Electrically insulating PBO/MXene film with superior thermal conductivity, mechanical properties, thermal stability, and flame retardancy

Constructing flexible and robust thermally conductive but electrically insulating composite films for efficient and safe thermal management has always been a sought-after research topic. Herein, a nacre-inspired high-performance poly(p-phenylene-2,6-benzobisoxazole) (PBO)/MXene nanocomposite film was prepared by a sol-gel-film conversion method with a homogeneous gelation process. Because of the as-formed optimized brick and mortar structure, and the strong bridging and caging effects of the fine PBO nanofibre network on the MXene nanosheets, the resulting nanocomposite film is electrically insulating (2.5 × 109 Ω cm), and exhibits excellent mechanical properties (tensile strength of 416.7 MPa, Young’s modulus of 9.1 GPa and toughness of 97.3 MJ m−3). More importantly, the synergistic orientation of PBO nanofibres and MXene nanosheets endows the film with an in-plane thermal conductivity of 42.2 W m−1 K−1. The film also exhibits excellent thermal stability and flame retardancy. This work broadens the ideas for preparing high-performance thermally conductive but electrically insulating composites.

spectroscopy (EDS) on JEOL JSM 7100F was used to analyze the element distribution of the fracture nanocomposite film.The crystalline structure of the film was characterized by an Xray diffractometer (XRD, Empyrean).The Raman spectra were recorded from 100 to 4000 cm −1 on a LabRAM HR Evolution using a 532 nm NeHe laser.The Anton Paar MCR 302 rheometer was used for the rheological characterization of gels.Tensile tests of the film (~ 3 mm width) were performed on a UTM-16555 (Shenzhen Suns Technology Stock Co., Ltd.) at a tensile speed of 1 mm min −1 .Tensile tests were carried out using the single-edge notched samples (film width of 5 mm, notched length of 1 mm, tensile speed of 1 mm min −1 ) to investigate the fracture energy.The fracture energy was calculated according to the following equation: where   is the fracture strain of the notched sample,  is the notch length and  is the integration of stress-strain curve until   for the unnotched sample.X-ray Photoelectron Spectroscopy (XPS) measurements were carried out with a Thermo Scientific ESCALab 250Xi.
The electrical conductivity of the films was measured by a high-precision four-probe instrument (MCP-T700, Measurement range: 0.001×10 -4 ~ 9.999×10 7 Ω).Volume resistance of the films was measured by a Keithley 6517B.Thermogravimetric analysis (TGA) was performed on a PE Pyris 1 with a heating rate of 10 °C min −1 from 30 to 800 °C in an air atmosphere.The ignition and flame shielding properties of films were tested using the oxygen index (JF-3A) and vertical combustion (TTech-GBT2408) according to ISO 4589-2 and ISO 9773-1998 standard, respectively.The thermal conductivity (TC) was calculated according to the following formula: where  ,  and   are the thermal diffusivity, density, and specific heat capacity of the nanocomposite film, respectively;  was measured with the laser-flash method (LFA 447, NETZSCH, Germany);  was obtained by weighing; and   was measured by DSC (TA-Q2000) using the sapphire method.Temperature changes in thermal management were recorded dynamically with the Fluke thermal imager TiS65.Two-dimensional wide-angle X-ray scattering (2D WAXS) measurements were carried out on a SmartLab X-ray diffractometer with HyPix 3000 detector.The incident beam was almost parallel to the film face.The in-plane orientation of PBO nanofibres with and without MXene nanosheets was calculated from the azimuthal profile of (200) reflection using the following equations: where  and I are the azimuthal angle and the corresponding integral intensity, respectively.
Notably,  is also the angle between the film face and the PBO nanofibres.The increase in  from 0 to 1 indicates that the nanofibres are distributed from completely random to completely parallel to the film face.Meanwhile, there is a finer and denser nanofibre network adhered to the nanosheets.

Figure 1 .
Characterization of MXene nanosheets.a SEM and b AFM images of the as-formed Ti3C2Tx MXene.c XRD patterns of Ti3AlC2 MAX and Ti3C2Tx MXene.The inset in (a) displays Tindal effect of the obtained dispersion of MXene nanosheets, indicating good hydrophilicity and dispersibility.

Supplementary Figure 2 . 4 . 5 .Supplementary Figure 6 .
The rheological properties of gels.Variation of elastic moduli (G′) and loss moduli (G″) with angular frequency for PBO, PM20 and PM50 alcohol gels.The result demonstrates the formation of the elastic networks in PBO and PBO/MXene alcohol gels.Furthermore, G' and G'' of the gels increased with MXene concentration, indicating the enhancing effect of MXene on the network strength.The cross-section SEM images of the films.a PBO.b PM10.c PM20. d PM30.e PM50.f PM70.It can be seen that the interlayer stacking density of composite film reduces with the increase of the MXene content, indicating a decrease in the interaction between the layers.Elemental mapping images of the fracture of PBO/MXene film (PM20), showing a homogeneous distribution of MXene in the laminar composite film.Morphology of PBO/MXene gel networks.a SEM image of PM50 gel network prepared by proton-consumption-induced gelation and b size distribution of the corresponding PBO nanofibres.c SEM image of the control sample (W-PM50) gel network prepared by water-vapor-induced gelation and d size distribution of the corresponding PBO nanofibres.It can be seen that both the PBO nanofibres and MXene nanosheets show a lower degree of aggregation in the PBO/MXene gels by proton-consumption-induced gelation.

Supplementary Figure 7 .
XPS analysis of MXene and PBO films.a The high-resolution O 1s spectrum of MXene film and b the high-resolution C 1s spectrum of PBO film.The fitted O 1s spectra indicate the presence of the polar group Ti−C−OH on the surface of MXene nanosheets.The fitted C 1s spectra indicate the presence of the polar groups −C=O and −C=N on the surface of the PBO nanofires.

Supplementary Figure 8 .
Structural stability investigation of the films.a Water contact angle (CA) of MXene, PBO and PM20 films.b Optical photographs of MXene, PBO and PM20 films before and after ultrasonic treatment.PBO/MXene composite film has a similar wettability to PBO film, further confirming the hydrophilic MXene nanosheets in the composite film were caged by the relatively hydrophobic PBO nanofibre network.The caging effect and the strong interaction between PBO and MXene endow the composite film with better stability against ultrasonication than pristine MXene film.The improved hydrophobicity also protects the film backbone from being weakened by water, and mitigates the shear damage to the film caused by the cavitation bubbles generated by ultrasonication.

Table 1 .
Properties of commercially available thermally conductive plastics.

Table 3 .
Comparison of mechanical properties of nacre-inspired PBO/MXene films with those of other polymer/2D inorganic nanosheet composites.
-: no data presented in the cited literature.Supplementary

Table 5 .
Comparison of the multifunction of nacre-inspired PBO/MXene films with other polymer/2D inorganic nanosheet composites.
-: no data presented in the cited literature.