Novel Ultraviolet-Light-Curable Polyimides
Polyimides have found broad application in fiber-reinforced composites for aerospace components and as thin films for electronics packaging. Typical routes to processing these materials require temperatures above 200 C. As a result, tooling costs for fabricating components from these composites can be quite high. Recent efforts within the Polymers Branch at the NASA Lewis Research Center have been aimed at developing radiationcurable (with light or electron beams) polyimides. Such materials may enable the processing of polymers and composites at or near room temperature, leading to reduced tooling requirements and costs. A new Diels-Alder route to polyimides has been developed that employs ultraviolet light (UV), rather than heat, to effect polymerization. This approach, which can be carried out at room temperature, is based on a well-known photochemical reaction--the photoenolization of o-methylphenyl ketones. Irradiation of o-methylphenyl ketones, such as 1 in the preceding figure, with UV wavelengths above 300 nm produces a photoenol, 2. This photoenol is unstable, but it persists long enough to undergo Diels-Alder reactions with good dienophiles, such as maleimide, 3. By utilizing a diketone, such as 2,5-dibenzoylp-xylene, 5, and a bismalemide, 6, this chemistry has been used to make a number of polyimides, 7.