Strain rate, temperature, and microstructure-dependent yield stress of poly(ethylene terephthalate)

Yield stress of a series of poly(ethylene terephthalate) (PET) filament samples, which differ in molecular draw ratio, was observed at a range of strain rate and temperature to reveal the relation between microstructure and yield stress behavior. Based on the observations, a novel correlation between (quasistatic) tensile yield behavior and dynamic mechanical properties was proposed. Above secondary relaxation temperature and below primary relaxation temperature, changes in yield stress with strain rate and temperature could be described successfully with one activated rate process, Eyring's model of yielding, regardless of the microstructure of PET samples. The activation enthalpy of Eyring's model increased almost linearly with crystallinity, while the activation volume decreased with crystallinity in a sigmoidal fashion. We suggest that crystalline structural development elevates the energy barrier for yield, and that the scale of molecular motion for yield decreases with microstructural development, not only because of the decrease in molecular volume. Such a scale of molecular motion as Eyring's activation volume could possibly interpret the loss tangent (tan δ), i.e., as the activation volume increases, loss tangent maxima of both primary and secondary relaxation increase almost linearly.