Role of molecular weight in shish-kebab formation during drawing by small-angle neutron and X-ray scattering

The role of molecular weight during shish-kebab formation is an important issue in flow-induced polymer crystallization. In our previous study on the shish-kebab formation of polyethylene [Macromolecules 40, 3650-3654 (2007)], we have shown that ultrahigh molecular weight components are dominantly included in the shish. In contrast, in the same month (May 2007), a completely opposite experimental result was published for isotactic polypropylene [Science 316, 1014-1017 (2007)], wherein more low molecular weight components were found to be included in the shish than high molecular weight components. To understand this contradiction, we conducted small-angle neutron and X-ray-scattering experiments using blends of deuterated polyethylene and hydrogenated polyethylene with various molecular weights drawn just below the melting temperature at three drawing rates. We found that more low molecular weight components than high molecular weight components were included in the shish, even in polyethylene under such experimental conditions, and this tendency was enhanced as the drawing rate decreased. The results suggested that there are various mechanisms for shish-kebab formation that depend on the factors such as the type of flow fields, flow rate, type of polymer, temperature, molecular weight and molecular weight distribution.