Latently reactive end caps have been investigated as improved means to increase the thermo-oxidative stability of polyimides of the polymerization of monomeric reactants (PMR) type, which are often used as the matrix resins of high-temperature-resistant composite materials. The present end-cap compounds are candidates to supplant the norbornene end cap (NE) compound that, heretofore, has served to limit molecular weights during oligomerization and, at high temperatures, to form crosslinks that become parts of stable network molecular structures.
NE has been important to processability of high-temperature resins because (1) in limiting molecular weights, it enables resins to flow more readily for processing and (2) it does not give off volatile byproducts during final cure and, therefore, enables the production of void-free composite parts. However, with respect to ability of addition polymers to resist oxidation at high temperature, NE has been a "weak link." Consequently, for example, in order to enable norbornene-end-capped polyimide matrices to last for lifetimes up to 1,000 hours, it is necessary to limit their use temperatures to ≤315 °C. The present improved end caps are also subject to oxidation at high temperature, but they oxidize in a different manner, such that the long-term stability of a polymer made with one of these end caps exceeds the long-term stability of the corresponding polymer made with NE. Hence, use temperatures and/or lifetimes can be increased.
Prior attempts to increase thermo-oxidative stability of PMR polyimides were oriented toward formulation of end caps that are inherently more stable than is the nadic end cap. The results were not satisfactory in that the end caps thus formulated adversely affected processability, the nature of the crosslinks, and, in some cases, the thermomechanical properties of the resulting polymers. In the present approach, one does not attempt to formulate end caps that are inherently more stable; instead, one seeks nadic derivatives that exploit one of the modes of the thermo-oxidative degradation of the nadic end cap in such a way as to retard the overall thermo-oxidative degradation of the affected polymers.
Research on the aging of PMR-15 polyimide has revealed that the degradation of the nadic end cap can occur via two primary reaction paths, designated A and B (see Figure 1). On path A, degradation proceeds through initial scission and oxidative opening of the norbornyl ring to form a 2-hydroxy substituted maleimide. On path B, degradation proceeds through oxidation of the bridging methylene of the norbornene moieties, followed by carbon monoxide extrusion. Aromatization of the resulting biradical leads to substituted phthalimides, and related secondary degradation products. The oxidation products of path A (including the 2-hydroxy substituted maleimide) are cleavage products that are most likely formed concomitantly with large loss of weight from the affected polymer. In contrast, the products of path B are more oxidatively stable and form with very little weight loss.
Therefore, in the present approach, one seeks to formulate end caps that preserve desirable processing properties of NE while favoring path B strongly, leading to lower weight loss and thus less shrinkage and cracking in the thermally oxidized layer of the affected polymer. Figure 2 depicts a generic molecular structure for a class of end caps that become thermo-oxidatively degraded primarily along path B. In this structure, X maintains its stability during imidization (at a temperature of 200 °C) and cross-linking (at 315 °C). Nevertheless, following these critical steps, X is spontaneously converted, upon aging, to a thermally stable capping group.
This work was done by Mary Ann B. Meador and Aryeh A. Frimer of Glenn Research Center. For further information, access the Technical Support Package (TSP) free on-line at www.nasatech.com/tsp under the Materials category.
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Attn: Steve Fedor,
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Refer to LEW-16987.