Peculiar dielectric properties of poly(diphenylene phthalide) at elevated temperatures

We report on the results of analysis of dielectric properties of poly(diphenylene phthalide) in the frequency range from 25 Hz to 1 MHz at temperatures of 373 and 653 K. It is found that at $T<$ 423 K and at frequencies below 100 kHz, the values of $\varepsilon'$ and $\varepsilon''$ weakly depend on frequency and temperature and are equal approximately to 2.8 and 0.01, respectively. At $T >$ 423 K, the value of $\varepsilon''$ sharply increases with temperature, attaining a value of about 40 at $T$ = 653 K. We believe that this increase in $\varepsilon''$ is due to an increase in the concentration of electrons appearing in poly(diphenylene phthalide) due to thermionic emission. The height of the potential barrier at the aluminum cathode-poly(diphenylene phthalide) interface controlling the ternionic current is estimated as $\sim$0.7 eV.