Kinetic study of the imidization of a poly(ester amic acid) by FT-Raman spectroscopy

Abstract

The kinetics of thermal imidization of a poly(ester amic acid) in solid state was investigated in the temperature range 120-180 degrees C. FT-Raman spectroscopy was used for the measurement of imidization. Data on changes in intensity were collected on compressed thin specimens that had been isothermally cured. FT-Raman spectral changes as a function of imidization time are adequately modeled by assuming two first-order imidization reactions. Activation energies for the first and second ring closure processes were almost the same, 20.5 and 19.9 kcal/mol, respectively. However, the activation energy increased slightly (24.6 kcal/mol) when the second-order reaction model was adopted for the first reaction. Increase in activation energy was screened by the increase of the pre-exponential factor, implying the absence of significant changes in the reaction rate ratio. A new time dependent model is proposed which is easy to apply and shows good agreement with the experimental data, both quantitatively and qualitatively. The activation energy calculated by the new model was 24.3 kcal/mol. It takes into consideration the evolution of different amic acid stages. The proposed model also correlates the two-step imidization reaction very well.