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.