Flow-Induced Nucleation as a Stabilizing Mechanism for Polymers against Edge Fracture*

We show here a universal indicator from rheology of flow-induced nucleation that gives insight into how flow enhances crystallization in semi-crystalline polymers. We study the effects of an interval of shear flow on the crystallization kinetics of a high density polyethylene (HDPE) and monodisperse polyethylene oxide (PEO) blends with a high molecular weight fraction. We characterize the crystallization of polymer using oscillatory shear linear viscoelasticity, cone & plate shear rheology, cone-partitioned plate shear rheology and capillary rheometry. We quantify crystallization by the Winter-Pogodina criterion that the storage modulus (G’) crosses the loss modulus (G”) in an oscillatory shear time sweep at a fixed angular frequency. We apply intervals of shear both above and below the melting temperature of the polymers. The intervals of shear are immediately followed by oscillatory shear time sweeps to quantify the effect of various shear intervals on crystallization kinetics when HDPE and PEO are sheared and crystallized at the same temperature. We find that applying flow at lower temperatures exhibits different crystallization kinetics than applying flow at higher temperatures. The most important take away is that flow-induced nucleation is closely connected to the first normal stress difference and acts as the universal nucleation indicator by stabilizing the melt against edge fracture which is very apparent only at high temperatures where crystals and their precursors cannot form.