EFFECTS OF EQUAL BIAXIAL STRETCHING ON THE ELECTRIC PROPERTIES OF POLYIMIDE FILMS

The influence of thermal treatment and stretching on properties of poly(4,4 prime -oxydiphenylene pyromellitimide) films has been studied by mechanical, thermally stimulated current (t. s. c) and dielectric measurements. Equal biaxial stretching of the films was achieved by the bubble-inflation technique. Thermal treatment at high temperatures may affect the onset temperature of the conduction loss of the samples in dielectric measurements. In addition to the improvement of tensile strength due to molecular orientation, the effect of stretching is to help further imidization of the residual reactive groups and the decrease of structural defects. An equally biaxially stretched PI film of average-area draw ratio 1. 70 is a good insulating material of low dissipation factor and high stability of dielectric properties in the temperature range from minus 120 degree C to 220 degree C.