Flow-Induced Dendritic β-Form Isotactic Polypropylene Crystals in Thin Films

Flow-induced dendritic β-form isotactic polypropylene (iPP) lamellae in thin films have been investigated using optical microscopy (OM) and atomic force microscopy (AFM). Shear flow in a thin film was induced by scratching supercooled iPP melt with a sharp scalpel at a constant rate. After subsequent isothermal crystallization, an extremely high nucleation density of edge-on α-iPP crystals oriented perpendicular to the flow direction was observed. Increasing shear flow temperature from 130 to 190 °C led to a decrease in both number density of flow-induced edge-on α-iPP lamellae and subsequent β-iPP crystals. Interestingly, two distinct paths for the change in orientation of edge-on crystals could be identified. The most probable route was the transition from edge-on to leaf-shaped α-iPP flat-on crystals. The transition from edge-on α-iPP lamellae to β-iPP flat-on crystals with dendritic shape occurred less frequently. We suggest a stepwise process for flow-induced growth of β-iPP crystals associated with the initial nucleation of edge-on α-iPP crystals via aligned polymer chains induced by flow and a subsequent transition to the growth of β-iPP flat-on crystals.