'Shish' polymer structure unraveled

Physical properties of polyolefins, the most widely used family of synthetic polymers, are strongly affected by the polymers' semicrystalline morphology. The "shish-kebab" form in particular is stiffer and less permeable than other forms. Scientists previously believed that when a polymer melt is sheared, the longer polymer chains stretch into a rodlike "shish" core while shorter chains aggregate into platelike "kebabs." Now, a group led by Julia A. Kornfield of California Institute of Technology and Yoshinobu Nozue of Sumitomo Chemical, in Japan, has demonstrated that, although the longer chains are critical to shish formation, shorter chains are included in the shish as well (Science 2007, 316, 1014). Also, rather than the longer chains forming the shish first and then acting as crystallization seeds for shorter chains, it appears they work by getting chains of all lengths to organize simultaneously, the researchers note. Better understanding of shish formation could lead to expansion of the material properties that can be achieved with polyolefins.