TY - JOUR
T1 - Transition in Crystal Morphology for Flow-Induced Crystallization of Isotactic Polypropylene
AU - Hamad, Fawzi G.
AU - Colby, Ralph H.
AU - Milner, Scott T.
N1 - Funding Information:
This work was funded by NSF CBET-1067554.
Publisher Copyright:
© 2016 American Chemical Society.
PY - 2016/8/9
Y1 - 2016/8/9
N2 - 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 Wc, 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 Wc, 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 Wsat, remaining constant thereafter. Similarly, the isothermal crystallization time decreases with increasing applied work up to Wsat 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.
AB - 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 Wc, 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 Wc, 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 Wsat, remaining constant thereafter. Similarly, the isothermal crystallization time decreases with increasing applied work up to Wsat 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.
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U2 - 10.1021/acs.macromol.6b00303
DO - 10.1021/acs.macromol.6b00303
M3 - Article
AN - SCOPUS:84981206891
VL - 49
SP - 5561
EP - 5575
JO - Macromolecules
JF - Macromolecules
SN - 0024-9297
IS - 15
ER -