Synthesis and characterization of some novel thermally stable poly(amide-imide)s

Two new diamines with built-in amide linkage were synthesized from p-nitrobenzoyl chloride in two steps and characterized by their melting points, elemental analyses, FT-IR and 1H–NMR spectral studies. One of the diamine precursors was also characterized by single crystal X-ray analysis indicating the trans orientation of amide bonds. The planes of the aromatic rings formed dihedral angles in the range of 11.7°–56.2° and the crystal packing was stabilized by N–H···O hydrogen bonds. The synthesized diamines were then polymerized with three commercially available dianhydrides either by one-step solution technique or by two-step procedure involving ring-opening poly-addition to give polyamic acid followed by cyclic dehydration. The resulting poly(amide–imide)s were characterized by their elemental analyses and FTIR spectroscopy. The polymers possessed excellent chemical resistance and were found insoluble in almost all the common solvents tested except conc. H2SO4, in which these showed inherent viscosities in the range 0.47–0.73 dL g−1 at 28 °C. The thermal behavior of the polymers, investigated by the DSC and TG analyses, revealed that the products were stable up to 300 °C in nitrogen. The activation energies of pyrolysis, entropy, and enthalpy values for each of the synthesized polymers were also estimated by Horowitz and Metzger method using thermal degradation data. These values were high exhibiting the remarkable thermal stability of the poly(amide–imide)s. Wide-angle X-ray studies of the polymers were also carried out which provided information about their morphology.