Paper Title
Percolation Behavior on Electric Conductivity by Water Absorption for Polymer Films with High-Temperature Resistance

Abstract
Polymer films used in food packaging films, adhesive tapes and speaker cones have properties such as temperature and chemical resistance. Polyimide (PI), 6-nylon and polyethylene terephthalate (PET) are examples of the heat resistant polymers. We have investigated, so far, the effect of water absorption on the electrical resistivity of PI films. PI film cannot be used as an insulating material because the resistivity decreases when it absorbs water. If a highly insulating film with high- temperature resistance could be made, its applications would be dramatically expanded. The purpose of this study is to investigate the effect of water absorption on the electrical properties, such as volume resistivity and dielectric constant, for PET and PI films. PI and PET films with a thickness of 125 µm in 4cm square were used in this study. The films were immersed in pure water at room temperature for 60 min to 480 min. The volume resistivity was measured for the films using an ultra-insulation meter and the dielectric constant was measured by an LCR meter from 50 Hz to 8 MHz at room temperature. Both films showed that the water absorption increased up to 180 min and then took a constant value. The water absorption rates at the equilibrium absorption for PI and PET were 2.2% and 0.4%, respectively. The volume resistivity decreased with the absorption time and became constant at 180 min for PI and 360 min for PET. The values of the critical volume fraction at where the electric conductivity rapidly increases were 0.02 for PI and 0.003 for PET. A plot using the maximum absorption rate revealed that the mechanism of electric conduction is same for these films. It was also found that the absorption sites for PET are finely dispersed in the film compared to those for PI. Additionally, both films after water absorption showed low dielectric constant indicating that the water absorbed in the films cannot be polarized due to the strong interaction between water and the polymers.