Influence of shearing history on polymer properties. III

As a continuation of the investigation of the effects of shearing history on the subsequent properties of polypropylene, the changes of crystalline structure induced by differences in shearing history were examined. It was found that even extremely high shear in a capillary does not increase the crystal orientation in the extrudate. Such orientation is increased by either extremely high rates of extension of the molten extrudate or by slight plastic deformation occurring in the solidified extrudate as a result of imposed stress. The size of crystallites was found to decrease with increasing shear rate experienced by the polymer prior to the crystallization process. A possible explanation for these changes in polymer properties related to shearing history is proposed as an extension of the cluster flow theory of Busse. This explanation takes into account the size of the ball-like clusters, their internal structure, as well as the type, number, and length of the intercluster connections. The changes induced in a polymer by shearing are of technologic importance in connection with both melt flow characteristics and solid-state properties subsequently developed.