Study on the Preparation of Polyphenylene Oxide Foams with Ultralow Dielectric Properties through Cell Structure Regulation

Abstract

In millimeter-wave signal transmission scenarios, materials must exhibit excellent signal stability─that is, a low dielectric constant and low dielectric loss over a wide frequency band─along with outstanding hydrophobic properties. In this study, polyphenylene oxid (PPO)/polystyrene (PS) blends were chosen as the matrix to capitalize on the excellent dielectric properties of PPO. Additionally, polytetrafluoroethylene (PTFE) was introduced into the blend to further optimize the system. The weak interactions between fluorine atoms and CO2 promoted CO2 dissolution; moreover, PTFE enhanced the melt viscoelasticity. Under a one-step depressurization using supercritical carbon dioxide as the blowing agent, the expansion ratio increased from 9.27 for the unmodified system to 12.77. Furthermore, by adjusting the second-stage holding pressure and holding time in a two-step depressurization, numerous large cells were introduced, successfully constructing a bimodal foam with an expansion ratio as high as 26.04─a 181% increase compared with the unmodified foam (9.27). This bimodal foam exhibited outstanding dielectric stability over the 1–40 GHz frequency range: the dielectric constant was below 1.06 and the dielectric loss remained stable at 0.0003; at 10 GHz, the dielectric constant and dielectric loss respectively reached as low as 1.05 and 0.0003. Meanwhile, the strong hydrophobicity of PTFE combined with the rough surface after foaming significantly improved the hydrophobicity. The water contact angle of the unmodified unimodal foam reached a maximum of 96.4°, whereas the bimodal foam obtained by adding PTFE and employing the two-step depressurization exhibited a maximum contact angle of 118.75°. This work provides a fabrication strategy for low-density, ultralow dielectric PPO foam materials, which are promising for advanced high-frequency communication applications such as millimeter-wave communication radomes and aerospace wave-transparent structures.