'Lifetime of Flow-Induced Precursors in Isotactic Polypropylene
'Lifetime of Flow-Induced Precursors in Isotactic Polypropylene
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DOI：
10.1021/acs.macromol.5b01408
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发表时间：
2015-10-13
期刊：
影响因子：
5.5
通讯作者：
Milner, Scott T.
中科院分区：
文献类型：
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作者：
Hamad, Fawzi G.;Colby, Ralph H.;Milner, Scott T.
半结晶聚合物熔体中强流动拉伸链的短暂间隔，形成流动诱导的前体，加速结晶动力学并改变形态。使用商业全同立构聚丙烯，流动诱导前体的持久性和寿命进行了研究，专注于比功和退火时间和温度的影响。前体通过在旋转流变仪中剪切聚丙烯，然后淬火至所需温度而形成。使用新的流变学和DSC实验的剪切样品的结晶时间和结晶温度进行了研究。对于足够大的剪切速率，流动诱导的前体的外观控制的应用工作。与前体相关的许多性质的质的变化发生在临界功值W-c处，对于我们的iPP材料约为7 MPa。为了消除流动诱导核的持久“记忆”，样品必须在低于210摄氏度（>3000 mm）下退火很长时间或在高于210摄氏度（例如，在250 ° C下300分钟）。在霍夫曼-威克斯温度（208摄氏度）以上退火，显然更快地消除了流动诱导的前体，尽管在210摄氏度下仍然需要比最长链的爬行时间长50倍的时间。
Brief intervals of strong flow stretch chains in a semicrystalline polymer melt to form flow-induced precursors, which accelerate crystallization kinetics and transform the morphology. Using commercial isotactic polypropylene, the persistence and lifetime of flow-induced precursors were investigated, focusing on the effects of specific work and annealing time and temperature. Precursors were formed by shearing polypropylene in a rotational rheometer and then quenching to a desired temperature. The crystallization time and crystallization temperature of the sheared samples were investigated using novel rheology and DSC experiments. For sufficiently large shear rates, the appearance of flow-induced precursors is controlled by the applied work. A qualitative change in many properties related to precursors takes place at a critical work value W-c, about 7 MPa for our iPP material. To erase the persistent "memory" of flow-induced nuclei, samples must be annealed for a very long time below 210 degrees C (>3000 mm) or a shorter time above 210 degrees C (e.g., 300 min at 250 degrees C). Annealing above the Hoffman-Weeks temperature (208 degrees C) evidently erases flow-induced precursors much faster, although at 210 degrees C still requiring times a factor of SO longer than the reptation times of the longest chains.