The mechanism of the thermal imidization of solid-state polyamic acid was studied using Fourier transform infrared (FT-IR) spectroscopy using the two-dimensional correlation spectroscopy method. It is assumed that two isomers exist in a polyamic acid segment: one is called the <i>para</i>-segment, which favors imidization reaction, the other is the <i>meta</i>-segment, which is not in favor for imidization unless the temperature is high enough. The results show that the imidization process differs for the two states of polyamic acid segments. The <i>para</i>-segment is more sensitive to the heat environment for the formation of imide ring, and it will take several intermediate steps to complete the ring closure at the aid of the solvent. As for the <i>meta</i>-segment, the ring will be closed before the imide formation due to the powerful energy provided in the high-temperature environment, and the ever-increasing chain rigidity and the loss of solvent during the heating process make this path the only option to continue the imidization process.

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