Stress oscillation-induced modulated phase transformation and yielding in syndiotactic polypropylene

The polymorphic behavior and yielding mechanisms developed in syndiotactic polypropylene (sPP) subjected to step-wise strain has been spatially resolved by simultaneous small- and wide-angle X-ray microdiffraction using synchrotron radiation. The scan carried out along adjacent transparent/opaque bands generated by ‘stress oscillation’ shows a gradual and non reversible crystal–crystal transformation related to a permanent deformation, in contrast to the elastic behaviour showed by sPP after several cycles of stretching and relaxing at constant velocity. However there is no change in the degree of crystallinity along the scan (∼30%). The μSAXS results reveal a modulation in the process of void formation with a higher concentration of microvoids in the opaque band than in the transparent one.