Study of the dielectric β‐relaxation in poly(ethylene terephthalate) and ethylene isophthalate terephthalate copolyesters

The dielectric β‐relaxation in poly(ethylene terephthalate) and ethylene isophthalate terephthalate copolyesters with various ethylene isophthalate contents was investigated over a wide range of frequencies. Isothermal spectra and normalized plots show that the dielectric β‐relaxation is characterized by the molecular structure between the aromatic rings. The equilibrium polarizability of the motional segment, which was estimated from the dielectric increment obtained by the best fit of the Havriliak‐Negami equation to the Cole‐Cole plot, indicates that torsional vibrations of the main chain are correlated over a length corresponding to one monomer.