Flow Memory and Stability of Shear-Induced Nucleation Precursors in Isotactic Polypropylene

The dissolution of flow-induced nucleation precursors in isotactic polypropylene is investigated indirectly by means of in-situ rheo-SAXS measurements. The progress of crystallization and the evolution of crystal orientation are recorded in isothermal conditions after a controlled shear step followed by an annealing step of different duration at various melt temperatures. The results confirm that the survival time of shear-induced nucleation precursors is extremely large compared to typical rheological relaxation times and it is longer for the precursors originated at higher shear rate. Most important, we show that the effect of flow on the development of oriented morphologies is lost much earlier than that on the overall crystallization kinetics. A schematic model for precursors’ dissolution involving gradual transformation from row into point-like nuclei is proposed.