Mechanism for Shish Formation under Shear Flow: An Interpretation from an in Situ Morphological Study

Isotactic polypropylene has been systematically studied with in situ optical microscopy to obtain a real space view of the morphology evolution of shear induced crystallization as a function of shear rates and shear time to elucidate the mechanism of shish formation. The critical shear time for inception of shish formation on the shear rate have provided an important understanding of the molecular and entangled network relaxation in relation to the shish formation. Also the observation of a typical shear time dependent comet like shish-kebab structure formation on the interface and the morphological growth of the shish from a fiber inside our sample have led to a new hypothesis that the shish is formed through a multiple discrete steps instead of forming directly to the final most stable state, this means that a transition state may be existed before shish growth. Two steps shear experiments with various time intervals between each step were designed to verify our proposed transition state mechanism, which can be observed much directly and obviously through real-space morphology, especially at low shear rate with long shear time. Another time dependent relaxation time is introduced base on shear rate dependent experiment after the first step shear and has close relationship with the existence of the critical shear time. A general framework for the shish formation has been established which can capture all the observed morphological features well, including the existence of the critical shear time at a given shear rate.