There are two opposing views on the role of the melting of low-entangled polyethylenes on the obtained entanglement density in the molten state and the time required for its equilibration, namely, instantaneous recovery to the equilibrium melt state upon melting (chain explosion) versus slow recovery (melt memory effect). A series of rheological studies have shown that slow heating of low-entangled nascent ultrahigh-molecular-weight polyethylene (UHMWPE) powders to temperatures above the equilibrium melting temperature causes the formation of “the disentangled nonequilibrium melt” due to consecutive detachment of chain stems from the edges of crystals keeping the largely intact low-entangled middle part of chains in the melt. Contrary to the rheology findings, studies of several mechanical properties of recrystallized UHMWPE have found that the equilibrium entanglement density is reached instantly upon the melting of low-entangled UHMWPE, suggesting a chain explosion mechanism. To obtain additional information that can help in understanding the melt memory phenomenon better, several UHMWPE samples are studied by ¹H NMR T₂ relaxometry. The NMR experiments are performed for melts prepared from UHMWPE powders with different entanglement densities that were molten using fast (∼10 K/min) and slow (∼0.2 K/min) heating rates. In all cases, the existence of “the disentangled nonequilibrium melt” was not observed. The results are explained by the chain explosion mechanism that leads to the equilibrium volume-average entanglement density, already at the final stage of melting. Cautious rheological experiments also do not detect “the disentangled nonequilibrium melt”. Possible artifacts of previous rheological studies of disentangled UHMWPE melts are discussed. The conclusion of the present study is supported by a large number of previous investigations that are briefly reviewed. Is the disentangled melt state fictitious or real? The answer is yes and no. Chain explosion causes instantaneous equilibration of the volume-average entanglement density upon melting by the formation of local entanglements that play a major role in several volume-average properties, i.e., modulus, drawability, adhesion, segmental mobility, and some other properties. However, the uniform distribution of topological knots between chains is a slow process that is largely governed by chain reptation. The heterogeneity of the entanglement network as well as the impurities in UHMWPE can influence (1) the local nucleation phenomenon at crystallization that does not characterize the entanglement network and (2) deformation properties at ultimate strains. Therefore, the definition of melt memory and chain explosion should be specified to properties that are used for the characterization of low-entangled and equilibrium melt states.

中文翻译：

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超高分子量聚乙烯熔体解开：虚构还是真实？

关于低缠结聚乙烯的熔融对熔融状态下获得的缠结密度及其平衡所需时间的作用，存在两种相反的观点，即熔融时立即恢复到平衡熔融状态（链爆炸）与缓慢恢复到平衡熔融状态的观点不同。恢复（融化记忆效应）。一系列流变学研究表明，将低缠结初生超高分子量聚乙烯 (UHMWPE) 粉末缓慢加热至高于平衡熔融温度的温度，会导致“解缠结的非平衡熔体”的形成，这是由于链源于连续分离。晶体的边缘保持熔体中链的基本完整的低缠结中间部分。与流变学研究结果相反，对重结晶 UHMWPE 的几种机械性能的研究发现，低缠结 UHMWPE 熔化后立即达到平衡缠结密度，这表明存在链式爆炸机制。为了获得有助于更好地理解熔融记忆现象的附加信息，通过¹ H NMR T ₂弛豫测定法研究了几种 UHMWPE 样品。 NMR 实验针对由具有不同缠结密度的 UHMWPE 粉末制备的熔体进行，这些熔体使用快速（~10 K/min）和慢速（~0.2 K/min）加热速率进行熔化。在所有情况下，都没有观察到“解开的非平衡熔体”的存在。结果可以通过链式爆炸机制来解释，该机制在熔化的最后阶段就已经达到了平衡体积平均纠缠密度。谨慎的流变实验也没有检测到“解开的非平衡熔体”。讨论了先前解缠结 UHMWPE 熔体流变学研究中可能出现的假象。本研究的结论得到了之前大量调查的简要回顾的支持。解开的熔化状态是虚构的还是真实的？答案是肯定和否定。链式爆炸通过形成局部缠结，在熔化时导致体积平均缠结密度瞬时平衡，局部缠结在多种体积平均性能（即模量、可拉伸性、粘合性、链段迁移率和一些其他性能）中起主要作用。然而，链之间拓扑结的均匀分布是一个缓慢的过程，很大程度上受链蠕动的控制。缠结网络的异质性以及 UHMWPE 中的杂质会影响 (1) 结晶时的局部成核现象，而该现象并不表征缠结网络；以及 (2) 极限应变下的变形特性。因此，熔体记忆和链式爆炸的定义应指定为用于表征低纠缠和平衡熔体状态的属性。