Processable Polyimides with High Glass Transition Temperature and High Storage Modulus Retention at 400°C

Abstract

End-capped polyimide precursor solutions in ethyl alcohol were prepared by incorporating carbonyl groups into the polymer backbone of a second-generation PMR (in situ polymerization of monomer reactants) polyimide, PMR-II-50, which exhibited good features as a matrix resin for carbon-fibrereinforced composites. Cured polyimides were prepared in an analogous process as the PMR-II polyimides. The thermal and thermo-oxidative stability of the cured polyimides was systemically investigated. It was found that the cured polyimide containing a 1:1 mole ratio of the carbonyl group [–C(O)–] to the hexafluoroisopropyl group [–C(CF₃)₂–] in the polymer backbone possessed a very high glass transition temperature (440 °C) and an excellent thermal decomposition temperature (576 °C). The onset temperature of the storage modulus curve was measured at about 400 °C with high retention (>55%) of the storage modulus at 400 °C, compared to 10–15% of the corresponding PMR-II polyimide. Carbon-fibre-reinforced composites also showed a high onset temperature of the storage modulus (420 °C) and high storage modulus retention (>85%) at 400 °C. Preliminary results indicated that the composite laminate exhibited excellent mechanical strength retention at 371 °C, which was superior to that of CF/PMR-II composites.

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