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6F-Polyimides with a phenylethynyl endcap (HFPE) were fabricated into carbon fiber composites using the standard polymerization of monomer reactant (PMR) approach via the ester/acid route. T650-35/HFPE carbon fiber composites were evaluated against the corresponding T650-35/PMR-II-50 composites with nadic endcap at 315ï370 C (600ï700 F) for their physical and mechanical properties as well as thermo-oxidative stability. In addition, concentrated HFPE monomer solutions were infused into stitched AS4 and T650-35 preforms and their mechanical properties were compared with those of the commercial BMI-5270 composites from cryogenic temperature to 343 C (650 F). The stitched HFPE composites out-performed BMI-5270 composites in terms of property retention at elevated temperature and microcrack resistance during thermal cycling from ï54 to 288 C (ï65 to 550 F). The stitched composites also showed more resistance toward blistering and delamination than conventional laminate composites during a rapid heating rate simulating launch and re-entry of reusable launch vehicles (RLV).

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X-ray crystal structures of three substituted 4,4′-biphenyldiamines, namely, 2,2′,6,6′tetramethylbenzidine (TMBZ), 2,2′-dimethylbenzidine (DMBZ) and 2,2′-bis(trifluoromethyl)benzidine (BFBZ), were determined. The torsional angles between the two phenyl rings for TMBZ, DMBZ, and BFBZ are φ ) 83°, 75°, and 59°, respectively. A structure-property relationship between the threedimensional configuration of diamine monomers and the corresponding polyimides is discussed.