Even though the crystallization of long-chain branched polypropylene (LCB-PP) has been widely studied, the role of long-chain branches within isotactic polypropylene (iPP) under shear-free conditions and a comprehensive understanding of its isothermal crystallization kinetics over a wide crystallization temperature (T_c) range are still lacking. The samples in this work were prepared by solution casting to eliminate any possible shear effects. Then, the impact of LCB on the crystallization behavior of iPP with varying content of long-chain branches (LCB) was investigated in the entire crystallization window range. Polarized light optical microscopy (PLOM) and scanning electron microscopy (SEM) revealed that the iPP nucleation density increased with branching content. Standard differential scanning calorimetry (DSC) and ultrafast scanning chip calorimetry (or FSC, fast scanning calorimetry) showed that the overall crystallization rates had different dependence on the branching content across a wide temperature range (5–136 °C). Above 95 °C, only the α-phase of iPP formed, with an increasing crystallization rate with branching content. In addition to the typical heterogeneous nucleation, the LCBs provoke a significant increase in the nucleation density through a homogeneous nucleation process triggered by the topological constraints of the segments linked to the branch point. These constraints stabilize the PP chain’s helical structure and facilitate the self-assembly of these neighboring chains into primary nuclei. Between 45 and 95 °C, the crystallization rates with varying branching content did not show a specific trend, as there was a competition between primary nucleation and diffusion. Below 45 °C, where supercooling was high and homogeneous nucleation became dominant, the crystallization rates were similar for LCB-PP and neat linear iPP. This work deepens the understanding of the role of LCB in iPP crystallization, providing critical insights into both the nucleation mechanisms and crystallization kinetics of polymers over a broader T_c range.

中文翻译：

---

长链支化在较宽的温度范围内对聚丙烯成核和结晶的影响，不受剪切的影响


尽管长链支链聚丙烯 （LCB-PP） 的结晶已被广泛研究，但长链支链在等规聚丙烯 （iPP） 中在无剪切条件下的作用以及对其在较宽结晶温度 （T_c） 范围仍然缺失。这项工作中的样品是通过固溶铸造制备的，以消除任何可能的剪切效应。然后，在整个结晶窗口范围内研究了 LCB 对长链支链 （LCB） 含量不同 iPP 结晶行为的影响。偏振光光学显微镜 （PLOM） 和扫描电子显微镜 （SEM） 显示 iPP 成核密度随支化含量的增加而增加。标准差示扫描量热法 （DSC） 和超快扫描芯片量热法（或 FSC，快速扫描量热法）表明，在较宽的温度范围 （5–136 °C） 内，总结晶速率与支化含量具有不同的依赖性。在 95 °C 以上，仅形成 iPP 的 α 相，结晶速率随着支化含量的增加而增加。除了典型的异质成核外，LCB 还通过由与分支点相连的片段的拓扑约束触发的均匀成核过程引起成核密度的显着增加。这些约束稳定了 PP 链的螺旋结构，并促进了这些相邻链自组装成初级原子核。在 45 到 95 °C 之间，不同支化含量的结晶速率没有显示出特定的趋势，因为初级成核和扩散之间存在竞争。 在 45 °C 以下，过冷度高且均匀成核占主导地位，LCB-PP 和纯线性 iPP 的结晶速率相似。这项工作加深了对 LCB 在 iPP 结晶中的作用的理解，为在更广泛的 T_c 范围内对聚合物的成核机制和结晶动力学提供了重要见解。