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 imidization 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 2.0°C·min–1 showed that it begins to imidize at 124°C, undergoes the major portion of imidization up to 210°C, and has completed imidization by 310°C. Anhydride rings are found to be present between 93°C (i. e., below the onset of imidization) and 310°C (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.