Linear viscoelastic properties of polyethylene terephthalate and its related polymers

The frequency dispersion in the complex dynamic modulus functions of polyethylene terephthalate and its related polymers over a frequency range from 0.05 to 50 cps was measured at various temperatures from −170° to 200°C covering the secondary glass transition, primary glass transition, and crystal disordering temperatures of these polymers. The results obtained were analyzed from the phenomenological view point of linear viscoelasticity on the basis of the following three assumptions: (1) additivity for assessing the contribution of each relaxation mechanism to the viscoelastic functions, (2) validity of the time-temperature superposition hypothesis within each relaxation mechanism independently of other relaxation mechanisms, (3) symmetrical loss modulus function of each relaxation mechanism with respect to logarithmic frequency. At least three distinctive mechanical dispersions associated with the secondary and primary glass transitions and the crystal grain boundary phenomena or crystal disordering transition were observed in the glassy and leathery state of these polymers. Besides the primary dispersion associated with the primary glass to leathery transition due to the onset of thermal diffusion of noncrystalline chain segments, the dispersion in the glassy state was separated into three relaxation mechanisms arising probably from localized hindered rotations of methylene sequences and from two types of hindered vibrations of carboxylate groups coupling with the hindered rotations of methylene sequences, whereas the dispersion in the leathery state was separated into two relaxation mechanisms arising probably from either the orientation or disordering of crystals in, at least, two different states of order.