Step‐like melting mechanisms of isothermally crystallized isotactic polypropylene
Step‐like melting mechanisms of isothermally crystallized isotactic polypropylene
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DOI：
10.1002/polb.21551
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发表时间：
2008-10
期刊：
影响因子：
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通讯作者：
F. Medellín-Rodríguez;Manuel Mata-Padilla;S. Sánchez-Valdes;S. Vega-Díaz;O. Dávalos-Montoya
中科院分区：
文献类型：
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作者：
F. Medellín-Rodríguez;Manuel Mata-Padilla;S. Sánchez-Valdes;S. Vega-Díaz;O. Dávalos-Montoya
本文研究了等温结晶后等规聚丙烯的复杂熔融行为，并将其应用于高温聚合物的阶梯状熔融机理。熔融过程的形态特征也进行了研究，作为分子量的函数，并观察到与高温聚合物的密切相似之处。低分子量样品的正双折射晶体在三个步骤中出现双熔融行为。第一个熔化步骤被分配到连续熔化的二次交叉线反转laminates，连同再结晶的其余等温晶体。在第二熔融阶段（第一熔融吸热），由于二次反向分支的熔融，晶体倾向于失去其原始的粗负双折射。这种效应产生了新的、更精细的纹理，但仍然是负双折射晶体。在第三熔融步骤（第二熔融吸热）中，存在两种晶体群的熔融的组合，一种由反转初晶的剩余部分组成，另一种由非反转初晶组成。因此，提出了一个crosshatch二次分枝模型来解释整体结果。高分子量样品的混合双折射球晶表现出类似的，虽然成比例，在相同的结晶和熔融条件下的行为证实了建议的熔融机制。
The complex melting behavior of isotactic polypropylene, after isothermal crystallization, was studied within the context of step-like melting mechanisms which were previously proposed for high temperature polymers. The morphological characteristics of the melting process were also studied as a function of molecular weight, and close similarities were observed with respect to high temperature polymers. Positive birefringence crystals of low molecular weight samples developed double melting behavior in three steps. The first melting step was assigned to continuous melting of secondary crosshatch reversing lamellae, together with recrystallization of the remaining isothermal crystals. In the second melting step (first melting endotherm), crystals tended to lose their original coarse negative birefringence due to melting of secondary reversing branching. This effect rendered new, finer texture, but still negative birefringence crystals. In the third melting step (second melting endotherm), there was a combination of melting of two crystal populations, one consisting of the remaining fraction of reversing primary crystals, and the other consisting of nonreversing primary crystals. A crosshatch secondary branching model was therefore proposed to explain the overall results. Mixed birefringence spherulites of high molecular weight samples displayed similar, although proportional, behavior under identical crystallization and melting conditions corroborating the proposed melting mechanism.