Non-Solvent-Induced Self-Assembly of Para-Aramid Nanofibers with Tunable Molecular Weight for Enhanced Thermal Stability and Electrical Insulation

The para-aramid nanofibers have attracted increasing research interest in recent years. However, current fabrication methods often lack control over key polymer properties, such as molecular weight (MW). Herein, we propose a nonsolvent-induced self-assembly (NISA) strategy for preparing poly(p-phenylene terephthalamide) (PPTA) nanofibers with tunable MW. The process involves dissolving PPTA in the N,N′-dimethylpropylene urea/LiCl solvent system, followed by water addition as a nonsolvent to trigger oriented self-assembly via hydrophobic interactions. Simultaneously, a high-shear dispersion is applied to facilitate the formation of homogenized nanofibers (NANF) with uniform diameter distributions (20–30 nm). Crucially, this top-down approach maintains the intrinsic MW of original polymers, achieving direct control over nanofiber MW by selecting PPTA with different initial MW, a significant advancement over conventional degradation-dependent methods. The nanopapers prepared from NANF of varying MW under identical conditions are further tested and evaluated. Papers derived from low-MW nanofibers exhibited superior mechanical strength and electrical insulation, while those from high-MW nanofibers demonstrated enhanced thermal stability (>500 °C) and flame retardancy. This nonsolvent-induced self-assembly method offers a versatile strategy for the controlled fabrication of aramid nanofibers tailored for high-performance applications such as the high-temperature resistant insulating aramid paper and related composite materials.