Transition in Crystal Morphology for Flow-Induced Crystallization of Isotactic Polypropylene

Deformation applied to a semicrystalline polymer melt prior to quenching can dramatically increase the concentration of nuclei and even transform the final crystalline morphology, phenomena collectively known as flow-induced crystallization (FIC). Using polarized optical microscopy and atomic force microscopy, we image the changes in morphology of an isotactic polypropylene (iPP) sample previously sheared in a rotational rheometer. Sufficiently large deformations promote the formation of "rice grain" shaped crystalline domains. These anisotropic structures are randomly oriented and are about 1.5-3.0 μm long (depending on applied work), with an aspect ratio of about 2:1. Well below the critical specific work threshold W_c, the crystalline morphology of our iPP sample is predominately composed of spherulites, with a fraction of rice grains that increases as the applied work increases. Above W_c, rice grains predominate, with no visible spherulites. The size of spherulites appears to be independent of applied work. In contrast, the size of rice grains decreases with increasing applied work, up to a saturation specific work W_sat, remaining constant thereafter. Similarly, the isothermal crystallization time decreases with increasing applied work up to W_sat and is constant thereafter. The effects of flow-induced precursors persist after repeated melting and recrystallizations, observable by elevated freezing temperatures in DSC experiments. Here, we probe directly the robustness of these precursors in optical microscopy, in which we repeatedly melt, anneal, and recrystallize a thin slice of an FIC sample. We observe rice grain domains appearing over and over, with the first domains to crystallize tending to reappear near the same locations.