End Caps for More Thermo-Oxidative Stability in Polyimides

John H. Glenn Research Center, Cleveland, Ohio

An additional class of end-cap compounds that increase the thermo-oxidative stability of polyimides of the polymerization of monomeric reactants (PMR) type has been invented. The prior class of end-cap compounds in this line of development is described in the preceding article.

Figure 1. The Thermo-Oxidative Degradation of a norbornenyl end cap follows paths A and B. The products of path B are more stable than are those of path A.

PMR polyimides are often used as matrix resins of high-temperature-resistant composite materials. Like the end-cap compounds described in the cited previous article, 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 cross-links that become parts of stable network molecular structures.

To recapitulate from the previous article: 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.

Figure 2. End Caps of This Molecular Structure are alternatives to previously reported end caps for increasing the thermo-oxidative stability of polyimides. R1 or R2 or both can be alkyl, alkoxy, aryl, flouro, chloro, carbomethoxy, nitro, or another substituent.

Like NE and like the end caps described in the prior article, the present 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 to the present line of development, 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 cross-links, 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 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 and with less shrinkage and cracking in the oxidized layer.

The present end caps are formulated to preserve the desirable processing properties of NE and to undergo thermo-oxidative degradation primarily or exclusively along path B. Figure 2 depicts a generic molecular structure for the present class of end caps. In this structure, the end cap contains no bridging methylene. Rather, the end cap is a 1,2,3,6-tetrahydrophthalic anhydride, substituted in such a way as to lower the cross-linking temperature from >415 °C to between 280 and 350 °C. The end cap maintains its stability during imidization (at 200 °C) and cross-linking. After the foregoing critical steps, the end cap is spontaneously converted, upon aging, to thermally stable capping groups.

This work was done by Mary Ann B. Meador 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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