Molecular Design Enables Dope-Dyed Porous Polyimide Fibers with Thermal Insulation and Flame Retardancy

Polyimide (PI) fibers are widely regarded for their exceptional thermal stability, mechanical strength, and chemical resistance, making them highly promising for aerospace, protective textiles, and flexible thermal insulation applications. However, their intrinsic brown–yellow tone and traditional carrier dyeing processes suffer from poor color fastness, structural damage, and wastewater pollution, which severely restrict their application expansion in multifunctional integration and green manufacturing. To overcome these limitations, this study proposes a molecular-level design strategy by precisely regulating the steric hindrance and electron cloud distribution of dianhydride monomers, leading to the development of a colorless poly(amic acid) (PAA, polyimide precursor) solution. It was demonstrated that increasing the concentration of PAA and storing it in solid form significantly delayed degradation while preserving favorable spinnability. Building upon this, a dope-dyeing strategy was employed by directly incorporating organic dyes (Dye), inorganic nanosheets, or thermochromic precursors (polyacrylonitrile (PAN)) into the PAA matrix. Using phase-separation-based wet spinning followed by thermal imidization, these functional agents were uniformly embedded within the fiber interiors. This approach enabled the formation of porous PI composite fibers with tunable pore distributions and stable coloration while benefiting from in situ structural reinforcement. Among them, PI/Preoxidized filament (panof, produced by preoxidizing PAN during the thermal imidization process) fibers showed the most outstanding performance in mechanical strength (22.34 MPa), flame retardancy (limiting oxygen index 53.0%), and thermal insulation (thermal conductivity 0.02907 W/m·K), due to molecular entanglement. This comprehensive strategy provides a feasible path for the green engineering of multifunctional thermal protection fibers and lays the theoretical foundation for their application in military thermal insulation textiles and extreme environment clothing.