NMR RELAXATION STUDY OF THERMAL-DEGRADATION IN CURED PMR POLYIMIDES

We have studied cross-linking and thermal degradation of high-performa nce first- and second-generation PMR-15 polyimides, both thermoset and thermoplastic versions, by performing nonspectroscopic NMR solid echo T-2 relaxation measurements at temperatures up to 430 degrees C usin g probes built for this purpose. We employ signal averaging and automa ted decomposition of the relaxation decays into two Gaussian component s, the slower of which gradually appears above 300 degrees C. Tracking the molecular mobility spectrum in terms of the relative intensity of the components and their relaxation times as temperature is cycled, w e detect essentially no irreversible effects below the glass transitio n, measure permanent mobility reductions attributable to completion of cure, and find that exposure to temperatures above 380 degrees C on t he order of 1 h is required for substantial thermal degradation to occ ur. These results are closely supported by thermal and mechanical meas urements on parallel specimens. Second-generation PMR resins appear to have higher microscopic rigidity and reduced viscous fraction at high temperatures. (C) 1995 John Wiley & Sons, Inc.