New thermostable systems: 2. Structure/properties relationship of nadimide-terminated fluorinated polyimides

The thermomechanical properties of 12 nadimide endcapped oligomers synthesi zed by polycondensation of nadic anhydride (NA), hexafluoroisopropylidene-2 ,2'-diphthalic acid anhydride (HFDA) and phenylene diamines, meta (mPDA), p ara (pPDA) or 9,9-bis(4-aminophenyl)fluorene (cardo diamine) have been stud ied. The rheological curves obtained showed the presence of two relaxations : cc, associated with the glass transition temperature, and beta, due to lo cal molecular movements with the glass transition. The temperature of the l oss modulus G " associated with these two relaxations varied in the domain of molecular weight (M) over bar(n) according to a Fox-Flory relation. By studying the relationship between chemical structures and mechanical pro perties, the influence of endo/exo isomerization of the nadimide end groups on the glass transition temperature T-g has been demonstrated. For a given oligomer, the T-g value increases with increasing percentage of exo isomer s. The thermo-oxidative stability of the HFDA, PDA and NA oligomers increas es when the molecular weight increases.