Strong, Tough, and Recyclable Aramid Aerogel Fibers with Homogeneous Nanoscale Structures for High-Temperature Thermal Insulation Applications

Aramid aerogel fibers (AAFs) have attracted tremendous attention recently because of their lightweight, temperature resistance, and flame retarding characteristics, which hold great promise in wearable thermal insulation applications. However, there is still a lack of an efficient method for the fabrication of high-performance AAFs with homogeneous nanoscale structures and optimized thermal/mechanical properties. This paper reports a scalable approach for the continuous spinning of heterocyclic AAFs (HAAFs) by the combination of the "ropes bind rods" reinforcement concept and the "redox responsive sol-gel transition" strategy. The redox reaction responsiveness of the coordination property between cobalt ions and benzimidazole ligand contained in heterocyclic aramid (PABI) is exploited as triggering for sol-gel transition in the spinning process, where the highly dynamic PABI/Co2+ complex in spinning dope rapidly converts into a highly stable PABI/Co3+ complex upon extruding into an oxidation bath and forms gel fibers. Incorporating a small amount of flexible benzimidazole-containing polymer further promotes the gelation of PABI/Co2+ system and strengthens the PABI/Co3+ cross-linked networks on the basis of a "ropes bind rods" concept on the nanoscale level, which ensures the continuous spinning of HAAFs. The resultant HAAFs demonstrated a homogeneous nanoscale porous structure thanks to the in situ constructed cross-linked networks that prevent the gel skeleton from distortion in subsequent treatment. As a result, the HAAFs show mechanical strength among the highest reported values for AAFs and excellent toughness that is superior to all previous AAFs. The AAFs also exhibit good thermal stability and flame retardancy for thermal insulation applications. The recyclability of HAAFs is finally demonstrated based on the reversible feature of metal coordination bonds. This work is expected to provide an efficient strategy for the design and scalable fabrication of aramid and other high-performance aerogel fibers.