Density of Physical Network Junctions in the Amorphous Phase of Polyethylene and Its Copolymers: The Effect of Crystallinity, Domain Sizes, and Molecular Weight,Macromolecules

当前位置： X-MOL 学术 › Macromolecules › 论文详情

Our official English website, www.x-mol.net, welcomes your feedback! (Note: you will need to create a separate account there.)

Density of Physical Network Junctions in the Amorphous Phase of Polyethylene and Its Copolymers: The Effect of Crystallinity, Domain Sizes, and Molecular Weight
Macromolecules ( IF 5.1 ) Pub Date : 2024-09-02 , DOI: 10.1021/acs.macromol.4c01200
Shiyan Li _{1,

2} , Victor M. Litvinov _{1,

3} , Yongfeng Men _{1,

2}

Affiliation

The ¹H NMR T₂ relaxation method is explored in the present study for the characterization of the effect of molecular weight, crystallinity, and comonomer units on the density of the network of physical junctions in the amorphous phase of polyethylene (PE) and its copolymers. The build-up of the network of physical junctions in the amorphous phase was studied during isothermal crystallization using real-time NMR experiments, which recorded the free induction decay of the crystalline phase and Hahn-echo decay of the amorphous phase. Two high-density polyethylenes (HDPEs) with similar M_n and different molecular weight distributions, ultrahigh-molecular-weight polyethylene (UHMWPE), and three ethylene copolymers with largely different amounts of short-chain branches were selected for the study. The density of crystal-induced physical junctions, which form tie molecules, increases linearly with the crystallinity increase in UHMWPE and PE copolymers. A partial disentangling is observed at the initial stage of the crystallization of low-molecular-weight HDPE. The density of tie molecules depends on the molecular weight of polymer chains and the ratio between the radius of gyration of polymer chains and the thickness of crystalline and amorphous domains. The lower the molecular weight and the smaller the ratio, the larger the fraction of chain end segments residing in interlamellar amorphous layers and the smaller the density of tie molecules. The increase in the fraction of short-chain branches in PE copolymers causes a large decrease in the long-range periodicity of the lamellar structure, a decrease in the lamellar thickness and the thickness of interlamellar amorphous layers, as shown by small-angle X-ray scattering experiments. The decrease leads to an increase in the relative fraction of network chains in the amorphous phase and the density of network junctions. The NMR method will be further explored in a follow-up study of model polyethylenes to establish the effect of molecular weight characteristics on the density of physical network junctions. Obtaining this knowledge could help in better understanding of stress–strain behavior and several other mechanical properties of polyolefins.

中文翻译：

聚乙烯及其共聚物非晶相中物理网络结的密度：结晶度、畴大小和分子量的影响

本研究探讨了 ¹H NMR T₂ 弛豫方法，用于表征分子量、结晶度和共聚单体单位对聚乙烯（PE）及其共聚物非晶相中物理结网络密度的影响。使用实时 NMR 实验在等温结晶过程中研究了非晶相中物理结网络的积累，该实验记录了结晶相的自由感应衰变和非晶相的 Hahn 回波衰变。该研究选择了两种具有相似 M_n 和不同分子量分布的高密度聚乙烯（HDPEs）、超高分子量聚乙烯（UHMWPE）和三种短链支链数量大致不同的乙烯共聚物。在 UHMWPE 和 PE 共聚物中，形成连接分子的晶体诱导物理结的密度随着结晶度的增加而线性增加。在低分子量 HDPE 结晶的初始阶段观察到部分解缠。连接分子的密度取决于聚合物链的分子量以及聚合物链的回转半径与结晶和非晶域的厚度之间的比率。分子量越低，比率越小，位于层间非晶层中的链端段的分数就越大，连接分子的密度就越小。正如小角 X 射线散射实验所示，PE 共聚物中短链支链分数的增加导致层状结构的长程周期性大幅降低，层状厚度和层间非晶层厚度的减小。这种降低导致非晶相中网络链的相对分数和网络结密度的增加。在模型聚乙烯的后续研究中，将进一步探索 NMR 方法，以确定分子量特性对物理网络结密度的影响。获得这些知识有助于更好地了解聚烯烃的应力-应变行为和其他几种机械性能。

更新日期：2024-09-02

点击分享查看原文

点击收藏

阅读更多本刊新发论文本刊介绍/投稿指南