Impact of the phenyl thioether contents on the high frequency dielectric loss characteristics of the modified polyimide films

With the combination of the phenyl thioether group into the molecular chain of polyimide, the high frequency dielectric loss of the modified polyimide film can be effectively reduced, which will avail this insulating material of application in high frequency electrical equipment. In the proposed study, polyimide films containing different proportions of phenyl thioether were prepared via alternating thermal imidization method and the impact of phenyl thioether contents on the high frequency dielectric loss characteristics investigated. Frequency-domain dielectric spectrum analysis indicated that the dielectric loss of the modified films decreased in the first instance and then roared again with increased phenyl thioether content, among which cases the lowest dielectric loss factor (tan delta = 0.000929 at 1 kHz) occurred in the film with phenyl thioether content of 40%. Both X-ray diffraction (XRD) and Ultraviolet-visible spectroscopy (UV-Vis) measurements, with reactive force field-based molecular dynamics simulation, were utilized to further elucidate the influencing mechanism of phenyl thioether content on dielectric loss characteristics. When the content of phenyl thioether was < 40%, the charge transfer effect decreased with the increased phenyl thioether content, gradually reducing the degree of micro-crystallinity in the modified polyimide, and thereby increasing the dispersion of dipole moment that directly led to the reduced dielectric loss. When the phenyl thioether content is higher than 40%, the charge transfer effect reversed, which results in the PI film with increased crystalline region caused by the tighten molecular stacking and has the increased dielectric loss. Synthesis of the low-dielectric-loss polyimide films can be then optimized for high frequency applications.