Unveiling the Viscoelastic Response of Nonequilibrium Low-Entangled Polymer Melt during Its Equilibration,Macromolecules

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Unveiling the Viscoelastic Response of Nonequilibrium Low-Entangled Polymer Melt during Its Equilibration
Macromolecules ( IF 5.1 ) Pub Date : 2024-09-09 , DOI: 10.1021/acs.macromol.4c01116
Huaqin Yang ₁ , Ameur Louhichi ₂ , Dario Romano _{1,

2} , Sanjay Rastogi _{1,

2}

Affiliation

New rheological experiments are presented to investigate the equilibration mechanism of low-entangled, high-molar-mass linear polyethylene upon melting. The evolution of linear viscoelastic spectra is monitored over time. Though the plateau modulus and the conventional crossover point remain stable with time, the terminal flow region exhibits intriguing temporal evolution. A low-frequency second crossover point is observed that moves toward higher moduli and frequencies over time. A phenomenological model is proposed to explain these complex spectra, which correlates strongly with the molecular mechanism proposed by McLeish that rationalizes the kinetics involved in the melting of these materials. We propose a two-step equilibration process. The first step involves equilibration of the nonequilibrium polymer melt, as reflected by the plateau modulus. This equilibration is driven by the mixing of chains upon melting of the low-entangled crystals, resulting in a heterogeneous distribution of topological constraints. This difference in constraints is delineated by a core–shell structure in the polymer melt. The second step addresses the dynamics at larger length scales, suggesting cooperative diffusion of a core–shell heterogeneous structure similar to colloidal dynamics. This second step is more pronounced and experimentally accessible in relatively low molecular weight samples due to the gradual decrease in core size, whereas high molecular weight samples exhibit critical gel-like dynamics at large length scales.

中文翻译：

揭示非平衡低缠结聚合物熔体平衡过程中的粘弹性响应

提出了新的流变实验来研究低缠结、高摩尔质量线性聚乙烯熔化时的平衡机制。随着时间的推移，监测线性粘弹性光谱的演变。尽管平台模量和传统交叉点随时间保持稳定，但终端流动区域表现出有趣的时间演变。观察到低频第二交叉点随着时间的推移向更高的模量和频率移动。提出了一个现象学模型来解释这些复杂的光谱，该模型与麦克利什提出的分子机制密切相关，该分子机制合理化了这些材料熔化所涉及的动力学。我们提出了一个两步平衡过程。第一步涉及非平衡聚合物熔体的平衡，如平台模量所反映的。这种平衡是由低缠结晶体熔化时链的混合驱动的，导致拓扑约束的异质分布。这种约束差异由聚合物熔体中的核壳结构来描述。第二步解决了更大长度尺度的动力学问题，表明类似于胶体动力学的核壳异质结构的协同扩散。由于核心尺寸逐渐减小，第二步在相对低分子量的样品中更加明显，并且在实验上更容易实现，而高分子量样品在大长度尺度上表现出关键的凝胶状动力学。

更新日期：2024-09-09

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