Effect of Ultra-High Molecular Weight Chain On Flow-Induced Crystallization of Polymers

Under shear flow, the morphology of crystalline polymers changes dramatically, leading to the formation of a unique structure known as the “shish-kebab.” The mechanism of shish formation has been under debate for decades. The current belief of shish formation is followed by a two-step mechanism: point nuclei is formed first and followed by shish growing from it, also researchers realizes concentration of high molecular weight chains in the shish is the same as in the bulk. Besides these progress, mechanism at molecular level is still not clear and few molecular theory is given to explain why high molecular long chains greatly enhance the shish formation. Methods such as in-situ X-ray scattering, microscopy, and rheology suffer from relatively poor time resolution, making birefringence a very powerful and complementary tool to study what happens during flow. We investigated the flow behavior of a series of polyethylene blends with different lengths and concentrations of ultra-high molecular weight chains using birefringence and rheology measurements. We found that even with very low concentration, birefringence response is different, which reflects the chain configuration during flow and leads to different final morphology.