Molecular mechanism leading to memory effect of mesomorphic isotactic polypropylene
Molecular mechanism leading to memory effect of mesomorphic isotactic polypropylene
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DOI：
10.1002/polb.24051
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发表时间：
2016-08
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影响因子：
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通讯作者：
Xiangyang Li;Yanping Liu;Xingyou Tian;Kunpeng Cui
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文献类型：
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作者：
Xiangyang Li;Yanping Liu;Xingyou Tian;Kunpeng Cui
采用偏光显微镜和小角X射线散射研究了介晶等规聚丙烯（iPP）的记忆效应。与经典的记忆效应不同，介晶iPP熔体具有较高的生长速率和记忆温度。相对生长速率随晶化温度的升高而增大。Lauritzen-Hoffman图表明，增加的生长速率是由于减少了表面成核势垒。最高记忆温度估计为185 °C，接近iPP晶体的平衡熔点。此外，小角X射线散射测量表明，液晶层可能存在于层状和非晶层之间。在此基础上提出了一个结晶模型。在介晶iPP熔体中，除α相晶体残留物外，还存在与β相结构相似的聚集体，不能作为成核点或通过表面成核转变为β晶。在晶体生长过程中，聚集体的唯一途径是向α晶转变。聚集体降低了表面成核势垒，促进了螺旋生长，导致更高的生长速率。只有当聚集体在较高的温度下通过增稠而松弛成聚合物线团时，记忆效应才能消除。© 2016 Wiley Periodicals，Inc. J.波利姆科学，部分B：聚合物物理2016，54，1573-1580
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