Deformation Failure Mechanisms and Post-Impact Thermal Insulation of Density-Graded Polyimide Aerogels Subjected to Axial Quasi-Static Compression and Low-Velocity Impact

Polyimide (PI) aerogels represent a class of organic aerogels endowed with exceptional integrated mechanical and thermal properties. Despite their substantial promise as structural components for mechanical-thermal-coupled protective applications, PI aerogels suffer from a stiffness mismatch with other rigid skins when integrated into structural assemblies. This study fabricates density-graded PI aerogels with positive gradient (PG) and negative gradient (NG) configurations. The mechanical and thermal properties of the aerogels under quasi-static compression (0.001-1 s^-1) and low-velocity impact (100 s^-1) were investigated. Results demonstrate that gradient PI aerogels exhibit strain-rate insensitivity, which is in sharp contrast to the pronounced strain-rate dependence of homogeneous PI aerogels. PG configurations achieve a superior energy absorption performance over NG counterparts via ordered layer-by-layer collapse with the volumetric energy absorption up to 3.46 MJ·m^-3 and gravimetric energy absorption up to 19.80 J·g^-1. In addition, the high-density regions of gradient PI aerogels retain excellent thermal insulation performance after impact. This study elucidates the critical role of density gradient direction and layer configuration in regulating the mechanical and thermal performances of PI aerogels. The findings provide mechanistic insights for the rational design of gradient PI aerogel-based protective structures requiring balanced lightweight, energy absorption, and thermal insulation under static and dynamic loading conditions.