High T(g) Polyimides

The use of high temperature polymer matrix composites in aerospace applications has expanded steadily over the past 30 years, due to the increasing demand of replacing metal parts with light weight composite materials for fuel efficiency and bigger payloads in the aircraft and the space transportation vehicles. Polyimide/carbon fiber composites, especially, have been regarded as major high temperature matrix materials, based on their outstanding performance in terms of heat resistance, high strength-to-weight ratio and property retention compared with epoxies (177 C/350 F) and bismaleimides (232 C/450 F). Traditional, then-neoplastic polyimides were prepared from dianhydrides and diamines in N-methyl-2-pyrrolidinone (NMP) at room temperature to form the polyamic acids, which were then imidized at 150 C to yield polyimides. However, the high-boiling solvent (NMP, BP= 202 C) is very difficult to remove, leading to the formation of voids during composite fabrication. In the early 1970's, PMR addition curing polyimides with reactive endcaps were developed at the Lewis Research Center (renamed NASA Glenn) to ensure the easy processing of imide oligomers in methanol during composite fabrication.