2, National Aeronautics and Space Administration, Langley Research Center, Hampton, Virginia, United States
3, National Aeronautics and Space Administration, George C. Marshall Space Flight Center, Huntsville, Alabama, United States
4, College of William and Mary, Williamsburg, Virginia, United States
5, Liberty University, Lynchburg, Virginia, United States
Polyimides have been widely used in many applications because of their superior properties, such as excellent thermal and chemical stability, excellent dielectric properties, and mechanical strength. Their applications range from microelectronics and aerospace to the medical and nuclear industries. However, even though polyimides show superior durability in harsh conditions compared with other polymers such as polyolefin and polyesters, the polyimides, being organic materials, will decompose under high doses of ionizing radiation. The resulting degradation of physical properties limits the reliable operation lifetime of systems composed of unprotected polyimide materials in this environment.
Recently, NASA has developed new inorganic/polyimide hybrid composites for long duration aerospace structural applications requiring a radiation resistant capability against high energy electrons, protons, neutrons, heavy ions, vacuum ultraviolet (VUV), gamma-rays, and X-rays from solar particle events (SPE) and galactic cosmic rays (GCR). In this study, the effect of inorganic particles on the radiation induced degradation of polyimides, and the mechanism of degradation under high energy radiation, was initiated. LaRC SITM polyimide was chosen as a baseline matrix, and several different inorganic nanoparticles were selected as additives. To evaluate the effect of ionizing radiation, the hybrid polyimide composites were exposed to energetic electrons (about 50-60 keV and 9 nA/cm2) or high energy photons (VUV, 184.9 nm and 253.7 nm). The thermal, optical, and mechanical properties were characterized before and after exposure. The pristine polyimide showed a weight loss of about 50% after high energy photon exposure, while the hybrid polyimide composite showed a weight loss of only about 29%. The hybrid polymer composite showed a retarded degradation in mechanical properties (about 20% decrease in elongation at break of the hybrid polyimide composite versus about 85% decrease in elongation at break of the pristine polyimide). The radiation induced molecular degradation mechanism is discussed using various experimental techniques, including Fourier transform-infrared (FT-IR) spectroscopy, dynamic mechanical analysis (DMA), and electron paramagnetic resonance (EPR) spectroscopy. In addition, potential applications in the nuclear and aerospace industries will be discussed.