OXIDATIVE-DEGRADATION OF NADIC-END-CAPPED POLYIMIDES .2. EVIDENCE FORREACTIONS OCCURRING AT HIGH-TEMPERATURES

The oxidative degradation of PMR (for olymerization of monomeric react ants) polyimides at elevated temperatures was followed by cross-polari zed magic angle spinning (CP-MAS) NMR. C-13 labeling of selected sites in the polymers allowed for direct observation of the transformations arising from oxidation processes. As opposed to model compound studie s, the reactions were followed directly in the polymer. The labeling e xperiments confirm the previously reported oxidation of the methylene carbon to ketone in the methylenedianiline portion of the polymer chai n. They also show the formation of two other oxidized species, acid an d ester, from this same carbon. In addition, the technique provides th e first evidence of the kind of degradation reactions that are occurri ng in the nadic end caps. Several PMR formulations containing moieties determined to be present after oxidation, as suggested by the labelin g study, were synthesized. Weight loss, FTIR, and natural abundance NM R of these derivatives were followed during aging. In this way, weight loss could be related to the observed transformations.