Breaking the Performance–Processability Trade-Off in Thermosetting Polyimides for Fiber-Reinforced Plastics

Polyimide-based PMR (Polymerization of Monomer Reactants) resins are a representative class of ultra-high-temperature thermosetting materials originally developed to meet the demanding requirements of the aerospace industry in the 1960s and 1970s. Designed to maintain mechanical integrity and long-term durability under extreme thermal conditions, PMR resins emerged through NASA programs for supersonic aircraft and aerospace structures and subsequently became key matrix materials for high-temperature composite applications. Among PMR-derived systems, phenylethynyl-terminated imide (PETI) resins have gained significant attention as next-generation thermosetting polyimides. Compared with conventional PMR formulations, PETI resins offer improved melt processability, reduced volatile evolution during curing, lower processing defects, high glass-transition temperatures, excellent thermo-oxidative stability, and outstanding mechanical performance. The combination of the thermally stable imide backbone and phenylethynyl end-group crosslinking enables PETI-based materials to satisfy the stringent requirements of advanced aerospace structures, high-temperature adhesives, and high-performance composite matrices. This invited lecture will review the historical development of PMR resins and their technological evolution from PMR-15 to modern PETI systems. Particular emphasis will be placed on the structure–property relationships of high-temperature thermosetting polyimides, including curing behavior, melt processability, and molecular design strategies. Recent advances in enhancing thermal resistance and structural performance through the control of molecular rigidity, reactive end-group chemistry, and free-volume characteristics will be highlighted. Finally, future opportunities and challenges for thermosetting polyimides in high-temperature composite matrices, heat-resistant adhesives, and next-generation aerospace structural materials will be discussed.