Dielectric properties of poly(ethylene terephthalate) and poly(ethylene 2,6-naphthalate)

Dielectric behavior was compared experimentally between polyethylene terephthalate (PET) and poly(ethylene 2,6-naphthalate) (PEN). Due to their similar chemical structures, the two polymers exhibit many parallel dielectric properties. While the two polymers exhibit fairly similar thermally stimulated polarization and depolarization currents (TSPC and TSDC), the temperature at which TSPC or TSDC starts to increase rapidly is about 20 °C higher in PEN than in PET, mostly likely reflecting the difference in their glass transition temperatures (T_g's). At temperatures about 30 °C lower than T_g, the two polymers show a hump in their first-run TSPC spectra, probably originating from impurity or moisture. Both the real and imaginary parts of complex permittivity, ε_r' and ε_r", increase significantly at temperatures above their T_g's for both PEN and PET, since molecular motion becomes active. Shoulders and plateaux clearly appear in ε_r' and ε_r" spectra of PEN, which move toward higher frequencies with an increase in temperature. To further analyze them, complex electric modulus M∗ was introduced. As a result, it has become clear that electric conduction dominates the dielectric behavior of PET and PEN at temperatures above T_g, especially at low frequencies.