Electrospinning-impregnation: Producing hydrophobic polyimide composites with superior dielectric properties

Polyimide (PI) with ultralow dielectric constant (D_k) and dielectric loss (D_f) is highly desired for high-frequency and high-speed electromagnetic communications. Herein, PI_fm/PI composites with excellent dielectric and hydrophobic properties were successfully prepared by impregnating an electrospun PI nanofiber membrane (PI_fm) with polyamic acid (PAA) solutions prior to re-imidization process. Evenly distribution of impregnated PAA in the PI_fm endows the imitated PI matrix to bind with PI fibers tightly. The abundant internal stress at the interfaces of PI nanofibers and PI matrix results in a higher chain packing density, which greatly inhabits dipole polarization in the PI_fm/PI composites. Specifically, the PI_fm/PI-15% composite demonstrates an average ultralow D_k of 1.756 and D_f of 0.004 in high-frequency range (8.2–12.4 GHz), together with a quite high dielectric breakdown strength reaching 226.47 kV mm⁻¹ and sufficient tensile strength of 40.3 MPa. The T_d5% is higher than 560 °C while keeping the coefficient of linear thermal expansion lower than 50 ppm °C⁻¹. Moreover, the PI_fm/PI composites possess a hydrophobic property with contact angle above 108.0°, ensuring the dielectric stability even in a high-temperature or humid environment. The obtained fluorine-free PI_fm/PI composites with excellent dielectric and hydrophobic properties via electrospinning-impregnation method will greatly boost the advancement of PI nanofiber composites in high-frequency applications.