Kinetic Process of Shish Formation: From Stretched Network to Stabilized Nuclei

On the basis of the duality of the shish-kebab superstructure, coil-stretch transition (CST) is well recognized as the molecular mechanism for shish-kebab formation in polymer melts, which, however, is challenged by recent results in flow-induced crystallization (FIC). In this work, we perform a real time investigation on FIC of polyethylene bimodal blends by combing a unique homemade extensional rheometer and synchrotron radiation small-angle X-ray scattering. The results show that the critical strain for shish formation decreases with increasing long chain concentration, which contradicts the role of CST but agrees well with stretched network model (SNM). Quantitative analyses indicate that the formation of shish is determined by the degree of network deformation rather than solely by strain or long chain concentration at a specific temperature. In addition, three types of shish with different stability are observed sequentially by increasing strain. On the basis of our results, strong support is given to the idea that shish formation is a kinetic process. When stretched to a critical deformation degree, the aligned segments couple with each other to form fibrillar-like type I shish, which further transform into type II shish embedded with sporadic lamellae and type III shish embedded with well-defined periodic lamellae sequentially by increasing flow intensity. Our results and the resulting conceptual model not only demonstrates that shish formation is derived from SNM but unveils its kinetic process from initial chain configuration to final stable nuclei.