Tensile strength of polyester fiber estimated by molecular-chain extension prior to structure formation

The combination of laser irradiation heating and synchrotron X-ray sources has made it possible to observe the fiber-structure development that occurs at sub-millisecond timescales after necking during continuous drawing. Through wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) analysis of poly(ethylene terephthalate) fibers of three different molecular weights drawn under equivalent stresses, a good correlation was observed between the d-spacing of smectic (001') diffraction extrapolated to the necking point and the strength of the drawn fiber. This indicates that the molecular chains that bear the drawing stress also bear most of the applied stress during tensile testing of the resultant fiber. In addition, considering the drawing-stress dependence of the d-spacing and the molecular weight distribution of the fiber revealed that molecular chains with molecular weights over 23,000 g/mol bear the majority of tensile force applied to the fiber.