Cure reaction pathways of bismaleimide polymers:: A solid-state 15N NMR investigation

The cure reaction pathways of a bismaleimide resin with a diamine are investigated using solid-state N-15 NMR spectroscopic techniques with specifically labeled monomers. These techniques provide clear identification of at least three different reaction pathways in the curing resin, one of which has previously been only postulated. In the homopolymerized bismaleimide resin system, maleimide ring addition has previously been shown to be the only observable reaction. When co-reacted with an amine, Michael addition of the amine to the maleimide ring has also been observed. Furthermore, a ring-opening aminolysis reaction, which has been observed in solution with specific reagents and conditions, has been suggested to occur under cure conditions. We show conclusively that this aminolysis reaction occurs to a significant extent during the cure of the neat resin and that this product can remain in the network structure even after a high-temperature postcure treatment. The existence of the amide product is demonstrated using bismaleimide resin formulations selectively labeled with C-13 and N-15 at specific sites. The N-15 chemical shifts and the C-13-N-15 scalar couplings are consistent with and confirm the amide product formation. Furthermore, under certain cure and postcure conditions, the aminolysis reaction is reversible which may significantly affect the final network structure.