The role of long molecules and nucleating agents in shear induced crystallization of isotactic polypropylenes

Short term shearing at low degrees of supercooling with subsequent monitoring of the crystallization process of isotactic polypropylenes leads to highly oriented surface layers and fine grained layers that reach further into the core of the solidified samples. This process is highly dependent on the content of long polymer chains or nucleating agents. The present paper shows a first experimental evidence of these facts together with theoretical considerations. If long polymer chains are present, as it is the case in reactor grades of iPP, shear induced crystallization is more sensitive to shear treatment. As a consequence, the orientation of the surface layers is much higher in these materials. Additionally, the surface layers-both, highly oriented and fine grained-are more distinct and, therefore, allow an easier quantitative determination. If the materials contain nucleating agents, highly oriented layers occur in materials containing long polymer chains as well as in rheology controlled grades. A first theoretical description of the possible role of nucleating agents is given.