Lightweight polybenzoxazole aerogels with high compressive strength, ultralow dielectric constants, and excellent thermal stability

Abstract

Aerogel materials with high thermostability, lightweight properties, and high strength have been widely used as thermal protection materials for spacecraft, materials for preventing the impact and vibration of aerospace spacecraft during high-speed flight, and structural materials for manufacturing light spacecraft. Polybenzoxazole is a high-temperature-resistant and high-performance polymer, but it is difficult to process into other shapes because it is only soluble in strong proton acids. Therefore, there is no report on directly preparing polybenzoxazole aerogels with polybenzoxazole resin. In this study, we used a new strategy to prepare real polybenzoxazole aerogels. We first synthetized poly-2,2-bis(3-amino-4-hydroxylphenyl) terephthalamide (PHA), then prepared PHA aerogels, and finally transformed the PHA aerogels into polybenzoxazole aerogels through heat treatment. According to the different bridging groups in PHA, we successfully prepared three kinds of aerogels, named BAP, 6FAP and DHB aerogel respectively. The DHB aerogel exhibited heat resistance with a 5% thermal decomposition temperature of 600 °C, a low thermal conductivity (0.036 W m⁻¹ K⁻¹), and excellent mechanical properties (stress at 50% strain: 31.02 MPa and compressive modulus: 79.20 MPa), and the 6FAP aerogel exhibited a low dielectric constant (1.47 at 1 MHz), and low dielectric loss (0.0025 at 1 MHz). Additionally, the DHB aerogel produces neither naked light nor smoke when burned in flames. It immediately extinguishes after exiting the flame, exhibiting good flame retardancy. This behavior is attributable to the intrinsic properties of polybenzoxazole and the high porosity of aerogels. Since PHA has good solubility and can be used to prepare homogeneous solutions with various solid contents, the proposed preparation strategy avoids the drawback of directly using polybenzoxazole resin. The preparation process is simple, efficient, and versatile.