| Abstract
| - The evolution of chemical composition and structure during the thermal imidization of an ester-type polyimideprecursor, poly(4,4‘-oxydiphenylene p-pyromellitamic diethyl ester), in micrometer scale films were studiedfor a heating rate of 2.0 °C/min with time-resolved synchrotron X-ray diffraction, in-situ infrared spectroscopy,and modulated differential scanning calorimetry. Our analyses show that the precursor polymer undergoesimidization in a two-step process. In the first step, the precursor polymer is decomplexed from the residualsolvent molecules, and in the second step, it undergoes imide ring formation with the release of ethanol asa byproduct. The imidization reaction starts around 210 °C and continues up to 320 °C. The thermal imidizationreaction induces the structural evolution of the film. As the imidization reaction proceeds, the coherent lengthalong the polymer chain axis increases. This imidization-induced structural evolution was found to occur viathree steps: (i) initiation, (ii) the first crystallization, and (iii) the second crystallization. The initiation stepis necessary prior to the evolution of the crystalline structure to increase the chain mobility of the precursorpolymer chains, and it requires thermal heating up to at least 238 °C at which point 22.5% of the imidizationis complete. Thereafter, the first crystallization occurs up to 310 °C, at which point 98.3% of the imidizationis complete. In the range 310−380 °C, the second crystallization occurs and produces almost completeimidization of the polymer chains.
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