Effects of the nozzle flow on the ultra-SAXS-scale orientation and tensile toughness of melt-spun poly(ethylene terephthalate) fibers

Poly(ethylene terephthalate) (PET) fibers are produced through melt spinning, drawing, and annealing processes. The fiber tensile properties are determined by the processing conditions. In this study, we investigated the effect of the flow in the spinning nozzle on the mesoscale orientation and tensile properties of the resulting fiber. In particular, the effects of the nozzle length-to-diameter ratio (L/D) and throughput rates were investigated. Ultra-small-angle X-ray scattering (USAXS) was used to investigate higher-order structure with a scale from several tens to hundreds of nanometers for extrudates exiting from the nozzle, as well as for as-spun, drawn, and annealed fibers. In addition, USAXS images were obtained for extrudates from a capillary rheometer. Equatorially oriented scattering was observed in all the USAXS images of the extrudates. The orientation factor determined from the images decreased with increasing L/D for extrudates from both the spinning machine and the capillary rheometer. With increasing L/D, the layer periodicity observed in the USAXS images of the drawn and annealed fibers became less distinct, and the tensile toughness of the as-spun and drawn fibers increased. This result indicates that shear deformation in the nozzle weakened density fluctuations at the mesoscale and improved the fiber toughness. The effect of the orientation in the nozzle extrudates on the mesoscale structure and tensile properties persisted in the drawn fiber. This result indicates that USAXS-scale orientation is an important factor in the design of mechanical properties for melt-spun PET fibers. • USAXS was used to evaluate mesoscale orientation in PET molecular networks. • The flow in a spinning nozzle affected the USAXS pattern and fiber tensile properties of PET. • Increasing the nozzle L/D weakened the mesoscale orientation of the extrudate after die-swell. • Increasing the nozzle L/D obscured the layered structure in drawn and annealed fibers. • Weakening the mesoscale orientation in the extrudate resulted in fibers with high toughness.