This study examines for the first time how matrix crosslinking affects the composite physical and mechanical properties of a graphite fiber reinforced PMR polyimide composite during long‐term isothermal aging. Unidirectional composite specimens of Celion 6000/PMR‐P1 were isothermally exposed at 288°C in air for various time periods up to 5000 h. The matrix crosslink densities were estimated from the kinetic theory of rubber elasticity and shifts in the glass transition temperatures (Tgs). The Tg, coefficient of thermal expansion, density, weight loss, moisture absorption, and elevated temperature flexural and interlaminar shear properties were also determined. Several linear relationships were found between the matrix crosslink density and composite physical and mechanical properties. The Tg, initial weight loss and density, and elevated temperature interlaminar shear strength increase with an increase in crosslink density. Conversely, the initial moisture absorption and coefficient of thermal expansion decrease with increasing crosslink density. As expected, the elevated temperature flexural strength and modulus show no direct correlations with crosslink density. Further, after achieving the highest matrix crosslink density, several of the composite properties begin to decrease rapidly. These findings suggest that time‐temperature dependent nature of attaining the maximum matrix crosslinking is closely linked to the onset of the composite property degradation. Though much more work is needed, a fundamental understanding of the relationships between matrix crosslinking and composite physical and mechanical property can provide a scientific basis for the prediction of the extent of composite service life not only for PMR polyimides but also for other thermosetting matrix resins, such as epoxies and bismaleimides.

Three geometric isomers from the thermal isomerization of methylene‐4,4′‐bis(endo‐N‐phenylbicyclo‐[2.2.1]hept‐2‐ene‐5,6‐dicarboximide) (I) were chromatographically separated and isolated. Nmr spectroscopy was used to assign endo‐endo (I), endo‐exo (II), and exo‐exo (III) configurations to each compound. The three isomers, which serve as model compounds for norbornene end‐capped polyimides, were then characterized by several chromatographic, spectroscopic, and thermal techniques. Upon heating, each isomer was found to establish an equilibrium mixture of all three isomers. The possibility that the compounds react by different mechanisms in air and in nitrogen is proposed.

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