Dielectric and thermoanalytical behavior of moisture and water in aromatic polyimide and polyimide films

Specimens of an aromatic polyamide(Aramid) film as well as a polyimide film, which were subjected to various degrees of moisture adsorption as well as water immersion, were investigated both dielectrically and thermoanalytically in order to compare how much the adsorbed moisture and the stored water affect each dielectric and thermoanalytical behavior. It became clear that the polyimide film is more hydrophilic than the Aramid film and there is apparently no free water detected in both films. It followed from the thermogravimetry (TG) measurements that the polyimide film appears extraordinary in such a manner that its weight loss occurs dominantly at room temperature and then does not occur almost above 200°C reaching totally about half of the water content value measured by the weight increase. It means that there is a large discrepancy existing between the weight loss and weight increase of the polyimide film, being demonstrated by the calculation of dissipated moisture from the polyimide film during the DSC measurements, too. The results for the dielectric measurements at room temperature show that in the whole frequency range both relative permittivity and dielectric dissipation factor values increase along with their water contents and that in the low frequency region there are large increases in both values particularly for their soaked specimens. Comparing the relationships between their relative permittivity/dielectric dissipation factor values and water contents, both values of the Aramid film were more dependent on the water contents than those of the polyimide film particularly in the low frequency region. Finally, it was concluded that, while in the Aramid film the moisture can be assumed to exist mainly in a state of bound water in the same way as that in humid Aramid papers, in the polyimide film the moisture can be assumed to exist in a state of surface attaching at room temperature up to about 80°C and then the biggest part of the residual moisture is thought to be transformed to a strongly bound state with the imide linkages over 200°C