Broadband dielectric spectroscopy of BPDA/ODA polyimide films

Dielectric spectroscopy of a high-temperature photosensitive polyimide was investigated in wide temperature and frequency ranges during heating and cooling cycles (from -150 to 370 degrees C and from 0.1 to 1 MHz). During the heating phase measurements two sub-glass relaxation processes were observed, noted as gamma and beta relaxations. The gamma relaxation appears at a low temperature (around -60 degrees C at 1 kHz) with an activation energy of 0.44 eV during the heating phase and disappears during the cooling one, indicating that the peak is initially related to the presence of water in the polyimide films. The beta relaxation appears at higher temperatures (around 180 degrees C at 1 kHz) with a higher activation energy of about 1.5 eV. The beta peak location and intensity for low temperatures (between 100 degrees C and 120 degrees C) change slightly on comparing the heating and cooling spectra, indicating also the effect of water molecules, which may act as a plasticizer. However, for higher temperatures, the beta peak does not show any significant effect of the thermal cycle, and the relaxation is mainly attributed to the non-cooperative relaxation of the polyimide molecules. The ac conductivity (sigma') values show that the electronic hopping process is influenced by the dynamics of the segmental and macromolecular chains of the polyimide in the gamma and beta relaxation regions. At high temperatures (>250 degrees C) a plateau region appears in the ac conductivity allowing the extraction of the dc conductivity values, which are not affected between the heating and cooling measurements. This leads us to conclude that there are no significant morphological or chemical changes in the polyimide even for temperatures higher than its glass transition one under N-2 for short periods. For temperatures above 300 degrees C an increase in the values of relative permittivity is observed and referred to the Maxwell-Wagner-Sillars or to the electrode polarization phenomena. In this range the activation energy of the polarization peak frequency, conductivity relaxation peak frequency and the dc conductivity is the same and equal to 2.4 eV, indicating that those three parameters are governed by the same underlying mechanism.