Some Aspects of the Cure of RP-46,a Nadic End-capped Polyimide,and Phenyl Nadimide and Bis-Nadic-3,4'-ODA Model Compounds

Infrared studies of the initial cure and post-cure of RP-46 resin, a nadic end-capped polyimide and a model bisnadimide compound N, N' -(oxydi-3,4 phenylene) di-5,6-norbornene-2,3-dicarboximide (Bis-nadic-3,4'-ODA) were conducted at 316, 325 and 350° C for various time periods. Infrared studies of another model compound, N-phenylnadimide, were conducted at lower temperatures, from 100 to 270° C. N-Phenylnadimide cures at much lower temperatures than Bis-nadic-3,4 -ODA and RP-46. The crosslinking reaction was followed by monitoring the absorption peaks at 841 and 785 cm^-1, the endo and exo bands in the 2,3 positions of the nadic end caps and cyclo-aliphatic and cyclo-olefinic peaks on the region 1000 to 600 cm^-1 associated with the nadic end-cap curing reaction. Weight losses and changes in the glass transition temperature of the RP-46 resin due to cure and post-cure, were also examined. The infrared changes suggest that complete cross-linking requires a temperature of 325° C or above, and that several new infrared bands are generated in the process. The simple thermally induced reverse Diels–Alder cyclopentadiene N-substituted maleimide recombination reaction or norbornene to norbornene, norbornene to cyclopentadiene or norbornene to substituted maleimide addition reactions may adequately define the cross-linked products derived from the initial cross-linking reaction at 316° C for 1 to 2 h. However, postcure of RP-46 resin or the nadic-3,4 -ODA model compound at 325 or 350° C for several hours generates additional infrared bands in the cyclo-aliphatic and cyclo-olefinic regions (1000–600 cm^-1), which are absent in material cured at 316° C for 1 to 2 h. Therefore, completely cured resin consists of simple addition reaction products from the initial cure at 316° C and more complex products from the elevated temperature post-cures. A post-cure at 325° C for about 8 h or at 350° C for 2 h is required for a maximumTgof 391° C. This is accompanied by a stabilization weight loss of about 2–3%. The weight loss and Tgdata are supporting evidence for the infrared studies.