Polymer fibrous aerogels for thermal insulation: from molecular building blocks to structural reliability.

Polymer fibrous aerogels combine the low density and high porosity of conventional aerogels with the continuity and deformability of fibrous networks, offering a route to lightweight thermal protection that can remain mechanically compliant. Their design is intrinsically coupled: reducing solid content, refining pores, strengthening junctions, or introducing functional phases can suppress one heat-transfer pathway while intensifying another or destabilizing the network. This review adopts retained thermal insulation after service exposure, rather than the minimum conductivity of the pristine specimen, as its central evaluation criterion. We trace how polymer chemistry, fiber formation, three-dimensional assembly, interfiber stabilization, and drying govern solid and gas conduction, thermal radiation, residual convection, and moisture-mediated transport, and how these pathways evolve after deformation, humidity, heating, and fire exposure. Cellulose-based, other bio-derived, aramid, PBO, polyimide, polyamide-imide, thermoplastic, and char-forming systems are compared using measurable structural descriptors and matched test conditions rather than composition labels alone. Across these material classes, durable insulation emerges within a structural window of sparse load-bearing fibers, localized mechanically effective junctions, controlled pore connectivity, and spatially confined functional phases. The resulting processing-structure-transport-retention framework identifies the evidence required to translate polymer fibrous aerogels from record-setting laboratory specimens into reproducible insulation systems with verifiable service reliability.