Dielectric relaxation in amorphous poly(ethylene terephthalate) and poly(ethylene 2,6-naphthalene dicarboxylate) and their copolymers

The dielectric loss spectra of poly(ethylene 2,6-naphthylene dicarboxylate) (PEN) and several copolymers of 2,6-naphthylene dicarboxylic acid and terephthalic acid with ethylene glycol have been studied over a wide range of frequency and temperature. Previously, Ezquerra, Balta-Calleja, and Zachmann have reported the presence, in isochronal temperature scans of dielectric loss, of a subglass process (beta*) in PEN homopolymer in addition to the subglass process (beta) similar to that in poly(ethylene terephthalate) (PET). In the present work both the beta and beta* processes in PEN and PET/PEN copolymers are resolved and characterized in isothermal frequency scans. It was found that the lower temperature beta loss peak in PEN and the copolymers has a complex or composite frequency domain structure requiring two Cole-Cole processes in its representation. The beta* process in the copolymers is found to shift to higher frequency isothermally (or lower temperature isochronally) with increasing terephthalic acid content. Analysis of previous dielectric data for PET shows that its beta subglass process is also complex in the log frequency axis but must be represented by three Cole-Cole processes. The two higher frequency components are essentially identical to the two Cole-Cole processes making up the beta process in PEN. The third and lowest frequency component is interpreted as of the same origin as the beta* process in PEN and the copolymers but shifted to higher frequency isothermally (or lower temperature isochronally) to where it overlaps with the beta process.