Experiments and analysis of stress-induced stiffening of a polypropylene

Describing the solidification process is very important in polymer processing. In polypropylene (PP), the increase of viscosity, named stiffening or hardening, is determined by a rise in crystallinity. When PP flows in a channel or is stretched on a chill roll, the stress induces an anticipated crystallization and thus can lead to an unexpected solidification. This study explores how flow fields influence the crystallization behavior of PP. A controlled-stress rheometer was used to investigate the effect of short shear stress steps on crystallization kinetics. The results revealed that applying a stress step significantly increased the rate of crystallization compared to a non-stressed sample. This acceleration is attributed to the stress-induced orientation of macromolecules, which promotes nucleation. Furthermore, longer durations of applied stress led to a faster increase in viscosity, indicating a higher nucleation density with increasing stress exposure. A mastercurve approach validated the consistency of the model describing the stress-crystallization relationship. The calculated parameter relating to nucleation density confirmed a linear increase with stress duration, allowing estimation of the nucleation rate during shear.