ELECTRON-PARAMAGNETIC-RESONANCE SPECTROSCOPIC STUDIES OF THERMALLY GENERATED FREE-RADICALS IN PMR-15 POLYIMIDE RESINS

PMR-15 is a high-performance thermoset polyimide resin that is used in many high-temperature applications. Postcured PMR-15 produces room-temperature electron paramagnetic resonance (EPR) spectra from stable free-radical species that are formed during the postcuring stages. The variable-temperature EPR spectral intensities show a minimum at T(min). in the range -60 to -40-degrees=C, and a maximum at T(max) in the range 80-120-degrees-C. The EPR intensities follow the inverse temperature dependence of Curie's law below T(min) and are due to a stable free radical. The intensities then increase with increasing temperature between T(min) and T(max). The free radical with such temperature dependence is not present below T(min) and is undetectable by EPR at temperatures above T(max). These free radicals are generated during the postcuring process at elevated temperature above 310-degrees-C. The thermo-oxidative degradation involves free radicals generated during the postcuring process in the presence of oxygen gas.