| Structure-property-processing interrelationships ate characterized for a variety of polyimide composite matrices used in advanced aerospace structural applications. The relationships between chemical/physical structure and (i) fabrication conditions, (ii) mechanical, thermal, and physical properties, and (iii) hydrolytic stability, are investigated. Major findings determined from this research are presented below.; Carbon fiber/bismaleimide (BMI) cross-ply composite laminates are microcracked after standard cure and postcure procedures. Using a unique in-situ characterization of microcracking, it is demonstrated that extended cure times at low temperatures (177°C) prior to postcure can shift the microcracking threshold in these composites. Ultimately cute induced microcracking can be prevented under standard fabrication postcure temperatures as a result of a reduction in composite residual stress and an improved fiber/matrix interphase.; Structure-property-processing characterization of BMI, polyetherimide (PEI), poly(4,4′-oxydiphenylene pyromellitimide) (POPPI), and a phenylethynyl terminated imide oligomer (PETI-5) demonstrates that deformation in thermoplastic polyimides is controlled primarily by free volume. In thermosets, deformation is controlled by network defects and free volume. Interestingly, PETI-5 was shown to crystallize under certain time-temperature cure cycles, which results in dramatic changes in neat resin mechanical properties. The observed crystallization behavior appears to be a result of liquid crystal-like ordering due to the phenylethynyl end groups.; With respect to hygrothermal durability, thermoset polyimides prove to be more resistant to blistering and associated macroscopic damage than thermoplastics under hygrothermal excursions unless hydrolytic degradation induced chain scission has occurred. Accelerated hygrothermal exposure also demonstrated that the hydrolytic stability of polyimides is strongly dependent on the chemical nature of the polyimide and the end caps in thermosets. Phenylethynyl terminated imide oligomers displayed dramatically higher hydrolytic stability than norbornene terminated polyimides as a result of hydrolytic attack of the crosslinks associated with the Michael addition reaction.; Compilation of the results presented provides insight into controlling the thermal, physical, and mechanical properties as well as the hydrolytic stability of polyimides based on chemical structure and processing conditions. |
