Drawn commercial yarns of poly ethylene terephthalate (PET) multifilament were heat treated (secondary setting) by infra red heat source on a three-factor heat-treat equipment made by our laboratory. By means of wide-angle X-ray Diffraction (WAXS)

Small-Angle X-ray Scattering (SAXS) and IB Specfcroscopy

heat shrinkage

crystallinity

crystallite orientation and size

non-crystalline orientation

long period and regular folded conformation band intensity of the treated yarns were measured. The change of these structural parameters showed that the tension was not able to preventmolecular chain refolding above 200℃ . Under constant tension treatment a critical value of tension fcritical=0.066 g/dtex was found

above which the tension will prevent chain refolding.