Contribution of the dynamic mechanical spectroscopy for studying the microstructure of polyimides

Different polyimide films based on various aromatic diamines and dianhydrides have been studied by dynamic mechanical thermal analysis. Polyimides exhibit three mechanical relaxations related to specific molecular motions. We have analyzed the sub-glass gamma relaxation which appears at 1Hz in −140°C to −50°C temperature range. This relaxation originates from water molecules in polyimides. Its temperature location strongly depends on the chemical structure of polyimides. Then, the temperature of this relaxation process was correlated to microstructural parameters. It was found that the gamma relaxation shifts towards higher temperatures with: (i) decreasing the free volume; (ii) decreasing the intersegmental distance determined X-ray diffraction; (iii) increasing the wavelengths of 50% transmission determined by UV-visible spectroscopy.