Investigation of Extensional Flow-induced Crystallization in Entangled Polymer Melts

To investigate the extensional flow-induced crystallization in polymer melts by means of synchrotron X-rays, a unique cross-slot flow apparatus has been devised and tested. In-situ wide-angle X-ray diffraction (WAXD) results of isotactic polypropylene ($i$PP) showed that the formation of crystallization precursor structure (i.e., shish-kebab structure) under extensional flow is strongly governed by applied strain as well as strain rate. A critical strain,$\varepsilon _{c }$exists in the formation of flow-induced crystallization precursor structure. Below $\varepsilon _{c}$, no shish-kebab structure can be formed even when the applied strain rate,$\dot {\varepsilon }$ is higher than the critical strain rate,$\dot {\varepsilon }_{c}$. Results also confirmed that the crystalline shish precursor structure is formed first and it induces the growth of folded-chain lamellae, i.e. kebabs.