Molecular mechanism leading to memory effect of mesomorphic isotactic polypropylene |
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Authors: | Xiangyang Li Yanping Liu Xingyou Tian Kunpeng Cui |
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Institution: | 1. Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, China;2. National Center for International Research of Micro‐nano Molding Technology & Key Laboratory for Micro Molding Technology of Henan Province, Zhengzhou University, Zhengzhou, China;3. National Synchrotron Radiation Lab and College of Nuclear Science and Technology, University of Science and Technology of China, Hefei, China |
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Abstract: | In this study, memory effect of mesomorphic isotactic polypropylene (iPP) was investigated using polarized optical microscope and small‐angle X‐ray scattering. Differing from classical memory effect, mesomorphic iPP melt had a higher growth rate and a higher memory temperature. The relative growth rate increased with increasing crystallization temperature. Lauritzen–Hoffman plots indicated that the increased growth rate arose from reduced surface nucleation barrier. The highest memory temperature was estimated to be 185 °C, which was close to the equilibrium melting point of iPP crystal. Additionally, Small‐angle X‐ray scattering measurements showed that a liquid crystal layer might exist between lamellar and amorphous layers. Based on above results, a crystallization model was proposed. In the mesomorphic iPP melt, there exist aggregates structurally similar to β phase except α‐phase crystal residuals, which cannot act as nucleation sites or transform to β crystal through surface nucleation. The only way for the aggregate is to transform to α crystal during crystal growth. The aggregate decreases the surface nucleation barrier and promotes the helical growth, leading to higher growth rate. Only when the aggregate relaxes to polymer coils through thickening at a higher temperature, can the memory effect be erased. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1573–1580 |
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Keywords: | activation energy crystallization growth rate lamellar memory effect mesomorphic iPP poly(propylene) (PP) SAXS small‐angle X‐ray scattering surface nucleation |
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