Thermal Stability of Shear-Induced Precursors and Their Effect on the Crystalline Structure of -Nucleated Isotactic Polypropylene

The thermal stability and lifetime of shear-induced precursors under various annealing temperatures, as well as the influence of their relaxation on the crystalline modification in -nucleated isotactic polypropylene (iPP), are investigated using an ARES rheometer. The wide-angle X-ray diffraction results show that the -crystal content of sheared -nucleated iPP samples gradually increases with thermal treatments. The relaxation of shear-induced precursors during annealing which caused the decrease of shear nuclei may restrain the counteraction effect between the shear flow and -nucleation agent as well as result in the increase of -crystal content. At the early stage of relaxation, the relaxation degree is closely related to the increase of -crystals, for which the deeper relaxed shear-induced precursors result in the more restoration of -crystals. However, when the relaxation degree exceeded a certain limitation, where the -crystals reached the maximum, the relaxation of shear-induced precursors will no longer influence the crystal structure of -nucleated iPP.