Kinetics of imidization and crosslinking in PMR polyimide resin

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Abstract
Infrared spectroscopy and differential scanning calorimetry were used to study the imidization and crosslinking kinetics of norbornenyl‐capped addition type of polyimide resins (designated PMR for polymerization of monomer reactants). The spectral and thermal analyses were performed on resin specimens that had been isothermally aged at temperatures appropriate for imidization (120–204°C) and crosslinking (275–325°C). Imidization occurs rapidly (about 10−2 min−1) at short times, but, at times longer than about 0.5 hr, the rate decreases significantly (about 10−4 min−1). The crosslinking reaction exhibits first‐order kinetics during the initial part of the reaction, and its rate appears to be limited by the reversion of the norbornenyl Diels‐Alder adduct. Both the first‐order dependence and the activation energy (about 44 kcal) are consistent with this interpretation. The total heat evolved per mole of end cap during crosslinking shows an inverse dependence on the molecular weight of the imido prepolymers. This reflects the effect of end‐cap dilution and decreased mobility of the larger oligomers.

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