Zhao Yan, Liang Yuling, Yao Yingjie, Wang Hao, Lin Tong, Gao Yun, Wang Xiaoliang, Xue Gi
Key Laboratory of High-Performance Polymer Materials and Technology of Ministry of Education, Department of Polymer Science and Engineering, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, China.
South China Advanced Institute for Soft Matter Science and Technology, School of Emergent Soft Matter, South China University of Technology, Guangzhou 510640, China.
Polymers (Basel). 2023 Apr 16;15(8):1910. doi: 10.3390/polym15081910.
Melts of ultrahigh molecular weight polyethylene (UHMWPE) entangled significantly, suffering processing difficulty. In this work, we prepared partially disentangled UHMWPE by freeze-extracting, exploring the corresponding enchantment of chain mobility. Fully refocused H free induction decay (FID) was used to capture the difference in chain segmental mobility during the melting of UHMWPE with different degrees of entanglement by low-field solid-state NMR. The longer the polyethylene (PE) chain is in a less-entangled state, the harder the process of merging into mobile parts after detaching from crystalline lamella during melting. H double quantum (DQ) NMR was further used to obtain information caused by residual dipolar interaction. Before melting, the DQ peak appeared earlier in intramolecular-nucleated PE than in intermolecular-nucleated PE because of the strong constraints of crystals in the former one. During melting, less-entangled UHMWPE could keep disentangled while less-entangled high density polyethylene (HDPE) could not. Unfortunately, no noticeable difference was found in DQ experiments between PE melts with different degrees of entanglement after melting. It was ascribed to the small contribution of entanglements compared with total residual dipolar interaction in melts. Overall, less-entangled UHMWPE could reserve its disentangled state around the melting point long enough to achieve a better way of processing.
超高分子量聚乙烯（UHMWPE）熔体的缠结程度很高，加工难度大。在本研究中，我们通过冷冻萃取制备了部分解缠结的UHMWPE，以探究链流动性的相应变化。利用全聚焦氢自由感应衰减（FID），通过低场固态核磁共振捕捉不同缠结程度的UHMWPE在熔融过程中链段流动性的差异。聚乙烯（PE）链处于缠结程度较低状态的时间越长，在熔融过程中从结晶片晶脱离后合并到可移动部分的过程就越困难。进一步利用氢双量子（DQ）核磁共振来获取由残余偶极相互作用引起的信息。在熔融前，由于分子内成核PE中晶体的强约束作用，其DQ峰比分子间成核PE中出现得更早。在熔融过程中，缠结程度较低的UHMWPE能够保持解缠结状态，而缠结程度较低的高密度聚乙烯（HDPE）则不能。遗憾的是，在熔融后不同缠结程度的PE熔体之间的DQ实验中未发现明显差异。这归因于与熔体中总残余偶极相互作用相比，缠结对结果的贡献较小。总体而言，缠结程度较低的UHMWPE能够在熔点附近长时间保持其解缠结状态，从而实现更好的加工方式。
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