Shear-induced crystallization of model polymers
The properties of a product made of semi-crystalline polymers strongly depend on both molecular parameters and on the processing conditions applied. This dissertation is focused on flow-induced structure formation correlating the material and crystallization time scales with the structure development in isotactic polypropylene (iPP) after a temperature quench. Morphological changes during the quiescent (no flow prior to crystallization) and shear-induced (with flow prior to crystallization) isothermal crystallization have been monitored with optical microscopy, light scattering and rheology. The experiments have the purpose to identify the effects of shear rate, total strain, molar mass and high molecular weight fraction on the crystallization of iPP. Dimensionless quantities are introduced to correlate the characteristic time scales with structure development; namely, the Weissenberg number ( We), consisting of the product of the applied shear rate and a material characteristic relaxation time ([special characters omitted]) is introduced to demonstrate the transition from spherulitic to oriented structure. A dimensionless characteristic crystallization time scale (τ), defined as the ratio of the experimental time and a characteristic crystallization time, and allows for comparison of crystallization patterns of samples with different molecular specifications (molecular weight, molecular weight distribution) and at different thermo-mechanical conditions. Wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) show the characteristics of the crystalline populations present in the cooled samples after completion of crystallization.