Poly(3,4 '-oxydiphenylene pyromellitamic acid), 1 - Time-resolved infrared spectroscopic study of thermal imidization in thin films

Abstract

A time-resolved infrared spectroscopic investigation was made into the thermal imidization behavior of poly(3,4'-oxydiphenylene pyromellitamic acid) precursor in a microscaled film. The CNH bend-stretch (amide-II band) was found to be the most useful tool for examining the imidization mechanism in the precursor film Examination of the behavior of this band showed that the precursor undergoes a two-step imidazation process: (i) decomplexation of the amide linkage from residual solvent molecules and other intra- and intermolecular amic acid groups, and (ii) subsequent imide-ring formation. In addition, it was found that, among the vibrational peaks in the precursor and resultant polyimide, the imide CN stretch is the most accurate tool for determining the degree of imidization. Experiments in which the precursor was heated at 2.0degreesC; min(-1) showed that it begins to imidize at 124degreesC, and has completed imidization by 310degreesC. Anhydride rings are found to be present between 93degreesC (i.e. below the onset of imidization) and 310degreesC (the temperature at which imidization is completed). The presence of anhydride rings originates from the nature of the equilibrium between the precursor and its constituent anhydride- and amino-terminated species. Conjugation along the chemical repeat unit during imidization was evident from the imide CN stretch and aromatic CH symmetric bend of the 3,4'-ODA unit, which is evidence that the precursor chains are converted to the extended polyimide chains that are necessary for the formation of a highly-ordered crystalline structure.