Abstract
Brief intervals of strong flow stretch chains in a semicrystalline polymer melt to form flow-induced precursors, which accelerate crystallization kinetics and transform the morphology. Using commercial isotactic polypropylene, the persistence and lifetime of flow-induced precursors were investigated, focusing on the effects of specific work and annealing time and temperature. Precursors were formed by shearing polypropylene in a rotational rheometer and then quenching to a desired temperature. The crystallization time and crystallization temperature of the sheared samples were investigated using novel rheology and DSC experiments. For sufficiently large shear rates, the appearance of flow-induced precursors is controlled by the applied work. A qualitative change in many properties related to precursors takes place at a critical work value Wc, about 7 MPa for our iPP material. To erase the persistent “memory” of flow-induced nuclei, samples must be annealed for a very long time below 210 °C (>3000 min) or ...
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