Flow-Induced Shish Nucleation in Lightly Crosslinked Polyethylene: Connecting Polymer Properties and Strain to the Final Shish-Kebab Structure

Shish appears under certain conditions in flow-induced crystallization of polymers. The morphology of shish is determined by experimental conditions, such as strain and polymer properties. Moreover, shish distribution significantly affects the morphology and arrangement of subsequent epitaxial kebabs. Therefore, quantitatively clarifying the chain relation of experimental conditions-shish nucleation-final shish- kebab structure is of scientific and practical importance. We investigate this problem by measuring lightly crosslinked high-density polyethylene samples with small-angle X-ray scattering and an extensional rheometer. It is found that the critical strain for shish formation (epsilon(1)) is the one fully extending the crosslinked strand in the network. The shish radius (R-s) is determined by the spacing (S-||) between adjacent parallel stretched strands. Based on these observations, the mechanism for shish nucleation in the stretched network is discussed. As the strain further increases and exceeds the one fully stretching the whole chain (62), the limit on the growth of shish is suppressed by overstretching, resulting in a further increase of the shish radius. The packing of shish also undergoes a transition at the strain of (epsilon(2)) from an ordered state to a sparse and disordered state. The lateral ordering of shish induces the interlocked shish-kebab structure, which is a candidate for polymeric materials with ultrahigh moduli.