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-te mperature electron paramagnetic resonance (EPR) spectra from stable fr ee-radical species that are formed during the postcuring stages. The v ariable-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 ran ge 80-120-degrees-C. The EPR intensities follow the inverse temperatur e dependence of Curie's law below T(min) and are due to a stable free radical. The intensities then increase with increasing temperature bet ween T(min) and T(max). The free radical with such temperature depende nce is not present below T(min) and is undetectable by EPR at temperat ures above T(max). These free radicals are generated during the postcu ring process at elevated temperature above 310-degrees-C. The thermo-o xidative degradation involves free radicals generated during the postc uring process in the presence of oxygen gas.