The structural characterization of poly(ethy1ene terephthalate) (PET) fibers has received considerable attention in order to understand better the relationship between structure and fiber properties and because of interest in how the polymer crystallization process is influenced by parameters such as stress and temperature. In particular, the variation in structure of the as-spun fiber with increasing wind-up speed has been studied extensively.13 At relatively low (4000 m/min) extensive crystallization occurs and the properties of the fibers begin to resemble those of a drawn fiber (i.e., higher breaking strength, lower elongation to break).ls3v5