Dow, Lake Jackson, Texas 77566, United States.
Bruker BioSpin AG, Industriestrasse 26, CH-8117 Fällanden, Switzerland.
Anal Chem. 2020 Jun 16;92(12):8350-8355. doi: 10.1021/acs.analchem.0c00834. Epub 2020 May 22.
Polyolefin microstructures, for example, short chain branching (SCB) and short chain branch distribution (SCBD), have a direct impact on properties and thus ultimately influence end-use applications. The H NMR approach to analyze SCB and SCBD is particularly useful when only a limited amount of sample is available, for example, polyolefin film layers or the fractions from polyolefin separation techniques, such as gel permeation chromatography (GPC), crystallization elution fractionation (CEF), high temperature liquid chromatography (HTLC), and thermal gradient interaction chromatography (TGIC). In this paper, we discuss the best approach to find a good decoupling frequency and propose an improved H pulse sequence with homonuclear decoupling for better measuring SCB. With this new pulse it is possible to reach a S/N of 10 (level of quantification) for the methyl signal from SCB in an ethylene-hexene copolymer (EH, 3.6 mol % H) in 3.5 min with 0.5 μg of sample. We also show an easy method to calculate SCB/1000C and demonstrate the proper use of heteronuclear single quantum coherence (HSQC) to measure SCB in a complicated system. A very quick approach to examine the presence of a small amount of LDPE in a polyolefin sample is also suggested, which can reduce NMR acquisition time from a couple of days to a few minutes.
聚烯烃的微观结构，例如短支链（SCB）和短支链分布（SCBD），对性能有直接影响，因此最终影响最终用途的应用。当只有有限量的样品可用时，例如聚烯烃薄膜层或聚烯烃分离技术（如凝胶渗透色谱（GPC）、结晶洗脱分级（CEF）、高温液相色谱（HTLC）和热梯度相互作用色谱（TGIC）的馏分，采用 H NMR 方法分析 SCB 和 SCBD 特别有用。在本文中，我们讨论了找到良好去耦频率的最佳方法，并提出了一种改进的带有同核去耦的 H 脉冲序列，以更好地测量 SCB。使用这种新的脉冲，可以在 3.5 分钟内，用 0.5μg 的样品，达到乙烯-己烯共聚物（EH，3.6mol%H）中 SCB 的甲基信号的 S/N 为 10（定量水平）。我们还展示了一种简单的方法来计算 SCB/1000C，并演示了如何在复杂体系中使用异核单量子相干（HSQC）来测量 SCB。还提出了一种快速检查聚烯烃样品中少量 LDPE 存在的方法，这可以将 NMR 采集时间从几天缩短到几分钟。
