Abstract— Kinetics and 18O‐labeling studies have provided evidence for the involvement of dioxirane intermediates (V) in the ketone‐catalysed decomposition of peroxomonosulfate (Caroate) HSO;. Reaction rates depend on ketone structure. In competition with catalysis of peroxide decomposition, the dioxirane intermediate is capable of oxidizing several organic and inorganic substrates. Thus, cis‐ and rrans‐cinnammic acids could be converted stereospecifically into the corresponding epoxides. Also, oxidation of phenylpropiolic acid, a substrate which is representative of weakly nucleophilic alkynes, could be carried out using the Caroatehetone oxidizing system. Under the conditions adopted, no oxidation of the substrates examined was found to occur in the absence of ketone. The possibility that formation of dioxathiirane intermediates (XXII) occurs following a side pathway in the reaction of Caroate with sulfoxides (which produces sulfones in high yield) has been explored. Preliminary experiments using 18O‐labeling of p‐tolyl phenylsulfoxide, however. failed to support this hypothesis. pointing out the need for further detailed studies.

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LARC™-RP46 resin system is a PMR type polyimide and is prepared by replacing methylenedianiline in the PMR-15 composition with 3,4′-oxydianiline. This resin system retains the same processing characteristics as PMR-15 but also offers enhanced fracture toughness. Rheological measurements were conducted on pre-imidized LARC™-RP46 moulding powder subjected to various ramp and hold temperature schemes. Adequate flow properties were found with theoretical (formulated) molecular weight 6 ≤1500 g mol−1. Critical transition temperatures for optimizing the process cycle were identified. They included the resin softening point, the imidization reaction peak, the isomerization reaction peak and the gelation point. Utilizing this information, 1.72 × 10 6 Pa (250 psi) cure cycles were designed for B-staged (dry) and unstaged (wet) prepregs. Composite laminates were fabricated which exhibited excellent consolidation and a void content below 0.1–0.2% as measured by image analysis. IM7/LARC™-RP46 exhibited higher composite mechanical properties than IM7/PMR-15. Short-beam shear strength, flexural strength and flexural modulus were measured at room temperature, 93, 150 and 177 °C. Composite engineering properties were also obtained including longitudinal tension, logitudinal compression, interlaminar shear, short block compression, open hole compression (OHC) and compression strength after impact (CAI). Excellent longitudinal tensile and compressive strengths were obtained and the CAI strength was 40% higher than that for PMR-15. Over 80% retention of all RT strengths were noted at 177 °C.

The thermally induced cross-linking of nadic-end-capped polyimides has been studied by a variety of spectroscopic techniques. Two nadic-end-capped polyimides were subjected to 371 °C heat treatment for 100 h in order to study cross-linking resulting from elevated temperature. The glass transition temperature of the treated samples increased to greater than 400 °C, indicating that both polyimides underwent extensive chemical changes during heat treatment. 13C spin counts were measured before and after heat treatment by 13C cross-polarization magic angle spinning nuclear magnetic resonance. Electron spin resonance measurements showed an increase in the concentration of stable free radicals in the treated samples. Dynamic nuclear polarization and electron spin resonance results show that two species of free radicals are present in the treated samples; the first is located on the polymer backbone and the second on the cross-linking network that arises from the reaction of the nadic end-capping groups. On the basis of these results, new cross-linking structures of nadic-end-capped polyimides before and after heat treatment at 371 °C are proposed.

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