Role of Entanglement Species in the Formation of Shish-Like Fibril Crystals Elucidated by Cyclic Polyethylene

The influence of chain entanglements in shear-induced crystallization remains unclear. It is of particular interest the difference in entanglement species between cyclic and linear polymers. To clarify the role of entanglement species in the formation of shish-like fibril crystals, the shear-induced crystallization behaviors of cyclic polyethylene (C-PE), linear polyethylene (L-PE), and C-PE and L-PE blends were investigated using a hot stage equipped with a polarizing optical microscope. The shear-induced crystallization behavior of the C-PE(115k) and L-PE(65k) homopolymers, where the values in parentheses represent the molecular weights (Mw), indicated that the formation rate of the shish-like fibril crystals of the former was one order of magnitude smaller than that of the latter. This implies that it is more difficult for C-PE to form shish-like fibril crystals than L-PE due to the topological effect of C-PE; that is, C-PE chains are more difficult to elongate by shear than L-PE chains due to the lack of chain ends. The shear-induced crystallization behavior of the blended C-PE and L-PE samples provided further insight into the effect of entanglement species on the formation of shish-like fibril crystals. The addition of small amounts of L-PE to C-PE remarkably enhanced the formation rate and density of the shish-like fibril crystals. The influence of the novel entanglement formed by the penetration of the L-PE molecule into the C-PE ring on the formation of the oriented melt was speculated to be significant.