In situ analysis of hierarchal structure changes to the tensile breakage of poly(ethylene terephthalate) fiber

The higher-order structure of poly(ethylene terephthalate) fibers is changed by plastic deformation prior to tensile breakage. However, the relationship between structure changes and macroscopic fiber breakage has not yet been elucidated. In the present study, hierarchal structure changes during fiber stretching were investigated using in situ wide-angle X-ray diffraction (WAXD), small-angle X-ray scattering (SAXS), and ultra-small-angle (USAXS) X-ray scattering. Both the intensity of crystal diffraction and the long-period peak obtained by WAXD and SAXS decreased monotonically from fiber yield to breakage, whereas the USAXS pattern changed in a complicated manner. That is, a sharp equatorial streak first appeared with an intensity increase of significantly less than 0.03 nm −1 , then a wider equatorial streak overlapped it with an intensity increase of significantly more than 0.03 nm −1 . These results indicate that interfibrillar slippage occurs initially and inter-microfibrillar slippage occurs subsequently; i.e., the slippage mechanism changes between the fiber yield and breakage. In addition, the intensity of the layer-like scattering increased initially, whereas it subsequently decreased to dissipation with the appearance of a strong meridional streak. The latter observation indicates crack growth beyond the fibril boundary, which causes fiber breakage. • ・Hierarchal structure changes were investigated during fiber stretching to breakage • ・WAXD/SAXS patterns changed monotonically, whereas USAXS changes were more complicated • ・Fibril slippage must be considered separately from microfibril slippage • ・Cracks suddenly grew beyond fibril boundaries accompanied by fibril subdivision