Development of Fibrillar Morphology in a Polypropylene Under Time‐Dependent Shear Flow

Polyolefins are among the most widely used polymers due to their versatile applications. The physical properties of these semicrystalline materials, such as crystallinity and crystal orientation, are significantly influenced by processing conditions, particularly under flow. Flow-induced crystallization modifies the polymer morphology from spherulitic to highly oriented fibrillar structures, often named “shish kebab”. Most of the literature studies focus on the effect of steady-state flow on crystallization, whereas in many industrial processes, the stress conditions are not steady with time. This study investigates the effects on the crystallization kinetics and morphology evolution of isotactic polypropylene (iPP) of two different isothermal shear flows, having the same maximum shear rate and the same overall mechanical work, but a different time evolution. Experimental results reveal that the two tests have a different effect on morphology, thus showing that neither a critical shear rate nor a critical work can be suitable criteria to describe the evolution of fibrillar structures. Vice versa, a kinetic model recently proposed that accounts for the evolution of spherulites and fibers, correctly describes the experimental data.