However, the tensile strength gradually decreases with increasing MXene fraction. These results possibly originate from the fact that the excessive MXene would prevent the PI chains from packing tightly, thereby increasing the free volume, reducing the interaction between PI chains and increasing the brittleness of the MXene/PI lm. 39 Simultaneously, as shown in Fig. 3(b), the elongation at break of the MXene-10/PI lm reaches 45 AE 2%, while the excessive MXene makes PI chains more rigid, resulting in increased brittleness of the lm. 39 In addition, the tensile modulus of the MXene/PI lm gradually increases due to the fact that the density increases with the MXene content (Fig. 4(c)), 40 and thus the MXene-40/PI lm exhibits the highest modulus of 17.5 AE 0.8 GPa.
To further demonstrate the synergistic effect of MXene sheets and PI chains in the composite lms, the tensile fracture morphologies were characterized by SEM. As shown in Fig. 3(c  and d), a large number of pull out aky structures can be observed in the fracture surface of the MXene-10/PI lm, while the fracture of the MXene-40/PI lm is brittle like and shows a small and at pull out structure (Fig. 3(e and f)), indicating that the MXene/PI lm can absorb the breaking energy to enhance the tensile strength during the tensile process. 41,42 Fig. 3(g) demonstrates the possible fracture model: on the one hand, PI, as a binder, forms hydrogen bonds between PAA chains and Ti 3 C 2 T x sheets, which enhances the interaction and the stress transfer between MXene sheets, further leading to an evenly dispersed tensile stress. 41 On the other hand, the MXene/ PI lm exhibits a compact and well-ordered layer-by-layer structure due to the secondary orientation strategy (freezedrying method and hot-pressing process), as proven by the analysis of XRD. The stacked MXene interlocking structure (Fig. 2(d)) is conducive to increasing the contact area and friction between sheets, leading to greater energy consumption during the relative slippage of the MXene sheets, and improves the break strength of the MXene/PI lm further. 42 Herein, the MXene/PI lm shows good exibility, as shown in the inset of Fig. 3(a).

Thermal management performance of MXene/PI lms
The development of multifunction and miniaturization in modern electronics generates increasingly serious heat accumulation, which affects the efficiency and reliability. 22,38 Therefore, efficient heat management is important for dissipating excessive heat. The thermal conductivity was evaluated by the laser ash method. Fig. 4(a and b) show the thermal conductivities and thermal diffusion coefficients of the MXene/ PI lm with different MXene weight ratios in the axial and radial directions, respectively. It can be seen that the thermal conductivities and thermal diffusion coefficients in both the axial and radial directions enhance with increasing weight fraction of MXene. Simultaneously, the MXene/PI lm exhibits clearly anisotropic thermal characteristics. Apparently, corresponding to the in-plane efficient thermal diffusivity of 3.1 AE 0.22 mm 2 s À1 , the in-plane and through-plane thermal conductivities of the MXene/PI lm reach 5.12 AE 0.37 W m À1 K À1 and 0.28 AE 0.006 W m À1 K À1 , which exhibit an enhancement of approximately 5.7 times and 3 times compared to the PI lm, respectively. Besides, Fig. 4(c) shows that the density of the MXene/PI lm gradually increases with the increase of MXene content, and the specic heat capacity of the MXene/PI lm is about 1.125 J g À1 K À1 . 40 In this work, we compared the thermal conductivity and mechanical tensile strength of different PI composite materials (Table S2 †). In fact, the heat transfer of two-dimensional nanomaterials depends on the heat  conduction performance of the materials and the formation of thermal networks strongly. 43 Therefore, the reasons for the high thermal conductivity of the MXene/PI lm may be as follows: 17,[22][23][24]43 (1) MXene has good dispersibility in the mixture solution and forms hydrogen bonds with PAA, which improves the interfacial compatibility between the ller and matrix. (2) PI can plug the gap between adjacent MXene sheets and enhance the interfacial adhesion. So, the interfacial thermal resistance can be reduced to the greatest extent due to a large contact area between MXene sheets in the in-plane direction. (3) The overlapped MXene sheets in the MXene/PI lm can provide a highspeed channel for phonon conduction. Thus, the heat ow can be rapidly transmitted along the direction of the MXene layers.
(4) The MXene/PI lm exhibits a dense and well-ordered layerby-layer structure due to the secondary orientation strategy, which results in more MXene sheets in the plane and further increases the conduction paths for heat transfer. To sum up, the MXene/PI lm has a relatively high in-plane thermal conductivity. On the contrary, PI chains exist between MXene sheets along the direction of through-plane, resulting in slow heat transfer and low through-plane thermal conductivity.
To further characterize the heat dissipation performance, the temperature variation of PI and MXene/PI lms at different times was recorded with an infrared camera. Herein, for accurate comparison, the center temperature is kept at the same level, and both surfaces of samples are coated with a thin layer of graphite paint to maintain the consistent surface emissivity and heat radiation. 32,44,45 As shown in Fig. 4(d), the MXene/PI lm exhibits a much faster cooling rate than the PI lm under the same cooling conditions. Aer 30 seconds, the temperature of the MXene-40/PI lm drops from 39.8 C to 21 C, while that of the PI lm only decreased to 24.17 C in the same situation. As shown in Fig. 4(e), all lms are heated by a point heater under the same environment. With time goes up, the lateral thermal region increases and the central colors of all samples become brighter. Moreover, compared with PI, the composite lms have a wider lateral area in the same time, indicating that the as-prepared MXene/PI lm has better heat-spreading performance. As a result, the MXene/PI lm has good heat transfer performance and the potential to be regarded as a promising candidate for thermal management.

Flame resistance of MXene/PI lms
The risk of re is fatal for electronic equipment, especially for miniature and multifunctional devices, which are in danger of local overheating and short circuit. 7,46 If the thermal management material has ame resistance, it can act as a barrier to prevent re from spreading to a certain extent. Herein, the ame retardant tests of PI and MXene/PI lms were carried out by burning them in the ame (800 C) produced by an alcohol burner. 47,48 Fig. 5(a and b) show the combustion processes of MXene/PI and PI lms under the consistent time and environment, respectively. And the whole demonstration process is shown in the Movies S1 and S2 of ESI. † It can be seen that both of them exhibit ame resistance for a long time, whereas the ame diffusion speed and burning area of the PI lm are larger than those of the MXene/PI lm. In addition, two new absorption peaks at 507 cm À1 and 671 cm À1 symbolizing the Ti-O stretching vibrations appeared in the burned MXene/PI lm, suggesting the formation of TiO 2 (Fig. 5(c)). 22 However, in the Raman spectrum of Fig. 5(d), for the burned MXene/PI lm, the amorphous and graphitic carbon peaks at 1610 cm À1 (G band) and 1385 cm À1 (D band) disappeared aer burning, while the peaks belonging to TiO 2 are obviously found at different positions (448 cm À1 , 615 cm À1 , and 813 cm À1 ). 49 Besides, Fig. 5(e) shows that MXene has a weight loss of 1.3% at about 160 C, indicating the formation of TiO 2 according to the literature, 50,51 and the nal weight loss of the MXene/PI lm is obviously less than that of the PI lm. Fig. 5(f) shows the X-ray photoelectron spectroscopy (XPS) survey spectra of the burned and unburned MXene/PI lm. The peaks of F 1s and N 1s can be found in the burned MXene/PI lm, indicating that the removal of uorinated and amino functional groups during the burning process was incomplete. Meanwhile, compared with the XPS spectrum of MXene/PI before burning, the peaks of O 1s and Ti 2p 3 of burned MXene/PI become stronger, indicating the evolution of Ti 3 C 2 T x MXene toward TiO 2 during burning. And the peak of C 1s becomes weaker which may be mainly because the combustion products of PI were covered by the TiO 2 layer during burning. Furthermore, the ame retardant behavior of PI and MXene/PI lms was measured using a Micro-scale Combustion Calorimeter (MCC). As shown in Fig. 5(g) and Table S1, † PI and MXene/PI lms show a sharp spike of the heat release rate (HRR) curve. With the addition of 40 wt% MXene, HRR decreased by 45 W g À1 , the peak HRR decreased to 12.8 W g À1 and the corresponding maximum temperature increased by 16.7 C, indicating that the ame retardancy of the composite lm was improved. As shown in 5(h), it is clear that the layer structure of the MXene/PI lm was kept well aer combustion, and the formation of TiO 2 can organize a physical protective layer to wrap the residue formed by PI carbonization. In addition, the results of SEM energy dispersive spectroscopy (EDS) of the MXene/PI lm before and aer burning (Fig. S2 †) also demonstrate the protection effect of TiO 2 during burning. There may be two reasons for these results: 22,38 on the one hand, compared with the PI lm, the MXene/PI lm exhibits excellent thermal conductivity leading to rapid heat diffusion. On the other hand, the formation of TiO 2 can organize a physical protective layer on the MXene-PI interface, which wraps the residue formed aer PI carbonization, effectively preventing the combustion of the underlying materials and the propagation of the ame.

Conclusion
In summary, the MXene/PI lm was fabricated using the secondary orientation strategy including the freeze-drying method and hot-pressing process. The resultant MXene/PI lm exhibited a compact and well-ordered layer-by-layer structure, increasing the heat transfer path and improving the interfacial interaction in the MXene/PI lm. Simultaneously, the composite lm has good exibility and ame retardancy owing to the synergistic effect between MXene sheets and PI chains. Therefore, those properties combined with high thermal and mechanical properties broaden their application elds and provide a novel and effective idea for preparing exible, ameretardant polymeric lms with outstanding thermal conductivity and mechanical strength.

Synthesis of the water-soluble PI precursor (PAA) solution
Poly(amic acid) (PAA) solution was synthesized using a polycondensation procedure in N,N-dimethylacetamide (DMAc) using an equivalent molar ratio of 4,4 0 -diaminodiphenyl ether (4,4 0 -ODA) and pyromellitic dianhydride (PMDA). The specic preparation process is detailed in the previous report of our group. 32

Synthesis of MXene (Ti 3 C 2 T x ) sheets
MXene (Ti 3 C 2 T x ) sheets were synthesized by etching the aluminum (Al) layer from the Ti 3 AlC 2 MAX phase according to the literature. 34 Firstly, 1.6 g of lithium uoride (LiF, 99%, Shanghai Aladdin Biochemical Co., Ltd) was dissolved in 20 mL of 9 M hydrochloric acid (HCl, 37%, Shanghai Aladdin Biochemical Co., Ltd) in a Teon vessel. Then to etch its Al layer, 1 g of MAX powder (Ti 3 AlC 2 , 400 mesh, Shanghai Macklin Biochemical Co., Ltd) was slowly added into the LiF solution, and the reaction lasted for 30 h at 50 C under stirring. Next, the resultant Ti 3 C 2 T x was repeatedly washed with deionized (DI) water and centrifuged at 3500 rpm for 5 min until pH $ 6. Finally, the sediment was centrifuged at 1500 rpm for 30 min to obtain the self-delaminated Ti 3 C 2 T x sheets and the homogeneous MXene (Ti 3 C 2 T x ) sheet suspension ($10 mg mL À1 ) was collected for further use.

Preparation of the MXene/PI lm
The MXene/PI lm was fabricated by means of two steps of the freeze-drying method and hot-pressing process, that is, the secondary orientation strategy. Fig. 1 shows the detailed preparation process of the MXene/PI lm. Firstly, the PAA solution (50 mg mL À1 ) was mixed with the MXene (Ti 3 C 2 T x ) suspension (10 mg mL À1 ) by stirring for 2 h. Then the resultant mixture was frozen and lyophilized for 96 h in a freeze-dryer (GZL-20, Beijing Songyuan Huaxing Science and Technology Development Co., Ltd.) to obtain the anisotropic MXene/PAA foam. The samples with different weight ratios of 10%, 20%, 30%, and 40% of MXene were denoted as MXene-x/PAA, where x was 10, 20, 30, and 40, respectively. Finally, the as-prepared MXene/PAA foam was subjected to hot-pressing treatment for 1 h in a (300 C and 25 MPa) resistance vacuum hot-pressing furnace (ZYD-30-80, Jinzhou Aerospace Vacuum Equipment Co., Ltd) and the MXene/PI lm was obtained.

Measurement and characterization
A eld emission scanning electron microscope (SEM, model SU8000, Hitachi, Japan) was used to research the morphology and structure of the samples. X-ray diffraction (XRD) patterns were recorded with a Shimadzu XRD-7000s diffraction instrument (Cu Ka radiation, l ¼ 0.154 nm, 40 kV, 40 mA). Fourier transform infrared spectroscopy (FTIR) spectra were recorded using a Bruker Tensor-27 FTIR spectrometer. Raman spectroscopy (Renishaw InVia, England) was used to describe the production of Ti 3 C 2 T x . Thermal gravimetric analyses (TGA) were performed on a TG-DTA7300 thermal analyzer at a heating rate of 10 C min À1 under an air atmosphere. The mechanical performance of the lm was measured with a crosshead speed of 0.1 mm min À1 (Instron 5944, USA). Thermal conductivity (K) was calculated by K ¼ a Â C p Â r, where C p , a and r are the specic heat capacity, thermal diffusivity and density, respectively. The Netzsch LFA 467 light ash apparatus was used to measure the C p and a at 27 C. The infrared photos were recorded with an infrared camera (VarioCAM HD 880, German).

Conflicts of interest
The authors declare no competing nancial interest.