High‐temperature resistant polyetherimide dielectric nanocomposite films reinforced with polyurea‐coated boron nitride nanosheets with core‐shell structure

Abstract Incorporating functional fillers with core‐shell structures into polymer can significantly enhance the comprehensive properties of nanocomposites, yet there was limited research on the impact of filler shell thickness on the performance. Herein, BNNS@PUA functional fillers with different shell thicknesses were obtained by controlling the ratio between BNNS and polyuria (PUA), and then incorporated into polyetherimide (PEI) to fabricate BNNS@PUA/PEI nanocomposites. The successful synthesis of BNNS@PUA was confirmed using TEM, XRD, FTIR, and Raman characterizations. Testing for the nanocomposites through mechanical, TG, and dielectric analysis revealed that increasing the shell thickness improved the performance of the nanocomposites. BNNS@PUA served as crosslinking sites, forming crosslinks between fillers and significantly enhancing the mechanical properties of the nanocomposites. The elastic modulus of 3 wt.% BNNS@PUA PEI reached 1122.76 MPa. After optimizing the shell thickness (the shell thickness was 18 nm), the highest dielectric constant of the nanocomposites reached 3.8, the dielectric loss was as low as 0.019, and the breakdown strength increased to 105.63 kV/mm. Finally, calculations showed that the maximum energy storage density reached 0.194 J/cm 3 , a 4.62‐fold increase compared to 0.042 J/cm 3 for PEI. The enhanced dielectric properties were attributed to the increase of the shell thickness, which increased the polarization between the filler and the matrix. By fitting the experimental data through the Cole‐Cole model, a more in‐depth exploration of the polarization mechanism and the optimization of the thickness of the filler shell were achieved. This work will provide a reference for the preparation of dielectric nanocomposite containing core‐shell structure fillers. Highlights Functional BNNS fillers with core‐shell structure were designed and synthesized. The effects of different shell thicknesses and nanofillers were systematically investigated. Interactions between BNNS@PUA and PEI contributed to the property improvements. The polarization mechanism was explored systematically by the Cole‐Cole model.