The crystallization and melting of semicrystalline polymers are one of the most fundamental issues of polymer physics. However, how crystal orientation and thickness relate to the multiple melting behavior is not well understood. Here, well-oriented thick lamellae (22–37 nm) have been produced in isotactic polypropylene (iPP) by crystallization under the combined effects of flow (30 s−1) and pressure (50, 75, 100, 125 MPa). The melting of these oriented thick lamellae is followed by real-time wide- and small-angle X-ray scattering measurements. The results show that the melting of oriented thick lamellae of iPP follows the sequential melting model at the early stage, and then the melting of the entire lamellar stacks at high temperature. More importantly, the temperature dependence of the reciprocal lamellar thickness of these oriented thick lamellae is shown to follow the multistage model. A common melting line is obtained which coincides with that of the unoriented lamellae crystallized under quiescent conditions, giving an equilibrium melting temperature of 183 °C and a folded surface free energy of (86−122)×10−7 J/cm2. Somewhat unexpectedly, the oriented thick lamellae thicken continuously on the heating, whereas the relatively thin lamellae obtained from quiescent crystallization melt directly without thickening. This phenomenon is discussed in terms of the irreversible thermodynamic process of lamellar thickening. The present work sheds light on the crystallization and melting behavior of oriented thick lamellae in iPP, and generalizes the multistage model to iPP crystallized under external fields.

中文翻译：

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等规聚丙烯中取向厚片的结晶和多次熔融行为


半结晶聚合物的结晶和熔化是聚合物物理学中最基本的问题之一。然而，晶体取向和厚度与多次熔融行为的关系尚不清楚。在这里，在流动 （30 s-1） 和压力 （50、75、100、125 MPa） 的共同作用下，通过结晶在等规聚丙烯 （iPP） 中产生了取向良好的厚薄片 （22-37 nm）。这些取向的厚薄片熔化后，进行实时广角和小角 X 射线散射测量。结果表明，iPP的取向厚片的熔融在早期遵循顺序熔融模式，然后在高温下熔化整个层层。更重要的是，这些定向厚薄片的倒数层层厚度的温度依赖性被证明遵循多级模型。获得一条公共熔线，该线与在静止条件下结晶的无取向薄片的熔线一致，平衡熔化温度为 183 °C，折叠表面自由能为 （86-122）×10-7 J/cm2。有点出乎意料的是，取向的厚片在加热时不断增厚，而从静止结晶得到的相对较薄的薄片直接熔化而不增厚。根据层状增厚的不可逆热力学过程讨论了这种现象。这项工作阐明了 iPP 中取向厚片的结晶和熔融行为，并将多级模型推广到在外场下结晶的 iPP。