The Sandwich Structure of a Polyimide/PTFE Composite\nFilm Designed to Achieve a Low Dielectric Constant and Excellent Thermal\nPerformance at High Frequency

As\n5G technology rapidly progresses, the demand for insulating\ndielectric materials with low dielectric constants, minimal dielectric\nloss, and enhanced thermal and mechanical properties has become increasingly\ncritical. In order to combine the advantages of both fluoropolyimide\n(FPI) and polytetrafluoroethylene (PTFE), this study successfully\ndeveloped an FPI/PTFE/FPI composite film with a sandwich structure\nthat seamlessly integrated a commercial hydrophilic PTFE porous fibrous\nfilm and FPI. This structure capitalizes on the permeation effect\nof the FPI wet film to significantly improve interfacial interactions\ncompared to traditional sandwich-structured composites. Consequently,\nthe composite films have an excellent combination of properties. When\nthe proportion of PTFE reaches 50%, the composite film remains stable\nacross high-frequency bands (8.2 to 12.4 GHz), with an impressively\nlow value of 1.5 at 9 GHz. The composite film also exhibits exceptional\nelectrical breakdown strength (385.2 kV mm<sup>–1</sup>), tensile\nmechanical robustness (σ<sub>b</sub> = 64 MPa, <i>E</i> = 2.1 GPa), thermal stability (<i>T</i><sub>d5%</sub> =\n496.1 °C, <i>T</i><sub>g</sub> = 306.2 °C), and\na low coefficient of thermal expansion (51.7 ppm/K). Notably, the\ncomposite film still maintains good hydrophobic properties, with a\nwater contact angle of 81.2° and a water absorption rate of 3.1%.\nThis research also details an efficient coating method to fabricate\nflexible dielectric films, proposing a strategy for the development\nof advanced insulating materials suitable for high-frequency and high-speed\ncommunication applications.