Bo Shen, California Institute of Technology
Room 528, Chemistry Building A
Under shear flow, the morphology of crystalline polymers changes from sphrerulity to orientated “shish-kebab”. The current belief of how shish is formed, involves a two-step mechanism: first point nuclei is formed and then followed by shish growing from it.
Researchers realized that the concentration of high molecular weight chains in the shish is the same as in the bulk. Besides this progress, mechanisms at the molecular level are still unclear and few molecular theories have been 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 concentrations of ultra-high molecular weight chains using birefringence and rheology measurements. We found that even with same birefringence during the shear flow, which means the same average chain orientation, the later birefringence response is different.
Under the same first step point-nuclei formation, high molecular weight chain has evolved into the second step the shish formation.