Lifetime of Flow-Induced Precursors in Isotactic Polypropylene

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 W_c, 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 a shorter time above 210°C (e.g., 300 min at 250°C). Annealing above the Hoffman-Weeks temperature (208°C) evidently erases flow-induced precursors much faster, although at 210°C still requiring times a factor of 50 longer than the reptation times of the longest chains.