Extension decelerated crystallization in γ-irradiated isotactic polypropylene: The role of asymmetric chain relaxation

Abstract

Extension induced crystallization of γ-irradiated isotactic polypropylene (γ-iPP) was studied with the combination of extensional rheology and in-situ wide-angle X-ray scattering (WAXS). The high gel content and markedly frequency-dependent storage modulus of γ-iPP indicates the occurrence of slight crosslinking accompanied by formation of free branching/dangling chains during irradiation. With WAXS, it is found that an abnormal slowing down of crystallization occurs when engineering stress passes its maximum. Lamellar network built of parent and daughter crystal also appears in this region, where daughter crystallization is commonly frustrated under such strong flow. Further increasing strain, crystallization is accelerated gradually and the acceleration becomes more significant after occurrence of strain-hardening. Interestingly, the orientation of crystal increases monotonically during this process. It is supposed that the difference in relaxation rate of cross-linked network and free branching/dangling chains is the origin of the abnormal deceleration and unique structure morphology.