Quiescent and shear-induced crystallization of Ziegler-Natta and metallocene-based isotactic polypropylenes

The quiescent and shear-induced crystallization of Ziegler-Natta and metallocene based isotactic polypropylenes (i-PP's) of comparable molecular weights were studied. For quiescent crystallization, the isothermal and nonisothermal rate of crystallization, induction time, and spherulite growth rate were measured. The i-PP's slabs were quenched and the gapwise spherulite size distribution in the quenched slabs was measured. Simulations of the temperature field and microstructure in slabs during quenching were performed. Simulated results for temperature traces during quenching and spherulite size distribution in quenched slabs were compared with corresponding measured values. For shear-induced crystallization, the thickness of the shear-induced crystallization layer, spherulite size distribution, crystallinity distribution and birefringence distribution of the injection molded samples were obtained. Simulation of the injection molding process was performed. Predicted pressure traces, shear-induced crystallization layer thickness, crystallinity distribution and spherulite size distribution in moldings were compared with the corresponding experimental results. The mechanical properties were also obtained and related to the microstructure and processing conditions.