Dielectric analysis of polyethylene terephthalate (PET) and polyethylene naphthalate (PEN) films

This paper deals with the dielectric analysis and comparison of two polymers (the PET and PEN films). The structure of the macromolecular chain of the PET film is almost the same as that of the PEN film, but the PEN film has naphthalate instead of phenyl group in the chain. The measurement was performed by the dielectric spectroscopy method. The research was focused on comparison of the changes of their real and imaginary part of the permittivity in dependence on temperature (from -50 °C to 160 °C) and frequency (from 50 mHz to 1 MHz) and description of these changes from the mathematical and chemical points of view. Special attention was paid to the temperature dependence at frequency 50 Hz. Observations showed that in the measured temperature and frequency range the PEN film has three relaxation processes (α, β, β*) and the PET film has only two (α, β). Furthermore, the measurement showed the PEN film has the real and imaginary part of the permittivity lower than the PET film, because naphthalate group gives much higher rigidity to the macromolecular chain and lower polarization than the phenyl group in the PET film. The PET film has no β*-relaxation, because it is only caused by the movements of the naphthalate group. Also β*-relaxation is significantly lower than α and β-relaxations. The results showed further that the losses of the PET film are lower from 20 °C to 95 °C, while the losses of the PEN film are lower at temperatures below 20 °C and above 95 °C.