Kinetics and mechanisms of thermal imidization of a polyamic acid studied by ultraviolet-visible spectroscopy

The kinetics and the mechanisms of thermal imidization of polyamic acid made from a conjugated diamine (p,p'-diaminoazobenzene) and a nonconjugated dianhydride (6F-DA) were investigated both in dilute solution and in solid state in the temperature range 150-190 deg C. UV-vis absorption spectroscopy was the main tool used, even though IR spectroscopy was also used for comparison with the results from UV-vis studies. UV-vis spectral changes as a function of imidization time were analyzed on the basis of the previously reported model compound studies to obtain the composition of various imidization species (Macromolecules , 1987, 20, 1414). In 1% N -methylpyrrolidone solution, dissociation and the first imide ring closure proceed faster than the second imide ring closure of polyamic acid/imide at all three temperatures studied (150, 170, and 190 deg C). This trend is attributed to the reduced basicity of the amide group in polyamic acid/imide in comparison to the analogous group in polyamic acid. Activation energies for the fast process and the second ring closure were 11 and 18 kcal/mol, respectively. During the early stages of imidization in dilute solution, IR spectra suggest some dissociation of polyamic acid and imidization. On the basis of these findings, the mechanism of imidization for dilute solution has been proposed. In solid-state imidization, the apparent imidization rate was initially faster than in dilute solution due to the catalytic effect of the neighboring amic acid group but levels off, probably due to vitrification. Therefore, the mechanism of solid-state imidization must take into account the catalytic effect of the neighboring amic acid groups and the decreasing mobility effect as a function of cure time. Modeling studies to predict spectral changes as a function of the ratio of the two imidization rate constants are consistent with the observed results in dilute solutions. The results are adequately modeled by assuming a first-order, two-step imidization reaction, assuming a small extent of dissociation. On the other hand, solid film results seem to be reasonably simulated by assuming a second-order reaction for the first step, followed by a first-order reaction for the second step to account for the decrease in the concentration of the conformationally favorable catalytic amic acid group. Finally, there is a good correlation between UV-vis results and IR results, in regard to the overall extent of reaction in solid films. 19 ref.--AA