Structure development during shear flow induced crystallization of i-PP: In situ wide-angle X-ray diffraction study
Structure development during shear flow induced crystallization of i-PP: In situ wide-angle X-ray diffraction study
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DOI：
10.1021/ma0106191
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发表时间：
2001-08-14
期刊：
影响因子：
5.5
通讯作者：
Tsou, AH
中科院分区：
文献类型：
--
作者：
Somani, RH;Hsiao, BS;Tsou, AH
采用同步辐射广角X射线衍射（WAY-D）技术对等规聚丙烯（i-PP）在140 ℃过冷熔体中分步剪切后的结晶过程进行了原位监测。使用平行板剪切装置在三种不同的剪切速率（10、57和102 s（-1））下使熔体经受1430%的剪切应变。WAXD结果用于确定i-PP晶体的类型（α-和β-晶体）、取向和相应的质量分数。结果发现，形成定向α晶体后立即发生的剪切场的应用。随后，观察到主要未取向的β晶体的生长。WAXD图谱清楚地表明，β-晶体生长后，只有在剪切的i-PP熔体中形成取向的α-晶体。在高剪切速率（57和102 s（-1））下，β-晶体对总晶相的贡献高达65-70%，而在低剪切速率（10 s（-1））下，β-晶体对总晶相的贡献较低（20%），这归因于在不同剪切速率下产生的取向α-晶体圆柱体的表面积的量不同。早先已经提出了与取向的α-形式晶体组装体的表面积相关的β-晶体的生长。此外，从WAXD实验确定的β-晶体的无取向性质和快速生长提供了对无取向结构的结晶动力学的2个数量级增加的解释，这是先前在我们的结晶研究中通过小角X射线散射（SAXS）观察到的（但未解释）。
In situ synchrotron wide-angle X-ray diffraction (WAY-D) was used to monitor crystallization of isotactic polypropylene (i-PP) in the subcooled melt at 140 degreesC after step shear. The melt was subjected to a shear strain of 1430% at three different shear rates (10, 57, and 102 s(-1)) using a parallel-plate shear apparatus. WAXD results were used to determine the type (alpha- and beta -crystals), orientation, and corresponding mass fractions of i-PP crystals. It was found that formation of oriented alpha -crystals occurred immediately after application of the shear field. Subsequently, growth of primarily unoriented beta -crystals was observed. WAXD patterns clearly showed that beta -crystals grew only after the formation of oriented alpha -crystals in the sheared i-PP melt. The contribution of beta -crystals to the total crystalline phase was as high as 65-70% at high shear rates (57 and 102 s(-1)) and low (20%) at low shear rates (10 s(-1)), which was attributed to the different amount of surface area of oriented alpha -crystal cylindrites generated at different shear rates. The growth of beta -crystals which is related to the surface area of the oriented alpha -form crystalline assembly has been proposed earlier. Also, the unoriented nature and fast growth of the beta -crystals determined from WAXD experiments provide an explanation for the 2 orders of magnitude increase in the kinetics of crystallization of the unoriented structures, which was previously observed (but not explained) in our crystallization study by small-angle X-ray scattering (SAXS).