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