Nanostructure evolution during unixial deformation of PET : a simultaneous WAXS and SAXS study using synchrotron radiation

Abstract

In this work, the structural evolution and damage of PET during stretching is assessed by wide- and small-angle X-ray scattering (respectively, WAXS and SAXS) experiments in specimens pre-deformed at different strain levels (ex-situ characterization). Injection moulded PET rectangular tensile specimens were stretched (at 2 mm/min) into the plastic domain in a universal test machine at different strain levels at room temperature. The structure of the central zone of the deformed specimens was then characterized by WAXS and SAXS experiments using an X-ray synchrotron source. PET was initially (before stretching) amorphous. A strong molecular orientation in the stretching direction is quickly developed for the initial plastic deformation levels, evidenced by strong equatorial WAXS reflections. This orientation rapidly levels off, remaining constant during further stretching. The WAXS patterns are accompanied with no reflections on SAXS, evidencing a local ordering phenomenon, typical of nematic liquid-crystalline structures. The SAXS patterns evidence the occurrence of some voiding in the cold drawing regime just after yielding. These anisotropic voids are oriented perpendicular to the stretching direction, as in a craze-like structure. The void size drastically increases at the onset of the strong strain hardening behaviour.