Tailoring polyimide aerogel performance: the role of solid content in influencing microstructure, mechanical, thermal, and acoustic behaviors

In this study, polyimide aerogels (PIAs) with solid contents of 3%, 4%, 5%, and 6% were fabricated using non-directional freezing and freeze-drying techniques. A comprehensive investigation was carried out on their microstructures, mechanical, thermal insulation, and acoustic properties. As the solid content increased, the density and linear shrinkage of PIAs rose, while porosity decreased from 97.62% (PIA-3%) to 94.54% (PIA-6%). Microstructural analysis showed that high-solid-content PIAs had smaller pore sizes and denser network structures. Thermogravimetric analysis (TGA) tests indicated excellent thermal stability with initial decomposition temperatures above 500 °C, with little influence from solid content. In terms of mechanical properties, the Young's modulus and specific Young's modulus of high-solid-content aerogels were notably enhanced, with PIA-6% reaching 5703 kPa. The thermal conductivity of PIAs increased with solid content, from 40.81 mW m^-1 K^-1 for PIA-3% to 48.63 mW m^-1 K^-1 for PIA-6%, due to increased skeleton density and reduced porosity. Regarding acoustic properties, low-solid-content aerogels had better sound absorption, <i>e.g.</i>, a 2-cm PIA-3% sample had an average sound absorption coefficient of 0.602 and NRC of 0.393 in 250-6300 Hz, while for PIA-6%, they dropped to 0.416 and 0.311. Conversely, high-solid-content aerogels had superior sound insulation, with PIA-6% achieving an average STL of 30.38 dB compared to 11.94 dB for PIA-3%. Overall, the performance of PIAs can be tailored by adjusting the solid content, with low-solid-content materials suitable for sound absorption and high-solid-content ones for sound insulation and high-strength applications. This research offers new insights for developing high-performance acoustic materials and promotes the practical application of PIAs in aerospace, architectural acoustics, and traffic noise reduction.