Note: Departure from Newtonian Behavior of Molten Poly(ethylene terephthalate)

Departure from Newtonian behavior of molten poly(ethylene terephthalate) (PET) flowing at steady state in a capillary is shown to occur at a constant shear stress of ∼9.65×105 dynes/cm2. However, the shear rate for departure of PET from Newtonian behavior [γ̇w (departure)] is a function of temperature and molecular weight. An equation for γ̇w (departure) as a function of temperature and molecular weight is derived; γ̇w (departure) increases as melt temperature increases for a given molecular weight and decreases as molecular weight increases for a given temperature. Maximum relaxation times for viscoelastic response of molten PET can be approximated from the reciprocal of γ̇w (departure), at least for the ranges of molecular weight and temperature investigated.