Dielectric effects of moisture in polyimide

Measurements of the complex permittivity of polyimide (PI) films in a temperature range from 80 to 325 K are reported. Two loss peaks show the pronounced influence of absorbed water on this part of the relaxation spectrum. Since these loss peaks can be removed by drying the film, it is concluded that the water dipole causes this relaxation process and not an intrinsic dipole of the PI chain. Different sites in neighboring PI chains are proposed to explain the two loss peaks and the temperature dependence of the relaxation strength. The activation energies expected for the proposed sites agree well with the activation energies determined by the measurements. The relaxational strength calculated by assuming noninteracting water molecules which relax as point defects is in good agreement with the measurements. A simple model, the hindered rotation of the water dipoles, is able to explain all the features of the measured loss curves without requiring complicated movements of chain segments.<
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