New thermostable systems: 2. Structure/properties relationship of nadimide-terminated fluorinated polyimides
Résumé
The thermomechanical properties of 12 nadimide endcapped oligomers synthesized by polycondensation of nadic anhydride (NA), hexafluoroisopropylidene-2,2′-diphthalic acid anhydride (HFDA) and phenylene diamines, meta (mPDA), para (pPDA) or 9,9-bis(4-aminophenyl)fluorene (cardo diamine) have been studied. The rheological curves obtained showed the presence of two relaxations: α, associated with the glass transition temperature, and β, due to local molecular movements with the glass transition. The temperature of the loss modulus G″ associated with these two relaxations varied in the domain of molecular weight M̄n according to a Fox-Flory relation. By studying the relationship between chemical structures and mechanical properties, the influence of endo/exo isomerization of the nadimide end groups on the glass transition temperature Tg has been demonstrated. For a given oligomer, the Tg value increases with increasing percentage of exo isomers. The thermo-oxidative stability of the HFDA, PDA and NA oligomers increases when the molecular weight increases.