Shear-Induced Phase Separation of Entangled Polymer Solutions: Formation of Optically Anisotropic Strings as Precursor Structures of Shish-Kebab |
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Authors: | Takeji Hashimoto Hiroki Murase Yasuo Ohta |
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Institution: | 1. Advanced Science Research Center, Japan Atomic Energy Agency, Tokai, Naka-gun, Ibaraki 319-1195, and Department of Polymer Chemistry, Kyoto University, Kyoto 615-8510, Japan;2. TOYOBO Co., Ltd., Research Center, 2-1-1, Katata, Ohtsu-shi, Shiga 520-092, Japan |
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Abstract: | Shear-induced structures were investigated for both ultrahigh molecular weight atactic polystyrene (UHMWaPS) and linear polyethylene (UHMWPE) solutions, which were entangled but homogeneous without shear flow, as a function of shear rate ( ) or time after a step-up shear flow. For the PE solutions, the shear flow was imposed at 124 °C which is higher than the nominal melting temperature Tnm of the solution without shear flow. At sufficiently high shear rates both solutions commonly formed highly optically anisotropic string-like structures which are composed of a series of phase-separated domains interconnected by bundles of stretched chains and aligned along the flow direction. After cessation of the shear flow the string-like structures completely disappeared in the UHMWaPS solution, recovering a homogeneous solution, while the UHMWPE solution exhibited shish-kebab structure. The results reveal a new kinetic pathway for shish-kebab formation for the entangled crystallizable solution sheared at T > Tnm which involves first formation of the phase-separated string-like domains and subsequent crystallization into shish from the bundles of stretched chain and then kebab in the demixed domains composed of essentially random coils. |
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Keywords: | crystallization phase separation rheo-optics semidilute polymer solutions shear flow shish-kebab |
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