| Organic-matrix composites provide advantages over other material systems because of their high strength, high stiffness, and light weight. The use of matrix materials stable to high temperatures allows for utilization of these materials in applications such as aircraft engines, hot structural components, and exhaust-washed structures. Norbornene-terminated addition polyimides are particularly attractive for high temperature applications because of their high glass transition temperatures, high thermal oxidative stability, and ease of processing. Polyimide composites used in aircraft engines and hot structural applications are often exposed to aggressive environments that include elevated temperatures and rapid heating as well as water and other aircraft fluids. The presence of water is of particular concern in high temperature applications. Norbornene-terminated addition polyimides absorb as much as 5 weight percent water under humid conditions and experience rapid heat-up in many service applications. This leads to chemical degradation and blistering of the laminates. This work studied the mechanism and kinetics of the chemical degradation process.; Initially, a processing study was conducted using 15N CP-MAS NMR to determine the processing dependence on the chemical structure of norbornene-terminated resin. It was shown that the relative amount of aminolysis and/or Michael addition reaction products formed is a function of the cure conditions.; Model compound studies were then conducted using FTIR to determine the mechanism of hydrolytic degradation. The backbone and crosslink structures were all shown to be hydrolytically stable with the exception of the Michael addition reaction product which readily degraded in moisture at 150°C.; Kinetic studies of the hydrolytic degradation were conducted using material property changes such as glass transition temperature and sample weight along with chemical changes measured 15N CP-MAS NMR. |
