AbstractThe structure of skin layer in injection‐molded polypropylen which displayed a clear two‐phase structure of skin and core has been studied by means of wide‐angle x‐ray diffraction, small‐angle x‐ray scattering, melting behavior, density, dynamic viscoelasticity, and tensile test. In skin layer, the c‐axis and a*‐axis were highly oriented to the machine direction (MD), and the plane of the lamellar structure of about 160 Å in thickness was in normal to MD. The density was about 0.907 g/cm3, which was nearly the same as that of core layer. Although the majority of crystallites melted in the same temperature range as in that of the core layer, there was about 5.3 higher temperature melting structure (Tm= 182°C). The dynamic tensile modulusE′ in MD decreased more slowly with increasing temperature than that of the core layer and held high modulus in the range of ca. 30°C, just above the temperature at whichE′ of the core layer suddenly dropped.E′ in MD was higher than that in TD in the temperature range below 33°C, which was slightly higher than the primary absorption temperature, and the order reversed above 33°C. The tensile yield stress in MD was 1.5 times higher than that of the core layer. The skin layer in MD ruptured just after yielding and did not show necking. The tensile yield stress in TD was about half of that in MD about 0.7 times that of the core layer. The necking stress in TD was about 0.6 times that of the core layer. In general, a polypropylene melt crystallizes under a high shear stress in injection molding. From these facts, it was concluded that the skin layer is composed of so‐called “shishkebab”‐like main skeleton structures, whose axis is parallel to MD, piled epitaxially with a*‐axis‐or
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